CN110212247A - Battery cell - Google Patents

Abstract

The invention provides a battery cell. The battery core comprises a negative plate, a positive plate and an isolating membrane, wherein the negative plate comprises a negative current collector and a negative diaphragm which is arranged on the surface of the negative current collector and contains a negative active substance, the positive plate comprises a positive current collector and a positive diaphragm which is arranged on the surface of the positive current collector and contains a positive active substance, and the isolating membrane is arranged between the adjacent negative plate and the positive plate. The surface of the negative electrode diaphragm is also provided with a metal lithium layer, the weight of the metal lithium layer is 0.5% -5% of the total weight of the negative electrode diaphragm, the negative electrode capacity per unit area/the positive electrode capacity per unit area is 1.2-2.1, and the negative electrode capacity per unit area/(the positive electrode capacity per unit area + the metal lithium layer capacity per unit area x 80%) is more than or equal to 1.10. The battery cell has better cycle performance and storage performance.

Description

Batteries

technical field

The present invention relates to the field of batteries, and more particularly to a battery cell.

Background technique

Among secondary batteries, lithium-ion secondary batteries have a higher energy density advantage compared to other types of secondary batteries, making them occupy a mainstream position in the market. Among them, lithium ion secondary batteries with lithium iron phosphate as the positive active material are widely used in electric bus power systems due to their high safety, low cost, and long life, and have broad application prospects in the field of large-scale energy storage.

In recent years, based on the consideration of the cost per kilowatt hour, the requirements for the life of lithium-ion secondary batteries are getting higher and higher. Although lithium iron phosphate has high structural stability, the dissolution-repair equilibrium of the solid-electrolyte interface film (SEI film) occurs on the surface of the graphite anode, resulting in the continuous reduction of active lithium ions that can be used for shuttle between the anode and cathode. , so that capacity loss inevitably occurs. The lithium ion secondary battery using lithium titanate as the negative electrode active material and lithium iron phosphate as the positive electrode active material can avoid the capacity loss caused by the negative electrode side reaction caused by the dissolution-repair balance of the SEI film because it does not generate an SEI film, but The higher voltage platform of lithium titanate leads to a lower discharge voltage platform of lithium ion secondary batteries, and the energy density is too low, and the expensive unit price of lithium titanate leads to high unit Wh cost. Therefore, effective technologies are needed to solve lithium ion secondary batteries. The long life of the secondary battery.

At present, the main means to improve the life of lithium ion secondary batteries are: selecting lithium iron phosphate and graphite types with good cycle performance and storage performance, optimizing electrolyte formula (changing organic solvents, additives), optimizing positive electrode diaphragm and negative electrode diaphragm formula , optimize the SEI film forming conditions, etc. All of these measures are considered from the viewpoint of suppressing the negative side reactions caused by the dissolution-repair balance of the SEI film, and the reduction of active lithium ions is delayed by throttling, so they can play a limited role, and the cycle life of lithium ion secondary batteries is the highest. It can achieve about 5,000 to 6,000 cycles, which is still far from the target of more than 10,000 cycles of long-life electric buses and large-scale energy storage systems.

SUMMARY OF THE INVENTION

In view of the problems existing in the background art, the purpose of the present invention is to provide a battery cell with better cycle performance and storage performance.

In order to achieve the above purpose, the present invention provides a battery cell, which includes a negative electrode sheet, a positive electrode sheet and a separator, the negative electrode sheet includes a negative electrode current collector and a negative electrode membrane sheet disposed on the surface of the negative electrode current collector and containing a negative electrode active material The positive electrode sheet includes a positive electrode current collector and a positive electrode membrane sheet disposed on the surface of the positive electrode current collector and containing a positive electrode active material, and the separator is spaced between adjacent negative electrode sheets and positive electrode sheets. The surface of the negative electrode membrane is also provided with a metal lithium layer, and the weight of the metal lithium layer is 0.5% to 5% of the total weight of the negative electrode membrane, and the capacity of the negative electrode per unit area/the capacity of the positive electrode per unit area=1.2 to 2.1 , the capacity of the negative electrode per unit area/(the capacity of the positive electrode per unit area + the capacity of the metal lithium layer per unit area×80%)≥1.10.

Compared with the prior art, the beneficial effects of the present invention are:

The battery cell of the invention has better cycle performance and storage performance.

Description of drawings

FIG. 1 is a graph of the normal temperature cycle performance of Example 1 and Comparative Example 1, wherein the curve on the left side represents Comparative Example 1, and the curve on the right side represents Example 1.

Detailed ways

The cell according to the present invention will be described in detail below.

The battery cell according to the present invention includes a negative electrode sheet, a positive electrode sheet and a separator, the negative electrode sheet includes a negative electrode current collector and a negative electrode membrane sheet disposed on the surface of the negative electrode current collector and containing a negative electrode active material, the positive electrode sheet includes a positive electrode current collector and a positive electrode membrane that is disposed on the surface of the positive electrode current collector and contains a positive electrode active material, and the separator is spaced between adjacent negative electrode sheets and positive electrode sheets. The surface of the negative electrode membrane is also provided with a metal lithium layer, and the weight of the metal lithium layer is 0.5% to 5% of the total weight of the negative electrode membrane, and the capacity of the negative electrode per unit area/the capacity of the positive electrode per unit area=1.2 to 2.1 , the capacity of the negative electrode per unit area/(the capacity of the positive electrode per unit area + the capacity of the metal lithium layer per unit area×80%)≥1.10.

It should be noted that the battery cell of the present invention refers to the battery cell that has not been soaked in the electrolyte solution (ie, the structure before the electrolyte solution is injected). In a narrow sense, the battery cell of the present invention may only include a positive electrode sheet, a separator and a negative electrode sheet, but in a broad sense, the battery cell of the present invention may also include other structures, such as terminal assemblies, protective shell assemblies, and the like.

In the battery cell of the present invention, a metal lithium layer accounting for 0.5% to 5% of the total weight of the negative electrode film is arranged on the surface of the negative electrode film, and the ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area is controlled to be 1.2～5%. Within the range of 2.1, while ensuring that the ratio of the negative electrode capacity per unit area to the sum of the positive electrode capacity per unit area and 80% of the metal lithium layer capacity per unit area is greater than or equal to 1.10, the cycle performance and storage performance of the cell can be effectively improved. The vacancies provided by the negative active material are required to accommodate the intercalation of lithium ions from the metallic lithium layer after liquid injection and to receive all lithium ions from the positive active material during the first charging process. If the content of the metal lithium layer is too high, the content of the negative electrode active material is too small, or the positive electrode active material content is too large, the above relationship will not be satisfied, which will lead to the formation of lithium deposition in the negative electrode during the full charging process of the battery, which will affect the cycle of the battery cell. performance and storage performance.

In the battery cell according to the present invention, since the capacity of the metal lithium layer on the surface of the negative electrode membrane is difficult to fully exert, when designing the battery cell, different positive and negative electrode capacities are designed according to 80% of the capacity of the metal lithium layer. , and then a cell with better cycle performance and storage performance is obtained.

In the battery cell of the present invention, the negative electrode lithium intercalation occurs inside the battery cell after liquid injection, and the metal lithium layer on the surface of the negative electrode membrane exists in the form of a lithium intercalation compound. The voltage after discharge is close. After full charge, the negative electrode active material has enough vacancies to receive all the lithium ions from the positive electrode active material, and stores excess lithium ions in the negative electrode after full discharge. Therefore, after the discharge is terminated, the negative electrode still has 5% to 97% ( Lithium ions that can shuttle between the positive and negative electrodes relative to the positive electrode capacity. In other words, the lithium-rich state of the cell after the first cycle of charge and discharge is 105% to 197% (relative to the positive electrode capacity), so it can effectively reduce the capacity. loss, improve the cycle performance and storage performance of the cell.

In the battery cell according to the present invention, the negative electrode membrane can be arranged on one surface of the negative electrode current collector, or can be arranged on both surfaces of the negative electrode current collector.

In the battery cell according to the present invention, the negative electrode active material can be selected from materials that can accept and extract lithium ions. Preferably, the negative electrode active material is selected from one or more of carbon-based materials, silicon-based materials, tin-based materials, and lithium titanate. The carbon-based material can be selected from one or more of natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon microspheres, nano-carbon, and carbon fiber, and the silicon-based material can be selected from elemental silicon, silicon oxide compounds. , one or more of silicon-carbon composites, and silicon alloys, and the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, tin-carbon composites, and tin alloys.

In the battery cell of the present invention, the negative electrode film further includes a binder and an optional conductive agent, and the types of the binder and the conductive agent are not specifically limited and can be selected according to actual needs. Preferably, the binder can be selected from one or more of styrene-butadiene rubber emulsion (SBR) and sodium carboxymethyl cellulose (CMC). The conductive agent may be selected from one or more of conductive carbon black, superconducting carbon black, conductive graphite, acetylene black, Ketjen black, graphene, and carbon nanotubes.

In the battery cell according to the present invention, the positive electrode membrane can be arranged on one surface of the positive electrode current collector, or can be arranged on both surfaces of the positive electrode current collector.

In the battery cell according to the present invention, the positive electrode active material can be selected from materials that can accept and extract lithium ions. Preferably, the positive active material can be selected from one or more of lithium transition metal oxides, and compounds obtained by adding other transition metals or non-transition metals or non-metals to lithium transition metal oxides. Specifically, the positive active material can be selected from lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, olivine structure One or more of lithium-containing phosphates, wherein the general formula of the olivine-structured lithium-containing phosphates can be LiFe _1-xy Mn _x M' _y PO ₄ , 0≤x≤1, 0≤y≤0.1 , 0≤x+y≤1, M' is selected from one or more of other transition metal elements or non-transition metal elements except Fe, Mn, M' is preferably selected from Cr, Mg, Ti, Al, Zn , W, Nb, Zr one or more. Preferably, the olivine-structured lithium-containing phosphate may be lithium iron phosphate, lithium manganese phosphate and lithium iron manganese phosphate. Among the cathode active materials, the olivine-structured lithium-containing phosphate itself has high structural stability, and will not cause capacity loss due to structural changes during cycling like other cathode active materials. The capacity attenuation of the battery cell containing lithium phosphate is mainly due to the loss of active lithium ions inside the battery that can shuttle between the positive and negative electrodes, while the battery cell of the present invention still has 5% to 97% (relative to the positive electrode) after the discharge is terminated. capacity) lithium ions that can shuttle between the positive and negative electrodes, thus, when the positive electrode active material contains an olivine-structured lithium-containing phosphate, the capacity loss of the cell can be effectively reduced, and the cycle performance and storage of the cell can be greatly improved. performance. However, the positive electrode active material of the present invention is not limited to the olivine-structured lithium-containing phosphate.

In the battery cell of the present invention, the positive electrode film further includes a conductive agent and a binder, and the types of the binder and the conductive agent are not specifically limited, and can be selected according to actual needs. Preferably, the binder can be selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer One or more of tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin. The conductive agent may be selected from one or more of conductive carbon black, superconducting carbon black, conductive graphite, acetylene black, Ketjen black, graphene, and carbon nanotubes.

In the battery cell according to the present invention, preferably, the capacity of the negative electrode per unit area/the capacity of the positive electrode per unit area=1.3˜2.1.

In the battery cell according to the present invention, preferably, the weight of the metal lithium layer may be 1% to 5% of the total weight of the negative electrode membrane.

In the battery cell of the present invention, the source and form of the metal lithium layer are not limited, and the source of the metal lithium layer can be selected from one or more of the forms of lithium powder, lithium ingot, and lithium flake. The metallic lithium layer on the surface of the negative electrode membrane is not necessarily completely dense and uniformly distributed on the surface of the negative electrode membrane. For example, when the source of the metal lithium layer is in the form of lithium powder, there may be a certain gap between the lithium powder particles on the surface of the negative film, and the size of the gap can be controlled between 1 μm and 5000 μm; when the source of the metal lithium layer is lithium ingot or In the form of lithium sheets, a gap setting method can also be used to make multiple lithium sheets (or lithium strips) intermittently cover the surface of the negative electrode membrane, and the distance between each lithium sheet (or lithium strip) can be controlled within 1μm～5000μm After injecting the electrolyte, after a long enough time of infiltration and diffusion, the lithium in the lithium-rich area (lithium sheet position) will diffuse to the lithium-poor area (interstitial position), and finally the lithium content in the negative film can still reach uniformity. Achieving uniform lithium enrichment.

In the battery cell of the present invention, a metal lithium layer can be provided on the surface of the negative electrode film by rolling, and the metal lithium layer can be formed by the intermolecular force between lithium metal and negative electrode active material (such as graphite). It is stably fixed on the surface of the negative electrode membrane.

In the battery cell according to the present invention, capacity of negative electrode per unit area = coating weight of negative electrode per unit area × weight ratio of negative electrode active material × reversible gram capacity of negative electrode active material, capacity of positive electrode per unit area = coating weight of positive electrode per unit area × The weight ratio of the positive electrode active material×the reversible gram capacity of the positive electrode active material, and the capacity of the metal lithium layer refers to the theoretical capacity of the metal lithium.

In the battery core of the present invention, the specific type of the isolation film is not particularly limited, and can be any isolation film material used in the prior art, such as polyethylene, polypropylene, polyvinylidene fluoride and many of them. layer composite films, but not limited to these.

The present application will be further described below with reference to the embodiments. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application.

Example 1

(1) Preparation of positive electrode sheet

The positive active material lithium iron phosphate (reversible gram capacity is 139mAh/g), the conductive agent acetylene black, and the binder PVDF are mixed in a weight ratio of 94:4:2, the solvent N-methylpyrrolidone is added, and the mixture is fully stirred and mixed to obtain The positive electrode slurry is then coated on both surfaces of the positive electrode current collector aluminum foil, wherein the coating weight of the positive electrode slurry is 0.198g/1540.25mm ² (calculated by weight without solvent), then dried, cold pressed , get the positive electrode.

(2) Preparation of negative electrode sheet

Mix the negative active material artificial graphite (with a reversible gram capacity of 340mAh/g), the conductive agent acetylene black, and the binder SBR+CMC according to the weight ratio of 95:1.5:3.1:0.4, add the solvent deionized water, fully stir and mix evenly The negative electrode slurry was obtained, and then coated on the two surfaces of the negative electrode current collector copper foil, wherein the coating weight of the negative electrode slurry was 0.120g/1540.25mm ² (in terms of the weight without solvent), after drying, cooling After pressing, a negative electrode film is obtained, and then the lithium sheet (theoretical gram capacity is 3861.3mAh/g) is compounded to the surface of the negative electrode film by rolling, wherein the weight of the lithium sheet is 3.05mg/1540.25mm ² , to obtain a negative electrode sheet .

(3) Electrolyte preparation

In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DMC) in a weight ratio of EC:PC:DMC=3:3:3 After mixing, a mixed organic solvent is obtained, and the fully dried lithium salt LiPF ₆ is dissolved in the above mixed organic solvent, and after stirring uniformly, an electrolyte solution is obtained, wherein the concentration of LiPF ₆ is 1 mol/L.

(4) Preparation of separator

A polyethylene porous membrane is used as the separator.

(5) Preparation of lithium ion secondary battery

The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence, so that the separator is in the middle of the positive electrode and the negative electrode to play a role of isolation, and the battery core is obtained by winding. The battery cell is placed in an outer package, and the prepared electrolyte is injected and packaged to obtain a lithium ion secondary battery.

in,

The weight percentage content of the metallic lithium layer on the surface of the negative electrode membrane=(3.05mg/1540.25mm ² )/(0.120g/1540.25mm ² )×100%=2.54%.

Negative capacity per unit area (calculated as an area of 1540.25mm ² , the following examples are similar)=0.120g×95%×340mAh/g=38.76mAh.

Unit area (calculated as 1540.25mm ² in area, similar to the following examples) positive electrode capacity=0.198g×94%×139mAh/g=25.87mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=38.76mAh/25.87mAh=1.50.

The capacity of negative electrode per unit area/(capacity of positive electrode per unit area+capacity of metal lithium layer per unit area×80%)=38.76mAh/(25.87mAh+(3.05mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[25.87mAh+(3.05mg×3861.3mAh/g)-38.76mAh×8%]/25.87mAh×100%=133.5%.

Example 2

The preparation process of the lithium ion secondary battery is the same as that of Example 1, except that the coating weight of the positive electrode slurry is 0.198g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.104g/1540.25mm ² , and the surface of the negative electrode membrane is 0.104g/1540.25mm 2 . The weight of the lithium sheet is 1.52mg/1540.25mm ² .

The weight percentage content of the metal lithium layer on the surface of the negative electrode membrane=(1.52mg/1540.25mm ² )/(0.104g/1540.25mm ² )×100%=1.46%.

Negative capacity per unit area=0.104g×95%×340mAh/g=33.59mAh.

Positive electrode capacity per unit area=0.198g×94%×139mAh/g=25.87mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=33.59mAh/25.87mAh=1.30.

Negative capacity per unit area/(positive electrode capacity per unit area+capacity of metal lithium layer per unit area×80%)=33.59mAh/(25.87mAh+(1.52mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[25.87mAh+(1.52mg×3861.3mAh/g)-33.59mAh×8%]/25.87mAh×100%=112.3%.

Example 3

The preparation process of the lithium ion secondary battery is the same as that of Example 1, except that the coating weight of the positive electrode slurry is 0.224g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , the lithium ion on the surface of the negative electrode membrane The weight of the tablet was 3.45 mg/1540.25 mm ² .

The weight percentage content of the metal lithium layer on the surface of the negative electrode membrane=(3.45mg/1540.25mm ² )/(0.136g/1540.25mm ² )×100%=2.54%.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.224g×94%×139mAh/g=29.27mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/29.27mAh=1.50.

Negative capacity per unit area/(positive electrode capacity per unit area+capacity of metal lithium layer per unit area×80%)=43.93mAh/(29.27mAh+(3.45mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[29.27mAh+(3.45mg×3861.3mAh/g)-43.93mAh×8%]/29.27mAh×100%=133.5%.

Example 4

The preparation process of the lithium ion secondary battery is the same as that of Example 1, except that the coating weight of the positive electrode slurry is 0.259g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the surface of the negative electrode membrane is 0.259g/1540.25mm 2 . The weight of the lithium sheet is 1.99 mg/1540.25 mm ² .

The weight percentage content of the metallic lithium layer on the surface of the negative electrode membrane=(1.99mg/1540.25mm ² )/(0.136g/1540.25mm ² )×100%=1.46%.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.259g×94%×139mAh/g=33.84mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/33.84mAh=1.30.

Negative capacity per unit area/(positive capacity per unit area+capacity of metal lithium layer per unit area×80%)=43.93mAh/(33.84mAh+(1.99mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[33.84mAh+(1.99mg×3861.3mAh/g)−43.93mAh×8%]/33.84mAh×100%=112.3%.

Example 5

The preparation process of the lithium ion secondary battery is the same as that of Example 1, except that the coating weight of the positive electrode slurry is 0.177g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the The weight of the lithium sheet was 5.44 mg/1540.25 mm ² .

The weight percentage content of the metal lithium layer on the surface of the negative electrode membrane=(5.44mg/1540.25mm ² )/(0.136g/1540.25mm ² )×100%=4.0%.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.177×94%×139mAh/g=23.13mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/23.13mAh=1.90.

Negative capacity per unit area/(positive capacity per unit area+capacity of metal lithium layer per unit area×80%)=43.93mAh/(23.13mAh+(5.44mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[23.13mAh+(5.44mg×3861.3mAh/g)-43.93mAh×8%]/23.13mAh×100%=175.6%.

Example 6

The preparation process of the lithium ion secondary battery is the same as that in Example 1, except that the coating weight of the positive electrode slurry is 0.160g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the The weight of the lithium sheet was 6.16 mg/1540.25 mm ² .

The weight percentage content of the metal lithium layer on the surface of the negative electrode membrane=(6.16mg/1540.25mm ² )/(0.136g/1540.25mm ² )×100%=4.53%.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.160g×94%×139mAh/g=20.91mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/20.91mAh=2.10.

Negative capacity per unit area/(positive capacity per unit area+capacity of metal lithium layer per unit area×80%)=43.93mAh/(20.91mAh+(6.16mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[20.91mAh+(6.16mg×3861.3mAh/g)−43.93mAh×8%]/20.91mAh×100%=197.0%.

Example 7

The preparation process of the lithium ion secondary battery is the same as that in Example 1, except that the coating weight of the positive electrode slurry is 0.280g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the surface of the negative electrode membrane is 0.280g/1540.25mm 2 . The weight of the lithium sheet is 1.10 mg/1540.25 mm ² .

The weight percentage content of the metal lithium layer on the surface of the negative electrode membrane=(1.10mg/1540.25mm ² )/(0.136g/1540.25mm ² )×100%=0.81%.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.280g×94%×139mAh/g=36.58mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/36.58mAh=1.20.

The capacity of negative electrode per unit area/(capacity of positive electrode per unit area+capacity of metal lithium layer per unit area×80%)=43.93mAh/(36.58mAh+(1.10mg×3861.3mAh/g)×80%)=1.10.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[36.58mAh+(1.10mg×3861.3mAh/g)−43.93mAh×8%]/36.58mAh×100%=102.0%.

Comparative Example 1

The preparation process of the lithium ion secondary battery is the same as that in Example 1, except that the coating weight of the positive electrode slurry is 0.198g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.120g/1540.25mm ² , and the Lithium sheets are not provided on the surface.

Negative capacity per unit area=0.120g×95%×340mAh/g=38.76mAh.

Positive electrode capacity per unit area=0.198g×94%×139mAh/g=25.87mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=38.76mAh/25.87mAh=1.50.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[25.87mAh-38.76mAh×8%]/25.87mAh×100%=88.0%.

Comparative Example 2

The preparation process of the lithium ion secondary battery is the same as that in Example 1, except that the coating weight of the positive electrode slurry is 0.198g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.094g/1540.25mm ² , and the Lithium sheets are not provided on the surface.

Negative capacity per unit area=0.094g×95%×340mAh/g=30.36mAh.

Positive electrode capacity per unit area=0.198g×94%×139mAh/g=25.87mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=30.36mAh/25.87mAh=1.17.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[25.87mAh-30.36mAh×8%]/25.87mAh×100%=90.6%.

Comparative Example 3

The preparation process of the lithium ion secondary battery is the same as that of Example 1, except that the coating weight of the positive electrode slurry is 0.224g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the Lithium sheets are not provided on the surface.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.224g×94%×139mAh/g=29.27mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/29.27mAh=1.50.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[29.27mAh-43.93mAh×8%]/29.27mAh×100%=88.0%.

Comparative Example 4

The preparation process of the lithium ion secondary battery is the same as that of Example 1, except that the coating weight of the positive electrode slurry is 0.287g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the Lithium sheets are not provided on the surface.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.287g×94%×139mAh/g=37.50mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/37.50mAh=1.17.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[37.50mAh-43.93mAh×8%]/37.50mAh×100%=90.6%.

Comparative Example 5

The preparation process of the lithium ion secondary battery is the same as that in Example 1, except that the coating weight of the positive electrode slurry is 0.280g/1540.25mm ² , the coating weight of the negative electrode slurry is 0.136g/1540.25mm ² , and the surface of the negative electrode membrane is 0.280g/1540.25mm 2 . The weight of the lithium sheet is 1.99 mg/1540.25 mm ² .

The weight percentage content of the metallic lithium layer on the surface of the negative electrode membrane=(1.99mg/1540.25mm ² )/(0.136g/1540.25mm ² )×100%=1.46%.

Negative capacity per unit area=0.136g×95%×340mAh/g=43.93mAh.

Positive electrode capacity per unit area=0.280g×94%×139mAh/g=36.58mAh.

CB value=negative electrode capacity per unit area/positive electrode capacity per unit area=43.93mAh/36.58mAh=1.20.

Negative electrode capacity per unit area/(positive electrode capacity per unit area + metal lithium layer capacity per unit area×80%)=43.93mAh/(36.58mAh+(1.99mg×3861.3mAh/g)×80%)=1.03.

The lithium-rich state of the lithium-ion secondary battery after the first cycle of charge and discharge=[36.58mAh+(1.99mg×3861.3mAh/g)−43.93mAh×8%]/36.58mAh×100%=111.4%.

Next, the test procedure of the lithium ion secondary battery will be described.

(1) Normal temperature cycle performance test of lithium ion secondary battery

At 25°C, the lithium-ion secondary battery was first charged with a constant current of 1C to a voltage of 3.65V, and then charged with a constant voltage of 3.65V to a current of 0.05C. After standing for 5 minutes, the lithium-ion secondary battery was charged with a constant current of 1C. The current is discharged to a voltage of 2.5V, which is a charge-discharge cycle process, and the discharge capacity of this time is the discharge capacity of the first cycle. The lithium ion secondary battery is subjected to multiple cycle charge-discharge tests according to the above method, until the discharge capacity of the lithium ion secondary battery decays to 80%, and the cycle times of the lithium ion secondary battery are recorded.

(2) High temperature cycle performance test of lithium ion secondary battery

At 60°C, the lithium-ion secondary battery was first charged with a constant current of 1C to a voltage of 3.65V, and then charged with a constant voltage of 3.65V to a current of 0.05C. After standing for 5 minutes, the lithium-ion secondary battery was charged with a constant current of 1C. The current is discharged to a voltage of 2.5V, which is a charge-discharge cycle process, and the discharge capacity of this time is the discharge capacity of the first cycle. The lithium-ion secondary battery was subjected to multiple cycle charge-discharge tests according to the above method, and the discharge capacity of the 500th cycle was detected.

The capacity retention rate of the lithium ion secondary battery after 500 cycles at 60° C.=(discharge capacity at the 500th cycle/discharge capacity at the first cycle)×100%.

(3) Storage performance test of lithium ion secondary battery

First, at 25°C, the lithium-ion secondary battery was charged at a constant current of 0.5C to a voltage of 3.65V, and then charged at a constant voltage of 3.65V to a current of 0.05C. After standing for 5 minutes, the lithium-ion secondary battery was charged with 0.5C constant current discharge to a voltage of 2.5V, the discharge capacity this time is the discharge capacity before storage; then the lithium ion secondary battery is fully charged with a charging current of 0.5C, and left at 60 ℃ for 90 days, then Take it out and let it stand at 25°C for 2 hours, then discharge it with 0.5C constant current until the voltage is 2.5V, after standing for 5 minutes, charge it with 0.5C constant current until the voltage is 3.65V, and then charge it with 3.65V constant voltage until the voltage is 3.65V. The current was 0.05C, and after standing for 5 minutes, the lithium-ion secondary battery was discharged at a constant current of 0.5C to a voltage of 2.5V, and the discharge capacity at this time was the discharge capacity after 90 days of storage.

The capacity retention rate of the lithium ion secondary battery after storage at 60° C. for 90 days=(discharge capacity after storage for 90 days/discharge capacity before storage)×100%.

Table 1 Performance test results of Examples 1-7 and Comparative Examples 1-5

Note: The unit area in the table is calculated as an area of 1540.25mm ² .

FIG. 1 is a graph showing the normal temperature cycle performance of Example 1 and Comparative Example 1. FIG. It can be seen from Fig. 1 that the lithium ion secondary battery of Comparative Example 1 is expected to be cycled up to about 6000 times when the capacity decays to 80% in the normal temperature environment, while the lithium ion secondary battery of Example 1 can be cycled for 6000 times at most. It has a reversible discharge capacity of 90%, and is expected to be able to cycle about 16,000 times when the capacity decays to 80%, so it can meet the needs of long-life electric buses and large-scale energy storage systems.

It can be seen from the comparison of Comparative Examples 1-2 that under the condition of a certain positive electrode capacity, increasing the negative electrode capacity has little effect on the cycle life and storage life of the lithium ion secondary battery. From the comparison of Examples 1-2, it can be known that after the negative electrode film is provided with a metal lithium layer, increasing the negative electrode capacity will significantly improve the cycle life and storage life of the lithium ion secondary battery, and with the increase of the metal lithium layer content , the cycle life and storage life of lithium ion secondary batteries are further extended, so by adjusting the negative electrode capacity and the content of the metal lithium layer, the customization of the cycle life and storage life of lithium ion secondary batteries can be realized.

From the comparison of Comparative Examples 3-4, it can be known that under the condition of a certain negative electrode capacity, reducing the positive electrode capacity has little effect on the cycle life and storage life of the lithium ion secondary battery. From the comparison of Examples 3-7, it can be known that after the negative electrode membrane is provided with a metal lithium layer, reducing the positive electrode capacity will significantly improve the cycle life and storage life of the lithium ion secondary battery, and with the increase of the metal lithium layer content , the cycle life and storage life of lithium-ion secondary batteries are further extended, so the customization of the cycle life and storage life of lithium-ion secondary batteries can be achieved by adjusting the cathode capacity and the content of the metal lithium layer.

In Comparative Example 5, if the ratio of the capacity of the negative electrode per unit area to the sum of the capacity of the positive electrode per unit area and the capacity of the metal lithium layer per unit area is too low, the cycle performance and storage performance of the lithium ion secondary battery are both poor. Because the ratio is too low, lithium precipitation occurs in the negative electrode when lithium is inserted for the first time, which leads to gas and liquid leakage of the lithium ion secondary battery, and deteriorates the cycle performance and storage performance of the lithium ion secondary battery.

Claims (10)

1. A cell, comprising: A negative electrode sheet, including a negative electrode current collector and a negative electrode membrane sheet disposed on the surface of the negative electrode current collector and containing a negative electrode active material; A positive electrode sheet, including a positive electrode current collector and a positive electrode diaphragm provided on the surface of the positive electrode current collector and containing a positive electrode active material; and a separator, which is spaced between adjacent negative electrode sheets and positive electrode sheets; It is characterized in that, The surface of the negative electrode membrane is provided with a metallic lithium layer; The weight of the metal lithium layer is 0.5% to 5% of the total weight of the negative electrode membrane; Negative capacity per unit area/positive capacity per unit area=1.2～2.1; The capacity of the negative electrode per unit area/(the capacity of the positive electrode per unit area + the capacity of the metal lithium layer per unit area×80%)≥1.10. 2 . The battery cell according to claim 1 , wherein the negative electrode active material is selected from materials that can accept and extract lithium ions. 3 . 3. The battery according to claim 2, characterized in that, The negative active material is selected from natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon microspheres, nano carbon, carbon fiber, elemental silicon, silicon oxide compound, silicon carbon composite, silicon alloy, elemental tin, tin oxide One or more of compounds, tin-carbon composites, tin alloys, and lithium titanate. 4 . The battery cell according to claim 1 , wherein the positive electrode active material is selected from materials that can accept and extract lithium ions. 5 . 5. The battery according to claim 4, wherein the positive electrode active material is selected from the group consisting of lithium transition metal oxides, lithium transition metal oxides and compounds obtained by adding other transition metals or non-transition metals or non-metals. one or more. 6. The battery cell according to claim 5, wherein the positive electrode active material is selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide , one or more of lithium nickel cobalt aluminum oxide and olivine structure lithium-containing phosphate. 7 . The battery according to claim 1 , wherein the weight of the metal lithium layer is 1% to 5% of the total weight of the negative electrode membrane. 8 . 8 . The battery cell according to claim 1 , wherein the source of the metal lithium layer is one or more selected from the group consisting of lithium powder, lithium ingot, and lithium sheet. 9 . 9 . The battery cell according to claim 1 , wherein a metal lithium layer is provided on the surface of the negative electrode film by means of rolling. 10 . 10. The battery according to claim 1, characterized in that, The negative electrode film also includes a binder and an optional conductive agent; The positive electrode film further includes a conductive agent and a binder.