Electrolyte, lithium ion battery and electric device

Abstract

The application discloses a kind of electrolyte for lithium ion battery and lithium ion battery, the electrolyte includes non-water organic solvent, lithium salt and additive, the additive includes lithium difluoro (oxalato) borate, propylene trifluorocarbonate and fluorinated film forming additive;The propylene trifluorocarbonate accounts for a % of total mass of electrolyte, fluorinated film forming additive accounts for b % of total mass of electrolyte, lithium difluoro (oxalato) borate accounts for c % of total mass of electrolyte, wherein a, b, c meet the relationship: 1 < a+b+c < 8, 0.2 ≤ a / b ≤ 60.Compared with prior art, the electrolyte provided by the application can effectively improve the stability of electrolyte system and positive and negative electrodes by using multiple electrolyte additives in combination and controlling the content ratio relationship, thereby improving the cycle performance of the battery, thereby effectively improving the problems of poor cycle expansion and high-temperature stability of silicon system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to an electrolyte, a lithium ion battery and a power utilization device. BACKGROUND

[0002] As a high-efficiency energy storage device, the development of lithium ion batteries has attracted extensive attention. With the progress of science and technology and the increasing application demand, the traditional lithium ion battery has gradually been unable to meet the market demand for higher energy density, faster charging speed and safer performance. Under this background, silicon-based negative electrode materials are considered as a strong candidate for the next generation of lithium ion battery negative electrode materials due to their high theoretical specific capacity. However, silicon-based negative electrode materials have problems such as serious volume expansion during charging and discharging, low electrical conductivity, short cycle life, and low coulomb efficiency, which limit their commercial application.

[0003] In order to solve these problems, researchers have conducted a lot of work, including structural design and modification of silicon-based materials, development of new binders, and research on electrolyte additives. Through these methods, the volume expansion of silicon-based materials can be inhibited to some extent, and their electrical conductivity and cycle stability can be improved. For example, by introducing a metal oxide coating layer on the surface of the silicon-based negative electrode material, such as amorphous TiO2 coating, the volume expansion of silicon during charging and discharging can be effectively alleviated, and the structural stability can be improved. In addition, developing a binder with sufficient adhesion to maintain good electron transmission between silicon and the current collector and achieve a stable electrode structure is also an important strategy to improve the performance of silicon-based negative electrodes. However, due to the excessive use of functional additives, the viscosity of the electrolyte is increased, which affects the ion transmission rate, greatly affecting the charging performance of the battery, so it is very difficult to improve the fast charging performance. SUMMARY

[0004] The purpose of the present application is to provide an electrolyte for lithium ion batteries, which can improve the poor stability of the current high-silicon negative electrode system battery electrolyte and the problem of easy cycle expansion of the battery.

[0005] In order to achieve the above-mentioned purpose, the following technical scheme is adopted in the present application: an electrolyte, comprising a non-aqueous organic solvent, a lithium salt and an additive, the additive comprising lithium difluoro(oxalato)borate, trifluoro(propylene carbonate) and a fluorinated film-forming additive; the trifluoro(propylene carbonate) accounts for a% of the total mass of the electrolyte, the fluorinated film-forming additive accounts for b% of the total mass of the electrolyte, and the lithium difluoro(oxalato)borate accounts for c% of the total mass of the electrolyte, wherein a, b and c satisfy the relationship: 1

[0006] The fluorinated film-forming additive comprises a compound represented by structural formula I:

[0007]

[0008] wherein R1, R2, R3, R4 are each independently selected from one of H, F, =C, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C1-C12 alkoxy, substituted or unsubstituted C2-C12 alkenyl, substituted or unsubstituted C2-C12 alkenyloxy; or, any two of R1, R2, R3, R4 are linked to form a ring.

[0009] Preferably, the fluorinated film-forming additive comprises at least one of the following compounds:

[0010]

[0011] Preferably, at least one of the following (1)-(3) is further included:

[0012] (1) the value of a ranges from 0.1 to 5;

[0013] (2) the value of b ranges from 0.05 to 4;

[0014] (3) the value of c ranges from 0.01 to 1.

[0015] Preferably, the non-aqueous organic solvent comprises at least one of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, propyl ethylene carbonate, methyl acetate, ethyl acetate, ethyl propionate, propyl propionate, 1,3-dioxolane, tetrahydrofuran, γ-butyrolactone, 1,2-dimethoxyethane, 2-methyltetrahydrofuran.

[0016] Preferably, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethylsulfonate and lithium tris(trifluoromethylsulfonyl)methide.

[0017] Preferably, the electrolyte further comprises at least one additive of lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tetrafluoroborate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, ethylene sulfate, methanediyl bis(malonate), propene sultone, citraconic anhydride, ethylene glycol bis(propionitrile) ether and hexyne trinitrile.

[0018] In addition, the present application also provides a lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and the above-mentioned electrolyte.

[0019] Preferably, the porosity of the separator is d%, wherein the relationship between a, b and d satisfies: 14≤d-4*(a+b+c)≤46.

[0020] Preferably, the value of d ranges from 30 to 60.

[0021] The application also provides a power utilization device comprising the lithium ion battery.

[0022] The application has the beneficial effect that: the application can effectively improve the solventation structure of the electrolyte, improve the stability of the positive and negative electrodes and the cycle performance of the battery, and achieve the effect of improving the cycle expansion at high and low temperatures by using the additives lithium difluoro(oxalato)borate, trifluoro(propylene carbonate) and fluorinated film-forming additives in the electrolyte in a synergistic manner and controlling the content ratio relationship. Among them, LiODFB can participate in film formation at the positive and negative electrodes, reduce the positive electrode impedance, improve the CEI stability, reduce the side reaction between the positive electrode and the solvent, and at the same time, the film formation product at the negative electrode can reduce the energy required for the fluorinated film-forming additive to form a film at the negative electrode, plus the influence of TFPC on the solventation structure of the electrolyte, improving the solventation participation ability of DFEC, making it easy to form a SEI layer rich in LiF. DETAILED DESCRIPTION

[0023] In order to make the technical solutions and advantages of the application clearer, the technical solutions of the application will be described clearly and completely below in combination with specific embodiments. Obviously, the described embodiments are only some of the embodiments of the application, not all. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0024] In the first aspect according to the application, the application provides an electrolyte comprising a non-aqueous organic solvent, a lithium salt and an additive, the additive comprising lithium difluoro(oxalato)borate, trifluoro(propylene carbonate) and a fluorinated film-forming additive; the trifluoro(propylene carbonate) accounts for a% of the total mass of the electrolyte, the fluorinated film-forming additive accounts for b% of the total mass of the electrolyte, and the lithium difluoro(oxalato)borate accounts for c% of the total mass of the electrolyte, wherein a, b and c satisfy the relationship: 1

[0025] The fluorinated film-forming additive comprises a compound shown in structural formula I:

[0026]

[0027] wherein R1, R2, R3 and R4 are each independently selected from one of H, F, *=C, substituted or unsubstituted C1-C12 alkyl, substituted or unsubstituted C1-C12 alkoxy, substituted or unsubstituted C2-C12 alkenyl and substituted or unsubstituted C2-C12 alkenyloxy; or, any two of R1, R2, R3 and R4 are connected to form a ring.

[0028] In some embodiments, the fluorinated film-forming additive comprises at least one of the following compounds:

[0029]

[0030] The lithium difluoro(oxalato)borate (LiODFB) as a film-forming additive in the electrolyte of lithium-ion batteries can improve the thermal stability of the electrolyte and help form a better solid electrolyte interface (SEI) on the anode and cathode, thereby enhancing the thermal stability of the electrolyte and the electrochemical performance of the battery. Under high pressure, it effectively inhibits the decline of electrode cycle performance. At the same time, lithium difluoro(oxalate)borate can form a solid, dense and conductive CEI layer on the positive electrode, thereby significantly improving the rate capability and cycle stability.

[0031] The fluorinated film-forming additive has a lower reduction potential, which can form a stable solid electrolyte interface (SEI) layer on the surface of the anode of the battery, improve the interface stability inside the battery, reduce the growth of lithium dendrites, and thereby reduce the risk of short circuit of the battery. Under the synergistic effect of LiODFB, the energy required for the film formation of the fluorinated additive can be reduced, and the reducibility of the fluorinated additive can be further improved. In addition, propylene carbonate trifluoride can maintain the conductivity of the electrolyte and ensure that the battery has good charge and discharge performance in a wide temperature range.

[0032] Propylene carbonate trifluoride (TFPC) is an organic compound that can significantly improve the cycle performance and low-temperature performance of lithium-ion batteries as an electrolyte additive. Its addition can increase the flash point of the electrolyte, enhance safety, and improve the overall performance of the battery. As a dual-functional dilution cosolvent, it can participate in the interfacial chemical reaction of the electrolyte and help build a more compact and stable solid electrolyte interface (SEI) and positive electrode / electrolyte interface (CEI). This helps to improve the cycle stability and safety of lithium metal batteries. The use of TFPC under high pressure conditions can effectively inhibit the oxidative decomposition of the electrolyte and improve the stability of the battery under high voltage. Studies have shown that the addition of TFPC can protect the surface of lithium metal and lithium cobaltate cathodes under high voltage, inhibit the occurrence of side reactions, and thereby improve the cycle performance of the battery.

[0033] The combination of the above three can effectively improve the safety performance of the battery electrolyte solvent.

[0034] In some embodiments, the value of a ranges from 0.1 to 5; the value of b ranges from 0.05 to 4; and the value of c ranges from 0.01 to 1.

[0035] In some embodiments, the non-aqueous organic solvent includes at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propyl ethylene carbonate, methyl acetate, ethyl acetate, ethyl propionate, propyl propionate, 1,3-dioxolane, tetrahydrofuran, gamma-butyrolactone, 1,2-dimethoxyethane, 2-methyltetrahydrofuran. Preferably, the mixed solvent of ethylene carbonate (EC), propylene carbonate (PC) and diethyl carbonate (DEC). The mixed electrolyte solvent obtained by mixing the above three electrolyte solvents has more stable chemical properties.

[0036] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethylsulfonate or lithium tris(trifluoromethylsulfonyl)methide. Preferably, lithium hexafluorophosphate, which has moderate ion transference number and dissociation constant in commonly used electrolyte organic solvents, ensures the conductivity and capacity of the battery; during the first charge and discharge process of the battery, lithium hexafluorophosphate participates in the formation of a stable SEI layer, which can protect the negative electrode of the battery and prevent the continuous decomposition of the electrolyte, thereby improving the cycle stability and safety of the battery, and lithium hexafluorophosphate has good electrochemical stability, with a cathode stable voltage of up to 5.1V, which is much higher than the required 4.2V of lithium ion batteries. It does not corrode the current collector and matches well with various positive and negative electrode materials, so it becomes the most important electrolyte lithium salt used in commercial lithium ion batteries.

[0037] In some embodiments, the electrolyte further includes at least one additive of lithium bis(oxalato)borate, lithium difluorosulfonimide, lithium difluoro oxalate borate, lithium tetrafluoroborate, lithium difluorophosphate, lithium difluoro bis(oxalate)phosphate, fluoroethylene carbonate, vinylene carbonate, 1,3-propane sultone, ethylene sulfate, methylene methane disulfonate, propylene sultone, citraconic anhydride, ethylene glycol bis(propionitrile) ether and hexane trinitrile.

[0038] In the second aspect according to the present application, the present application further provides a lithium ion battery, including a positive electrode sheet, a negative electrode sheet, a separator arranged between the positive electrode sheet and the negative electrode sheet, and the above-mentioned electrolyte.

[0039] Specifically, the porosity of the separator is d%, and the relationship between a, b and d satisfies 14≤d-4*(a+b+c)≤46. Wherein, the value range of d is 30-60.

[0040] The specific value of the porosity d of the diaphragm can be 30, 35, 40, 45, 50, 55, 60, and preferably 50. The porosity of the diaphragm determines the number of ion transmission channels it can provide. A higher porosity means more ion channels, which helps to improve the ion conductivity of the battery and the charge and discharge performance of the battery. However, too high a porosity can lead to a decrease in the mechanical strength of the diaphragm at high temperatures, affecting its integrity and isolation effect under high-temperature conditions, thereby affecting the safety of the battery. When the battery experiences abnormal thermal runaway, the porosity and pore size of the diaphragm affect the speed of thermal runaway development. An appropriate porosity helps to control the heat release rate during thermal runaway.

[0041] In the present application, the diaphragm porosity is combined with the electrolyte additive. The diaphragm porosity affects the adsorption and retention of the electrolyte. A diaphragm with high porosity can adsorb more electrolyte, thereby ensuring the stability of the electrolyte and ion migration of the battery under different temperature conditions for a long time. The porosity of the diaphragm determines the number of ion transmission channels it can provide.

[0042] In some embodiments, the diaphragm substrate includes any one of a polyolefin film, a cellulose film, a polyimide film, and a polyester film.

[0043] The present application will be further described below through specific examples.

[0044] Example 1

[0045] Preparation of electrolyte:

[0046] In an argon-filled glove box, ethylene carbonate, methyl ethyl carbonate and diethyl carbonate were mixed in a mass ratio of EC:PC:DEC = 1:1:3, then 14.5wt% lithium hexafluorophosphate (LiPF6) based on the total weight of the electrolyte was slowly added to the mixed solution, and finally 3wt% TFPC (trifluoropropylene carbonate), 0.5wt% LiODFB (lithium difluoro(oxalato)borate), 0.5wt% compound A, 8wt% fluoroethylene carbonate (FEC) and 1wt% vinyl sulfate (DTD) based on the total weight of the electrolyte were added. After stirring uniformly, the lithium ion battery electrolyte of Example 1 was obtained.

[0047] Preparation of negative electrode sheet:

[0048] Raw materials: hard carbon, silicon carbon (SiC), conductive agent carbon nanotube (CNT), thickening agent sodium carboxymethyl cellulose (CMC), binder polyaniline (PANI) and binder polyurethane (PU). The hard carbon and silicon carbon are mixed as a mixture of hard carbon: silicon carbon = 75:25 by weight ratio as the negative electrode active material, and after mixing each substance in a weight ratio of negative electrode active material: CNT: CMC: PANI: PU = 97.6:0.5:0.7:0.7:0.5, deionized water is added and mixed uniformly to prepare a negative electrode slurry; the negative electrode slurry is coated on the current collector copper foil, dried at 85°C, then cold-pressed, and then edge-cutting, cutting, and striping are performed, and then dried at 85°C under vacuum for 12h to obtain a lithium ion battery negative electrode sheet. The specific capacity of the hard carbon used is 380mAh / g, the specific capacity of the silicon carbon negative electrode is 1810mAh / g, and the total specific capacity of the negative electrode sheet is 737.5mAh / g.

[0049] Preparation of the positive electrode sheet:

[0050] Raw materials: cathode active material lithium cobaltate LiCoO2, conductive agent Super P, and binder polyvinylidene fluoride PVDF. After mixing each substance in a weight ratio of LiCoO2: Super P: PVDF = 97.5:1:1.5, N-methyl pyrrolidone (NMP) is added, and the mixture is uniformly mixed to prepare a lithium ion battery positive electrode slurry; the positive electrode slurry is coated on the current collector aluminum foil, dried at 85°C, then cold-pressed, and then edge-cutting, cutting, and striping are performed, and then dried at 85°C under vacuum for 4h, and the tab is welded to prepare a lithium ion battery positive electrode sheet.

[0051] Preparation of the battery:

[0052] The above-prepared positive electrode sheet, PE separator (wet process, Enji), and negative electrode sheet are stacked in order with the separator between the positive and negative electrode sheets, and wound to obtain a bare cell; the bare cell is placed in an aluminum plastic film outer package, and the above-prepared electrolyte is injected into the dried battery, and then packaged, left to stand, formed, shaped, and separated to complete the preparation of the lithium ion soft-pack battery.

[0053] Examples 2-9 and Comparative Examples 1-9: The difference from Example 1 is that the additive of the electrolyte is added and / or the porosity of the separator is different, as shown in Table 1. The others are the same as Example 1, which will not be repeated here.

[0054] Table 1

[0055]

[0056]

[0057] The batteries prepared in the examples and comparative examples are respectively tested for the following performances:

[0058] (1) High-temperature cycle performance test: after the battery was divided, it was charged at 45℃ to 4.45V at 0.7C constant current and constant voltage, and the cutoff current was 0.05C, then discharged to 3.0V at 0.5C constant current, and the cycle was repeated, and the capacity retention rate at the 500th cycle was calculated after 500 cycles of charge and discharge, and the calculation formula was as follows:

[0059] The capacity retention rate at the 500th cycle (%) = (the 500th cycle discharge capacity / the first cycle discharge capacity) x 100%.

[0060] (2) 60℃ 14d high-temperature storage test: the battery was charged and discharged at room temperature at 0.5C (4.45V-3.0V) once, and the discharge capacity C0 before storage was recorded, then the battery was charged to 4.45V full state, and the thickness d1 of the battery before high-temperature storage was tested using PPG battery thickness tester (500g), the battery was placed in a 60℃ constant temperature oven for 14 days, and after the storage was completed, the battery was taken out and the battery thickness d2 after storage was tested, and the thickness expansion rate of the battery after 14 days of 60℃ storage was calculated; after the battery was cooled at room temperature for 24h, the battery was discharged again at 0.5C constant current to 3.0V, and then charged at 0.5C constant current and constant voltage to 4.45V, and the discharge capacity C1 and the charge capacity C2 after storage were recorded, and the capacity retention rate and the recovery rate of the battery after 14 days of 60℃ storage were calculated, and the calculation formula was as follows:

[0061] The thickness expansion rate after 14 days of 60℃ storage = (d2-d1) / d1*100%;

[0062] The capacity retention rate after 14 days of 60℃ storage = C1 / C0*100%;

[0063] The capacity recovery rate after 14 days of 60℃ storage = C2 / C0*100%.

[0064] (3) Thermal shock performance: discharge at 25℃ environment condition to 3.0V at a given current of 0.2C; stand for 5min; charge to 4.45V at a charge current of 0.2C, when the cell voltage reaches 4.45V, change to 4.45V constant voltage charging until the charge current is less than or equal to the given cutoff current of 0.05C; after standing for 1h, put the cell into the oven, the oven temperature is increased to 135±2℃ at a speed of 5±2℃ / min, and keep for 30min, then stop, and the judgment standard is that the cell does not catch fire and does not explode.

[0065] The battery performance test results of examples 1-9 and comparative examples 1-9 are shown in table 2.

[0066] Table 2

[0067]

[0068]

[0069] According to the test results in the above table, when the combination of LiODFB, fluorinated film-forming additive and TFPC in the embodiment of the present application meets 1 < a + b + c < 8, 0.2 ≤ a / b ≤ 60, and 14 ≤ d - 4*(a + b + c) ≤ 46, the stability of the electrolyte system and the positive and negative electrodes can be effectively improved, and the cycle performance of the battery is improved, thereby effectively improving the problems of cycle expansion and poor high-temperature stability of the silicon system.

[0070] Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will be able to make changes and modifications to the above embodiments. Therefore, the present application is not limited to the specific embodiments described above, and any obvious improvements, replacements or modifications made by those skilled in the art based on the present application shall fall within the scope of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience of description and do not constitute any limitation on the present application.

Claims

1. A lithium-ion battery, characterized by: The lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator arranged between the positive electrode sheet and the negative electrode sheet, and an electrolyte, wherein the electrolyte comprises a non-aqueous organic solvent, a lithium salt, and an additive; The additive comprises lithium difluoro(oxalato)borate, propylene carbonate trifluoride, and a fluorinated film-forming additive; The propylene carbonate trifluoride accounts for a% of the total mass of the electrolyte, the fluorinated film-forming additive accounts for b% of the total mass of the electrolyte, the lithium difluoro(oxalato)borate accounts for c% of the total mass of the electrolyte, and the porosity of the separator is d%, wherein a, b, c, and d satisfy the following relationships: 1 The fluorinated film-forming additive comprises at least one of the following compounds: Compound A, Compound B, Compound C, Compound D, Compound E; The negative electrode sheet contains a silicon-based negative electrode material.

2. The lithium-ion battery of claim 1, wherein: At least one of the following (1) and (2) is further included: (1) The value of a is in the range of 0.1-5; (2) The value of b is in the range of 0.05-4.

3. The lithium-ion battery of claim 1, wherein: The non-aqueous organic solvent comprises at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propyl ethyl carbonate, methyl acetate, ethyl acetate, ethyl propionate, propyl propionate, 1,3-dioxolane, tetrahydrofuran, gamma-butyrolactone, 1,2-dimethoxyethane, and 2-methyltetrahydrofuran.

4. The lithium-ion battery of claim 1, wherein: The lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium trifluoromethylsulfonate, and lithium tris(trifluoromethylsulfonyl)methide.

5. The lithium-ion battery of claim 1, wherein: The electrolyte further comprises at least one of the following additives: lithium bis(oxalato)borate, lithium difluorosulfonimide, lithium difluoro(oxalato)borate, lithium tetrafluoroborate, lithium difluorophosphate, lithium difluorobis(oxalato)phosphate, fluorinated ethylene carbonate, vinylene carbonate, 1,3-propane sultone, ethylene sulfate, methanediyl dimethanesulfonate, propene sultone, citraconic anhydride, ethylene glycol bis(propionitrile) ether, and hexane trinitrile.

6. The lithium-ion battery of claim 1, wherein: The value of d is in the range of 30-60.

7. An electrical device, characterized by The lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator arranged between the positive electrode sheet and the negative electrode sheet, and an electrolyte, wherein the electrolyte comprises a non-aqueous organic solvent, a lithium salt, and an additive;

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