Electrolyte for low-temperature lithium batteries

Abstract

The present invention provides an electrolyte for low-temperature lithium batteries, comprising: a non-aqueous solvent, an electrolyte lithium salt, a borate ester additive, and other additives. The other additives are any one or a mixture of several of vinylene carbonate (VC), biphenyl (BP), triphenyl phosphite (TPP), 1,3-propane sultone (PS), 1,4-butane sultone (BS), succinic anhydride (SA), and fluoroethylene carbonate (FEC). The structural general formula of the borate ester additive is: Through the technical solution of the present invention, the viscosity of the electrolyte can be effectively reduced, the ionic conductivity of the lithium battery can be improved, the impedance of the electrolyte under low-temperature conditions can be reduced, and the performance of the lithium battery under low-temperature conditions can be enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery electrolytes, and more specifically, to an electrolyte for low-temperature lithium batteries. Background Art

[0002] Lithium-ion batteries dominate the current energy storage field due to their high energy density, high power density, and long cycle life in commercial secondary batteries. Nowadays, the room-temperature energy density of commercial lithium-ion batteries has tripled compared to their initial commercialization, but the problem of their significant performance degradation at low temperatures remains prominent. This undoubtedly limits the application of lithium batteries in high-altitude and high-latitude regions and is also one of the major obstacles in defense and space applications.

[0003] In existing lithium-ion batteries, when the external temperature drops to -20°C or even lower, the ionic conductivity of the electrolyte rapidly decreases or even the electrolyte freezes. The interfacial charge transfer kinetics becomes slower, and the transport of Li+ in the SEI and inside the electrode becomes more difficult. Most lithium-ion batteries based on ethylene carbonate (EC) electrolytes will experience a significant loss of energy / power density. In addition, lithium plating is likely to occur during low-temperature charging, which also brings some safety problems. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art or related technologies.

[0005] To this end, the object of the present invention is to provide an electrolyte for low-temperature lithium batteries. By adding borate additives with special structures, the viscosity of the electrolyte can be effectively reduced, the ionic conductivity of the lithium battery can be increased, the impedance of the electrolyte under low-temperature conditions can be reduced, and the performance of the lithium battery under low-temperature conditions can be improved.

[0006] To achieve the above object, the technical solution of the present invention provides an electrolyte for low-temperature lithium batteries, including: a non-aqueous solvent, an electrolyte lithium salt, a borate additive, and other additives. The other additives are any one or a mixture of several of vinylene carbonate (VC), biphenyl (BP), triphenyl phosphite (TPP), 1,3-propane sultone (PS), 1,4-butane sultone (BS), succinic anhydride (SA), and fluoroethylene carbonate (FEC). The structural general formula of the borate additive is:

[0007]

[0008] Wherein, R 1 ~R 2Each independently is hydrogen, cyano, halo (C1-C10) alkyl(ene), (C1-C10) alkyl(ene), (C1-C10) alkoxy, (C1-C10) alkoxycarbonyl, (C3-C12) cycloalkyl, (C3-C12) heterocycloalkyl, (C6-C12) aryl, (C3-C12) heteroaryl or (C6-C12) aryl(C1-C10) alkyl, pyridine aromatic group or (unsaturated) aliphatic group.

[0009] In this technical solution, by adding a borate additive with a special structure to the electrolyte for low-temperature lithium batteries, the viscosity of the electrolyte can be reduced, the conductivity of the electrolyte can be improved, a stable interfacial film can be formed on the electrode surface more efficiently, the impedance of the SEI film and the CEI film can be reduced. At the same time, the interfacial film with low impedance can effectively alleviate the lithium plating phenomenon during low-temperature charging. Therefore, the low-temperature performance of the lithium battery can be improved. Moreover, such a borate additive has good compatibility in the electrolyte, can improve the thermal stability of the electrolyte, reduce interfacial side reactions, and significantly improve the safety performance of the battery.

[0010] In the above technical solution, preferably, the borate additive is selected from the following structures:

[0011]

[0012] In this technical solution, the special structure of the borate additive contains multiple B-O bonds, and there are double bonds in the boron-oxygen heterocycle, which can more effectively reduce the viscosity of the electrolyte, improve the conductivity of the electrolyte, more efficiently form a stable interfacial film on the electrode surface, and the formed interfacial film has low impedance, which can effectively alleviate the lithium plating phenomenon during low-temperature charging. Moreover, the borate additive has good compatibility in the electrolyte, can significantly improve the conductivity of the electrolyte for lithium batteries under low-temperature conditions, and can significantly improve the cycling performance and storage performance of lithium-ion batteries under low-temperature conditions.

[0013] In any of the above technical solutions, preferably, the borate additive is dimethyl 3,4-dihydro-1,2,5-oxaborolane The dosage of the borate additive is 0.01%-5% of the total mass of the electrolyte.

[0014] In this technical solution, the electrolyte for low-temperature lithium batteries is further optimized. Dimethyl 3,4-dihydro-1,2,5-oxaborole is easily obtained. When its dosage is 0.01%-5% of the total mass of the electrolyte, its compatibility in the electrolyte is better. The moisture and acidity prepared in this way can meet the standards. At low temperature (-20°C), the conductivity is significantly improved. When the addition amount of dimethyl 3,4-dihydro-1,2,5-oxaborole is 2%, the conductivity at low temperature (-20°C) can reach 5.57 ms / cm. Compared with the electrolytes in the prior art, the conductivity is significantly improved. As a result, the electrolyte has a lower impedance at low temperature, can effectively alleviate the lithium plating phenomenon during low-temperature charging, and solves the problems of easy discharge and difficult charging of lithium batteries under low-temperature conditions. Moreover, the thermal stability of the electrolyte is further improved, and the interfacial side reactions are controlled. Therefore, the safety performance of the battery can be significantly improved.

[0015] In any of the above technical solutions, preferably, the electrolyte lithium salt is any one or a mixture of several of lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), and lithium bis(oxalato)borate (LiBOB).

[0016] In any of the above technical solutions, preferably, the electrolyte lithium salt is lithium hexafluorophosphate (LiPF6), and the concentration of the electrolyte lithium salt is 1.2 mol / L.

[0017] In any of the above technical solutions, preferably, the non-aqueous solvent is any one or a mixture of several of dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), and methyl propyl carbonate (MPC).

[0018] In any of the above technical solutions, preferably, the non-aqueous solvent is a mixture of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC). The dosage of the non-aqueous solvent is 60%-80% of the total mass of the electrolyte, and the mass ratio of dimethyl carbonate (DMC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) is 1:1:1.

[0019] In any of the above technical solutions, preferably, the other additive is a mixture of 1,4-butanesultone (BS) and fluoroethylene carbonate (FEC), and the dosage of the other additive is 0.01%-5% of the total mass of the electrolyte.

[0020] In any of the above technical solutions, preferably, the mass fraction of 1,4-butanesultone (BS) is 1.2%, and the mass fraction of fluoroethylene carbonate (FEC) is 1.2%.

[0021] In this technical solution, the electrolyte ratio is further optimized, so that the electrolyte has high compatibility, further improves the thermal stability of the electrolyte, and can give full play to the synergistic effect of borate additives and other substances, further effectively reduce the viscosity of the electrolyte, improve the conductivity of the electrolyte, further enhance the thermal stability of the electrolyte, significantly improve the safety performance of the battery, improve the low-temperature performance of the battery. The capacity retention rate of the lithium battery can reach 76.48% after 100 cycles at -20°C, and the capacity retention rate can reach 84.37% after storing for 7 days at -20°C.

[0022] The technical solution of the present invention also provides a low-temperature lithium battery, which uses the low-temperature type lithium battery electrolyte in the above technical solution. Therefore, it has all the beneficial technical effects of the low-temperature type lithium battery electrolyte in the above technical solution, which will not be elaborated here.

[0023] The low-temperature type lithium battery electrolyte proposed by the present invention has the following beneficial technical effects:

[0024] (1) The low-temperature type lithium battery electrolyte proposed by the present invention is added with borate additives with special structures, which can effectively reduce the viscosity of the electrolyte, improve the ionic conductivity of the lithium battery, make the electrolyte have a lower impedance under low-temperature conditions. Moreover, the borate additives have good compatibility in the electrolyte and can cooperate with other substances in the electrolyte to effectively improve the low-temperature performance of the electrolyte.

[0025] (2) The low-temperature type lithium battery electrolyte proposed by the present invention is added with borate additives with special structures. The borate additives contain multiple B-O bonds, and there are double bonds in the boron-oxygen heterocycle, which can more efficiently form a stable interfacial film on the electrode surface, reduce the impedance of SEI and CEI. Therefore, the low-temperature performance of the lithium battery is improved. The capacity retention rate of the lithium battery can reach 76.48% after 100 cycles at -20°C, and the capacity retention rate can reach 84.37% after storing for 7 days at -20°C. At the same time, the interfacial film with low impedance helps to alleviate the lithium plating phenomenon during low-temperature charging, effectively solving the problem that lithium batteries are easy to discharge and difficult to charge under low-temperature conditions.

[0026] (3) The low-temperature type lithium battery electrolyte proposed by the present invention is added with borate additives with special structures. The B-O bonds contained in the borate additives can improve the electrochemical and thermodynamic stability of the lithium battery, control interfacial side reactions, and thus improve the safety performance of the lithium battery.

[0027] Additional aspects and advantages of the present invention will be given in the following description section, some will become apparent from the following description, or be learned through the practice of the present invention. Detailed Embodiments

[0028] The present invention discloses an electrolyte for low-temperature lithium batteries. Those skilled in the art can draw on the content herein and appropriately modify process parameters to achieve it. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments. Relevant personnel can obviously make changes or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0029] The following further elaborates the present invention in conjunction with embodiments:

[0030] Example 1

[0031] Under a nitrogen-sealed protection atmosphere with moisture < 10 ppm, non-aqueous solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were sequentially added in a mass ratio of 1:1:1 for mixing. A condenser was used to cool the mixed solution to ensure that the temperature did not exceed 10 °C. Lithium hexafluorophosphate was slowly added to ensure that the lithium salt concentration was 1.2 mol / L. Then, 1,4-butanesultone (BS) with a mass fraction of 1.2% and fluoroethylene carbonate (FEC) with a mass fraction of 1.2% were added. Dimethyl 3,4-dihydro-1,2,5-oxaborolane with a mass fraction of 0.5% was added and stirring continued until the solution became clear.

[0032] Example 2

[0033] The difference from Example 1 was that dimethyl 3,4-dihydro-1,2,5-oxaborolane with a mass fraction of 1% was added and stirring continued until the solution became clear.

[0034] Example 3

[0035] The difference from Example 1 was that dimethyl 3,4-dihydro-1,2,5-oxaborolane with a mass fraction of 1.5% was added and stirring continued until the solution became clear.

[0036] Example 4

[0037] The difference from Example 1 was that dimethyl 3,4-dihydro-1,2,5-oxaborolane with a mass fraction of 2% was added and stirring continued until the solution became clear.

[0038] Example 5

[0039] The difference from Example 1 is that dimethyl 3,4-dihydro-1,2,5-oxaborole with a mass fraction of 2.5% is added and stirring is continued until the solution becomes clear.

[0040] Example 6

[0041] The difference from Example 1 is that dimethyl 3,4-dihydro-1,2,5-oxaborole with a mass fraction of 3% is added and stirring is continued until the solution becomes clear.

[0042] Comparative Example 1

[0043] Under a nitrogen sealed protection atmosphere with moisture < 10 ppm, non-aqueous solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are sequentially added and mixed in a mass ratio of 1:1:1. The mixed solution is cooled with a condenser to ensure that the temperature does not exceed 10°C. Lithium hexafluorophosphate is slowly added to ensure a lithium salt concentration of 1.2 mol / L. Then 1,4-butanesultone (BS) with a mass fraction of 1.2% and fluoroethylene carbonate (FEC) with a mass fraction of 1.2% are added and stirring is continued until the solution becomes clear.

[0044] Comparative Example 2

[0045] Under a nitrogen sealed protection atmosphere with moisture < 10 ppm, non-aqueous solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are sequentially added and mixed in a mass ratio of 1:1:1. The mixed solution is cooled with a condenser to ensure that the temperature does not exceed 10°C. Lithium hexafluorophosphate is slowly added to ensure a lithium salt concentration of 1.2 mol / L. Then 1,4-butanesultone (BS) with a mass fraction of 1.2% and fluoroethylene carbonate (FEC) with a mass fraction of 1.2% are added, and dimethyl borate with a mass fraction of 3% is added and stirring is continued until the solution becomes clear.

[0046] The electrolytes prepared in the above Examples 1 to 6 and Comparative Examples 1 and 2 are tested for moisture, acidity and conductivity, and the test results are shown in Table 1 below.

[0047] Table 1 Test results of moisture, acidity and conductivity of the electrolytes

[0048]

[0049]

[0050] As can be seen from Table 1, the moisture and acidity values of the electrolytes prepared in Examples 1 to 6 and Comparative Examples 1 and 2 are all qualified. At low temperature (-20°C), as the amount of borate ester additives increases, the conductivity of the electrolyte increases. However, when the conductivity of the electrolyte increases to a certain value, the increase in borate ester additives will instead cause the conductivity of the electrolyte to decrease. When the mass fraction of borate ester additives is 2%, the effect is better, and the conductivity can reach 5.57 ms / cm under low temperature conditions. Comparing Comparative Example 2 with Example 6, it can be seen that the conductivity of dimethyl 3,4-dihydro-1,2,5-oxaborolane is higher than that of traditional dimethyl borate. The special structure of dimethyl 3,4-dihydro-1,2,5-oxaborolane can more efficiently form a stable interfacial film on the electrode surface, reducing the impedance of SEI and CEI, thereby improving the low-temperature performance of lithium batteries. Compared with traditional dimethyl borate, dimethyl 3,4-dihydro-1,2,5-oxaborolane significantly improves the conductivity, and a relatively high conductivity can be obtained when the dosage is 2%, with less dosage and high compatibility in the electrolyte.

[0051] Lithium-ion batteries are prepared using the electrolytes formulated in Examples 1 to 6 and Comparative Examples 1 and 2 above.

[0052] The positive and negative electrodes respectively use ternary material LiNi8Co1Mn1O2, Super-P, PVDF (900, 5130), CNT; silicon-carbon material (450 mAh / g), Super-P, CMC, SBR, etc., which are mixed evenly in a certain proportion to prepare positive and negative electrode slurries with a certain viscosity. Then they are evenly coated on aluminum and copper current collectors respectively. After drying at 80°C, the electrodes are cut, rolled, slit, wound into the shell, etc. to complete the production of the battery core. Finally, after drying at 85°C for 48 h, the above electrolyte is injected, and the lithium-ion battery is completed after encapsulation. The lithium-ion batteries are subjected to low-temperature cycle tests at 0°C / -20°C and storage tests at 0°C / -20°C after hot pressing formation and vacuum secondary sealing. The test results are shown in Table 2 below.

[0053] Table 2 Results of 0°C / -20°C low-temperature cycle test and 0°C / -20°C storage test of lithium batteries

[0054]

[0055]

[0056] As can be seen from Table 2, the use of this borate additive significantly improves the cycling performance of lithium-ion batteries under low-temperature conditions, while being beneficial to enhancing the storage performance of lithium-ion batteries under low-temperature conditions, improving the thermal stability of the electrolyte, controlling interfacial side reactions, and improving the safety performance of the battery. Among them, when the mass fraction of dimethyl 3,4-dihydro-1,2,5-oxaborolane in Example 5 is 2%, the effect is better. The capacity retention rate of the lithium battery can reach 76.48% after 100 cycles at -20°C, and the capacity retention rate can reach 84.37% after storing for 7 days at -20°C. Moreover, compared with traditional dimethyl borate, the addition of dimethyl 3,4-dihydro-1,2,5-oxaborolane significantly improves the capacity retention rate after 100 cycles at -20°C and the capacity retention rate after storing for 7 days. The addition of borate additives with special structures can more efficiently form a stable interfacial film on the electrode surface, thereby enhancing the conductivity of lithium-ion batteries under low-temperature conditions, making the electrolyte have a lower impedance under low-temperature conditions, and forming a stable interfacial film on the electrode surface, thereby enhancing the low-temperature performance of lithium-ion batteries. In addition, the B-O bond contained in the borate additive can improve the electrochemical and thermodynamic stability of the lithium battery, and further improve the safety performance of the lithium battery.

[0057] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An electrolyte for a low-temperature lithium battery, Characterized in that, Comprising: A non-aqueous solvent, an electrolyte lithium salt, a borate ester additive, and other additives. The other additives are any one or a mixture of several of vinylene carbonate (VC), biphenyl (BP), triphenyl phosphite (TPP), 1,3-propane sultone (PS), 1,4-butane sultone (BS), succinic anhydride (SA), and fluoroethylene carbonate (FEC). The borate ester additive is dimethyl 3,4-dihydro-1,2,5-oxaborole The dosage of the borate ester additive is 0.01% - 5% of the total mass of the electrolyte.

2. The electrolyte for a low-temperature lithium battery according to claim 1, Characterized in that, The electrolyte lithium salt is any one of lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB) or a mixture of several of them.

3. The electrolyte for a low-temperature lithium battery according to claim 2, Characterized in that, The electrolyte lithium salt is lithium hexafluorophosphate (LiPF6), and the concentration of the electrolyte lithium salt is 1.2 mol / L.

4. The electrolyte for a low-temperature lithium battery according to claim 1, Characterized in that, The non-aqueous solvent is any one or a mixture of several of dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC) and methyl propyl carbonate (MPC).

5. The electrolyte for a low-temperature lithium battery according to claim 4, Characterized in that, The non-aqueous solvent is a mixture of dimethyl carbonate (DMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC), the amount of the non-aqueous solvent is 60%-80% of the total mass of the electrolyte, and the mass ratio of dimethyl carbonate (DMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) is 1:1:

1.

6. The electrolyte for a low-temperature lithium battery according to claim 1, Characterized in that, The other additives are a mixture of 1,4-butanesultone (BS) and fluoroethylene carbonate (FEC), and the amount of the other additives is 0.01%-5% of the total mass of the electrolyte.

7. The electrolyte for a low-temperature lithium battery according to claim 6, Characterized in that, The mass fraction of 1,4-butanesultone (BS) is 1.2%, and the mass fraction of fluoroethylene carbonate (FEC) is 1.2%.

8. A low-temperature lithium battery, Characterized in that, The electrolyte for a low-temperature lithium battery according to any one of claims 1 to 7 above is applied.

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