SECONDARY BATTERY

Abstract

Disclosed is a secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte comprises a lithium salt, an organic solvent, and an additive, wherein the additive comprises a fluorinated compound that accounts for 5-20% of the total mass of the electrolyte, and a polynitrile compound that accounts for 0.2-4% of the total mass of the electrolyte and a sulfate compound that accounts for 0.3-5% of the total mass of the electrolyte; and the negative electrode has a compacted density of not less than 1.55 g/cm3. When the secondary battery is used, the battery can still have both a high temperature characteristic and a low temperature charging ability under the conditions of a higher negative electrode density, thereby having market application prospects, and overcoming the problem of the performance of existing secondary batteries deteriorating when the compacted density of a negative electrode is improved.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode and an electrolyte, the electrolyte comprising a lithium salt, an organic solvent and additives, wherein the additives comprise a fluorinated compound that accounts for 5 to 20% of the total mass of the electrolyte, a polynitrile compound that accounts for 0.2 to 4% of the total mass of the electrolyte, and a sulfate compound that accounts for 0.3 to 5% of the total mass of the electrolyte, and the compaction density of the negative electrode is not less than 1.55 g/cm.sup.3.

2. The secondary battery according to claim 1, wherein the fluorinated compound is selected from the group consisting of fluorocarbonate, fluorocarboxylate, fluoroether, fluorophosphate, fluorophosphite, and combinations thereof.

3. The secondary battery according to claim 2, wherein the fluorocarbonate is selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, methyl trifluoroethyl carbonate, and combinations thereof; and/or, the fluorocarboxylate is selected from the group consisting of 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, ethyl trifluoroacetate, 2,2,2-trifluoroethyl trifluoroacetate, methyl trifluoropropionate, ethyl 3,3-difluoropropionate, ethyl 2,2,3,3-tetrafluoropropionate, 2,2,2-trifluoroethyl propionate, 2,2,2-trifluoroethyl n-butyrate, and combinations thereof; and/or, the fluoroether is selected from the group consisting of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 2,2,2-trifluoroethyl-1,1,2,3,3,3-hexafluoropropyl ether, 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, perfluorobutyl ethyl ether, perfluorobutyl methyl ether, heptafluoropropyl trifluorovinyl ether, and combinations thereof; and/or, the fluorophosphate is selected from the group consisting of tris(2,2,2-trifluoroethyl) phosphate and tris(1,1,1,3,3,3-hexafluoroisopropyl) phosphate; and/or, the fluorophosphate is selected from the group consisting of tris(2,2,2-trifluoroethyl) phosphite and tris(1,1,1,3,3,3-hexafluoroisopropyl) phosphite.

4. The secondary battery according to claim 1, wherein the polynitrile compound contains three or more cyano groups.

5. The secondary battery according to claim 4, wherein the polynitrile compound is selected from the group consisting of 1,2,3-propanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,3,5-cyclohexanetricarbonitrile, 1,2,3-tris (2-cyanoethoxy) propane and combinations thereof.

6. The secondary battery according to claim 1, wherein the sulfate compound is selected from the group consisting of ethylene sulfate, propylene sulfate, bis(trimethylsilyl) sulfate, bis(triethylsilyl) sulfate, and combinations thereof.

7. The secondary battery according to claim 1, wherein the lithium salt is selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, anhydrous lithium perchlorate, lithium bis(trifluoromethylsulfonyl)imide, lithium trifluoromethanesulfonate, lithium bis(oxalate) borate, lithium monooxalate difluoroborate, lithium bis(fluorosulfonyl)imide, and combinations thereof.

8. The secondary battery according to claim 1, wherein the mass of the lithium salt is from 10 to 20% of the total mass of the electrolyte.

9. The secondary battery according to claim 1, wherein the organic solvent is selected from the group consisting of ethylene carbonate, propylene carbonate, γ-butyrolactone, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl propionate, ethyl propionate, propyl propionate, methyl acetate, ethyl acetate, propyl acetate, methyl butyrate, ethyl butyrate, propyl butyrate, sulfolane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and combinations thereof.

10. The secondary battery according to claim 1, wherein the compaction density of the negative electrode is 1.55 g/cm.sup.3 or higher, and 1.75 g/cm.sup.3 or lower.

11. The secondary battery according to claim 1, wherein the compaction density of the negative electrode is not less than 1.6 g/cm.sup.3.

12. The secondary battery according to claim 1, wherein both the positive electrode and the negative electrode are able to absorb and desorb lithium ions.

13. The secondary battery according to claim 12, wherein the positive electrode contains a positive electrode active material selected from the group consisting of LiCoO.sub.2, LiNi.sub.xCo.sub.yMn.sub.1-x-yO.sub.2, LiNi.sub.xCo.sub.yAl.sub.1-x-yO.sub.2, and combinations thereof.

14. The secondary battery according to claim 12, wherein the negative electrode contains a negative electrode active material that is carbon material or/and silicon material.

15. The secondary battery according to claim 14, wherein the carbon material is selected from the group consisting of artificial graphite and natural graphite; the silicon material is selected from the group consisting of silicon-carbon composites, silicon-oxygen-carbon composites, silicon-lithium composites, and combinations thereof.

16. The secondary battery according to claim 7, wherein the mass of the lithium salt is from 10 to 20% of the total mass of the electrolyte.

17. The secondary battery according to claim 10, wherein the compaction density of the negative electrode is not less than 1.6 g/cm.sup.3.

Description

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0031] In the following, this application is further explained in detail combining with embodiments, but this application should not be limited to these embodiments. Unless otherwise specified herein, “%” means mass percentage.

Embodiment 1

Manufacture of Positive Electrode

[0032] LiCoO.sub.2, conductive carbon black and PVDF were weighed according to a mass ratio of 90:5:5, and an appropriate amount of NMP was added thereto, followed by thorough stirring to give a positive electrode slurry. The positive electrode slurry was coated on an aluminum foil, dried, and then rolled and cut to obtain a positive electrode. The density of the positive electrode other than the current collector (referred to as compaction density of the positive electrode) was 4.15 g/cm.sup.3.

Manufacture of Negative Electrode

[0033] Graphite, styrene-butadiene rubber and carboxymethylcellulose were weighed according to a mass ratio of 95:3:2, and an appropriate amount of deionized water was added thereto, followed by thorough stirring to give a negative electrode slurry. The negative electrode slurry was coated on a copper foil, dried, and then rolled and cut to obtain a negative electrode. The density of the negative electrode other than the current collector (referred to as compaction density of the negative electrode) was 1.65 g/cm.sup.3.

Preparation of Electrolyte

[0034] Ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl propionate (EP) were mixed according to a mass ratio of 30:35:35 to obtain a mixed solvent, and the charging amounts of the mixed solvent, lithium hexafluorophosphate and the additives were calculated according to a mass ratio of (85-a):15:a.

[0035] 7.5 parts of mixed solvent, 15 parts of lithium hexafluorophosphate, and 7.5 parts of additives were taken, where the composition of the additives were 5 parts of FEC, 2 parts of 1,3,6-hexanetricarbonitrile (HTN) and 0.5 parts of ethylene sulfate (ESA) to prepare an electrolyte.

Manufacture of Battery

[0036] The above positive electrode, negative electrode, and electrolyte were used, a PE separator having a thickness of 20 μm was selected, and a soft pack battery was manufactured by a winding process, and its model number was 053048.

Battery Performance Test

[0037] The battery was formed at a rate of 0.1 C, degassed and sealed; then charged and discharged at a rate of 0.2 C for 1 cycle (the voltage range was 3.0 to 4.45 V, the same below).

[0038] The high-temperature storage performance test was to first charge the battery at a constant current of 0.5 C to 4.45V at 25° C., charge at a constant voltage until the current drops to 0.05 C, and then store the battery in an oven at 60° C. for 14 days. The internal resistances of the battery (full charge state) before and after storage were tested. Internal resistance change rate=[(internal resistance after storage−internal resistance before storage)/internal resistance before storage]*100%.

[0039] The high-temperature cyclic performance test was to charge and discharge the battery at a rate of 0.7 C for 400 cycles at 50° C., and calculate the capacity retention rate. Capacity retention rate=(400-cycle discharge capacity/first-cycle discharge capacity)*100%.

[0040] The low-temperature charge capability test was to charge the battery at a constant current of 0.2 C to 4.45V at −5° C., and then charge at a constant voltage until the current drops to 0.05 C. After leaving the battery at room temperature for half an hour, it was disassembled in a drying room to observe the condition of the negative electrode interface.

Embodiments 2 to 23, and Comparative Examples 1 to 8

[0041] The battery was manufactured in the same manner as in Embodiment 1, except for the change in the additives of the electrolyte and the compaction density of the negative electrode. The battery performance tests were performed in the same manner as in Embodiment 1.

[0042] See Table 1 for the additives, addition amounts, the compaction densities of the negative electrodes and the test results of high-temperature performance of the batteries of Embodiments 1-23 and Comparative Examples 1-8.

[0043] Wherein, fluoroethylene carbonate is abbreviated as FEC, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether is abbreviated as FE1, 1,3,6-hexanetricarbonitrile is abbreviated as HTN, ethylene sulfate is abbreviated as ESA, propylene sulfate is abbreviated as PSA, 1,3-propane sultone is abbreviated as PS, ethyl trifluoroacetate is abbreviated as ETFA, tris(2,2,2-trifluoroethyl) phosphate is abbreviated as TTFP, tris(2,2,2-trifluoroethyl) phosphite is abbreviated as TTFPI, bis(trimethylsilyl) sulfate is abbreviated as BTMS, bis(triethylsilyl) sulfate is abbreviated as BTES, 1,3,5-cyclohexanetricarbonitrile is abbreviated as CHTCN, and 1,2,3-tris (2-cyanoethoxy) propane is abbreviated as TCEP.

TABLE-US-00001 TABLE 1 Compaction Capacity densities of Change rate retention rate Additive negative of internal after 400 cycles amount electrode resistance of circulation Additives (wt %) (g/cm.sup.3) (%) (%) Embodiment 1 FEC + HTN + ESA 5 + 2 + 0.5 1.65 14.6 78 Embodiment 2 FEC + HTN + ESA 5 + 2 + 1.5 1.65 12.4 83 Embodiment 3 FEC + HTN + ESA 5 + 2 + 3 1.65 10.2 84 Embodiment 4 FEC + HTN + ESA 5 + 2 + 5 1.65 15.6 85 Embodiment 5 FEC + HTN + ESA 5 + 4 + 1.5 1.65 10.7 86 Embodiment 6 FEC + HTN + PSA 5 + 2 + 3 1.65 13.1 81 Embodiment 7 FEC + HTN + PSA 5 + 3 + 2 1.65 9.8 83 Embodiment 8 FE1 + HTN + ESA 5 + 2 + 1.5 1.65 14.9 77 Embodiment 9 FE1 + HTN + ESA 10 + 2 + 1.5 1.65 12.3 79 Embodiment 10 FEC + FE1 + HTN + ESA 3 + 5 + 2 + 1.5 1.65 10.7 81 Embodiment 11 FEC + FE1 + HTN + ESA 5 + 5 + 2 + 1.5 1.65 8.5 87 Embodiment 12 FEC + FE1 + HTN + ESA 8 + 5 + 2 + 1.5 1.65 14.6 88 Embodiment 13 FEC + FE1 + HTN + ESA 5 + 12 + 1 + 1 1.65 11.3 89 Embodiment 14 FEC + ETFA + CHTCN + ESA 5 + 4 + 2 + 0.5 1.60 10.2 89 Embodiment 15 FEC + ETFA + TCEP + BTMS 5 + 5 + 2 + 3 1.60 10.6 88 Embodiment 16 FEC + TTFP + TCEP + BTMS 4 + 3 + 2 + 3 1.60 11.0 89 Embodiment 17 FEC + TTFPI + CHTCN + BTES 3 + 3 + 1 + 4 1.55 12.3 91 Embodiment 18 FEC + HTN + ESA 5 + 2 + 5 1.55 9.4 90 Embodiment 19 FEC + HTN + PSA 5 + 2 + 3 1.55 11.8 85 Embodiment 20 FEC + HTN + ESA 5 + 2 + 0.5 1.70 16.7 75 Embodiment 21 FEC + FE1 + HTN + ESA 3 + 5 + 2 + 1.5 1.70 14.0 78 Embodiment 22 FEC + HTN + ESA 5 + 2 + 0.5 1.75 19.6 70 Embodiment 23 FEC + FE1 + HTN + ESA 3 + 5 + 2 + 1.5 1.75 18.1 73 Comparative Example 1 FEC + PS 5 + 3 1.65 34.2 53 Comparative Example 2 FEC + PS 5 + 4 1.65 39.7 55 Comparative Example 3 FEC + PS 5 + 4 1.60 38.8 56 Comparative Example 4 FEC + PS 5 + 4 1.55 32.3 60 Comparative Example 5 FEC + PS 5 + 3 1.55 30.8 59 Comparative Example 6 FEC + PS 5 + 3 1.50 28.5 62 Comparative Example 7 FEC + PS 5 + 3 1.70 37.4 48 Comparative Example 8 FEC + PS 5 + 3 1.75 44.9 40

[0044] The data of the internal resistance change rate and capacity retention rate of the batteries of the embodiments and comparative examples in Table 1 shows that when using the same composition of electrolyte, the increase in the compaction density of the negative electrode will result in a higher internal resistance change rate after high-temperature storage and a worse capacity retention rate of the batteries; under the condition of higher compaction density of the negative electrode (≥1.55 g/cm.sup.3), using fluorinated compounds, polynitrile compounds and sulfates as additives and optimizing the combinations, the internal resistance change rate of the battery after high-temperature storage can be lower, and the capacity retention rate of the battery after high-temperature cycling is higher, that is, the high-temperature characteristics of the battery are improved.

[0045] The conditions of the negative electrode interfaces of the batteries of Embodiments 1 to 23 and Comparative Examples 1 to 8 after low-temperature charging are shown in Table 2. The condition of the negative electrode interface is divided into four situations: no lithium plating, slight lithium plating, moderate lithium plating, and severe lithium plating, wherein, no lithium plating means that the negative electrode interface is uniform golden yellow, slight lithium plating means that there are white or gray spot-like attachments on the interface (and the distribution area does not exceed 10% of the coated area), moderate lithium plating means that 10-30% of the area on the interface has white or gray attachments, and severe lithium plating means more than 30% of the area on the interface has white or gray attachments. No lithium plating indicates that the battery has excellent charging ability, and the more severe the degree of lithium plating, the worse the low-temperature charging ability.

TABLE-US-00002 TABLE 2 State of negative electrode interface Embodiment 1 No lithium plating Embodiment 2 No lithium plating Embodiment 3 No lithium plating Embodiment 4 Slight lithium plating Embodiment 5 No lithium plating Embodiment 6 Slight lithium plating Embodiment 7 No lithium plating Embodiment 8 No lithium plating Embodiment 9 No lithium plating Embodiment 10 No lithium plating Embodiment 11 No lithium plating Embodiment 12 Slight lithium plating Embodiment 13 No lithium plating Embodiment 14 No lithium plating Embodiment 15 No lithium plating Embodiment 16 No lithium plating Embodiment 17 No lithium plating Embodiment 18 No lithium plating Embodiment 19 No lithium plating Embodiment 20 Slight lithium plating Embodiment 21 Slight lithium plating Embodiment 22 Slight lithium plating Embodiment 23 Slight lithium plating Comparative Example 1 Moderate lithium plating Comparative Example 2 Severe lithium plating Comparative Example 3 Severe lithium plating Comparative Example 4 Moderate lithium plating Comparative Example 5 Slight lithium plating Comparative Example 6 No lithium plating Comparative Example 7 Severe lithium plating Comparative Example 8 No lithium plating

[0046] The conditions of the negative electrode interfaces of the batteries of the embodiments and comparative examples in Table 2 indicate that when using the same components of the electrolyte, the increase in the compaction density of the negative electrode will result in that lithium plating is more prone to occur in the negative electrode of the battery; under the same compaction density of the negative electrode, the increase in the content of the film-forming additive will cause lithium plating is more prone to occur in the negative electrode of the battery; after using fluorinated compounds, polynitrile compounds and sulfates as additives and optimizing the combinations, it can enable lithium plating not to occur in the negative electrode under the condition of higher compaction density of the negative electrode (≥1.55 g/cm.sup.3), that is, the low-temperature charging ability of the battery has been improved.

[0047] It can be seen from Table 1 and Table 2 that the battery of the present disclosure can still take into account good high-temperature characteristics and low-temperature charging ability under the condition of a higher compaction density of the negative electrode.

[0048] The embodiments described above are only for illustrating the technical concepts and features of the present disclosure, and are intended to make those skilled in the art being able to understand the present disclosure and thereby implement it, and should not be concluded to limit the protective scope of this disclosure. Any equivalent variations or modifications according to the spirit of the present disclosure should be covered by the protective scope of the present disclosure.