Electrolyte and lithium ion battery

Abstract

The application provides an electrolyte and a lithium ion battery. The electrolyte comprises a primary electrolyte and a secondary electrolyte, the primary electrolyte comprises a compound as shown in formula 1, an organic solvent, a lithium salt and an additive, the secondary electrolyte comprises the compound as shown in formula 1, an organic solvent and a lithium salt, the primary electrolyte is an electrolyte for primary injection, and the secondary electrolyte is an electrolyte for secondary injection, wherein R1-R5 are independently selected from any one of a hydrogen atom, a fluorine atom, an alkyl group, an alkyne group or a benzyl group. In the application, the compound as shown in formula 1 is used in the electrolyte in combination with the secondary injection mode, so that the content of the compound as shown in formula 1 in the electrolyte after formation is improved, the damaged SEI film in the cycle process is effectively repaired, and the cycle life of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, and relates to an electrolyte, and more particularly to an electrolyte and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, electric vehicles, and other fields due to their advantages such as high operating voltage, high energy density, long cycle life, and environmental friendliness. With the development of smart wearables, smartphones, and portable mobile office devices, high-energy-density lithium-ion batteries are currently a hot topic in the industry.

[0003] Improving the volumetric energy density of batteries within a limited space is a common goal for many products. The main methods for improving battery energy density within a certain space are: 1) increasing the working voltage of the positive electrode material to maximize the specific capacity of the active material; 2) increasing the specific capacity of the negative electrode, such as using silicon-carbon negative electrodes; 3) increasing the proportion of positive and negative electrode active materials per unit volume, which also means higher coating density and greater compaction density; 4) reducing the proportion of auxiliary materials, such as using thinner separators, aluminum-plastic films and copper foils; 5) reducing the amount of electrolyte injected.

[0004] Currently, lithium iron phosphate (LFP) is widely used as the safer and lower-cost cathode material for batteries, while high-capacity, high-compact graphite is typically used as the anode material. However, increasing the compaction density of both cathode and anode materials reduces electrolyte adsorption and electrolyte retention. Furthermore, lithium-ion battery manufacturing processes cannot tolerate excessively high electrolyte retention while maintaining good battery shape and rigidity. The advantage of low electrolyte retention is a lower electrolyte content, directly increasing both volumetric and gravimetric energy densities, and reducing battery softness and deformation. However, traditional electrolytes with low retention coefficients often result in compromised cycle performance, leading to frequent drops in electrolyte levels later in the cycle, and difficulty in achieving optimal high and low temperature performance. Therefore, ensuring long cycle life with a low electrolyte retention coefficient while maintaining excellent overall high and low temperature performance remains a significant challenge.

[0005] To achieve the same overall performance as electrolytes with high liquid retention coefficients, low liquid retention coefficient electrolytes need to increase their effective liquid retention (the amount of effective material per unit volume of electrolyte). For batteries, effective liquid retention refers to providing the necessary electrolyte components for the normal operation of the positive and negative electrodes while ensuring adequate wetting. This requires the electrolyte to meet the following requirements: 1) Sufficient lithium ions in the electrolyte; 2) Highly efficient wetting and low interfacial impedance; 3) Excellent negative electrode film-forming additives to ensure cycling and reduce negative electrode expansion; 4) Additives beneficial to the positive electrode to ensure the coulombic efficiency of the active material and suppress the increase in positive electrode impedance; 5) Stability of the electrolyte under different environmental conditions and inhibition of electrolyte decomposition.

[0006] CN108598488A discloses a high-energy-density lithium-ion battery and its electrolyte. It adds fluoroethylene carbonate, 1,2-difluoroethylene carbonate, and ethylene sulfate as negative electrode film-forming additives; nitrile compounds such as 1,3,6-ethanetrionitrile and 1,4-diacetonitrile-2-butene as additives to improve high-temperature performance; lithium difluorophosphate and lithium difluorooxalate borate as modifying additives for solid electrolyte membranes; and additives to inhibit electrolyte decomposition. By using these four additives in combination, a high-energy-density battery can achieve long cycle performance with low electrolyte retention within a limited space, while also exhibiting excellent high and low temperature performance. However, the addition of too many additives can easily have an adverse effect on the performance of the electrolyte.

[0007] CN1117693253A discloses a lithium-ion battery electrolyte and a lithium-ion battery for improving high and low temperature cycle performance. The lithium-ion battery electrolyte contains a sulfur-containing multifunctional electrolyte additive, which is lithium dithiooxalate borate or lithium dithiooxalate phosphate, thereby improving the battery's high and low temperature cycle performance. However, simply adding the sulfur-containing multifunctional electrolyte additive provides limited improvement in electrolyte performance.

[0008] Therefore, developing a fast-charging electrolyte and lithium-ion battery that balances high wetting, high and low temperatures, and low liquid retention is an important research direction in this field. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a fast-charging electrolyte and lithium-ion battery that balances high wetting, high and low temperatures, and low electrolyte retention.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] One objective of this invention is to provide an electrolyte comprising a primary electrolyte and a secondary electrolyte. The primary electrolyte comprises a compound as shown in Formula 1, an organic solvent, a lithium salt, and an additive. The secondary electrolyte comprises a compound as shown in Formula 1, an organic solvent, and a lithium salt. The primary electrolyte is for primary electrolyte injection, and the secondary electrolyte is for secondary electrolyte injection.

[0012]

[0013] R1-R5 are each independently selected from any one of hydrogen atom, fluorine atom, alkyl group, alkyne group or benzyl group.

[0014] The compound represented by Formula 1 in this invention has a fluorooxazolidinone structure. During the charging and discharging process of the battery, the fluorooxazolidinone structure of this compound can rapidly undergo ring-opening polymerization or decomposition during the formation stage, and preferentially form a dense and stable SEI film on the negative electrode surface compared to carbonate solvents. This SEI film has excellent elasticity, allowing for volume changes in the carbon material during charging and discharging while minimizing rupture. It effectively isolates the negative electrode from direct contact with the electrolyte, reducing the occurrence of side reactions. The SEI film formed on the negative electrode surface by the compound represented by Formula 1 of this invention is rich in LiF, which can effectively reduce the SEI interface impedance, which is beneficial for high-rate discharge and low-temperature discharge. This invention uses primary and secondary electrolytes for secondary electrolyte injection into the battery, increasing the content of the compound represented by Formula 1 in the electrolyte after formation. This effectively repairs the SEI film damaged during cycling, thereby significantly improving the cycle life of the battery.

[0015] As a preferred embodiment of the present invention, the compound of Formula 1 is selected from any one of the compounds shown in Formula 2 or Formula 3.

[0016]

[0017] As a preferred embodiment of the present invention, in the primary electrolyte, the mass fraction of the compound as shown in Formula 1 is 0.5% to 4.5%, wherein the mass fraction can be 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, or 4.5%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 2.0% to 3.0%.

[0018] Preferably, in the secondary electrolyte, the mass fraction of the compound as shown in Formula 1 is 8-18%, wherein the mass fraction can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, or 18%, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable, preferably 10-15%.

[0019] As a preferred technical solution of the present invention, the lithium salts in the primary electrolyte and the secondary electrolyte independently include any one or at least two combinations of LiPF6, LiClO4, LiBF4, LiPO2F2, LiODFB, LiTFSI or LiFSI. Typical but non-limiting examples of such combinations include: combinations of LiPF6 and LiClO4, combinations of LiClO4 and LiBF4, combinations of LiBF4 and LiPO2F2, combinations of LiPO2F2 and LiODFB, combinations of LiODFB and LiTFSI, or combinations of LiTFSI and LiFSI, etc.

[0020] As a preferred embodiment of the present invention, in the primary electrolyte, the mass fraction of the lithium salt is 10-15%, wherein the mass fraction can be 10%, 11%, 12%, 13%, 14% or 15%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0021] Preferably, in the secondary electrolyte, the mass fraction of the lithium salt is 10-15%, wherein the mass fraction can be 10%, 11%, 12%, 13%, 14% or 15%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0022] As a preferred embodiment of the present invention, the organic solvents in the primary electrolyte and the secondary electrolyte each independently include at least two of the following: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene sulfite, ethyl acetate, diethyl sulfite, or 1,3-propanesulfonate lactone, preferably including at least two of the following: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

[0023] As a preferred embodiment of the present invention, in the primary electrolyte, the mass fraction of ethylene carbonate is 20-30%, the mass fraction of methyl ethyl carbonate is 20-30%, and the mass fraction of diethyl carbonate is 40-60%. The mass fraction of ethylene carbonate can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%, the mass fraction of methyl ethyl carbonate can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%, and the mass fraction of diethyl carbonate can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, or 60%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0024] Preferably, in the secondary electrolyte, the mass fraction of ethylene carbonate is 20-30%, the mass fraction of methyl ethyl carbonate is 20-30%, and the mass fraction of diethyl carbonate is 40-60%. The mass fraction of ethylene carbonate can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%, the mass fraction of methyl ethyl carbonate can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%, and the mass fraction of diethyl carbonate can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, or 60%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0025] In this invention, the viscosity of diethyl carbonate is lower than that of ethylene carbonate and methyl ethyl carbonate, ensuring that the proportion of diethyl carbonate is above 40%, more preferably above 50%, which can greatly reduce the viscosity of the electrolyte and improve wettability.

[0026] As a preferred embodiment of the present invention, the additives include tris(trimethylsilyl)phosphite and methyl disulfonate.

[0027] Preferably, the (trimethylsilyl)phosphite in the primary electrolyte has a mass fraction of 0.1% to 1.0%, wherein the mass fraction may be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1.0%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0028] Preferably, in the primary electrolyte, the mass fraction of methylene disulfonate is 0.1% to 1.0%, wherein the mass fraction can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1.0%, etc., but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0029] As a preferred technical solution of the present invention, the mass ratio of the primary electrolyte to the secondary electrolyte is (80-90):(10-20), wherein the mass ratio can be 80:20, 82:18, 84:16, 86:14, 88:12 or 90:10, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0030] A second objective of the present invention is to provide a lithium-ion battery, comprising a positive electrode and a negative electrode, characterized in that the lithium-ion battery comprises an electrolyte as described in one objective.

[0031] In this invention, the positive electrode active material of the lithium-ion battery is lithium manganese iron phosphate.

[0032] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] In this invention, the SEI film generated by the compound shown in Formula 1 in the electrolyte during charge and discharge effectively isolates the negative electrode from direct contact with the electrolyte, reducing the occurrence of side reactions. The SEI film is rich in LiF, effectively reducing the SEI interface impedance, which is beneficial for high-rate discharge and low-temperature discharge. The secondary electrolyte injection method increases the content of the compound shown in Formula 1 in the electrolyte after formation, effectively repairing the damaged SEI film during cycling and improving the battery's cycle life. Specifically, the capacity retention rate after 1000 cycles at 25℃ FC / 1C can reach over 92%, and the capacity retention rate after 1000 cycles at 35℃ FC / 1C can reach over 88%. Detailed Implementation

[0035] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0036] Example 1

[0037] This embodiment provides an electrolyte, which includes a primary electrolyte and a secondary electrolyte.

[0038] The primary electrolyte contains 2.5% of the compound shown in Formula 2, 0.3% of tris(trimethylsilyl)phosphite (TMSP), and 0.3% of methylene disulfonate (DTD), with the balance being an organic solvent. The organic solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a mass ratio of 1:1:2. The lithium salt in the primary electrolyte is a mixture of LiPF6 and LiFSI, wherein the mass fraction of LiPF6 in the electrolyte is 12.5%, and the mass fraction of LiFSI in the electrolyte is 3%.

[0039] The secondary electrolyte is 15% of the compound shown in Formula 2. The organic solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a mass ratio of 1:1:2. The lithium salt in the secondary electrolyte is a mixture of LiPF6 and LiFSI, wherein the mass fraction of LiPF6 in the electrolyte is 12.5% ​​and the mass fraction of LiFSI in the electrolyte is 3%.

[0040] This embodiment also provides a lithium-ion battery and its preparation method:

[0041] The lithium-ion battery is a square aluminum-cased lithium iron phosphate battery with a room temperature capacity distribution of 160Ah, a charge / discharge voltage range of 2.5 to 3.65V, and can be continuously cycled at 1C rate at both room temperature and high temperature.

[0042] Preparation of the positive electrode sheet: LiFePO4:SP:CNT:PVDF = 95.0:2.0:0.5:2.5. The positive electrode gel is prepared with a solid content of 1.327%. The first step involves adding LiFePO4, SP, and NMP, rotating at 25±1 r / min, dispersing at 500±50 r / min, and stirring for 10 min, then rotating at 25±1 r / min, dispersing at 1000±50 r / min, and stirring at 45℃ for 90 min. The second step involves adding the conductive agent CNT slurry, rotating at 25±1 r / min, dispersing at 1000±50 r / min, under a vacuum of 0.080 kPa, and stirring at 45℃ for 60 min. The third step involves adding the positive electrode gel, rotating at 25±1 r / min, dispersing at 1000±50 r / min, and stirring at 45℃ for 60 min. The process involves stirring at 2500±50 r / min, with a vacuum of 0.080 kPa, at 45℃ for 90 min; the fourth step is viscosity adjustment, adding NMP to adjust the slurry viscosity; the fifth step is slow stirring at 15±1 r / min, dispersing at 500±50 r / min, with a vacuum of 0.080 kPa, stirring for 0.5 h and then cooling to ensure the positive electrode discharge viscosity is 20000±5000 mPa·s and the fineness is ≤15 μm. Deposits on the mixing tank wall and stirring rod are scraped off promptly at each step. The positive electrode sheet is then obtained by sieving, coating, cold pressing, and slitting.

[0043] Preparation of negative electrode sheet: graphite:SP:CMC:SBR = 95.5:1.5:1.2:1.8. Negative electrode slurry preparation: The slurry solid content is 8%. Step 1: Add graphite and SP, dry mix at 20±1 r / min, disperse at 1000±50 r / min, and stir for 1 h. Step 2: Add 50% of the negative electrode slurry, rotate at 20±1 r / min, disperse at 1000±50 r / min, and stir for 1.5 h. Step 3: Add another 50% of the negative electrode slurry, rotate at 25±1 r / min, disperse at 2000±50 r / min, vacuum degree 0.085 kPa, and stir for 1 h. Step 4: Adjust viscosity by adding deionized water. Step 5: Add aqueous dispersant SBR, rotate at 25±1 r / min, disperse at 800±50 r / min, vacuum degree 0.085 kPa, and stir for 1 h. Ensure the negative electrode discharge viscosity is 4000±1500 mPa·s and the fineness is ≤20μm. Scrape away any deposited material on the mixing tank wall and stirring rod at each step. The negative electrode sheet is then obtained through sieving, coating, cold pressing, and slitting.

[0044] The above-mentioned positive electrode, negative electrode, and electrolyte are assembled to form a lithium-ion battery. The electrolyte is designed with a liquid retention coefficient of 3.0 g / Ah, which is smaller than that of conventional square lithium iron phosphate batteries.

[0045] Examples 2-6 and Comparative Examples 1-4 differed from Example 1 in that the electrolyte content was modified. The specific parameters are shown in Table 1.

[0046] Table 1

[0047]

[0048]

[0049] The lithium-ion batteries prepared with the electrolytes in Examples 1-6 and Comparative Examples 1-4 were tested for cycle performance, and the test results are shown in Table 2.

[0050] Table 2

[0051]

[0052]

[0053] The table above shows that in Examples 1-2, the use of compounds as shown in Formula 2 or Formula 3 improved the cycle performance of the electrolyte to varying degrees. In Example 3, the absence of other additives in the primary electrolyte resulted in a slight decrease in battery cycle performance. In Example 4, the reduced amount of the compound shown in Formula 2 in the secondary electrolyte led to a decrease in battery cycle performance. Therefore, a concentration of 10-15% of the compound shown in Formula 2 in the secondary electrolyte maintains good cycle performance; lower concentrations have less significant impact on battery performance improvement. In Example 5, changing the content of the organic solvent resulted in a decrease in battery cycle performance. In Example 6, the use of only LiPF6 lithium salt, without a mixture of the two lithium salts, resulted in a decrease in battery cycle performance, indicating that the synergistic effect of the two lithium salts improved the electrolyte performance.

[0054] In Comparative Example 1, only one electrolyte injection was performed, resulting in a decrease in battery cycle performance, indicating that the addition of a secondary electrolyte effectively improved electrolyte performance. Compared to Example 1, in Comparative Example 2, no compound as shown in Formula 2 was added to the primary electrolyte, and the electrolyte cycle performance at 35°C significantly decreased. In Comparative Example 3, neither the primary nor secondary electrolyte contained the compound as shown in Formula 1, and the electrolyte cycle performance significantly decreased. In Comparative Example 4, a commonly used electrolyte in the prior art was used, resulting in a significant decrease in battery cycle performance, and high-temperature cycling was not possible.

[0055] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte includes a primary electrolyte and a secondary electrolyte. The primary electrolyte includes a compound as shown in Formula 1, an organic solvent, a lithium salt, and an additive. The secondary electrolyte includes a compound as shown in Formula 1, an organic solvent, and a lithium salt. The primary electrolyte is used for primary electrolyte injection, and the secondary electrolyte is used for secondary electrolyte injection. Formula 1; R1-R5 are each independently selected from any one of hydrogen atom, fluorine atom, alkyl group, alkyne group or benzyl group; In the primary electrolyte, the mass fraction of the compound as shown in Formula 1 is 0.5% to 4.5%; In the secondary electrolyte, the mass fraction of the compound as shown in Formula 1 is 8-18%; The additives include tris(trimethylsilyl)phosphite and methylene dimethylsulfonate.

2. The electrolyte according to claim 1, characterized in that, The compound described in Formula 1 is selected from any one of the compounds shown in Formula 2 or Formula 3. Equation 2, Formula 3.

3. The electrolyte according to claim 1 or 2, characterized in that, In the primary electrolyte, the mass fraction of the compound shown in Formula 1 is 2.0 to 3.0%.

4. The electrolyte according to claim 1, characterized in that, In the secondary electrolyte, the mass fraction of the compound shown in Formula 1 is 10-15%.

5. The electrolyte according to claim 1, characterized in that, The lithium salts in the primary electrolyte and the secondary electrolyte independently include any one or a combination of at least two of LiPF6, LiClO4, LiBF4, LiPO2F2, LiODFB, LiTFSI, or LiFSI.

6. The electrolyte according to claim 1, characterized in that, In the primary electrolyte, the mass fraction of the lithium salt is 10-15%.

7. The electrolyte according to claim 1, characterized in that, In the secondary electrolyte, the mass fraction of the lithium salt is 10-15%.

8. The electrolyte according to claim 1, characterized in that, The organic solvents in the primary electrolyte and the secondary electrolyte each independently include at least two of the following: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene sulfite, ethyl acetate, diethyl sulfite, or 1,3-propanesulfonate lactone.

9. The electrolyte according to claim 8, characterized in that, The organic solvents in the primary electrolyte and the secondary electrolyte are each independently at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

10. The electrolyte according to claim 8, characterized in that, In the primary electrolyte, the mass fraction of ethylene carbonate is 20-30%, the mass fraction of methyl ethyl carbonate is 20-30%, and the mass fraction of diethyl carbonate is 40-60%.

11. The electrolyte according to claim 8, characterized in that, In the secondary electrolyte, the mass fraction of ethylene carbonate is 20-30%, the mass fraction of methyl ethyl carbonate is 20-30%, and the mass fraction of diethyl carbonate is 40-60%.

12. The electrolyte according to claim 1, characterized in that, The mass fraction of the tris(trimethylsilyl)phosphite in the primary electrolyte is 0.1~1.0%.

13. The electrolyte according to claim 1, characterized in that, In the primary electrolyte, the mass fraction of methylene methyl disulfonate is 0.1-1.0%.

14. The electrolyte according to claim 1, characterized in that, The mass ratio of the primary electrolyte to the secondary electrolyte is (80~90):(10~20).

15. A lithium-ion battery, comprising a positive electrode and a negative electrode, characterized in that, The lithium-ion battery includes the electrolyte as described in any one of claims 1-14.

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