ELECTROLYTE ADDITIVE, NON-AQUEOUS ELECTROLYTE, AND LITHIUM ION BATTERY USING SAME

Abstract

The present invention relates to an electrolyte additive, a non-aqueous electrolyte and a lithium ion battery using same. The electrolyte additive includes a compound represented by Formula 1:

##STR00001##

In Formula 1, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are each independently hydrogen atom, halogen atom, a substituted or unsubstituted chain C.sub.1-C.sub.12 alkyl group, a substituted or unsubstituted chain C.sub.2-C.sub.12 alkenyl group, a substituted or unsubstituted chain C.sub.2-C.sub.12 alkynyl group, or groups represented by RC.sub.nH.sub.2n+1. R is each independently oxygen atom or sulfur atom, and n is positive integer. The electrolyte additive has a special structure. During the first charge and discharge process, redox products formed by oxidation-reduction reaction of multiple conjugated olefin structures adhere to the positive and negative electrode surfaces to form solid electrolyte interface films. The films have low impedance and high lithium ion conductivity, so the lithium ion battery has excellent rate performance and low temperature performance.

Claims

1. An electrolyte additive comprising a compound represented by Formula 1: ##STR00006## wherein, in Formula 1, R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are each independently hydrogen atom, halogen atom, a substituted or unsubstituted chain C.sub.1-C.sub.12 alkyl group, a substituted or unsubstituted chain C.sub.2-C.sub.12 alkenyl group, a substituted or unsubstituted chain C.sub.2-C.sub.12 alkynyl group, or groups represented by RC.sub.nH.sub.2n+1, R is each independently oxygen atom or sulfur atom, and n is positive integer.

2. The electrolyte additive according to claim 1, wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are each independently hydrogen atom, fluorine atom, a substituted or unsubstituted chain C.sub.1-C.sub.3alkyl group, or groups represented by RC.sub.nH.sub.2+1, R is each independently oxygen atom or sulfur atom, and n is positive integer less than 5.

3. The electrolyte additive according to claim 1, wherein the compound represented by Formula 1 comprises at least one of the compounds A to J below: ##STR00007## ##STR00008##

4. A non-aqueous electrolyte comprising a lithium salt, a non-aqueous organic solvent, and the electrolyte additive according to claim 1.

5. The non-aqueous electrolyte according to claim 4, wherein weight percentage of the compound represented by Formula 1 in the non-aqueous electrolyte is 0.1˜2%.

6. The non-aqueous electrolyte according to claim 4, wherein the lithium salt is at least one selected from groups consisting of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(oxalate)borate, lithium oxalyldifluoroborate, lithium difluoro bis(oxalato)phosphate, lithium tetrafluoroborate, lithium tetrafluoro(oxalato)phosphate, lithium bistrifluoromethanesulfonimide, lithium bis(fluorosulfonyl)imide, and lithium tetrafluoromalonate phosphate.

7. The non-aqueous electrolyte according to claim 4, wherein the non-aqueous organic solvent is at least one selected from groups consisting of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, fluoroethylene carbonate, butyl acetate, γ-butyrolactone, n-propyl propionate, ethyl propionate, and ethyl butyrate.

8. The non-aqueous electrolyte according to claim 4, further comprising a supplemental additive, wherein the supplemental additive is at least one selected from groups consisting of methyl trifluoroethyl carbonate, ethyl-2,2,2-trifluoroethyl carbonate, propyl-2,2,2-trifluoroethyl carbonate, vinylene carbonate, diethyl pyrocarbonate, 1,3-propane sultone, dioxathiolane 2,2-dioxide, 1,2-difluoro-ethylene carbonate, tris(trimethylsilyl)-phosphate, tris(trimethylsilyl)-phosphite, 4,4′-Bi-1,3-dioxolane]-2,2′-dione (BDC), 3,3-Bi-1,3,2-Dioxathiolane 2,2-dioxide (BDTD), 4,4-Bi-1,3,2-Dioxathiolane 2,2-dioxide, triallyl phosphite, tripropargyl phosphate, succinonitrile, adiponitrile, 1,3,6-hexanetricarbonitrile, and ethylene glycol bis(propionitrile) ether.

9. A lithium ion battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte according to claim 4, and the maximum charging voltage being 4.5V.

10. The lithium ion battery according to claim 9, wherein the positive electrode is lithium cobalt oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings facilitate an understanding of the various embodiments of this invention. In such drawings:

[0022] FIG. 1 shows an oxidation potential diagram of Example 1 and Comparative Example 1;

[0023] FIG. 2 shows a reduction potential diagram of Example 1 and Comparative Example 1;

[0024] FIG. 3 shows a positive electrode impedance diagram of Example 1 and Comparative Examples 1-3;

[0025] FIG. 4 shows a negative electrode impedance diagram of Example 1 and Comparative Examples 1-3; and

[0026] FIG. 5 shows dQ/dV-V curves of Example 1 and Comparative Example 1 under the formation stage.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

[0027] The present invention will be described with reference to the specific embodiments. The following description of the example(s) is illustrative in nature and is not intended to limit the disclosure, its application, or uses.

Example 1

[0028] Electrolyte Preparation

[0029] All samples were prepared in a nitrogen atmosphere glovebox (<1 ppm of O.sub.2 and H.sub.2O) by mixing dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate in a mass ratio of 4:5:3 to obtain 79.7 g non-aqueous organic solvent, and 0.3 g compound A was added to the non-aqueous organic solvent to obtain a mixed solution. Then the mixed solution was sealed, placed and then frozen for 2h in the freezing chamber (−4° C.). Then, 20 g LiPF.sub.6 was slowly added to the mixed solution with stirring until a homogeneous solution in a nitrogen atmosphere glovebox (<1 ppm of O.sub.2 and H.sub.2O) and the electrolyte was obtained.

[0030] The electrolyte formulations in Examples 2-20 and Comparative Examples 1-8 are shown in Table 1 below. The method of preparing the electrolyte is the same as that in Example 1.

TABLE-US-00001 TABLE 1 Non-aqueous organic lithium salt/ Additive/Mass Supplemental Examples solvent/Mass (g) Mass (g) (g) additive/Mass (g) Ex.1 DMC/DEC/EMC LiPF.sub.6/20 Compound (4:5:3)/79.7 A/0.3 Ex.2 DMC/DEC/EMC LiPF.sub.6/20 Compound (4:5:3)/79.9 I/0.1 Ex.3 DMC/DEC/EMC LiPF.sub.6/19.5 Compound (4:5:3)/79.7 LiBF.sub.4/0.5 A/0.3 Ex.4 EC/PC/EMC/DEC LiPF.sub.6/20 Compound (2:3:5:2)/79.7 A/0.3 Ex.5 DMC/DEC/EMC LiPF.sub.6/20 Compound VC/0.3 (4:5:3)/79.4 A/0.3 Ex.6 DMC/DEC/EMC LiPF.sub.6/20 Compound PS/0.3 (4:5:3)/79.4 A/0.3 Ex.7 DMC/DEC/EMC LiPF.sub.6/20 Compound FEC/0.3 (4:5:3)/79.4 A/0.3 Ex.8 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/79.7 A/0.3 Ex.9 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/79.5 A/0.5 Ex.10 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/78 A/2 Ex.11 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/79.7 C/0.3 Ex.12 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/79.7 D/0.3 Ex.13 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/79.7 G/0.3 Ex.14 DMC/DEC/EMC/FEC LiPF.sub.6/19.5 Compound (4:5:3:1)/79.7 LiDFOP/0.5 A/0.3 Ex.15 DMC/DEC/EMC/FEC LiPF.sub.6/19.5 Compound (4:5:3:1)/79.7 LiPO.sub.2F.sub.2/0.5 A/0.3 Ex.16 DMC/DEC/EMC/FEC LiPF.sub.6/19.5 Compound (4:5:3:1)/79.7 LiBOB/0.5 A/0.3 Ex.17 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound VC/0.5 (4:5:3:1)/79.2 A/0.3 Ex.18 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound DTD/0.5 (4:5:3:1)/79.2 A/0.3 Ex.19 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound TMSP/0.5 (4:5:3:1)/79.2 A/0.3 Ex.20 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound PS/0.5 (4:5:3:1)/79.2 A/0.3 Com.1 DMC/DEC/EMC LiPF.sub.6/20 (4:5:3)/80 Com.2 DMC/DEC/EMC LiPF.sub.6/20 ADN/0.3 (4:5:3)/79.7 Com.3 DMC/DEC/EMC LiPF.sub.6/20 Compound (4:5:3)/79.7 K/0.3 Com.4 DMC/DEC/EMC/FEC LiPF.sub.6/20 (4:5:3:1)/80 Com.5 DMC/DEC/EMC/FEC LiPF.sub.6/20 SN/0.3 (4:5:3:1)/79.7 Com.6 DMC/DEC/EMC/FEC LiPF.sub.6/20 ADN/0.3 (4:5:3:1)/79.7 Com.7 DMC/DEC/EMC/FEC LiPF.sub.6/20 HTCN/0.3 (4:5:3:1)/79.7 Com.8 DMC/DEC/EMC/FEC LiPF.sub.6/20 Compound (4:5:3:1)/79.7 K/0.3

[0031] Compound K is represented by the following formula:

##STR00005##

[0032] Lithium ion batteries were prepared by using lithium cobalt oxide with the highest charging voltage of 4.5V as the positive electrode material, and natural graphite as the negative electrode material. Lithium cobalt oxide was obtained from Tianjin B&M Science and Technology Co., Ltd. Lithium ion batteries were prepared using the electrolytes in Examples 1-20 and Comparative Examples 1-8 as reference to the conventional preparation method of lithium battery. The battery in each embodiment was tested for normal temperature cycle performance, high temperature cycle performance, low temperature discharge performance, high temperature storage performance, FEC remaining capacity test, and 3 C rate discharge performance. Further, the oxidation potential test, reduction potential test, positive electrode impedance test, and negative electrode impedance test were performed on Example 1 and Comparative Example 1. The dQ/dV-V curve of the formation stage was characterized. The test conditions are as follows, and the test results are shown in Table 2 and in the drawings.

[0033] Normal Temperature Cycle Performance Test

[0034] The lithium ion batteries were placed in a room (25° C.), charged to 4.5V at a constant current of IC, then charged at a constant voltage until the current reached 0.05 C, then discharged to 3.0V at a constant current of IC, and then that cycle was repeated 500 times. The discharge capacity of the first cycle and the discharge capacity of the last cycle were recorded. The capacity retention rate under normal temperature cycle was calculated by the following formula.

Capacity retention rate (%)=Discharge capacity of the last cycle/Discharge capacity of the first cycle×100%

[0035] High Temperature Cycle Performance Test

[0036] The lithium ion batteries were placed in a room (45° C.), charged to 4.5V at a constant current of 1 C, then charged at a constant voltage until the current reached 0.05 C, then discharged to 3.0V at a constant current of IC, and then that cycle was repeated 500 times. The discharge capacity of the first cycle and the discharge capacity of the last cycle were recorded. The capacity retention rate under high temperature cycle was calculated by the following formula.

Capacity retention rate (%)=Discharge capacity of the last cycle/Discharge capacity of the first cycle×100%

[0037] Low Temperature Discharge Performance Test

[0038] At room temperature, the lithium ion batteries were charged to 4.5V at a constant current of 0.5 C, and then charged at a constant voltage until the current reached 0.05 C. Then the lithium ion batteries were placed in a constant temperature box (−20° C.) and discharged at a constant current of 0.5 C to 3.0V voltage.

Capacity retention rate (%)=Retention capacity/Initial capacity×100%

[0039] High Temperature Storage Performance Test

[0040] The formatted batteries were charged to 4.5V at a constant current of IC and constant voltage at room temperature, and the initial capacity and the initial battery thickness were measured. Then the batteries were stored at 85° C. for 8 hours and then discharged to 3.0V at a constant current of IC to measure the retention capacity and the recovery capacity and battery thickness after storage. The results were calculated by the following formula.

Capacity retention rate (%)=Retention capacity/Initial capacity×100%

Capacity recovery rate (%)=Recovery capacity/Initial capacity×100%

Thickness expansion (%)=(Battery thickness after storage−Initial battery thickness)/Initial battery thickness×100%

[0041] FEC Remaining Capacity Test

[0042] The batteries after the 500 high temperature cycles were disassembled, the positive electrode, the negative electrode and the separator were soaked in dichloromethane for 24 hours, and then the extract liquor was taken out to detect the FEC content by GC.

[0043] 3 C Rate Discharge Performance Test

[0044] At room temperature, the batteries were charged with a constant current of 0.5 C to 4.5V, then charged at a constant voltage until the current reached 0.05 C, and then discharged at a constant current of 3 C to 3.0V.

Capacity retention rate (%)=Retention capacity/Initial capacity×100%

[0045] Oxidation Potential Test

[0046] The electrolytes prepared in Example 1 and Comparative Example 1 were added in a three-electrode system in which the working electrode was a platinum electrode, the reference electrode and the counter electrode were lithium electrodes. Then the three electrodes were placed on the electrochemical workstation for linear sweep voltammetry, the scanning voltage was 3V-7V and the scanning speed was 1 mV/s. The results are shown in FIG. 1.

[0047] Reduction Potential Test

[0048] The electrolytes prepared in Example 1 and Comparative Example 1 were used to prepare lithium cobalt oxide|electrolyte|graphite button-type batteries. The button-type batteries were placed on the electrochemical workstation for linear sweep voltammetry, the scanning voltage was 3V-0V, and the scanning speed was 1 mV/s. The results are shown in FIG. 2.

[0049] Positive Electrode Impedance Test

[0050] The electrolytes prepared in Example 1 and Comparative Examples 1-3 were used to prepare lithium cobalt oxide|electrolyte|lithium plate button-type half batteries. And the button-type half batteries were placed on the electrochemical workstation for EIS test, and the results are shown in FIG. 3.

[0051] Negative Electrode Impedance Test

[0052] The electrolytes prepared in Example 1 and Comparative Examples 1-3 were used to prepare graphite|electrolyte|lithium plate button-type half batteries. And the button-type half batteries were placed on the electrochemical workstation for EIS test, and the results are shown in FIG. 4.

[0053] dQ/dHV-V Curve on the Formation Stage

[0054] The voltage and capacity information of the batteries on formation stage in Example 1 and Comparative Example were extracted to create a dQ)/dV-V curve. The results are shown in FIG. 5.

TABLE-US-00002 TABLE 2 Cycle performance and high-low temperature performance, FEC remaining capacity test results after the 500 high temperature cycles Low temperature FEC remaining High temperature storage Capacity discharge capacity after performance (85° C., 8 h) retention rate performance 500 high 3 C rate Capacity Capacity after 500 cycles (−20° C., temperature discharge retention recovery Thickness Examples 25° C. 45° C. 0.5 C) cycles performance rate rate expansion Ex. 1 74.2% 63.9% 72.8% / 77.4% 84.2% 88.2% 3.5% Ex. 2 75.1% 64.2% 72.3% / 76.3% 83.5% 87.9% 2.9% Ex. 3 73.8% 65.1% 71.4% / 75.1% 85.3% 89.7% 2.0% Ex. 4 75.6% 64.7% 72.1% / 77.8% 83.5% 88.8% 3.7% Ex. 5 78.4% 69.2% 71.5% / 77.6% 87.5% 89.8% 0.9% Ex. 6 77.4% 73.5% 72.3% / 81.2% 88.9% 91.3% 0.7% Ex. 7 80.6% 72.1% 73.9% 0.2% 81.3% 88.2% 91.4% 1.0% Ex. 8 88.9% 85.4% 76.7% 4.8% 85.9% 92.1% 95.4% 0.2% Ex. 9 88.2% 86.7% 76.4% 4.9% 86.1% 93.5% 95.0% 0.3% Ex. 10 87.8% 88.1% 74.3% 5.1% 85.4% 93.1% 94.9% 0.2% Ex. 11 89.5% 87.4% 77.9% 4.7% 87.1% 94.2% 96.6% 0.2% Ex. 12 89.3% 88.0% 78.1% 4.8% 88.3% 93.5% 96.7% 0.1% Ex. 13 88.9% 88.1% 78.6% 5.0% 87.1% 93.4% 96.5% 0.3% Ex. 14 90.2% 89.5% 81.4% 4.9% 89.5% 93.7% 96.1% 0.2% Ex. 15 90.1% 89.5% 81.2% 4.8% 89.7% 94.3% 95.9% 0.2% Ex. 16 90.3% 88.7% 80.2% 4.9% 89.0% 94.4% 96.1% 0.1% Ex. 17 90.1% 89.5% 74.6% 4.8% 86.9% 93.5% 95.4% 0.2% Ex. 18 89.6% 86.9% 80.1% 4.7% 87.8% 92.9% 94.7% 0.4% Ex. 19 90.2% 85.8% 81.6% 5.2% 88.4% 93.5% 96.7% 0.1% Ex. 20 92.1% 90.9% 81.0% 5.4% 89.3% 95.2% 97.2% 0.1% Com. 1 62.3%  <10% 54.2% / 60.3% 72.4% 75.2% 10.5% Com. 2 65.9%  <10% 50.3% / 57.6% 79.5% 83.7% 8.4% Com. 3 68.7%  <10% 52.9% / 56.4% 82.0% 85.8% 7.6% Com. 4 78.3% 73.4% 71.5% 1.3% 78.3% 75.1% 80.9% 7.9% Com. 5 82.5% 80.0% 65.6% 1.5% 70.5% 83.4% 87.8% 5.5% Com. 6 81.9% 81.6% 66.8% 1.4% 71.2% 84.3% 88.7% 4.4% Com. 7 83.8% 82.1% 63.4% 1.8% 68.1% 85.8% 89.0% 4.2% Com. 8 84.1% 80.9% 67.5% 2.0% 72.3% 84.9% 90.1% 3.9%

[0055] The results in Table 2 show that compared to Comparative Examples 1-8, the normal temperature cycle performance, high temperature cycle performance, low temperature discharge performance, high temperature storage performance, and rate capability of Examples 1 to 20 are all at a better level. Because the electrolyte additive with a special structure is used in this invention, which is a stable conjugated olefin structure formed by a nitrile group and a quinone-like structure. Compared with the prior art, during the first charge and discharge process, the conjugated olefin structure formed by the nitrile group and the quinone-like structure undergoes a redox reaction to form a redox product attached to the surface of the positive electrode and the negative electrode to form a solid electrolyte interface film with a stable skeleton. The positive electrode interface film can alleviate the particle breakage of the positive electrode active material and the catalytic oxidative decomposition of the electrolyte under high voltage, and the negative electrode interface film can inhibit the reduction and decomposition of the electrolyte at the negative electrode interface. More importantly, the solid electrolyte interface membrane has low impedance, and its lithium ion conductivity is much higher than that of conventional solid electrolyte interface membranes, which makes the lithium ion battery have excellent rate performance, low temperature performance and cycle performance. Therefore, the electrolyte additive of the present invention can improve battery cycle performance while taking into account both rate and low temperature performance.

[0056] In Comparative Examples 2 and 5-7, although nitrile additives were added, which can improve the cycle performance to a certain extent, especially in Comparative Examples 4-7, the cycle performances are better under the action of FEC and nitrile additives. But low temperature performance and rate performance in Comparative Examples 2 and 5-7 are obviously inferior to the examples containing the compounds represented by Formula 1. Although compound K, whose structure is similar to the compound represented by Formula 1, was added in Comparative Example 3 and Comparative Example 8, compound K also does not have the conjugated olefin structure of the compound represented by Formula 1, and contains a benzene ring structure. So the compound K also fails to improve low temperature performance and rate performance.

[0057] From Example 1 and Examples 7-8, it can be seen that when both the compound represented by Formula 1 and a high content of FEC are contained, the cycle performance of the high voltage lithium cobalt oxide batteries are more significantly improved. This is because the compound represented by Formula 1 in the present invention can synergize with FEC to adjust the decomposition rate of FEC to form an electronic insulation interface layer containing LiF, which is beneficial to improve the cycle performance and high-low temperature performance of lithium ion batteries under high voltage. Furthermore, from the results of FEC remaining capacity after 500 high temperature cycles in Table 1, it can be seen that when both the compound represented by Formula 1 and high content of FEC are added, the detected FEC content is higher, indicating that the compound represented by Formula 1 inhibits the decomposition of FEC.

[0058] Comparing Example 8 with Examples 14-16, it can be seen that adding other lithium salt along with lithium hexafluorophosphate, the batteries have better cycle performance and high-low temperature performance.

[0059] From Examples 5-6 and Examples 17-20, it can be seen that the cycle performance and high temperature performance of the batteries with PS are better than those of the batteries with other additives, which is due to 1, 3-Propane sultone (PS) has good film-forming performance as a promoter. PS can form a large number of CEI films containing sulfonic acid groups at the positive electrode interface. The CEI films inhibit the decomposition of FEC at high temperature, and increase the capacity loss of lithium ion batteries during the first charge and discharge, which helps to increase the reversible capacity of lithium ion batteries, thereby improving the high-temperature performance and long-term cycle performance of lithium ion batteries.

[0060] It can be seen from the oxidation potential results in FIG. 1 that the electrolyte containing compound A in Example 1 is oxidized prior to the blank electrolyte in Comparative Example 1 at about 5.8 v. It can be seen from the reduction potential results in FIG. 2 that the electrolyte containing compound A in Example 1 takes part in the reduction reaction on the negative electrode prior to the blank electrolyte in Comparative Example 1 at about 2.0V, and the decomposition of the electrolyte is inhibited. It can be seen from the results of positive and negative electrode impedances in FIG. 3 and FIG. 4 that the positive and negative electrode impedances of the electrolyte containing compound A in Example 1 after formation are obviously smaller than those of the blank electrolyte in Comparative Example 1, the electrolyte containing ADN in Comparative Example 2, and the electrolyte containing compound K in Comparative Example 3. According to the dQ/dV-V curves under the formation stage in FIG. 5, compound A is preferentially taken part in the film formation at 3.0V, and the reaction of solvent in the film formation at 3.2V is inhibited.

[0061] While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention.