CN118867131A - Negative electrode sheet for lithium secondary battery, preparation method thereof, lithium secondary battery and electric device - Google Patents

Abstract

The invention relates to a negative electrode plate for a lithium secondary battery, a preparation method thereof, the lithium secondary battery and an electric device. The negative electrode tab for a lithium secondary battery includes a negative electrode base layer, wherein the negative electrode base layer includes at least one metal sodium or sodium alloy inactive layer. According to the invention, by arranging the metal sodium or sodium alloy inactive layer and optionally adjusting the content of sodium metal in the negative electrode substrate layer, the energy density of the lithium secondary battery can be improved, the manufacturing cost can be reduced, the growth of lithium dendrites on the surface of the negative electrode plate can be avoided, and the formation of lithium dendrites can be eliminated.

Description

Negative electrode sheet for lithium secondary battery, preparation method thereof, lithium secondary battery and power-using device

Technical Field

The present invention relates to the field of lithium secondary batteries, and in particular to a negative electrode sheet for a lithium secondary battery, a preparation method thereof, a lithium secondary battery and an electrical device.

Background Art

Lithium secondary batteries are important devices for electric energy transfer and have been widely used in new energy vehicles, portable electronic products, energy storage and other fields. As the negative electrode active material of lithium secondary batteries, the low theoretical specific capacity of graphite limits the further improvement of battery energy density. In contrast, metal negative electrodes have ultra-high theoretical specific capacity and extremely low redox potential, and have become the focus of research in recent years. In particular, metal lithium negative electrodes have extremely high theoretical specific capacity and the lowest electrode potential, which has received widespread attention.

At present, there are still some technical problems in directly using metallic lithium as the negative electrode. Metallic lithium has extremely high reactivity, and will react chemically/electrochemically with most electrolytes and cause rapid consumption of metallic lithium and electrolytes, reducing the coulombic efficiency of the battery. In addition, lithium atoms have an oriented growth trend during the deposition process, which makes metallic lithium tend to be deposited in a dendritic manner, which leads to huge volume deformation of metallic lithium during the cycle, making it difficult to form a stable solid electrolyte interface layer (SEI) on the surface of metallic lithium, and there is a risk of dendrites piercing the diaphragm and causing battery short circuit. In addition, the uneven deposition/stripping of lithium ions causes lithium dendrites to easily peel off from the main body of metallic lithium and generate dead lithium, resulting in continuous loss of battery capacity and easy battery explosion. The SEI formed by metallic lithium during the cycle is conducive to blocking the mutual consumption between metallic lithium and electrolyte, but conventional electrolytes cannot form a stable solid electrolyte interface film on the metallic lithium electrode, thereby failing to provide long-term, high-efficiency protection for metallic lithium batteries.

Considering that the intrinsic deposition law of metallic lithium leads to the growth of lithium dendrites, there is a proposal to interfere with the growth mode of metallic lithium dendrites by co-deposition of other metal ions. Although the redox potential of lithium is the most negative, it is difficult for ordinary ions to co-deposit with lithium ions, but the redox potentials of sodium and lithium are relatively close. If lithium ions and sodium ions exist in the electrolyte at the same time, lithium and sodium, as active substances, may co-deposit or co-extract within a certain potential range in the process of jointly participating in charge transfer, thereby breaking the intrinsic growth law of metallic lithium dendrites, thereby weakening or even eliminating the formation of lithium dendrites. However, the kinetic characteristics of sodium ion transmission in the battery are weaker than those of lithium ions, and the kinetic characteristics of batteries with lithium ions and sodium ions as mixed carriers are weaker than those of batteries with pure lithium ion carriers. At the same time, when lithium ions and sodium ions are co-extracted from the negative electrode, the negative electrode structure is difficult to maintain stability. In addition, lithium ions and sodium ions acting as carriers together narrow the range of choices for the positive electrode materials of the battery and increase the difficulty of managing the battery cells. It is worth mentioning that the energy density of sodium ion batteries is generally lower than that of lithium secondary batteries.

CN107093706A discloses a method for preparing a negative electrode of a lithium battery, which relates to the field of solid-state batteries and liquid batteries. It solves the problems of low specific capacity, easy growth of dendrites, low initial coulomb efficiency, and low battery energy density of traditional graphite negative electrodes. However, the negative electrode of the lithium battery has the following disadvantages in terms of structure and preparation method: (1) Complex structure. In order to inhibit the growth of lithium dendrites, the intermediate material layer of the negative electrode needs to play a role. It uses the intermediate material layer to regulate the distribution of lithium ions on the surface of the negative electrode, avoid the aggregation of lithium ions in time and space, and achieve uniform distribution of lithium ions on the surface of the negative electrode, thereby inhibiting the appearance of lithium dendrites. In the negative electrode structure of the lithium battery, it is necessary to rely on the intermediate material layer to play a role in order to inhibit the generation of lithium dendrites; (2) It is difficult to avoid the consumption of active lithium. For lithium batteries, the negative electrode protective film (SEI) has a repeated regeneration phenomenon during the use of the battery. In this negative electrode structure, as an active material, the metal covering the surface of the intermediate material layer cannot ensure that the negative electrode SEI does not consume lithium during the regeneration process, thereby reducing the capacity of the battery; (3) The intermediate material layer is located below the metal layer, which makes it difficult to effectively increase the specific surface area of the metal layer, increases the current density on the negative electrode surface during the battery cycle, and easily leads to excessive formation of SEI and excessive passivation of the negative electrode by SEI.

Summary of the invention

In order to solve the problems existing in the prior art, the present application provides a negative electrode plate with a simple process and an inactive layer of metallic sodium or a sodium alloy, and optionally by adjusting the specific surface area, shape structure of the metallic sodium or the sodium alloy, or adding a negative electrode protective film and/or a negative electrode enhancement film, it is possible to effectively solve the problems of lithium dendrite growth, lithium consumption by forming a negative electrode protective film (SEI), short battery cycle life, high complexity of the existing process, and low electrode conductivity.

Furthermore, the present application generates a sodium-based negative electrode protective film (SEI) by a self-limiting reaction between the inactive negative electrode base layer and the electrolyte, participates in the formation and regeneration of the sodium-based negative electrode protective film by consuming sodium elements, and reduces the consumption of active lithium. The negative electrode protective film contains inorganic fluoride, which has a stable structure and is difficult to dissolve in the electrolyte. It can inhibit the continuous reaction of the negative electrode base layer and the electrolyte, and has a strong lithium ion conduction ability. The negative electrode base layer serves as a substrate for lithium deposition and can inhibit the formation of lithium dendrites.

In lithium secondary batteries, the active material undergoes a valence change during the battery charge and discharge process, and a reversible electrochemical reaction occurs. In the present invention, lithium ions are reduced in the form of metallic lithium during the charging process, and metallic lithium is oxidized to lithium ions during the discharge process, so the active material is metallic lithium. Specifically, sodium is not embedded in the positive electrode material or the positive electrode material preferentially embeds lithium, so sodium ions cannot be reversibly embedded and extracted in the positive electrode material, that is, sodium ions cannot reversibly gain or lose electrons during the battery charge and discharge process, that is, they cannot reversibly participate in the battery charge and discharge. Metallic sodium does not undergo a reversible valence change during the charge and discharge process, so the metallic sodium or sodium alloy is an inactive material, and the metallic sodium or sodium alloy layer is an inactive layer. The metallic sodium is a functional material for achieving uniform deposition of metallic lithium. This functional material is a key material for forming a sodium-based negative electrode protective film and dynamically repairing the negative electrode protective film, and is also one of the core points of the present invention.

The negative electrode plate of the present application can be used in common batteries such as liquid, semi-solid, and solid.

According to a first aspect of the present application, a negative electrode plate for a lithium secondary battery is provided, wherein the negative electrode plate comprises a negative electrode base layer, wherein the negative electrode base layer comprises at least one metallic sodium or sodium alloy inactive layer.

In some embodiments of the present application, the negative electrode plate further includes a sodium-based negative electrode protection film located on at least a portion of the surface of the negative electrode base layer.

In some embodiments of the present application, the negative electrode plate further comprises a negative electrode enhancement film located on at least a portion of the surface of the sodium-based negative electrode protection film. In some embodiments of the present application, the negative electrode plate satisfies at least one of the following conditions:

(1) The specific surface area S _negative of the metallic sodium or sodium alloy inactive layer satisfies 0.001 ^{m 2} /g≤S _{negative≤100} ^{m 2} /g, preferably, 0.01 m ² /g≤S _{negative≤50} m ² /g;

(2) The surface roughness Ra of the metallic sodium or sodium alloy inactive layer satisfies 0.01 μm ≤ Ra ≤ 250 μm, preferably 0.1 μm ≤ Ra ≤ 100 μm; or

(3) The porosity K negative of the metallic sodium or sodium alloy inactive layer _satisfies 0% ≤ K _{negative ≤} 50%, preferably 0.1% ≤ K _negative ≤ 30%.

In some embodiments of the present application, the negative electrode enhancement film includes at least one component selected from non-metallic elements, oxides, fluorides, carbides, carbon materials, nitrides and silicides, and is at least partially embedded in the metallic sodium or sodium alloy inactive layer.

In some embodiments of the present application, the metallic sodium or sodium alloy inactive layer is in the form of a sheet, layer, strip and/or particle structure.

In some embodiments of the present application, the metallic sodium or sodium alloy inactive layer is in the form of a three-dimensional structure.

In some embodiments of the present application, the negative electrode plate includes a current collector for supporting the negative electrode base layer.

In some embodiments of the present application, the surface density C negative of the negative electrode substrate layer _satisfies : 0.2 mg/cm ² ≤ C _negative ≤ 50 mg/cm ² .

In some embodiments of the present application, the thickness d negative of the negative electrode base layer _satisfies : 2 μm≤d _{negative≤500} μm.

In some embodiments of the present application, the metallic sodium or sodium alloy particles are in the form of nanoparticles and/or micron particles, and the median particle size D _{50 −} satisfies 0.05 μm ≤ _{D 50 − ≤} 200 μm.

In some embodiments of the present application, the strip-shaped structure of the metallic sodium or sodium alloy is distributed on the negative electrode substrate in a uniform or non-uniform manner, and the size of the smallest dimension is between 0.05 μm and 200 μm. The smallest dimension refers to the thickness of the metallic sodium or sodium alloy strip under general conditions.

In some embodiments of the present application, the negative electrode base layer further comprises lithium element supplemented in at least one metallic sodium or sodium alloy inactive layer, and the lithium element is 0-70 mass % based on the total mass of the negative electrode base layer.

According to a second aspect of the present application, a lithium secondary battery is provided, comprising a positive electrode plate, an electrolyte and the above-mentioned negative electrode plate. The electrolyte comprises a liquid, semi-solid, solid electrolyte and the like.

In some embodiments of the present application, the lithium secondary battery further includes an isolation membrane, and a second negative electrode reinforcement membrane disposed on at least a portion of the isolation membrane; and/or the isolation membrane itself is at least partially a second negative electrode reinforcement membrane; the second negative electrode reinforcement membrane is at least partially embedded in the metallic sodium or sodium alloy inactive layer.

In some embodiments of the present application, the positive electrode plate includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate and lithium manganese phosphate.

In some embodiments of the present application, at least one anion contained in the electrolyte is selected from fluorine-containing anions; preferably, at least one selected from bis(fluorosulfonyl)imide, hexafluorophosphate, bis(trifluoromethanesulfonyl)imide, difluorooxalatoborate, tetrafluoroborate, hexafluoroarsenate and difluorophosphate.

In some embodiments of the present application, the solvent of the electrolyte is selected from at least one of an ether compound, an acetal compound and a ketal compound.

According to a third aspect of the present application, a method for manufacturing the negative electrode sheet is provided, the method comprising:

(1) pressing metallic sodium or sodium alloy onto the surface of the current collector by cold pressing or hot pressing to form a negative electrode substrate layer having at least one metallic sodium or sodium alloy inactive layer; or

(2) by spraying or sputtering, metal sodium or sodium alloy is compounded onto the surface of the current collector to form a negative electrode substrate layer of metal sodium or sodium alloy inactive layer with a particle structure or a three-dimensional structure; or

(3) Metal sodium or sodium alloy is compounded onto the surface of the current collector by extrusion to obtain a negative electrode substrate layer having a strip-shaped metal sodium or sodium alloy inactive layer.

In some embodiments of the present application, the sodium-based negative electrode protection film is formed by an in-situ reaction of a metallic sodium or sodium alloy inactive layer and an electrolyte.

In some embodiments of the present application, the negative electrode reinforcement film is compounded on the surface of the negative electrode base layer by cold pressing or hot pressing.

In some embodiments of the present application, the isolation film containing the second reinforcement film is compounded on the surface of the negative electrode base layer by cold pressing or hot pressing.

According to a fourth aspect of the present application, there is provided an electrical device comprising the lithium secondary battery.

Compared with the prior art, the present application relies on a metallic sodium or sodium alloy layer as an inactive material layer to inhibit the growth of metallic lithium dendrites, protect active lithium, and reduce the consumption of active lithium. The process is simple and time-consuming. The formation and regeneration of the sodium-based negative electrode protection film (SEI) consumes sodium elements, which reduces the consumption of active lithium and increases the service life of the battery. Moreover, the negative electrode enhancement film can increase the specific surface area of the negative electrode sheet, effectively reducing the current density on the negative electrode surface during the battery cycle, thereby inhibiting the excessive growth of SEI and the excessive passivation of the negative electrode sheet by SEI. The negative electrode enhancement film can inhibit the excessive formation of SEI under high temperature conditions and further optimize the capacity retention rate, kinetic characteristics and service life of the battery.

These and other features and advantages will become apparent by reading the following detailed description and by reference to the associated drawings.It is to be understood that the foregoing general description and the following detailed description are illustrative only and are not restrictive of the aspects of what is claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

1 is a schematic diagram of the structure of a negative electrode sheet in a specific embodiment of the present application, which includes at least one layer of layered metallic sodium or sodium alloy inactive material as a negative electrode base layer, and at least a portion of the negative electrode reinforcement film layer is embedded in a sodium-based negative electrode protection film and/or a negative electrode base layer.

FIG. 2 is a SEM image of the negative electrode substrate layer in Example 3 of the present application after the battery cycle is completed.

FIG3 is a SEM image of the negative electrode substrate layer in Comparative Example 1 of the present application after the battery cycle is completed.

DETAILED DESCRIPTION

The present application is described in detail below in conjunction with the accompanying drawings, and the features of the present application will be further revealed in the following specific description.

"Scope" disclosed herein is defined in the form of lower limit and upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope defined in this way can include or exclude end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a particular parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range "a-b" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

In this application, if there is no special description, all embodiments and preferred embodiments mentioned herein can be combined with each other to form a new technical solution. In this application, if there is no special description, all technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.

In this application, unless otherwise specified, the terms "include" and "comprising" mentioned herein may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

In the description herein, unless otherwise specified, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

It should be understood that, in the description of the present invention, the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

It should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "disposed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

As shown in FIG1 , the negative electrode plate 1 includes a current collector 11, at least one layer of metallic sodium or sodium alloy inactive material formed on the surface of the current collector 11 as a negative electrode base layer 12, a sodium-based negative electrode protection film 13 formed on at least a portion of the surface of the negative electrode base layer 12, and a negative electrode reinforcement film 14 formed on at least a portion of the surface of the sodium-based negative electrode protection film 13.

In the present invention, the metallic sodium or sodium alloy inactive layer of the negative electrode substrate layer and metallic lithium can diffuse and fuse with each other rapidly, and lithium and sodium have different redox reaction potentials. When lithium ions are deposited on the negative electrode surface in the form of metallic lithium dendrites, the lithium dendrites and metallic sodium or sodium alloy with a high specific surface area form a primary battery in a local area. Driven by the redox potential difference between lithium and sodium, the lithium in the lithium dendrites dissolves in the form of ions, and regains electrons on the sodium surface for reduction deposition. The metallic lithium and the metallic sodium or sodium alloy diffuse and fuse with each other to form a uniform sodium alloy, and the final result is: when the metallic lithium is deposited on the surface of the metallic sodium or sodium alloy, no lithium dendrites are generated.

In order to improve the charge and discharge performance of a lithium secondary battery and avoid the growth of lithium dendrites, the present invention provides a negative electrode plate for a lithium secondary battery, wherein the negative electrode plate comprises a negative electrode base layer, wherein the negative electrode base layer comprises at least one metallic sodium or sodium alloy inactive layer. In a preferred embodiment of the present invention, the negative electrode substrate layer can meet the following conditions: (1) Under the premise of being able to fully eliminate lithium dendrites, the surface density C of the negative electrode substrate layer _is as small as possible, thereby improving the energy density of the battery, while reducing the average distance of lithium ion transmission in the negative electrode, optimizing the kinetic characteristics of the battery, and reducing the manufacturing cost of the battery; (2) Under the premise of not consuming too much electrolyte for forming a sodium-based negative electrode protective film, the specific surface area S of the negative electrode substrate layer _is as large as possible, which is beneficial to increase the number of sites or channels for lithium ion embedding in the negative electrode substrate layer while reducing the negative electrode current density and the thickness of the SEI film, and further optimizing the capacity retention rate and rate performance of the battery; (3) Under the premise of not consuming too much electrolyte for forming a sodium-based negative electrode protective film, the porosity K of the negative electrode substrate layer _is as large as possible, which is beneficial to increase the contact area between the negative electrode and the electrolyte, reduce the negative electrode current density and the thickness of the SEI film, promote the diffusion of electrolyte and ions, and promote the fusion of metallic lithium and metallic sodium or sodium alloy, thereby further reducing the probability of lithium dendrite formation. In a preferred embodiment of the present invention, when the negative electrode base layer has a granular structure, under the premise of not consuming too much electrolyte for forming a sodium-based negative electrode protective film, the particles constituting the negative electrode base layer are as small as possible, that is, the _{D50 of the median particle size of the particles is} as small as possible, which is beneficial to reducing the diffusion distance of lithium ions inside the negative electrode particles, increasing the comparative area of the negative electrode sheet, reducing the negative electrode current density and the thickness of the SEI film, reducing the probability of lithium dendrite formation, and further optimizing the rate performance of the battery.

In some embodiments of the present application, the negative electrode plate also includes a sodium-based negative electrode protective film located on at least part of the surface of the negative electrode base layer (metal sodium or sodium alloy inactive layer), and the sodium-based negative electrode protective film has good lithium ion conduction ability and can hinder the passage of solvent molecules, which can avoid the generation of lithium dendrites and avoid the continuous side reaction between electrolyte and metal sodium or sodium alloy. Unlike the SEI of traditional lithium secondary batteries, the sodium-based negative electrode protective film (SEI) described in the present application mainly consumes sodium rather than lithium during the formation and regeneration process, which can reduce the consumption of active lithium. Moreover, the metal sodium or sodium alloy layer can be used as a source of sodium in the sodium-based negative electrode protective film, and can continuously provide sodium for the regeneration of the sodium-based negative electrode protective film. Metal sodium or sodium alloy is an inactive layer, and the sodium ions therein do not participate in the charge transfer during the battery charging and discharging process as carriers. Sodium does not undergo a reversible valence change during the battery charging and discharging process, and has the effect of dissolving and protecting active lithium and inhibiting the generation of lithium dendrites. The sodium-based negative electrode protective film in the present invention is a type of solid electrolyte interface film (SEI) located on the surface of the negative electrode substrate layer. In the present invention, the sodium-based negative electrode protective film is formed by in-situ reaction of metallic sodium or sodium alloy and electrolyte, or is artificially constructed on the surface of metallic sodium or sodium alloy.

In some embodiments of the present application, the negative electrode plate further includes a negative electrode enhancement film located on at least part of the surface of the sodium-based negative electrode protection film and/or metallic sodium or sodium alloy, for increasing the specific surface area of the negative electrode substrate layer. In some embodiments of the present application, the negative electrode enhancement film is a substance that does not chemically react with the electrolyte. Preferably, the negative electrode enhancement film may further be a substance that does not chemically react with the electrolyte, metallic lithium and metallic sodium or sodium alloy at the same time. The negative electrode enhancement film includes at least one component selected from non-metallic elements, oxides (e.g., aluminum oxide), fluorides (e.g., sodium fluoride), carbides (e.g., tungsten carbide), carbon materials (e.g., carbon nanotubes), nitrides (e.g., silicon nitride) and silicides (e.g., titanium silicide), and is at least partially embedded in the metallic sodium or sodium alloy inactive layer. Preferably, the components of the negative electrode enhancement film are sodium fluoride, lithium fluoride, aluminum oxide, boron nitride, carbon nanotubes, carbon black or a combination thereof. When the negative electrode enhancement film is selected within the above range, the consumption rate of active materials in the battery is low, the structure of the negative electrode enhancement film is stable, which is beneficial to maintaining a high specific surface area of the negative electrode base layer, reducing the current density of the negative electrode, reducing the probability of short circuit between the positive and negative electrodes of the battery, and increasing the service life of the battery.

In some embodiments of the present application, the specific surface area S _negative of the metallic sodium or sodium alloy inactive layer satisfies 0.001m ² /g≤S _{negative≤100m} ² /g, 0.01m ² /g≤S _{negative≤50m} ² /g, 0.1m ² /g≤S _{negative≤30m} ² /g, 1m ² /g≤S _{negative≤10m} ² /g or any numerical range consisting of the end values of the above range. It should be particularly emphasized that although the above numerical values are listed in parallel, it does not mean that the numerical range consisting of any two of the above numerical values as endpoints can obtain equivalent or similar performance. The same applies to the numerical ranges mentioned below. Regarding the preferred embodiments of the present application, the selection is made only based on the specific discussion below and specific experimental data. In an optional embodiment of the present application, 0.01m ² /g≤S _{negative≤50m} ² /g, wherein S _negative is the specific surface area of the negative electrode substrate layer, in units of m ² /g. In the present invention, when the specific surface area S of the negative electrode substrate layer is _controlled within the above range, the process of lithium element dissolving into the negative electrode substrate layer can be made easier, which is beneficial to the rapid elimination of lithium dendrites in the negative electrode substrate layer and improves the high current charge and discharge performance of the secondary battery. Moreover, a higher specific surface area is beneficial to reducing the actual current density of the negative electrode during the battery cycle, reducing the thickness of the SEI film, preventing excessive passivation of the negative electrode by SEI, and further optimizing the capacity retention rate of the secondary battery.

In some embodiments of the present application, the surface roughness of the inactive layer of the metallic sodium or sodium alloy satisfies 0.01 μm ≤ Ra ≤ 100 μm, 0.01 μm ≤ Ra ≤ 50 μm, 0.01 μm ≤ Ra ≤ 10 μm, 0.1 μm ≤ Ra ≤ 250 μm, 0.05 μm ≤ Ra ≤ 40 μm. In the present invention, a higher roughness is conducive to reducing the actual current density of the negative electrode sheet, preventing excessive passivation of the negative electrode by SEI, further reducing the probability of lithium dendrite formation, and reducing the processing and manufacturing cost of the negative electrode.

In some embodiments of the present application, the porosity K _negative of the metallic sodium or sodium alloy inactive layer satisfies 0% ≤ K _{negative ≤} 50%, 5% ≤ K _negative ≤ 40%, 10% ≤ K _negative ≤ 30%, 20% ≤ K _negative ≤ 25% or any numerical range composed of the end values of the above ranges. In an optional embodiment of the present application, 0% ≤ K _negative ≤ 50%, wherein K _{negative is} the surface porosity of the negative electrode substrate layer, in units of %. In the present invention, by providing a negative electrode substrate layer with a certain porosity, it is possible to promote the diffusion of electrolytes and ions in the negative electrode, promote the alloying reaction of the negative electrode, and further reduce the probability of lithium dendrite formation. Reduce the current density of the negative electrode, prevent excessive passivation of the negative electrode by SEI, and reduce the thickness of SEI.

In some embodiments of the present application, the metallic sodium or sodium alloy inactive layer includes a lamellar, strip-shaped and/or granular structure. In some embodiments of the present application, the metallic sodium or sodium alloy inactive layer is in the form of a three-dimensional structure. In the present invention, by further setting the shape of the negative electrode substrate layer, the specific surface area of the negative electrode substrate layer is increased, that is, the contact area between the electrolyte and the negative electrode substrate layer is increased, thereby further improving the lithium dissolving performance of the negative electrode substrate layer, further reducing the probability of lithium dendrite formation, reducing the actual current density of the negative electrode, reducing the thickness of the SEI, and preventing SEI from excessive passivation of the negative electrode. The present invention uses a lamellar, strip-shaped, granular, three-dimensional structure metallic sodium or sodium alloy inactive layer as the negative electrode substrate layer, which inhibits the growth of lithium dendrites. At the same time, the above-mentioned negative electrode substrate material has the advantages of large specific surface area, large pore volume and good conductivity, which can improve the energy density, cycle life, rate performance and safety performance of the battery.

In some embodiments of the present application, the negative electrode plate includes a current collector for supporting the negative electrode base layer, such as copper foil.

In some embodiments of the present application, the surface density C _negative of the negative electrode substrate layer satisfies: 0.2mg/cm ² ≤C _negative ≤50mg/cm ² , 1mg/cm ² ≤C _negative ≤20mg/cm ² , 4mg/cm ² ≤C _negative ≤35mg/cm ² , 7mg/cm ² ≤C _negative ≤50mg/cm ² or any numerical range composed of the end values of the above range. In an optional embodiment of the present application, 1mg/cm ² ≤C _negative ≤10mg/cm ² ; C _{negative is} the surface density of the negative electrode substrate layer, in mg/cm ^2. In the present invention, the surface density of the negative electrode substrate layer needs to be controlled within the range of 0.2 to 50mg/cm ^2. If the surface density is too low, the negative electrode substrate layer cannot effectively inhibit the generation of lithium dendrites. If the surface density is too high, the thickness of the negative electrode substrate layer increases, resulting in an increase in the diffusion distance of lithium ions during the charge and discharge process, thereby reducing the fast charging capability, and the energy density of the battery cell will become lower. Controlling the surface density of the negative electrode substrate within the above range can inhibit the generation of lithium dendrites, improve the fast charging capability, and ensure that the battery energy density is not too low. The surface density of the _negative electrode can be adjusted by controlling the parameters of the coating machine. For example, when using extrusion coating, the coating gap, slit gap, flow channel width, flow rate (pump speed), coating speed, etc. can be controlled to control the surface density.

In some embodiments of the present application, the thickness d negative of the negative electrode base layer _satisfies : 2μm≤d _{negative≤500μm} , 10μm≤d _{negative≤400μm} , 20μm≤d _{negative≤300μm} , 50μm≤d _{negative≤200μm} or any numerical range consisting of the end values of the above range. In an optional embodiment of the present application, 10μm≤d _{negative≤100μm} , wherein d _negative is the thickness of the negative electrode base layer in μm. When the thickness of the negative electrode base layer is within the above range, the negative electrode base layer can effectively inhibit the generation of lithium dendrites, improve the service life of the battery, and the battery has a high energy density and reduced manufacturing costs.

In some embodiments of the present application, the metal sodium or sodium alloy particles are in the form of nanoparticles or micron particles, and the median particle size D _{50 negative} satisfies 0.05 μm ≤ _{D 50 negative} ≤ 200 μm, 0.5 μm ≤ D 50 _negative ≤ 150 μm, 1 μm ≤ D ₅₀ negative ≤ 100 μm, 10 μm ≤ D 50 negative _≤ 50 μm or any numerical range composed of the end values of the above range. In an optional embodiment of the present application, 1 μm ≤ _{D 50 negative} ≤ 100 μm; D _{50 negative} is the median particle size of the particles constituting the negative electrode substrate layer, in μm. In the present invention, smaller metal sodium or sodium alloy particles can increase the specific surface area of the negative electrode substrate layer, promote the fusion of lithium and sodium, thereby further reducing the probability of lithium dendrite formation, reducing the current density of the negative electrode, and reducing the thickness of SEI. If the particle size of the metal sodium or sodium alloy particles is too large, the speed of full fusion of lithium and sodium slows down, and the metal sodium or sodium alloy particles are easily broken, resulting in increased internal resistance. In the present invention, the smaller the particle size of the metal sodium or sodium alloy particles, the easier it is to fully and evenly fuse with the negative electrode substrate layer when lithium is embedded in the negative electrode, and the smaller the particles, the more channels for lithium ion embedding and extraction, which is more conducive to improving the speed of lithium-sodium fusion, and the smaller the degree of polarization under high-rate charge and discharge conditions, that is, the better the high-current charge and discharge performance. However, if the particle size of the metal sodium or sodium alloy particles is too small, the specific surface area that can be in contact with the electrolyte is too large, and the SEI film formed or regenerated during the charge and discharge process consumes more metal sodium or sodium alloy and electrolyte, which reduces the service life of the battery.

In some embodiments of the present application, the metallic sodium or sodium alloy particles are distributed on the negative electrode base layer in a uniform or non-uniform manner. In some embodiments, the particle size distribution of the particles in the negative electrode base layer conforms to the statistical normal distribution characteristics. In some embodiments, the size of the smallest dimension of the metallic sodium or sodium alloy particles is between 0.05μm-200μm, between 0.5μm-150μm, between 5μm-100μm, between 30μm-50μm, or any numerical range composed of the end values of the above ranges. In an optional embodiment of the present application, the size of the smallest dimension of the negative electrode base layer is between 1μm-100μm. In some embodiments of the present application, when the negative electrode base layer is in a strip structure, the size of the smallest dimension is the thickness of the strip structure of the negative electrode base layer.

In some embodiments of the present application, a certain amount of lithium may be pre-supplemented into the negative electrode base layer, and the lithium element is 0-70 mass%, 1-60 mass%, 10-50 mass%, 20-30 mass% or any numerical range consisting of the end values of the above ranges, based on the total mass of the negative electrode base layer. In an optional embodiment of the present application, the metallic lithium is 0.1-30 mass% based on the total mass of the negative electrode base layer. In the present invention, the pre-supplemented lithium can compensate for the loss of active lithium in the secondary battery during a long cycle, while keeping the negative electrode from growing lithium dendrites, and improving the service life of the secondary battery. When the content of lithium pre-supplemented in the negative electrode base layer is within the above range, the lithium and sodium in the negative electrode base layer fuse quickly, the diffusion resistance of lithium ions in the negative electrode base layer is small, and the formation of lithium dendrites is effectively suppressed.

In some embodiments of the present application, the atomic number of the sodium element in the negative electrode base layer accounts for 10%-100%, 20%-90%, 30%-80%, 40%-70%, 50%-60%, or any numerical range composed of the end values of the above ranges. In an optional embodiment of the present application, the atomic number of the sodium element in the negative electrode base layer accounts for 50%-100%. In the present invention, the use of sodium alloys (for example, sodium-lithium alloys, sodium-magnesium alloys, sodium-potassium alloys) as the negative electrode base layer is conducive to reducing the manufacturing cost of the negative electrode plate, further improving the ability of the negative electrode plate to suppress lithium dendrites, further improving the ability of the negative electrode base layer to dissolve lithium, further reducing the consumption of active lithium, and further extending the cycle life of the secondary battery.

According to the second aspect of the present application, a lithium secondary battery is provided, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte spaced between the positive electrode sheet and the negative electrode sheet. In one embodiment of the present invention, the solvent of the electrolyte has low reactivity with the metallic sodium or sodium alloy layer, which is one of the reasons why the metallic sodium or sodium alloy layer is called an inactive layer. In the present invention, the lithium secondary battery uses lithium ions as carriers to participate in the charge transfer during the battery charging and discharging process. Different from the mechanism in which lithium/sodium ion secondary batteries in the prior art use lithium ions and sodium ions as carriers, the sodium element in the negative electrode substrate layer of the present invention does not act as a carrier in the electrolyte of the secondary battery, and will not reversibly participate in the transmission or transfer of charge during the battery charging and discharging process, which is one of the reasons why the metallic sodium or sodium alloy is called the metallic sodium or sodium alloy inactive layer in the present invention. The metallic sodium or sodium alloy in the present invention is used to dissolve lithium elements, eliminate lithium dendrites, and form a sodium-based negative electrode protective film. The present invention uses cations of a single metal as carriers, which is conducive to reducing the manufacturing, use, and management costs of the battery. In the prior art, sodium ions in batteries must act as carriers and reversibly participate in charge transmission or transfer during the battery charging and discharging process together with lithium ions. The participation of sodium ions in electrochemical reactions will lead to a decrease in the kinetic performance of secondary batteries, increasing the cost and difficulty of battery manufacturing, use, and management.

In some embodiments of the present application, the lithium secondary battery further comprises a second negative electrode reinforcement film disposed on at least a portion of the separator; and/or

The isolation film itself is at least partially a second negative electrode enhancement film;

The second negative electrode enhancement film is at least partially embedded in the metallic sodium or sodium alloy inactive layer. In the present invention, the negative electrode enhancement film can be compacted above the negative electrode base layer (or facing the positive electrode plate side) or below the negative electrode base layer (or away from the positive electrode plate side).

In some embodiments of the present application, the positive electrode plate includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate and lithium manganese phosphate.

In some embodiments of the present application, the electrolyte comprises a solvent and a fluorine-containing salt. The at least one anion contained in the electrolyte is selected from fluorine-containing anions; preferably, at least one selected from bis(fluorosulfonyl)imide, hexafluorophosphate, bis(trifluoromethanesulfonyl)imide, difluorooxalate borate, tetrafluoroborate, hexafluoroarsenate and difluorophosphate. The fluorine-containing salt includes a lithium salt, including lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium difluorophosphate or a combination thereof. In the present application, the fluorine-containing anion reacts with the negative electrode substrate layer to generate metal fluoride through a self-limiting reaction, which is insoluble in the electrolyte, which is beneficial to improve the stability and ion conductivity of the negative electrode protective film, further inhibit the side reaction between the negative electrode substrate layer and the electrolyte, further inhibit the formation of lithium dendrites, and thus further improve the service life of the battery.

In some embodiments of the present application, the solvent of the electrolyte is selected from at least one of an ether compound, an acetal compound and a ketal compound. The solvent of the electrolyte used in the present invention has low reactivity with the inactive layer of metallic sodium or sodium alloy. In a preferred embodiment, the solvent is an ether solvent. More preferably, the solvent is ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, polyethylene glycol dimethyl ether, ethylene glycol methyl ether ethyl ether, ethylene glycol diethyl ether, diethylene glycol methyl ether ethyl ether, diethylene glycol diethyl ether, triethylene glycol methyl ether ethyl ether, triethylene glycol diethyl ether, tetraethylene glycol methyl ether ethyl ether, tetraethylene glycol diethyl ether, ethylene glycol methyl ether propyl ether, ethylene glycol dipropyl ether, diethylene glycol methyl ether propyl ether, diethylene glycol ethyl ether propyl ether, diethylene glycol dipropyl ether, triethylene glycol methyl ether propyl ether, triethylene glycol ethyl ether At least one of propyl ether, triethylene glycol dipropyl ether, tetraethylene glycol methyl ether propyl ether, tetraethylene glycol ethyl ether propyl ether, tetraethylene glycol dipropyl ether, ethylene glycol methyl ether butyl ether, ethylene glycol ethyl ether butyl ether, ethylene glycol propyl ether butyl ether, ethylene glycol dibutyl ether, diethylene glycol methyl ether butyl ether, diethylene glycol ethyl ether butyl ether, diethylene glycol propyl ether butyl ether, diethylene glycol dibutyl ether, triethylene glycol methyl ether butyl ether, triethylene glycol ethyl ether butyl ether, triethylene glycol propyl ether butyl ether, triethylene glycol dibutyl ether, tetraethylene glycol methyl ether butyl ether, tetraethylene glycol ethyl ether butyl ether, tetraethylene glycol propyl ether butyl ether, and tetraethylene glycol dibutyl ether.

In some embodiments of the present application, the concentration of the fluoride salt in the electrolyte is 0.5mol/L-7mol/L, 1mol/L-5mol/L, 2mol/L-4mol/L or any numerical range consisting of the end values of the above range. In some embodiments of the present application, the electrolyte also includes additives, and the additives include, but are not limited to: 1,3-dioxolane, 1,4-dioxane, trioxymethylene, lithium nitrate, lithium fluorosulfonate, sulfur dioxide, fluoroethylene carbonate, 12-crown ether-4, 15-crown ether-5 and 18-crown ether-6. In some embodiments, the mass content of the additive in the electrolyte is 0.1% to 20%, 1% to 15% or 5% to 10%.

According to the third aspect of the present application, a method for manufacturing the negative electrode sheet is provided, the method comprising: heating metallic sodium, or a mixture of metallic sodium and a certain proportion of other metal powders to completely melt and fully and evenly stirring for 1.5 to 3 hours under an inert atmosphere, and cooling to obtain a negative electrode substrate material. In some embodiments, the negative electrode substrate material is compounded to the surface of the current collector by cold pressing or hot pressing to form a negative electrode substrate having at least one metallic sodium or sodium alloy inactive layer (in a layered structure). In some embodiments, metallic sodium or sodium alloy is compounded to the surface of the current collector by atomization spraying or sputtering to form a negative electrode substrate layer of a metallic sodium or sodium alloy inactive layer with a granular structure or a three-dimensional structure. In some embodiments, metallic sodium or sodium alloy is compounded to the surface of the current collector by extrusion to obtain a negative electrode substrate layer having a metallic sodium or sodium alloy inactive layer with a strip structure. In the present invention, by setting the shape of the metallic sodium or sodium alloy inactive layer/negative electrode base layer, the specific surface area of the negative electrode base layer is increased, and the contact area between the electrolyte and the negative electrode base layer is increased, thereby further increasing the rate of mutual fusion of lithium and sodium in the negative electrode base layer, reducing the current density of the negative electrode, and reducing the thickness of the SEI.

In some embodiments of the present application, the sodium-based negative electrode protective film is formed by an in-situ reaction between a metallic sodium or sodium alloy inactive layer and an electrolyte, or is artificially constructed on the surface of the negative electrode substrate layer. When the sodium-based negative electrode protective film is formed by an in-situ reaction between metallic sodium or a sodium alloy and an electrolyte, at least one anion in the electrolyte is at least one of bisfluorosulfonyl imide, hexafluorophosphate, bistrifluoromethanesulfonyl imide, difluorooxalate borate, tetrafluoroborate, hexafluoroarsenate, and difluorophosphate. Preferably, at least one anion in the electrolyte is a perfluoro anion. The solvent in the electrolyte is at least one of an ether compound, an acetal compound, or a ketal compound. In the present invention, when at least one anion in the electrolyte is a fluorine-containing anion, the sodium-based negative electrode protective film has a good lithium ion conduction ability, and can hinder the passage of solvent molecules, reducing the consumption of lithium ions in the formation and regeneration process of the sodium-based negative electrode protective film. The main component of the sodium-based negative electrode protective film includes sodium fluoride. The sodium-based negative electrode protective film may also contain a certain amount or a small amount of lithium fluoride.

In some embodiments of the present application, the negative electrode enhancement film is composited on the surface of the negative electrode base layer by cold pressing or hot pressing. In some embodiments of the present application, the isolation film containing the second negative electrode enhancement film is composited on the surface of the negative electrode base layer by cold pressing or hot pressing.

According to a fourth aspect of the present application, there is provided an electrical device comprising the lithium secondary battery.

Lithium secondary battery

The lithium secondary battery of the present application includes a positive electrode (pole sheet), a negative electrode (pole sheet), an electrolyte (including liquid, semi-solid, solid electrolyte, etc.). During the battery charging and discharging process, lithium ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet.

[Positive electrode]

In the secondary battery of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode film layer (or positive electrode active material layer) disposed on at least one surface of the positive electrode current collector and including a positive electrode active material. For example, the positive electrode current collector has two surfaces opposite to each other in the thickness direction of itself, and the positive electrode film layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector. In the secondary battery of the present application, the positive electrode current collector may be a metal foil or a composite current collector, for example, the metal foil may be an aluminum foil, and the composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and its copolymers, etc.).

In the secondary battery of the present application, the positive electrode active material (substance) may be a positive electrode active material for a secondary battery known in the art. For example, the positive electrode active material may include one or more of the following: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for secondary batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO ₂ ), lithium nickel oxide (such as LiNiO ₂ ), lithium manganese oxide (such as LiMnO ₂ , LiMn ₂ O ₄ ), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM333), LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (NCM523), LiNi _0.5 Co _0.25 Mn _0.25 O ₂ (NCM211), LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (NCM622), LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811)), lithium nickel cobalt aluminum oxide (such as LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) and one or more of its modified compounds, etc. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate (such as LiFePO ₄ (LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. In an embodiment of the present application, the second positive electrode active material and the third positive electrode active material are the same or different, and are selected from at least one of lithium iron phosphate (LFP), lithium manganate (LMO), lithium nickel cobalt manganate (NCM), lithium cobaltate (LCO), and lithium nickel cobalt aluminum (NCA).

In some embodiments, the positive electrode film layer may also optionally include a binder. Non-limiting examples of binders that can be used for the positive electrode film layer may include one or more of the following: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin. In an embodiment of the present application, the first positive electrode active material layer and/or the second positive electrode active material layer each independently contain a binder selected from polyvinylidene fluoride, polyacrylic acid, polytetrafluoroethylene, polyimide and combinations thereof.

In some embodiments, the positive electrode film layer may also optionally include a conductive agent. Examples of conductive agents for the positive electrode film layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. In an embodiment of the present application, the first positive electrode active material layer and/or the second positive electrode active material layer each independently contain a conductive agent of graphite, carbon black, acetylene black, graphene, carbon nanotubes and a combination of the above substances.

In one embodiment of the present application, the positive electrode can be prepared in the following manner: the above-mentioned components for preparing the positive electrode, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[Negative electrode]

In the secondary battery of the present application, the negative electrode plate does not exclude other additional functional layers in addition to the negative electrode base layer. For example, in some embodiments, the negative electrode plate of the present application may also include a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode base layer and disposed on the surface of the negative electrode current collector. In some other embodiments, the negative electrode plate of the present application may also include a covering protective layer covering the surface of the second negative electrode film layer.

In the secondary battery of the present application, the negative electrode current collector may be a metal foil or a composite current collector, for example, the metal foil may be a copper foil, a silver foil, an iron foil, or a foil composed of an alloy of the above metals. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer, and may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material base layer (such as a base layer made of materials such as polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and copolymers thereof).

[Electrolytes]

The electrolyte plays the role of conducting ions between the positive electrode plate and the negative electrode plate. The electrolyte can be selected from at least one of a solid electrolyte, a semi-solid electrolyte and a liquid electrolyte. In one embodiment of the present application, the electrolyte may also optionally contain additives. For example, the additives may include one or more of the following: negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain properties of the battery, such as additives that improve the overcharge performance of the battery, additives that improve the high temperature performance of the battery, additives that improve the low temperature performance of the battery, etc.

[Isolation film]

In one embodiment of the present application, the secondary battery further comprises a separator, which separates the anode side of the secondary battery from the cathode side, and provides selective permeation or blocking for substances of different types, sizes and charges in the system. For example, the separator can insulate electrons, physically isolate the positive and negative electrodes of the secondary battery, prevent internal short circuits and form an electric field in a certain direction, and allow ions in the battery to pass through the separator and move between the positive and negative electrodes. In one embodiment of the present application, the material used to prepare the separator may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer may be the same or different. In an embodiment of the present application, the separator is selected from polyolefin separators, polyester separators, polyimide separators, polyamide separators and cellulose separators.

In one embodiment of the present application, the positive electrode sheet, the negative electrode sheet and the separator can be made into an electrode assembly/bare cell by a winding process or a lamination process.

In one embodiment of the present application, the secondary battery may include an outer package, which can be used to encapsulate the above-mentioned electrode assembly and electrolyte. In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. In other embodiments, the outer package of the secondary battery can be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

The shape of the secondary battery of the present application may be cylindrical, square or any other shape. The outer packaging may include a shell and a cover plate, and the shell may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell has an opening connected to the receiving cavity, and the cover plate can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can form an electrode assembly through a winding process or a lamination process, and the electrode assembly is encapsulated in the receiving cavity, and the electrolyte is impregnated in the electrode assembly. The number of electrode assemblies contained in the secondary battery may be one or more.

In one embodiment of the present application, a plurality of secondary batteries may be assembled together to form a battery module, wherein the battery module includes two or more secondary batteries, and the specific number depends on the application of the battery module and the parameters of a single battery module.

In the battery module, the plurality of secondary batteries may be arranged in sequence along the length direction of the battery module. Of course, they may also be arranged in any other manner. The plurality of secondary batteries may further be fixed by fasteners. Optionally, the battery module may further include a housing having a storage space, and the plurality of secondary batteries are stored in the storage space.

In one embodiment of the present application, two or more of the above-mentioned battery modules can be assembled into a battery pack, and the number of battery modules contained in the battery pack depends on the application of the battery pack and the parameters of the individual battery modules. The battery pack may include a battery box and a plurality of battery modules disposed in the battery box, and the battery box includes an upper box body and a lower box body, and the upper box body can cover the lower box body and match well with it to form a closed space for accommodating the battery modules. Two or more battery modules can be arranged in the battery box in a desired manner.

In one embodiment of the present application, an exemplary battery pack may include a battery box and a plurality of battery modules disposed in the battery box. The battery box includes an upper box body and a lower box body, wherein the upper box body is used to cover the lower box body and form a closed space for accommodating the battery modules. The plurality of battery modules may be arranged in the battery box in any manner.

Electrical devices

In one embodiment of the present application, the electric device of the present application includes at least one of the secondary battery, battery module, or battery pack of the present application, and the secondary battery, battery module, or battery pack can be used as a power source of the electric device, and can also be used as an energy storage unit of the electric device. The electric device includes but is not limited to mobile digital devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

In the following, the influence of the lithium secondary battery manufactured according to the implementation mode of the present application on the performance of the electrochemical device is characterized based on specific examples, but it should be pointed out that the protection scope of the present application is defined by the claims and is not limited to the above specific implementation modes.

Example

Unless otherwise specified, all raw materials used in the present invention are analytically pure and all water is deionized water.

1. Preparation of positive electrode

10 wt % of polyvinylidene fluoride was dissolved in N-methylpyrrolidone solvent as a binder, 80 wt % of lithium iron phosphate positive electrode active material and 10 wt % of conductive carbon black were added to prepare a uniformly dispersed positive electrode slurry, the positive electrode slurry was evenly coated on the surface of the aluminum foil current collector, transferred to a vacuum oven for thorough drying, and the obtained positive electrode sheet was rolled and punched to obtain the target positive electrode disc.

2. Preparation of negative electrode sheet

In an Ar atmosphere, metallic sodium, or metallic sodium and other alloy component powders (see Table 4 for details) were placed in a stainless steel crucible, heated until completely melted, and stirred for 2 hours to ensure that the alloy components and metallic sodium were evenly mixed. After cooling, the sodium alloy negative electrode base layer material was obtained.

The metallic sodium or sodium alloy substrate material is compounded on the copper foil surface by cold pressing or hot pressing to obtain a layered negative electrode substrate. Alternatively, the metallic sodium or sodium alloy substrate material is compounded on the copper foil surface by atomization spraying or sputtering to obtain a granular negative electrode substrate. Alternatively, the metallic sodium or sodium alloy substrate material is compounded on the copper foil surface by extrusion to obtain a strip-shaped negative electrode substrate. Alternatively, the metallic sodium or sodium alloy substrate material is compounded on the copper foil surface by atomization spraying or sputtering to obtain a three-dimensional negative electrode substrate.

The negative electrode reinforcement film material is compounded on the surface of the negative electrode base layer by cold pressing or hot pressing to obtain a negative electrode plate.

3. Preparation of Isolation Membrane

A polypropylene porous polymer film is used as the isolation membrane.

4. Preparation of Electrolyte

Under Ar atmosphere, fully dried fluorine-containing salt (for details, see Table 1 and Table 2, hexafluorophosphate, hexafluoroarsenate or tetrafluoroborate) or non-fluorine-containing salt (nitrate) is dissolved in ethylene glycol dipropyl ether organic solvent, fully stirred and uniformly prepared to prepare an electrolyte.

5. Preparation of Button Cells

The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is between the positive and negative electrode sheets to play an isolating role, and the electrolyte is added to assemble into a button battery.

The negative electrode sheets used in Examples 1-39 and Comparative Examples 1-15 were prepared according to the above-mentioned preparation method, wherein the specific parameters of the negative electrode sheet formula are shown in Table 1-9.

According to the test results of Examples 1 to 5 and Comparative Examples 1 to 2 in Table 1 and the SEM results of Figures 2 and 3, when the surface density of the negative electrode substrate layer is in the range of 0.2mg/ ^cm2-50mg / ^cm2 , the secondary battery has a higher energy density and good cycle performance, and the negative electrode substrate layer can effectively inhibit the formation of lithium dendrites. According to the test results of Comparative Example 1 and the SEM results of Figure 3, when the surface density of the negative electrode substrate layer is too low, it is difficult for the negative electrode substrate layer to effectively inhibit the formation of lithium dendrites, and the cycle performance of the battery deteriorates. According to the test results of Comparative Example 2, when the surface density of the negative electrode substrate layer is too high, the energy density of the battery is low, the diffusion distance and diffusion resistance of lithium dissolved in metallic sodium or sodium alloy increase, and it is difficult to fully participate in the cycle process of the battery, thereby deteriorating the cycle performance of the battery.

After the corresponding battery test of Example 3 is completed, the positive electrode of the battery in a 100% discharge state is taken out, and the electrode is fully rinsed with ethylene glycol dipropyl ether, and then elemental analysis is performed. The results show that the positive electrode does not contain sodium element, or the sodium element content is lower than the detection limit of the test equipment, thereby confirming that no sodium ions are embedded in the positive electrode.

Table 1

According to the test results of Examples 6 to 7 in Table 2 and Comparative Example 3, when the electrolyte contains at least one anion that is a fluorine-containing anion, the main component of the sodium-based negative electrode protective film is sodium fluoride, the secondary battery has good cycle performance, the transmission resistance of lithium ions in the sodium-based negative electrode protective film is small, and the sodium-based negative electrode protective film can effectively block the side reaction between the negative electrode base layer and the electrolyte, and reduce the consumption of active substances in the battery. According to the test results of Comparative Example 3, when the composition of the sodium-based negative electrode protective film is not mainly sodium fluoride, the resistance of lithium ions passing through the sodium-based negative electrode protective film is large, and the side reaction between the negative electrode base layer and the electrolyte is aggravated, resulting in the formation of lithium dendrites and the consumption of active substances, which deteriorates the cycle performance of the battery.

Table 2

According to the test results of Examples 8 to 12 and Comparative Examples 4 to 5 in Table 3, when the atomic number of sodium in the negative electrode base layer is in the range of 10% to 100%, the lithium and sodium in the negative electrode base layer fuse rapidly, which can effectively inhibit the growth of lithium dendrites, reduce the consumption of active lithium during the regeneration of the sodium-based negative electrode protective film, and improve the cycle performance of the battery. According to the test results of Comparative Examples 4 to 5, when the sodium content in the negative electrode base layer is less than 10%, the ability of the negative electrode base layer to dissolve lithium is weak, the effect of inhibiting the formation of lithium dendrites is poor, and the cycle performance of the battery is poor.

Table 3

According to the test results of Example 3 and Examples 13 to 17 in Table 4 and Comparative Examples 6 to 7, when the mass percentage of lithium in the negative electrode substrate layer is in the range of 0%-70%, the lithium consumption during the battery cycle is suppressed, and the battery cycle performance is good. According to the test results of Comparative Examples 6 to 7, when the mass percentage of lithium in the negative electrode substrate layer is greater than 70%, the lithium-sodium fusion in the negative electrode substrate layer is slow, the diffusion ability of lithium in the negative electrode substrate layer is weak, and the negative electrode substrate layer is difficult to effectively suppress the formation of lithium dendrites, and the battery cycle performance deteriorates. At the same time, as shown in Comparative Example 7, when the negative electrode does not contain a metallic sodium or sodium alloy negative electrode substrate layer, its effect on dendrite suppression is poor, and the battery cycle performance deteriorates.

Table 4

According to the test results of Examples 18 to 21 and Comparative Example 8 in Table 5, when the component of the negative electrode enhancement film is a substance that does not react with metallic lithium and/or metallic sodium or sodium alloy, the consumption rate of active lithium in the battery is low, and the negative electrode enhancement film has a stable structure, thereby increasing the specific surface area of the negative electrode base layer, stabilizing the surface state of the negative electrode base layer, reducing the actual current density of the negative electrode, and improving the cycle performance of the battery. According to the test results of Comparative Example 8, when the component of the negative electrode enhancement film can react with metallic lithium or metallic sodium or sodium alloy, the consumption rate of active lithium in the battery is high, and the structure of the negative electrode enhancement film is unstable, thereby causing the structure of the negative electrode base layer to be unstable, which deteriorates the cycle performance of the battery.

Table 5

According to the test results of Examples 22 to 26 and Comparative Examples 9 to 10 in Table 6, when the particle size of the particles constituting the negative electrode base layer is in the range of 0.05 μm to 200 μm, the negative electrode base layer has a strong dissolution ability for lithium elements, and the negative electrode base layer will not consume too much electrolyte when forming SEI, and has a strong ability to inhibit the formation of lithium dendrites, and the battery has good first cycle coulomb efficiency and cycle performance. According to the test results of Comparative Example 9, when the particle size of the particles constituting the negative electrode base layer is too small, the negative electrode base layer consumes too much electrolyte when forming SEI, which deteriorates the first cycle coulomb efficiency and cycle performance of the battery. According to the test results of Comparative Example 10, when the particle size of the particles constituting the negative electrode base layer is too large, the diffusion distance and diffusion resistance of lithium in the negative electrode base layer increase, the battery impedance increases, and the ability of the negative electrode base layer to inhibit the formation of lithium dendrites decreases, thereby deteriorating the cycle performance of the battery.

Table 6

According to the test results of Examples 27 to 31 and Comparative Examples 11 to 12 in Table 7, when the negative electrode substrate layer is a strip structure and the minimum dimension of the strip is in the range of 0.05 μm to 200 μm, the negative electrode substrate layer has a strong solubility for lithium elements, and the negative electrode substrate layer will not consume too much electrolyte in the process of forming SEI, and has a strong ability to inhibit the formation of lithium dendrites. The battery has a higher first-cycle coulomb efficiency and better cycle performance. According to the test results of Comparative Example 11, when the minimum dimension of the stripes constituting the negative electrode substrate layer is too small, the negative electrode substrate layer consumes too much electrolyte in the process of forming SEI, resulting in a decrease in the first-cycle coulomb efficiency of the battery and deterioration in the cycle performance. According to the test results of Comparative Example 12, when the minimum dimension of the stripes constituting the negative electrode substrate layer is too large, the diffusion distance and diffusion resistance of the negative electrode substrate layer for lithium elements in the negative electrode substrate layer increase, deteriorating the cycle performance of the battery.

Table 7

According to the test results of Examples 32 to 35 and Comparative Example 13 in Table 8, when the negative electrode substrate layer is a three-dimensional structure and the porosity of the negative electrode substrate layer is in the range of 0%-50%, the energy density of the battery is high, the negative electrode substrate layer does not consume too much electrolyte in the process of forming SEI, the lithium element has a strong solubility in the negative electrode substrate layer, and the negative electrode substrate layer has a strong ability to inhibit the formation of lithium dendrites, and the battery has good cycle performance. According to the test results of Comparative Example 13, when the porosity of the negative electrode substrate layer is too high, the energy density of the battery is low, the negative electrode substrate layer consumes too much electrolyte in the process of forming SEI, and the first cycle coulomb efficiency and cycle capacity of the battery are deteriorated.

Table 8

According to the test results of Examples 36 to 39 and Comparative Examples 14 to 15 in Table 9, when the electrolyte concentration is in the range of 0.5 mol/L to 7 mol/L, the electrolyte has good lithium ion conductivity and film-forming effect, can effectively inhibit the formation of lithium dendrites, and the battery has a high cycle life. According to the test results of Comparative Example 14, when the concentration of the electrolyte is too low, the ionic conductivity of the electrolyte is low, the concentration polarization of the battery during the cycle is large, and the cycle life of the battery is reduced. According to the test results of Comparative Example 15, when the concentration of the electrolyte is too high, the viscosity of the electrolyte is too high, the ionic conductivity of the electrolyte is low, and the cycle life of the battery is reduced.

Table 9

Test Method

1. Specific surface area S _negative

According to the national standard "GB/T 19587-2004 Method for Determining the Specific Surface Area of Solids by Gas Adsorption BET Principle", the test is performed using an adsorption characterization analyzer. The powder sample to be tested is placed in a U-shaped sample tube, and a mixed gas containing a certain proportion of adsorbate flows through the sample. The adsorption amount of the adsorbate molecule (N ₂ ) of the sample to be tested is determined based on the change in gas concentration before and after adsorption, thereby obtaining the specific surface area S _negative .

2. Median particle size D _{50 negative}

The median particle size D _{50 negative} is the median particle size of the metal sodium or sodium alloy particles constituting the negative electrode substrate layer, in μm. _{D 50 negative is} the particle size corresponding to when the cumulative volume percentage of the metal sodium or sodium alloy particles reaches 50%. According to the national standard "GB/T19077-2016 Particle Size Distribution Laser Diffraction Method", the sample is dispersed under pressure, and the particle size of the material is characterized by a laser particle size analyzer. The test results use the median particle size D _{50 negative} to represent the average particle size.

3. Roughness Ra

The roughness in this article refers to the microscopic geometric features composed of small spacing and tiny peaks and valleys that the surface of the processed inactive layer still has. As a parameter representing the surface roughness of industrial products, the "roughness Ra" in this article is expressed as the "arithmetic mean roughness Ra" specified in the Japanese Industrial Standard (JIS B 0601-1994), which can be measured using, for example, a surface roughness meter, a confocal laser microscope, etc. The surface roughness of the inactive layer of metallic sodium or sodium alloy satisfies Ra 0.01μm≤Ra≤250μm.

4. Porosity

The volume of the negative electrode plate is calculated according to its size, and the dry mass m _p of the negative electrode plate is weighed to calculate the dry density ρ _p of the negative electrode plate. Then, the negative electrode plate is immersed in ethylene glycol dibutyl ether for 10 hours, the negative electrode plate is taken out, the flowing liquid on the surface of the negative electrode plate is wiped off, and its wet weight is weighed. The mass m _a of ethylene glycol dibutyl ether absorbed by the negative electrode plate as a whole is determined based on the difference between the wet weight and the dry weight. The density ρ _a of ethylene glycol dibutyl ether is a known parameter. Based on the above parameters, the porosity of the negative electrode plate is calculated.

It can be seen that the overall porosity takes into account the total porosity of the substrate layer and the coating. However, when other components in the substrate layer and the coating of each different negative electrode plate remain unchanged, the total porosity very intuitively reflects the porosity of the negative electrode base layer of the negative electrode plate of the present invention.

5. Surface density of negative electrode substrate

The square negative electrode substrate layer with a size of 10 cm*10 cm was weighed to obtain its mass, which was then divided by its area to obtain the surface density.

6.Thickness of negative electrode base layer

A square negative electrode sheet with a size of 10 cm*10 cm is placed between two stainless steel sheets with a size of 10 cm*10 cm, uniform thickness and a certain thickness _H1 in a way that the edges and corners are aligned. The total thickness _H2 of the two stainless steel sheets plus the negative electrode sheet is measured and obtained. The thickness _H3 of the negative electrode sheet is obtained according to the formula _{H3 = H2-2H1} . When the negative electrode sheet is coated on both sides, the _approximate thickness _H5 of the negative electrode base layer is obtained according to the formula H5 = ( _H3 - _H4 )/2, where _H4 is the thickness of the current collector; when the negative electrode sheet is coated on one side, the approximate thickness _H5 of the negative electrode base layer is obtained according to the formula _H5 = _H3 - _H4 , where _H4 is the thickness of the current collector.

7. Cycle performance test

At 25°C, the lithium secondary batteries prepared in the examples and comparative examples were subjected to full charge and discharge cycle tests at a rate of 0.5C. The cycles were operated in this way, and the number of cycles when the capacity of the battery was less than or equal to 80% of the initial capacity was recorded as the nth cycle. The larger n is, the better the cycle performance of the battery. n greater than 530 cycles is excellent cycle performance, n in the range of 500-530 cycles is average cycle performance, and n less than 500 cycles is poor cycle performance.

Claims (22)

1. A negative electrode plate for a lithium secondary battery, the negative electrode plate comprising a negative electrode base layer, wherein the negative electrode base layer comprises at least one metallic sodium or sodium alloy inactive layer. 2 . The negative electrode plate for a lithium secondary battery according to claim 1 , characterized in that the negative electrode plate further comprises a sodium-based negative electrode protective film located on at least a portion of the surface of the negative electrode base layer. 3 . The negative electrode plate for a lithium secondary battery according to claim 2 , characterized in that the negative electrode plate further comprises a negative electrode reinforcement film located on at least a portion of the surface of the sodium-based negative electrode protection film. 4. The negative electrode sheet for a lithium secondary battery according to any one of claims 1 to 3, characterized in that the negative electrode sheet satisfies at least one of the following conditions: (1) the specific surface area S _negative of the metallic sodium or sodium alloy inactive layer satisfies 0.001 ^{m 2} /g≤S _{negative≤100} ^{m 2} /g; or (2) the surface roughness Ra of the metallic sodium or sodium alloy inactive layer satisfies 0.01 μm ≤ Ra ≤ 250 μm; or (3) The porosity K negative of the metallic sodium or sodium alloy inactive layer _satisfies 0% ≤ K _negative ≤ 50%. 5. The negative electrode plate for a lithium secondary battery according to claim 3, characterized in that the negative electrode enhancement film includes at least one component selected from non-metallic elements, oxides, fluorides, carbides, carbon materials, nitrides and silicides, and is at least partially embedded in the metallic sodium or sodium alloy inactive layer. 6 . The negative electrode sheet for a lithium secondary battery according to claim 1 , wherein the metallic sodium or sodium alloy inactive layer is in the form of a lamellar structure, a strip shape, a three-dimensional structure and/or a granular structure. 7 . The negative electrode sheet for a lithium secondary battery according to claim 1 , wherein the negative electrode sheet comprises a current collector for supporting the negative electrode base layer. 8 . The negative electrode sheet for a lithium secondary battery according to claim 1 , wherein the surface density C _negative of the negative electrode base layer satisfies: 0.2 mg/cm ² ≤ C _{negative ≤} 50 mg/cm ² . 9. The negative electrode sheet for a lithium secondary battery according to claim 1, characterized in that the thickness d _negative of the negative electrode base layer satisfies: 2 μm ≤ d _{negative ≤} 500 μm. 10 . The negative electrode sheet for a lithium secondary battery according to claim 6 , wherein the metallic sodium or sodium alloy particles are in the form of nanoparticles and/or micron particles, and the median particle size D _{50 −} satisfies 0.05 μm ≤ _{D 50 − ≤} 200 μm. 11 . The negative electrode sheet for a lithium secondary battery according to claim 6 , wherein the strip structure of the metallic sodium or sodium alloy is distributed on the negative electrode substrate layer in a uniform or non-uniform manner, and the size in the smallest dimension is between 0.05 μm and 200 μm. 12. The negative electrode plate for a lithium secondary battery according to claim 1, characterized in that the negative electrode base layer also includes lithium elements supplemented in at least one metallic sodium or sodium alloy inactive layer, and the lithium element is 0-70 mass % based on the total mass of the negative electrode base layer. 13. A lithium secondary battery comprising a positive electrode sheet, an electrolyte and the negative electrode sheet according to any one of claims 1 to 12. 14. The lithium secondary battery according to claim 13, characterized in that the lithium secondary battery further comprises a separator and a second negative electrode reinforcement film disposed on at least a portion of the separator; and/or the separator itself is at least partially the second negative electrode reinforcement film. 15 . The lithium secondary battery according to claim 13 , wherein the positive electrode plate comprises at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate and lithium manganese phosphate. 16. The lithium secondary battery according to claim 13, characterized in that the at least one anion contained in the electrolyte is selected from fluorine-containing anions; preferably, at least one selected from bis(fluorosulfonyl)imide, hexafluorophosphate, bis(trifluoromethanesulfonyl)imide, difluorooxalatoborate, tetrafluoroborate, hexafluoroarsenate and difluorophosphate. 17 . The lithium secondary battery according to claim 13 , wherein the solvent of the electrolyte is selected from at least one of an ether compound, an acetal compound and a ketal compound. 18. A method for manufacturing the negative electrode sheet according to any one of claims 1 to 12, the method comprising: (1) pressing metallic sodium or sodium alloy onto the surface of the current collector by cold pressing or hot pressing to form a negative electrode substrate layer having at least one metallic sodium or sodium alloy inactive layer; or (2) by spraying or sputtering, metal sodium or sodium alloy is compounded onto the surface of the current collector to form a negative electrode substrate layer of metal sodium or sodium alloy inactive layer with a particle structure or a three-dimensional structure; or (3) Metal sodium or sodium alloy is compounded onto the surface of the current collector by extrusion to obtain a negative electrode substrate layer having a strip-shaped metal sodium or sodium alloy inactive layer. 19. The method according to claim 18, characterized in that the sodium-based negative electrode protective film is formed by an in-situ reaction between an inactive layer of metallic sodium or a sodium alloy and an electrolyte. 20. The method according to claim 18, characterized in that the negative electrode enhancement film is compounded on the surface of the negative electrode base layer by cold pressing or hot pressing. 21 . The method according to claim 18 , wherein the isolation film containing the second negative electrode enhancement film is composited on the surface of the negative electrode base layer by cold pressing or hot pressing. 22. An electrical device comprising the lithium secondary battery according to any one of claims 13 to 17.