CN114300735B - Lithium secondary battery - Google Patents
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- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
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- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
- H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
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Abstract
In order to overcome the defect of poor cycle performance of the existing high-voltage power Chi Gaowen, the application provides a lithium secondary batteryA battery comprising a positive electrode, a negative electrode, and a nonaqueous 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, the nonaqueous electrolyte comprising a solvent, an electrolyte salt, and a compound represented by structural formula 1:wherein R is 1 Selected from unsaturated hydrocarbon groups having 3 to 6 carbon atoms, R 2 Selected from hydrocarbylene groups having 2-5 carbon atoms, n=1 or 2; the lithium secondary battery satisfies the following conditions: a x (b+c) is more than or equal to 0.05 and less than or equal to 3; according to the lithium secondary battery provided by the application, the compound shown in the 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, and can also alleviate the transmission problem of lithium ions in the battery charge-discharge cycle process caused by doping metal elements, reduce the battery impedance and battery polarization and alleviate the lithium precipitation of the battery, so that the battery has better high-temperature cycle performance.
Description
Technical Field
The application belongs to the technical field of energy storage battery devices, and particularly relates to a lithium secondary battery.
Background
The lithium ion battery has the advantages of high working voltage, wide working temperature range, high energy density and power density, no memory effect, long cycle life and the like, and is widely applied to the fields of 3C digital products such as mobile phones, notebook computers and the like and the fields of new energy automobiles. In recent years, with the continuous development of thinning of 3C digital products, the requirement of the battery industry for high energy density of lithium ion batteries is also increasing. There is therefore a need to increase the energy density of lithium ion batteries.
At present, two methods for improving the energy density of a battery are mainly used, namely, the method for improving the charging cut-off voltage of a positive electrode and the method for pressurizing an active material layer of an electrode to realize high density. However, after the positive electrode charge cut-off voltage is increased, the activity of the positive electrode is further increased, and side reactions between the positive electrode and the electrolyte are also increased, so that the transition metal ions of the positive electrode are dissolved out, thereby degrading the high-temperature performance of the battery. In addition, when the high-compaction electrode is adopted, the porosity of the high-compaction electrode is low, the liquid retention amount of the battery is reduced, so that the electrolyte is difficult to permeate at the interface of the low-porosity electrode plate, the contact internal resistance between the electrolyte and the electrode is further increased, and in the long-term circulation process, the charge-discharge electrode is increased, so that the situation of sudden water jump caused by lithium precipitation occurs. Therefore, how to improve the long-term cycle performance of high-voltage, high-compaction lithium ion batteries is an industry problem, and needs to be improved from various levels of electrode materials, electrolytes, and the like.
In the aspect of the positive electrode material, surface coating modification is an important means for improving the performance of the positive electrode material of the lithium ion battery. The surface coating material can effectively reduce the corrosion of electrolyte to the anode and reduce the dissolution of metal ions. Meanwhile, the surface coating material can isolate the electrolyte from the positive electrode surface active material in physical space. However, in the case of using a positive electrode surface coating material such as alumina or the like, there is a problem in that the coated metal oxide having no electrochemical activity inhibits the transmission of lithium ions at the positive electrode interface, resulting in an increase in resistance, which is disadvantageous for long-term cycling of the lithium ion battery.
There is currently no good solution in terms of electrolyte. Therefore, for a high-voltage high-compaction high-energy density battery, the long-term cycle performance of the battery is ensured from the viewpoint of electrolyte, and the fact that lithium is not separated out in the later period of cycle is ensured to be a great difficulty of the high-voltage solid lithium ion battery.
Disclosure of Invention
Aiming at the problems of poor high-temperature cycle performance and increased impedance of a high-voltage solid lithium ion battery in the prior art, the application provides a lithium secondary battery.
The technical scheme adopted by the application for solving the technical problems is as follows:
the application provides a lithium secondary battery, comprising a positive electrode, a negative electrode and a nonaqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises LiCoO 2 And a doped or 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 comprises at least one of Ti, cr, zr, mo, la, ce and rare earth elements;
the nonaqueous electrolyte includes a lithium salt, an organic solvent, and a compound represented by structural formula 1:
wherein R is 1 Selected from unsaturated hydrocarbon groups having 3 to 6 carbon atoms, R 2 Selected from hydrocarbylene groups having 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 content of a compound shown in a structural formula 1 in the nonaqueous electrolyte, and the unit is weight percent;
b is the relative LiCoO of the first metal element 2 The doping amount or coating amount of the mass is expressed as weight percent,
c is the second metal element relative LiCoO 2 The doping amount or cladding amount of the mass is expressed as weight percent.
Further, the lithium secondary battery satisfies the following conditions:
0.08≤a×(b+c)≤2。
further, the compound shown in the structural formula 1 is selected from one or more of the following compounds:
further, the mass percentage content a of the compound shown in the structural formula 1 in the nonaqueous electrolyte is 0.1-5wt%; preferably, the mass percentage content a of the compound represented by structural formula 1 in the nonaqueous electrolyte solution in the lithium secondary battery is 0.1 to 2wt%.
Further, the first metal element is relative to LiCoO 2 The doping amount or coating amount b of the mass is 0.1-1.2 wt%; preferably, the first metal element is relative to LiCoO 2 The doping amount or coating amount b of the mass is 0.5-0.8 wt%.
Further, the second metal element is relative to LiCoO 2 The doping amount or coating amount c of the mass is 0.001-0.1wt%; preferably, the second metal element is relative to LiCoO 2 The doping amount or coating amount c is 0.005-0.05 wt%.
Further, the lithium secondary battery has a charge cutoff voltage of 4.45V or more.
Further, the positive electrode material layer has a compacted density of 3.8g/cm or more 3 The method comprises the steps of carrying out a first treatment on the surface of the Preferably, the positive electrode material layer has a compacted density of 3.9g/cm 3 -4.15g/cm 3 ；
The negative electrode material layer has a compacted density of 1.6g/cm or more 3 The method comprises the steps of carrying out a first treatment on the surface of the Preferably, the negative electrode material layer has a compacted density of 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 of the following;
the concentration of the lithium salt in the nonaqueous electrolyte is 0.5-3.5mol/L.
Further, the nonaqueous electrolyte also comprises an auxiliary additive, wherein the auxiliary additive is at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound and a nitrile compound;
further, the addition amount of the auxiliary additive is 0.01 to 30% based on 100% of the total mass of the nonaqueous electrolytic solution.
Further, the cyclic sulfate compound is selected from at least one of vinyl sulfate, propylene sulfate or vinyl methyl sulfate;
the sultone compound is at least one selected from 1, 3-propane sultone, 1, 4-butane sultone or 1, 3-propylene sultone;
the cyclic carbonate compound is at least one selected from ethylene carbonate, fluoroethylene carbonate or a compound shown in a structural formula 2,
in the structural formula 2, R 21 、R 22 、R 23 、R 24 、R 25 、R 26 Each independently selected from one of a hydrogen atom, a halogen atom, a C1-C5 group;
the unsaturated phosphate compound is at least one compound shown in a structural formula 3:
in the structural formula 3, R 31 、R 32 、R 32 Each independently selected from the group consisting of C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si (C) m H 2m+1 ) 3 M is a natural number of 1 to 3, and R 31 、R 32 、R 33 At least one of them is an unsaturated hydrocarbon group;
the nitrile compound comprises one or more of succinonitrile, glutaronitrile, ethylene glycol bis (propionitrile) ether, hexanedinitrile, adiponitrile, pimelic nitrile, suberonitrile, nonyldinitrile and decyldinitrile.
The application has the beneficial effects that:
the lithium secondary battery provided by the application is characterized in that LiCoO 2 The positive electrode active material is coated or doped with a compound containing a first metal element and a second metal element, and the compound shown in the structural formula 1 is added into a non-aqueous electrolyte, so that a great amount of researches show that the positive electrode active material has the following characteristicsThe doping amount or coating amount of the first metal and the second metal element doped or coated on the surface of the material is closely related to the content of the compound shown in the structural formula 1 in the electrolyte, when the content of a (b+c) is less than or equal to 0.05 and less than or equal to 3, the cobalt ion can be effectively inhibited from being dissolved out and the damage of cobalt ion dissolved out of the positive electrode to the negative electrode SEI film can be repaired, and meanwhile, the doping amount or coating amount of the first metal element and the second metal element is controlled to be stable in LiCoO 2 The high-voltage characteristic of the positive electrode active material can slow down the transmission problem of lithium ions in the battery charging and discharging cycle process, and reduce the battery impedance and the battery polarization, so that the battery has high energy density and good high-temperature cycle performance.
Detailed Description
In order to make the technical problems, technical schemes and beneficial effects solved by the application more clear, the application is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the application.
The embodiment of the application provides a lithium secondary battery, which comprises a positive electrode, a negative electrode and a nonaqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises LiCoO 2 And a doped or coated compound containing a first metal element and a second metal element;
the first metal element includes at least one of Mg, al, zr, W;
the second metal element comprises at least one of Ti, cr, mo and rare earth elements;
the negative electrode includes a negative electrode material layer; the nonaqueous electrolyte includes a lithium salt, an organic solvent, and a compound represented by structural formula 1:
wherein R is 1 Selected from unsaturated hydrocarbon groups having 3 to 6 carbon atoms, R 2 Selected from hydrocarbylene groups of 2-5 carbon atoms, n1 or 2;
the lithium secondary battery satisfies the following conditions:
0.05≤a×(b+c)≤3；
wherein a is the mass percentage content of a compound shown in a structural formula 1 in the nonaqueous electrolyte, and the unit is weight percent;
b is the relative LiCoO of the first metal element 2 The doping amount or coating amount of the mass is expressed as weight percent,
c is the second metal element relative LiCoO 2 The doping amount or cladding amount of the mass is expressed as weight percent.
In some embodiments, the LiCoO of the lithium secondary battery 2 The positive electrode is doped or coated with Mg, al, W, ti, cr, zr, mo, la, ce and rare earth elements, compared with LiCoO which is not doped or coated 2 Positive electrode, mg, al, W and other elements to raise LiCoO 2 The stability of the anode under high temperature and high pressure, ti, cr, zr, mo, la, ce, rare earth elements and the like form unique layered distribution due to slow diffusion kinetics in the sintering process, and the anode is mainly enriched on the top surface and can form strong ionic bonding with oxygen ions, so that the loss of surface oxygen is inhibited, and the high-pressure structural stability of the anode active material particles containing cobalt and lithium compounds can be further improved. However, there is a problem in that the coated metal element having no electrochemical activity inhibits the transmission of lithium ions at the interface of the positive electrode, resulting in an increase in the resistance of the positive electrode, which is disadvantageous for long-term cycling of the lithium secondary battery.
Based on the above-mentioned shortcomings, the nonaqueous electrolyte of the lithium secondary battery of the present application contains a substance represented by the structural formula 1, and the mechanism of action is not very clear, but the inventors speculate that the substance can be effectively complexed on the surface of the positive electrode to inhibit the dissolution of cobalt ions, and meanwhile, the destruction of cobalt ions dissolved out of the positive electrode to the negative electrode SEI film can be effectively repaired due to the special structure, so that the impedance increase of the battery in the high-temperature and high-pressure cycle process can be effectively inhibited.
The inventor finds that a certain relation exists between the content of the compound shown in the structural formula 1 and the doping coating amount in the electrolyte, the compound shown in the structural formula 1 can effectively inhibit the dissolution of cobalt ions of the positive electrode, and indirectly improves the stability of the positive electrode, so that the doping coating amount of the positive electrode material can be properly reduced, the consumption of non-electrochemical active metal elements is indirectly reduced, the problem of inhibiting lithium ion conduction caused by the non-electrochemical active metal elements is relieved, the high-temperature high-pressure cycle performance of the battery is improved, and the excessive consumption of the compound shown in the structural formula 1 can cause the deterioration of the battery performance due to too high viscosity. According to a large number of researches, when the doping amount or cladding amount of the first metal and the second metal elements doped or clad on the surface of the positive electrode active material and the content of the compound shown in the structural formula 1 in the electrolyte meet 0.05-a (b+c) to be less than or equal to 3, the compound shown in the structural formula 1 in the electrolyte can effectively inhibit cobalt ions from being dissolved out and repair damage of the dissolved cobalt ions to a negative electrode SEI film, and meanwhile, the doping amount or cladding amount of the first metal element and the second metal element can slow down the transmission problem of lithium ions in a battery charge-discharge cycle process caused by the stable high-voltage characteristic of the positive electrode active material, and reduce battery impedance and battery polarization, so that the battery can have good high-temperature cycle performance.
In a preferred embodiment, the lithium secondary battery satisfies the following conditions:
0.08≤a×(b+c)≤2；
in some embodiments, the lithium secondary battery provided by the application has the advantages that the content of the compound shown in the structural formula 1 of the nonaqueous electrolyte is related to the doping amount or the coating amount of the compound containing the first metal element and the second metal element on the surface of the positive electrode active material in the lithium secondary battery, so that the influence of the first metal element, the second metal element and the compound shown in the structural formula 1 on the battery performance can be integrated to a certain extent, namely, the battery polarization is weakened, the battery impedance is reduced, and the lithium precipitation of the battery is slowed down, so that the battery has better high-temperature cycle performance.
In some embodiments, the compound of formula 1 is selected from one or more of the following compounds:
in some embodiments, the firstMetal element relative LiCoO 2 The doping amount or coating amount b of the mass is 0.1-1.2 wt%.
In a preferred embodiment, the first metal element is relative to LiCoO 2 The doping amount or coating amount b of the mass is 0.5-0.8 wt%.
In some embodiments, the second metal element is relative to LiCoO 2 The doping amount or coating amount c of the mass is 0.001-0.1 wt%.
In a preferred embodiment, the second metal element is relative to LiCoO 2 The doping amount or coating amount c is 0.005-0.05 wt%.
In the positive electrode active material LiCoO 2 The surface is doped or coated with a compound containing the first metal element and the second metal element, so that the battery voltage can be effectively increased, but excessive metal elements are doped or coated to prevent lithium in the positive electrode active material from being inserted or extracted, thereby reducing the battery capacity and increasing the impedance of the battery. The inventors have found through extensive research that the first metal element in the positive electrode material layer is relative to LiCoO 2 The doping amount or coating amount b of the mass reaches 0.1 to 1.2 weight percent, and the second metal element is relative to LiCoO 2 The doping amount or the coating amount c of the mass reaches 0.001-0.1 wt%, and the first metal element, the second metal element and the compound shown in the structural formula 1 in the electrolyte cooperate with each other, so that the battery voltage can be effectively improved, and meanwhile, the lithium ion transmission of the positive electrode active material can be relieved and inhibited.
In some embodiments, the mass percentage content a of the compound shown in the structural formula 1 in the nonaqueous electrolyte is 0.1-5 wt%.
In a preferred embodiment, the mass percentage content a of the compound shown in the structural formula 1 in the nonaqueous electrolyte is 0.1-2 wt%.
The positive electrode active material contains the lithium cobaltate compound, so that the lithium secondary battery has better multiplying power performance, but when the mass percentage of Co element in the positive electrode material layer is too large, the positive electrode active material is easy to react with non-aqueous electrolyte under the condition of high voltage to cause dissolution of Co element, the compound shown in the structural formula 1 is added to effectively inhibit the reaction between the non-aqueous electrolyte and the positive electrode active material, and further inhibit the dissolution of Co element in the positive electrode active material, but the compound shown in the structural formula 1 is not excessively added, the initial impedance of an SEI film formed by a negative electrode of the battery is larger, the impedance of the battery is increased, and the excessive addition of the compound shown in the structural formula 1 increases the viscosity of the electrolyte and reduces the conductance, so that the cycle performance of the battery is unfavorable.
The above analysis is based only on the effect on the battery of each parameter or of the parameters alone, but the three parameters are interrelated and inseparable during actual battery application. The relation provided by the application relates the three, and the three affect the electrochemical performance of the battery together, so that the mass percent content a of the compound shown in the structural formula 1 and the relative LiCoO of the first metal element in the positive electrode material layer are regulated 2 Doping or cladding amount b of mass, second metal element in positive electrode material layer relative to LiCoO 2 The doping amount or the cladding amount c of the mass is not more than 0.05 a× (b+c) not more than 3, so that the high-temperature cycle performance of the lithium secondary battery can be improved while the lithium secondary battery has higher stable voltage characteristic; beyond the above range, deterioration of the kinetics of the battery occurs, so that the cycle life of the battery is shortened under high temperature conditions, and even failure occurs.
Further, the lithium secondary battery has a charge cutoff voltage of 4.45V or more. The metal element for isolating the electrolyte from contacting with the positive electrode surface active material is doped or coated in the positive electrode of the battery, and the compound shown in the structural formula 1 is added in the electrolyte to assist in matching, so that the charge cut-off voltage of the battery is more than or equal to 4.45V, and the requirement on high energy density of the lithium ion battery is met.
Further, the positive electrode material layer has a compacted density of 3.8g/cm or more 3 The method comprises the steps of carrying out a first treatment on the surface of the Preferably, the positive electrode material layer has a compacted density of 3.9g/cm 3 -4.15g/cm 3 ；
The negative electrode material layer has a compacted density of 1.6g/cm or more 3 The method comprises the steps of carrying out a first treatment on the surface of the Preferably, the negative electrode material layer has a compacted density of 1.65g/cm 3 -1.8g/cm 3 ；
The porosity of the positive electrode material layer and the negative electrode material layer is 50% or less; preferably, the porosity of the positive electrode material layer and the negative electrode material layer is 10% -35%.
The compacted density of the positive electrode material layer and the compacted density of the negative electrode material layer have a certain influence on the electrical performance of the secondary battery. For the compaction density of the positive electrode material layer, the compaction density of the positive electrode material layer is generally too low, and the positive electrode material is easy to fall off in the preparation process of the pole piece, so that the consistency of the overall battery capacity is affected; if the compaction density of the positive electrode material layer is too high, the pole piece is easy to break during the preparation process, the production process problem occurs, the production efficiency is reduced, meanwhile, lithium ions are difficult to separate out during the charge and discharge process of the battery, the impedance of the battery is increased, the capacity of the battery is reduced, and the cycle performance of the battery is reduced. For the compaction density of the anode material layer, the compaction density of the anode material layer is generally too low, and abnormal processes such as powder dropping and the like are more likely to occur in the pole piece preparation process, so that the production efficiency is reduced; the compaction density of the negative electrode material layer is too high, so that the immersion of the electrolyte is influenced, the embedding of the electrolyte is influenced, the battery capacity is reduced, the lithium precipitation condition of the battery is increased, and the safety performance of the battery is reduced. The larger the porosity of the positive and negative electrode material layers is, the more favorable the extraction or intercalation of lithium ions is, but the larger the porosity is, the volume of the battery is increased, and the energy density of the battery is reduced.
In addition, the solid density of the positive electrode material layer, the solid density of the negative electrode material layer, the void ratio of the positive electrode material layer and the negative electrode material layer are mutually related to the compound shown in the structural formula 1 of the electrolyte, and after the compound shown in the structural formula 1 is added into the nonaqueous electrolyte, the ionic conductivity, the viscosity and the like of the nonaqueous electrolyte are changed, so that the permeability of the nonaqueous electrolyte to the positive electrode material layer and the negative electrode material layer is influenced, and the intercalation and deintercalation efficiency of lithium ions is further influenced; the compaction density of the positive electrode material layer and the negative electrode material layer also affects the compaction degree of the SEI film formed by decomposing the compound shown in the structural formula 1 on the surface of the negative electrode material layer, and affects the protection effect of the SEI film on free Co.
In some embodiments, the lithium salt in the nonaqueous 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 (SO) 2 F) 2 In a preferred embodiment, the lithium salt is LiPF 6 。
In the nonaqueous electrolyte, lithium ions are contained in the lithium salt, so that electrochemical reaction of the battery is facilitated in the charging and discharging process of the battery, consumption of a small amount of lithium ions is supplemented, and the cycle performance of the battery is promoted.
In some embodiments, the non-aqueous electrolyte has a conductivity of 6mS/cm or greater at 25 ℃. The conductivity of the nonaqueous electrolyte has a certain influence on the cycle performance and storage performance of the lithium secondary battery. The inventor has found through a great deal of research that when the conductivity of the nonaqueous electrolyte is low under the normal temperature condition, the conductivity in the electrolyte is low, the ion transmission rate is reduced, concentration polarization is increased in the charging and discharging process of the lithium secondary battery, the battery impedance is also increased, and the cycle performance and the storage performance of the battery are reduced. It is found that when the conductivity of the nonaqueous electrolyte is higher than 6mS/cm at the normal temperature of 25 ℃, concentration polarization can be reduced to the greatest extent without affecting lithium ion transmission, and the nonaqueous electrolyte can act together with the compound shown in the structural formula 1, so that cobalt ions are effectively inhibited from being dissolved out, damage of the dissolved cobalt ions to the negative electrode SEI film is repaired, and high-temperature circulation and high-temperature storage performance of the battery are improved.
In some embodiments, the concentration of the lithium salt in the nonaqueous electrolyte is 0.5-3.5mol/L; in a preferred embodiment, the concentration of the lithium salt in the nonaqueous electrolyte is 0.8 to 2.0mol/L.
In the nonaqueous electrolyte, the concentration of lithium salt is too low, so that concentration polarization is increased in the electrochemical reaction of the battery, and the electrochemical reaction of the battery is not facilitated; the concentration of lithium salt is too high, the concentration of the nonaqueous electrolyte is increased, the ion transmission efficiency of the compound shown in the structural formula 1 is affected, the damage of the compound shown in the structural formula 1 to the cathode SEI film caused by the dissolved cobalt ions is repaired, and the dissolution efficiency of the cobalt ions is inhibited from being reduced.
In some embodiments, the organic solvent in the nonaqueous electrolyte is one or more of cyclic carbonate, linear carbonate, carboxylate, and ether; preferably, the cyclic carbonate comprises one or more of vinylene carbonate, propylene carbonate and ethylene carbonate; preferably, the linear carbonate comprises one or more of dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate; preferably, the carboxylic acid ester comprises one or more of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate and ethyl trimethylacetate; preferably, the ethers include one or more of ethylene glycol dimethyl ether, 1, 3-dioxolane, and 1, 2-tetrafluoroethyl-2, 3-tetrafluoropropyl ether.
In some embodiments, the nonaqueous electrolyte further includes an auxiliary additive, and the auxiliary additive is at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound, and a nitrile compound.
In a preferred embodiment, the cyclic sulfate 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 having fluorine atoms, such as fluoroethylene carbonate and/or bisfluoroethylene carbonate; preferably, the cyclic carbonate compound includes, but is not limited to, a cyclic carbonate compound having a carbon-carbon unsaturated bond, and may be, for example, one or more of ethylene carbonate, vinyl ethylene carbonate, methyl ethylene carbonate, or a compound represented by structural formula 2;
in the structural formula 2, R 21 、R 22 、R 23 、R 24 、R 25 、R 26 Each independently selected from one of a hydrogen atom, a halogen atom, a C1-C5 group.
The unsaturated phosphate compound is at least one compound shown in a structural formula 3:
in the structural formula 3, R 31 、R 32 、R 32 Each independently selected from the group consisting of C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si (C) m H 2m+1 ) 3 M is a natural number of 1 to 3, and R 31 、R 32 、R 33 At least one of them is an unsaturated hydrocarbon group.
In a preferred embodiment, the unsaturated phosphate compound may be at least one of tripropylethyl phosphate, dipropargyl methyl phosphate, dipropargylethyl phosphate, dipropargylpropyl phosphate, dipropargyl trifluoromethyl phosphate, dipropargyl-2, 2-trifluoroethyl phosphate, dipropargyl-3, 3-trifluoropropyl phosphate, dipropargyl hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2, 2-trifluoroethyl phosphate, diallyl-3, 3-trifluoropropyl phosphate, diallyl hexafluoroisopropyl phosphate.
Preferably, the sultone compound includes, but is not limited to, cyclic sultone compounds, for example, at least one of 1, 3-propane sultone, propylene sulfite, 1, 4-butane sultone or 1, 3-propylene sultone;
preferably, the nitrile compound may be, but is not limited to, one or more of succinonitrile, adiponitrile, ethylene glycol bis (propionitrile) ether, hexanetrinitrile, adiponitrile, pimelic nitrile, suberic nitrile, nonyldinitrile, and decyldinitrile.
In general, the addition amount of any one of the optional substances in the auxiliary additive to the nonaqueous electrolytic solution is 0.05 to 10%, preferably 0.1 to 5%, and more preferably 0.1 to 3%, unless otherwise specified. Specifically, the addition amount of any optional substance in the auxiliary additive may 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%, 8.5%, 9%, 9.5%, 10%.
In some embodiments, when the auxiliary additive is selected from fluoroethylene carbonate, the fluoroethylene carbonate is added in an amount of 0.05% to 30% based on 100% of the total mass of the nonaqueous electrolytic solution.
The content of the auxiliary additive in the nonaqueous electrolyte is set in the above range, so that the decrease in conductivity due to the decrease in dielectric constant of the nonaqueous electrolyte can be avoided, and the high-current discharge characteristic, stability against the negative electrode, and cycle characteristic of the nonaqueous electrolyte battery can be easily brought into good ranges; the oxidation/reduction resistance of the nonaqueous electrolyte can be improved, thereby contributing to improvement of the stability of the battery during high-temperature cycle and high-temperature storage.
In the nonaqueous electrolyte, compared with the single addition or the combination of other existing additives, the compound shown in the structural formula 1 and the auxiliary additive are added together, the nonaqueous electrolyte has obvious synergistic improvement effect on the aspect of improving the high-temperature storage performance of the battery, and the fact that the compound shown in the structural formula 1 and the auxiliary additive form a film together on the surface of an electrode can make up for the film forming defect of the single addition, so that a passivation film which is more stable under the high-temperature condition is obtained.
The application is further illustrated by the following examples.
The compounds of structural formula 1 used in the following examples and comparative examples are shown in the following table:
examples 1 to 30 and comparative examples 1 to 12
This example is for illustrating the battery and the method for preparing the same disclosed in the present application, in which the positive electrode active material is exemplified by lithium cobaltate, and the negative electrode active material is exemplified by graphite, and comprises the following steps:
(1) Preparation of positive plate
The positive electrode active material is LiCoO 2 The doping amount or coating amount of the first metal element doped or coated by the positive electrode active material is b, the doping amount or coating amount of the second metal element doped or coated by the positive electrode active material is c, wherein the values of b and c are referred to each table, and the positive electrode active material LiCoO with the doped or coated surface is obtained 2 Dispersing the conductive agent and the binder PVDF into a solvent NMP, and uniformly mixing to obtain anode slurry; uniformly coating the positive electrode slurry on a positive electrode current collector aluminum foil, and drying, rolling and cutting to obtain a positive electrode plate, wherein the compaction density of the obtained positive electrode plate is 4.1g/cm 3 The mass ratio of the positive electrode active material to the conductive carbon black to the binder PVDF is 96:2:2.
(2) Preparation of negative electrode sheet
Dispersing negative electrode active material graphite, a conductive agent, a binder CMC and SBR in deionized water according to a mass ratio of 96:1:1:2, and stirring to obtain negative electrode slurry; uniformly coating the negative electrode slurry on a negative electrode current collector copper foil, and drying, rolling and cutting to obtain a compact density of 1.7g/cm 3 Is a negative electrode sheet.
(3) Preparation of electrolyte
Uniformly mixing Ethylene Carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 3:7, and then adding lithium hexafluorophosphate (LiPF) 6 ) To a molar concentration of 1mol/L, based on 100% by weight of the total nonaqueous electrolytic solution, then additives were added according to the respective tables.
(4) Preparation of lithium secondary battery
The positive plate, the isolating film and the negative plate are sequentially laminated by adopting a lamination process, and the soft-package battery is manufactured after the procedures of top side sealing, injection of a certain amount of electrolyte and the like.
Performance testing
The lithium secondary battery prepared by the method is subjected to the following performance test:
1. high temperature cycle performance test: and (3) charging the formed battery to a cut-off voltage at 45 ℃ with a constant current and a constant voltage of 1C, charging the battery to a constant voltage until the current is reduced to 0.05C, discharging the battery to 4.45V with a constant current of 1C, and recording the discharge capacity of the 1 st time and the discharge capacity of the last 1 time by circulating 500 circles.
The capacity retention for the high temperature cycle was calculated as follows:
capacity retention = last 1 discharge capacity/1 st discharge capacity x 100%.
2. Internal resistance increase rate after 500 cycles of high temperature cycle: and (3) at the temperature of 45 ℃, the formed battery is charged to a cut-off voltage at constant current of 1C, then is charged at constant current and constant voltage until the current is reduced to 0.05C, the internal resistance of the battery is tested, then is discharged to 4.45V at constant current of 1C, and after the battery is circulated for 500 circles in this way, the internal resistance of the battery which is fully charged for the 1 st time and the internal resistance of the battery which is fully charged for the last 1 time are recorded.
The battery high temperature cycle 500 turns internal resistance increase rate= (internal resistance of battery after the last 1 time of battery fully charged-internal resistance of battery after the 1 st time of fully charged)/internal resistance of battery after the 1 st time of fully charged x 100%.
The test results obtained in 1.1, examples 1 to 23 and comparative examples 1 to 8 are filled in Table 2.
TABLE 2
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From the test results of examples 1 to 23 and comparative examples 1 to 8, it is understood that the lithium secondary battery has higher high-temperature cycle performance when the mass percentage content a of the compound 1 in the electrolyte, the doping amount b of the first metal element in the positive electrode active material, and the doping amount c of the second metal element of the positive electrode active material satisfy the preset relationship of 0.05 < a× (b+c) < 3.
From the test results of examples 1 to 23, it was revealed that as the value of a× (b+c) increases, the high-temperature cycle performance of the lithium secondary battery was improved and then reduced, indicating that the content of the compound represented by structural formula 1 in the electrolyte, the doping amount of the first metal element in the lithium cobaltate positive electrode, and the doping amount of the second metal element were correlated with the high-temperature performance of the lithium secondary battery, and in particular, when 0.08.ltoreq.a× (b+c). Ltoreq.2, the lithium ion battery had 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 low, although the increase in internal resistance is not particularly remarkable, it is possible that the compound represented by structural formula 1 in the electrolyte is not contained in a high content, which is insufficient to suppress the dissolution of cobalt ions in the positive electrode, the stability of the positive electrode is poor, resulting in a large rate of increase in resistance and low high-temperature cycle performance of the battery. In the lithium secondary batteries of comparative examples 5 to 8, the effect of suppressing the increase in the high-temperature cycle performance and the increase in the internal resistance is far lower than the effect when the battery satisfies the preset relationship of 0.05 < a× (b+c) < 3, probably because the compound content shown by structural formula 1 in the electrolyte is high, the viscosity of the electrolyte is easily increased, the electric conductivity is reduced, and the cycle performance of the battery is not facilitated, and simultaneously, the intercalation and deintercalation of lithium in the positive electrode active material is hindered due to the high contents of the first metal element and the second metal element in the positive electrode, the increase in the battery impedance is remarkable, and the high-temperature cycle performance of the battery is not good.
The test results obtained in examples 12, 24 to 26 are shown in Table 3.
TABLE 3 Table 3
As can be seen from the test results of table 3, for the different compounds shown in structural formula 1, when the content of the first metal element, the content of 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 of 0.05 < a× (b+c) < 3, the functions are similar, and the improvement effect on the internal resistance increase and the high temperature cycle performance of the battery is provided, which indicates that the relational expression provided by the application is applicable to the different compounds shown in structural formula 1.
1.3, examples 27 to 30, comparative examples 9 to 12 are shown in Table 4.
TABLE 4 Table 4
As can be seen from the test results of table 4, in the battery provided by the present application, the above additives PS (1, 3-propane sultone), VC (ethylene carbonate), DTD (ethylene sulfate) or FEC (fluoroethylene carbonate) are added to the nonaqueous electrolyte, so that the high-temperature cycle performance of the battery can be further improved, and the increase of the thermal internal resistance can be reduced, presumably, the compound shown in structural formula 1 and the above additives are involved in the formation of the passivation film on the electrode surface together, so that the passivation film with excellent thermal stability is obtained, the dissolution of cobalt ions is suppressed, the stability of the positive electrode is improved, the reaction of the electrolyte on the electrode surface is further effectively reduced, and the high-temperature cycle performance of the battery is improved.
The foregoing description of the preferred embodiments of the application is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the application.
Claims (10)
1. A lithium secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer comprising a positive electrode active material comprising LiCoO 2 And a doped or 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 comprises at least one of Ti, cr, zr, mo, la, ce and rare earth elements;
the nonaqueous electrolyte includes a lithium salt, an organic solvent, and a compound represented by structural formula 1:
structure 1
Wherein R is 1 Selected from unsaturated hydrocarbon groups having 3 to 6 carbon atoms, R 2 Selected from hydrocarbylene groups having 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 content of a compound shown in a structural formula 1 in the nonaqueous electrolyte, and the unit is weight percent;
b is the relative LiCoO of the first metal element 2 The doping amount or coating amount of the mass is expressed as weight percent,
c is the second metal element relative LiCoO 2 The doping amount or cladding amount of the mass is expressed as weight percent;
a is 0.1 to 5 weight percent, b is 0.1 to 1.2 weight percent, c is 0.001 to 0.1 weight percent;
the lithium secondary battery has a charge cutoff voltage of 4.45V or more.
2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery satisfies the following condition:
0.08≤a×(b+c)≤2。
3. the lithium secondary battery according to claim 1, wherein the compound represented by structural formula 1 is selected from one or more of the following compounds:
4. the lithium secondary battery according to claim 1, wherein,
the mass percentage content a of the compound shown in the structural formula 1 in the nonaqueous electrolyte is 0.1-2 wt%.
5. The lithium secondary battery according to claim 1, wherein,
the first metal element is relative to LiCoO 2 The doping amount or coating amount b of the mass is 0.5 to 0.8 weight percent, and the second metal element phaseFor LiCoO 2 The doping amount or coating amount c is 0.005-0.05 wt%.
6. The lithium secondary battery according to claim 1, wherein the positive electrode material layer has a compacted density of 3.8g/cm or more 3 The negative electrode material layer has a compacted density of 1.6g/cm or more 3 。
7. The lithium secondary battery according to claim 6, wherein the positive electrode material layer has a compacted density of 3.9g/cm 3 -4.15g/cm 3 ；
The negative electrode material layer had a compacted density of 1.65g/cm 3 -1.8g/cm 3 。
8. 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 ) 2 And LiN (SO) 2 F) 2 One or more of the following;
the concentration of the lithium salt in the nonaqueous electrolyte is 0.5-3.5mol/L.
9. The lithium secondary battery according to claim 1, wherein the nonaqueous electrolyte further comprises an auxiliary additive including at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound, and a nitrile compound;
the addition amount of the auxiliary additive is 0.01% -30% based on 100% of the total mass of the nonaqueous electrolyte.
10. The lithium secondary battery according to claim 9, wherein the cyclic sulfate compound is selected from at least one of vinyl sulfate, propylene sulfate, or vinyl methyl sulfate;
the sultone compound is at least one selected from 1, 3-propane sultone, 1, 4-butane sultone or 1, 3-propylene sultone;
the cyclic carbonate compound is at least one selected from ethylene carbonate, fluoroethylene carbonate or a compound shown in a structural formula 2,
in the structural formula 2, R 21 、R 22 、R 23 、R 24 、R 25 、R 26 Each independently selected from one of a hydrogen atom, a halogen atom, a C1-C5 group;
the unsaturated phosphate compound is at least one compound shown in a structural formula 3:
in the structural formula 3, R 31 、R 32 、R 33 Each independently selected from the group consisting of C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si (C) m H 2m+1 ) 3 M is a natural number of 1 to 3, and R 31 、R 32 、R 33 At least one of them is an unsaturated hydrocarbon group;
the nitrile compound comprises one or more of succinonitrile, glutaronitrile, ethylene glycol bis (propionitrile) ether, hexanedinitrile, adiponitrile, pimelic nitrile, suberonitrile, nonyldinitrile and decyldinitrile.
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