CN118589019A - A battery - Google Patents

Abstract

The invention provides a battery, which comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the positive plate comprises a positive current collector and a positive active layer arranged on at least one side surface of the positive current collector, the positive active layer comprises a lithium cobaltate material, and the mass percentage of cobalt element in the positive active layer is W _Co; the electrolyte comprises fluoroacetonitrile and polynitrile compounds; based on the mass of the electrolyte, the mass percentage of the fluoroacetonitrile is W _a, and the mass percentage of the polynitrile compound is W _b; wherein, W _a+W_b)/W_Co is less than or equal to 0.12 and less than or equal to 1.12, the invention uses the fluoro-acetonitrile and the polynitrile compound in the electrolyte in a matching way, and controls the content of the fluoro-acetonitrile and the polynitrile compound and the cobalt element content in the positive electrode active layer to meet the above relation, thereby leading the battery to have excellent cycle performance, quick charge performance and storage performance under high voltage.

Description

A battery

Technical Field

The invention belongs to the technical field of batteries and relates to a battery.

Background Art

With the development of electric vehicles, energy storage systems and other high-energy consumption applications, the energy density and charging speed requirements of lithium-ion batteries are getting higher and higher.

The energy density of lithium-ion batteries is closely related to the energy density of their positive and negative electrode materials and the operating voltage of the battery. Therefore, improving the compaction density and gram capacity of the positive and negative electrode materials of the battery, as well as increasing the operating voltage of the lithium-ion battery, are effective ways to increase the energy density of the battery. Compared with other positive electrode materials, lithium cobalt oxide materials have the advantages of high specific capacity, high voltage and high rate discharge, which is conducive to increasing the energy density of the battery. However, the structural stability of lithium cobalt oxide materials themselves is poor. As the voltage increases, the lithium cobalt oxide materials will undergo phase changes, causing the volume of the material particles to expand and contract. This volume deformation not only affects the structural stability of the material, but also causes the destruction of the electrode material and the attenuation of the battery cell performance, making the battery's cycle performance at high voltage worse.

The charging speed of lithium-ion batteries is closely related to the conductivity of the electrolyte. The electrolytes commonly used in the field for high-voltage fast-charging systems usually include polynitrile compounds, which have a wide electrochemical window and ionic conductivity, and can form a good protective film at the positive electrode to avoid side reactions between the positive electrode material and the electrolyte. However, these compounds have poor compatibility with the negative electrode material, resulting in large interfacial impedance and high internal resistance of the interfacial impedance battery, which is not conducive to the battery's cycle performance and storage performance.

Therefore, how to make the battery have both good cycle performance and storage performance at high voltage is a technical problem that needs to be solved urgently in this field.

Summary of the invention

The present invention provides a battery, which uses fluoroacetonitrile and a polynitrile compound in an electrolyte and controls the contents of the two and the cobalt content in a positive electrode active layer to satisfy the above relationship, thereby enabling the battery to have excellent cycle performance, fast charging performance and storage performance at high voltage.

The present invention provides a battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode active layer comprises a lithium cobalt oxide material; the mass percentage of the cobalt element in the positive electrode active layer is W _Co ;

The electrolyte comprises fluoroacetonitrile and a polynitrile compound; based on the mass of the electrolyte, the mass percentage of the fluoroacetonitrile is W _a , and the mass percentage of the polynitrile compound is W _b ;

Among them, 0.12≤(W _a +W _b )/W _Co ≤1.12.

In the present invention, the polynitrile compound refers to a nitrile compound containing at least two cyano groups.

The cyano group is a highly active functional group that can coordinate with the cobalt element in the positive electrode active layer, thereby forming a good protective film at the interface between the positive electrode and the electrolyte, avoiding the side reaction between the lithium cobalt oxide material and the electrolyte. However, the negative electrode usually uses carbon materials or silicon materials as the negative electrode active material. The carbon materials or silicon materials are highly inert and are not easy to have a strong interaction force with the cyano functional group. Therefore, the traditional polynitrile compounds in the electrolyte have poor compatibility with the negative electrode, resulting in a large interface impedance between the negative electrode and the electrolyte, increasing the internal resistance of the battery, and being detrimental to the battery's cycle performance and storage performance.

Based on this, the present invention adds fluoroacetonitrile to the electrolyte. Compared with traditional nitrile compounds in the electrolyte, the F atom contained in fluoroacetonitrile is extremely electronegative and can react with the relatively inert carbon material or silicon material in the negative electrode to form a stable C-F or Si-F bond, thereby enhancing the compatibility of the electrolyte with the negative electrode, reducing the internal resistance of the battery, and being beneficial to improving the battery cycle performance and storage performance.

In addition, fluoroacetonitrile and polynitrile compounds are compounded. Fluoroacetonitrile has a lower viscosity and is easy to transport in the electrolyte. It can form a film on the positive electrode surface better than polynitrile compounds, and the formed film contains C-F bonds, which is beneficial to the transmission of lithium ions. On the basis of the film formation of fluoroacetonitrile, the polynitrile compounds coordinate with the cobalt ions in the positive electrode material to form a film, block the metal dissolution channel, and enhance the structural stability of the lithium cobalt oxide material. Therefore, the inner protective film formed by fluoroacetonitrile and the outer protective film formed by the polynitrile compounds work synergistically, without affecting the transmission of lithium ions, but also effectively avoiding the dissolution of the cobalt element and reducing the side reaction between the positive electrode material and the electrolyte.

In summary, the inventors have studied the synergistic effect of the fluoroacetonitrile content _Wa , the polynitrile compound content _Wb and the cobalt element content _Wc , and found that when the three satisfy 0.12≤( _Wa + _Wb ) _/WCo≤1.12 , the electrolyte and the negative electrode can have a lower interface impedance, and the structural stability of the positive electrode material can be enhanced to avoid the dissolution of the cobalt element, thereby making the battery have excellent cycle performance and storage performance.

The polynitrile compounds of the present invention include, but are not limited to, succinonitrile (SN), glutaronitrile, adiponitrile (AND), 1,5-dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 1,8-dicyanooctane, 1,9-dicyanononane, 1,10-dicyanodecane, 1,12-dicyanododecane, tetramethylsuccinonitrile, 2-methylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 1,4-dicyanopentane, 2,6-dicyanoheptane, 2,7-dicyanooctane, 2,8-dicyanononane, 1,9 ... ,6-dicyanodecane, 1,2-dicyanobenzene, 1,3-dicyanobenzene, 1,4-dicyanobenzene, 3,5-dioxa-heptanenitrile, 1,4-bis(cyanoethoxy)butane, ethylene glycol di(2-cyanoethyl) ether, diethylene glycol di(2-cyanoethyl) ether, triethylene glycol di(2-cyanoethyl) ether, tetraethylene glycol di(2-cyanoethyl) ether, 3,6,9,12,15,18-hexaoxaicosanoic acid dinitrile, 1,3-bis(2-cyanoethoxy)propane, 1,4-bis(2-cyanoethoxy)butane, 1,5-bis(2-cyanoethyl) 1,4-dicyano-2-butene, 1,4-dicyano-2-methyl-2-butene, 1,4-dicyano-2-ethyl-2-butene, 1,4-dicyano-2,3-dimethyl-2-butene, 1,4-dicyano-2,3-diethyl-2-butene, 1,6-dicyano-3-hexene, 1,6-dicyano-2-methyl-3-hexene and 1,6-dicyano-2-methyl-5-methyl-3-hexene, 1,3,5-pentanetricarbonitrile, 1,2,3-propanetricarbonitrile, 1,3,6 - one or more of hexanetrinitrile (HTCN), 1,2,6-hexanetrinitrile, 1,2,3-tris(2-cyanoethoxy)propane, 1,2,4-tris(2-cyanoethoxy)butane, 1,1,1-tris(cyanoethoxymethylene)ethane, 1,1,1-tris(cyanoethoxymethylene)propane, 3-methyl-1,3,5-tris(cyanoethoxy)pentane, 1,2,7-tris(cyanoethoxy)heptane, 1,2,6-tris(cyanoethoxy)hexane, tris(cyanoethyl)phosphate and 1,2,5-tris(cyanoethoxy)pentane.

In a preferred embodiment, (W _a ² -W _b ² )/W _a ≤0.80. Fluoroacetonitrile and polynitrile compounds satisfy this relationship. Fluoroacetonitrile can form a stable surface film on the positive electrode surface in preference to polynitrile compounds, thereby enhancing the structural stability of the positive electrode material and optimizing the transmission path of lithium ions on the surface of the positive electrode material, thereby reducing the transmission resistance. On this basis, the polynitrile compound coordinates with the cobalt ions in the positive electrode material to block the metal dissolution channel. At the same time, the mass transfer interface film formed on the inner layer effectively protects the positive electrode interface without affecting the transmission performance of lithium ions, thereby ensuring the rapid transmission of lithium ions under fast charging conditions.

When the content of fluoroacetonitrile in the electrolyte is low, the viscosity of the electrolyte is high and the compatibility with the negative electrode is poor; when the content of fluoroacetonitrile is high, the cyanide content in the electrolyte is low, which is not conducive to coordination with the positive electrode material and has limited protective effect on the positive electrode. In order to have both low electrolyte viscosity and maintain a suitable concentration of fluorine atoms and cyanide in the electrolyte, 5%≤W _a ≤85%, preferably, 10%≤W _a ≤60%. Exemplarily, _Wa can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or a range consisting of any two of these values.

For the purpose of making the electrolyte have both low viscosity and suitable cyanide concentration, 2%≤W _b ≤10%, preferably, 4%≤W _b ≤6%. Exemplarily, W _b can be 2%, 4%, 4.5%, 5%, 5.5%, 6%, 8%, 10% or a range consisting of any two of them.

The higher the cobalt content in the positive electrode active layer, the more conducive it is to improving the battery energy density; however, the higher the cobalt content, the worse the structural stability of the positive electrode sheet. In order to achieve both good positive electrode stability and battery energy density, 35% ≤ W _Co ≤ 65%, preferably, 54.8% ≤ W _Co ≤ 63.1%.

In a preferred embodiment, the electrolyte further includes an additive shown in Formula I:

A-B-C formula I;

In Formula I, A is selected from the structure shown in Formula Ia or Formula Ib, B is selected from a single bond, H or the structure shown in Formula Ic, and C is selected from H, the structure shown in Formula Id or the structure shown in Formula Ie;

In formula Ia, X ₁ is selected from -O- or -CH ₂ -, and n ₁ is an integer between 0 and 4;

In Formula Ib, X ₂ is selected from -O- or -CH ₂ -, and n ₂ is an integer between 0 and 3;

In Formula Ic, X ₃ and X ₄ are each independently selected from -O- or -CH ₂ -, and Y ₁ is selected from -C(O)- or -S(O) ₂ -;

In Formula Id, X ₅ and X ₆ are each independently selected from -O- or -CH ₂ -, Y ₂ is selected from -C(O)- or -S(O) ₂ -, and n ₃ is an integer between 0 and 4;

In Formula Ie, X ₇ is selected from -O- or -CH ₂ -, and n ₄ is an integer between 0 and 3.

In the above structural formula, Indicates that it is bonded to other groups at this position.

Lithium ions have a high degree of solvation in fluoroacetonitrile solvents, resulting in greater resistance to desolvation when embedded in the positive and negative electrodes, and the sulfur atoms in the additive shown in Formula I can provide lone pairs of electrons to form strong coordination bonds with lithium ions, thereby improving the coordination environment between lithium ions and fluoroacetonitrile. This coordination adjustment can weaken the solvation ability of lithium ions in fluoroacetonitrile and reduce the interaction force between lithium ions and fluoroacetonitrile, thereby increasing the deintercalation rate of lithium ions in fluoroacetonitrile solvents, better improving the kinetic properties of ions in the electrolyte, and thereby improving the fast charging performance of the battery.

In addition, the additive shown in Formula I can also form a _{SEI film mainly composed of Li2SO3 on} the surface of the negative electrode, and the film has high density and strength. Fluoroacetonitrile can form LiF on the surface of the negative electrode. LiF and the SEI film mainly composed of _Li2SO3 work together to make the SEI film have better mechanical strength, making the SEI film not easily damaged during the cycle process, _and forming better protection for the negative electrode interface.

In addition, the sulfonic acid group in the additive shown in Formula I can form a CEI film on the surface of the positive electrode, protect the positive electrode interface, avoid side reactions between the electrolyte and the positive electrode material, further inhibit the cycle expansion of the battery, and extend its cycle life.

In a preferred embodiment, the additive represented by formula I is selected from one of the following compounds:

Taking the compound of formula I-2 as an example, the additive shown in formula I can be prepared by referring to the following method:

2,4:3,5-di-O-benzylidene-L-iditol and 2,2-dimethoxypropane are placed in a reaction container at the same time, and a catalyst p-TsOH is added thereto. After the reaction, an intermediate product compound 1 is obtained. The intermediate product compound 1 is mixed with dimethyl carbonate and TBD and reacted to obtain an intermediate product compound 2. p-TsOH is then added to the reaction container to obtain compound 3. Dichlorothionyl is added thereto to obtain compound 4. Compound 4 is then oxidized using RuCl ₃ as a catalyst and NaIO ₄ as an oxidant to obtain the compound shown in formula I-2.

In a preferred embodiment, the electrolyte further includes a lithium salt, and based on the mass of the electrolyte, the mass percentage of the lithium salt is W _d , and the mass percentage of the additive represented by formula I is W _c , wherein W _d /W _c ≥3.

As mentioned above, when the contents of the lithium salt and the additive shown in Formula I in the electrolyte satisfy the above relationship, it is further beneficial to improve the negative electrode interface stability and enhance the cycle performance of the battery.

The lithium salt of the present invention includes, but is not limited to, one or more of lithium hexafluorophosphate (LiPF ₆ ), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium difluorooxalatoborate (LiDFOB), lithium bis(trifluoromethylsulfonylimide), lithium difluorobis(oxalatophosphate), lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, tris(trifluoromethylsulfonyl)methyllithium or lithium bis(trifluoromethylsulfonyl)imide.

In a preferred embodiment, 0.5%≤W _c ≤10%. Exemplarily, the mass percentage of the additive represented by Formula I is 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range consisting of any two of these values.

In a preferred embodiment, 5%≤W _d ≤30%, preferably, 10%≤W _d ≤20%, more preferably, 12%≤W _d ≤17%.

In a preferred embodiment, the lithium cobalt oxide material includes first particles and second particles, and the ratio of the particle sizes of the first particles and the second particles is N, wherein 3≤N≤6.5.

Lithium cobalt oxide materials with larger particles are conducive to providing better electronic conductivity and higher charge and discharge rates, which are beneficial to the overall power output of the battery, while smaller particles can provide a larger specific surface area, which is conducive to increasing the contact area between the electrode and the electrolyte, and improving the ion transmission efficiency and material capacity. Therefore, the use of first particles with larger particle sizes and second particles with smaller particle sizes within the above particle size range can balance the electronic and conductive performance and ion transmission efficiency.

Further, the D50 particle size of the first particles is 15 to 35 μm; and/or, the D50 particle size of the second particles is 5 to 20 μm. Exemplarily, the D50 particle size of the first particles is 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, or a range value consisting of any two values therein, and the D50 particle size of the second particles is 5 μm, 10 μm, 15 μm, 20 μm, or a range value consisting of any two values therein.

In a preferred embodiment, W _a /N ≥ 0.012. When the content of fluoroacetonitrile in the electrolyte and the ratio of the D50 particle size of the first particle and the second particle in the lithium cobalt oxide material meet the above ratio, there is good contact between the interface of the lithium cobalt oxide material and the interface of the electrolyte, which shortens the ion transmission distance and thus reduces the internal resistance of the battery, so that the battery still has a good cycle capacity retention rate when discharged at a high rate.

In a specific embodiment, the electrolyte of the present invention further includes carbonate solvents and/or carboxylate solvents. The carbonate solvents are selected from one or more of fluorinated or unsubstituted ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, diethyl carbonate (DEC), and ethyl methyl carbonate; the carboxylate solvents are selected from one or more of fluorinated or unsubstituted propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, propyl propionate (PP), ethyl propionate (EP), methyl butyrate, and ethyl butyrate.

The positive electrode current collector of the present invention can be selected from aluminum foil.

In a specific embodiment, the positive electrode active layer includes 80% to 99.8% of positive electrode active material, 0.1% to 10% of conductive agent, and 0.1% to 10% of binder by mass percentage. Preferably, the positive electrode active layer includes 90% to 99.6% of positive electrode active material, 0.2% to 5% of conductive agent, and 0.2% to 5% of binder by mass percentage.

In a specific embodiment, the positive electrode active layer of the present invention may include, in addition to lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel manganese oxide, lithium iron phosphate, ternary materials and other positive electrode active materials.

Furthermore, elements may be doped into the lithium cobalt oxide material to enhance the internal structural stability of the lithium cobalt oxide material, wherein the doping elements are selected from one or more of Ti, Mg, Al, and Ni.

The conductive agent includes but is not limited to at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber.

The above-mentioned binder includes but is not limited to at least one of sodium carboxymethyl cellulose, styrene-butadiene latex, polytetrafluoroethylene, and polyethylene oxide.

The positive electrode sheet of the present invention can be prepared by the following method: dispersing the positive electrode active material, the conductive agent and the binder in a solvent to obtain a positive electrode active material layer slurry, then coating the slurry on the positive electrode collector, and drying to obtain the positive electrode sheet.

The negative electrode sheet of the present invention comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface of the negative electrode current collector, wherein the negative electrode current collector can be selected from copper foil.

In a specific embodiment, the negative electrode active layer includes 80% to 99.8% of negative electrode active material, 0.1% to 10% of conductive agent, and 0.1% to 10% of binder by mass percentage. Preferably, the negative electrode active layer includes 90% to 99.6% of negative electrode active material, 0.2% to 5% of conductive agent, and 0.2% to 5% of binder by mass percentage.

The negative electrode active material of the present invention includes but is not limited to at least one of artificial graphite, natural graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon-carbon material, and silicon-oxygen material.

The conductive agent in the negative electrode active layer includes, but is not limited to, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, and carbon fiber.

The binder in the negative electrode active layer includes but is not limited to at least one of sodium carboxymethyl cellulose, styrene-butadiene latex, polytetrafluoroethylene, and polyethylene oxide.

The negative electrode sheet of the present invention can be prepared by the following method: dispersing the negative electrode active material, conductive agent and binder in a solvent to form a negative electrode active slurry; then coating the negative electrode active slurry on the negative electrode collector, and drying and rolling to obtain the negative electrode sheet.

The function of the separator is to separate the positive electrode plate from the negative electrode plate and provide a channel for lithium ion migration. The separator of the present invention can be selected from polypropylene separators, polyethylene separators, and the like.

In a specific embodiment, the battery is prepared by the following method: the positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence, so that the separator is located between the positive electrode sheet and the negative electrode sheet, and the battery cell is obtained by a stacking or winding process, and then the battery of the present invention is obtained through baking, liquid injection, formation, packaging and other processes.

The implementation of the present invention has at least the following beneficial effects:

1) The electrolyte of the present invention includes fluoroacetonitrile, in which the extremely strong electronegativity of fluorine atoms allows it to react with the inert negative electrode active material to form a stable covalent bond, thereby enhancing the compatibility between the electrolyte and the negative electrode, improving the interface impedance, and reducing the internal resistance of the battery. Fluoroacetonitrile also has a low viscosity and a high dielectric constant, which improves the conductivity of the electrolyte, thereby enabling the battery to have good cycle performance, fast charging performance and storage performance.

2) Fluoroacetonitrile and polynitrile compounds are used in the electrolyte of the present invention, so that there are sufficient cyano groups in the electrolyte to coordinate with the cobalt element in the positive electrode material to form a protective film on the positive electrode surface, thereby enhancing the structural stability of the lithium cobalt oxide material, inhibiting the battery thickness expansion caused by the phase change of its structure under high voltage, avoiding the side reaction between the positive electrode material and the electrolyte, and inhibiting the gas production of the battery.

3) The present invention studies the synergistic effect of the fluoroacetonitrile content _Wa , the polynitrile compound content _Wb and the cobalt element content _Wc in the electrolyte to satisfy a specific relationship, so that the battery has excellent high-voltage resistance, cycle performance, storage performance and fast charging performance.

DETAILED DESCRIPTION

In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

The battery provided by the present invention will be further described in detail below through specific embodiments.

Unless otherwise specified, the reagents, materials and instruments used in the following examples are conventional reagents, conventional materials and conventional instruments in the art and can be obtained commercially. The reagents involved can also be synthesized by conventional methods in the art.

Examples 1 to 25 and Comparative Examples 1 to 6

Examples 1 to 25 and Comparative Examples 1 to 6 provide a battery, and the preparation method is as follows:

1. Preparation of positive electrode

The positive electrode active materials lithium cobalt oxide (LiCoO ₂ ), polyvinylidene fluoride (PVDF), SP (superP) and carbon nanotubes (CNT) are mixed in a mass ratio of 96:2:1.5:0.5, N-methylpyrrolidone (NMP) is added, and the mixture is stirred under the action of a vacuum mixer until the mixed system becomes a positive electrode active slurry with uniform fluidity; the positive electrode active slurry is evenly coated on both surfaces of an aluminum foil; the coated aluminum foil is dried, and then rolled and cut to obtain the desired positive electrode sheet.

The lithium cobalt oxide is obtained by mixing first particles with a particle size D50 of 30.7 μm and second particles with a particle size D50 of 8.3 μm in a mass ratio of 4:1, and the mass percentage content of the cobalt element W _Co in the positive electrode active layer is 57.8%.

2. Preparation of negative electrode sheet

A mixture of artificial graphite and silicon carbon in a mass ratio of 4:1 is used as the negative electrode active material, wherein the outer surface of the silicon carbon has a coating layer, and the coating layer includes carbon elements;

The negative electrode active material, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black and carbon nanotubes are uniformly mixed in a mass ratio of 98.2:0.5:0.5:0.2:0.6, deionized water is added, and the negative electrode active slurry is obtained under the action of a vacuum mixer; the negative electrode active slurry is uniformly coated on both surfaces of the copper foil; the coated copper foil is dried at room temperature, and then transferred to an oven at 80° C. for drying for 10 hours, and then cold pressed and cut to obtain a negative electrode sheet, wherein the weight content of elemental sodium in the negative electrode active material layer is 0.048%.

3. Preparation of electrolyte

In a glove box filled with argon (H ₂ O <0.1 ppm, O ₂ <0.1 ppm), fully dried lithium salt is added to a mixture of a solvent and an additive, and the electrolyte is obtained after dissolution.

The solvent is obtained by mixing EC, PC, EP and PP in a mass ratio of 1:1:2:6, and the type of lithium salt in the electrolyte is lithium hexafluorophosphate, and the mass content _Wd is 15%.

The additives include 8% FEC and other additives. The types and mass contents of the other additives are listed in Table 1.

4. Preparation of lithium-ion batteries

The positive electrode sheet of step 2, the negative electrode sheet of step 3 and the separator (9 μm PP separator) are stacked in the order of positive electrode sheet, separator and negative electrode sheet, and then wound to obtain a battery cell; the battery cell is placed in an outer packaging aluminum foil, and the electrolyte of step 1 is injected into the outer packaging aluminum foil. After vacuum packaging, standing, formation, shaping, sorting and other processes, a lithium-ion battery is obtained.

Table 1

Embodiments 26 to 32

Embodiments 26 to 32 provide a battery, and the preparation method thereof is basically the same as that of Embodiment 1, except that, in the preparation of the electrolyte, the mass content of the added fluoroacetonitrile is 7%, and in the preparation of the positive electrode sheet, the lithium cobalt oxide of Embodiment 26 is obtained by mixing the first particles with a particle size D50 of 30.7 μm and the second particles with a particle size D50 of 8.3 μm in a mass ratio of 4:1, the lithium cobalt oxide of Embodiment 27 includes only the first particles with a particle size D50 of 30.7 μm, the lithium cobalt oxide of Embodiment 28 includes only the second particles with a particle size D50 of 8.3 μm, and the lithium cobalt oxide of Embodiment 29 includes the second particles with a particle size D50 of 30 μm. The first particle and the second particle with a particle size D50 of 6 μm are mixed in a mass ratio of 4:1. The lithium cobalt oxide of Example 30 is obtained by mixing the first particle with a particle size D50 of 35 μm and the second particle with a particle size D50 of 5 μm in a mass ratio of 4:1. The lithium cobalt oxide of Example 31 is obtained by mixing the first particle with a particle size D50 of 30.7 μm and the second particle with a particle size D50 of 3 μm in a mass ratio of 4:1. The lithium cobalt oxide of Example 32 is obtained by mixing the first particle with a particle size D50 of 40 μm and the second particle with a particle size D50 of 8.3 μm in a mass ratio of 4:1. The specific change factors are listed in Table 2.

Table 2

Test Case

1. Cycle performance

A. Capacity retention rate after 800 cycles at 25℃

The battery was charged and discharged for 800 cycles at 25°C at a rate of 5C within the charge and discharge cut-off voltage range of 3.0 to 4.55V. The discharge capacity in the first week of the test was calculated as x1mAh, and the discharge capacity in the 800th cycle was calculated as y1mAh. The capacity retention rate after 800 cycles at 25°C was calculated by y1/x1×100%.

B. Capacity retention rate after 800 cycles at 45℃

The battery was charged and discharged for 800 cycles at 45°C at a rate of 5C in the charge and discharge cut-off voltage range of 3.0 to 4.55V. The discharge capacity in the first week of the test was calculated as m1 mAh, and the discharge capacity after 800 cycles was calculated as n1 mAh. The cycle capacity retention rate after 800 cycles at 45°C was calculated by n1/m1×100%.

2. Rate performance

Test method: First, let the battery stand for 10 minutes after being divided into capacities, then discharge it at 0.2C to the lower limit voltage of 3V, the discharged capacity is marked as the initial discharge capacity C0, let it stand for 10 minutes, then charge it at 0.7C to the upper limit voltage of 4.55V, and then charge it at the upper limit voltage to 0.025C cutoff, finally discharge it at 5C current to the lower limit voltage of 3.0V, record the battery's discharge capacity C1, and calculate the 5C discharge rate capacity retention rate by C1/C0.

3. High temperature storage performance

Test method: At room temperature, record the initial thickness T1 of the battery after capacity division, then place the fully charged battery in a constant temperature box at 85°C for 15 days, take it out after storage and cool it back to room temperature, record the thickness T2 of the battery, and calculate the thickness expansion rate of the battery stored at 85°C for 15 days by T2/T1×100%.

The above test results are listed in Table 3.

Table 3

1) From the comparison of Examples 1, 2, 4, 7, 22, and 26, it can be seen that when the content of fluoroacetonitrile in the electrolyte is too low, less than 10%, the battery's 25°C and 45°C cycle performance, fast charging performance, and high-temperature storage performance will deteriorate.

2) From the comparison of Examples 1, 3, 5, and 6, it can be seen that when the content of the polynitrile compound is within the optimal range of 4% to 6%, the battery can have more excellent comprehensive performance, but when it is less than 4% or greater than 7%, the battery cycle, rate and high-temperature storage performance will be slightly reduced.

3) From the comparison of Examples 1, 8, 9, 23, 24, and 25, it can be seen that the use of different types of polynitrile compounds can enable the battery to have excellent high and low temperature cycle performance, fast charging performance, and high temperature storage performance.

4) From the comparison of Examples 1 and 10 to 18, it can be seen that the addition of cyclic sulfonate compounds of Formula I of different substitution types can improve the high and low temperature cycle performance, fast charging performance and high temperature storage performance of the battery, especially the improvement of the fast charging performance is more significant.

5) From the comparison of Examples 1 and 19 to 21, it can be seen that when the content of the cyclic sulfonate compound represented by Formula I is less than 0.5% or greater than 10%, the high and low temperature cycle performance, fast charging performance and high temperature storage performance of the battery will deteriorate.

6) From the comparison of Example 1 and Comparative Examples 1 to 3, it can be seen that when no fluoroacetonitrile or polynitrile compound is added to the electrolyte, the high and low temperature cycle performance, high and low temperature cycle performance, fast charging performance and high temperature storage performance of the battery are significantly poor.

7) It can be seen from Comparative Examples 4 to 6 that when the content Wa of fluoroacetonitrile, the content Wb of the polynitrile compound and the content of the Co element in the positive electrode active layer do not meet the range of 0.12 to 1.12, the high and low temperature cycle performance, high and low temperature cycle performance, fast charging performance and high temperature storage performance of the battery will also be significantly deteriorated, especially when the cyclic sulfonate compound shown in Formula I is not added to Comparative Example 6, the fast charging performance deteriorates more significantly.

8) From the comparison of Examples 26 to 28, it can be seen that the use of lithium cobalt oxide materials with first particles and second particles of matching particle size is beneficial to making the battery have better 25°C cycle performance, 45°C storage performance, 5C discharge performance and high temperature storage performance; from the comparison of Examples 26, 29 to 32, it can be seen that when the D50 particle size of the first particles used is too large, or the D50 particle size of the second particles is too small, or the ratio N of the particle sizes D50 of the two is not in the range of 3 to 65, or _Wa /N≤0.012, the high and low temperature cycle performance, 5C rate performance, and 85°C high temperature storage performance of the battery are all poor compared to those that meet the above conditions.

The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some or all of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims (10)

1. A battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector, the positive electrode active layer comprises a lithium cobalt oxide material; the mass percentage of the cobalt element in the positive electrode active layer is W _Co ; The electrolyte comprises fluoroacetonitrile and a polynitrile compound; based on the mass of the electrolyte, the mass percentage of the fluoroacetonitrile is W _a , and the mass percentage of the polynitrile compound is W _b ; Among them, 0.12≤(W _a +W _b )/W _Co ≤1.12. 2 . The battery according to claim 1 , wherein (W _a ² −W _b ² )/W _a ≤ 0.80. 3. The battery according to claim 1 or 2, characterized in that 5%≤W _a ≤85%, preferably 10%≤W _a ≤60%; and/or, 2%≤W _b ≤10%, preferably, 4%≤W _b ≤6%; And/or, 35%≤W _Co ≤65%, preferably, 54.8%≤W _Co ≤63.1%. 4. The battery according to any one of claims 1 to 3, characterized in that the electrolyte further comprises an additive represented by formula I: A-B-C formula I; In Formula I, A is selected from the structure shown in Formula Ia or Formula Ib, B is selected from a single bond, H or the structure shown in Formula Ic, and C is selected from H, the structure shown in Formula Id or the structure shown in Formula Ie; In formula Ia, X ₁ is selected from -O- or -CH ₂ -, and n ₁ is an integer between 0 and 4; In Formula Ib, X ₂ is selected from -O- or -CH ₂ -, and n ₂ is an integer between 0 and 3; In Formula Ic, X ₃ and X ₄ are each independently selected from -O- or -CH ₂ -, and Y ₁ is selected from -C(O)- or -S(O) ₂ -; In Formula Id, X ₅ and X ₆ are each independently selected from -O- or -CH ₂ -, Y ₂ is selected from -C(O)- or -S(O) ₂ -, and n ₃ is an integer between 0 and 4; In Formula Ie, X ₇ is selected from -O- or -CH ₂ -, and n ₄ is an integer between 0 and 3. 5. The battery according to claim 4, characterized in that the additive represented by formula I is selected from one or more of the following compounds: 6. The battery according to claim 4 or 5, characterized in that the electrolyte further comprises a lithium salt, and based on the mass of the electrolyte, the mass percentage of the lithium salt is W _d , and the mass percentage of the additive represented by formula I is W _c , wherein W _d /W _c ≥3. 7. The battery according to claim 6, characterized in that 0.5%≤W _c ≤10%; And/or, 5%≤W _d ≤30%, preferably, 10%≤W _d ≤20%. 8. The battery according to any one of claims 1 to 7, characterized in that the lithium cobalt oxide material comprises first particles and second particles, and the ratio of the D50 particle size of the first particles to the second particles is N; Among them, 3≤N≤6.5. 9 . The battery according to claim 8 , wherein the D50 particle size of the first particles is 15 to 35 μm; and/or the D50 particle size of the second particles is 5 to 20 μm. 10. The battery according to claim 7, characterized in that _Wa /N≥0.012.