Lithium battery electrolyte, Secondary battery

Abstract

The application provides a lithium battery electrolyte and a secondary battery. The electrolyte comprises an organic solvent, a lithium salt, an ionic liquid and a low-resistance additive, the ionic liquid is 1-methyl-3-ethoxymethyl imidazole bistrifluoromethylsulfonylimide salt and / or 1-methyl-3-butyl imidazole bistrifluoromethylsulfonylimide salt, and the low-resistance additive is vinylene carbonate and / or methanediyl dimethanesulfonate. The application can improve the stability and conductivity of the electrolyte by adding the ionic liquid in the lithium battery electrolyte, the electrochemical window is obviously widened, so that the high and low temperature discharge performance and cycle performance of the lithium battery are significantly improved; meanwhile, the low-resistance additive is added to further reduce the internal resistance of the battery, and the ionic liquid and the low-resistance additive can synergistically improve the cycle performance of the lithium battery in a wide temperature range of high temperature, normal temperature and low temperature.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery, in particular to a lithium battery electrolyte and a secondary battery. BACKGROUND

[0002] Lithium ion battery has been widely used in the field of new energy electric vehicles due to its high voltage, long storage and cycle life, strong charge retention ability, no environmental pollution and wide working range. However, under low temperature conditions, the lithium ion power battery with conventional electrolyte system has increased viscosity, reduced conductivity and increased electrode interface impedance of electrolyte, which often causes low battery charge and discharge capacity and lithium precipitation, and further causes electrode reaction polarization, reduced discharge platform and energy attenuation of lithium ion battery. Under high temperature conditions, the conventional electrolyte composed of LiPF6 is prone to oxidative decomposition, HF generated thereby corrodes the positive electrode material to cause partial dissolution of metal ions and deposition at the negative electrode, which changes the composition and structure of the negative electrode electrolyte interface film, causes instability of the electrolyte interface and increases the impedance of the SEI film, and further causes battery swelling, performance deterioration and even safety hazards. Moreover, the existing lithium ion battery electrolyte cannot achieve high and low temperature performance, and the low temperature performance is deteriorated when the high temperature performance is satisfied, and the high temperature performance is deteriorated when the low temperature performance is satisfied. SUMMARY

[0003] The main purpose of the present application is to provide a lithium battery electrolyte and a secondary battery to solve the problem that the existing lithium ion battery electrolyte cannot achieve high and low temperature performance.

[0004] In order to achieve the above purpose, according to one aspect of the present application, a lithium battery electrolyte is provided, which comprises an organic solvent, a lithium salt, an ionic liquid and a low impedance additive, the ionic liquid is 1-methyl-3-ethoxymethyl imidazole bis-trifluoromethyl sulfonimide salt and / or 1-methyl-3-butyl imidazole bis-trifluoromethyl sulfonimide salt, and the low impedance additive is vinylene carbonate and / or methanediol methane disulfonate.

[0005] Further, the mass of the ionic liquid accounts for 3-10% of the total mass of the electrolyte, and the mass of the low impedance additive accounts for 0.5-2% of the total mass of the electrolyte.

[0006] Further, the low impedance additive is vinylene carbonate and methanediol methane disulfonate; preferably, the mass ratio of vinylene carbonate to methanediol methane disulfonate is (0.5-2):1.

[0007] Further, the mass of the organic solvent accounts for 89-96% of the total mass of the electrolyte; preferably, the organic solvent is a cyclic ester and / or a chain ester.

[0008] Further, the cyclic ester is selected from one or more of ethylene carbonate, propylene carbonate and gamma-butyrolactone; the chain ester is selected from one or more of dimethyl carbonate, butylene carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and propyl propionate.

[0009] Further, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium oxalate phosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tetrafluoroborate and lithium bis(fluorosulfonyl)imide.

[0010] According to another aspect of the present application, a secondary battery is provided, comprising a positive electrode, a negative electrode, a separator and an electrolyte, the electrolyte being the electrolyte of the present application.

[0011] Further, the positive electrode comprises a positive electrode active material, the positive electrode active material being selected from one or more of LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4, LiMn 2-y M y O4, Li2Mn 1-x O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, F and Y, 0≤a<0.2, x<1, y<2.

[0012] Further, the negative electrode comprises a negative electrode active material, the negative electrode active material being selected from one or more of natural graphite, artificial graphite, soft carbon, hard carbon, lithium titanate, silicon, silicon-carbon alloy; wherein the soft carbon is one or more of petroleum coke, needle coke, carbon fiber and carbon microsphere, and the hard carbon is one or more of resin carbon, organic polymer pyrolytic carbon and carbon black.

[0013] Further, the separator is a PE membrane and / or a PP-PE-PP membrane; preferably, the separator is a PE membrane and / or a PP-PE-PP membrane coated with ceramic or PVDF on the surface.

[0014] The application can improve the stability and conductivity of the electrolyte, significantly widen the electrochemical window, and significantly improve the high and low temperature discharge performance and cycle performance of the lithium battery by adding 1-methyl-3-ethoxymethyl imidazole bistrifluoromethyl sulfonimide salt and / or 1-methyl-3-butyl imidazole bistrifluoromethyl sulfonimide salt in the lithium battery electrolyte. The battery internal resistance can be further reduced by adding low impedance additives vinylene carbonate and / or methanedimethylenesulfonate, and the cycle performance of the lithium battery in a wide temperature range of high temperature, normal temperature and low temperature can be improved by the synergistic effect of the ion liquid. DETAILED DESCRIPTION

[0015] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments.

[0016] As described in the background of the present application, the prior art has the problem that the lithium ion battery electrolyte cannot balance the high and low temperature performance. In order to solve the above problem, in a typical embodiment of the present application, a lithium battery electrolyte is provided, which comprises an organic solvent, a lithium salt, an ion liquid and a low impedance additive, the ion liquid is 1-methyl-3-ethoxymethyl imidazole bistrifluoromethyl sulfonimide salt and / or 1-methyl-3-butyl imidazole bistrifluoromethyl sulfonimide salt, and the low impedance additive is vinylene carbonate and / or methanedimethylenesulfonate.

[0017] The two ion liquids in the electrolyte of the present application have high solubility to the lithium salt, good thermal stability, wide electrochemical window, high conductivity when added to the electrolyte, and can significantly improve the high and low temperature discharge performance and cycle performance of the lithium battery. The above low impedance additive can also improve the SEI film structure of the lithium ion battery, further reduce the SEI low temperature resistance, and further improve the charge-discharge performance and cycle performance of the lithium battery in high and low temperature environments in cooperation with the aforementioned ion liquid.

[0018] In particular, compared with the 1-ethyl-3-methyl imidazole bistrifluoromethyl sulfonimide salt or 1-octyl-2,3-dimethyl imidazole trifluoromethane sulfonate commonly used in the prior art, the ion liquid of the present application is easier to synthesize, has better thermal stability, a wider electrochemical window, and can better improve the high and low temperature performance of the battery in combination with the low impedance additive. In summary, the present application can improve the stability and conductivity of the electrolyte by adding 1-methyl-3-ethoxymethyl imidazole bistrifluoromethyl sulfonimide salt and / or 1-methyl-3-butyl imidazole bistrifluoromethyl sulfonimide salt in the lithium battery electrolyte, significantly widen the electrochemical window, and further reduce the battery internal resistance by adding low impedance additives vinylene carbonate and / or methanedimethylenesulfonate, and the cycle performance of the lithium battery in a wide temperature range of high temperature, normal temperature and low temperature can be improved by the synergistic effect of the ion liquid.

[0019] Since the above-mentioned ionic liquid and low-resistance additive of the present application have good performance in a wide temperature range, a small amount of addition in the electrolyte can further improve the conductivity and ion diffusion rate of the lithium battery electrolyte. In a preferred embodiment, the mass of the ionic liquid accounts for 3-10% of the total mass of the electrolyte, and the mass of the low-resistance additive accounts for 1-2% of the total mass of the electrolyte. The ionic liquid and low-resistance additive in the above-mentioned content range can better improve the stability and conductivity of the electrolyte, further reduce the SEI high-low temperature resistance, and further reduce the internal resistance of the lithium battery, so that the lithium battery has better cycle performance at high and low temperatures.

[0020] In a preferred embodiment, the low-resistance additive is vinylene carbonate and methylene methane disulfonate. Preferably, the mass ratio of vinylene carbonate to methylene methane disulfonate is (0.5-2):1. When the low-resistance additive is in the above-mentioned content range, the SEI film impedance formed in the lithium battery is smaller, the positive and negative electrode interface can be better protected, the migration impedance of lithium ions is smaller, the insertion and extraction speed is faster, and the cycle performance at high and low temperatures is better.

[0021] The present application does not have special limitations on the specific type of solvent, which can be selected according to actual needs. In a preferred embodiment, the mass of the organic solvent accounts for 89-96% of the total mass of the electrolyte. Preferably, the organic solvent is a cyclic ester and / or a chain ester, so that the electrolyte has high conductivity in high and low temperature environments, better solubility for lithium salt, and further improved wettability of the electrolyte.

[0022] Specifically, in a preferred embodiment, the cyclic ester is selected from one or more of vinyl carbonate, propylene carbonate and γ-butyrolactone; and the chain ester is selected from one or more of dimethyl carbonate, butylene carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and propyl propionate.

[0023] The addition amount of lithium salt is 1-3 mol of lithium salt per liter of electrolyte. In a preferred embodiment, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluorooxalate phosphate, lithium oxalate phosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate and lithium bisfluorosulfonimide. These lithium salts have relatively high thermal stability and are not easy to decompose, and the anion and cation are easy to dissociate, so that the lithium battery has better cycle performance.

[0024] In another typical embodiment of the present application, a secondary battery is also provided, which comprises a positive electrode, a negative electrode, a separator and an electrolyte, and the electrolyte is the electrolyte of the present application. Due to the use of the electrolyte of the present application, the SEI film structure of the lithium ion battery is improved, the stability and conductivity of the electrolyte are greatly improved, and the internal resistance of the lithium battery is reduced, so that the above-mentioned lithium battery has good cycle performance in a wide temperature range of high temperature, normal temperature and low temperature.

[0025] In order to further improve the cycle performance and rate performance of the lithium ion battery, in a preferred embodiment, the positive electrode comprises a positive electrode active material, and the positive electrode active material is selected from one or more of LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+ a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4, LiMn 2-y M y O4, Li2Mn 1-x O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, F and Y, 0≤a<0.2, x<1, y<2, and the above-mentioned positive electrode active material will not affect the high-temperature storage performance and cycle stability of the lithium ion battery.

[0026] In order to further reduce the impedance of the material at high and low temperatures, in a preferred embodiment, the negative electrode comprises a negative electrode active material, and the negative electrode active material is selected from one or more of natural graphite, artificial graphite, soft carbon, hard carbon, lithium titanate, silicon, silicon-carbon alloy; wherein the soft carbon is one or more of petroleum coke, needle coke, carbon fiber and carbon microsphere, and the hard carbon is one or more of resin carbon, organic polymer pyrolytic carbon and carbon black. The above-mentioned negative electrode active material can simultaneously improve the liquid absorption performance, increase the high and low temperature discharge voltage platform of the lithium battery, and better improve the high and low temperature cycle performance of the lithium battery.

[0027] The separator can use conventional materials in the art, and in a preferred embodiment, the separator is a PE film and / or a PP-PE-PP film; preferably, the separator is a PE film and / or a PP-PE-PP film coated with ceramic or PVDF, which is more suitable for the electrolyte of the present application.

[0028] The present application will be further described in detail below in conjunction with specific examples, which cannot be understood as limiting the scope of the present application.

[0029] It should be noted that the percentages in the following examples are all mass percentages.

[0030] Example 1

[0031] (1) Preparation of the positive electrode sheet: the positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.6 Co 0.1 Mn 0.3 O2), conductive agent Super-P, carbon nanotube CNT, binder PVDF were mixed in a solvent N-methyl pyrrolidone in a mass ratio of 96:1:1:2 to form a positive electrode slurry, then the positive electrode slurry was uniformly coated on the current collector aluminum foil, dried and rolled, trimmed, cut, then dried at 85°C under vacuum, the tab was welded, and the positive electrode sheet of the lithium ion secondary battery was prepared.

[0032] (2) Preparation of the negative electrode sheet: the negative electrode active material artificial graphite, conductive agent Super-P, thickening agent CMC, binder SBR were dispersed in deionized water in a mass ratio of 97:2:1 to form a negative electrode slurry, then the negative electrode slurry was uniformly coated on the current collector copper foil, dried and rolled, trimmed, cut, then dried at 85°C under vacuum, the tab was welded, and the negative electrode sheet of the lithium ion secondary battery was prepared.

[0033] (3) Preparation of the electrolyte: 1 mol / L LiPF6 was used as the lithium salt, and a mixture of ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 1:1:1 was used as the non-aqueous organic solvent, the solvent ratio was 96%, 1-methyl-3-butyl imidazole bis-trifluoromethanesulfonylimide salt was 3%, and low-impedance additives vinylene carbonate and methanedimethanesulfonate were each 0.5wt.%.

[0034] (4) Preparation of the lithium ion secondary battery: the positive electrode sheet, the negative electrode sheet, and the separator PE film of the lithium ion secondary battery prepared according to the foregoing process were subjected to a lamination process to form a soft-pack battery cell, after water removal, the electrolyte was injected, and the battery was allowed to stand for 24h, then it was charged at 45°C with a constant current of 0.1C to 3.8V, high-temperature aging was performed at 45°C for 72h, it was then charged at 45°C with a constant current of 0.33C to 4.2V, then constant-voltage charging was performed until the current dropped to 0.05C, then it was discharged at 0.33C to 2.8V, and the charging and discharging was repeated twice to complete the preparation of the lithium ion secondary battery.

[0035] Example 2

[0036] The lithium ion secondary battery of Example 2 was prepared according to the same method as Example 1, except that the proportion of the organic solvent in the electrolyte was 94%, 1-methyl-3-butyl imidazole bis-trifluoromethanesulfonylimide salt was 5%, and low-impedance additives vinylene carbonate and methanedimethanesulfonate were each 0.5wt.%.

[0037] Example 3

[0038] A lithium ion secondary battery of Example 3 was prepared in the same manner as Example 1, except that the proportion of the organic solvent in the electrolyte was 89%, the 1-methyl-3-butylimidazole bis-trifluoromethanesulfonimide salt was 10%, and the low impedance additives, vinylene carbonate and methylene methanedisulfonate, were each 0.5 wt.%.

[0039] Example 4

[0040] A lithium ion secondary battery of Example 4 was prepared in the same manner as Example 1, except that the proportion of the organic solvent in the electrolyte was 95.5%, the 1-methyl-3-butylimidazole bis-trifluoromethanesulfonimide salt was 3%, and the low impedance additives, vinylene carbonate 0.5 wt.% and methylene methanedisulfonate 1.0 wt.%.

[0041] Example 5

[0042] A lithium ion secondary battery of Example 5 was prepared in the same manner as Example 1, except that the proportion of the organic solvent in the electrolyte was 95.5%, the 1-methyl-3-butylimidazole bis-trifluoromethanesulfonimide salt was 3%, and the low impedance additives, vinylene carbonate 1.0 wt.% and methylene methanedisulfonate 0.5 wt.%.

[0043] Example 6

[0044] A lithium ion secondary battery of Example 6 was prepared in the same manner as Example 1, except that the proportion of the organic solvent in the electrolyte was 95%, the 1-methyl-3-butylimidazole bis-trifluoromethanesulfonimide salt was 3%, and the low impedance additives, vinylene carbonate 1.0 wt.% and methylene methanedisulfonate 1.0 wt.%.

[0045] Example 7

[0046] A lithium ion secondary battery of Example 7 was prepared in the same manner as Example 1, except that the ionic liquid was 1-methyl-3-ethoxymethylimidazole bis-trifluoromethanesulfonimide salt.

[0047] Comparative Example 1

[0048] A lithium ion secondary battery of Comparative Example 1 was prepared in the same manner as Example 1, except that no ionic liquid was added to the electrolyte, and the low impedance additives, vinylene carbonate and methylene methanedisulfonate, were each 0.5 wt.%.

[0049] Comparative Example 2

[0050] A lithium ion secondary battery of Comparative Example 2 was prepared in the same manner as in Example 1, except that no low impedance additive was added to the electrolyte and the proportion of the carbonate solvent was 97% and the proportion of 1-methyl-3-butyl imidazole bis-trifluoromethylsulfonylimide was 3%.

[0051] Comparative Example 3

[0052] A lithium ion secondary battery of Comparative Example 2 was prepared in the same manner as in Example 1, except that no low impedance additive was added to the electrolyte and the proportion of the carbonate solvent was 97% and the proportion of 1-methyl-3-butyl imidazole bis-trifluoromethylsulfonylimide was 3%.

[0053] Comparative Example 4

[0054] A lithium ion secondary battery of Comparative Example 2 was prepared in the same manner as in Example 1, except that no low impedance additive was added to the electrolyte and the proportion of the carbonate solvent was 97% and the proportion of 1-methyl-3-butyl imidazole bis-trifluoromethylsulfonylimide was 3%.

[0055] The lithium ion secondary batteries in Examples 1 to 7 and Comparative Examples 1 to 4 were subjected to cycle performance tests, and the results are shown in Table 1.

[0056] Cycle test method:

[0057] 1) High temperature cycle: The lithium ion secondary batteries prepared were subjected to 1C / 1C cycle tests at 45°C, and the effect of the addition of different contents of ionic liquid and additive on the cycle performance of the batteries was analyzed.

[0058] The capacity retention rate (%) of the lithium ion secondary battery after 500 cycles = (discharge capacity at the 500th cycle ÷ discharge capacity at the 1st cycle) x 100%.

[0059] 2) Normal temperature cycle: The lithium ion secondary batteries prepared were subjected to 1C / 1C cycle tests at 25°C, and the effect of the addition of different contents of ionic liquid and additive on the cycle performance of the batteries was analyzed.

[0060] 3) Low temperature cycle: The lithium ion secondary batteries prepared were subjected to 0.05C / 0.33C cycle tests at -20°C, and the effect of the addition of different contents of ionic liquid and additive on the cycle performance of the batteries was analyzed.

[0061] Table 1

[0062]

[0063]

[0064] From the above, compared with the comparative examples, the examples 1 to 6 can obviously improve the high temperature, normal temperature and low temperature cycle performance of the battery by adding different proportions of ionic liquid and low impedance additive in the electrolyte, both of which are used in combination. The cycle performance of the electrolyte only added with ionic liquid or low impedance additive is relatively worse than that of the battery simultaneously added with ionic liquid and low impedance additive. It can be seen that the present application can improve the stability and conductivity of the electrolyte by adding ionic liquid in the lithium battery electrolyte, the electrochemical window is obviously widened, the high and low temperature discharge performance and cycle performance of the lithium battery are significantly improved, and the addition of low impedance additive further reduces the internal resistance of the battery, which can improve the cycle performance of the lithium battery in a wide temperature range of high temperature, normal temperature and low temperature in cooperation with ionic liquid.

[0065] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. The present application can have various modifications and changes for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A lithium battery electrolyte, characterized in that: The electrolyte comprises an organic solvent, a lithium salt, an ionic liquid and a low-impedance additive, wherein the ionic liquid is 1-methyl-3-ethoxymethylimidazolium bis(trifluoromethanesulfonyl)imide salt, and the low-impedance additive is vinylene carbonate and methylene methanedisulfonate in a mass ratio of (0.5-2):1; The mass of the ionic liquid accounts for 3-10% of the total mass of the electrolyte, and the mass of the low-impedance additive accounts for 1-2% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, characterized in that The mass of the organic solvent accounts for 89-96% of the total mass of the electrolyte.

3. The electrolyte according to claim 2, characterized in that The organic solvent is a cyclic ester and / or a chain ester.

4. The electrolyte according to claim 3, characterized in that The cyclic ester is selected from one or more of ethylene carbonate, propylene carbonate and γ-butyrolactone; the chain ester is selected from one or more of dimethyl carbonate, butylene carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and propyl propionate.

5. The electrolyte according to claim 1 or 2, characterized in that The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobisoxalatophosphate, lithium tetrafluorooxalatophosphate, lithium oxalatephosphate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium tetrafluoroborate and lithium bisfluorosulfonyl imide.

6. A secondary battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that: The electrolyte is the electrolyte according to any one of claims 1 to 5.

7. The secondary battery according to claim 6, characterized in that The positive electrode comprises a positive electrode active material selected from the group consisting of LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li 1+a Mn 1-x M x O2、LiCo 1-x M x O2、LiFe 1-x M x PO4、LiMn 2-y M y O4、Li2Mn 1-x One or more of O4, wherein M is selected from one or more of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B, F and Y, 0≤a<0.2, x<1, y<2.

8. The secondary battery according to claim 6, wherein The negative electrode includes a negative electrode active material, which is selected from one or more of natural graphite, artificial graphite, soft carbon, hard carbon, lithium titanate, silicon, and silicon-carbon alloy; wherein the soft carbon is one or more of petroleum coke, needle coke, carbon fiber, and carbon microspheres, and the hard carbon is one or more of resin carbon, organic polymer pyrolytic carbon, and carbon black.

9. The secondary battery according to claim 6, wherein The separator is a PE film and / or a PP-PE-PP film.

10. The secondary battery according to claim 6, wherein The diaphragm is a PE film and / or PP-PE-PP film with a surface coated with ceramic or PVDF.

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