NON-AQUEOUS ELECTROLYTE FOR LITHIUM-ION BATTERY AND LITHIUM-ION BATTERY

Abstract

A non-aqueous electrolyte for a lithium-ion battery and a lithium-ion battery. The non-aqueous electrolyte includes an unsaturated phosphate compound and a cyclic unsaturated carboxylic anhydride compound. The unsaturated phosphate compound has a structure represented by structural formula (4). R.sub.13, R.sub.11 and R.sub.12 are each independently selected from a hydrocarbon group having 1 to 5 carbon atoms, and at least one of R.sub.13, R.sub.11 and R.sub.12 is an unsaturated hydrocarbon group having a double bond or a triple bond. The unsaturated cyclic carboxylic anhydride compound having a structure represented by Structural Formula 5. R.sub.14 is selected from the group consisting of an alkenylene group having 2 to 4 carbon atoms or a fluorinated alkenylene group having 2 to 4 carbon atoms. By means of the synergistic effect of two compounds, the non-aqueous electrolyte has excellent high-temperature cycling performance and storage performance, and also has lower impedance and good low-temperature performance.

##STR00001##

Claims

1. A non-aqueous electrolyte for lithium-ion battery, comprising Component B, wherein Component B comprises an unsaturated phosphate compound and a cyclic unsaturated carboxylic anhydride compound, the unsaturated phosphate compound having a structure represented by Structural Formula 4, ##STR00013## wherein R.sub.13, R.sub.11 and R.sub.12 are each independently selected from a hydrocarbon group having 1 to 5 carbon atoms, and at least one of R.sub.13, R.sub.11 and R.sub.12 is an unsaturated hydrocarbon group having a double bond or a triple bond; and the unsaturated cyclic carboxylic anhydride compound having a structure represented by Structural Formula 5, ##STR00014## wherein R.sub.14 is selected from the group consisting of an alkenylene group having 2 to 4 carbon atoms or a fluorinated alkenylene group having 2 to 4 carbon atoms.

2. The non-aqueous electrolyte according to claim 1, wherein the hydrocarbon group having 1 to 5 carbon atoms of the unsaturated phosphate compound is selected from the group consisting of methyl, ethyl, propyl, vinyl, allyl, 3-butenyl, isobutenyl, ethynyl, propargyl, 3-butynyl, and 1-methyl-2-propynyl group.

3. The non-aqueous electrolyte according to claim 1, wherein the unsaturated phosphate compound is one or more selected from the group consisting of tripropargyl phosphate, dipropargyl methyl phosphate, dipropargylethyl phosphate, dipropargylpropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, and propargyl dipropyl phosphate.

4. The non-aqueous electrolyte according to claim 3, wherein the unsaturated phosphate compound is one or more selected from the group consisting of: ##STR00015##

5. The non-aqueous electrolyte according to claim 1, wherein the cyclic unsaturated carboxylic anhydride is one or more selected from the group consisting of maleic anhydride, 2-methylmaleic anhydride, succinic anhydride, and glutaric anhydride.

6. The non-aqueous electrolyte according to claim 5, wherein the cyclic unsaturated carboxylic anhydride is one or more selected from the group consisting of maleic anhydride and 2-methylmaleic anhydride.

7. The non-aqueous electrolyte according to claim 1, wherein in the non-aqueous electrolyte, the unsaturated phosphate compound accounts for 0.1% to 3% of the total weight of the non-aqueous electrolyte for lithium-ion battery, and the cyclic unsaturated carboxylic anhydride compound accounts for 0.1% to 3% of the total weight of the non-aqueous electrolyte for lithium-ion battery.

8. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte further comprises at least one selected from the group consisting of an unsaturated cyclic carbonate, a cyclic sultone, and a cyclic sulfate.

9. The non-aqueous electrolyte according to claim 8, the unsaturated cyclic carbonate compound accounts for 0.1% to 5% of the total weight of the non-aqueous electrolyte, or the cyclic sultone compound accounts for 0.1% to 5% of the total weight of the non-aqueous electrolyte, or the cyclic sulfate compound accounts for 0.1% to 5% of the total weight of the non-aqueous electrolyte.

10. The non-aqueous electrolyte according to claim 8, wherein the unsaturated cyclic carbonate is at least one selected from the group consisting of vinylene carbonate and vinylethylene carbonate.

11. The non-aqueous electrolyte according to claim 8, the cyclic sultone is at least one selected from the group consisting of 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone and methylene methanedisulfonate.

12. The non-aqueous electrolyte according to claim 8, the cyclic sulfate is one or both selected from the group consisting of vinyl sulfate and propylene sulfate.

13. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte comprises a non-aqueous organic solvent and a lithium salt.

14. The non-aqueous electrolyte according to claim 13, wherein the non-aqueous organic solvent being at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate.

15. The non-aqueous electrolyte according to claim 13, wherein and the lithium salt being at least one selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis (trifluoromethanesulfonyl)imide, and lithium difluorosulfonimide.

16. A lithium-ion battery, comprising a cathode, an anode, a separator interposed between the cathode and the anode, and an electrolyte, wherein the electrolyte is the non-aqueous electrolyte according to claim 1.

17. The lithium-ion battery according to claim 16, wherein the cathode comprises a cathode active material, the cathode active material being at least one selected from the group consisting of LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, LiCo.sub.1-yM.sub.yO.sub.2, LiNi.sub.1-yM.sub.yO.sub.2, LiMn.sub.2-yM.sub.yO.sub.4 and LiNi.sub.xCo.sub.yMn.sub.zM.sub.1-x-y-zO.sub.2; wherein M is at least one selected from the group consisting of Fe, Co, Ni, Mn, Mg, Cu, Zn, Al, Sn, B, Ga, Cr, Sr, V, and Ti, and 0≤y≤1, 0≤x≤1, 0≤z≤1, and x+y+z≤1.

18. The lithium-ion battery according to claim 16, wherein the cathode comprises a cathode active material, the cathode active material being at least one selected from the group consisting of LiNi.sub.xCo.sub.yMn.sub.zL.sub.(1-x-y-z)O.sub.2, LiCo.sub.x′L.sub.(1-x′)O.sub.2 and LiNi.sub.x″L′.sub.y′Mn.sub.(2-x″-y′)O.sub.4, wherein L is Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe, 0≤x≤1, 0≤y≤1, 0≤z≤1, 0<x+y+z≤1, 0<x′≤1, 0.3≤x″≤0.6, and 0.01≤y′≤0.2, and L′ is Co, Al, Sr, Mg, Ti, Ca, Zr, Zn, Si or Fe.

19. The lithium-ion battery according to claim 16, wherein the lithium-ion battery has a charge cut-off voltage of greater than or equal to 4.3V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0093] FIG. 1 is a capacity differential graph for the initial charging of a blank electrolyte, Example 6 and Comparative Example 1; and

[0094] FIG. 2 is an alternating current impedance graph of a blank electrolyte, Example 6 and Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0095] The present application will be further described in detail below by reference to particular examples and the accompanying drawings. The following examples are only intended to further illustrate the application and are not to be construed as limiting the invention.

Examples

Technical Solution I:

[0096] In this technical solution, electrolytes were prepared according to the components and ratios shown in Table 1. A plurality of non-aqueous electrolytes for lithium-ion battery according to the present application as well as a plurality of Comparative Examples were designed, as shown in Table 1 in detail.

[0097] In this technical solution, lithium hexafluorophosphate was used as the lithium salt. It is appreciated that the lithium salt used in this technical solution served only as a particular embodiment. Other lithium salts used in the art, such as LiBF.sub.4, LiBOB, LiDFOB, LiPO.sub.2F.sub.2, LiSbF.sub.6, LiAsF.sub.6, LiN(SO.sub.2CF.sub.3).sub.2, LiN(SO.sub.2C.sub.2F.sub.5).sub.2, LiC(SO.sub.2CF.sub.3).sub.3 and LiN(SO.sub.2F).sub.2, can also be used in this technical solution, and are not specifically limited herein.

[0098] The electrolytes in this technical solution were prepared by preparing a non-aqueous organic solvent according to a volume ratio of EC/EMC/DEC=1/1/1 (volume ratio), and then adding lithium hexafluorophosphate to the solvent to a final concentration of 1.0 mol/L, and then adding the additive according to Table 1. The percentage in Table 1 was percentage by weight, i.e., the percentage of the additive based on the total weight of the electrolyte.

TABLE-US-00001 TABLE 1 The components and their contents in the electrolytes Unsaturated cyclic Other carboxylic additive anhydride and Unsaturated phosphate and content content Example compound thereof thereof Example 1 Tripropargyl phosphate: 0.5% CA: 0.1% Example 2 Tripropargyl phosphate: 0.5% CA: 0.5% Example 3 Tripropargyl phosphate: 0.5% CA: 1% Example 4 Tripropargyl phosphate: 0.5% CA: 2% Example 5 Tripropargyl phosphate: 0.1% CA: 0.5% Example 6 Tripropargyl phosphate: 1% CA: 0.5% Example 7 Tripropargyl phosphate: 2% CA: 0.5% Example 8 Tripropargyl phosphate: 1% CA: 0.1% Example 9 Tripropargyl phosphate: 0.1% CA: 1% Example 10 Tripropargyl phosphate: 0.5% MA: 0.5% Example 11 Tripropargyl phosphate: 0.5% MA: 1% Example 12 Tripropargyl phosphate: 0.5% CA: 0.5% Vinylene carbonate: 1% Example 13 Tripropargyl phosphate: 0.5% CA: 0.5% 1,3-propane sultone: 1% Example 14 Tripropargyl phosphate: 0.5% CA: 0.5% Vinyl sulfate: 1% Example 15 Diallylethyl phosphate: 0.5% CA: 0.5% Example 16 Diallylethyl phosphate: 1% CA: 0.5% Example 17 Diallylethyl phosphate: 2% CA: 0.5% Example 18 Diallylethyl phosphate: 1% CA: 0.1% Comparative Tripropargyl phosphate: 1% Example 1 Comparative Diallylethyl phosphate: 1% Example 2

[0099] In the lithium-ion batteries in this technical solution, the cathode active material used was LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, the anode used was artificial graphite, and the separator used was a three-layer separator of polypropylene, polyethylene and polypropylene. Specifically, lithium-ion batteries were made as follows.

[0100] Preparation of the cathode: Cathode active material LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, conductive carbon black and binder polyvinylidene fluoride were mixed in a mass ratio of 96.8:2.0:1.2. The mixture was dispersed in N-methyl-2-pyrrolidone to obtain a cathode slurry. The cathode slurry was uniformly coated onto both sides of an aluminum foil, which was then subjected to oven drying, calandering and vacuum drying, followed by welding of aluminum lead wires by an ultrasonic welder to obtain the cathode plate, the thickness of the plate being in the range of 120-150 μm.

[0101] Preparation of the anode: Graphite, conductive carbon black and binders styrene-butadiene rubber and carboxymethyl cellulose were mixed in a mass ratio of 96:1:1.2:1.8. The mixture was dispersed in deionized water to obtain an anode slurry. The anode slurry was coated onto both sides of a copper foil, which was then subjected to oven drying, calandering and oven drying, followed by welding of nickel lead wires by an ultrasonic welder to obtain the anode plate, the thickness of the plate being in the range of 120-150 μm.

[0102] Preparation of the separator: a three-layer separator of polypropylene, polyethylene and polypropylene was used, the thickness being 20 μm.

[0103] Battery assembling: the three-layer separator having a thickness of 20 μm was placed between the cathode plate and the anode plate, and the resulting sandwich structure composed of the cathode plate, the anode plate and the separator was wound. The wound structure was flattened and placed into an aluminum foil packing bag, and baked at 75° C. for 48 hours to obtain a battery core, which was to be injected with electrolyte. Then, the battery core was injected with an electrolyte prepared as above, and was vacuum-packed and allowed to stand for 24 hours.

[0104] Battery formation: 0.05 C constant current charging for 180 min, 0.1 C constant current charging to 3.95V, vacuum packing again and standing at 45° C. for 48h, then further, 0.2 C constant current charging to 4.4V, and 0.2 C constant current discharging to 3.0V.

[0105] The batteries having the respective electrolytes in this technical solution were subjected to capacity retention rate test of 1 C cycling for 300 cycles at 45° C. and 1 C cycling for 500 cycles at normal temperature; capacity retention rate, capacity recovery rate, and thickness expansion rate tests after storage at 60° C. for 30 days; 1 C discharging efficiency test at −20° C.; and normal and low temperature direct current impedance test. The specific test methods are as follows:

[0106] (1) The capacity retention rate test of 1 C cycling for 300 cycles at 45° C. was in fact to measure the high-temperature cycling performance of the battery. The specific test method comprised: subjecting, at 45° C., the formed battery to 1 C constant current and constant voltage charging to 4.35V, with the cut-off current being 0.01 C, followed by 1 C constant current discharging to 3.0V. After 300 cycles of charging/discharging, the capacity retention rate at the 300.sup.th cycle was calculated to evaluate the high-temperature cycling performance. The formula for calculating the capacity retention rate after 1 C cycling for 300 cycles at 45° C. is as follows:

Capacity retention rate at the 300.sup.th cycle (%)=(discharge capacity at the 300.sup.th cycle/discharge capacity at the 1.sup.st cycle)×100%.

[0107] (2) Normal temperature cycling performance test: At 25° C., the formed battery was subjected to 1 C constant current constant voltage charging to 4.35 V, then 1 C constant current discharging to 3.0 V. The capacity retention rate at the 500.sup.th cycle after 500 cycles of charging/discharging was calculated to evaluate the normal temperature cycling performance. The formula for calculation is as follows:

Capacity retention rate at the 500.sup.th cycle (%)=(discharge capacity at the 500.sup.th cycle/discharge capacity at the 1.sup.st cycle)×100%.

[0108] (3) The test method of capacity retention rate, capacity recovery rate and thickness expansion rate after storage at 60° C. for 30 days comprised: subjecting, at a normal temperature, the formed battery to 1 C constant current constant voltage charging to 4.35 V, with the cut-off current being 0.01 C; followed by 1 C constant current discharging to 3.0 V, at which time the initial discharge capacity of the battery was measured, followed by 1 C constant current constant voltage charging to 4.35V, with the cut-off current being 0.01 C, at which time the initial thickness of the battery was measured; followed by storage of the battery at 60° C. for 30 days, at which time the thickness of the battery was measured; followed by 1 C constant current discharging to 3.0V, at which time the retention capacity of the battery was measured; followed by 1 C constant current constant voltage charging to 4.35V, with the cut-off current being 0.01 C; and followed by 1 C constant current discharging to 3.0 V, at which time the recovery capacity was measured.

[0109] The formulas for calculating the capacity retention rate, capacity recovery rate, and thickness expansion rate are as follows:

Battery capacity retention rate (%)=retention capacity/initial capacity×100%

Battery capacity recovery rate (%)=recovery capacity/initial capacity×100%

Battery thickness expansion rate (%)=(thickness after 30 days−initial thickness)/initial thickness×100%.

[0110] (4) Low-temperature discharge performance test: At 25° C., the formed battery was subjected to 1 C constant current constant voltage charging to 4.35 V, then 1 C constant current discharging to 3.0 V, at which time the discharge capacity was recorded. Then, the battery was subjected to 1 C constant current constant voltage charging to full capacity, allowed to stand in an environment of −20° C. for 12 hours, then subjected to 1 C constant current discharging to 3.0 V, at which time the discharge capacity was recorded.

Low-temperature discharge efficiency value at −20° C.=1 C discharge capacity (−20° C.)/1 C discharge capacity (25° C.).

[0111] (5) Normal temperature direct current impedance (DCIR) performance test: Subjecting, at 25° C., the formed battery to 1 C charging to SOC=50%, followed by respectively subjecting the battery to 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C charging and discharging for 10 seconds and respectively recording the charge and discharge cut-off voltage. Then, a linear relationship plot (unit: mV) was prepared by plotting the charge and discharge currents at different rates on the abscissa (unit: A) and plotting the cut-off voltages corresponding to the charge and discharge currents on the ordinate.

[0112] Discharge DCIR value=slope of the linear plot of different discharge currents vs corresponding cut-off voltages.

[0113] The test results are shown in Table 2.

TABLE-US-00002 TABLE 2 Test results Cycling capacity retention rate/% High-temperature storage 1 C 45° C. Normal performance (60° C., 30 days) discharge (1 C/1 C, temperature Capacity Capacity Thickness efficiency 25° C. 300 (1 C/1 C, retention recovery expansion at discharge Test item cycles) 500 cycles) rate/% rate/% rate/% −20° C. DCIR/mΩ Comparative 72.1 82.8 73.5 78.9 13.5 40.3 145.6 Example 1 Comparative 70.2 80.1 70.3 74.6 15.5 39.3 140.2 Example 2 Example 1 75.3 83.8 77.6 80.1 10.3 42.7 129.2 Example 2 82.2 85.6 81.3 85.2 5.8 47.6 126.7 Example 3 81.9 84.6 80.1 83.4 6.8 43.5 128.9 Example 4 81.1 83.4 80.2 83.9 7.5 42.3 131.2 Example 5 73.5 83.4 76.8 79.2 11.3 53.2 118.7 Example 6 84.3 83.6 85.3 88.3 3.8 45.3 132.7 Example 7 85.5 83.8 85.9 88.6 5.3 43.2 135.3 Example 8 78.5 81.2 79.5 83.2 8.5 42.3 137.2 Example 9 72.5 73.5 73.5 79.2 14.8 48.7 125.3 Example 10 80.2 81.3 78.9 84.2 8.8 45.5 125.8 Example 11 78.9 82.3 78.2 82.3 9.7 46.3 126.8 Example 12 83.3 87.5 83.2 84.5 7.8 45.6 127.8 Example 13 80.2 82.3 80.3 83.5 5.2 47.2 125.3 Example 14 83.9 86.6 84.3 87.2 4.2 48.5 123.5 Example 15 80.1 84.6 78.3 81.3 6.8 45.5 127.2 Example 16 83.3 82.4 82.3 85.3 4.2 44.2 129.6 Example 17 85.9 84.2 85.1 87.6 5.2 42.3 137.6 Example 18 76.5 81.9 76.5 79.2 10.5 40.3 136.2

[0114] Through the tests, an initial charging capacity differential plot (as shown in FIG. 1) and an AC impedance plot (as shown in FIG. 2) were obtained for the blank electrolyte, Example 6 and Comparative Example 1.

[0115] It can be seen from FIG. 1 and FIG. 2 that the unsaturated phosphate (Compound 1) began to form a film on the anode at about 2.7 V during the initial charging process, and the film formation on the anode at this time caused a significant increase in the impedance of the anode. When the cyclic unsaturated carboxylic anhydride compound (CA) was added on the basis of the unsaturated phosphate (Compound 1), the cyclic unsaturated carboxylic anhydride compound (CA) preferentially formed a film on the surface of the anode at about 1.5 V and 2 V, and the film preferentially formed by the cyclic unsaturated carboxylic anhydride compound (CA) inhibited the film formation by the unsaturated phosphate (Compound 1) at the subsequent 2.7 V, thereby further lowering the impedance of the anode.

[0116] By comparing the test results of Comparative Examples 1-2, it can be found that when the unsaturated phosphate compound was used alone, the cycling performance and the high-temperature storage were good, but the impedance was high and the low-temperature performance was poor. When the unsaturated cyclic carboxylic anhydride compound was used alone, the impedance was low and the low-temperature performance was good, but the cycling performance and the high-temperature storage were poor.

[0117] Among the test results of Examples 1-18 of the present application, by comparing Comparative Example 1 with Examples 2, 6, and 8, it can be found that addition of the unsaturated cyclic carboxylic anhydride compound on the basis of the unsaturated phosphate compound not only significantly improved the cycling performance and the high-temperature performance, but also significantly improved the low-temperature performance, and significantly lowered the impedance.

[0118] Also, among the test results of Examples 1 to 18 of the present application, it can be found that relative to Comparative Example 1, all the Examples containing both the unsaturated phosphate compound and the unsaturated cyclic carboxylic anhydride compound had improved high-temperature performance and low-temperature performance. Comparing Examples 2, 5, 6 and 7, it can be found that as the content of the unsaturated phosphate compound increased, the high-temperature performance improved, but the low-temperature performance was relatively degraded, and especially, as the content increased, the impedance increased accordingly. In particular, when the content of the unsaturated phosphate compound was very high and the content of the unsaturated cyclic carboxylic anhydride compound was very low, the impedance was high and the low-temperature performance was obviously insufficient.

[0119] In summary of the above, the present application used the unsaturated phosphate compound and the unsaturated cyclic carboxylic anhydride compound in combination, which, in suitable ratios, allowed the battery to have excellent high-temperature performance and cycling performance as well as good low-temperature performance.

Technical Solution II:

[0120] Electrolytes were prepared according to the components and ratios shown in Table 3. A plurality of non-aqueous electrolytes for lithium-ion battery according to the present application as well as a plurality of Comparative Examples were designed, as shown in Table 3 in detail.

[0121] The electrolytes in this technical solution were prepared by preparing a non-aqueous organic solvent according to the volume ratio shown in Table 3, and then adding lithium hexafluorophosphate to the solvent to a final concentration of 1.0 mol/L, and then adding the additive according to Table 3. The percentage in Table 3 was percentage by weight, i.e., the percentage of the additive based on the total weight of the electrolyte. The lithium salt content of the electrolyte was 12.5%, and others were solvent grade additives.

TABLE-US-00003 TABLE 3 The components and their contents in the electrolytes Total content, composition and weight ratio of the solvent Additive and its content Example 19 Total content: 87% Tripropargyl phosphate: 0.5% FEC/PC/EC = 2/1/1 Example 20 Total content: 86.5% Tripropargyl phosphate: 1% FEC/PC/DEC = 2/1/1 Example 21 Total content: 86.5% Hexafluoroisopropyldipropargyl FEC/PC/EMC = 2/1/1 phosphate: 1% Example 22 Total content: 86.5% Dipropargyl methyl phosphate: FEC/PC/DEC/EC = 1% 2/1/1/1 Example 23 Total content: 86.5% MA: 1% FEC/DFEA = 2/1 Example 24 Total content: 86.5% CA: 1% FEC/DFEA/EC = 2/1/1 Example 25 Total content: 86.5% Tripropargyl phosphate: 0.5%, FEC/DFEA/DEC = 2/1/1 CA: 0.5% Example 26 Total content: 86.5% Tripropargyl phosphate: 0.5% FEC/PC/DEC = 2/1/1 VC: 0.5% Example 27 Total content: 86.5% Tripropargyl phosphate: 0.5% FEC/PC/DEC = 2/1/1 PS: 0.5% Example 28 Total content: 85.5% Tripropargyl phosphate: 1% FEC/PC/DEC = 2/1/1 DTD: 1% Example 29 Total content: 84.5% Tripropargyl phosphate: 1% FEC/PC/DEC = 2/1/1 DTD: 2% Example 30 Total content: 85.5% CA: 1% FEC/PC/DEC = 2/1/1 DTD: 1% Example 31 Total content: 85% Tripropargyl phosphate: 1% FEC/PC/DEC = 2/1/1 CA: 0.5 DTD: 1% Example 32 Total content: 86.5% Tripropargyl phosphate: 0.5% FEC/DFEA = 2/1 CA: 0.5% Example 33 Total content: 85.5% Tripropargyl phosphate: 1% FEC/DFEA = 2/1 DTD: 1% Example 34 Total content: 85.5% CA: 1% FEC/DFEA = 2/1 DTD: 1% Example 35 Total content: 85% Tripropargyl phosphate: 1% FEC/DFEA = 2/1 CA: 0.5 DTD: 1% Example 36 Total content: 86.5% Tripropargyl phosphate: 0.5% FEC/PC/DFEA = 2/1/1 CA: 0.5% Example 37 Total content: 84.5% Tripropargyl phosphate: 1% FEC/PC/DFEA = 2/1/1 DTD: 2% Example 38 Total content: 85.5% CA: 1% FEC/PC/DFEA = 2/1/1 DTD: 1% Comparative Total content: 87.5% Example 3 FEC/PC/DEC = 2/1/1 Comparative Total content: 87.5% Example 4 FEC/DFEA = 2/1 Comparative Total content: 87.5% Example 5 FEC/PC/DFEA = 2/1/1 Comparative Total content: 87.5% Example 6 EC/DEC = 2/1 Comparative Total content: 86.5% Tripropargyl phosphate: 1% Example 7 EC/DEC = 2/1 Comparative Total content: 86.5% CA: 1% Example 8 EC/DEC = 2/1

[0122] In the lithium-ion batteries in this technical solution, the cathode active material used was LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, the anode used was artificial graphite and conductive carbon black, and the separator used was a three-layer separator of polypropylene, polyethylene and polypropylene. Specifically, lithium-ion batteries were made as follows.

[0123] Preparation of the cathode: Cathode active material LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, conductive carbon black and binder polyvinylidene fluoride were mixed in a mass ratio of 96.8:2.0:1.2. The mixture was dispersed in N-methyl-2-pyrrolidone to obtain a cathode slurry. The cathode slurry was uniformly coated onto both sides of an aluminum foil, which was then subjected to oven drying, calandering and vacuum drying, followed by welding of aluminum lead wires by an ultrasonic welder to obtain the cathode plate, the thickness of the plate being in the range of 120-150 μm.

[0124] Preparation of the anode: Graphite, conductive carbon black and binders styrene-butadiene rubber and carboxymethyl cellulose were mixed in a mass ratio of 96:1:1.2:1.8. The mixture was dispersed in deionized water to obtain an anode slurry. The anode slurry was coated onto both sides of a copper foil, which was then subjected to oven drying, calandering and oven drying, followed by welding of nickel lead wires by an ultrasonic welder to obtain the anode plate, the thickness of the plate being in the range of 120-150 μm.

[0125] Preparation of the separator: a three-layer separator of polypropylene, polyethylene and polypropylene was used, the thickness being 20 μm.

[0126] Battery assembling: the three-layer separator having a thickness of 20 μm was placed between the cathode plate and the anode plate, and the resulting sandwich structure composed of the cathode plate, the anode plate and the separator was wound. The wound structure was flattened and placed into an aluminum foil packing bag, and baked at 85° C. for 24 hours to obtain a battery core, which was to be injected with electrolyte. Then, the battery core was injected with an electrolyte prepared as above, and was vacuum-packed and allowed to stand for 24 hours.

[0127] Battery formation: 0.05 C constant current charging for 180 min, 0.1 C constant current charging to 3.95V, vacuum packing again and standing at 45° C. for 48h, then further, 0.2 C constant current charging to 4.4V, and 0.2 C constant current discharging to 3.0V.

[0128] The batteries having the respective electrolytes in this technical solution were subjected to capacity retention rate test of 1 C cycling for 400 cycles at 45° C., and capacity retention rate, capacity recovery rate, and thickness expansion rate tests after storage at 60° C. for 30 days. By “after storage at 60° C. for 30 days” was meant for the electrolytes of the Comparative Examples, the lithium-ion batteries were tested after storage at 60° C. for 30 days, and the Test Examples were tested after storage at 60° C. for 30 days. The specific test methods are as follows:

[0129] (1) The capacity retention rate test of 1 C cycling for 400 cycles at 45° C. was in fact to measure the high-temperature cycling performance of the battery. The specific test method comprised: subjecting, at 45° C., the formed battery to 1 C constant current and constant voltage charging to 4.35V, with the cut-off current being 0.01 C, followed by 1 C constant current discharging to 3.0V. This was conducted for 400 cycles. The formula for calculating the capacity retention rate is as follows:

Capacity retention rate (%)=(discharge capacity at the 400.sup.th cycle/discharge capacity at the 1.sup.st cycle)×100%.

[0130] (2) The test method of capacity retention rate, capacity recovery rate and thickness expansion rate after storage at 60° C. for 30 days comprised: subjecting, at a normal temperature, the formed battery to 1 C constant current constant voltage charging to 4.4 V, with the cut-off current being 0.01 C; followed by 1 C constant current discharging to 3.0 V, at which time the initial discharge capacity of the battery was measured, followed by 1 C constant current constant voltage charging to 4.4V, with the cut-off current being 0.01 C, at which time the initial thickness of the battery was measured; followed by storage of the battery at 60° C. for 30 days, at which time the thickness of the battery was measured; followed by 1 C constant current discharging to 3.0V, at which time the retention capacity of the battery was measured; followed by 1 C constant current constant voltage charging, with the cut-off current being 0.01 C; and followed by 1 C constant current discharging to 3.0 V, at which time the recovery capacity was measured. The formulas for calculations are as follows:

Battery capacity retention rate (%)=retention capacity/initial capacity×100%

Battery capacity recovery rate (%)=recovery capacity/initial capacity×100%

Battery thickness expansion rate (%)=(thickness after 30 days−initial thickness)/initial thickness×100%.

[0131] (3) Low-temperature discharge performance test:

[0132] At 25° C., the formed battery was subjected to 1 C constant current constant voltage charging to 4.4 V, followed by constant voltage charging until the current decreased to 0.01 C, followed by 1 C constant current discharging to 3.0 V, at which time the normal-temperature discharge capacity was recorded. Then, the battery was subjected to 1 C constant current charging to 4.4 V, followed by constant voltage charging until the current decreased to 0.01 C, followed by allowing the battery to stand in an environment of −20° C. for 12 hours, and followed by 0.2 C constant current discharging to 3.0 V, at which time the discharge capacity at −20° C. was recorded.

Low-temperature discharge efficiency at −20° C.=0.2 C discharge capacity (−20° C.)/1 C discharge capacity (25° C.)×100%

[0133] The test results are shown in Table 4.

TABLE-US-00004 TABLE 4 Test results Storage at 60° C. for 30 days 0.2 C Capacity Capacity Thickness discharge 400 cycles retention recovery expansion efficiency at at 45° C. rate rate rate −20° C. Example 19 80.1% 83.4% 88.5% 17.8% 72.6% Example 20 84.1% 85.6% 90.1% 15.5% 65.5% Example 21 80.5% 84.6% 89.1% 16.5% 76.5% Example 22 79.5% 81.4% 86.7% 19.1% 76.8% Example 23 76.6% 78.6% 84.2% 20.2% 70.1% Example 24 78.6% 80.2% 86.1% 18.2% 69.4% Example 25 82.2% 84.6% 88.1% 16.1% 70.3% Example 26 81.4% 82.4% 87.3% 21.8% 70.4% Example 27 80.5% 84.6% 89.3% 16.2% 71.2% Example 28 85.5% 85.2% 91.1% 14.4% 74.1% Example 28 85.7% 88.6% 94.8% 13.2% 74.9% Example 30 82.1% 83.4% 88.7% 17.4% 76.2% Example 31 87.6% 88.7% 94.3% 10.4% 73.6% Example 32 83.4% 84.6% 89.5% 18.2% 70.1% Example 33 86.4% 86.5% 91.8% 15.6% 75.1% Example 34 83.5% 84.6% 89.4% 18.5% 75.3% Example 35 88.4% 87.8% 94.1% 12.5% 74.1% Example 36 86.5% 90.7% 95.4% 13.2% 70.3% Example 37 88.7% 89.2% 94.6% 13.1% 75.2% Example 38 83.4% 84.2% 89.6% 13.6% 76.3% Comparative   40% 44.5% 50.1% 53.4%   76% Example 3 Comparative 34.3% 35.2% 40.5% 70.4%   78% Example 4 Comparative 50.5% 53.5% 60.3% 63.4%   75% Example 5 Comparative 30.1% 60.1% 68.4% 40.1%   70% Example 6 Comparative 62.1% 73.4% 78.6% 25.3%   30% Example 7 Comparative 52.6% 65.8% 71.5% 32.4%   35% Example 8

[0134] It can be seen from the test results in Table 4 that compared with the carbonate solvent, although the fluorinated solvent could improve the high-temperature cycling performance and the low-temperature discharge performance of the battery, the gas production during high-temperature storage was high, which was a safety hazard. Although the unsaturated phosphate and/or the cyclic carboxylic anhydride additive could simultaneously improve the high-temperature cycling performance and the high-temperature storage performance, the extent of improvement was limited and needed to be further increased, and moreover the low-temperature discharge performance was poor. The combination of the fluorinated solvent with the unsaturated phosphate and/or the cyclic carboxylic anhydride could significantly improved the high-temperature storage performance and the high-temperature cycling performance of the battery, without compromising the low-temperature discharge performance. Since there is a certain synergistic effect between the fluorinated solvent and the unsaturated phosphate and/or cyclic carboxylic anhydride, an effect not achievable with the respective single component can be obtained. Further addition of the unsaturated cyclic carbonate or the cyclic sultone or the cyclic sulfate could further improve the high-temperature storage performance and the high-temperature cycling performance of the battery.

Technical Solution III:

[0135] In a series of studies on the electrolyte, it was found that the first compound, when used as a non-aqueous organic solvent, decomposes and produces gas at the anode, which poses a safety hazard; and although the second compound can improve the high-temperature performance, it undergoes polymerization reaction on the surface of the cathode and the anode to form a passivation film, which has a high impedance, resulting in reduction in the low-temperature discharge performance and the rate performance of the battery. After extensive research and experimentation, the present applicant proposed that the first compound and the second compound are used in combination to act in synergy, such that the respective advantages and functions of the first compound and the second compound are maintained, and at the same time, the safety hazard of the first compound decomposing at the anode to produce gas is overcome and the influence of the second compound on the low-temperature discharge performance and the rate performance of the battery is alleviated, which greatly improves the performances of the battery.

[0136] Electrolytes were prepared according to the components and ratios shown in Table 5, in which a plurality of non-aqueous electrolytes for lithium-ion battery according to the present application and a plurality of comparative examples were designed, as shown in detailed in Table 5.

[0137] The respective electrolyte in this technical solution was prepared by preparing a non-aqueous organic solvent in the proportion shown in Table 5, then adding lithium hexafluorophosphate to the solvent to a final concentration of 1.0 mol/L, and then adding the additive according to Table 5. The percentage in Table 5 was percentage by weight, i.e., the percentage of the additive based on the total weight of the electrolyte. The lithium salt content in the electrolyte was 12.5%, the other being solvent-grade additive.

TABLE-US-00005 TABLE 5 The components and amounts thereof in the electrolyte Total content, composition Compound of structure and weight ratio of the formula 4 and amounts Other additive and solvent thereof amounts thereof Comparative Content: 86.5% Tripropargyl — Example 9 EC/DEC = 1/2 phosphate: 1% Comparative Content: 87.5% — — Example 10 EC/DFEA = 1/2 Comparative Content: 86.5% — Tris(hexafluoroisopropyl) Example 11 EC/DFEA = 1/2 phosphate: 1% Comparative Content: 86.5% — Tris(trifluoroethyl) Example 12 EC/DFEA = 1/2 phosphate: 1% Comparative Content: 86.5% — Tris(isopropyl)phosphate: Example 13 EC/DFEA = 1/2 1% Comparative Content: 86.5% — Tris(hexafluoroisopropyl) Example 14 FEC/DFEA = 1/2 phosphate: 1% Comparative Content: 86.5% — Tris(hexafluoroisopropyl) Example 15 FEC/DFEA = 1/3 phosphate: 1% Comparative Content: 86.5% — Tris(hexafluoroisopropyl) Example 16 EC/FEC/DFEA = 1/1/2 phosphate: 1% Comparative Content: 86.5% — Tris(hexafluoroisopropyl) Example 17 PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 39 Content: 86.5% Tripropargyl — EC/DFEA = 1/2 phosphate: 1% Example 40 Content: 86.5% Tripropargyl — EC/DFEP = 1/2 phosphate: 1% Example 41 Content: 86.5% Di(propargyl)ethyl — EC/DFEA = 1/2 phosphate: 1% Example 42 Content: 86.5% Di(propargyl) — EC/DFEA = 1/2 hexafluoroisopropyl phosphate: 1% Example 43 Content: 86.5% Tripropargyl — FEC/DFEA = 1/2 phosphate: 1% Example 44 Content: 86.5% Tripropargyl — FEC/DFEA = 1/3 phosphate: 1% Example 45 Content: 86.5% Tripropargyl — EC/FEC/DFEA = 1/1/2 phosphate: 1% Example 46 Content: 86.5% Tripropargyl — PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 47 Content: 86.5% Di(propargyl)ethyl PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 48 Content: 86.5% Di(propargyl) PC/FEC/DFEA = 1/1/2 hexafluoroisopropyl phosphate: 1% Example 49 Content: 86.5% Tripropargyl PC/FEC/DFPA = 1/1/2 phosphate: 1% Example 50 Content: 86.5% Tripropargyl PC/FEC/DFEP = 1/1/2 phosphate: 1% Example 51 Content: 86.5% Tripropargyl PC/FEC/DFPP = 1/1/2 phosphate: 1% Example 52 Content: 86.5% Tripropargyl — SL/FEC/DFEA = 1/1/2 phosphate: 1% Example 53 Content: 86.5% Tripropargyl — GBL/FEC/DFEA = 1/1/2 phosphate: 1% Example 54 Content: 85.5% Tripropargyl PS: 1% PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 55 Content: 85.5% Tripropargyl BS: 1% PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 56 Content: 85.5% Tripropargyl PST: 1% PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 57 Content: 85.5% Tripropargyl MMDS: 1% PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 58 Content: 85.5% Tripropargyl DTD: 1% PC/FEC/DFEA = 1/1/2 phosphate: 1% Example 59 Content: 85.5% Tripropargyl CA: 1% PC/FEC/DFEA = 1/1/2 phosphate: 1%

[0138] In the lithium-ion batteries in this technical solution, the cathode active material used was LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, the anode used was graphite and conductive carbon black, and the separator used was a three-layer separator of polypropylene, polyethylene and polypropylene. Specifically, lithium-ion batteries were made as follows.

[0139] Preparation of the cathode: Cathode active material LiNi.sub.0.5Co.sub.0.2Mn.sub.0.3O.sub.2, conductive carbon black and binder polyvinylidene fluoride were mixed in a mass ratio of 96.8:2.0:1.2. The mixture was dispersed in N-methyl-2-pyrrolidone to obtain a cathode slurry. The cathode slurry was uniformly coated onto both sides of an aluminum foil, which was then subjected to oven drying, calandering and vacuum drying, followed by welding of aluminum lead wires by an ultrasonic welder to obtain the cathode plate, the thickness of the plate being in the range of 120-150 μm.

[0140] Preparation of the anode: Graphite, conductive carbon black and binders styrene-butadiene rubber and carboxymethyl cellulose were mixed in a mass ratio of 96:1:1.2:1.8. The mixture was dispersed in deionized water to obtain an anode slurry. The anode slurry was coated onto both sides of a copper foil, which was then subjected to oven drying, calandering and oven drying, followed by welding of nickel lead wires by an ultrasonic welder to obtain the anode plate, the thickness of the plate being in the range of 120-150 μm.

[0141] Preparation of the separator: a three-layer separator of polypropylene, polyethylene and polypropylene was used, the thickness being 20 μm.

[0142] Battery assembling: the three-layer separator having a thickness of 20 μm was placed between the cathode plate and the anode plate, and the resulting sandwich structure composed of the cathode plate, the anode plate and the separator was wound. The wound structure was flattened and placed into an aluminum foil packing bag, and baked at 75° C. for 48 hours to obtain a battery core, which was to be injected with electrolyte. Then, the battery core was injected with an electrolyte prepared as above, and was vacuum-packed and allowed to stand for 24 hours.

[0143] Battery formation: 0.05 C constant current charging for 180 min, 0.1 C constant current charging to 3.95V, vacuum packing again and standing at 45° C. for 48h, then further, 0.2 C constant current charging to 4.4V, and 0.2 C constant current discharging to 3.0V.

[0144] The lithium-ion batteries having the respective electrolytes in this technical solution were subjected to the test for the number of cycles when the capacity retention rate decreased to 80% during 1 C cycling at 45° C., and the tests for the capacity retention rate, capacity recovery rate, and thickness expansion rate after storage at 60° C. for 14 days, wherein storage at 60° C. for a number of days means that the lithium-ion batteries comprising the respective electrolytes in the Comparative Examples were tested after storage at 60° C. for 7 days, and the lithium-ion batteries comprising the respective electrolytes in the Examples were tested after storage at 60° C. for 14 days. The specific test methods are as follows:

[0145] The number of cycles when the capacity retention rate decreased to 80% during 1 C cycling at 45° C. in fact represented the high-temperature cycling performance of the battery. The specific test method comprised: subjecting, at 45° C., the formed battery to 1 C constant current and constant voltage charging to 4.4V, with the cut-off current being 0.01 C, followed by 1 C constant current discharging to 3.0V. This cycling was conducted until the capacity retention rate decreased to 80%, at which time the number of cycles was counted. The formula for calculating the capacity retention rate is as follows:

Capacity retention rate (%)=(discharge capacity at the N.sup.th cycle/discharge capacity at the 1.sup.st cycle)×100%.

[0146] (2) The test method of capacity retention rate, capacity recovery rate and thickness expansion rate after storage at 60° C. for 14 days comprised: subjecting, at a normal temperature, the formed battery to 1 C constant current constant voltage charging to 4.4 V, with the cut-off current being 0.01 C; followed by 1 C constant current discharging to 3.0 V, at which time the initial discharge capacity of the battery was measured, followed by 1 C constant current constant voltage charging to 4.4V, with the cut-off current being 0.01 C, at which time the initial thickness of the battery was measured; followed by storage of the battery at 60° C. for 14 days, at which time the thickness of the battery was measured; followed by 1 C constant current discharging to 3.0V, at which time the retention capacity of the battery was measured; followed by 1 C constant current constant voltage charging, with the cut-off current being 0.01 C, and followed by 1 C constant current discharging to 3.0V, at which time the recovery capacity was measured. The formulas for calculation are as follows:

Battery capacity retention rate (%)=retention capacity/initial capacity×100%

Battery capacity recovery rate (%)=recovery capacity/initial capacity×100%

Battery thickness expansion rate (%)=(thickness after 14 days−initial thickness)/initial thickness×100%.

[0147] (3) Low-temperature discharge performance test

[0148] At 25° C., the formed battery was subjected to 1 C constant current constant voltage charging to 4.4 V, followed by constant voltage charging until the current dropped to 0.01 C, followed by 1 C constant current discharging to 3.0 V, at which time the discharge capacity at normal temperature was recorded. Then, the battery was subjected to 1 C constant current charging to 4.4V, followed by constant voltage charging until the current dropped to 0.01 C, followed by allowing the battery to stand in an environment of −20° C. for 12 hours, followed by 0.2 C constant current discharging to 3.0 V, at which time the discharge capacity at −20° C. was recorded.

Low-temperature discharge efficiency at −20° C.=0.2 C discharge capacity (−20° C.)/1 C discharge capacity (25° C.)×100%.

[0149] The test results are shown in Table 6.

TABLE-US-00006 TABLE 6 Test results Number of cycles when the capacity retention rate decreased to Storage at 60° C. for 14 days 0.2 C 80% during Capacity Capacity Thickness discharge 1 C cycling retention recovery expansion efficiency at 45° C. rate rate rate at −20° C. Comparative 290   47%   51%   15%   40% Example 9 Comparative 250 20.1% 25.3%   42% 77.5% Example 10 Comparative 365 78.2% 82.3% 20.6% 73.4% Example 11 Comparative 354 76.4% 80.7% 24.5% 72.5% Example 12 Comparative 330 74.4% 78.6% 28.4% 73.2% Example 13 Comparative 400 80.5% 84.5% 32.4% 74.1% Example 14 Comparative 387 79.5% 84.1% 27% 73.2% Example 15 Comparative 390 80.1% 84.7% 26.4% 71.3% Example 16 Comparative 420 82.1% 87.2% 26.5% 74.5% Example 17 Example 39 430 83.5% 87.5% 12.4% 68.1% Example 40 445 84.5% 88.4% 11.8% 69.4% Example 41 425 81.2% 85.1% 14.4% 72.5% Example 42 420 79.1% 84.6% 16.5% 73.9% Example 43 510 85.2% 89.3% 18.2% 70.5% Example 44 505 84.6% 89.1% 15.7% 72.1% Example 45 526 86.7% 90.5% 15.5% 71.6% Example 46 574 88.1% 92.2% 13.5% 73.4% Example 47 554 86.3% 90.7% 14.8% 74.5% Example 48 535 84.4% 88.4% 16.6% 75.1% Example 49 589 88.9% 92.6% 12.5% 73.6% Example 50 612 90.1% 94.2% 12.1% 74.6% Example 51 631 91.1% 95.2% 11.5% 75.1% Example 52 512 83.1% 87.5% 20.5% 65.3% Example 53 522 84.4% 88.5% 17.5% 64.2% Example 54 590 89.5% 93.5% 12.1% 70.4% Example 55 595 89.2% 93.7% 13.3% 70.8% Example 56 620 90.5% 94.9% 12.4% 68.5% Example 57 650 90.9% 95.2% 12.3% 74.1% Example 58 655 90.5% 94.5% 13.6% 76.2% Example 59 662 89.4% 92.1% 11.1% 69.6%

[0150] According to the results in Table 6, it can be seen that in Comparative Example 9 that only used the second compound as an additive and did not use the first compound as a solvent, the high-temperature cycling performance was weak, the capacity retention rate left was 80% after 290 cycles, and the storage capacity and recovery capacity after storage at 60° C. for 14 days were not satisfactory either, especially the low-temperature discharge performance was relatively poor. In Comparative Example 10 that used the first compound as a solvent and did not use the second compound as an additive, the high-temperature storage performance and the high-temperature storage performance were both very poor. In Comparative Example 11-17 that used the first compound as a solvent, used saturated phosphate as an additive, and also optimized the solvent combination, although the high-temperature cycling performance and the high-temperature storage performance of the batteries were greatly improved, the batteries still failed to meet the requirements, needed to be further improved. In Examples 39-59 that used the first compound as a solvent and the second compound as an additive and also optimized the solvent combination and the additive combination, the high-temperature cycling performance and the high-temperature storage performance were both markedly improved, without compromising the low-temperature discharge performance. In Example 59, the high-temperature cycling performance was the best, reduction of the capacity retention rate to 80% only resulted after 662 cycles, and the high-temperature storage performance was also excellent.

[0151] The above is a further detailed description of the present application in conjunction with particular embodiments, and the specific implementation of the present application is not to be construed as limiting to such description. It will be apparent to those skilled in the art that several simple derivations and substitutions can be made without departing from the concept of the present application and such derivations and substitutions shall be deemed to fall within the scope of protection of the present application.