CN103441304B - Lithium rechargeable battery and its electrolyte - Google Patents

Abstract

The invention discloses a lithium-ion secondary battery and its electrolyte. The electrolyte includes a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive contains a compound represented by general formula (1), In the general formula (1), m is an integer of 0 to 2, n is an integer of 1 to 3, R ₁ , R ₂ , and R ₃ are hydrogen atoms, fluorine atoms, or C1-C6 straight-chain or branched-chain alkyl groups or fluorine Substituted alkyl group, Rf ₁ , Rf ₂ , Rf ₃ , Rf ₄ are hydrogen atom or fluorine atom. Since the compound represented by the general formula (1) can effectively improve the positive electrode/electrolyte interface and inhibit the oxidative decomposition of the electrolyte on the surface of the positive electrode, it can improve the storage performance of lithium-ion secondary batteries under high temperature conditions without causing battery cycling. The deterioration of the performance makes the electrical performance of the lithium ion secondary battery of the present invention significantly improved.

Description

Lithium-ion secondary battery and its electrolyte

technical field

The invention belongs to the field of lithium-ion secondary batteries, more specifically, the invention relates to a lithium-ion secondary battery with ideal high-temperature storage performance and its electrolyte.

Background technique

Lithium-ion secondary batteries are widely used due to their advantages of high working voltage, long life and fast charging speed. However, with the continuous development of technology, people put forward higher requirements for the energy density of lithium-ion secondary batteries.

On the one hand, in a fully charged state, the negative electrode of the lithium-ion secondary battery has high reducing properties, and the positive electrode has high oxidizing properties. On the other hand, in actual use, factors such as the continuous use of electronic products and the increase in the temperature of the battery environment may cause the battery to be in a high temperature state, which further enhances the reactivity of the positive and negative electrodes of the lithium-ion secondary battery, and its electrolysis The liquid will react with the negative and positive electrodes to generate gas. Not only will it cause battery expansion and damage, but it will also cause damage to the equipment using the battery. In severe cases, it may even cause an internal short circuit due to battery expansion and deformation, or electrolyte leakage due to battery packaging rupture, fire and other safety accidents. It can be seen that the decomposition of electrolyte and battery flatulence have become problems that need to be solved urgently in the industry.

At present, in order to further improve the energy density of lithium-ion secondary batteries, some positive electrode materials with high nickel content, such as lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide with high nickel content, etc., have been developed for lithium ion secondary batteries. Secondary batteries; in addition, increasing the charging cut-off voltage of lithium-ion secondary batteries, so that a higher proportion of lithium ions can be extracted from the positive electrode can also obtain higher capacity, so as to meet the needs of increasing the energy density of the battery. However, the above two methods will increase the oxidation ability of the positive electrode, resulting in a more serious oxidation problem of the electrolyte. Therefore, for such high-energy cathode materials or higher voltage applications, it is particularly urgent to solve the problem of electrolyte decomposition.

The decomposition of the electrolyte is mainly due to the fact that in the charged lithium-ion secondary battery, the metal oxide as the positive electrode active material shows very strong oxidation at high potential, and it is easy to undergo an oxidation reaction with the electrolyte to decompose it. , It can be seen that inhibiting the oxidation reaction between the electrolyte and the positive electrode material is the key to solving the high-temperature storage performance of lithium-ion secondary batteries.

In the prior art, the method of adding succinonitrile or adiponitrile to the electrolyte of lithium-ion secondary batteries is usually used to improve its high-temperature storage performance, but the improvement effect is very limited; in addition, it has been proposed to use olefin nitrile compounds As electrolyte additives for lithium-ion secondary batteries, although such additives can effectively improve the high-temperature storage performance of lithium-ion secondary batteries, they deteriorate their cycle performance.

In view of this, it is necessary to provide a lithium-ion secondary battery and its electrolyte that have good storage performance and cycle performance under high-voltage and high-temperature conditions.

Contents of the invention

The object of the present invention is to provide a lithium-ion secondary battery and its electrolyte with good storage performance and cycle performance under high-voltage and high-temperature conditions, so as to effectively improve the electrical performance of the lithium-ion secondary battery.

In order to achieve the purpose of the above invention, the inventors have made painstaking research and found that adding the compound represented by the general formula (1) into the electrolyte of lithium-ion secondary batteries can significantly improve the storage characteristics of the battery at high temperature. Accordingly, the present invention provides a lithium ion secondary battery electrolyte, which includes lithium salts, non-aqueous organic solvents and additives, and the additives contain compounds represented by general formula (1),

In the general formula (1), m is an integer of 0 to 2, n is an integer of 1 to 3, R ₁ , R ₂ , and R ₃ are hydrogen atoms, fluorine atoms or C1 to C6 straight or branched chain alkyl groups, wherein , the hydrogen atoms on C1-C6 linear or branched alkyl groups can be partially or completely replaced by fluorine atoms, Rf ₁ , Rf ₂ , Rf ₃ , Rf ₄ are hydrogen atoms or fluorine atoms; m is greater than 2 or n is greater than 3 , will reduce the high temperature storage performance of the battery.

The mechanism by which the compound shown in general formula (1) improves the high-temperature storage characteristics of the battery without affecting the cycle performance of the battery is not yet clear. It is believed that the possible reasons are: 1) the cyano group in the compound structure can effectively combine with the high-valent metal atom (Ni , Co, Mn, etc.) complexation, the complexation of N atoms and high-valent metal atoms (Ni, Co, Mn, etc.) effectively reduces the ability of high-valent metal atoms to oxidize the electrolyte; 2) When the cyano group in the compound structure and the positive electrode During complexation, the olefin functional group can form a passivation film on the surface of the positive electrode through polymerization reaction, which further reduces the ability of high-valent metal atoms to oxidize the electrolyte; The interfacial effect of the liquid makes the battery have good cycle performance. Therefore, the compound represented by the general formula (1) reduces the reaction between the positive electrode and the electrolyte, thereby effectively improving the storage performance of the battery at high temperature without affecting the cycle performance of the battery.

As an improvement of the lithium ion secondary battery electrolyte of the present invention, the compound represented by the general formula (1) is preferably a compound represented by the general formula (2),

In the general formula (2), m, n, R ₁ , R ₂ , and R ₃ are the same as those in the general formula (1).

As an improvement of the lithium ion secondary battery electrolyte of the present invention, the compound represented by the general formula (1) is preferably a compound represented by the general formula (3),

In the general formula (3), n is a natural number of 1-3, and R ₁ , R ₂ , R ₃ are C1-C4 linear or branched alkyl groups.

As an improvement of the lithium-ion secondary battery electrolyte of the present invention, the compound represented by the general formula (1) is preferably a compound represented by the general formula (4)-(25) or a combination thereof,

As an improvement to the electrolyte solution of the lithium ion secondary battery of the present invention, the weight percentage of the compound represented by the general formula (1) in the electrolyte solution is 0.2% to 5%, preferably 0.5% to 2.5%. This is because, if the content of the compound shown in the general formula (1) in the electrolyte is too much, the C=C in its structure forms an overly thick passivation film through polymerization, causing the impedance of the battery to increase and affecting the cycle characteristics of the battery; If the content of the compound represented by the general formula (1) in the electrolyte is too small, the C=C functional group in its structure cannot form an effective and dense passivation film, and it cannot effectively prevent the reaction between the electrolyte and the positive electrode sheet, thereby failing to effectively improve the high-temperature storage performance of the battery.

As an improvement of the lithium ion secondary battery electrolyte of the present invention, the non-aqueous organic solvent contains cyclic carbonates and chain carbonates, and the weight percentage of cyclic carbonates in the electrolyte is 10% to 70%. %, the weight percentage of chain carbonate in the electrolyte is 15% to 80%. This is because the cyclic carbonate has a high dielectric constant and can form solvated lithium ion molecules with lithium ions; the linear ester has a low viscosity and can improve the fluidity and wettability of the electrolyte.

As an improvement of the lithium ion secondary battery electrolyte of the present invention, the cyclic carbonate and chain carbonate of the non-aqueous organic solvent are selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, butyl lactone , butylene carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethylene propyl carbonate or combinations thereof.

As an improvement of the lithium ion secondary battery electrolyte of the present invention, the lithium salt is selected from LiPF ₆ , LiBF ₄ , LiN(C _x F2 _x+1 SO ₂ )(C _y F _2y+1 SO ₂ ), LiBOB , LiAsF ₆ , Li(CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO ₃ , LiClO ₄ or combinations thereof, wherein x, y are natural numbers.

As an improvement to the electrolyte solution of the lithium ion secondary battery of the present invention, the concentration of the lithium salt is 0.5M-2M.

As an improvement of the lithium ion secondary battery electrolyte of the present invention, the electrolyte also contains organic sulfone compounds, sulfonate compounds, halogenated cyclic carbonate compounds, carbonic alkylene compounds, cyano compounds or combinations thereof.

In order to achieve the above-mentioned purpose of the invention, the present invention also provides a lithium ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator spaced between the positive electrode sheet and the negative electrode sheet, and an electrolyte, wherein the electrolyte is the above The electrolyte solution described in any paragraph.

Compared with the prior art, the compound shown in the general formula (1) added in the lithium-ion secondary battery electrolyte of the present invention can effectively improve the positive electrode/electrolyte interface, inhibit the oxidation and decomposition of the electrolyte on the surface of the positive electrode, thereby improving lithium The storage performance of the ion secondary battery under high temperature conditions will not lead to deterioration of battery cycle performance, so the electrical performance of the lithium ion secondary battery of the present invention is significantly improved.

Example

In order to make the objectives, technical solutions and beneficial technical effects of the present invention clearer, the present invention will be further described in detail below in conjunction with examples. It should be understood that the examples described in this specification are only for explaining the present invention, not for limiting the present invention, and the formulas and proportions of the examples can be selected according to local conditions without substantial influence on the results.

Example 1

Preparation of positive electrode sheet: Lithium cobalt oxide, conductive agent SuperP, and binder PVDF are mixed uniformly at a mass ratio of 96:2.0:2.0 to make a certain viscosity lithium ion secondary battery positive electrode slurry, which is coated on the current collector aluminum foil. The coating amount is 0.0194g/cm ² , after drying at 85°C, cold pressing is carried out; then trimming, cutting, and stripping are carried out, and drying is carried out at 85°C for 4 hours under vacuum conditions, and the tabs are welded. A cathode sheet of a lithium-ion secondary battery that meets the requirements.

Preparation of negative electrode sheet: graphite, conductive agent SuperP, thickener CMC, and binder SBR are made into a slurry at a mass ratio of 96.5:1.0:1.0:1.5, coated on the current collector copper foil and baked at 85°C Dry, the coating amount is 0.0089g/cm ² ; after trimming, cutting into pieces, and slitting, dry at 110°C for 4 hours under vacuum conditions, and weld the tabs to make a lithium-ion secondary battery negative electrode sheet that meets the requirements .

Preparation of the electrolyte: the electrolyte uses lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1M as a lithium salt, and a mixture of ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC) as a solvent, each carbonic acid The mass ratio of esters is EC:PC:DEC=30:30:40. In addition, the electrolyte solution also contains 1% by mass of the compound represented by the general formula (5) as an additive.

Preparation of lithium-ion secondary battery: The positive electrode sheet, negative electrode sheet, and separator prepared according to the aforementioned process are made into a lithium-ion secondary battery with a thickness of 4.2mm, a width of 34mm, and a length of 82mm through a winding process, and vacuum-baked at 75°C Baked for 10 hours, filled with electrolyte, and left to stand for 24 hours, charged to 4.2V with a constant current of 0.1C (160mA), and then charged at a constant voltage of 4.2V until the current dropped to 0.05C (80mA); then charged with 0.1C ( 160mA) to 3.0V, repeat charge and discharge twice, and finally charge the battery to 3.85V with 0.1C (160mA) to complete the battery production.

Example 2

A lithium ion secondary battery was prepared with reference to the method of Example 1, except that the compound represented by general formula (4) was used instead of the compound represented by general formula (5) in Example 1 when preparing the lithium ion secondary battery electrolyte.

Example 3

The lithium ion secondary battery was prepared with reference to the method of Example 1, except that the compound represented by the general formula (6) was used instead of the compound represented by the general formula (5) in Example 1 when preparing the lithium ion secondary battery electrolyte.

Example 4

The lithium ion secondary battery was prepared with reference to the method of Example 1, except that the compound represented by general formula (11) was used instead of the compound represented by general formula (5) in Example 1 when preparing the lithium ion secondary battery electrolyte.

Example 5

The lithium ion secondary battery was prepared with reference to the method of Example 1, except that the compound represented by the general formula (19) was used instead of the compound represented by the general formula (5) in Example 1 when preparing the lithium ion secondary battery electrolyte.

Example 6

The lithium-ion secondary battery was prepared with reference to the method of Example 1, except that when preparing the lithium-ion secondary battery electrolyte, the amount of the compound represented by the general formula (5) was 0.2% of the mass percentage of the electrolyte.

Example 7

The lithium ion secondary battery was prepared with reference to the method of Example 1, but when preparing the lithium ion secondary battery electrolyte, the usage amount of the compound shown in general formula (5) was 5% of the mass percentage of the electrolyte.

Example 8

The lithium-ion secondary battery was prepared with reference to the method of Example 1, except that when preparing the lithium-ion secondary battery electrolyte, the usage amount of the compound represented by the general formula (5) was 0.5% of the mass percentage of the electrolyte.

Example 9

The lithium ion secondary battery was prepared with reference to the method of Example 1, except that when preparing the lithium ion secondary battery electrolyte, the usage amount of the compound represented by the general formula (5) was 2.5% of the mass percentage of the electrolyte.

Comparative example 1

A lithium-ion secondary battery was prepared with reference to the method of Example 1, except that no additives were added when preparing the electrolyte for the lithium-ion secondary battery.

Comparative example 2

The lithium ion secondary battery was prepared with reference to the method of Example 1, except that when preparing the lithium ion secondary battery electrolyte, the compound represented by the general formula (5) in Example 1 was replaced by acrylonitrile with a mass percentage of 1%. as an additive.

Comparative example 3

The lithium ion secondary battery was prepared with reference to the method of Example 1, except that when preparing the lithium ion secondary battery electrolyte, the compound represented by the general formula (5) in Example 1 was replaced by acrylonitrile with a mass percentage of 2.5% as an additive.

The high-temperature storage performance and cycle performance of the lithium-ion secondary battery using the electrolyte additive for the lithium-ion secondary battery of the present invention are described below through experimental data.

High temperature storage characteristics test

For the lithium-ion secondary batteries obtained in Examples 1-9 and Comparative Examples 1-3, first charge the lithium-ion secondary battery to 4.2V with a constant current of 0.1C (160mA), and then charge the lithium-ion secondary battery at a constant voltage of 4.2V Until the current is less than 0.05C (80mA).

The thickness of the lithium-ion secondary battery before storage was tested, then stored in an environment of 85°C, and its thickness was measured again after 24 hours. The high-temperature storage performance of a lithium-ion secondary battery is evaluated by its thickness expansion rate. The calculation formula for the thickness expansion rate is: thickness expansion rate (%)=[(thickness after storage-thickness before storage)/thickness before storage ]*100%; the experimental results are shown in Table 1.

Table 1 High temperature storage characteristics test results

Battery additive Mass percentage Thickness expansion rate (%) Example 1 Compound shown in general formula (5) 1.0% 14 Example 2 Compound shown in general formula (4) 1.0% 16 Example 3 Compound shown in general formula (6) 1.0% 17 Example 4 Compound shown in general formula (11) 1.0% 17 Example 5 Compound shown in general formula (19) 1.0% 15 Example 6 Compound shown in general formula (5) 0.2% twenty two Example 7 Compound shown in general formula (5) 5.0% 8 Example 8 Compound shown in general formula (5) 0.5% 17 Example 9 Compound shown in general formula (5) 2.5% 11 Comparative example 1 none 0% 56 Comparative example 2 Acrylonitrile 1.0% 12 Comparative example 3 Acrylonitrile 2.5% 9

It can be seen from Table 1 that adding the compound represented by the general formula (1) as an additive in the electrolyte of the lithium-ion secondary battery can effectively improve the high-temperature storage performance of the lithium-ion secondary battery. As can be seen from the comparison of Examples 1, 6-9 and Comparative Example 1: the addition of 0.2% of the compound represented by the general formula (5) in the lithium-ion secondary battery electrolyte can not improve the lithium-ion secondary battery well. The high-temperature storage performance of the secondary battery; when the mass percentage of the compound represented by the general formula (5) in the electrolyte of the lithium-ion secondary battery increases to 1%, the high-temperature storage performance of the lithium-ion secondary battery can be effectively improved. From Example 1, Example 9, and Comparative Examples 2-3, it can be seen that the high-temperature storage performance of the compound represented by the general formula (5) at the same content is slightly better than that of acrylonitrile.

Cycle performance test

For the lithium-ion secondary batteries prepared in Examples 1-9 and Comparative Examples 1-3, the lithium-ion secondary batteries were first charged to 4.2V with a constant current of 0.7C (1120mA) at 25°C and 45°C. Then charge at a constant voltage of 4.2V until the current is less than 0.05C (80mA), and then discharge the lithium-ion secondary battery to 3.0V at a constant current of 0.5C (800mA). This discharge capacity is the first cycle discharge capacity. The battery was subjected to a cycle charge and discharge test in the above manner, and the discharge capacity of the 400th cycle was taken.

The high-temperature cycle performance of lithium-ion secondary batteries is evaluated by its capacity retention rate. The calculation formula of capacity retention rate is: capacity retention rate (%)=[discharge capacity of the 400th cycle/discharge capacity of the first cycle]* 100%; the experimental results are shown in Table 2.

Table 2 Cycle characteristic test results

It can be seen from Table 2:

1) From the comparison of Examples 1 to 5, 6, 8, 9 and Comparative Examples 1 to 3, it can be seen that the compound shown in the general formula (1) is added as an additive in the lithium-ion secondary battery electrolyte, not only will not It leads to the deterioration of the cycle performance of the lithium-ion secondary battery, but improves its cycle performance at 45°C;

2) From the comparison of Examples 1, 6 to 9 and Comparative Example 1, it can be seen that adding 2.5% of the compound shown in the general formula (5) in the lithium ion secondary battery electrolyte will not significantly deteriorate the lithium ion secondary battery. The 25 ℃ cycle performance of secondary battery; But, when the mass percentage content of the compound shown in general formula (5) increases to 5% in lithium ion secondary battery electrolyte, then obviously deteriorated the 25 ℃ of lithium ion secondary battery Cycle performance: Adding the compound represented by the general formula (5) in the electrolyte of the lithium-ion secondary battery can improve the cycle performance of the battery at 45°C.

Combining Table 1 and Table 2, it can be seen from Example 1, Example 9, Comparative Example 2 and Comparative Example 3: when the same content of 3-hydroxypropionitrile acrylate and acrylonitrile are used as additives, the high-temperature storage performance of the battery is the same Excellent, but the use of acrylic acid deteriorates the cycle performance of the battery, and the battery using 3-hydroxypropionitrile acrylate has good cycle performance.

It should be pointed out that although the additives of the lithium-ion secondary battery electrolyte of the present invention are described by taking general formulas (4) to (6), (11) and (19) as examples in the examples of this specification, In other embodiments of the present invention, the additive of lithium ion secondary battery electrolyte also can be wherein any one or the mixture of two or more compounds represented by general formula (1); LiPF ₆ is taken as an example to describe the lithium ion secondary battery electrolyte of the present invention, but, according to other implementation modes of the lithium ion secondary battery of the present invention, the lithium salt in the electrolyte can also be LiN(C _x F _x+1 SO ₂ ) (C _y F _2y+1 SO ₂ ), LiPF ₆ , LiBF ₄ , LiBOB, LiAsF ₆ , Li(CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO ₃ , LiClO ₄ or a combination of the above lithium salts; 1M also It is only the common concentration of lithium salt LiPF ₆ in lithium ion secondary batteries, and the concentration of lithium salt in different embodiments can be any value between 0.5M～2M; In addition, the non-aqueous organic solvent can be selected from ethylene carbonate, Propylene carbonate, dimethyl carbonate, butyl lactone, butylene carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethylene propyl carbonate or combinations thereof.

According to the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also make appropriate changes and modifications to the above embodiment. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims (11)

1. a lithium ion secondary battery electrolyte, comprising lithium salt, non-aqueous organic solvent and additive, is characterized in that: described additive contains the compound shown in general formula (1), In the general formula (1), m is an integer of 0 to 2, n is an integer of 1 to 3, R ₁ , R ₂ , and R ₃ are hydrogen atoms, fluorine atoms or C1 to C6 straight or branched chain alkyl groups, wherein , the hydrogen atoms on the C1-C6 linear or branched alkyl groups may be partially or completely replaced by fluorine atoms, and Rf ₁ , Rf ₂ , Rf ₃ , and Rf ₄ are hydrogen atoms or fluorine atoms. 2. lithium ion secondary battery electrolyte according to claim 1, is characterized in that: the compound shown in described general formula (1) is the compound shown in general formula (2), In the general formula (2), m, n, R ₁ , R ₂ , and R ₃ are the same as those in the general formula (1). 3. The lithium ion secondary battery electrolyte according to claim 1, characterized in that: the compound shown in the general formula (1) is a compound shown in the general formula (3), In the general formula (3), n is a natural number of 1-3, and R ₁ , R ₂ , R ₃ are C1-C4 linear or branched alkyl groups. 4. The lithium ion secondary battery electrolyte according to claim 1, characterized in that: the compound represented by the general formula (1) is a compound represented by the general formula (4) to (25) or a combination thereof, 5 . The lithium-ion secondary battery electrolyte according to claim 1 , wherein the weight percentage of the compound represented by the general formula (1) in the electrolyte is 0.2%˜5%. 6 . The lithium-ion secondary battery electrolyte according to claim 1 , wherein the weight percentage of the compound represented by the general formula (1) in the electrolyte is 0.5% to 2.5%. 7. lithium-ion secondary battery electrolyte according to claim 1, is characterized in that: described non-aqueous organic solvent contains cyclic carbonate and chain carbonate, and the weight percent of cyclic carbonate in electrolytic solution The content is 10%-70%, and the weight percent content of the chain carbonate in the electrolytic solution is 15%-80%. 8. lithium ion secondary battery electrolyte according to claim 7, is characterized in that: the cyclic carbonate of described non-aqueous organic solvent and chain carbonate are selected from ethylene carbonate, propylene carbonate, dimethyl carbonate ester, butyl lactone, butylene carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethylene propyl carbonate or combinations thereof. 9. The lithium ion secondary battery electrolyte according to claim 1, wherein the lithium salt is selected from LiN(C _x F2 _x+1 SO ₂ )(C _y F _2y+1 SO ₂ ), LiPF _6. LiBF ₄ , LiBOB, LiAsF ₆ , Li(CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO ₃ , LiClO ₄ or combinations thereof, wherein x and y are natural numbers, and the concentration of the lithium salt is 0.5M-2M. 10. The lithium ion secondary battery electrolyte according to claim 1, characterized in that: said electrolyte also contains organic sulfone compounds, sulfonate compounds, halogenated cyclic carbonate compounds, Alkenyl compounds, cyano compounds, or combinations thereof. 11. A lithium-ion secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator between the positive electrode sheet and the negative electrode sheet, and an electrolyte, characterized in that: the electrolyte is any one of claims 1 to 10 One of the described electrolytes.