Nonaqueous electrolyte and battery containing same

Abstract

The non-aqueous electrolyte includes an organic solvent containing a carbonate solvent and 10 to 50% by weight of methyl acetate based on the total weight of the organic solvent, and a plurality of lithium salts dissolved in the organic solvent. The plurality of lithium salts include 0.25 to 4.5% by weight of lithium difluorophosphate based on the total weight of the non-aqueous electrolyte, 0.25 to 3% by weight of lithium bis(oxalato)borate based on the total weight of the non-aqueous electrolyte, and lithium hexafluorophosphate.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63 / 315,228, filed March 1, 2022, the contents of which are incorporated by reference in their entirety herein. [Technical field]

[0002] The present disclosure relates to a non-aqueous electrolyte and a lithium ion battery comprising the non-aqueous electrolyte. Lithium ion batteries are widely used. Some applications require high stability at high temperatures while at the same time providing high rate charging and discharging capability at low temperatures. There remains a need for improved electrolytes that enable high rate charging and discharging at low temperatures and improve stability at high temperatures. Summary of the Invention

[0003] The above and other technical problems are solved by a non-aqueous electrolyte comprising an organic solvent, the organic solvent comprising a carbonate solvent, 10% by weight or more than 10% to 50% by weight of methyl acetate based on the total weight of the organic solvent, and a plurality of lithium salts dissolved in the organic solvent, the plurality of lithium salts comprising 0.25 to 4.5% by weight of lithium difluorophosphate based on the total weight of the non-aqueous electrolyte, 0.25 to 3% by weight of lithium bis(oxalato)borate based on the total weight of the non-aqueous electrolyte, and lithium hexafluorophosphate. The non-aqueous electrolyte is prepared by preparing an organic solvent and the plurality of lithium salts, and contacting the organic solvent with the plurality of lithium salts.

[0004] In another aspect, a lithium ion cell includes a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and the non-aqueous electrolyte described above. The lithium ion cell is manufactured by preparing the non-aqueous electrolyte and adding the non-aqueous electrolyte to an assembly including a negative electrode, a positive electrode, and a separator between the negative electrode and the positive electrode.

[0005] Also disclosed is a method for preparing a non-aqueous electrolyte, the method comprising the steps of: preparing an organic solvent containing a carbonate solvent and 10% by weight, or more than 10% to 50% by weight, of methyl acetate, based on the total weight of the organic solvent; preparing a plurality of lithium salts including lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate; and contacting the organic solvent with the plurality of lithium salts to prepare a non-aqueous electrolyte, in which lithium difluorophosphate is present in an amount of 0.25 to 4.5% by weight and lithium bis(oxalato)borate is present in an amount of 0.25 to 3% by weight, based on the total weight of the non-aqueous electrolyte. [Brief description of the drawings]

[0006] The following description should not be construed as limiting in any way. [Figure 1A] FIG. 1A is a graph showing voltage (volts, V) at the end of a 15 second 5C pulse discharge versus cycle number for cells having example electrolyte compositions of the present disclosure, as evaluated using a pulse discharge thermal loop. [Figure 1B] FIG. 1A is a graph showing voltage (volts, V) at the end of a 15 second 5C pulse discharge versus cycle number for cells having example electrolyte compositions of the present disclosure, as evaluated using a pulse discharge thermal loop. [Figure 2A] FIG. 2A is a graph showing voltage (volts, V) at the end of a 15 second 5C pulse discharge versus cycle number for a cell having a comparative electrolyte composition, as evaluated using a pulse discharge thermal loop. [Figure 2B] FIG. 2B is a graph showing voltage (volts, V) at the end of a 15 second 5C pulse discharge versus cycle number for a cell having a comparative electrolyte composition, as evaluated using a pulse discharge thermal loop. [Diagram 3]FIG. 3 is a graph of the voltage (volts, V) at the end of a 0.5 second, 0.6 watt (W) pulse at −17° C. before aging (initial), after the pulse discharge thermal loop, and after the pulse discharge thermal loop and 130 hours storage at 71° C. [Figure 4] FIG. 4 is a graph showing the voltage (volts, V) at the end of 0.2 second 119A pulse tests at -17°C versus test number, with the cells aged by charge / discharge cycling at 55°C for 2 weeks between each pulse test. [Diagram 5] FIG. 5 is a graph showing the voltage (volts, V) vs. test number at the end of a 29.5A discharge for 5 seconds at -40°C after charging to 4.1V at 30°C, with the cells aged at 55°C for 2 weeks between each pulse test. [Figure 6] FIG. 6 is a table summarizing the C / 5 capacity at 25° C., the voltage at the end of a 10 second C / 2 pulse at −30° C., and the C / 10 capacity at −30° C. before and after storage for 13 days at 55° C. for cells having the electrolytes of Examples 6-10 and Comparative Examples 7-11, respectively. [Figure 7] FIG. 7 is a table summarizing the C / 5 capacity at 25° C., the voltage at the end of a 10 second C / 2 pulse at −30° C., and the C / 10 capacity at −30° C. before and after storage for 13 days at 55° C. for the cells having the electrolytes of Examples 11-13, respectively. [Figure 8] FIG. 8 shows the C / 10 capacity (milliamp hours, mAh) at -30° C. versus storage time (weeks) for electrolytes with different methyl acetate contents, with the cells aged at 55° C. for 2 weeks by charge-discharge cycling between each pulse test. [Figure 9] FIG. 9 is a graph showing voltage (volts, V) at the end of a 10 second 1C pulse at 95% state of charge (SOC) at −30° C. versus storage time (weeks) for cells containing electrolytes with different methyl acetate contents, with the cells aged by cycling at 55° C. for 2 weeks between each pulse test. [Figure 10] FIG. 10 shows the C / 5 capacity (milliamp hours, mAh) at 25° C. versus storage time (weeks) for cells containing electrolytes with different methyl acetate contents, with the cells aged at 55° C. for 2 weeks between pulse tests. [Figure 11] FIG. 11 shows a graph of C / 10 capacity (milliamp hours, mAh) versus storage time (weeks) at −30° C. comparing methyl acetate vs. ethyl propionate and lithium bis(oxalato)borate vs. LiPF2(oxalate) in the electrolyte, with the cells aged at 55° C. for 2 weeks between pulse tests. [Figure 12] FIG. 12 is a graph of voltage (volts, V) at the end of a 10 second 1C pulse at 95% SOC at −30° C. versus storage time (weeks) comparing methyl acetate vs. ethyl propionate and lithium bis(oxalato)borate vs. LiPF2(oxalate)2 in the electrolyte, with the cells aged at 55° C. for 2 weeks between each pulse test. [Figure 13] FIG. 13 shows a graph of C / 5 capacity (milliamp hours, mAh) versus storage time (weeks) at 25° C. comparing methyl acetate vs. ethyl propionate, and lithium bis(oxalato)borate vs. LiPF2(oxalate) in the electrolyte, with the cells aged at 55° C. for 2 weeks between pulse tests. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Many applications for lithium-ion batteries require a wide range of operating temperatures, for example from -40 °C to +70 °C. The performance of lithium-ion cells, especially at low temperatures, is often limited by the electrolyte. The ionic conductivity of the electrolyte is one of the transport properties that helps determine how quickly the cell can be charged and discharged. One factor that determines the conductivity of an electrolyte is the viscosity of the electrolyte; solutions with lower viscosity have greater ion mobility and therefore greater conductivity.

[0008] Liquid electrolytes include a mixture of organic carbonate solvents with desirable and undesirable viscosity, dielectric constant, melting point, and ability to form a stable solid electrolyte interface (SEI) layer. For example, dimethyl carbonate (DMC) has a lower viscosity than ethyl methyl carbonate (EMC), and its melting point is 5°C compared to -53°C for EMC. Additionally, ethylene carbonate (EC) is used to form a stable SEI layer that allows for good battery life, but its melting point is 34°C and its viscosity is unfavorable for low temperature performance.

[0009] Efforts to improve the performance of lithium-ion batteries and make them applicable to non-consumer applications have generally focused on increasing the conductivity of the electrolyte at low temperatures, e.g., −20°C to −40°C. Aliphatic esters such as methyl acetate (MA) can be added to the organic carbonate mixture to extend the operating temperature range to low temperatures, in part because aliphatic esters have lower viscosities and melting points than organic carbonates. However, the presence of MA in the electrolyte can also reduce capacity retention and increase cell resistance as the cell ages, and capacity retention continues to decrease and resistance increases with increasing MA concentration. Thus, although electrolytes with higher MA concentrations may improve low-temperature performance, the presence of MA in the electrolyte can also limit cell life and reduce high-temperature stability.

[0010] The use of certain electrolyte additives can affect the performance of lithium-ion batteries. Without wishing to be bound by theory, it is believed that a solid electrolyte interface (SEI) film on the surface of the negative electrode, which is ionically conductive to lithium ions and electrically insulating, may prevent continued electrolyte reduction and loss of capacity. If the SEI film is too thick or has a high resistance, the rate performance of the cell may be poor. During cycling, if the film is not dense and adequately stable, the SEI will eventually dissolve or break down, and the exposed negative electrode will react with the electrolyte, consuming the electrolyte and resulting in loss of cyclable lithium and loss of capacity. Thus, while the inclusion of electrolyte additives can modify the SEI to improve cell life, including offsetting some of the degraded life effects due to the presence of MA, the increased SEI film resistance that accompanies the improved life may increase the cell resistance as the movement of lithium ions through the more stable SEI is more resisted, which may adversely affect high rate charge / discharge performance, especially at low temperatures.

[0011] The present inventors have discovered a non-aqueous electrolyte that provides an unexpected improved combination of high rate performance at low temperatures while providing enhanced stability at high temperatures.

[0012] The non-aqueous electrolyte includes an organic solvent and a plurality of lithium salts dissolved in the organic solvent, including lithium difluorophosphate (LDFP) in an amount of 0.25 to 4.5 wt % relative to the total weight of the non-aqueous electrolyte, lithium bis(oxalato)borate (LiBOB) in an amount of 0.25 to 3 wt % relative to the total weight of the non-aqueous electrolyte, and lithium hexafluorophosphate.

[0013] The lithium salts preferably contain 0.3 to 3 wt% LDFP and 0.3 to 2.5 wt% LiBOB, or 0.5 to 2.5 wt% or 1.5 to 2.5 wt% LDFP and 0.3 to 1.5 wt% or 0.3 to 0.8 wt% LiBOB, based on the total weight of the non-aqueous electrolyte. Lithium hexafluorophosphate can be present in the non-aqueous electrolyte in an amount of 0.3 to 1.5 mol / liter (M), 0.6 to 1.2 M, or 0.7 to 1 M.

[0014] The organic solvent contains 10% by weight, preferably more than 10 to 50% by weight, 15 to 50% by weight, 20 to 50% by weight, 25 to 40% by weight, or 28 to 32% by weight of methyl acetate based on the total weight of the organic solvent. In one embodiment, the organic solvent contains 10 to 40% by weight, 15 to 35% by weight, 20 to 32% by weight, or 22 to 30% by weight of methyl acetate based on the total weight of the organic solvent.

[0015] The organic solvent comprises a carbonate solvent. The carbonate solvent comprises ethylene carbonate (EC) and may comprise dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate (EMC), propylene carbonate (PC), butylene carbonate, or a combination thereof. Based on the total weight of the organic solvent, the carbonate solvent can be present in an amount of 50-90 wt%, 50-75 wt%, 50-80 wt%, 60-75 wt%, or 68-72 wt%. In one embodiment, the carbonate solvent comprises 10-50 wt%, 15-50 wt%, or 17-35 wt% ethylene carbonate based on the total weight of the organic solvent. The organic solvent may consist of methyl acetate and a carbonate solvent.

[0016] As a specific example, the organic solvent contains 20-40% by weight of methyl acetate, 10-30% by weight of ethylene carbonate, 10-30% by weight of dimethyl carbonate, and 15-35% by weight of ethyl methyl carbonate, based on the total weight of the organic solvent. As another specific example, the organic solvent contains 20-25% by weight or 25-35% by weight of methyl acetate, 15-30% by weight of ethylene carbonate, 15-25% by weight of dimethyl carbonate, and 20-30% by weight of ethyl methyl carbonate, based on the total weight of the organic solvent.

[0017] In one embodiment, the total weight of MA and DMC in the organic solvent is 45 to 90% by weight, 50 to 80% by weight, or 50 to 70% by weight, based on the total weight of the organic solvent.

[0018] The non-aqueous electrolyte may further include vinylene carbonate (VC), which may be present in an amount of 0.25 to 3 wt %, 0.5 to 3 wt %, 0.5 to 2.5 wt %, or 1 to 2 wt %, based on the total weight of the non-aqueous electrolyte.

[0019] In one embodiment, the non-aqueous electrolyte comprises an organic solvent, a plurality of lithium salts dissolved in the organic solvent, and 0.5 to 3 wt % or 1 to 2 wt % of VC relative to the total weight of the non-aqueous electrolyte, the organic solvent comprises 20 to 40 wt % or 25 to 35 wt % of methyl acetate, 10 to 30 wt % or 15 to 30 wt % of ethylene carbonate, 10 to 30 wt % or 15 to 25 wt % of dimethyl carbonate, and 15 to 35 wt % or 20 to 30 wt % of ethyl methyl carbonate, each based on the total weight of the organic solvent, and the lithium salt comprises 1 to 3 wt % or 1.5 to 2.5 wt % of LDFP relative to the total weight of the non-aqueous electrolyte, 0.25 to 1 wt % or 0.3 to 0.8 wt % of LiBOB relative to the total weight of the non-aqueous electrolyte, and further comprises 0.6 to 1.2 M or 0.7 to 1 M of lithium hexafluorophosphate.

[0020] The non-aqueous electrolyte can be prepared by contacting an organic solvent with the lithium salts and the optional vinylene carbonate. For example, the lithium salts and the optional vinylene carbonate can be added to the organic solvent separately or in combination.

[0021] The capacity retention of cells with non-aqueous electrolyte is greater than 94%, or greater than 95%, after being held charged at 71° C. for 5 days.

[0022] In one aspect, a lithium ion cell, such as a lithium ion battery, includes the non-aqueous electrolyte described herein. The cell includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and the non-aqueous electrolyte described above.

[0023] The positive electrode includes a positive electrode active material that is not particularly limited. The positive electrode preferably includes, as an active material, lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium nickel cobalt manganese oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), lithium nickel cobalt oxide (LCO), lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP), or a combination thereof. The positive electrode may include a metal current collector. The current collector for the positive electrode may include aluminum.

[0024] The negative electrode may include a negative electrode active material such as graphite, hard carbon, silicon, or a combination thereof.

[0025] The negative electrode may include a metal current collector. The negative electrode current collector may include copper.

[0026] The positive electrode and the negative electrode may each independently further comprise a binder, a conductive agent, or a combination thereof.

[0027] Representative binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene-rubber, fluororubber, or copolymers thereof. Combinations comprising at least one of these may also be used.

[0028] Representative conductive agents include Ketjen black, carbon black, graphite, carbon nanotubes, carbon fibers, mesoporous carbon, mesocarbon microbeads, oil furnace black, superconducting black, acetylene black, lamp black, etc. Combinations comprising at least one of these may also be used.

[0029] The cell includes a microporous separator disposed between the cathode and anode. Microporous polyolefin membranes including polypropylene, polyethylene, or copolymers thereof may be used.

[0030] The cell may be fabricated by preparing a non-aqueous electrolyte and adding the non-aqueous electrolyte to an assembly having a cathode, an anode, and a separator between the anode and cathode.

[0031] The above and other features are illustrated by the following examples in which, unless otherwise specified, the percentages of components are by weight based on the total weight of the composition.

[0032] Working Example The cells used in Examples 1-5 and Comparative Examples 1-6 used lithium nickel cobalt aluminum oxide as the positive electrode active material, graphite as the negative electrode active material, and a microporous polypropylene separator. Each cell was filled with electrolyte, sealed under vacuum, and restrained between flat plates prior to cycling. Each cell had an initial capacity of approximately 14 milliamp hours.

[0033] The base electrolyte contained lithium hexafluorophosphate, 22.5 wt % of EC, 20 wt % of DMC, 27.5 wt % of EMC, and 30 wt % of MA based on the total weight of organic carbonates (ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC)) and methyl acetate (MA). The concentration of lithium hexafluorophosphate in the base electrolyte was 0.85 molar (M). The electrolytes used in Examples 1 to 5 and Comparative Examples 1 to 6 were prepared by adding lithium difluorophosphate (LDFP), lithium bis(oxalato)borate (LiBOB), vinylene carbonate (VC), fluoroethylene carbonate (FEC), or lithium bis(trifluoromethanesulfonimide) (LFSI) to the base electrolyte. The compositions of the electrolytes in Examples 1 to 5 and Comparative Examples 1 to 6 are shown in Tables 1 and 2. The content of LDFP, LiBOB, or VC added for these examples and comparative examples is based on the weight of the base electrolyte, i.e., 0.85 mole (M) lithium hexafluorophosphate, 22.5 wt% EC, 20 wt% DMC, 27.5 wt% EMC, and 30 wt% MA, where 100 g of base electrolyte solution containing organic solvent and LiPF6 contains 1 wt% LDFP and 1 wt% LiBOB, and has 1 g of LDFP and LiBOB, respectively, for a total weight of 102 g of solution. In other words, the percentage of additives in Tables 1 and 2 can be considered as parts by weight per 100 parts by weight of base electrolyte.

[0034] [Table 1]

[0035] [Table 2]

[0036] The cells were initially charged and cycled between 4.1V and 3V at a rate of 7 milliamps (mA). After charging to 4.1V at 30°C, the cells were cooled to -17°C, allowed to reach thermal equilibrium, and then discharged using a 0.6W pulse for 0.5 seconds. Each cell was then pulse discharged in a pulse discharge thermal loop at different temperatures according to Table 3. Each pulse discharge cycle consisted of one 0.4C pulse for 15 seconds, five 4C pulses for 4 seconds, one 0.0165 Ampere-hour (Ah) discharge at 1C, one 5C discharge for 15 seconds, and a charge to 4.1V at C / 2. After the pulse discharge thermal loop, the cells reached thermal equilibrium at 30°C, charged to 4.1V, cooled to -17°C, allowed to reach thermal equilibrium, and a 0.6W pulse discharge for 0.5 seconds was performed. The cell was then charged to 4.1 V and stored at open circuit for 5 days at 71° C. After storage at 71° C., the cell was recharged to 4.1 V at 30° C., cooled to −17° C., allowed to reach thermal equilibrium, and discharged using a 0.6 W pulse of 0.5 s.

[0037] [Table 3]

[0038] Effect of electrolyte additives For Examples 1 and 2 and Comparative Examples 1 to 6, the initial capacity, the capacity after pulse discharge thermal loop before and after charge holding at 71° C., and the capacity retention are shown in Table 4. The capacity was determined by discharge at C / 2.

[0039] [Table 4]

[0040] The results in Table 4 show that the capacity retention of the cell with LDFP (Comparative Example 1) is 93.4%, and that the capacity retention of the cell with the combination of LDFP and LiBOB is improved to 96% (Example 1). The capacity retention of the cell with the combination of LDFP, LiBOB, and VC (Example 2) is further improved to 97.2%, while the capacity retention of the comparative cell with LDFP and VC but without LiBOB (Comparative Example 3), the cell with LiBOB and VC but without LDFP (Comparative Examples 5 and 6), and the cell with LDFP as the only additive (Comparative Example 1) is about 93%, 93.7%, 94.3%, and 93.4%, respectively. These results show that the combination of MA with LDFP and LiBOB unexpectedly improves the high temperature stability (capacity retention after thermal loop cycling and charge hold at 71° C.), and that the high temperature stability can be further improved by adding VC.

[0041] The results in Table 4 also show that FEC (Comparative Example 2) and LFSI (Comparative Example 4), when combined with LDFP, worsen rather than improve capacity retention compared to using LDFP alone. The adverse effects of FEC and LFSI demonstrate the unpredictability of the synergistic effects observed when using MA with LDFP and LiBOB.

[0042] 1A and 1B, and 2A and 2B show cell voltage at the end of a 15 second 5C pulse discharge versus cycle number at 30°C, 0°C, -26°C, and 55°C. The electrolytes of the example compositions (FIGS. 1A and 1B) and the comparative compositions (FIGS. 2A and 2B) are separated in the graphs for clarity. As shown in these figures, the results have overlapping symbols. As shown in FIG. 1A and 1B, FIG. 2A and 2B, the cells with LDFP have a higher voltage at low temperature compared to the cells without LDFP. As shown in Table 4, the cells with the disclosed electrolyte also have improved capacity retention after charge hold at 71°C.

[0043] FIG. 3 shows the voltage at the end of a 0.5 second 0.6 W pulse discharge at −17° C. initially, after the pulse discharge thermal loop, and after the pulse discharge thermal loop and an additional 130 hours of storage at 71° C. These results show that the use of a combination of MA, LDFP, and LiBOB, as in Examples 1 and 2, improves the cell voltage at the end of the pulse compared to cells without such a combination, e.g., using electrolytes with only VC and LiBOB (Comparative Examples 5 and 6). Thus, the combination of LDFP and LiBOB (Example 1) and the combination of LDFP, LiBOB, and VC (Example 2) achieve both good capacity retention (Table 4) and improved cell voltage during pulse discharge (FIG. 3). These results show that the use of MA together with LDFP, LiBOB, and optionally VC can provide high power output at low temperatures while also providing improved high temperature capacity retention.

[0044] The synergistic effect of MA, LDFP, and LiBOB is further illustrated in 5.5 Ah prismatic cells in Examples 3 to 5, compared to Comparative Example 6. Materials are otherwise similar to Example 1.

[0045] For Examples 3 to 5 and Comparative Example 6, the cells were charged to 4.1 V at 30° C., and then the cell voltages were determined at the end of a 119 A pulse discharge for 0.5 seconds at −17° C., and at the end of a 29.5 A discharge for 15 seconds at −40° C. The cells were then cycled at 55° C. for 2 weeks. Each cycle at 55° C. was 1. 2.75A charge up to 4.1V, 0.11A hold at 4.1V; 2. Open circuit for 10 minutes, 3. 1Ah discharge at 11A, 4. 10 minutes open circuit It consists of:

[0046] Every two weeks (approximately 400 cycles), the cell voltage at the end of a 119 A pulse discharge for 0.5 seconds at -17°C (Figure 4) and at the end of a 29.5 A discharge for 15 seconds at -40°C (Figure 5) were determined. The results in Figures 4 and 5 show that at both -17°C and -40°C, the cells using the electrolytes of the examples (Examples 3 to 5) achieved higher voltages at the end of each pulse discharge compared to Comparative Example 6.

[0047] Examples 6 to 17 and Comparative Examples 7 to 12 Examples 6-17 and Comparative Examples 7-12 further demonstrate the effect of LDFP, LiBOB, and MA in improving high temperature stability and low temperature pulse discharge voltage when using various organic carbonates. The cells used in these examples are multi-layer pouch cells (Pred Materials International) with a capacity of about 170 mAh. The cells contained an NCA cathode, an artificial graphite anode, and a microporous polyethylene separator with a 4 μm ceramic coating on the cathode side. The dry cells were filled with 1 mL of electrolyte and sealed under vacuum. The cells were then cycled between 4.2 V and 3 V, degassed, and resealed under vacuum.

[0048] Tests of these cells were performed at 25°C and -30°C at 2 week intervals. At both temperatures, the cells were charged to 4.2V at 34mA (C / 5) and held constant until the current dropped to 8.5mA before discharging. Tests at 25°C were discharged to 3.0V at C / 5 rate. Tests at -30°C were discharged at 17mA (C / 10) for 10 seconds at 170mA (1C) at 95% state of charge, and continued to discharge to 3.0V after the 10 second pulse. The total capacity at the time of discharge from 4.2 to 3.0V and the voltage at the end of the 10 second 170mA pulse were recorded. The cells were then charged to 4.2V at C / 5 and held constant until the current dropped to C / 20. The cells were then stored at open circuit voltage (OCV) at 55°C for 13 days or for 2 weeks if the test was to be repeated.

[0049] For Examples 6-10 and Comparative Examples 7-11, the base electrolyte contained 22.5 wt% EC, 20 wt% DMC, 27.5 wt% EMC, and 30 wt% MA based on the total weight of 0.85 M lithium hexafluorophosphate, organic carbonates (ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC)), and methyl acetate (MA). LDFP, LiBOB, and VC were added to the base electrolyte to prepare the electrolytes shown in Table 5. The results are summarized in FIG. 6.

[0050] [Table 5]

[0051] Comparative examples containing only LDFP (Comparative Example 7) or only LiBOB (Comparative Example 8) as additives were inferior to Examples 6-10 containing both LDFP and LiBOB. Comparative Example 7 containing 2% LDFP and no LiBOB had a capacity retention at 25°C after storage at 55°C of 170.4mAh, which was less than the corresponding values ​​for Examples 6-10 which ranged from 172.1mAh to 176.9mAh. At -30°C, the C / 10 capacity of Comparative Example 7 was reduced by 5.7mAh relative to the initial capacity, whereas Examples 6-10 did not increase by 0.2mAh or reduce by a maximum of 4.8mAh relative to the initial capacity, demonstrating the stabilizing effect of the combination of LDFP and LiBOB. Comparative Example 8 containing LiBOB and no LDFP had a pulse voltage of 2.99V at the end of storage at 55°C, which was less than the comparative values ​​of Examples 6-10, 3.01V to 3.23V. Also, the capacity of Comparative Example 8 at -30°C after storage at 55°C was 137.2mAh, which is smaller than the corresponding values ​​of Examples 6 to 10 (which vary between 137.7mAh and 147.8mAh). The electrolytes with LDFP and LiBOB concentrations of 7% by weight and 5% by weight, respectively (Comparative Examples 9 and 10) did not dissolve completely. Comparative Example 11 had a capacity of 124.4mAh at -30°C and a pulse end voltage of 2.91V after storage at 55°C. These are all smaller than the comparative values ​​of Examples 6 to 10, showing the effect of LDFP concentrations greater than 4.5%, for example 5%.

[0052] As shown in Table 6, Examples 11-13 each contained the same amount of LDFP, LiBOB, vinylene carbonate (VC), and 30 wt% MA, but each contained a different combination of carbonates. The concentration of lithium hexafluorophosphate in the base electrolyte was 0.85 molar (M).

[0053] [Table 6]

[0054] Results before and after storage at 55°C are shown in Figure 7. The results show that unexpected improvements in capacity and pulse discharge capability after storage at 55°C are achieved for various carbonate compositions shown in Examples 11-13. Examples 11 and 12 show that DEC can also be used as an organic carbonate. Examples 12 and 13 further demonstrate that quaternary and binary organic carbonate mixtures, respectively, are suitable organic carbonate mixtures.

[0055] Examples 4 and 14-16 and Comparative Examples 11 and 12 each have the same ratio of organic carbonate (EC:DMC:EMC weight ratio 22.5:20:27.5), and LDFP, LiBOB, and VC were kept constant at 2%, 0.5%, and 1.5% as percentages of organic carbonate, methyl acetate, and LiPF6 salt, respectively. The concentration of methyl acetate differs between these Examples and Comparative Examples, and the percentage of methyl acetate shown in Table 7 is the percentage relative to the combined weight of methyl acetate and organic carbonate. The concentration of lithium hexafluorophosphate in the base electrolyte is 0.85 molar (M).

[0056] [Table 7]

[0057] The results are shown in Figures 8 to 10. As shown in Figures 8 and 9, the cells with MA of more than 10 wt% (Example 4, Examples 14 to 17, and Comparative Example 12) have a higher discharge capacity and a higher voltage at the end of a 10-second 1C pulse at -30°C than cells with MA concentrations of 10 wt% or less, such as Example 14 and Comparative Example 11. As the MA content increased to less than 40%, the cells had better performance at -30°C (higher voltage during pulse, higher capacity), but the capacity loss at 25°C also increased (Example 4, Examples 14 to 15, Comparative Example 11). When MA was contained at 40 wt%, 50 wt%, or 60 wt% (Example 16, Example 17, and Comparative Example 12), the performance at -30°C did not change much, but the capacity retention at 25°C continued to decrease with increasing MA. The capacity retention at room temperature (FIG. 10) was greatest for cells with electrolytes with MA concentrations below 40% (Example 4, Example 14, Example 15, and Comparative Example 11). These results indicate that MA reduces cell life and improves low temperature performance. These results also indicate that MA concentration with this additive combination balances low temperature performance and long term life. These results indicate that increasing the MA concentration up to below 40% can improve low temperature performance with minimal loss to capacity retention, and that MA concentrations of 40%, 50%, and 60% are detrimental to capacity retention at ambient temperature after storage at elevated temperatures without significantly improving low temperature performance.

[0058] Example 4 and Comparative Example 13 had the same composition, except that Comparative Example 13 had 30 wt% ethyl propionate (EP) instead of 30 wt% MA in Example 4. Comparative Example 14 had 0.5 wt% LiPF2(oxalate)2 as an additive instead of LiBOB in Example 4. The results are shown in Figures 11 to 13. Figures 11 and 12 show that the electrolyte containing MA and LiBOB has excellent capacity retention and voltage response at 1C pulse at -30°C when aged at 55°C. The electrolyte containing EP (Comparative Example 13) has excellent room temperature capacity, but when the cell is aged at 55°C, the capacity retention and voltage response at -30°C are worse than the cell with MA. The electrolyte containing LiPF2(oxalate)2 (Comparative Example 14) has room temperature capacity almost the same as the cell containing LiBOB even after aging, but when the cell is aged at 55°C, the capacity retention and voltage response at -30°C are worse than the cell containing LiBOB.

[0059] Various aspects of the present disclosure are set forth below.

[0060] Aspect 1 A non-aqueous electrolyte comprising an organic solvent comprising a carbonate solvent, 10% by weight or more than 10% by weight to 50% by weight of methyl acetate, based on the total weight of the organic solvent, and a plurality of lithium salts dissolved in the organic solvent, the plurality of lithium salts comprising 0.25 to 4.5% by weight of lithium difluorophosphate, 0.25 to 3% by weight of lithium bis(oxalato)borate, and lithium hexafluorophosphate, based on the total weight of the non-aqueous electrolyte.

[0061] Aspect 2 The non-aqueous electrolyte according to any one of the above embodiments, wherein the plurality of lithium salts include lithium difluorophosphate in an amount of 0.3 to 3 wt % relative to the total weight of the non-aqueous electrolyte, lithium bis(oxalato)borate in an amount of 0.3 to 2.5 wt % relative to the total weight of the non-aqueous electrolyte, and lithium hexafluorophosphate, and the lithium hexafluorophosphate may be present in the non-aqueous electrolyte in an amount of 0.3 to 1.5 M, 0.6 to 1.2 M, or 0.7 to 1 M.

[0062] Aspect 3 The non-aqueous electrolyte according to any one of the above embodiments, wherein the plurality of lithium salts include lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate.

[0063] Aspect 4 The non-aqueous electrolyte according to any one of the above embodiments, wherein the organic solvent contains 20 to 40% by weight of methyl acetate based on the total weight of the organic solvent.

[0064] Aspect 5 The non-aqueous electrolyte according to any one of the above embodiments, wherein the carbonate solvent is present in an amount of 60 to 80% by weight based on the total weight of the organic solvent.

[0065] Aspect 6 The nonaqueous electrolyte according to any one of the above embodiments, wherein the carbonate solvent comprises ethylene carbonate and may comprise dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, or a combination thereof, and the ethylene carbonate is present in an amount of 10 to 50% by weight, based on the total weight of the organic solvent.

[0066] Aspect 7 The nonaqueous electrolyte according to any one of the above embodiments, wherein the organic solvent contains 20 to 40 wt % of methyl acetate, 10 to 30 wt % of ethylene carbonate, 10 to 30 wt % of dimethyl carbonate, and 15 to 35 wt % of ethyl methyl carbonate, each of which is based on the total weight of the organic solvent.

[0067] Aspect 8 The non-aqueous electrolyte according to any one of the preceding aspects, further comprising vinylene carbonate.

[0068] Aspect 9 The non-aqueous electrolyte according to any one of the above embodiments, wherein the vinylene carbonate is present in an amount of 0.5 to 3% by weight based on the total weight of the non-aqueous electrolyte.

[0069] Aspect 10 The non-aqueous electrolyte according to any of the preceding embodiments, wherein a cell having the non-aqueous electrolyte has a capacity retention of greater than 94% after being held at 100% charged state for 5 days at 71° C.

[0070] Aspect 11 A lithium ion cell having a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte, the non-aqueous electrolyte comprising an organic solvent including a carbonate solvent, and 10% by weight or more than 10% by weight to 50% by weight of methyl acetate relative to the total weight of the organic solvent, and a plurality of lithium salts dissolved in the organic solvent, the plurality of lithium salts including 0.25 to 4.5% by weight of lithium difluorophosphate relative to the total weight of the non-aqueous electrolyte, 0.25 to 3% by weight of lithium bis(oxalato)borate relative to the total weight of the non-aqueous electrolyte, and lithium hexafluorophosphate.

[0071] Aspect 12 The lithium ion cell according to any one of the above embodiments, wherein the plurality of lithium salts include lithium difluorophosphate in an amount of 0.25 to 4.5 wt % based on the total weight of the non-aqueous electrolyte, lithium bis(oxalato)borate in an amount of 0.25 to 3 wt % based on the total weight of the non-aqueous electrolyte, and lithium hexafluorophosphate, and the lithium hexafluorophosphate may be present in the non-aqueous electrolyte in an amount of 0.3 to 1.5 M, 0.6 to 1.2 M, or 0.7 to 1 M.

[0072] Aspect 13 The lithium-ion cell according to any of the above embodiments, wherein the plurality of lithium salts consists of lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate.

[0073] Aspect 14 The lithium ion cell according to any one of the above aspects, wherein the organic solvent contains 20 to 40% by weight of methyl acetate, based on the total weight of the organic solvent.

[0074] Aspect 15 The lithium-ion cell of any of the above aspects, wherein the carbonate solvent comprises ethylene carbonate, and may comprise dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, or a combination thereof, and the ethylene carbonate is present in an amount of 10 to 50 wt %, based on the total weight of the organic solvent.

[0075] Aspect 16 The lithium-ion cell according to any of the above embodiments, wherein the carbonate solvent is present in an amount of 60 to 80% by weight, based on the total weight of the organic solvent.

[0076] Aspect 17 The lithium ion cell according to any one of the above embodiments, wherein the organic solvent contains 20 to 40 wt % of methyl acetate, 10 to 30 wt % of ethylene carbonate, 10 to 30 wt % of dimethyl carbonate, and 15 to 35 wt % of ethyl methyl carbonate, each of which is based on the total weight of the organic solvent.

[0077] Aspect 18 4. The lithium ion cell according to any of the preceding aspects, wherein the non-aqueous electrolyte further comprises vinylene carbonate.

[0078] Aspect 19 The lithium ion cell according to any of the above embodiments, wherein the vinylene carbonate is present in an amount of 0.5 to 3% by weight, based on the total weight of the non-aqueous electrolyte.

[0079] Aspect 20 The lithium ion cell of any of the preceding aspects, wherein the lithium ion cell has a capacity retention of greater than 94% after being held charged at 71° C. for 5 days.

[0080] Aspect 21 A method for preparing a non-aqueous electrolyte, comprising the steps of: preparing an organic solvent comprising a carbonate solvent and 10% by weight, or greater than 10% to 50% by weight, of methyl acetate, based on the total weight of the organic solvent; preparing a plurality of lithium salts including lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate; and contacting the organic solvent with the plurality of lithium salts to prepare a non-aqueous electrolyte, wherein lithium difluorophosphate is present in an amount of 0.25 to 4.5% by weight, and lithium bis(oxalato)borate is present in an amount of 0.25 to 3% by weight, based on the total weight of the non-aqueous electrolyte.

[0081] Aspect 22 A method for producing a lithium ion cell, comprising: preparing the nonaqueous electrolyte according to any one of the above aspects; and adding the nonaqueous electrolyte to an assembly having a cathode, an anode, and a separator between the cathode and the anode, to produce a lithium ion cell.

[0082] The compositions, methods, and articles of the present disclosure can alternatively comprise, consist, or consist essentially of any suitable materials, steps, or ingredients described in the present disclosure. The compositions, methods, and articles of the present disclosure can additionally, or alternatively, be made to be free, or substantially free, of materials (or species), steps, or ingredients that are not necessary to the function or achievement of the purpose of the compositions, methods, and articles.

[0083] All ranges described in this disclosure are inclusive of the endpoints, and the endpoints can be combined independently of each other (e.g., the range "up to 25 wt.%, or, more specifically, 5 wt.% to 20 wt.%" is inclusive of the endpoints and includes all intermediate values ​​in the range "5 wt.% to 25 wt.%, etc.). "Combination" includes blends, mixtures, alloys, reaction products, and the like. The terms "a", "an", and "the" do not denote limitations of quantity and are intended to be inclusive of both the singular and plural unless otherwise indicated or clearly contradicted by context. "Or" means "and / or" unless expressly indicated otherwise. "Combinations of these" are open and include any combination containing at least one of the recited components or properties, optionally with similar or equivalent unrecited components or properties.

[0084] All patents, patent applications, and other documents cited are incorporated herein by reference in their entirety. If a term in this application contradicts or conflicts with a term in an incorporated document, the term in this application takes precedence over the conflicting term in the incorporated document.

[0085] While the embodiments have been set forth for illustrative purposes, the above description should not be deemed to be limiting on the scope of the present specification. Thus, various modifications, adaptations, and substitutions may occur to those skilled in the art without departing from the spirit and scope of the present specification.

Claims

1. an organic solvent comprising a carbonate solvent and 10 to 50% by weight of methyl acetate based on the total weight of the organic solvent; a plurality of lithium salts dissolved in the organic solvent, The lithium salts are 0.25 to 4.5% by weight of lithium difluorophosphate based on the total weight of the nonaqueous electrolyte; 0.25 to 3 wt % of lithium bis(oxalato)borate based on the total weight of the nonaqueous electrolyte; and lithium hexafluorophosphate.

2. The lithium salts are 0.3 to 3 wt % of lithium difluorophosphate based on the total weight of the nonaqueous electrolyte; 0.3 to 2.5 wt % of lithium bis(oxalato)borate based on the total weight of the nonaqueous electrolyte; Lithium hexafluorophosphate, The non-aqueous electrolyte according to claim 1 .

3. The lithium salts include lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate. The non-aqueous electrolyte according to claim 1 .

4. The organic solvent comprises 20 to 40% by weight of the methyl acetate based on the total weight of the organic solvent. The non-aqueous electrolyte according to claim 1 .

5. The carbonate solvent is present in an amount of 60 to 80% by weight based on the total weight of the organic solvent; The non-aqueous electrolyte according to claim 4 .

6. The carbonate solvent comprises ethylene carbonate, and may comprise dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, or combinations thereof; The ethylene carbonate is present in an amount of 10 to 50% by weight based on the total weight of the organic solvent. The non-aqueous electrolyte according to claim 1 .

7. The organic solvent is, based on the total weight of the organic solvent, 20 to 40% by weight of said methyl acetate; 10 to 30% by weight of ethylene carbonate; 10 to 30% by weight of dimethyl carbonate; 15 to 35% by weight of ethyl methyl carbonate; The non-aqueous electrolyte according to claim 1 .

8. Further comprising vinylene carbonate, The non-aqueous electrolyte according to claim 1 .

9. The vinylene carbonate is present in an amount of 0.5 to 3 wt %, based on the total weight of the non-aqueous electrolyte. The non-aqueous electrolyte according to claim 8 .

10. The cell having the non-aqueous electrolyte has a capacity retention of greater than 94% after being held at 100% charge for 5 days at 71° C. The non-aqueous electrolyte according to claim 1 .

11. A positive electrode and A negative electrode; a separator interposed between the positive electrode and the negative electrode; A non-aqueous electrolyte; 1. A lithium ion cell having The non-aqueous electrolyte is an organic solvent comprising a carbonate solvent and 10 to 50% by weight of methyl acetate based on the total weight of the organic solvent; a plurality of lithium salts dissolved in the organic solvent; The lithium salts are 0.25 to 4.5% by weight of lithium difluorophosphate based on the total weight of the nonaqueous electrolyte; 0.25 to 3 wt % of lithium bis(oxalato)borate based on the total weight of the nonaqueous electrolyte; and lithium hexafluorophosphate.

12. The lithium salts are 0.25 to 4.5% by weight of lithium difluorophosphate based on the total weight of the nonaqueous electrolyte; 0.25 to 3 wt % of lithium bis(oxalato)borate based on the total weight of the nonaqueous electrolyte; Lithium hexafluorophosphate, 12. The lithium ion cell of claim 11.

13. The lithium salts include lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate.

12. The lithium ion cell of claim 11.

14. The organic solvent comprises 20 to 40% by weight of the methyl acetate based on the total weight of the organic solvent.

12. The lithium ion cell of claim 11.

15. The carbonate solvent comprises ethylene carbonate, and may comprise dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, propylene carbonate, butylene carbonate, or combinations thereof; The ethylene carbonate is present in an amount of 10 to 50% by weight based on the total weight of the organic solvent.

12. The lithium ion cell of claim 11.

16. the carbonate solvent is present in an amount of 60 to 80% by weight based on the total weight of the organic solvent; 16. The lithium ion cell of claim 15.

17. The organic solvent is, based on the total weight of the organic solvent, 20 to 40% by weight of said methyl acetate; 10 to 30% by weight of ethylene carbonate; 10 to 30% by weight of dimethyl carbonate; 15-35 wt. % ethyl methyl carbonate.

18. The non-aqueous electrolyte further contains vinylene carbonate.

12. The lithium ion cell of claim 11.

19. The vinylene carbonate is present in an amount of 0.5 to 3 wt %, based on the total weight of the non-aqueous electrolyte.

20. The lithium ion cell of claim 18.

20. The lithium ion cell has a capacity retention of greater than 94% after being held charged at 71° C. for 5 days.

12. The lithium ion cell of claim 11.

21. providing an organic solvent comprising a carbonate solvent and 10-50 wt. % methyl acetate based on the total weight of the organic solvent; Providing a plurality of lithium salts including lithium difluorophosphate, lithium bis(oxalato)borate, and lithium hexafluorophosphate; and contacting the organic solvent with the plurality of lithium salts to prepare a nonaqueous electrolyte, The lithium difluorophosphate is present in an amount of 0.25 to 4.5 wt % and the lithium bis(oxalato)borate is present in an amount of 0.25 to 3 wt %, each based on the total weight of the non-aqueous electrolyte. A method for preparing a non-aqueous electrolyte.

22. A step of preparing the nonaqueous electrolyte according to claim 1; and adding the non-aqueous electrolyte to an assembly including a negative electrode, a positive electrode, and a separator between the negative electrode and the positive electrode to manufacture a lithium ion cell. How lithium-ion cells are manufactured.

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