CN114171795B - Lithium ion battery electrolyte and lithium ion battery - Google Patents
Lithium ion battery electrolyte and lithium ion battery Download PDFInfo
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- 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
- H01M10/0566—Liquid materials
- H01M10/0567—Liquid materials characterised by the additives
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- H—ELECTRICITY
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- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
- H01M10/4235—Safety or regulating additives or arrangements in electrodes, separators or electrolyte
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Abstract
The invention provides lithium ion battery electrolyte and a lithium ion battery, comprising lithium salt, an organic solvent, a first additive and a second additive, wherein the first additive has a structural formula shown in a formula I; the second additive has a structural formula shown in a formula II; the two additives can perform synergistic effect to generate stable and low-impedance interface films on the surfaces of the anode and the cathode respectively, so that the hot box performance and the low-temperature performance of the battery are improved, and the cycle performance and the storage performance of the battery are improved.
Description
Technical Field
The invention relates to the field of lithium batteries, in particular to lithium ion battery electrolyte and a lithium ion battery.
Background
Lithium ion batteries are widely used by people due to the characteristics of high working voltage, large specific energy, long cycle life, no memory effect and the like. At present, the lithium ion battery is generally applied to the fields of 3C digital consumer electronic products, power batteries and the like. The electrolyte is used as blood vessel of the lithium ion battery, is one of important constituent materials of the lithium ion battery, plays a role in transporting lithium ions between the anode and the cathode, and plays a vital role in the performance of the lithium ion battery.
The electrolyte adopted at present generally comprises a solvent, lithium salt and an additive, and the substances play an important role in the low-temperature, circulation, storage and safety performances of the lithium ion battery. However, the current electrolyte still has the following problems: 1) Low temperature performance is poor: under the low temperature condition, the linear carbonic ester and the carboxylic ester have low dielectric constant due to the self structural characteristics, and the lithium salt has low solubility at low temperature, so that the migration rate of lithium ions is reduced, and the low temperature performance is poor; 2) The hot box performance is poor: the hot box test is one of the important test items of the lithium ion battery at present, the electrolyte can not achieve the effect of relieving at the limit temperature (more than or equal to 130 ℃), the chemical reaction inside the battery is continuously aggravated, heat can not be released, and the lithium battery is caused to fire or even explode, so that serious potential safety hazards are brought.
Aiming at the problem of thermal box performance, the prior solution is mainly to add a stable film forming additive to improve the stability of the positive electrode material and reduce side reaction, however, the stable film forming additive can cause the impedance of a battery system to be increased, and the dynamic performance is affected.
Therefore, it is necessary to develop an electrolyte having more excellent performance.
Disclosure of Invention
One of the objects of the present invention is: the invention provides the lithium ion battery electrolyte, which solves the problem that the conventional electrolyte cannot effectively improve the hot box performance and the low temperature performance of a battery.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
a lithium ion battery electrolyte comprises lithium salt, an organic solvent, a first additive and a second additive, wherein the first additive has a structural formula shown in a formula I; the second additive has a structural formula shown in a formula II;
wherein R is 1 、R 3 Each independently selected from any one of C1-C10 alkyl and its substituent, C1-C10 olefine base and its substituent; r is R 2 Is any one of a hydrogen atom, a halogen atom, a C1-C10 alkyl group and a substituent thereof, a C1-C10 alkoxy group and a substituent thereof, a C1-C10 haloalkane and a C1-C10 haloaryl group; x1 is a C1-C10 alkylene group;x2 to X4 are each independently selected from any one of a hydrogen atom, a halogen atom, an alkoxy group, a nitrile group and an aryl group.
Preferably, the first additive is at least one of the following structural formulas:
preferably, the content of the first additive is 0.1 to 0.3wt%, 0.3 to 0.5wt%, 0.5 to 0.7wt%, 0.7 to 1wt%, 1 to 1.2wt%, 1.2 to 1.5wt%, 1.5 to 1.8wt%, 1.8 to 2wt%, 2 to 2.2wt%, 2.2 to 2.5wt%, 2.5 to 2.8wt%, 2.8 to 3wt%, 3 to 3.5wt%, 3.5 to 4wt%, 4 to 4.5wt%, 4.5 to 5wt%, 5 to 8wt% or 8 to 10wt% of the total mass of the electrolyte.
Preferably, the second additive is at least one of the following structural formulas:
preferably, the content of the second additive is 0.1 to 0.3wt%, 0.3 to 0.5wt%, 0.5 to 0.7wt%, 0.7 to 1wt%, 1 to 1.2wt%, 1.2 to 1.5wt%, 1.5 to 1.8wt%, 1.8 to 2wt%, 2 to 2.2wt%, 2.2 to 2.5wt%, 2.5 to 2.8wt%, 2.8 to 3wt%, 3 to 3.5wt%, 3.5 to 4wt%, 4 to 4.5wt%, 4.5 to 5wt%, 5 to 8wt% or 8 to 10wt% of the total mass of the electrolyte.
Preferably, the electrolyte further comprises a third additive, wherein the third additive is one or more of fluoroethylene carbonate (FEC), vinylene Carbonate (VC), 1, 3-Propane Sultone (PS), ethylene sulfate (DTD), methylene Methane Disulfonate (MMDS), propylene Sultone (PST), citraconic anhydride (MSDS), succinonitrile (SN), hexadinitrile (ADN), ethylene glycol bis (propionitrile) ether (EGBE) and hexane dinitrile (HTCN). Preferably, the third additive is at least two additives.
Preferably, the total content of the third additive is 0.1-20 wt% of the total mass of the electrolyte. Specifically, the content of the third additive may be 0.1 to 1wt%, 1 to 3wt%, 3 to 5wt%, 5 to 6wt%, 6 to 8wt%, 8 to 10wt%, 10 to 12wt%, 12 to 15wt%, or 15 to 20wt% of the total mass of the electrolyte.
Preferably, the organic solvent is at least one of Ethylene Carbonate (EC), propylene Carbonate (PC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), ethylpropionate (EP), propylpropionate (PP), ethylacetate (EA), ethyln-butyrate (EB) and γ -butyrolactone (GBL); the organic solvent may be present in an amount of 20 to 30wt%, 30 to 40wt%, 40 to 50wt%, 50 to 60wt%, 60 to 65wt%, or 65 to 70wt% based on the total mass of the electrolyte.
Preferably, the lithium salt is lithium hexafluorophosphate (LiPF 6 ) Lithium difluorophosphate (LiPF) 2 O 2 ) Difluoro lithium bis (oxalato) phosphate (LiPF) 2 (C 2 O 4 ) 2 ) Lithium tetrafluorooxalate phosphate (LiPF) 4 C 2 O 4 ) Lithium oxalate phosphate (LiPO) 2 C 2 O 4 ) Lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiODFB), lithium tetrafluoroborate (LiBF) 4 ) At least two of lithium bis-fluorosulfonimide (LiTFSI) and lithium bis-fluorosulfonimide (LiFSI); the lithium salt may be contained in an amount of 0.1 to 1wt%, 1 to 2wt%, 2 to 3wt%, 3 to 4wt%, 4 to 5wt%, 5 to 6wt%, 6 to 7wt%, 7 to 8wt%, 8 to 9wt%, 9 to 10wt%, 10 to 11wt%, 11 to 12wt%, 12 to 13wt%, or 13 to 15wt% based on the total mass of the electrolyte.
The second object of the present invention is to provide a lithium ion battery, comprising a positive plate, a negative plate, a separator and an electrolyte, wherein the separator is arranged between the positive plate and the negative plate, and the electrolyte is any one of the above lithium ion battery electrolytes.
Compared with the prior art, the invention has the beneficial effects that:
1) The electrolyte provided by the invention is added with a first additive, and the first additive with a structure of formula I is provided with a silica structure, on one hand, si in the first additive can be used as an anion receptor to capture hydrofluoric acid and water; on the other hand, the silicon oxide structure can change the SEI film structure generated on the surface of the negative electrode, so that the generated SEI film is thinner, the migration rate of lithium ions is further improved, and the interface impedance of the negative electrode is reduced. In addition, the first additive also contains a thiophene structure, so that the conductive polymer film can be generated by oxidation of the positive electrode, the interface impedance is further improved, meanwhile, the stable polymer film can improve the material stability at the limiting temperature, and the side reaction of the positive electrode is reduced.
2) The electrolyte provided by the invention is further added with a second additive, and the second additive with a structure of formula II is provided with a nitrile sulfate structure, wherein C=C double bonds and thiophene structures in the first additive act together to be beneficial to forming a stable and compact organic polymer film on the surfaces of the positive electrode and the negative electrode, so that the cycle performance is improved, meanwhile, sulfur-containing compounds participate in film formation, the solubility of LiF on the surface of the positive electrode can be improved, and the interface impedance of the positive electrode is reduced; the nitrile structure has high self stability and stronger coordination function, can be complexed with high-valence transition metal in the anode material, further inhibit the dissolution of transition metal ions, and the sulfate can be further complexed with the transition metal ions, so that an N-type complex formed by the nitrile and the sulfate is attached to the surface of the anode to form a stable CEI film, thereby improving the stability of the material.
3) The electrolyte provided by the invention has the advantages that the additive comprises the first additive with the structure shown in the formula I and the second additive with the structure shown in the formula II, and the first additive and the second additive can be in synergistic effect to respectively generate the stable and low-impedance interface film on the surfaces of the positive electrode and the negative electrode, so that the hot box performance and the low-temperature performance of the battery are improved, and the cycle performance and the storage performance of the battery are improved.
Detailed Description
In one aspect, the invention provides lithium ion battery electrolyte, which comprises lithium salt, an organic solvent, a first additive and a second additive, wherein the first additive has a structural formula shown in a formula I; the second additive has a structural formula shown in a formula II;
wherein R is 1 、R 3 Each independently selected from any one of C1-C10 alkyl and its substituent, C1-C10 olefine base and its substituent; r is R 2 Is any one of a hydrogen atom, a halogen atom, a C1-C10 alkyl group and a substituent thereof, a C1-C10 alkoxy group and a substituent thereof, a C1-C10 haloalkane and a C1-C10 haloaryl group; x1 is a C1-C10 alkylene group; x2 to X4 are each independently selected from any one of a hydrogen atom, a halogen atom, an alkoxy group, a nitrile group and an aryl group.
Further, the first additive is at least one of the following structural formulas:
further, the content of the first additive is 0.1 to 0.3wt%, 0.3 to 0.5wt%, 0.5 to 0.7wt%, 0.7 to 1wt%, 1 to 1.2wt%, 1.2 to 1.5wt%, 1.5 to 1.8wt%, 1.8 to 2wt%, 2 to 2.2wt%, 2.2 to 2.5wt%, 2.5 to 2.8wt%, 2.8 to 3wt%, 3 to 3.5wt%, 3.5 to 4wt%, 4 to 4.5wt%, 4.5 to 5wt%, 5 to 8wt% or 8 to 10wt% of the total mass of the electrolyte. The first additive with proper content can not only relieve the serious problem of volume expansion of materials and improve the stability of a battery system, but also reduce the interface impedance of the positive electrode and further improve the stability of the system. And if the content of the additive is added less, the content is insufficient, the improvement of the battery performance is limited; if the content is large, the reaction is caused, and the performance of the battery cannot be effectively improved. Further preferably, the content of the first additive is 0.5 to 0.7wt%, 0.7 to 1wt%, 1 to 1.2wt%, 1.2 to 1.5wt%, 1.5 to 1.8wt%, 1.8 to 2wt%, 2 to 2.2wt%, 2.2 to 2.5wt%, 2.5 to 2.8wt% and 2.8 to 3wt% of the total mass of the electrolyte.
Further, the second additive is at least one of the following structural formulas:
further, the content of the second additive is 0.1 to 0.3wt%, 0.3 to 0.5wt%, 0.5 to 0.7wt%, 0.7 to 1wt%, 1 to 1.2wt%, 1.2 to 1.5wt%, 1.5 to 1.8wt%, 1.8 to 2wt%, 2 to 2.2wt%, 2.2 to 2.5wt%, 2.5 to 2.8wt%, 2.8 to 3wt%, 3 to 3.5wt%, 3.5 to 4wt%, 4 to 4.5wt%, 4.5 to 5wt%, 5 to 8wt% or 8 to 10wt% of the total mass of the electrolyte. Further preferably, the content of the first additive is 0.5 to 0.7wt%, 0.7 to 1wt%, 1 to 1.2wt%, 1.2 to 1.5wt%, 1.5 to 1.8wt%, 1.8 to 2wt%, 2 to 2.2wt%, 2.2 to 2.5wt%, 2.5 to 2.8wt% and 2.8 to 3wt% of the total mass of the electrolyte. The inventor finds that compared with the first additive, the second additive has more obvious improvement effect in the electrolyte, which is mainly beneficial to the nitrile sulfate structure and the carbon-carbon double bond structure, an N-type complex formed by nitrile groups and sulfate can be attached on the surface of the positive electrode to form a stable CEI film, and C=C double bonds can also contribute to the formation of an organic polymer film on the surface of the positive electrode, and meanwhile, sulfur-containing compounds participate in film formation, so that the solubility of LiF on the surface of the positive electrode can be improved, and the interface impedance of the positive electrode can be reduced. After further intensive research, the inventors have found that when the first additive and the second additive are used in combination, the effect of 1+1>2 on improving each performance of the battery is achieved, so that the application range of the electrolyte is wider.
Further, the electrolyte also comprises a third additive, wherein the third additive is one or more of fluoroethylene carbonate (FEC), vinylene Carbonate (VC), 1, 3-Propane Sultone (PS), ethylene sulfate (DTD), methylene Methane Disulfonate (MMDS), propylene Sultone (PST), citraconic anhydride (MSDS), succinonitrile (SN), hexadinitrile (ADN), ethylene glycol bis (propionitrile) ether (EGBE) and hexane dinitrile (HTCN). Preferably, the third additive is at least two additives. More preferably, the third additive is FEC and maleic anhydride, and in a silicon battery system, the two additives are more matched with the first additive of the invention, so that the applicability is better, and the cycle performance of the battery cell can be obviously improved. This is mainly because the first additive, FEC and maleic anhydride of the present invention are film-forming additives, and there is a synergistic effect between them to affect the film formation on the positive and negative electrode surfaces, thereby improving the performance of the silicon-based battery to the best.
Further, the total content of the third additive is 0.1-20wt% of the total mass of the electrolyte. Specifically, the content of the additive may be 0.1 to 1wt%, 1 to 3wt%, 3 to 5wt%, 5 to 6wt%, 6 to 8wt%, 8 to 10wt%, 10 to 12wt%, 12 to 15wt%, or 15 to 20wt% of the total mass of the electrolyte.
Further, the organic solvent is at least one of Ethylene Carbonate (EC), propylene Carbonate (PC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), ethylpropionate (EP), propylpropionate (PP), ethylacetate (EA), ethyln-butyrate (EB) and γ -butyrolactone (GBL); the organic solvent may be present in an amount of 20 to 30wt%, 30 to 40wt%, 40 to 50wt%, 50 to 60wt%, 60 to 65wt%, or 65 to 70wt% based on the total mass of the electrolyte. The inventors found that when the organic solvent is proportioned with EC/PC/dec=1/1/3, it is more suitable for the first, second, third additives.
Further, the lithium salt is lithium hexafluorophosphate (LiPF 6 ) Lithium difluorophosphate (LiPF) 2 O 2 ) Difluoro lithium bis (oxalato) phosphate (LiPF) 2 (C 2 O 4 ) 2 ) Lithium tetrafluorooxalate phosphate (LiPF) 4 C 2 O 4 ) Lithium oxalate phosphate (LiPO) 2 C 2 O 4 ) Lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiODFB), lithium tetrafluoroborate (LiBF) 4 ) At least two of lithium bis-fluorosulfonimide (LiTFSI) and lithium bis-fluorosulfonimide (LiFSI); the content of the lithium saltCan be 0.1 to 1wt%, 1 to 2wt%, 2 to 3wt%, 3 to 4wt%, 4 to 5wt%, 5 to 6wt%, 6 to 7wt%, 7 to 8wt%, 8 to 9wt%, 9 to 10wt%, 10 to 11wt%, 11 to 12wt%, 12 to 13wt%, or 13 to 15wt% of the total mass of the electrolyte. Further preferably, the lithium salt is a mixture of lithium hexafluorophosphate and lithium difluorooxalato borate, and the use of these two mixed lithium salts is more conducive to improving the cycle performance of the battery than a single lithium salt. In addition, the two lithium salts are mixed with the first additive and the second additive of the invention and are matched with the third additive FEC and DTD for use, and all substances are mutually and synergistically influenced, so that all electrochemical performances of the battery can be further improved.
The invention further provides a lithium ion battery, which comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the diaphragm and the electrolyte are arranged between the positive plate and the negative plate, and the electrolyte is any one of the lithium ion battery electrolytes.
The positive plate comprises a positive current collector and a positive active material layer coated on the positive current collector, wherein the positive active material layer comprises a positive active material, a positive conductive agent and a positive binder. The positive electrode active material may be of a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95.ltoreq.a.ltoreq.1.2, x)>0, y is greater than or equal to 0, z is greater than or equal to 0, and x+y+z=1, 0 is greater than or equal to b is greater than or equal to 1, M is selected from a combination of one or more of Mn, al, N is selected from a combination of one or more of F, P, S), the positive electrode active material may also be a combination of one or more of compounds including but not limited to LiCoO 2 、LiNiO 2 、 LiVO 2 、LiCrO 2 、LiMn 2 O 4 、LiCoMnO 4 、Li 2 NiMn 3 O 8 、LiNi 0.5 Mn 1.5 O 4 、LiCoPO 4 、 LiMnPO 4 、LiFePO 4 、LiNiPO 4 、LiCoFSO 4 、CuS 2 、FeS 2 、MoS 2 、NiS、TiS 2 And the like. The positive electrode active material can also be modified to change the positive electrode active materialMethods of sex treatment should be known to those skilled in the art, for example, coating, doping, etc. may be used to modify the positive electrode active material, and the material used for modification treatment may be a combination of one or more of Al, B, P, zr, si, ti, ge, sn, mg, ce, W, etc. The positive current collector is typically a structure or part for collecting current, and may be any of a variety of materials suitable in the art for use as a positive current collector for a lithium ion battery, for example, the positive current collector may be a material including, but not limited to, a metal foil, etc., and more particularly may be a material including, but not limited to, aluminum foil, etc.
The negative electrode sheet includes a negative electrode fluid and a negative electrode active material layer coated on the negative electrode current collector, the negative electrode active material layer including a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The negative electrode active material may be one or more of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microsphere, silicon-based material, tin-based material, lithium titanate, or other metals capable of forming an alloy with lithium, etc., including but not limited to. Wherein, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be one or more selected from simple substance silicon, silicon oxygen compound, silicon carbon compound and silicon alloy; the tin-based material can be selected from one or more of elemental tin, tin oxide and tin alloy. The negative current collector is typically a structure or part that collects current, and may be any of a variety of materials suitable in the art for use as a negative current collector for a lithium ion battery, for example, the negative current collector may be a material including, but not limited to, a metal foil, etc., and more particularly may be a material including, but not limited to, a copper foil, etc.
And the separator may be a variety of materials suitable for lithium ion battery separators in the art, for example, may be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, natural fibers, and the like.
In order to make the technical solution and advantages of the present invention more apparent, the present invention and its advantageous effects will be described in further detail below with reference to the specific embodiments, but the embodiments of the present invention are not limited thereto.
Example 1
The lithium ion battery comprises a positive plate, a negative plate, a diaphragm and electrolyte, wherein the diaphragm and the electrolyte are arranged between the positive plate and the negative plate, the positive plate adopts lithium cobaltate as a positive active substance, the negative plate adopts graphite as a negative active substance, and the diaphragm is a polypropylene diaphragm.
Preparation of electrolyte: ethylene carbonate, propylene carbonate and diethyl carbonate are mixed according to the mass ratio of EC to PC: dec=1:1:3, and then 14.5wt% of lithium hexafluorophosphate (LiPF) based on the total weight of the electrolyte was slowly added to the mixed solution 6 ) And 0.5wt% of lithium difluorooxalato borate (LiODFB), and finally adding 0.2wt% of a first additive with a structure shown in a formula I, 0.2wt% of a second additive with a structure shown in a formula II, 6wt% of fluoroethylene carbonate (FEC) and 1.0wt% of ethylene sulfate (DTD) based on the total weight of the electrolyte, and uniformly stirring to obtain the lithium ion battery electrolyte of the embodiment.
Preparation of a soft package battery: sequentially stacking the prepared positive plate, the diaphragm and the negative plate, enabling the diaphragm to be positioned between the positive plate and the negative plate, and winding to obtain a bare cell; and (3) filling the bare cell into an aluminum plastic film outer package, filling the prepared electrolyte into a dried battery, packaging, standing, forming, shaping and capacity division to prepare the lithium ion battery.
Examples 2 to 22 and comparative examples 1 to 9 were prepared according to the above-described preparation methods, except that the contents of each substance of the electrolyte were as shown in the following Table 1.
TABLE 1
Performance testing
The lithium ion batteries and the electrolytes obtained in examples 1 to 22 and comparative examples 1 to 9 were subjected to the performance test.
(1) And (3) testing the cycle performance: at 25 ℃, the battery after capacity division is charged to 4.45V according to a constant current and a constant voltage of 0.7C, the cut-off current is 0.05C, then the battery is discharged to 3.0V according to a constant current of 0.5C, the capacity retention rate at 500 weeks is calculated after 500 cycles of charge and discharge according to the circulation, and the calculation formula is as follows:
500 th cycle capacity retention (%) = (500 th cycle discharge capacity/first cycle discharge capacity) ×100%.
(2) High temperature storage test at 60 ℃ for 14 d: the battery is charged and discharged 1 time (4.45V-3.0V) at the normal temperature under the temperature of 0.5C, and the discharge capacity C before the battery is stored is recorded 0 Then the battery is charged to 4.45V full state at constant current and constant voltage, and the thickness d of the battery before high-temperature storage is tested by using a PPG battery thickness meter (500 g) 1 Placing the battery into a 60 ℃ incubator for storage for 14 days, taking out the battery after storage is completed, and testing the thermal thickness d of the stored battery 2 Calculating the thickness expansion rate of the battery after the battery is stored at 60 ℃ for 14 days; after the battery is cooled for 24 hours at room temperature, the battery is discharged to 3.0V at constant current at 0.5C, then is charged to 4.45V at constant current and constant voltage at 0.5C, and the discharge capacity C of the battery after storage is recorded 1 And charging capacity C 2 And calculating the capacity remaining rate and recovery rate of the battery after being stored at 60 ℃ for 14 days, wherein the calculation formula is as follows:
thickness expansion ratio= (d) after 14 days of storage at 60 DEG C 2 -d 1 )/d 1 *100％；
Capacity remaining rate after 14 days of storage at 60 ℃ =c 1 /C 0 *100％；
Capacity recovery rate after 14 days storage at 60 ℃ = C 2 /C 0 *100％。
(3) DCR (direct current impedance) test: at normal temperature (23 ℃ +/-3 ℃), constant current and voltage are carried out at 0.5 ℃ to 4.45V, the cut-off current is 0.02 ℃, then the discharge is carried out at 0.1C for 9 hours (adjusted to 10% SOC), then the discharge is carried out at 0.1C for 10 seconds, the recording end voltage V1 and the discharge at 1C for 1s are carried out, and the recording end voltage V2 is recorded;
DCR calculation formula: dcr= (V1-V2)/(1C-0.1C).
(4) Low temperature discharge performance test: discharging the battery with the capacity of 0.5C to 3.0V at the temperature of 25 ℃ and standing for 5min; charging the battery to 4.45V at 0.2C, changing to 4.45V constant voltage charging when the voltage of the battery core reaches 4.45V until the charging current is less than or equal to the given cutoff current of 0.05C, and standing for 5min; transferring the fully charged core into a high-low temperature box, setting the temperature to-10 ℃, and standing for 120min after the temperature of the box reaches; then discharging at 0.2C to a final voltage of 3.0V, and standing for 5min; then the temperature of the high-low temperature box is adjusted to 25+/-3 ℃, and the box is left for 60 minutes after the temperature of the box is reached; charging the battery to 4.45V at 0.2C, and changing the battery to 4.45V constant voltage charging when the voltage of the battery cell reaches 4.45V until the charging current is less than or equal to the given cutoff current of 0.05C; standing for 5min; the capacity retention rate of 3.0V discharge at-10 ℃ is calculated. The calculation formula is as follows:
-10 ℃ discharge 3.0V capacity retention (%) = (-10 ℃ discharge to 3.0V discharge capacity/25 ℃ discharge to 3.0V discharge capacity) ×100%.
(5) Thermal shock performance: discharging to 3.0V at a given current of 0.2C under ambient conditions of 25 ℃; standing for 5min; charging to 4.45V at a charging current of 0.2C, and changing to 4.45V constant voltage charging when the voltage of the battery cell reaches 4.45V until the charging current is less than or equal to a given cutoff current of 0.05C; placing the battery cell into an oven after the battery cell is placed for 1h, wherein the temperature of the oven is 135+/-2 ℃ at the speed of 5+/-2 ℃/min, and stopping after the battery cell is kept for 30min, and judging that the battery cell is not fired or exploded.
The test results are shown in Table 2 below.
TABLE 2
As can be seen from the above test results, when the examples of the first additive of structural formula i and the second additive of structural formula ii shown in the electrolyte of the present invention are contained, the hot box performance, the low temperature discharge performance, and the reversible capacity and cycle performance of the lithium ion battery are improved, and at the same time, the high temperature storage performance including the high temperature storage capacity retention rate, the high temperature storage capacity recovery rate, and the high temperature storage thickness expansion rate are improved, compared to comparative examples 1 to 8. The two additives mainly benefit from the mutual influence of the two additives, and the synergistic effect of the two additives can generate stable and low-impedance interface films on the surfaces of the anode and the cathode respectively, so that the hot box performance and the low-temperature performance of the battery are improved, and the cycle performance and the storage performance of the battery are improved.
As is apparent from the experimental comparison results of examples 1 to 5 and comparative example 4, the first additive is effective in improving the high-temperature storage performance and cycle performance of the battery, and the high-temperature storage performance and cycle performance of the battery are improved as the content of the first additive increases, but when the content is too large, the improvement of the performance starts to show a decreasing trend mainly because the excessive content adversely affects the formation of the polymer film on the surface of the positive and negative electrodes. As is apparent from the experimental results of comparative examples 2, 6 to 9 and comparative example 3, the second additive was effective in improving the low temperature performance and the hot box performance, and the low temperature performance of the battery was also improved with the increase of the content of the second additive, but the storage performance was deteriorated and the improvement of the low temperature performance was limited when the second additive was excessive. When both the first additive and the second additive are used in combination, the performance disadvantages associated with the single additive can be substantially offset, and the effect of the single additive improvement can be further enhanced.
In addition, as can be seen from the comparison of examples 2, 20 to 22 and comparative example 9, the kind and content of the third additive, the kind of lithium salt used, the proportion of the organic solvent, and the like also affect the electrochemical performance of the battery. Wherein, when the electrolyte adopts the third additive of the mixture of FEC and DTD, the battery has better electrochemical performance; the mixed lithium salt of lithium hexafluorophosphate and lithium difluorooxalato borate has better electrochemical performance than lithium hexafluorophosphate single lithium salt, and the content ratio of the organic solvent EC/PC/DEC is 1/1/3 and better electrochemical performance than 3/2/5. From this, it can be seen that when the electrolyte simultaneously controls the kind and content of the first additive, the kind and content of the second additive, the kind and content of the third additive, the kind of lithium salt used, and the ratio of the organic solvent, the electrochemical performance of the battery can be improved to be more excellent.
From the test results, it can be seen that when the electrolyte provided by the invention adopts the first additive with the structure of formula I and the second additive with the structure of formula II in the high-voltage lithium cobaltate system battery, the hot box performance and the low-temperature performance of the battery can be effectively improved, and the cycle performance and the storage performance of the battery are improved.
Variations and modifications of the above embodiments will occur to those skilled in the art to which the invention pertains from the foregoing disclosure and teachings. Therefore, the present invention is not limited to the above-described embodiments, but is intended to be capable of modification, substitution or variation in light thereof, which will be apparent to those skilled in the art in light of the present teachings. In addition, although specific terms are used in the present specification, these terms are for convenience of description only and do not limit the present invention in any way.
Claims (9)
1. The lithium ion battery electrolyte is characterized by comprising lithium salt, an organic solvent, a first additive and a second additive, wherein the first additive has a structural formula shown in a formula I;
the second additive is at least one of the following structural formulas:
wherein R is 1 、R 3 Each independently selected from any one of C1-C10 alkyl and its substituent, C1-C10 olefine base and its substituent; r is R 2 Is any one of a hydrogen atom, a halogen atom, a C1-C10 alkyl group and a substituent thereof, a C1-C10 alkoxy group and a substituent thereof, a C1-C10 haloalkane and a C1-C10 haloaryl group.
2. The lithium ion battery electrolyte of claim 1, wherein the first additive is at least one of the following structural formulas:
3. the lithium ion battery electrolyte according to claim 1 or 2, wherein the content of the first additive is 0.1-10 wt% of the total mass of the electrolyte.
4. The lithium ion battery electrolyte according to claim 1, wherein the content of the second additive is 0.1-10 wt% of the total mass of the electrolyte.
5. The lithium ion battery electrolyte of claim 1, further comprising a third additive that is one or more of fluoroethylene carbonate, vinylene carbonate, 1, 3-propane sultone, vinyl sulfate, methylene methane disulfonate, acrylsultone, citraconic anhydride, succinonitrile, hexanedinitrile, ethylene glycol bis (propionitrile) ether, and hexane tricarbonitrile.
6. The lithium ion battery electrolyte of claim 5, wherein the total content of the third additive is 0.1 to 20wt% of the total mass of the electrolyte.
7. The lithium ion battery electrolyte according to claim 1, wherein the organic solvent is at least one of ethylene carbonate, propylene carbonate, diethyl carbonate, methyl ethyl carbonate, ethyl propionate, propyl propionate, ethyl acetate, ethyl n-butyrate, and γ -butyrolactone; the content of the organic solvent is 20-70 wt% of the total mass of the electrolyte.
8. The lithium ion battery electrolyte of claim 1, wherein the lithium salt is at least two of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis-oxalato phosphate, lithium tetrafluorooxalato phosphate, lithium oxalato phosphate, lithium bis-oxalato borate, lithium difluorooxalato borate, lithium tetrafluoroborate, and lithium difluorosulfonimide; the content of the lithium salt is 0.1-15 wt% of the total mass of the electrolyte.
9. A lithium ion battery comprising a positive plate, a negative plate, a diaphragm and electrolyte, wherein the diaphragm and the electrolyte are arranged between the positive plate and the negative plate, and the lithium ion battery electrolyte is characterized in that the electrolyte is the lithium ion battery electrolyte as claimed in any one of claims 1 to 8.
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