An electrolyte and a battery comprising the same

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte and a battery comprising the electrolyte. The electrolyte provided by the application is added with a first additive imide phosphate compound, which has a lower LUMO and a higher HOMO, is helpful to stable film formation on the positive and negative electrode surfaces, avoids the side reaction between the positive and negative electrode materials and the electrolyte, and generates an interface film rich in N and P. The interface film has high thermodynamic stability and high ionic conductance, is beneficial to improving the high-temperature cycle performance and low-temperature performance of the battery under high voltage, and the phosphate groups in the phosphate groups can capture H· / OH· free radicals in the electrolyte, inhibit the diffusion reaction of free radicals in the battery combustion process. Meanwhile, the phosphate groups can also combine with Lewis acids such as PF5 in the electrolyte, so that the flame retardance of the electrolyte is further improved, and a battery with good thermal stability, i.e., safety performance, is obtained.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte and a battery comprising the electrolyte. BACKGROUND

[0002] As the blood of lithium ion batteries, electrolyte is an indispensable component material of lithium ion batteries and plays a crucial role in the performance of lithium ion batteries. In recent years, with the continuous development of lithium ion battery technology, high working voltage, long cycle life and high safety performance have become important performance indicators of lithium ion batteries. However, with the continuous increase of working voltage, the stability of positive and negative electrode materials of lithium ion batteries decreases at high voltage, and the oxidation and reduction side reactions of electrolyte on the surface of positive and negative electrodes are intensified, resulting in continuous growth of lithium ion battery impedance, significant deterioration of cycle life, low temperature performance and safety performance of the battery, and even battery swelling and fire, causing safety accidents and limiting the application of lithium ion batteries.

[0003] At present, researchers usually modify the positive and negative electrode materials by doping and coating, and add positive and negative electrode film-forming additives to the electrolyte to improve the stability of the system and inhibit the side reaction of electrolyte with positive and negative electrode materials.

[0004] However, the protective film generated by the commonly used electrolyte additives has insufficient stability at high voltage, and the additives that can stably form a film will bring high impedance, and the generated protective film will also intensify the side reaction under extreme working conditions, affecting the safety performance of the battery and leading to the inability to balance the performance of lithium ion batteries. Therefore, it has become a research focus to develop an electrolyte for lithium ion batteries that can not only stably form a film on the surface of positive and negative electrodes to improve the cycle life of the battery at high voltage, but also balance the low impedance of the formed film and the safety performance of the battery under extreme working conditions. SUMMARY

[0005] In order to improve the poor safety performance and the inability to balance the electrical performance of the battery at high voltage in the prior art, the present application provides an electrolyte and a battery comprising the electrolyte, which can stably form a film on the surface of positive and negative electrodes, improve the high temperature cycle life and low temperature performance of the battery at high voltage, and balance the safety performance of the battery.

[0006] The purpose of the present application is achieved by the following technical scheme:

[0007] An electrolyte, comprising a lithium salt, an organic solvent and an additive, wherein the additive comprises a first additive selected from imide phosphate compounds.

[0008] According to an embodiment of the present application, the imide phosphate compound is a compound containing an imide group and a phosphate group.

[0009] According to an embodiment of the present application, the imidophosphate compound is selected from at least one of the compounds shown in formula I:

[0010]

[0011] In formula I, X is O or S;

[0012] R1, R2 are the same or different and are independently selected from substituted or unsubstituted C 1～10 alkyl, substituted or unsubstituted C 2～10 alkenyl, substituted or unsubstituted C 2～10 alkynyl, substituted or unsubstituted C 6～10 aryl, if substituted, the substituents are halogen, nitrile, C 1～10 alkyl;

[0013] R3, R4 are the same or different and are independently selected from hydrogen atom, halogen, substituted or unsubstituted C 2～10 alkenyl, substituted or unsubstituted C 1～10 alkyl, substituted or unsubstituted C 1～10 alkoxy, if substituted, the substituents are halogen, nitrile, C 1～10 alkyl; R3, R4 can also be linked to each other to form a bond; R3, R4 can also form a fused ring with the nitrogen-containing five-membered ring.

[0014] According to an embodiment of the present application, in formula I, R1, R2 are the same or different and are independently selected from substituted or unsubstituted C 1～6 alkyl, substituted or unsubstituted C 2～6 alkenyl, substituted or unsubstituted C 2～6 alkynyl, substituted or unsubstituted C 6～8 aryl, if substituted, the substituents are halogen, nitrile, C 1～6 alkyl.

[0015] According to an embodiment of the present application, in formula I, R1, R2 are the same or different and are independently selected from substituted or unsubstituted C 1～3 alkyl, substituted or unsubstituted C 2～3 alkenyl, substituted or unsubstituted C 2～3 alkynyl, substituted or unsubstituted C 1～3 alkyl.

[0016] According to an embodiment of the present application, in formula I, R3, R4 are the same or different and are independently selected from hydrogen atom, halogen, substituted or unsubstituted C 2～6 alkenyl, substituted or unsubstituted C 1～6 alkyl, substituted or unsubstituted C 1～6 alkoxy, if substituted, the substituents are halogen, nitrile, C 1～6alkyl; R3, R4may also be connected to each other to form a bond; R3, R4may also be connected to each other to form a benzene ring.

[0017] According to embodiments of the present application, in formula I, R3, R4are the same or different, and are independently selected from a hydrogen atom, halogen, substituted or unsubstituted C 2～3 alkenyl, substituted or unsubstituted C 1～3 alkyl, substituted or unsubstituted C 1～3 alkoxy, if substituted, the substituent is halogen, nitrile, C 1～3 alkyl; R3, R4may also be connected to each other to form a bond; R3, R4may also be connected to each other to form a benzene ring.

[0018] According to embodiments of the present application, the compound of formula I is selected from at least one of the following compounds A1-A8:

[0019]

[0020]

[0021] According to embodiments of the present application, the mass of the first additive is 0.1-10wt% of the total mass of the electrolyte, preferably 0.5-3wt%, for example 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 1.2wt%, 1.3wt%, 1.5wt%, 1.6wt%, 1.8wt%, 2wt%, 2.2wt%, 2.4wt%, 2.5wt%, 2.6wt%, 2.8wt%, 3wt%, 3.3wt%, 3.5wt%, 3.8wt%, 4wt%, 4.2wt%, 4.5wt%, 4.8wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%.

[0022] According to embodiments of the present application, the lithium salt is at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPF2O2), lithium difluorophosphate bisoxalate (LiPF2(C2O4)2), lithium tetrafluorophosphate oxalate (LiPF4C2O4), lithium phosphate oxalate (LiPO2C2O4), lithium bisoxalate borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium bisfluorosulfonylimide (LiTFSI) and lithium bisfluorosulfonylimide (LiFSI).

[0023] According to embodiments of the present application, the mass of the lithium salt is 10-15wt% of the total mass of the electrolyte, for example 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%.

[0024] According to embodiments of the present application, the organic solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl propionate (EP), propyl propionate (PP), ethyl acetate (EA), ethyl n-butyrate (EB), and gamma-butyrolactone (GBL).

[0025] According to embodiments of the present application, the mass of the organic solvent is 20-60 wt% of the total mass of the electrolyte, for example 20 wt%, 25 wt%, 30 wt%, 40 wt%, 55 wt%, 60 wt%.

[0026] According to embodiments of the present application, the additive further comprises a second additive selected from at least one of fluoroethylene carbonate (FEC), lithium difluoro(oxalato)borate (LiODFB), vinylene carbonate (VC), 1,3-propane sultone (PS), vinyl sulfate (DTD), methylene methane disulfonate (MMDS), propene sultone (PST), maleic anhydride, diglycolic anhydride, succinic anhydride, succinonitrile (SN), adiponitrile (ADN), ethylene glycol bis(propionitrile) ether (EGBE), and hexane trinitrile (HTCN).

[0027] According to embodiments of the present application, the mass of the second additive is 0.1-15 wt% of the total mass of the electrolyte, for example 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.2 wt%, 1.3 wt%, 1.5 wt%, 1.6 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.4 wt%, 2.5 wt%, 2.6 wt%, 2.8 wt%, 3 wt%, 3.3 wt%, 3.5 wt%, 3.8 wt%, 4 wt%, 4.2 wt%, 4.5 wt%, 4.8 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%.

[0028] The present application also provides a battery comprising the electrolyte described above.

[0029] According to embodiments of the present application, the battery is a lithium ion battery.

[0030] According to embodiments of the present application, the battery further comprises a positive electrode sheet, a negative electrode sheet, and a separator interposed between the positive electrode sheet and the negative electrode sheet.

[0031] According to embodiments of the present application, the battery has a charge cut-off voltage of 4.45 V or higher.

[0032] Advantages of the present application:

[0033] The electrolyte provided by the present application adds the first additive imide phosphate compound, which has a lower LUMO and a higher HOMO, helps to form a stable film on the positive and negative electrode surfaces, avoids the side reaction between the positive and negative electrode materials and the electrolyte, and the generated interface film is rich in N and P. The interface film has high thermodynamic stability and high ionic conductivity, which is beneficial to improve the high-temperature cycle performance and low-temperature performance of the battery at high voltage. At the same time, the phosphate group in the phosphate group can capture H· / OH· free radicals in the electrolyte, inhibit the diffusion reaction of free radicals in the battery combustion process, and the phosphate group can also combine with the free PF5 Lewis acid in the electrolyte, so that the flame retardance of the electrolyte is further improved, and a battery with good thermal stability, i.e. safety performance, is obtained. DETAILED DESCRIPTION

[0034] The present application will be further described in detail below in conjunction with specific examples. It should be understood that the following examples are only illustrative and explanatory of the present application, and should not be interpreted as limiting the scope of protection of the present application. Any modification or equivalent replacement of the technical solutions of the present application without departing from the spirit of the technical solutions of the present application should be covered in the protection scope of the present application.

[0035] The preparation method of the lithium ion battery comprises:

[0036] [Preparation of positive electrode sheet]

[0037] Lithium cobalt oxide (LCO), a binder polyvinylidene fluoride (PVDF), conductive carbon black and single-walled carbon nanotubes are mixed in a weight ratio of 97.2:1.5:1.2:0.1, N-methyl pyrrolidone (NMP) is added, and stirring is performed under the action of a vacuum stirrer until the mixed system becomes a positive electrode slurry with uniform fluidity; the positive electrode slurry is uniformly coated on a current collector aluminum foil; the coated aluminum foil is baked in an oven with different temperature gradients in 5 stages, then dried in an oven at 120℃ for 8h, and then subjected to rolling and slitting to obtain the required positive electrode sheet.

[0038] [Preparation of negative electrode sheet]

[0039] Graphite, a thickening agent sodium carboxymethyl cellulose (CMC-Na), a binder styrene-butadiene rubber, and a conductive agent acetylene black are mixed in a weight ratio of 97:1:1:1, deionized water is added, and a negative electrode slurry is obtained under the action of a vacuum stirrer; the negative electrode slurry is uniformly coated on a high-strength carbon-coated copper foil to obtain an electrode sheet; the obtained electrode sheet is dried at room temperature, then transferred to a 80℃ oven for drying for 10h, and then subjected to rolling and slitting to obtain a negative electrode sheet.

[0040] [Preparation of electrolyte]

[0041] Ethylene carbonate, propylene carbonate and propyl propionate were mixed in a glove box filled with inert gas (H2O < 10 ppm, O2 < 5 ppm) at a mass ratio of EC:PC:PP = 1:1:3, then 14.38 wt% of lithium hexafluorophosphate (LiPF6) based on the total weight of the electrolyte was slowly added to the mixed solution. After passing the moisture and free acid detection, the base electrolyte was obtained. Different contents of the first additive and the second additive in Table 1 were added to the base electrolyte to obtain the electrolyte of the corresponding examples and comparative examples.

[0042] [Manufacture of the battery]

[0043] The above prepared positive electrode sheet, separator (9 micron thick PP film), and negative electrode sheet were stacked in order, ensuring that the separator was between the positive and negative electrode sheets to play a role of isolation. The bare cell was placed in an aluminum plastic film outer package, and the above prepared electrolyte was injected into the dried battery. After packaging, standing, formation, shaping, and capacity distribution, the preparation of the lithium ion soft package battery was completed.

[0044] Examples 1-14 and Comparative Examples 1-4

[0045] The batteries of Examples 1-14 and Comparative Examples 1-4 were prepared according to the above preparation method. The difference between different examples and comparative examples was only the composition of the electrolyte, which was shown in Table 1 as follows.

[0046] Table 1: Electrolyte composition of examples and comparative examples

[0047]

[0048]

[0049] The lithium ion batteries obtained in Examples 1-14 and Comparative Examples 1-4 were tested for relevant performance.

[0050] (1) High temperature cycle performance test:

[0051] At 45°C, the battery after capacity distribution was charged to 4.45V at 0.7C constant current and constant voltage, and the cutoff current was 0.05C. Then it was discharged to 3.0V at 0.5C constant current, and the cycle was repeated. After 500 cycles of charging and discharging, the cycle capacity retention rate at the 500th cycle was calculated, and the calculation formula was as follows:

[0052] Cycle capacity retention rate at the 500th cycle (%) = (cycle discharge capacity at the 500th cycle / first cycle discharge capacity) x 100%.

[0053] (2) Low temperature discharge performance test:

[0054] The discharged battery after being divided was discharged at 0.5C to 3.0V at 25℃ ambient condition, and was left for 5min; then was charged at 0.2C to 4.45V, when the voltage of the battery reached 4.45V, it was changed to 4.45V constant voltage charging, until the charging current was less than or equal to the given cutoff current 0.05C, and was left for 5min; the fully charged battery was transferred into the high-low temperature box, and was set at -10℃, after the temperature of the box reached, it was left for 120min; then was discharged at 0.2C to the terminal voltage 3.0V, and was left for 5min; then the temperature of the high-low temperature box was adjusted to 25℃±3℃, after the temperature of the box reached, it was left for 60min; was charged at 0.2C to 4.45V, when the voltage of the battery reached 4.45V, it was changed to 4.45V constant voltage charging, until the charging current was less than or equal to the given cutoff current 0.05C; was left for 5min; the capacity retention rate of -10℃ low temperature discharge to 3.0V was calculated. The calculation formula was as follows:

[0055] The capacity retention rate of -10℃ low temperature discharge to 3.0V (%) = (-10℃ discharge to 3.0V discharge capacity / 25℃ discharge to 3.0V discharge capacity) x 100%.

[0056] (3) Thermal shock performance:

[0057] The battery was discharged at a given current 0.2C to 3.0V at 25℃ ambient condition; was left for 5min; was charged at a charging current 0.2C to 4.40V, when the voltage of the battery reached 4.45V, it was changed to 4.45V constant voltage charging, until the charging current was less than or equal to the given cutoff current 0.05C; after being left for 1h, the battery was put into the oven, the temperature of the oven was increased to 132±2℃ at a speed of 5±2℃ / min, and was kept for 60min, then was stopped, and the judgment standard was that the battery did not catch fire and did not explode.

[0058] The results of the performance tests of the above items were shown in Table 2.

[0059] Table 2 Performance test results of the batteries of the examples and the comparative examples

[0060]

[0061] From the comparison of the test results of the comparative examples 2-4 and the examples 1-14 in Table 2, it could be known that the thermal shock performance and the low temperature performance of the lithium ion battery could be effectively improved, and the reversible capacity and the high temperature cycle performance of the battery could be improved by adding the imide phosphate compound.

[0062] Further, compared with the examples 2, 12-13, the synergistic effect between the imide phosphate compound and other film-forming additives, the film-forming performance of the electrolyte on the electrode surface was excellent, the combination of them in the electrolyte could improve the thermal shock performance of the lithium ion battery, while having good cycle performance, and the dynamic performance of the battery was also considered.

[0063] Further, comparative examples 1-4, comparative example 4, the imide phosphate compound can significantly improve the thermal box performance of the battery, and the improvement of the thermal box performance has significant correlation with the content.

[0064] Further, comparative examples 8-11, examples 2, 5-7, the imide phosphate compound with sulfur has better thermal box performance and low temperature performance, because the addition of sulfur atom can further improve the interface ion conductance and improve the interface high temperature stability.

[0065] In summary, it can be seen that the electrolyte provided by the application can improve the high temperature cycle life and low temperature performance of the battery, and improve the safety performance of the battery, and the obtained battery shows high application potential.

[0066] The above describes the embodiments of the application. However, the application is not limited to the above embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A battery, characterized by, The battery comprises a positive active material lithium cobaltate, a negative active material graphite and an electrolyte, the electrolyte comprises a lithium salt, an organic solvent and an additive, the additive comprises a first additive, fluoroethylene carbonate (FEC) and lithium difluoro(oxalato)borate (LiODFB), the first additive is selected from at least one of imide phosphate compounds shown in formula I: In formula I, X is O or S; R1, R2are the same or different, independently of one another selected from substituted or unsubstituted C 1～10 alkyl, substituted or unsubstituted C 2～10 alkenyl, substituted or unsubstituted C 2～10 alkynyl, substituted or unsubstituted C 6～10 aryl, if substituted, the substituents are halogen, nitrile, C 1～10 alkyl; R3, R4are the same or different, independently of one another selected from the group consisting of a hydrogen atom, halogen, substituted or unsubstituted C 2～10 alkenyl, substituted or unsubstituted C 1～10 alkyl, substituted or unsubstituted C 1～10 alkoxy, if substituted, the substituents are halogen, nitrile, C 1～10 alkyl; R3, R4may also form a fused ring with the nitrogen-containing five-membered ring; The total mass of the fluoroethylene carbonate (FEC) and lithium difluoro(oxalato)borate (LiODFB) accounts for 8-15wt% of the total mass of the electrolyte.

2. The battery of claim 1, wherein, In formula I, R1, R2 are the same or different and independently selected from substituted or unsubstituted C 1～6 alkyl, substituted or unsubstituted C 2～6 alkenyl, substituted or unsubstituted C 2～6 alkynyl, substituted or unsubstituted C 6～8 aryl, if substituted, the substituents are halogen, nitrile, C 1～6 alkyl.

3. The battery of claim 2, wherein, The compound shown in formula I is selected from at least one of the compounds shown in the structural formula of compounds A1-compound A8:

4. The battery according to any one of claims 1 to 3, characterized in that The mass of the first additive accounts for 0.1-10wt% of the total mass of the electrolyte.

5. The battery of claim 4, wherein, The mass of the first additive accounts for 0.5-3wt% of the total mass of the electrolyte.

6. The battery according to any one of claims 1 to 3, wherein The additive further comprises a second additive, the second additive is selected from at least one of vinylene carbonate (VC), 1,3-propane sulfone lactone (PS), vinyl sulfate (DTD), methane disulfonic acid methylene ester (MMDS), propylene sulfone lactone (PST), maleic anhydride, diglycolic anhydride, succinic anhydride, succinonitrile (SN), adiponitrile (ADN), ethylene glycol bis(propionitrile) ether (EGBE) and hexane trinitrile (HTCN).

7. The battery of claim 1, wherein, The charge cut-off voltage of the battery is 4.45V or above.

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