CN109935906B - Electrolyte and Lithium-Ion Batteries - Google Patents

Abstract

The invention relates to the field of lithium ion batteries, in particular to an electrolyte and a lithium ion battery. The electrolyte contains electrolyte lithium salt, a non-aqueous solvent and an electrolyte additive, wherein the electrolyte lithium salt is a combination of a first lithium salt and a second lithium salt, and the first lithium salt is LiPF₆The second lithium salt is one or more of lithium difluoro oxalate borate, lithium dioxalate borate and lithium difluoro phosphate; the electrolyte additive comprises a film forming agent and an additive A, wherein the additive A is one or more of alkyl sultone, alkenyl sultone and dialkyl sulfite. The components in the electrolyte of the invention can be matched to complete the charging and discharging process and maintain LiPF₆The lithium ion battery with higher energy density, cycle performance, rate discharge performance and high temperature resistance can be prepared.

Description

Electrolyte and Lithium-Ion Batteries

technical field

The present invention relates to the field of lithium ion batteries, in particular, to an electrolyte and a lithium ion battery.

Background technique

With the increasing development of electric vehicles, higher requirements are placed on high-energy-density batteries that can provide greater cruising range. Existing lithium-ion batteries include a positive electrode composed of ternary materials and a negative electrode composed of graphite. If the active material coating amount of the positive and negative electrode sheets is increased and the conductive agent carbon black is used, although the cell can obtain a higher energy density, However, the cycle performance and rate discharge performance are poor and cannot meet the requirements of EV power batteries. At the same time, due to the continuous release of heat by the battery during use, the temperature in the battery pack increases, making the temperature in the battery pack reach 50°C or even higher, which is bound to have a bad impact on the performance of the cell cycle. In the prior art, a cooling device is added to reduce the temperature of the battery pack, but this greatly reduces the energy density of the battery pack and affects the driving range of the electric vehicle.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a battery capable of obtaining both high energy density, cycle performance, rate of Electrolyte and lithium-ion battery for lithium-ion batteries with discharge performance and high temperature resistance.

In order to achieve the above object, the present invention provides an electrolyte, which contains an electrolyte lithium salt, a non-aqueous solvent and an electrolyte additive, wherein the electrolyte lithium salt is a combination of a first lithium salt and a second lithium salt, so The first lithium salt is LiPF ₆ , the second lithium salt is one or more of lithium difluorooxalate borate, lithium dioxalate borate and lithium difluorophosphate; the electrolyte additives include film-forming agents and additives A, the additive A is one or more of alkyl sultone, alkenyl sultone and dialkyl sulfite.

The invention also provides a lithium ion battery, the battery includes a pole core and an electrolyte, the pole core and the electrolyte are sealed in the battery casing, the pole core includes a positive electrode, a negative electrode and a separator, and the electrolyte is the above electrolyte.

The electrolyte of the present invention maintains the stability of LiPF ₆ by enabling the first lithium salt and the second lithium salt to coordinately complete the charging and discharging process under the action of a specific electrolyte additive, especially in combination with the hereinafter described in the present invention. Under the described positive electrode, negative electrode, etc., a lithium ion battery with higher energy density, cycle performance, rate discharge performance and high temperature resistance performance can be prepared.

Other features and advantages of the present invention will be described in detail in the detailed description that follows.

Detailed ways

Specific embodiments of the present invention will be described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, but not to limit the present invention.

The endpoints of ranges and any values disclosed herein are not limited to the precise ranges or values, which are to be understood to encompass values proximate to those ranges or values. For ranges of values, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values can be combined with each other to yield one or more new ranges of values that Ranges should be considered as specifically disclosed herein.

The present invention provides an electrolyte, which contains an electrolyte lithium salt, a non-aqueous solvent and an electrolyte additive, wherein the electrolyte lithium salt is a combination of a first lithium salt and a second lithium salt, and the first lithium salt LiPF ₆ , the second lithium salt is one or more of lithium difluorooxalate borate (LiODFB), lithium dioxalate borate (LiBOB) and lithium difluorophosphate (LiPF ₂ ); the electrolyte additive includes A film-forming agent and additive A, wherein the additive A is one or more of alkyl sultone, alkenyl sultone and dialkyl sulfite.

In the electrolyte of the present invention, by selecting specific electrolyte components, the electrode material and the electrolyte can form a tight structure layer on the solid-liquid interface without increasing the impedance, and the solid electrolyte interface film (SEI film) can be improved. ) stability, can prevent further decomposition of the electrolyte, improve the high temperature resistance of the electrolyte, effectively prevent the co-insertion of solvent molecules, and avoid the damage to the electrode material caused by the co-insertion of solvent molecules, thus greatly improving the lithium-ion battery. Energy density and service life.

According to the present invention, although the amount of the film-forming agent and the additive A can be varied within a wide range, in view of further improving the energy density, cycle performance, rate discharge performance and high temperature resistance performance of the lithium ion battery, preferably Based on the total weight of the electrolyte, the content of the film-forming agent is 2-8% by weight, preferably 3-5% by weight; the content of the additive A is 0.1-5% by weight, preferably 0.5-2% by weight % by weight, for example, 0.5-1.5% by weight.

According to the present invention, the additive A is selected from one or more of alkyl sultone, alkenyl sultone and dialkyl sulfite, and the alkyl sultone can be, for example, C1- C10 alkyl sultone, specifically 1,3-propane sultone, 1,4-butane sultone, etc.; for example, the alkenyl sultone can be C2-C10 alkenyl sultone Acid lactone, specifically 1,3-propene sultone, etc.; the dialkyl sulfite can be a di-C1-C10 alkyl ester of sulfite, specifically dimethyl sulfite, diethyl sulfite esters, etc.

Preferably, the additive A is 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, dimethyl sulfite and diethyl sulfite one or more of.

According to the present invention, the film-forming agent can be various film-forming agents conventionally used in the art, such as vinylene carbonate (VC), vinyl vinylene carbonate (VEC), vinyl sulfite (ES), One or more of propylene sulfite (PS) and butylene sulfite (BS), but in combination with the specific electrolyte and additives of the present invention, preferably, the film-forming agent is vinylene carbonate A combination of (VC) and propylene sulfite (PS), more preferably a combination of vinylene carbonate (VC) and propylene sulfite (PS) in a weight ratio of 1:0.5-5, preferably 1:1-2 .

According to the present invention, as described above, the first lithium salt and the second lithium salt have a good synergistic effect, preferably, the molar ratio of the first lithium salt and the second lithium salt is 1:0.01-0.5, It is preferably 1:0.02-0.2, more preferably 1:0.04-0.1. Wherein, the second lithium salt is preferably a combination of LiODFB and LiBOB, especially a combination of LiODFB and LiBOB with a molar ratio of 1:1-5.

In a preferred embodiment of the present invention, the concentration of the electrolyte lithium salt in the electrolyte is 0.5-2 mol/L, preferably 1-1.1 mol/L.

According to the present invention, the non-aqueous solvent can adopt various solvents conventional in the art, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC) , one or more of methyl propyl carbonate (MPC), fluoroethylene carbonate (FEC) and butyl methyl carbonate (BMC). But with the specific electrolyte and additives of the present invention, preferably, the non-aqueous solvent is ethylene carbonate (EC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and propylene carbonate (PC) ), more preferably ethylene carbonate (EC) and diethyl carbonate with a volume ratio of 10:10-30:5-20:1-5, preferably 10:15-20:8-10:1-2 (DEC), a combination of ethyl methyl carbonate (EMC) and propylene carbonate (PC).

The invention also provides a lithium ion battery, the battery includes a pole core and an electrolyte, the pole core and the electrolyte are sealed in the battery casing, the pole core includes a positive electrode, a negative electrode and a separator, and the electrolyte is the above electrolyte.

Wherein, the electrolyte solution is described above, which is not repeated here.

According to the present invention, generally, the negative electrode includes a negative electrode current collector and a negative electrode material formed on the negative electrode current collector, and the negative electrode material includes a negative electrode active material, a conductive agent and a binder.

Wherein, the negative electrode active material can be a conventional negative electrode active material that can intercalate and extract lithium in the field, such as graphite, artificial graphite, petroleum coke, organic cracked carbon, mesocarbon microspheres, carbon fiber, tin alloy, and silicon alloy. One or more of them, preferably graphite, such as natural graphite.

Wherein, the type and content of the negative electrode binder can be selected conventionally in the field, such as fluorine-containing resins and polyolefin compounds such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber ( One or more of SBR) and carboxymethyl cellulose (CMC), preferably styrene-butadiene rubber (SBR) and/or carboxymethyl cellulose (CMC).

Wherein, the negative electrode conductive agent can be a conventional conductive agent in the field, such as one or more of carbon black, acetylene black, furnace black, carbon fiber, graphene, carbon nanotube, conductive carbon black and conductive graphite, preferably It is carbon black and carbon fiber, more preferably carbon black and gas-phase generated carbon fiber.

According to the present invention, based on the total weight of the negative electrode active material, the negative electrode conductive agent and the negative electrode binder, the content of the negative electrode active material is 82-96% by weight, and the content of the negative electrode conductive agent is 82-96% by weight. It is 3-8 wt%, and the content of the negative electrode binder is 0.1-10 wt%.

According to the present invention, the current collector of the negative electrode can be a negative electrode current collector commonly used in lithium ion batteries, such as stamped metal, metal foil, mesh metal and foamed metal, preferably copper foil.

Wherein, the preparation method of the negative electrode can adopt a conventional preparation method. For example, the negative electrode can be obtained by mixing the negative electrode active material, the negative electrode conductive agent and the negative electrode binder with a solvent to form a negative electrode slurry, coating the negative electrode current collector, and then drying, rolling and slitting. Among them, the methods and conditions of drying, calendering and slitting can be conventionally selected in the field.

According to the present invention, generally the positive electrode comprises a positive electrode current collector and a positive electrode material layer on the surface of the positive electrode current collector, the positive electrode material layer containing a positive electrode active material, a composite conductive agent and a binder.

Wherein, in order to obtain higher energy density, cycle performance, rate discharge performance and high temperature resistance performance, preferably, the composite conductive agent contains the first carbon black, the second carbon black and the vapor-generated carbon fiber; wherein, the The apparent density of the first carbon black is 60-90kg/m ³ , the specific surface area is 60-80m ² /g, and the electrical conductivity is 10 ² -10 ⁴ S/m, for example, the apparent density can be satisfied to be 70-80kg/m ³ , the specific surface area is 70-75m ² /g, and the electrical conductivity is 10 ³ -10 ⁴ S/m. Preferably, the first carbon black is carbon black Super P.

The apparent density of the second carbon black is 17-50kg/m ³ , the specific surface area is 800-1000m ² /g, and the electrical conductivity is 10 ⁵ -10 ⁷ S/m, for example, the apparent density can be satisfied to be 20-35kg /m ³ , the specific surface area is 900-950 m ² /g, and the electrical conductivity is 10 ⁶ -10 ⁷ S/m. Preferably, the second carbon black is Ketjen black.

According to the present invention, in the composite conductive agent, in order to enhance the electrical conductivity of the composite conductive agent, the gas-phase carbon fiber is preferably prepared by chemical catalytic vapor deposition technology. Specifically, the gas-phase carbon fiber is prepared at 873-1473K , which is generated by cracking low-carbon hydrocarbon compounds such as methane, acetylene and benzene, using one of transition metals Fe, Co, Ni or their compounds as catalysts. Further preferably, the diameter of the vapor-generated carbon fiber may be 140-160 nm (for example, 145-155 nm), the length may be 5-10 μm (for example, 6-8 μm), and the tensile modulus may be 1-10 GPa (for example, 1-10 GPa). 2-6GPa), the density can be 80-100kg/m ³ (eg 85-95kg/m ³ ), the thermal expansion coefficient can be -0.5×10 ^-6 to -1×10 ^-6 , and the thermal conductivity can be 1000- 2000Wm ^-1 K ^-1 (eg 1200-1600Wm ^-1 K ^-1 ), the conductivity may be 10 ⁵ -10 ⁷ S/m (eg 10 ⁶ -10 ⁷ S/m).

According to the present invention, the binder can be a conventional binder used in the positive electrode material in the field, but in order to provide more microporous structure, so that the positive electrode of the lithium ion battery can obtain stronger liquid absorption and storage capacity. The ability of nano-electrolyte, thereby improving the cycle life and energy density of the battery, the binder is preferably polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), carboxymethyl cellulose At least one of sodium (CMC) and polyethylene (PE).

According to the present invention, although the content of the positive electrode active material, the composite conductive agent and the binder in the positive electrode material layer can be varied in a wide range, as long as the higher energy density, cycle performance, A lithium ion battery with rate discharge performance and high temperature resistance performance is sufficient. Preferably, based on the total weight of the positive electrode active material, the composite conductive agent and the binder, the content of the positive electrode active material is 85% -98% by weight, the content of the composite conductive agent is 1-10% by weight, and the content of the binder is 0.1-10% by weight. More preferably, based on the total weight of the positive electrode active material, the composite conductive agent and the binder, the content of the positive electrode active material is 96-98% by weight, and the content of the composite conductive agent is 1-5% by weight, and the content of the binder is 0.1-5% by weight.

According to the present invention, the positive electrode active material is a ternary positive electrode active material conventionally used in the field, although the positive electrode active material may satisfy the chemical formula LiCo _p Ni _q Mn _1-pq O ₂ (wherein, 0<p<1,0 One or more of any ternary materials represented by <q<1), but from the viewpoint of the coordination effect with other effective components of the positive electrode material layer, especially the conductive agent, preferably, the positive electrode active material is LiCo One or more of _0.2 Ni _0.5 Mn _0.3 O ₂ , LiCo _0.2 Ni _0.6 Mn _0.2 O ₂ , LiCo _0.1 Ni _0.8 Mn _0.1 O ₂ and LiCo _0.05 Ni _0.9 Mn _0.05 O ₂ .

According to the present invention, in the composite conductive agent, the first carbon black, the second carbon black and the vapor-generated carbon fiber can form a conductive network combining "dots" and "lines", which cooperates with the positive electrode activity Under the material, higher electrical properties are obtained. Preferably, the weight ratio of the content of the first carbon black, the second carbon black and the vapor-generated carbon fiber is 1:0.2-1:0.1-0.5, preferably 1:0.2-0.5:0.1-0.3.

In the present invention, the types of the positive electrode current collectors are not particularly limited, and can be conventionally selected. Specifically, the positive electrode current collector may be made of materials such as aluminum, copper, or steel. Usually, when the positive electrode is a positive electrode sheet, that is, when the positive electrode is a sheet, the positive electrode current collector is also made of a sheet-shaped material, such as aluminum foil, copper foil or punched steel strip, preferably aluminum foil. The thickness of the positive electrode current collector is not particularly limited, and can be appropriately adjusted according to the desired lithium ion battery. For example, the thickness of the positive electrode current collector is 10-20 μm, preferably 14-18 μm.

Considering cost and improving energy density, cycle performance and rate discharge performance, preferably, the thickness ratio of the positive electrode current collector and the positive electrode material layer is 1:5-10. Under this condition, preferably, the thickness of the positive electrode material layer is 60-180 μm, preferably 80-160 μm, and more preferably 100-140 μm.

According to the present invention, the amount of the positive electrode material formed on the positive electrode current collector can be 34-38 mg/cm ² , so that the lithium ion battery can obtain higher energy density.

According to the present invention, the preparation of the positive electrode can be carried out according to the conventional lithium-ion battery preparation process in the art, for example, including: coating the positive electrode slurry on the positive electrode current collector, and then baking, rolling, and cutting to obtain the positive electrode.

The positive electrode slurry can be prepared by dispersing the material of the positive electrode material layer in a solvent, wherein the solvent used to form the positive electrode slurry can be various solvents conventionally used in the field. The slurry of the positive electrode material composition has a higher viscosity and makes the components more uniformly dispersed, and the solvent is selected from N-methylpyrrolidone, N-methylformamide, N-methylacetamide, acetonitrile and N , at least one of N-dimethylformamide, preferably N-methylpyrrolidone. The amount of the solvent can be varied in a wide range, in order to obtain better conductive effect and higher bonding fastness between the conductive active material and the current collector, preferably, relative to 1 kg of the positive electrode active material, The total weight of the composite conductive agent and the binder, and the amount of the solvent in the slurry is 400-800 mL.

According to the present invention, the method for coating the positive electrode slurry on the positive electrode current collector is not particularly limited in the present invention, and it can be performed on various equipments commonly used in the art. Generally, the positive electrode material can be coated on the positive electrode current collector by using a pulper.

According to the present invention, the baking conditions are not particularly limited, as long as the solvent in the positive electrode slurry can be fully removed, for example, the baking can be performed at a temperature of 80-140°C; preferably, the The baking can be carried out at a temperature of 110-130°C.

The purpose of the rolling is to form a dense coating of the positive electrode active material to meet the design requirements of the battery. In the present invention, the rolling conditions are not particularly limited, and can be appropriately selected according to the expected thickness and compaction density. In the present invention, the conditions of the rolling are such that the thickness of the positive electrode can be, for example, 100-140 μm (eg, 130 μm), and the compaction density is between 3.3-3.6 g/cm ³ .

The purpose of the cutting is to make the positive electrode meet the design requirements of the battery. In the present invention, the cutting conditions are not particularly limited, and can be appropriately selected according to the expected size of the positive electrode. In the present invention, the cutting is such that the width of the positive electrode is 150-160 mm (for example, 156 mm), and the length is 205-220 mm (for example, 212 mm).

According to the present invention, the preparation method of the lithium ion battery of the present invention can be a method known to those skilled in the art. Generally speaking, the method includes stacking the positive electrode, the separator and the negative electrode in a top-down lamination mode. Assemble, then weld the positive electrode to the aluminum tab, and the negative electrode to the nickel-plated copper tab, then heat-seal the aluminum-plastic film, inject the electrolyte, and vacuumize the battery to obtain the battery cell. After infiltration, formation, and vacuuming again, lithium is obtained ion battery.

The infiltration conditions include: the infiltration time is 20-40h.

The formation conditions include: the formation voltage is 2.75-4.4V.

The present invention will be described in detail below by means of examples.

In the following examples, the first carbon black is Super P, the apparent density is 63kg/m ³ , the specific surface area is 65m ² /g, and the electrical conductivity is 10 ³ S/m;

The second carbon black is Ketjen Black ECP, the apparent density is 35kg/m ³ , the specific surface area is 800m ² /g, and the electrical conductivity is 10 ⁵ S/m;

Vapor-generated carbon fiber VGCF has a diameter of 150nm, a length of 6μm, a tensile modulus of 2GPa, a density of 80kg/m ³ , a thermal expansion coefficient of -10 ^-6 , a thermal conductivity of 1500Wm ^-1 K ^-1 , and an electrical conductivity of 10 ⁵ S/m.

In the following examples, the coating amount (g) of the positive electrode material is calculated as follows:

m _coating = m ₂ -m ₁ ,

Wherein, m ₁ and m ₂ respectively represent the weight (g) of the same size aluminum foil before and after coating the positive electrode material.

Example 1

This example is used to illustrate the electrolyte of the present invention.

LiPF ₆ , LiODFB and LiBOB were dissolved in ethylene carbonate EC, diethyl carbonate DEC, ethyl methyl carbonate EMC and propylene carbonate PC (volume ratio EC:DEC:EMC:PC=20:35:20:3) In the mixed solvent, the obtained solution; then add electrolyte additives: film-forming agent VC and film-forming agent PS, and additive A (i.e. 1,3-propane sultone) are mixed evenly to obtain electrolyte A1, electrolyte A1 The concentration of LiPF ₆ is 1 mol/L, the concentration of LiODFB is 0.01 mol/L, the concentration of LiBOB is 0.03 mol/L, the content of film-forming agent VC is 1.5 wt%, and the content of film-forming agent PS is 1.5 wt% , the content of additive A is 0.5% by weight.

Example 2

This example is used to illustrate the electrolyte of the present invention.

LiPF ₆ , LiODFB and LiBOB were dissolved in ethylene carbonate EC, diethyl carbonate DEC, ethyl methyl carbonate EMC and propylene carbonate PC (volume ratio EC:DEC:EMC:PC=20:35:20:3) In the mixed solvent, the obtained solution; then add electrolyte additives: film-forming agent VC and film-forming agent PS, and additive A (i.e. 1,4-butane sultone), mix uniformly to obtain electrolyte A2, electrolyze In solution A2, the concentration of LiPF ₆ is 1 mol/L, the concentration of LiODFB is 0.02 mol/L, the concentration of LiBOB is 0.03 mol/L, the content of film-forming agent VC is 2 wt%, and the content of film-forming agent PS is 2 % by weight, the content of additive A is 1% by weight.

Example 3

This example is used to illustrate the electrolyte of the present invention.

LiPF ₆ , LiODFB and LiBOB were dissolved in ethylene carbonate EC, diethyl carbonate DEC, ethyl methyl carbonate EMC and propylene carbonate PC (volume ratio EC:DEC:EMC:PC=20:35:20:3) In the mixed solvent, the obtained solution; then add electrolyte additives: film-forming agent VC and film-forming agent PS, and additive A (i.e. 1,3-propane sultone), mix well to obtain electrolyte A3, electrolyte In A3, the concentration of LiPF ₆ is 1 mol/L, the concentration of LiODFB is 0.03 mol/L, the concentration of LiBOB is 0.05 mol/L, the content of film-forming agent VC is 2 wt %, and the content of film-forming agent PS is 3 wt % %, the content of additive A is 1.5% by weight.

Example 4

This example is used to illustrate the electrolyte of the present invention.

According to the electrolyte described in Example 1, the difference is that instead of using diethyl carbonate DEC, only ethylene carbonate EC, ethyl methyl carbonate EMC and propylene carbonate PC are used (volume ratio EC:EMC:PC= 45:45:7) mixed solvent was used as a solvent to obtain electrolyte solution A4.

Example 5

This example is used to illustrate the electrolyte of the present invention.

According to the electrolyte solution described in Example 1, the difference is that instead of using the film-forming agent VC, an equal amount of the film-forming agent PS is used instead of the film-forming agent VC, so as to obtain the electrolyte solution A5, in which the film-forming agent A5 is obtained. The content of the agent PS was 3% by weight.

Comparative Example 1

According to the electrolyte solution described in Example 1, the difference is that the additive A is not used, but the additive A is replaced with an equal amount of VC, so as to obtain the electrolyte solution DA1. In the electrolyte solution, the content of the film-forming agent VC is 2 wt. %.

Comparative Example 2

According to the electrolyte solution described in Example 1, the difference is that the additive A is not used, but the additive PS is replaced with an equal amount of the additive PS, thereby obtaining the electrolyte solution DA2. In the electrolyte solution, the content of the film-forming agent PS is 2 weight%.

battery preparation

(1) Preparation of positive electrode sheet

Mix 940g LiCo _0.2 Ni _0.6 Mn _0.2 O ₂ , 25g polyvinylidene fluoride PVDF, 20g carbon black Super P, 10g Ketjen Black ECP and 5g gas phase carbon fiber VCGF and disperse in 600mL nitrogen methyl pyrrolidone at 3000rpm. In NMP, the mixture was stirred for 4 h to obtain a positive electrode material with a solid content of 50% by weight. Double-sided dressing was applied on an aluminum foil with a thickness of 16 μm, and the coating was uniform, and the coating amount of the positive electrode material was 47 mg/cm ² . Drying at 90°C, calendering, cutting into positive electrode sheets, the size of the positive electrode sheet is 212mm (length)×156mm (width)×130μm (thickness), the compaction density is 3.5g/cm ³ , the coating amount of the battery positive electrode material was 36.6 mg/cm ² .

(2) Preparation of negative electrode sheet

945g graphite, 15g conductive agent Super P (particle size of D50 is 2-5μm, true density is 0.073±0.01g/cm ³ , specific surface area> 60m ² /g, oil absorption value is 20-40mL/5g), 23g Styrene rubber SBR (solid content of 40% by weight, viscosity of 5-50mPa.s, average particle size of 145nm, pH=7-8, the same below) and 17g of sodium carboxymethylcellulose CMC were uniformly mixed in deionized water , to obtain a negative electrode material with a solid content of 50% by weight. Double-sided dressing was applied on copper foil with a thickness of 8 μm and spread evenly. Drying at 90°C, rolling, and cutting into negative electrode sheets, the size of the negative electrode sheets is 213 mm (length)×157 mm (width)×110 μm (thickness), and the compaction density is 1.6 g/cm ³ .

(3) Assembly of the battery

The above-mentioned positive electrode sheet, polyethylene separator (thickness is 20 μm, porosity (JIS) is 320s, porosity is 45%, the following are the same) and the above-mentioned negative electrode sheet are stacked and assembled according to the top-down lamination mode, and then Weld the positive electrode to the aluminum tab, and the negative electrode to the nickel-plated copper tab, and then heat-seal the aluminum-plastic film. Subsequently, the electrolytes A1-A5 in the above-mentioned embodiments and the electrolytes DA1-DA3 in the comparative example were injected into the battery case in an amount of 3.3g/Ah, vacuum sealed, and then infiltrated for 30h, and formed into a 3.5V voltage. , vacuumed again to make a lithium-ion battery.

test case

(1) Battery volumetric energy density

The battery prepared in the above example was formed and charged to 4.2V at a constant current of 0.2C at 30°C, and then switched to constant voltage charging with a cut-off current of 0.05C; then, the battery was discharged to 2.75V at a constant current of 0.2C. , get the capacity of the battery at room temperature 0.2C current discharge to 2.75V, which is the battery capacity; then charge the battery to 4.2V with 0.2C current constant current, remove the battery and weigh the battery at this time.

Calculate the battery energy density according to the following formula: battery energy density (Wh/kg) = battery discharge capacity (mAh) ÷ battery weight (g) × working voltage (V).

(2) Cycle performance

At a high temperature of 55°C, the battery prepared in the experimental example was charged to 4.2V with a constant current of 1C, and then transferred to constant voltage charging with a cut-off current of 0.05C; then, the battery was discharged to 2.75V with a constant current of 1C. Repeat the above steps 250 times to obtain the capacity of the battery after 250 cycles of high temperature at 1C current to discharge to 2.75V, and calculate the battery capacity retention rate after the cycle.

Table 1 shows the performances of the positive electrode sheets of the examples and comparative examples and the batteries prepared therefrom.

Table 1

It can be seen from Table 1 that the battery prepared by using the electrolyte of the present invention has higher battery discharge capacity, battery energy density and battery capacity retention rate.

The preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the specific details of the above-mentioned embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention. These simple modifications All belong to the protection scope of the present invention.

In addition, it should be noted that the specific technical features described in the above-mentioned specific embodiments can be combined in any suitable manner unless they are inconsistent. In order to avoid unnecessary repetition, the present invention provides The combination method will not be specified otherwise.

In addition, the various embodiments of the present invention can also be combined arbitrarily, as long as they do not violate the spirit of the present invention, they should also be regarded as the contents disclosed in the present invention.

Claims (16)

1. an electrolyte, it is characterized in that, this electrolyte is made up of electrolyte lithium salt, non-aqueous solvent and electrolyte solution additive, wherein, described electrolyte lithium salt is the combination of the first lithium salt and the second lithium salt, described The first lithium salt is LiPF ₆ , the second lithium salt is composed of lithium difluorooxalate borate and lithium dioxalate borate with a molar ratio of 1:1-5; the electrolyte additive is composed of a film-forming agent and additive A, Described additive A is one or more in alkyl sultone, alkenyl sultone and dialkyl sulfite; The non-aqueous solvent is a combination of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and propylene carbonate with a volume ratio of 10:10-30:5-20:1-5; The film-forming agent is a combination of vinylene carbonate and propylene sulfite in a weight ratio of 1:0.5-5; The molar ratio of the first lithium salt and the second lithium salt is 1:0.02-0.2. 2 . The electrolyte according to claim 1 , wherein the concentration of the electrolyte lithium salt in the electrolyte is 0.5-2 mol/L. 3 . 3. The electrolyte according to claim 2, wherein the concentration of the electrolyte lithium salt in the electrolyte is 1-1.1 mol/L. 4 . The electrolyte according to claim 1 , wherein the additive A is 1,3-propane sultone, 1,4-butane sultone, 1,3- One or more of propylene sultone, dimethyl sulfite and diethyl sulfite. 5. The electrolyte according to any one of claims 1-3, wherein, based on the total weight of the electrolyte, the content of the film-forming agent is 2-8% by weight; The content is 0.1-5% by weight. 6. The electrolyte according to claim 5, wherein, based on the total weight of the electrolyte, the content of the film-forming agent is 3-5% by weight; the content of the additive A is 0.5-2% by weight %. 7 . The electrolyte according to claim 1 , wherein the molar ratio of lithium difluorooxalate borate to lithium dioxalate borate is 1:1.5-3. 8 . 8. a lithium ion battery, the battery comprises a pole core and an electrolyte, the pole core and the electrolyte are sealed in the battery casing, the pole core comprises a positive electrode, a negative electrode and a separator, and the electrolyte is claim 1 The electrolyte solution described in any one of -7. 9. The battery according to claim 8, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer on the surface of the positive electrode current collector, the positive electrode material layer containing a positive electrode active material, a composite conductive agent and a binding agent agent, the composite conductive agent contains the first carbon black, the second carbon black and the vapor-generated carbon fiber, Wherein, the apparent density of the first carbon black is 60-90kg/m ³ , the specific surface area is 60-80m ² /g, and the electrical conductivity is 10 ^{2-10 4} S/m; The apparent density of the second carbon black is 17-50 kg/m ³ , the specific surface area is 800-1000 m ² /g, and the electrical conductivity is 10 ⁵ -10 ⁷ S/m. 10. The battery of claim 9, wherein the first carbon black is carbon black Super P, and the second carbon black is Ketjen black. 11. The battery according to claim 9 or 10, wherein the gas-phase generated carbon fibers have a diameter of 140-160 nm, a length of 5-10 μm, a tensile modulus of 1-10 GPa, and a density of 80-100 kg/ ^m , the thermal expansion coefficient is -0.5×10 ^-6 to -1×10 ^-6 , the thermal conductivity is 1000-2000Wm ^-1 K ^-1 , and the electrical conductivity is 10 ⁵ -10 ⁷ S/m. 12. The battery according to claim 9 or 10, wherein the binder is at least one of polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, sodium carboxymethyl cellulose, and polyethylene. 13. The battery according to claim 9, wherein, based on the total weight of the positive electrode active material, the composite conductive agent and the binder, the content of the positive electrode active material is 85-98% by weight , the content of the composite conductive agent is 1-10% by weight, and the content of the binder is 0.1-10% by weight. 14. The battery according to claim 13, wherein, based on the total weight of the positive electrode active material, the composite conductive agent and the binder, the content of the positive electrode active material is 96-98% by weight , the content of the composite conductive agent is 1-5% by weight, and the content of the binder is 0.1-5% by weight. 15. The battery according to any one of claims 9, 10, 13 and 14, wherein the positive electrode active material is LiCo _p Ni _q Mn _1-pq O ₂ , wherein 0<p<1, 0<q<1. 16. The battery according to claim 15, wherein the positive active material is LiCo _0.2 Ni _0.5 Mn _0.3 O ₂ , LiCo _0.2 Ni _0.6 Mn _0.2 O ₂ , LiCo _0.1 Ni _0.8 Mn _0.1 O ₂ and LiCo _0.05 Ni _0.9 One or more of Mn _0.05 O ₂ .