阅读说明：本技术 一种非水电解液及包括所述非水电解液的锂二次电池 (Non-aqueous electrolyte and lithium secondary battery comprising same ) 是由 廖波 王海 李素丽 李俊义 徐延铭 于 2019-11-20 设计创作，主要内容包括：本发明提供了一种非水电解液及包括所述非水电解液的锂二次电池,所述非水电解液包括有机溶剂、添加剂和锂盐,所述添加剂至少含有多元腈类化合物；所述的添加剂的用量占非水电解液总重量的0.01-10wt％；所述多元腈类化合物由低元腈类化合物制备,所述多元腈类化合物中低元腈类化合物的含量为5-60ppm。本申请通过控制作为添加剂的多元腈类化合物中原料杂质的含量和该添加剂的用量制备得到了一种具有更好的高温循环性能的电解液,将其用于锂二次电池显著提高了其高温循环性能。(The invention provides a non-aqueous electrolyte and a lithium secondary battery comprising the same, wherein the non-aqueous electrolyte comprises an organic solvent, an additive and a lithium salt, and the additive at least contains a polynary nitrile compound; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the polynary nitrile compound is prepared from low-element nitrile compounds, and the content of the low-element nitrile compounds in the polynary nitrile compounds is 5-60 ppm. The electrolyte with better high-temperature cycle performance is prepared by controlling the content of raw material impurities in the polynary nitrile compound serving as the additive and the using amount of the additive, and the high-temperature cycle performance of the electrolyte is obviously improved when the electrolyte is used for a lithium secondary battery.)

1. A nonaqueous electrolyte solution comprising an organic solvent, an additive and a lithium salt, wherein the additive contains at least a polynitrile compound; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the polynary nitrile compound is prepared from low-element nitrile compounds, and the content of the low-element nitrile compounds in the polynary nitrile compounds is 5-60 ppm.

2. The nonaqueous electrolytic solution of claim 1, wherein the low nitrile compound is selected from compounds having a structure represented by formula (1):

in the formula (1), R₁And R₂Identical or different, independently of one another, from H, halogen, alkyl or aryl; r₃Selected from H, halogen, alkyl or aryl; when R is₁And R₃When selected from hydrocarbon groups, they may be linked to each other to form a ring, i.e., a compound having a structure represented by the following formula (2):

in the formula (2), R₂Is as defined in formula (1); r'₁And R'₃Are each R₁And R₃By removal of one H group, R₁And R₃Identical or different, independently of one another, from the group of hydrocarbon radicals.

3. The nonaqueous electrolytic solution of claim 1 or 2, which comprises an organic solvent, an additive containing at least 1,3, 6-hexanetricarbonitrile, and a lithium salt; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the content of acrylonitrile in the 1,3, 6-hexanetricarbonitrile is 5-60 ppm.

4. The nonaqueous electrolytic solution of any one of claims 1 to 3, wherein the content of acrylonitrile in the 1,3, 6-hexanetricarbonitrile is 5 to 58ppm, such as 7 to 55ppm, such as 8 to 53ppm, such as 8.99 to 51.99ppm, such as 9.99 to 31.99 ppm.

5. The nonaqueous electrolytic solution of any one of claims 1 to 4, wherein the additive is used in an amount of 0.1 to 5 wt%, for example, 0.5 to 2 wt%, based on the total weight of the nonaqueous electrolytic solution.

6. The nonaqueous electrolytic solution of any one of claims 1 to 5, wherein the organic solvent is at least one selected from the group consisting of a carbonate (e.g., a cyclic carbonate, a chain carbonate), a carboxylate (e.g., a cyclic carboxylate, a chain carboxylate), an ether compound (e.g., a cyclic ether compound, a chain ether compound), a phosphorus-containing compound, a sulfur-containing compound, and an aromatic fluorine-containing compound.

7. The nonaqueous electrolytic solution of any one of claims 1 to 6, wherein the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, lithium difluorooxalato borate, lithium bis (trifluoromethylsulfonyl) imide and lithium bis (oxalato) borate, lithium difluorooxalato phosphate, and the lithium salt accounts for 8 to 25 wt% of the total mass of the nonaqueous electrolytic solution.

8. A lithium secondary battery comprising the nonaqueous electrolytic solution of any one of claims 1 to 7.

9. The lithium secondary battery according to claim 8, wherein the lithium secondary battery further comprises a positive electrode tab, a negative electrode tab, and a separator interposed between the positive electrode tab and the negative electrode tab.

10. According to claimThe lithium secondary battery according to claim 9, wherein the specific surface area of the positive electrode active material in the positive electrode sheet is 0.1 to 1m²The specific surface area of the negative active material in the negative plate is 1-2 m²/g。

Technical Field

The invention belongs to the technical field of electrolyte, and particularly relates to a non-aqueous electrolyte and a lithium secondary battery comprising the same.

Background

Compared with lead-acid batteries, cadmium-nickel batteries and nickel-hydrogen batteries, the lithium secondary battery has the advantages of high working voltage, high specific energy, small self-discharge, long cycle life, no memory effect and the like, so that the lithium secondary battery is developed rapidly in recent years, and the performance index is improved continuously. As the demand for the endurance of the lithium secondary battery increases, developers have to further increase the charging voltage of the lithium secondary battery. However, this deteriorates the cycle performance of the lithium secondary battery.

The electrolyte additive has the outstanding advantage of being most economical and feasible for improving the performance of the lithium secondary battery. The polynary nitrile compound is a common additive for improving high-temperature cycle performance, and the high-temperature cycle performance of the lithium secondary battery can be obviously improved by adding a small amount of polynary nitrile compound into the electrolyte, so that the lithium secondary battery which is suitable for high voltage and has better high-temperature cycle performance is prepared.

However, even when the polynitrile compound is added, the lithium secondary battery obtained has a problem that the transition metal ions are eluted at a high voltage and the high-temperature performance is poor, and thus it is difficult to achieve an optimum state.

Disclosure of Invention

The invention aims to solve the problems of the existing lithium secondary battery that the transition metal ions are dissolved out under high voltage and the high-temperature performance is poor, and provides a non-aqueous electrolyte and a lithium secondary battery containing the non-aqueous electrolyte.

When the polynary nitrile compound is produced from a monovalent nitrile compound or a lower nitrile compound, the monovalent nitrile compound or the lower nitrile compound is generally contained as an impurity because it is difficult to completely remove the compound by a simple purification method. Taking 1,3, 6-hexanetricarbonitrile as an example, it is prepared from acrylonitrile, and generally contains a trace amount of acrylonitrile impurities, and even purified 1,3, 6-hexanetricarbonitrile inevitably contains residual acrylonitrile. The inventors have studied and found that when the polynitrile compound is used as an additive for an electrolyte solution, the content of raw material impurities (such as acrylonitrile) therein greatly affects the performance of a lithium secondary battery, and as a result, it has been found that the raw material impurities generally contain cyano-conjugated double bonds which are superior to the effect of the polynitrile compound in reacting at a positive electrode when the lithium secondary battery is first charged, and thus the polynitrile compound hardly realizes the effect of complexing with transition metal ions on the surface of the positive electrode to stabilize the transition metal on the surface. The inventor of the present application finds, through research, that an electrolyte having a better high-temperature cycle performance can be prepared by controlling the content of raw material impurities in the polynitrile compound. Specifically, when the content of raw material impurities is controlled within the range of 5-60ppm, the raw material impurities can be completely reduced at the negative electrode during the first charging, and the reaction at the positive electrode is not carried out any more, so that the effect of polynitrile compounds on the positive electrode protection is not influenced; in addition, the reduced raw material impurities can form a good interface film on the surface of the negative electrode, and nitrile groups (-CN) in the raw material impurities can be combined with transition metal on the surface of the negative electrode, so that the deposition of transition metal ions dissolved out from the positive electrode on the negative electrode is inhibited, and the decomposition of electrolyte on the negative electrode is inhibited; based on this, the high-temperature cycle performance of the lithium secondary battery is remarkably improved, and the purpose of the invention is realized.

The purpose of the invention is realized by the following technical scheme:

a nonaqueous electrolytic solution comprising an organic solvent, an additive and a lithium salt, wherein the additive contains at least a polynitrile compound; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the polynary nitrile compound is prepared from low-element nitrile compounds, and the content of the low-element nitrile compounds in the polynary nitrile compounds is 5-60 ppm.

Wherein the low-membered nitrile compound is selected from compounds having a structure represented by formula (1):

in the formula (1), R₁And R₂Identical or different, independently of one another, from H, halogen, alkyl or aryl; r₃Selected from H, halogen, alkyl or aryl; when R is₁And R₃When selected from hydrocarbon groups, they may be linked to each other to form a ring, i.e., a compound having a structure represented by the following formula (2):

in the formula (2), R₂Is as defined in formula (1); r'₁And R'₃Are each R₁And R₃By removal of one H group, R₁And R₃Identical or different, independently of one another, from the group of hydrocarbon radicals.

The nonaqueous electrolyte comprises an organic solvent, an additive and a lithium salt, wherein the additive at least contains 1,3, 6-hexanetrinitrile; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the content of acrylonitrile in the 1,3, 6-hexanetricarbonitrile is 5-60 ppm.

Wherein the 1,3, 6-hexanetricarbonitrile has an acrylonitrile content of 5 to 58ppm, such as 7 to 55ppm, such as 8 to 53ppm, such as 8.99 to 51.99ppm, such as 9.99 to 31.99 ppm.

Wherein the additive is used in an amount of 0.1 to 5 wt%, for example, 0.5 to 2 wt%, based on the total weight of the nonaqueous electrolytic solution.

Wherein the organic solvent is at least one selected from carbonate (such as cyclic carbonate and chain carbonate), carboxylate (such as cyclic carboxylate and chain carboxylate), ether compound (such as cyclic ether compound and chain ether compound), phosphorus-containing compound, sulfur-containing compound and aromatic fluorine-containing compound.

The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, lithium difluoro oxalate borate, lithium bis (trifluoromethanesulfonyl) imide, lithium bis (oxalato) borate and lithium difluoro oxalate phosphate, and accounts for 8-25 wt% of the total mass of the nonaqueous electrolyte.

The invention also provides a lithium secondary battery which comprises the nonaqueous electrolyte.

The lithium secondary battery further comprises a positive plate, a negative plate and a diaphragm, wherein the diaphragm is arranged between the positive plate and the negative plate.

Wherein the specific surface area of the positive active material in the positive plate is 0.1-1 m²The specific surface area of the negative active material in the negative plate is 1-2 m²/g。

The invention has the beneficial effects that:

the invention provides a non-aqueous electrolyte and a lithium secondary battery containing the same, wherein the electrolyte has better high-temperature cycle performance and can improve the charging performance of the lithium secondary battery.

The electrolyte with better high-temperature cycle performance can be prepared by controlling the content of raw material impurities in the polynitrile compound. Specifically, when the content of raw material impurities is controlled within the range of 5-60ppm, the raw material impurities can be completely reduced at the negative electrode during the first charging, and the reaction at the positive electrode is not carried out any more, so that the effect of polynitrile compounds on the positive electrode protection is not influenced; in addition, the reduced raw material impurities can form a good interface film on the surface of the negative electrode, and nitrile groups (-CN) in the raw material impurities can be combined with transition metal on the surface of the negative electrode, so that the deposition of transition metal ions dissolved out from the positive electrode on the negative electrode is inhibited, and the decomposition of electrolyte on the negative electrode is inhibited; based on this, the high-temperature cycle performance of the lithium secondary battery is significantly improved.

Drawings

Fig. 1 is a graph of cycle performance for example 1, comparative example 2, and comparative example 3.

Detailed Description

As described above, the present invention provides a nonaqueous electrolytic solution including an organic solvent, an additive, and a lithium salt, wherein the additive contains at least a polynitrile compound; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the polynary nitrile compound is prepared from low-element nitrile compounds, and the content of the low-element nitrile compounds in the polynary nitrile compounds is 5-60 ppm.

In the present invention, the low-membered nitrile compound means a nitrile compound in which the number of cyano CNs is smaller than that of the above-mentioned polybasic nitrile compound; for example, if the polynary nitrile compound is a ternary nitrile compound, the lower nitrile compound is a monovalent nitrile compound or a divalent nitrile compound.

Wherein the low-membered nitrile compound may be selected from compounds having a structure represented by formula (1):

in the formula (1), R₁And R₂Identical or different, independently of one another, from H, halogen, alkyl or aryl; r₃Selected from H, halogen, alkyl or aryl; when R is₁And R₃When selected from hydrocarbon groups, they may be linked to each other to form a ring, i.e., a compound having a structure represented by the following formula (2):

in the formula (2), R₂Is as defined in formula (1); r'₁And R'₃Are each R₁And R₃By removal of one H group, R₁And R₃Identical or different, independently of one another, from the group of hydrocarbon radicals.

Preferably, R₁And R₂Identical or different, independently of one another, from H, halogen, alkyl; r₃Selected from H, halogen, alkyl; when R is₁And R₃When selected from alkyl groups, they may be linked to each other to form a ring. Specifically, the R is₁、R₂、R₃Is selected from H.

Wherein the polynary nitrile compound is prepared from low-element nitrile compounds, and the low-element nitrile compounds at least comprise the compound with the structure shown in the formula (1). For example, when the low nitrile is selected from acrylonitrile, the poly nitrile is selected from 1,3, 6-hexanetrinitrile.

The "halogen" in the invention refers to fluorine, chlorine, bromine or iodine.

"hydrocarbyl" as used herein alone or as suffix or prefix, is for example a straight or branched chain saturated/unsaturated aliphatic radical, which may in particular be alkyl, alkenyl or alkynyl.

"alkyl" used herein alone or as suffix or prefix, is intended to include both branched and straight chain saturated aliphatic hydrocarbon groups having from 1 to 20, preferably from 1 to 6, carbon atoms. For example, "C1-6 alkyl" denotes straight and branched chain alkyl groups having 1,2, 3, 4, 5 or 6 carbon atoms. Examples of alkyl groups include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, pentyl, and hexyl.

"alkenyl" as used herein alone or as a suffix or prefix, is intended to include both branched and straight chain aliphatic hydrocarbon radicals containing alkenyl or alkene radicals having from 2 to 20, preferably 2-6, carbon atoms (or the particular number of carbon atoms if provided). For example, "C2-6 alkenyl" denotes alkenyl having 2,3, 4, 5 or 6 carbon atoms. Examples of alkenyl groups include, but are not limited to, vinyl, allyl, 1-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 2-methylbut-2-enyl, 3-methylbut-1-enyl, 1-pentenyl, 3-pentenyl, and 4-hexenyl.

"alkynyl" used herein alone or as a suffix or prefix is intended to include both branched and straight chain aliphatic hydrocarbon radicals containing alkynyl groups or alkynes having 2 to 20, preferably 2-6 carbon atoms (or the particular number of carbon atoms if provided). For example ethynyl, propynyl (e.g., l-propynyl, 2-propynyl), 3-butynyl, pentynyl, hexynyl and 1-methylpent-2-ynyl.

"aryl" used herein alone or as a suffix or prefix, refers to an aromatic ring structure made up of 5 to 20 carbon atoms. For example: the aromatic ring structure containing 5, 6, 7 and 8 carbon atoms may be a monocyclic aromatic group such as phenyl; the ring structure containing 8, 9, 10, 11, 12, 13 or 14 carbon atoms may be polycyclic, for example naphthyl. The aromatic ring may be substituted at one or more ring positions with substituents such as alkyl, halogen, and the like, e.g., tolyl. The term "aryl" also includes polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings (the rings are "fused rings"), wherein at least one of the rings is aromatic and the other cyclic rings can be, for example, cycloalkyl, cycloalkenyl, cycloalkynyl, aryl, and/or heterocyclyl. Examples of polycyclic rings include, but are not limited to, 2, 3-dihydro-1, 4-benzodioxine and 2, 3-dihydro-1-benzofuran.

In a specific embodiment, the nonaqueous electrolytic solution comprises an organic solvent, an additive and a lithium salt, wherein the additive at least contains 1,3, 6-hexanetricarbonitrile; the dosage of the additive accounts for 0.01 to 10 weight percent of the total weight of the nonaqueous electrolyte; the content of acrylonitrile in the 1,3, 6-hexanetricarbonitrile is 5-60 ppm.

In one embodiment of the invention, the 1,3, 6-hexanetricarbonitrile has an acrylonitrile content of from 5 to 58ppm, for example from 7 to 55ppm, for example from 8 to 53ppm, for example from 8.99 to 51.99ppm, for example from 9.99 to 31.99 ppm.

In one embodiment of the present invention, the 1,3, 6-hexanetricarbonitrile is used in an electrolyte solution, and can form a film on a positive electrode and inhibit elution of metal ions from the positive electrode. When the content of acrylonitrile in 1,3, 6-hexanetricarbonitrile is controlled to be within a range of 5-60ppm, acrylonitrile can be completely reduced at the negative electrode during first charging, and the effect of 1,3, 6-hexanetricarbonitrile on the positive electrode protection will not be affected.

As above, the additive is used in an amount of 0.01 to 10 wt% based on the total weight of the nonaqueous electrolytic solution; in one embodiment of the present invention, the amount is 0.1 to 5 wt%, and further, for example, 0.5 to 2 wt% based on the total weight of the nonaqueous electrolytic solution.

In one embodiment of the present invention, the organic solvent is at least one selected from the group consisting of a carbonate (e.g., cyclic carbonate, chain carbonate), a carboxylate (e.g., cyclic carboxylate, chain carboxylate), an ether compound (e.g., cyclic ether compound, chain ether compound), a phosphorus-containing compound, a sulfur-containing compound, and an aromatic fluorine-containing compound.

Wherein the carbonate is at least one selected from the group consisting of dimethyl carbonate, methylethyl carbonate, diethyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate, di-n-propyl carbonate, bis (fluoromethyl) carbonate, bis (difluoromethyl) carbonate, bis (trifluoromethyl) carbonate, bis (2-fluoroethyl) carbonate, bis (2, 2-difluoroethyl) carbonate, bis (2,2, 2-trifluoroethyl) carbonate, 2-fluoroethyl methyl carbonate, 2, 2-difluoroethyl methyl carbonate and 2,2, 2-trifluoroethyl methyl carbonate.

Wherein the carboxylic ester is at least one selected from methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, sec-butyl acetate, isobutyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, isopropyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, methyl isobutyrate, ethyl isobutyrate, methyl valerate, ethyl valerate, methyl pivalate and ethyl pivalate, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, butyl trifluoroacetate, trifluoroacetic acid and 2,2, 2-trifluoroethyl ester.

Wherein the ether compound is at least one selected from tetrahydrofuran, 2-methyltetrahydrofuran, 1, 3-dioxolane, 2-methyl-1, 3-dioxolane, 4-methyl-1, 3-dioxolane, 1, 3-dioxane, 1, 4-dioxane, dimethoxypropane, dimethoxymethane, 1-dimethoxyethane, 1, 2-dimethoxyethane, diethoxymethane, 1-diethoxyethane, 1, 2-diethoxyethane, ethoxymethoxymethane, 1-ethoxymethoxyethane and 1, 2-ethoxymethoxyethane.

Wherein the phosphorus-containing compound is selected from at least one of trimethyl phosphate, triethyl phosphate, dimethyl ethyl phosphate, methyl diethyl phosphate, ethylene methyl phosphate, ethylene ethyl phosphate, triphenyl phosphate, trimethyl phosphite, triethyl phosphite, triphenyl phosphite, tris (2,2, 2-trifluoroethyl) phosphate, and tris (2,2,3,3, 3-pentafluoropropyl) phosphate.

Wherein the sulfur-containing compound is at least one selected from sulfolane, 2-methylsulfolane, 3-methylsulfolane, dimethylsulfone, diethylsulfone, ethylmethylsulfone, methylpropylsulfone, dimethylsulfoxide, methyl methanesulfonate, ethyl methanesulfonate, methyl ethanesulfonate, ethyl ethanesulfonate, dimethyl sulfate, diethyl sulfate and dibutyl sulfate.

Wherein the aromatic fluorine-containing compound is selected from m-fluorotoluene, p-fluorotoluene, fluorobiphenyl and fluorobenzene.

In one embodiment of the invention, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, lithium difluoro oxalate borate, lithium bis (trifluoromethylsulfonyl) imide, lithium bis oxalate borate and lithium difluoro oxalate phosphate, and the lithium salt accounts for 8-25 wt% of the total mass of the nonaqueous electrolytic solution.

As described above, the present invention also provides a method for preparing the above nonaqueous electrolytic solution, the method comprising:

mixing an organic solvent, a lithium salt and an additive which at least contains the polynitrile compound and accounts for 0.01-10 wt% of the total weight of the non-aqueous electrolyte to prepare the electrolyte.

In one embodiment of the invention, the mixing is carried out at room temperature.

In one embodiment of the invention, the mixing is not limited by the order of addition.

As described above, the present invention also provides a lithium secondary battery comprising the above nonaqueous electrolytic solution.

In one aspect of the present invention, the lithium secondary battery further includes a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet. The diaphragm arranged between the positive plate and the negative plate can prevent the current short circuit caused by the contact of the two plates and can allow lithium ions to pass through.

In one aspect of the present invention, the anode includes an anode current collector and an anode active material layer disposed on one or both surfaces of the anode current collector.

Wherein, the negative current collector is selected from copper foil, such as electrolytic copper foil or rolled copper foil.

Wherein the anode active material layer includes an anode active material and an anode binder.

In some embodiments, the negative electrode active material is any material capable of deintercalating metal ions such as lithium ions.

In some embodiments, the negative active material may be one or more of graphite, a silicon material, a silicon-carbon composite material, a silicon-oxygen material, an alloy material, and a lithium-containing metal composite oxide material.

In some embodiments, the chargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent unintentional precipitation of lithium metal on the negative electrode during charging.

In some embodiments, the negative electrode binder includes, but is not limited to, one or more of styrene-butadiene rubber, fluorine-based rubber, and ethylene propylene diene, hydroxyalkyl methyl cellulose.

The negative electrode active material layer may further include a conductive agent, and the conductive agent may be at least one of graphite, carbon black, acetylene black, ketjen black, carbon nanotubes, and graphene.

In one aspect of the present invention, the positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on one or both surfaces of the positive electrode current collector.

Wherein the positive electrode current collector is selected from aluminum foil.

Wherein the positive electrode active material layer includes a positive electrode active material and a positive electrode binder.

In some embodiments, the positive active material is a lithium-containing compound. The lithium-containing compound includes one or more of a lithium transition metal composite oxide and a lithium transition metal phosphate compound.

In some embodiments, the lithium transition metal composite oxide comprises lithium and an oxide having one or more transition metal elements.

In some embodiments, the lithium transition metal phosphate compound comprises lithium and a phosphate compound having one or more transition metal elements.

In some embodiments, the transition metal element includes one or more of Co, Ni, Mn, and Fe, which may allow the electrochemical device to obtain a higher voltage. Specific examples can be lithium cobaltate, ternary nickel cobalt manganese, lithium iron phosphate and other materials.

In some embodiments, the positive electrode binder may be a polymeric material including, but not limited to, polyvinylidene fluoride and polyimide.

The positive electrode may further include a conductive agent, and the conductive agent may be at least one of graphite, carbon black, acetylene black, ketjen black, carbon nanotubes, and graphene.

In one aspect of the invention, the separator is selected from porous films.

Wherein, the diaphragm is a porous film made of polymer.

In some embodiments, the polymers include, but are not limited to: polyethylene terephthalate, polybutylene terephthalate, polyether, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene ether, polyphenylene sulfide, polyethylene naphthalene, high density polyethylene, low density polyethylene, linear low density polyethylene, ultra high molecular weight polyethylene, and polypropylene.

In some embodiments, the separator further comprises an organic or inorganic coating disposed on one or both surfaces of the porous membrane.

In some embodiments, the inorganic substances may specifically include, but are not limited to: BaTiO 2₃、Pb(Zr，Ti)O₃(PZT)、Pb_1-xLa_xZr_1-yTi_yO₃、PB(Mg₃Nb_2/3)O₃-PbTiO₃Hafnium oxide (HfO)₂)、SrTiO₃、SnO₂、CeO₂、MgO、NiO、CaO、ZnO、ZrO₂、SiO₂、Y₂O₃、Al₂O₃、SiC、TiO₂And mixtures thereof.

In some embodiments, the organic material may specifically include, but is not limited to: cyanoethylpullulan, cyanoethylpolyvinyl alcohol, cyanoethylcellulose, cyanoethylsucrose, polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate butyrate and cellulose acetate propionate, and mixtures thereof.

In one embodiment of the present invention, the specific surface area of the positive electrode active material is 0.1 to 1m²(ii)/g; further preferably 0.1 to 0.5m²/g。

In one embodiment of the invention, the specific surface area of the negative active material is 1-2 m²(ii)/g; further preferably 1.2 to 1.8m²Per g, preferably from 1.3 to 1.7m²/g。

The preparation method of the present invention will be described in further detail with reference to specific examples. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.

The experimental methods used in the following examples are all conventional methods unless otherwise specified; reagents, materials and the like used in the following examples are commercially available unless otherwise specified.

Example 1

A non-aqueous electrolyte consists of an organic solvent, a lithium salt and an additive, wherein the organic solvent is prepared from ethylene carbonate: propylene carbonate: diethyl carbonate (1: 2: 7) and the organic solvent accounted for 84.5 wt% of the total mass of the nonaqueous electrolytic solution. The lithium salt is lithium hexafluorophosphate accounting for 12.5 wt% of the total mass of the nonaqueous electrolyte. The additive was 1,3, 6-hexanetricarbonitrile containing 15.03ppm of acrylonitrile, which accounted for 3 wt% of the total mass of the nonaqueous electrolytic solution.

A lithium secondary battery includes a positive electrode sheet containing a positive electrode active material, a negative electrode sheet containing a negative electrode active material, a separator, and the above electrolyte. The positive active material is lithium cobaltate with a specific surface area of 0.34m²(ii) in terms of/g. The negative active material is graphite with a specific surface area of 1.41m²/g。

Example 2

A non-aqueous electrolyte consists of an organic solvent, a lithium salt and an additive, wherein the organic solvent is prepared from ethylene carbonate: propylene carbonate: propyl propionate: diethyl carbonate (2: 1:3: 4) and the organic solvent accounts for 82 wt% of the total mass of the nonaqueous electrolyte. The lithium salt is lithium hexafluorophosphate accounting for 15 wt% of the total mass of the nonaqueous electrolyte. The additive was 1,3, 6-hexanetricarbonitrile containing 45.68ppm of acrylonitrile, which accounted for 3 wt% of the total mass of the nonaqueous electrolytic solution.

The lithium secondary battery was fabricated by the same procedure as in example 1, except that the electrolyte of this example was used.

Example 3

A non-aqueous electrolyte consists of an organic solvent, a lithium salt and an additive, wherein the organic solvent is prepared from ethylene carbonate: propylene carbonate: ethyl propionate: propyl propionate: diethyl carbonate (2: 1:2:2: 3) and the organic solvent accounts for 84 wt% of the total mass of the nonaqueous electrolyte. The lithium salt is lithium hexafluorophosphate accounting for 13 wt% of the total mass of the nonaqueous electrolyte. The additive was 1,3, 6-hexanetricarbonitrile containing 33.38ppm of acrylonitrile, which accounted for 3 wt% of the total mass of the nonaqueous electrolytic solution.

A lithium secondary battery was fabricated by the same procedure as in example 1, except that the electrolyte of this example was used, and the specific surface area of the selected negative electrode active material was 1.6m²/g。

Comparative example 1

The electrolyte was prepared in the same manner as in example 1 except that 2.98ppm of acrylonitrile was contained in 1,3, 6-hexanetricarbonitrile.

The lithium secondary battery was fabricated by the same procedure as in example 1, except that the electrolyte of this example was used.

Comparative example 2

The electrolyte was prepared in the same manner as in example 1 except that 68.23ppm of acrylonitrile was contained in 1,3, 6-hexanetricarbonitrile.

The lithium secondary battery was fabricated by the same procedure as in example 1, except that the electrolyte of this example was used.

Comparative example 3

The electrolyte was prepared in the same manner as in example 1.

The lithium secondary battery was fabricated by the same procedure as in example 1, except that the negative active material having a specific surface area of 2.4m was used²/g。

Comparative example 4

The electrolyte was prepared in the same manner as in example 1.

The lithium secondary battery was fabricated by the same procedure as in example 1, except that the positive electrode active material having a specific surface area of 1.3m was used²/g。

Comparative example 5

The procedure for preparing an electrolyte was the same as in example 1 except that 1,3, 6-hexanetricarbonitrile was added in an amount of 15% by weight based on the total mass of the nonaqueous electrolyte.

The lithium secondary battery was fabricated by the same procedure as in example 1, except that the electrolyte of this example was used.

The lithium secondary batteries of examples and comparative examples were subjected to high-temperature cycle tests under the following specific test conditions:

high-temperature cycle test: the battery is placed at 45 ℃, the charge-discharge cycle is carried out by using 1C current in the charge-discharge voltage interval of 3-4.5V, the maximum capacity of the battery in the first three times is recorded as Q, and the capacity of the battery in the cycle of 300 weeks is selected as Q₂The capacity retention after high temperature cycling of the battery was calculated by the following formula: capacity retention (%) ═ Q₂/Q×100。

As shown in the above table and fig. 1, it can be seen from the results listed in the above table and fig. 1 that the examples have significant advantageous effects, specifically, the capacity retention rate after high temperature cycling is significantly improved, compared to the comparative examples, indicating that the lithium secondary battery obtained by using the electrolyte solution of the present invention has outstanding advantages.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.