WO2023093589A1 - Lithium secondary battery - Google Patents

Abstract

In order to overcome the problem of poor high-temperature cycle performance of existing high-voltage batteries, the present invention provides a lithium secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the surface of a positive electrode active material is doped or coated with a compound containing a first metal element and a second metal element, and the non-aqueous electrolyte comprises a solvent, an electrolyte salt, and a compound shown in structural formula 1:, wherein R₁ is selected from C3-C6 unsaturated hydrocarbyl groups, R₂ is selected from C2-C5 hydrocarbylene groups, and n=1 or 2; the lithium secondary battery satisfies the following condition: 0.05≤a×(b+c)≤3. According to the lithium secondary battery provided in the present invention, the compound shown in structural formula 1 in the electrolyte can effectively inhibit cobalt ions from dissolving out and repair the damage to a negative electrode SEI film by dissolved cobalt ions, and can mitigate the transfer problem of lithium ions in a battery charging and discharging cycle process caused by doping metal elements, reduce battery impedance and battery polarization, and slow down lithium precipitation of the battery, such that the battery can have good high-temperature cycle performance.

Description

lithium secondary battery technical field

The invention belongs to the technical field of energy storage battery devices, and in particular relates to a lithium secondary battery.

Background technique

Due to the advantages of high working voltage, wide working temperature range, high energy density and power density, no memory effect and long cycle life, lithium-ion batteries have been widely used in the field of 3C digital products such as mobile phones and notebook computers, as well as in the field of new energy vehicles. Applications. In recent years, with the continuous development of thinner and thinner 3C digital products, the battery industry has higher and higher requirements for high energy density of lithium-ion batteries. Therefore, there is an urgent need to increase the energy density of lithium-ion batteries.

At present, there are two main methods to increase the energy density of batteries, one is to increase the positive charge cut-off voltage, and the other is to pressurize the active material layer of the electrode to achieve high density. However, after increasing the cut-off voltage of the positive electrode, the activity of the positive electrode will be further improved, and the side reaction between the positive electrode and the electrolyte will also be intensified, which will lead to the dissolution of the transition metal ions of the positive electrode, resulting in the deterioration of the high temperature performance of the battery. In addition, when a high-pressure electrode is used, due to the low porosity of the high-pressure electrode, the liquid retention capacity of the battery will also decrease, making it difficult for the electrolyte to permeate at the interface of the low-porosity pole piece, thereby making the gap between the electrolyte and the electrode difficult. The contact internal resistance increases, and during the long-term cycle, the charging and discharging polarization becomes larger, resulting in a sudden dive due to lithium precipitation. Therefore, how to improve the long-term cycle performance of high-voltage, high-pressure lithium-ion batteries is an industry problem, which needs to be improved from various aspects such as electrode materials and electrolytes.

In terms of cathode materials, surface coating modification is an important means to improve the performance of cathode materials for lithium-ion batteries. The surface coating material can effectively reduce the corrosion of the positive electrode by the electrolyte and reduce the dissolution of metal ions. At the same time, the surface coating material can also physically isolate the contact between the electrolyte and the positive surface active material. However, in the case of using positive electrode surface coating materials such as alumina, there is a problem that the coated metal oxides that do not have electrochemical activity inhibit the transmission of lithium ions at the positive electrode interface, resulting in increased impedance, which is not conducive to Long-term cycling of lithium-ion batteries.

There is currently no good solution for the electrolyte. Therefore, for high-voltage, high-pressure, high-energy-density batteries, it is a major problem for high-voltage, high-pressure lithium-ion batteries to ensure the long-term cycle performance of the battery from the perspective of the electrolyte, and to ensure that lithium is not decomposed at the end of the cycle.

Contents of the invention

Aiming at the problems of poor high-temperature cycle performance and increased impedance of high-voltage and high-pressure lithium-ion batteries in the prior art, the invention provides a lithium secondary battery.

The technical solution adopted by the present invention to solve the problems of the technologies described above is as follows:

The invention provides a lithium secondary battery, comprising a positive pole, a negative pole and a non-aqueous electrolyte, the positive pole comprises a positive electrode material layer, the positive pole material layer comprises a positive pole active material, and the positive pole active material comprises _LiCoO2 and doped or a coated compound containing a first metal element and a second metal element;

The first metal element includes at least one of Mg, Al and W;

The second metal element includes at least one of Ti, Cr, Zr, Mo, La, Ce and rare earth elements;

Described non-aqueous electrolytic solution comprises the compound shown in lithium salt, organic solvent and structural formula 1:

Wherein, R ₁ is selected from unsaturated hydrocarbon groups with 3-6 carbon atoms, R ₂ is selected from alkylene groups with 2-5 carbon atoms, n=1 or 2;

The lithium secondary battery satisfies the following conditions:

0.05≤a×(b+c)≤3;

Wherein, a is the mass percentage of the compound shown in structural formula 1 in the non-aqueous electrolyte, and the unit is wt%;

b is the doping amount or coating amount of the first metal element relative to the quality of _LiCoO2 , the unit is wt%,

c is the doping amount or covering amount of the second metal element relative to the mass of LiCoO ₂ , and the unit is wt%.

Further, the lithium secondary battery satisfies the following conditions:

0.08≤a×(b+c)≤2.

Further, the compound represented by the structural formula 1 is selected from one or more of the following compounds:

Further, the mass percentage a of the compound represented by the structural formula 1 in the non-aqueous electrolyte is 0.1 to 5 wt %; preferably, the mass percentage of the compound represented by the structural formula 1 in the non-aqueous electrolyte in the lithium secondary battery is The component content a is 0.1 to 2 wt%.

Further, the doping amount or coating amount b of the first metal element relative to the mass of _LiCoO2 is 0.1 to 1.2 wt %; preferably, the doping amount or coating amount of the first metal element relative to the mass of _LiCoO2 b is 0.5 to 0.8 wt%.

Further, the doping amount or coating amount c of the second metal element relative to the _LiCoO2 mass is 0.001-0.1 wt %; preferably, the doping amount or coating amount c of the second metal element relative to the _LiCoO2 mass c is 0.005 to 0.05 wt%.

Further, the charging cut-off voltage of the lithium secondary battery is greater than or equal to 4.45V.

Further, the compacted density of the positive electrode material layer is greater than or equal to 3.8g/cm ³ ; preferably, the compacted density of the positive electrode material layer is 3.9g/cm ³ -4.15g/cm ³ ;

The compacted density of the negative electrode material layer is greater than or equal to 1.6g/cm ³ ; preferably, the compacted density of the negative electrode material layer is 1.65g/cm ³ -1.8g/cm ³ ;

Further, the lithium salt is selected from LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiBOB, LiClO ₄ , LiCF ₃ SO ₃ , LiDFOB, LiN(SO ₂ CF ₃ ) ₂ and LiN(SO ₂ F) ₂ one or more;

The concentration of the lithium salt in the non-aqueous electrolytic solution is 0.5-3.5 mol/L.

Further, the non-aqueous electrolyte also contains auxiliary additives, the auxiliary additives are cyclic sulfate compounds, sultone compounds, cyclic carbonate compounds, unsaturated phosphate compounds and nitriles at least one of the compounds;

Further, based on the total mass of the non-aqueous electrolyte solution being 100%, the amount of the auxiliary additive added is 0.01%-30%.

Further, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate or vinyl methyl sulfate;

The sultone compound is selected from at least one of 1,3-propane sultone, 1,4-butane sultone or 1,3-propene sultone;

The cyclic carbonate compound is selected from at least one of vinylene carbonate, ethylene carbonate, fluoroethylene carbonate or the compound shown in structural formula 2,

In the structural formula 2, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , and R ₂₆ are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group;

The unsaturated phosphate compound is selected from at least one of the compounds shown in structural formula 3:

In the structural formula 3, R ₃₁ , R ₃₂ , and R ₃₃ are each independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si(C _m H _2m+1 ) ₃ , m is 1 to A natural number of 3, and at least one of R ₃₁ , R ₃₂ , and R ₃₃ is an unsaturated hydrocarbon group;

Described nitrile compound comprises succinonitrile, glutaronitrile, ethylene glycol two (propionitrile) ether, hexanetrinitrile, adiponitrile, pimelonitrile, suberonitrile, azelanitrile, sebaconitrile one or more.

Beneficial effects of the present invention:

In the lithium secondary battery provided by the present invention, when the surface of _the LiCoO positive electrode active material is coated or doped with a compound containing the first metal element and the second metal element, a compound shown in structural formula 1 is added to the non-aqueous electrolyte, A large number of studies have found that the doping amount or coating amount of the first metal and the second metal element doped or coated on the surface of the positive electrode active material is closely related to the content of the compound shown in structural formula 1 in the electrolyte. When 0.05≤a×(b+c)≤3, it can effectively inhibit the dissolution of cobalt ions and repair the damage to the SEI film of the negative electrode caused by the cobalt ions dissolved in the positive electrode. At the same time, the doping amount or coating of the first metal element and the second metal element While stabilizing the high-voltage characteristics of the LiCoO ₂ positive electrode active material, it can also slow down the transmission problem of lithium ions during the battery charge and discharge cycle, reduce the battery impedance and battery polarization, so that the battery has a higher energy density. At the same time, it has good high temperature cycle performance.

Detailed ways

In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

An embodiment of the present invention provides a lithium secondary battery, including a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes _LiCoO2 and Doped or clad compounds containing a first metal element and a second metal element;

The first metal element includes at least one of Mg, Al, Zr, and W;

The second metal element includes at least one of Ti, Cr, Mo, and rare earth elements;

The negative pole includes a negative electrode material layer; the non-aqueous electrolytic solution includes a lithium salt, an organic solvent and a compound shown in structural formula 1:

Wherein, R ₁ is selected from unsaturated hydrocarbon groups with 3-6 carbon atoms, R ₂ is selected from alkylene groups with 2-5 carbon atoms, and n is 1 or 2;

The lithium secondary battery satisfies the following conditions:

0.05≤a×(b+c)≤3;

Wherein, a is the mass percentage of the compound shown in structural formula 1 in the non-aqueous electrolyte, and the unit is wt%;

b is the doping amount or coating amount of the first metal element relative to the quality of _LiCoO2 , the unit is wt%,

c is the doping amount or covering amount of the second metal element relative to the mass of LiCoO ₂ , and the unit is wt%.

In some embodiments, the _LiCoO2 positive electrode of the lithium secondary battery is doped or coated with metal elements such as Mg, Al, W, Ti, Cr, Zr, Mo, La, Ce and rare earth elements, compared to those without Doped or coated LiCoO ₂ positive electrode, Mg, Al, W and other elements improve the stability of LiCoO ₂ positive electrode at high temperature and high pressure, Ti, Cr, Zr, Mo, La, Ce and rare earth elements due to the sintering process Slow diffusion kinetics form a unique layered distribution, which is mainly enriched on the top surface and can form strong ionic bonds with oxygen ions, thereby inhibiting the loss of surface oxygen and further improving the high-pressure structural stability of positive electrode active material particles containing cobalt and lithium compounds sex. However, there is a problem that the coated metal elements that are not electrochemically active inhibit the transmission of lithium ions at the interface of the positive electrode, resulting in an increase in the impedance of the positive electrode, which is not conducive to the long-term cycle of the lithium secondary battery.

Based on the above-mentioned deficiencies, the non-aqueous electrolyte of the lithium secondary battery of the present invention contains the substance shown in structural formula 1, and its mechanism of action is not very clear, but the inventor speculates that it is because it can effectively complex on the surface of the positive electrode and inhibit the dissolution of cobalt ions At the same time, due to its special structure, it can effectively repair the damage to the SEI film of the negative electrode caused by the cobalt ions dissolved in the positive electrode, thereby effectively inhibiting the impedance growth of the battery during high temperature and high pressure cycling.

The inventor found through a lot of research that there is a certain relationship between the content of the compound shown in structural formula 1 in the electrolyte and the amount of doping coating, and the compound shown in structural formula 1 can effectively inhibit the dissolution of positive cobalt ions and indirectly improve the stability of the positive electrode. Therefore The amount of doping and coating of the positive electrode material can be appropriately reduced, thereby indirectly reducing the amount of non-electrochemically active metal elements, alleviating the problem of inhibiting lithium ion conduction caused by it, thereby improving the high-temperature and high-pressure cycle performance of the battery, but the structural formula 1 shows If the compound is used in too much amount, the battery performance will be deteriorated due to too high viscosity. Through a lot of research, it is found that when the doping amount or coating amount of the first metal and the second metal element doped or coated on the surface of the positive electrode active material and the content of the compound shown in the structural formula 1 in the electrolyte satisfy 0.05≤a When ×(b+c)≤3, the compound represented by structural formula 1 in the electrolyte can effectively inhibit the dissolution of cobalt ions and repair the damage of the dissolved cobalt ions to the negative electrode SEI film, while the doping of the first and second metal elements While stabilizing the high-voltage characteristics of the positive electrode active material, the amount or coating amount can also slow down the transmission problem of lithium ions during the battery charge and discharge cycle, reduce the battery impedance and battery polarization, so that the battery can have a better performance. High temperature cycle performance.

In a preferred embodiment, the lithium secondary battery meets the following conditions:

0.08≤a×(b+c)≤2;

In some embodiments, in the lithium secondary battery provided by the present application, the content of the compound shown in the non-aqueous electrolyte structural formula 1 is combined with the content of the first metal element and the second metal element on the surface of the positive electrode active material in the lithium secondary battery. The doping amount or coating amount of the compound is related, and to a certain extent, the effects of the first metal element, the second metal element, and the compound shown in structural formula 1 on the battery performance can be integrated, that is, to weaken the battery polarization, reduce the battery impedance, and slow down the battery performance. The battery decomposes lithium, so that the battery can have better high-temperature cycle performance.

In some embodiments, the compound represented by the structural formula 1 is selected from one or more of the following compounds:

In some embodiments, the doping amount or coating amount b of the first metal element relative to the mass of LiCoO ₂ is 0.1-1.2 wt%.

In a preferred embodiment, the doping amount or coating amount b of the first metal element relative to the mass of LiCoO ₂ is 0.5-0.8 wt%.

In some embodiments, the doping amount or coating amount c of the second metal element relative to the mass of LiCoO ₂ is 0.001-0.1 wt%.

In a preferred embodiment, the doping amount or coating amount c of the second metal element relative to the mass of LiCoO ₂ is 0.005-0.05 wt%.

Doping or coating the compound containing the first metal element and the second metal element on the surface of the positive electrode active material _LiCoO2 can effectively increase the battery voltage, but doping or coating excessive metal elements hinders the lithium in the positive electrode active material. Embedding or protruding, thereby reducing the battery capacity and increasing the impedance of the battery. After a lot of research, the inventor found that the doping amount or coating amount b of the first metal element relative to the _LiCoO2 mass in the positive electrode material layer reaches 0.1-1.2 wt%, and the doping amount or coating amount b of the second metal element relative to the _LiCoO2 mass When the coating amount c reaches 0.001-0.1wt%, the first metal element, the second metal element and the compound shown in the structural formula 1 in the electrolyte cooperate with each other, which can not only effectively increase the battery voltage, but also alleviate the inhibition of lithium ions in the positive electrode active material. transmission.

In some embodiments, the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte is 0.1-5 wt%.

In a preferred embodiment, the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte is 0.1-2 wt%.

The positive electrode active material contains lithium cobaltate compound so that the lithium secondary battery has better rate performance, but when the mass percentage of Co element in the positive electrode material layer is too large, it is easy to cause the positive electrode active material to be separated from the non-aqueous material under high voltage conditions. The reaction of the electrolyte leads to the dissolution of the Co element, and the addition of the compound shown in Structural Formula 1 can effectively inhibit the reaction between the non-aqueous electrolyte and the positive active material, thereby inhibiting the dissolution of the Co element in the positive active material, but as shown in Structural Formula 1 The addition amount of the compound should not be too large, the initial impedance of the SEI film formed on the negative electrode of the battery is relatively large, which will cause the battery impedance to increase, and excessive addition of the compound shown in structural formula 1 will increase the viscosity of the electrolyte and reduce the conductance, thereby It is not conducive to the cycle performance of the battery.

The above analysis is only based on the impact of each parameter or multiple parameters on the battery alone, but in the actual battery application process, the above three parameters are interrelated and inseparable. The relational formula given by the present invention relates the three, and the three affect the electrochemical performance of the battery together, so the mass percentage content a of the compound shown in structural formula 1, the first metal element in the positive electrode material layer relative to _the LiCoO quality ratio is adjusted. The doping amount or coating amount b, the doping amount or coating amount c of the second metal element in the positive electrode material layer relative to the quality of _LiCoO2 , so that 0.05≤a×(b+c)≤3, can guarantee lithium secondary While the battery has high stable voltage characteristics, it can also improve the high-temperature cycle performance of the battery; beyond the above-mentioned range, the battery will experience kinetic deterioration, which will shorten the cycle life of the battery under high-temperature conditions, and even fail.

Further, the charging cut-off voltage of the lithium secondary battery is greater than or equal to 4.45V. The positive electrode of the battery of the present invention is doped or coated with a metal element that isolates the contact between the electrolyte and the surface active material of the positive electrode, and the compound shown in structural formula 1 is added to the electrolyte, and the two cooperate to make the battery charge cut-off voltage Greater than or equal to 4.45V, meeting the demand for high energy density of lithium-ion batteries.

Further, the compacted density of the positive electrode material layer is greater than or equal to 3.8g/cm ³ ; preferably, the compacted density of the positive electrode material layer is 3.9g/cm ³ -4.15g/cm ³ ;

The compacted density of the negative electrode material layer is greater than or equal to 1.6g/cm ³ ; preferably, the compacted density of the negative electrode material layer is 1.65g/cm ³ -1.8g/cm ³ ;

The porosity of the positive electrode material layer and the negative electrode material layer is less than or equal to 50%; preferably, the porosity of the positive electrode material layer and the negative electrode material layer is 10%-35%.

The compaction density of the positive electrode material layer and the compaction density of the negative electrode material layer have a certain influence on the electrical performance of the secondary battery. Regarding the compaction density of the positive electrode material layer, in general, the compaction density of the positive electrode material layer is too low, and the positive electrode material is easy to fall off during the preparation of the electrode sheet, which affects the consistency of the overall battery capacity; if the compaction density of the positive electrode material layer If the temperature is too high, the strip is easy to be broken during the preparation of the pole piece, and the process problems will occur, which will reduce the production efficiency. At the same time, during the charging and discharging process of the battery, it will be difficult for lithium ions to come out, which will increase the battery impedance, reduce the battery capacity, and decrease the battery cycle performance. Regarding the compaction density of the negative electrode material layer, in general, if the compaction density of the negative electrode material layer is too low, process abnormalities such as powder drop are more likely to occur during the preparation of the electrode sheet, which reduces production efficiency; if the compaction density of the negative electrode material layer is too large , not only affects the immersion of the electrolyte, but also affects the embedding of the electrolyte, reduces the battery capacity, increases the lithium precipitation of the battery, and reduces the safety performance of the battery. The larger the porosity of the positive and negative electrode material layers, the more conducive to the extraction or insertion of lithium ions, but if the porosity is too large, the volume of the battery will increase and the energy density of the battery will decrease.

In addition, the compacted density of the positive electrode material layer, the compacted density of the negative electrode material layer, the porosity of the positive electrode material layer and the negative electrode material layer are related to the compound of the electrolyte structural formula 1, after adding the compound shown in the structural formula 1 in the non-aqueous electrolyte, Changes in its ionic conductivity and viscosity will affect the permeability of the non-aqueous electrolyte to the positive and negative material layers, thereby affecting the intercalation and extraction efficiency of lithium ions; while the compaction density of the positive and negative material layers also affects The density of the SEI film formed by the decomposition of the compound represented by structural formula 1 on the surface of the negative electrode material layer affects the protective effect of the SEI film on free Co.

In some embodiments, the lithium salt in the non-aqueous electrolyte is LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiBOB, LiClO ₄ , LiCF ₃ SO ₃ , LiDFOB, LiN(SO ₂ CF ₃ ) ₂ and LiN One or more of (SO ₂ F) ₂ , in a preferred embodiment, the lithium salt is LiPF ₆ .

In the non-aqueous electrolyte, the lithium salt contains lithium ions, which are beneficial to the electrochemical response of the battery during the charging and discharging process of the battery, supplementing the consumption of a small amount of lithium ions, and promoting the cycle performance of the battery.

In some embodiments, the conductivity of the non-aqueous electrolyte solution at 25° C. is greater than or equal to 6 mS/cm. The conductivity of the non-aqueous electrolyte has a certain influence on the cycle performance and storage performance of lithium secondary batteries. After a lot of research, the inventor found that when the conductivity of the non-aqueous electrolyte is low at room temperature, the conductivity in the electrolyte is low, the ion transmission rate is reduced, and the concentration polarization increases during the charging and discharging of the lithium secondary battery. Impedance also increases, reducing the cycle performance and storage performance of the battery. The study found that when the conductivity of the non-aqueous electrolyte is greater than 6mS/cm at room temperature of 25°C, it can reduce the concentration polarization to the greatest extent without affecting the transmission of lithium ions. Inhibit the dissolution of cobalt ions and repair the damage of the dissolved cobalt ions to the negative electrode SEI film, and improve the high-temperature cycle and high-temperature storage performance of the battery.

In some embodiments, the concentration of the lithium salt in the non-aqueous electrolyte is 0.5-3.5mol/L; in a preferred embodiment, the concentration of the lithium salt in the non-aqueous electrolyte is 0.8- 2.0mol/L.

In the non-aqueous electrolyte, the concentration of lithium salt is too low, and the concentration polarization increases in the electrochemical reaction of the battery, which is not conducive to the electrochemical reaction of the battery; The ion transport efficiency of the compound affects the compound shown in structural formula 1 to repair the damage of the dissolved cobalt ion to the SEI film of the negative electrode, and inhibits the reduction of the dissolution efficiency of the cobalt ion.

In some embodiments, the organic solvent in the non-aqueous electrolyte is one or more of cyclic carbonates, linear carbonates, carboxylates and ethers; preferably, the cyclic carbonates include One or more of vinylene carbonate, propylene carbonate and ethylene carbonate; preferably, the linear carbonate includes one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate species; preferably, the carboxylic acid esters include methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethyl acetate and trimethyl One or more of ethyl acetate; preferably, the ethers include ethylene glycol dimethyl ether, 1,3-dioxolane and 1,1,2,2-tetrafluoroethyl-2, One or more of 2,3,3-tetrafluoropropyl ether.

In some embodiments, the non-aqueous electrolyte also contains auxiliary additives, and the auxiliary additives are cyclic sulfate compounds, sultone compounds, cyclic carbonate compounds, unsaturated phosphate compounds and at least one of nitrile compounds.

In a preferred embodiment, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate or vinyl methyl sulfate;

The cyclic carbonate compounds include but are not limited to cyclic carbonate compounds with fluorine atoms, such as fluoroethylene carbonate and/or bisfluoroethylene carbonate; preferably, the cyclic carbonate compounds include But not limited to the cyclic carbonate compound of carbon-carbon unsaturated bond, for example, it can be vinylene carbonate, vinyl ethylene carbonate, vinyl ethylene carbonate, methyl vinylene carbonate or the compound shown in structural formula 2 one or more of

In the structural formula 2, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , and R ₂₆ are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group.

The unsaturated phosphate compound is selected from at least one of the compounds shown in structural formula 3:

In the structural formula 3, R ₃₁ , R ₃₂ , and R ₃₃ are each independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si(C _m H _2m+1 ) ₃ , m is 1 to 3, and at least one of R ₃₁ , R ₃₂ , and R ₃₃ is an unsaturated hydrocarbon group.

In a preferred embodiment, the unsaturated phosphoric acid ester compound may be tripropargyl phosphate, dipropargyl methyl phosphate, dipropargyl ethyl phosphate, dipropargyl propyl phosphate, Dipropargyl trifluoromethyl phosphate, Dipropargyl-2,2,2-trifluoroethyl phosphate, Dipropargyl-3,3,3-trifluoropropyl phosphate, Dipropargyl Hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl Phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, diallyl hexafluoroisopropyl phosphate at least one of the

Preferably, the sultone compounds include but are not limited to cyclic sulfonate compounds, such as 1,3-propane sultone, propylene sulfite, 1,4-butane sultone or at least one of 1,3-propene sultone;

Preferably, the nitrile compound can be but not limited to succinonitrile, adiponitrile, ethylene glycol bis(propionitrile) ether, hexanetrinitrile, adiponitrile, pimelonitrile, suberonitrile, azela One or more of nitrile and sebaconitrile.

It should be noted that, unless otherwise specified, in general, the addition amount of any optional substance in the auxiliary additive in the non-aqueous electrolyte is 0.05-10%, preferably, the addition amount is 0.1-5%, More preferably, the added amount is 0.1% to 3%. Specifically, the addition amount of any optional substance in the auxiliary additive can be 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2% %, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 7.8%, 8%, 8.5%, 9%, 9.5%, 10%.

In some embodiments, when the auxiliary additive is selected from fluoroethylene carbonate, based on 100% of the total mass of the non-aqueous electrolyte, the added amount of the fluoroethylene carbonate is 0.05%-30%.

The content of the auxiliary additive of the non-aqueous electrolyte is set in this range, which can avoid the decrease of the electrical conductivity due to the decrease of the dielectric constant of the non-aqueous electrolyte, and it is easy to make the high-current discharge characteristics of the non-aqueous electrolyte battery and the stability of the negative electrode , The cycle characteristics reach a good range; it can improve the oxidation/reduction resistance of the non-aqueous electrolyte, thereby helping to improve the stability of the battery during high-temperature cycle and high-temperature storage.

In the non-aqueous electrolyte, compared with a single addition or a combination of other existing additives, when the compound shown in structural formula 1 is added together with the auxiliary additive, it shows obvious synergy in improving the high-temperature storage performance of the battery. The promotion effect shows that the compound shown in structural formula 1 and the auxiliary additive can form a film together on the electrode surface to make up for the film-forming defect of a single addition, and obtain a more stable passivation film under high temperature conditions.

The present invention is further described by way of examples below.

The compound shown in the structural formula 1 adopted in the following examples and comparative examples is as follows:

Embodiment 1～30 and comparative example 1-12

This embodiment is used to illustrate the battery disclosed in the present invention and its preparation method. The positive electrode active material is lithium cobaltate as an example, and the negative electrode active material is graphite as an example. The following steps are included:

(1) Preparation of positive electrode sheet

The positive electrode active material is LiCoO ₂ , the positive electrode active material is doped or coated with the doping amount or coating amount of the first metal element is b, the positive electrode active material is doped or coated with the doping amount or coating amount of the second metal element is c, where the values of b and c refer to each table, disperse the surface-doped or coated positive electrode active material LiCoO ₂ , the conductive agent and the binder PVDF into the solvent NMP and mix them uniformly to obtain the positive electrode slurry; the positive electrode The slurry is uniformly coated on the aluminum foil of the positive electrode current collector, and after drying, rolling, and cutting, the positive electrode sheet is obtained. The compacted density of the obtained positive electrode sheet is 4.1g/cm ³ , and the positive electrode active material, conductive carbon black And the mass ratio of binder PVDF is 96:2:2.

(2) Preparation of negative electrode sheet

Disperse the negative electrode active material graphite, conductive agent, binder CMC and SBR in deionized water according to the mass ratio of 96:1:1:2 and stir to obtain the negative electrode slurry; evenly coat the negative electrode slurry on the negative electrode current collector copper foil, after drying, rolling, and cutting, a negative electrode sheet with a compacted density of 1.7 g/cm ³ was obtained.

(3) Preparation of electrolyte

Mix ethylene carbonate (EC) and diethyl carbonate (DEC) uniformly at a mass ratio of 3:7, then add lithium hexafluorophosphate (LiPF ₆ ) to a molar concentration of 1 mol/L, with the total weight of the non-aqueous electrolyte being 100%, and then add additives according to each table.

(4) Preparation of lithium secondary battery

Using the stacking process, the positive electrode sheet, separator and negative electrode sheet are laminated in sequence, and then the top and side sealing, injection of a certain amount of electrolyte and other processes are performed to make a soft pack battery.

Performance Testing

The following performance tests were carried out on the lithium secondary battery prepared above:

1. High temperature cycle performance test: At 45°C, charge the formed battery to the cut-off voltage with 1C constant current and constant voltage, then charge it at constant voltage until the current drops to 0.05C, and then discharge it at 1C constant current to 4.45V , so cycle 500 times, record the first discharge capacity and the last discharge capacity.

Calculate the capacity retention rate of high temperature cycle according to the following formula:

Capacity retention ratio = last discharge capacity / first discharge capacity × 100%.

2. The growth rate of internal resistance after 500 high-temperature cycles: at 45°C, charge the formed battery at room temperature with a constant current of 1C to the cut-off voltage, and then charge with constant current and constant voltage until the current drops to 0.05C. Test the internal resistance of the battery, and then discharge it to 4.45V at a constant current of 1C. After 500 cycles of this cycle, record the internal resistance of the battery that is fully charged for the first time and the internal resistance of the battery that is fully charged for the last time.

Battery internal resistance growth rate after 500 cycles of high-temperature cycle = (battery internal resistance after the last full charge of the battery - internal resistance of the first fully charged battery) / internal resistance of the first full charge of the battery × 100%.

1.1. Fill in Table 2 with the test results obtained in Examples 1-23 and Comparative Examples 1-8.

Table 2

From the test results of Examples 1-23 and Comparative Examples 1-8, it can be seen that when the mass percentage of compound 1 in the electrolyte a, the doping amount b of the first metal element in the positive electrode active material, and the second metal element in the positive electrode active material When the doping amount c of the element satisfies the preset relationship 0.05≦a×(b+c)≦3, the lithium secondary battery has higher high-temperature cycle performance.

From the test results of Examples 1-23, it can be seen that as the value of a×(b+c) increases, the high-temperature cycle performance of the lithium secondary battery first increases and then decreases, indicating that the content of the compound represented by the structural formula 1 in the electrolyte is , The doping amount of the first metal element and the doping amount of the second metal element in the lithium cobalt oxide positive electrode are related to the high temperature performance of the lithium secondary battery, especially when 0.08≤a×(b+c)≤2, Lithium-ion batteries have the best high-temperature cycle performance.

In the lithium secondary battery of Comparative Example 4, since the content of the first metal element in the positive electrode is very low, although the increase in internal resistance is not particularly obvious, it may be because the content of the compound represented by structural formula 1 in the electrolyte is not high enough to inhibit the positive electrode. The dissolution of cobalt ions leads to poor stability of the positive electrode, resulting in a large growth rate of impedance and poor high-temperature cycle performance of the battery. In the lithium secondary batteries of Comparative Examples 5-8, the improvement rate of high-temperature cycle performance and the suppression effect of internal resistance growth rate are far lower than the effect when the battery satisfies the preset relationship 0.05≤a×(b+c)≤3, This may be because the content of the compound shown in structural formula 1 in the electrolyte is relatively high, which easily increases the viscosity of the electrolyte and reduces the conductance, which is not conducive to the cycle performance of the battery. At the same time, due to the high content of the first metal element and the second metal element in the positive electrode , hinder the insertion and extraction of lithium in the positive electrode active material, the battery impedance increases significantly, and the high temperature cycle performance of the battery is not good.

1.2. Fill in Table 3 with the test results obtained in Examples 12, 24-26.

table 3

It can be seen from the test results in Table 3 that for different compounds shown in structural formula 1, when the content of the first metal element, the second metal element and the content of the compound shown in structural formula 1 in the positive electrode material layer satisfy the preset relationship 0.05≤ When a×(b+c)≤3, they have similar effects, both of which have a certain improvement effect on the internal resistance growth and high-temperature cycle performance of the battery, indicating that the relational formula provided by the present invention is applicable to different structural formulas shown in 1 compound of.

1.3. Fill in Table 4 with the test results obtained in Examples 27-30 and Comparative Examples 9-12.

Table 4

As can be seen from the test results in Table 4, in the battery provided by the invention, the above-mentioned additives PS (1,3-propane sultone), VC (ethylene carbonate), DTD (sulfuric acid) were added to the non-aqueous electrolyte Vinyl ester) or FEC (fluoroethylene carbonate), can further improve the high-temperature cycle performance of the battery, and reduce the growth of thermal internal resistance. It is speculated that the compound shown in structural formula 1 and the above-mentioned additives participate in the passivation film on the electrode surface Forming, a passivation film with excellent thermal stability can be obtained, which can inhibit the dissolution of cobalt ions and improve the stability of the positive electrode, thereby effectively reducing the reaction of the electrolyte on the electrode surface and improving the high-temperature cycle performance of the battery.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. within range.

Claims (19)

A kind of lithium secondary battery, it is characterized in that, comprises positive pole, negative pole and non-aqueous electrolytic solution, described positive pole comprises positive pole material layer, and described positive pole material layer comprises positive pole active material, and described positive pole active material comprises LiCoO ₂ and doped or a coated compound containing a first metal element and a second metal element; The first metal element includes at least one of Mg, Al and W; The second metal element includes at least one of Ti, Cr, Zr, Mo, La, Ce and rare earth elements; Described non-aqueous electrolytic solution comprises the compound shown in lithium salt, organic solvent and structural formula 1:

Wherein, R ₁ is selected from unsaturated hydrocarbon groups with 3-6 carbon atoms, R ₂ is selected from alkylene groups with 2-5 carbon atoms, n=1 or 2; The lithium secondary battery satisfies the following conditions: 0.05≤a×(b+c)≤3; Wherein, a is the mass percentage of the compound shown in structural formula 1 in the non-aqueous electrolyte, and the unit is wt%; b is the doping amount or coating amount of the first metal element relative to the quality of _LiCoO2 , the unit is wt%, c is the doping amount or covering amount of the second metal element relative to the mass of LiCoO ₂ , and the unit is wt%. The lithium secondary battery according to claim 1, wherein the lithium secondary battery satisfies the following conditions: 0.08≤a×(b+c)≤2. The lithium secondary battery according to claim 1, wherein the compound represented by the structural formula 1 is selected from one or more of the following compounds:

The lithium secondary battery according to claim 1, characterized in that the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte solution in the lithium secondary battery is 0.1-5 wt%. The lithium secondary battery according to claim 4, characterized in that the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte solution is 0.1-2 wt%. The lithium secondary battery according to claim 1, characterized in that, the doping amount or coating amount b of the _first metal element relative to the mass of LiCoO is 0.1 to 1.2 wt%, and the second metal element relative to LiCoO _2. The doping amount or coating amount c by mass is 0.001 to 0.1 wt%. The lithium secondary battery according to claim 6, characterized in that, the doping amount or coating amount b of the first metal element relative to the _LiCoO2 mass is 0.5-0.8 wt%, and the second metal element relative to the _LiCoO2 mass The doping amount or coating amount c is 0.005-0.05wt%. The lithium secondary battery according to claim 1, wherein the charging cut-off voltage of the lithium secondary battery is greater than or equal to 4.45V. The lithium secondary battery according to claim 1, wherein the compacted density of the positive electrode material layer is greater than or equal to 3.8 g/cm ³ ; the compacted density of the negative electrode material layer is greater than or equal to 1.6 g/cm ³ . The lithium secondary battery according to claim 9, wherein the compacted density of the positive electrode material layer is 3.9g/cm ³ -4.15g/cm ³ ; the compacted density of the negative electrode material layer is 1.65g /cm ³ -1.8g/cm ³ . The lithium secondary battery according to claim 1, wherein the lithium salt is selected from LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiBOB, LiClO ₄ , LiCF ₃ SO ₃ , LiDFOB, LiN(SO ₂ CF ₃ ) One or more of ₂ and LiN(SO ₂ F) ₂ . The lithium secondary battery according to claim 11, characterized in that the concentration of the lithium salt in the non-aqueous electrolyte is 0.5-3.5 mol/L. The lithium secondary battery according to claim 1, wherein the non-aqueous electrolytic solution further comprises auxiliary additives, and the auxiliary additives include cyclic sulfate ester compounds, sultone compounds, and cyclic carbonate compounds. At least one of compounds, unsaturated phosphate ester compounds and nitrile compounds. The lithium secondary battery according to claim 13, characterized in that, based on the total mass of the non-aqueous electrolyte solution as 100%, the amount of the auxiliary additive added is 0.01%-30%. The lithium secondary battery according to claim 14, wherein the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate or vinyl methyl sulfate. The lithium secondary battery according to claim 14, wherein the sultone compound is selected from 1,3-propane sultone, 1,4-butane sultone or 1,3 - at least one of propene sultones. The lithium secondary battery according to claim 14, wherein the cyclic carbonate compound is selected from the group consisting of vinylene carbonate, vinylethylene carbonate, fluoroethylene carbonate or compounds shown in structural formula 2 at least one,

In the structural formula 2, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , and R ₂₆ are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group. The lithium secondary battery according to claim 14, wherein the unsaturated phosphate compound is selected from at least one of the compounds shown in structural formula 3:

In the structural formula 3, R ₃₁ , R ₃₂ , and R ₃₃ are each independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si(C _m H _2m+1 ) ₃ , m is 1 to 3, and at least one of R ₃₁ , R ₃₂ , and R ₃₃ is an unsaturated hydrocarbon group. The lithium secondary battery according to claim 14, wherein the nitrile compound comprises succinonitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrinitrile, adiponitrile, heptane One or more of dinitrile, suberonitrile, azelanitrile and sebaconitrile.