Propylene carbonate-based electrolyte for lithium ion battery and lithium ion battery

Abstract

This invention provides an electrolyte for lithium-ion batteries based on propylene carbonate and a lithium-ion battery. The electrolyte includes a lithium salt, an organic solvent, and a first additive; wherein the organic solvent includes propylene carbonate; and the first additive is selected from at least one aromatic compound. The use of an aromatic compound as the first additive in the electrolyte of this invention has two advantages: firstly, the reduction potential of aromatic compounds is higher than the decomposition potential of polypropylene carbonate (PC), allowing the aromatic compound to form a stable SEI film on the negative electrode surface during the first charge-discharge process; secondly, the introduction of the aromatic compound also alters the properties of the lithium... + The solvation structure enables Li to be realized when PC is used as the main solvent. + It can be reversibly inserted and extracted into graphite. The electrolyte also includes a second additive, which, when used in conjunction with the first additive, can enhance the protective effect of the first additive on the negative electrode and improve cycle performance.

Description

Technical Field

[0001] This invention relates to a lithium-ion battery electrolyte based on propylene carbonate with high electrochemical compatibility and a lithium-ion battery including the electrolyte, belonging to the technical field of lithium-ion battery electrolytes. Background Technology

[0002] In recent years, lithium-ion batteries have been widely used in 3C products, electric vehicles, and other fields. However, the temperature of the operating environment still has a significant impact on the performance of lithium-ion batteries. For example, when the operating temperature is too low, the conductivity of the electrolyte will decrease significantly, the SEI film impedance will increase, and the transmission impedance of lithium ions in the electrodes will increase. This is because most electrolytes currently used are ethylene carbonate (EC) based electrolytes, and EC viscosity increases or even solidifies at low temperatures, resulting in a decrease in electrolyte conductivity.

[0003] Propylene carbonate (PC) possesses characteristics such as low melting point, high boiling point, wide operating temperature range, and wide electrochemical window. Replacing EC solvent with PC holds promise for improving the battery's operating temperature range and safety. However, PC readily reacts with Li... + Co-intercalation occurs in the graphite negative electrode, causing the graphite layer to peel off, which affects the use of the battery.

[0004] Currently, in existing commercial lithium-ion batteries, polycarbonate (PC) can only be added to the electrolyte in small amounts to improve the battery's high and low temperature performance, and cannot replace electrolyte polymer (EC) as the main solvent of the electrolyte. Therefore, exploring methods to inhibit PC intercalation into the graphite layer, thereby enabling PC to be used as the main solvent of the electrolyte, is of great guiding significance for the development of wide-temperature-range, high-safety lithium-ion batteries. Summary of the Invention

[0005] To improve the existing propylene carbonate's tendency to react with Li + To address issues such as the problem of graphite layer peeling due to co-intercalation in the graphite anode, rendering it unsuitable as the primary solvent for the electrolyte, this invention provides a lithium-ion battery electrolyte based on propylene carbonate (PC) with high electrochemical compatibility, and a lithium-ion battery incorporating this electrolyte. The use of this electrolyte enables the lithium-ion battery to exhibit a wide temperature range and high safety.

[0006] The present invention adopts the following technical solution:

[0007] An electrolyte comprising a lithium salt, an organic solvent, and a first additive; wherein the organic solvent comprises propylene carbonate; and the first additive is selected from at least one aromatic compound.

[0008] According to an embodiment of the present invention, the electrolyte further comprises a second additive, the second additive being selected from one or more of vinyl ethylene carbonate, lithium difluorophosphate, lithium difluorobis(oxalato) phosphate, dimethyl maleic anhydride, succinic anhydride, triargyl propargyl phosphate, ethoxypentafluorophosphazene, phenoxypentafluorophosphazene, tris(trimethylsilane)borate, tris(trimethylsilane) phosphate, ethylene ethylene carbonate, vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, vinyl sulfate, methanedisulfonate methylenecyclohexane, succinate, adiponitrile, glutaronitrile, 1,3,6-hexanetrionitrile, ethylene glycol bis(propionitrile) ether, and 1,2,3-tri-(2-cyanoethoxy)propane.

[0009] According to an embodiment of the present invention, the aromatic compound has the structure shown in Formula 1 or Formula 2:

[0010]

[0011] In Formula 1, X is selected from N or C-R6, and R6 is selected from any one of hydrogen atom, halogen atom, nitro group, alkyl group (e.g., alkyl group has 1 to 20 carbon atoms) or haloalkyl group (e.g., alkyl group has 1 to 20 carbon atoms).

[0012] R1 and R5 may be the same or different, and are independently selected from any one of hydrogen atoms, halogen atoms, nitro, alkyl (e.g., alkyl with 1 to 20 carbon atoms), alkoxy (e.g., alkoxy with 1 to 20 carbon atoms), haloalkyl (e.g., alkyl with 1 to 20 carbon atoms), phenyl, phenyl derivatives or N-containing 5- or 6-membered heterocyclic substituents;

[0013] R3 is selected from any one of hydrogen atom, halogen atom, nitro group, haloalkyl group (e.g., the number of carbon atoms in the alkyl group is 1 to 20), haloalkoxy group, haloalkylthio group, haloalkylsulfinyl group, and haloalkylacylsulfonyl group.

[0014] R2 and R4 may be the same or different, and are independently selected from any one of hydrogen atom, halogen atom, nitro group, alkyl group (e.g., alkyl group with 1 to 20 carbon atoms), alkoxy group (e.g., alkoxy group with 1 to 20 carbon atoms) or haloalkyl group (e.g., alkyl group with 1 to 20 carbon atoms);

[0015]

[0016] In Equation 2, R 11 R 12 R 13 R 14 R 15 R 16 R 17 and R18 They may be the same or different, and are independently selected from any one of hydrogen atoms, halogen atoms, nitro groups, alkyl groups (e.g., alkyl groups with 1 to 20 carbon atoms) or haloalkyl groups (e.g., alkyl groups with 1 to 20 carbon atoms).

[0017] According to an embodiment of the present invention, in Formula 1, X is selected from N or C-R6, R6 is selected from any one of hydrogen atom, halogen atom, alkyl (e.g., alkyl with 1 to 10 carbon atoms) or haloalkyl (e.g., alkyl with 1 to 10 carbon atoms); R1, R2, R4 and R5 are the same or different, and are independently selected from any one of hydrogen atom, nitro, halogen atom, alkyl (e.g., alkyl with 1 to 10 carbon atoms), alkoxy (e.g., alkoxy with 1 to 10 carbon atoms) or haloalkyl (e.g., alkyl with 1 to 10 carbon atoms); R3 is selected from any one of hydrogen atom, nitro, halogen atom, haloalkyl (e.g., alkyl with 1 to 10 carbon atoms), haloalkoxy, haloalkylthio, haloalkylsulfinyl, haloalkylacylsulfonyl.

[0018] According to an embodiment of the present invention, in Formula 1, X is selected from N or C-R6, and R6 is selected from any one of hydrogen atom, nitro group, halogen atom, alkyl group (e.g., alkyl group has 1 to 6 carbon atoms) or haloalkyl group (e.g., alkyl group has 1 to 6 carbon atoms); R1, R2, R3, R4 and R5 may be the same or different, and are independently selected from any one of hydrogen atom, nitro group, halogen atom or haloalkyl group (e.g., alkyl group has 1 to 6 carbon atoms).

[0019] According to an embodiment of the present invention, the aromatic compound represented by Formula 1 is selected from at least one of the following compounds:

[0020]

[0021]

[0022] According to an embodiment of the present invention, in formula 2, R 11 R 12 R 13 R 14 R 15 R 16 R 17 and R 18 They may be the same or different, and are independently selected from any one of hydrogen atoms, halogen atoms, nitro groups, alkyl groups (e.g., alkyl groups with 1 to 10 carbon atoms) or haloalkyl groups (e.g., alkyl groups with 1 to 10 carbon atoms).

[0023] According to an embodiment of the present invention, in formula 2, R 11 R 12R 13 R 14 R 15 R 16 R 17 and R 18 They may be the same or different, and are independently selected from any one of hydrogen atoms, halogen atoms, alkyl groups (e.g., alkyl groups having 1 to 6 carbon atoms) or haloalkyl groups (e.g., alkyl groups having 1 to 6 carbon atoms).

[0024] According to an embodiment of the present invention, in formula 2, R 11 R 12 R 13 R 14 R 15 R 16 R 17 and R 18 They may be the same or different, and are independently selected from either hydrogen atoms or halogen atoms.

[0025] According to an embodiment of the present invention, the aromatic compound represented by Formula 2 is selected from at least one of the following compounds:

[0026]

[0027] According to an embodiment of the present invention, the content of the first additive accounts for 0.1 to 10 wt% of the total mass of the electrolyte, for example, 0.1 wt%, 0.2 wt%, 0.5 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.7 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.4 wt%, 2.5 wt%, 2.7 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, and 10 wt%.

[0028] According to an embodiment of the present invention, the first additive may be obtained through commercial purchase or prepared using methods known in the art.

[0029] According to an embodiment of the present invention, the content of the second additive accounts for 0.1 to 25 wt% of the total mass of the electrolyte, for example, 0.1 wt%, 0.2 wt%, 0.5 wt%, 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.7 wt%, 1.8 wt%, 2 wt%, 2.2 wt%, 2.4 wt%, 2.5 wt%, 2.7 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, or 25 wt%.

[0030] According to an embodiment of the present invention, the second additive may be obtained through commercial purchase or prepared using methods known in the art.

[0031] According to embodiments of the present invention, the lithium salt includes one or more of LiPF6, LiTFSI, LiClO4, LiFSI, LiBOB, LiODFB, LiBF4 and LiAsF6.

[0032] According to an embodiment of the present invention, the molar concentration of the lithium salt is 0.5–5 mol / L. -1 For example, 1–3 mol / L -1 For example, 1 mol L -1 1.5 mol L -1 or 2 mol L -1 .

[0033] According to an embodiment of the present invention, the content of propylene carbonate is 5 to 60 wt% of the total mass of the electrolyte, for example, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, or 60 wt%.

[0034] According to embodiments of the present invention, the organic solvent further includes other solvents, which include one or more of chain carbonate organic solvents or carboxylic acid ester organic solvents; preferably, the chain carbonate organic solvents include one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), methyl propyl carbonate, and butenyl carbonate; the carboxylic acid ester organic solvents include one or more of ethyl acetate (EA), ethyl propionate, methyl acetate, propyl acetate, methyl propionate, methyl butyrate, and ethyl butyrate.

[0035] According to an embodiment of the present invention, the mass ratio of the other solvent to propylene carbonate is 40-95:60-5, for example 40:60, 45:55, 50:50, 55:45, 60:40, 65:35, 70:30, 75:25, 80:20, 85:15, 90:10 or 95:5.

[0036] The present invention also provides a method for preparing the above-mentioned electrolyte, the method comprising the following steps:

[0037] The electrolyte is prepared by mixing lithium salt, organic solvent and first additive.

[0038] The organic solvent includes propylene carbonate, and the first additive is selected from at least one aromatic compound.

[0039] Furthermore, the method includes the following steps: mixing lithium salt, organic solvent, first additive, and second additive to prepare the electrolyte;

[0040] Wherein, the organic solvent includes propylene carbonate; the first additive is selected from at least one aromatic compound; the second additive is selected from one or more of vinyl ethylene carbonate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, dimethyl maleic anhydride, succinic anhydride, triargyl propargyl phosphate, ethoxypentafluorophosphazene, phenoxypentafluorophosphazene, tris(trimethylsilane)borate, tris(trimethylsilane) phosphate, ethylene ethylene carbonate, vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, vinyl sulfate, methanedisulfonate methylenecyclohexane, succinate, adiponitrile, glutaronitrile, 1,3,6-hexanetrionitrile, ethylene glycol bis(propionitrile) ether, and 1,2,3-tri-(2-cyanoethoxy)propane.

[0041] The present invention also provides a lithium-ion battery, wherein the lithium-ion battery includes the electrolyte described above.

[0042] According to an embodiment of the present invention, the lithium-ion battery further includes a positive electrode sheet containing a positive electrode active material, a conductive agent, and a binder.

[0043] According to an embodiment of the present invention, the lithium-ion battery further includes a negative electrode sheet containing a negative electrode active material, a conductive agent, and a binder.

[0044] According to an embodiment of the present invention, the lithium-ion battery further includes a separator.

[0045] The positive electrode active material is lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium-rich manganese-based positive electrode material, or ternary material LiNi. x Co y Mn 1-x-yO2, ternary material LiNi x Co y Al 1-x-y O2, a mixture of one or more metal oxide cathode materials; the binder is one or more of PVDF, CMC, PAA, and SBR; the conductive agent is one or more of acetylene black, Ketjen black, Super P, and carbon nanotubes.

[0046] The negative electrode active material is one or more of natural graphite, artificial graphite, mesophase carbon microspheres, soft carbon, hard carbon, carbon nanotubes, graphene, silicon oxide, and silicon carbon; the binder is one or more of PVDF, CMC, PAA, and SBR; and the conductive agent is one or more of acetylene black, Ketjen black, Super P, and carbon nanotubes.

[0047] The beneficial effects of this invention are:

[0048] This invention provides a lithium-ion battery electrolyte based on propylene carbonate (PC) with high electrochemical compatibility and a lithium-ion battery including the electrolyte. In the electrolyte of this invention, an aromatic compound is used as a first additive. On the one hand, the reduction potential of the aromatic compound is higher than the decomposition potential of PC, allowing it to form a stable SEI film on the negative electrode surface during the first charge-discharge process. On the other hand, the introduction of the aromatic compound also alters the properties of Li... + The solvation structure enables Li to be realized when PC is used as the main solvent. + It can be reversibly inserted and extracted into graphite. The electrolyte also includes a second additive, which, when used in conjunction with the first additive, can enhance the protective effect of the first additive on the negative electrode and improve cycle performance.

[0049] Therefore, the lithium-ion battery prepared by this invention effectively avoids the graphite stripping problem caused by PC solvent co-intercalation, and improves the initial discharge capacity, cycle life and high and low temperature performance of the battery. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0051] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0052] Example 1

[0053] (1) Preparation of positive electrode

[0054] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2, polyvinylidene fluoride (PVDF) binder, and acetylene black conductive agent are mixed in a weight ratio of 97:1.5:1.5. N-methylpyrrolidone (NMP) is added, and the mixture is stirred under vacuum until a uniform and fluid positive electrode slurry is formed. The positive electrode slurry is then uniformly coated onto an aluminum foil with a thickness of 12 μm. The coated aluminum foil is baked in an oven with five different temperature gradients, and then dried in an oven at 120°C for 8 hours. Finally, it is rolled and slit to obtain the desired positive electrode sheet.

[0055] (2) Preparation of negative electrode sheet

[0056] Artificial graphite (anode active material), sodium carboxymethyl cellulose (CMC-Na) (thickener), styrene-butadiene rubber (binder), and acetylene black (conductive agent) were mixed in a weight ratio of 97:1:1:1. Deionized water was added, and the mixture was stirred in a vacuum mixer to obtain a cathode slurry. The cathode slurry was then uniformly coated onto a copper foil with a thickness of 8 μm. After the copper foil was dried at room temperature, it was transferred to an 80°C oven to dry for 10 hours. The resulting material was then cold-pressed and slit to obtain a cathode sheet.

[0057] (3) Electrolyte preparation

[0058] In a glove box filled with argon gas and with acceptable water and oxygen content (moisture <1ppm, oxygen <1ppm), PC and other solvents (ethyl methyl carbonate, EMC) are mixed uniformly at a mass ratio of 5:95 to form a mixed solvent. Then, 2wt% of 1,2-difluorobenzene and 2wt% of vinylene carbonate based on the total mass of the electrolyte are added to the mixed solvent and mixed uniformly. Then, LiPF6 electrolyte with a molar concentration of 1mol / L based on the electrolyte is slowly added to the mixed solution and stirred until it is completely dissolved. After passing the moisture and free acid tests, the desired electrolyte is obtained.

[0059] (4) Preparation of the separating membrane

[0060] An 8μm thick polyethylene separator membrane (provided by Asahi Kasei Corporation) was selected.

[0061] (5) Preparation of lithium-ion batteries

[0062] The prepared positive electrode, separator, and negative electrode are stacked in sequence, ensuring that the separator is between the positive and negative electrodes to provide isolation. Then, the unfilled bare cell is obtained by winding. The bare cell is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and sorting, the desired soft-pack lithium-ion battery is obtained.

[0063] Examples 2-11 and Comparative Examples 1-3

[0064] In the lithium-ion batteries of Examples 2-11 and Comparative Examples 1-3, except that the electrolyte components were added in the proportions shown in Table 1, everything else was the same as in Example 1.

[0065] Table 1. Composition and proportions of electrolytes in Examples 1-11 and Comparative Examples 1-3

[0066]

[0067] The lithium-ion batteries prepared in Examples 1-11 and Comparative Examples 1-3 were subjected to the following related tests:

[0068] (1) Room temperature cycle performance test: At 25℃, the capacity-graded pouch battery was charged to 4.2V at 0.5C with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C. The capacity retention rate after 100 charge-discharge cycles was calculated. The calculation formula is as follows: Capacity retention rate after 100 cycles (%) = (Cycle discharge capacity after 100 cycles / Cycle discharge capacity after 1 cycle) * 100%.

[0069] (2) High-temperature cycle performance test: At 70℃, the capacity-graded pouch cells were charged to 4.2V at 0.5C with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C. After 50 charge-discharge cycles, the capacity retention rate at the 50th cycle was calculated. The calculation formula is as follows: High-temperature cycle capacity retention rate at the 50th cycle (%) = (High-temperature cycle discharge capacity at the 50th cycle / High-temperature cycle discharge capacity at the 1st cycle) * 100%.

[0070] (3) Low-temperature cycle performance test: At 25℃, the capacity-graded pouch battery was charged to 4.2V at 0.5C with a cutoff current of 0.05C, and then discharged to 3.0V at a constant current of 0.5C. This cycle was repeated for 3 weeks, and the room temperature discharge capacity in the 3rd week was recorded. Then, at 25℃, it was charged to 4.2V at 0.5C with a cutoff current of 0.05C, and discharged to 3.0V at a constant current of 0.5C at -30℃. The calculation formula is as follows: Low-temperature capacity retention rate (%) = (Low-temperature discharge capacity / Room temperature discharge capacity in the 3rd week) * 100%.

[0071] (4) Temperature shock test: At 25°C, charge the soft pack battery after capacity testing to 4.2V with 0.5C and cut-off current of 0.05C. Then place it at 130°C for 30 minutes and observe whether the battery catches fire or explodes.

[0072] The results of the above performance tests are shown in Table 2.

[0073] Table 2 shows the performance test results of lithium-ion batteries corresponding to Examples 1-11 and Comparative Examples 1-3.

[0074]

[0075] Comparing the examples and comparative examples, it can be found that the first-week cycle discharge capacity at room temperature of Comparative Example 1 and Comparative Example 2 is about 1030 mAh, while the first-week cycle discharge capacity at room temperature of other examples and comparative examples is about 2960-3030 mAh. This is mainly because the introduction of the electrolyte additive of the present invention can effectively avoid the graphite stripping problem caused by PC solvent co-intercalation, thereby improving the initial discharge capacity of the battery.

[0076] As can be seen from the comparison of Comparative Examples 1-2 and Example 10, the safety and electrical performance of the battery are greatly improved after the introduction of the first additive into the electrolyte system.

[0077] As can be seen from the comparison of Examples 9, 11, and Comparative Example 3, when the range of PC is not within the scope of the present invention, the electrical performance of the battery will deteriorate, and if it does not contain PC at all, the safety performance of the battery will deteriorate.

[0078] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An electrolyte, said electrolyte comprising a lithium salt, an organic solvent, a first additive, and a second additive; wherein, The organic solvent is propylene carbonate and at least one of the following other organic solvents: diethyl carbonate (DEC), methyl ethyl carbonate (EMC), and ethyl acetate (EA); the mass ratio of the other organic solvent to propylene carbonate is 40-50:60-50. The first additive is selected from at least one aromatic compound; the aromatic compound has the structure shown in Formula 1: Formula 1 In Formula 1, X is selected from C-R6, and R6 is selected from any one of hydrogen atom, fluorine atom, and alkyl group; R1 and R5 may be the same or different, and are independently selected from any one of hydrogen atom, fluorine atom, and alkyl group; R3 is selected from any one of hydrogen atom and fluorine atom; R2 and R4 may be the same or different, and are independently selected from any one of hydrogen atom, fluorine atom, and alkyl group; and Formula 1 contains at least one fluorine atom. The content of the first additive accounts for 0.5~1.8 wt% of the total mass of the electrolyte; The electrolyte further includes a second additive selected from one or more of lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, dimethyl maleic anhydride, succinic anhydride, triargyl phosphate, ethoxypentafluorophosphazene, phenoxypentafluorophosphazene, tris(trimethylsilane)borate, tris(trimethylsilane) phosphate, ethylene ethylene carbonate, vinylene carbonate, fluoroethylene carbonate, 1,3-propanesulfonate lactone, 1,4-butanesulfonate lactone, vinyl sulfate, methanedisulfonate methylenecyclohexane, succinic anhydride, adiponitrile, glutaronitrile, 1,3,6-hexanetrionitrile, ethylene glycol bis(propionitrile) ether, and 1,2,3-tris(2-cyanoethoxy)propane.

2. The electrolyte according to claim 1, wherein, The aromatic compound represented by Formula 1 is selected from at least one of the following compounds: 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 。 3. The electrolyte according to claim 1, wherein, The content of the second additive accounts for 0.1 to 25 wt% of the total mass of the electrolyte.

4. The electrolyte according to any one of claims 1-3, wherein, The lithium salt includes one or more of LiPF6, LiTFSI, LiClO4, LiFSI, LiBOB, LiODFB, LiBF4, and LiAsF6.

5. A lithium-ion battery, the lithium-ion battery comprising the electrolyte according to any one of claims 1-4.

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