WO2018107746A1 - 电解液及二次电池 - Google Patents

电解液及二次电池 Download PDF

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WO2018107746A1
WO2018107746A1 PCT/CN2017/093378 CN2017093378W WO2018107746A1 WO 2018107746 A1 WO2018107746 A1 WO 2018107746A1 CN 2017093378 W CN2017093378 W CN 2017093378W WO 2018107746 A1 WO2018107746 A1 WO 2018107746A1
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carbonate
electrolyte
secondary battery
ethyl
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PCT/CN2017/093378
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English (en)
French (fr)
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郇凤
韩昌隆
朱建伟
周晓崇
刘继琼
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宁德时代新能源科技股份有限公司
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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
    • 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
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present invention relates to the field of battery technologies, and in particular, to an electrolyte and a secondary battery.
  • Lithium-ion secondary batteries have the advantages of high energy density, no memory effect, high operating voltage, etc., and thus become the first choice for the automotive industry to cope with energy crisis and environmental pressure.
  • the increasing demand for cruising range in the automotive industry means that the demand for high energy density of lithium ion secondary batteries is increasing, and thus conventional lithium ion secondary batteries are facing enormous challenges.
  • a high working voltage means that the positive active material needs to have a higher electrode potential, the positive active material is more oxidizing, the side reaction with the electrolyte is more and the reaction is more intense, resulting in secondary lithium ions in the late cycle.
  • the battery swells, the capacity retention rate after the lithium ion secondary battery is cycled decreases, and the volume expansion ratio after storage increases, and even the lithium ion secondary battery fails.
  • the current common method is to introduce a positive electrode additive to form a protective layer on the surface of the positive electrode active material, thereby reducing direct contact between the positive electrode active material and the electrolyte, and reducing the occurrence of side reactions between the positive electrode active material and the electrolyte.
  • a positive electrode additive usually causes some negative effects, such as a decrease in power performance of a lithium ion secondary battery.
  • an object of the present invention is to provide an electrolyte and a secondary battery which are capable of improving the cycle performance and storage performance of the secondary battery and reducing the internal resistance of the secondary battery.
  • the invention provides an electrolyte comprising: an organic solvent; an electrolyte salt, dissolved in an organic solvent; and an additive.
  • the additive includes an alpha-ketoester compound.
  • the present invention provides a secondary battery comprising the present invention An electrolyte as described on the one hand.
  • the electrolytic solution of the present invention includes an ⁇ -ketoester compound which can improve the cycle performance and storage performance of the secondary battery and lower the internal resistance of the secondary battery.
  • the electrolytic solution according to the first aspect of the invention includes: an organic solvent; an electrolyte salt, dissolved in an organic solvent; and an additive.
  • the additive includes an alpha-ketoester compound.
  • the oxygen atoms on the two carbonyl groups of the ⁇ -ketoester can coordinate with the transition metal ion on the surface of the positive electrode to form a stable protective film on the surface of the positive electrode, preventing the positive active material from being blocked.
  • the oxidation of the electrolyte effectively reduces the side reaction between the positive electrode active material and the electrolyte, lowers the internal resistance of the secondary battery, and at the same time improves the cycle performance and storage performance of the secondary battery.
  • the ⁇ -ketoester compound may be selected from one or more of the compounds represented by Formula 1.
  • R 1 and R 2 are each independently selected from an alkyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, an alkynyl group having 2 to 10 carbon atoms, and a carbon atom.
  • One of an aromatic group having 6 to 10 and a heteroaryl group having 5 to 10 carbon atoms, wherein H in the alkyl group, the alkenyl group, the alkynyl group, the aryl group or the heteroaryl group may be partially or wholly One or more substitutions of F, Cl, Br, cyano, carboxyl, carbonyl, sulfonate, aryl, heteroaryl.
  • the ⁇ -ketoester compound may be selected from one or more of the following compounds;
  • the content of the ⁇ -ketoester compound may be from 0.01% to 15% by weight based on the total weight of the electrolytic solution.
  • the weight percentage of the ⁇ -ketoester compound is more than 15%, the internal resistance of the secondary battery is not further lowered, and the effect of improving the cycle performance and storage performance of the secondary battery is not further increased.
  • the ⁇ -ketoester compound may be included in an amount of 0.1% to 10% by weight based on the total weight of the electrolyte.
  • the content of the ⁇ -ketoester compound may be from 0.5% to 8% by weight based on the total weight of the electrolyte.
  • the specific kind of the organic solvent can be selected according to actual needs.
  • a non-aqueous organic solvent is selected.
  • the organic solvent may include any kind of carbonate or carboxylate such as a cyclic carbonate, a chain carbonate, a cyclic carboxylate or a chain carboxylate.
  • the organic solvent may also include a halogenated compound of a carbonate.
  • the organic solvent may include ethylene carbonate (EC), propylene carbonate, butylene carbonate, pentene carbonate, dimethyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, and carbonic acid.
  • Ethyl ester methyl propyl carbonate, ethylene propyl carbonate, tetrahydrofuran, 1,4-butyrolactone, methyl formate, ethyl formate, propyl formate, butyl formate, ethyl acetate, methyl propionate
  • EMC Ethyl ester
  • the electrolyte salt may be selected from a lithium salt, a sodium salt or a zinc salt, which varies depending on the secondary battery to which the electrolyte is applied.
  • the content of the electrolyte salt may be the The total weight of the electrolyte is 6.25% to 25%.
  • the content of the electrolyte salt may be 6.25% to 18.8% of the total weight of the electrolyte.
  • the lithium salt may be selected from the group consisting of LiPF 6 , LiBF 4 , LiN(SO 2 F) 2 (abbreviated as LiFSI), LiClO 4 , LiAsF 6 , LiB (C 2 ) O 4 ) 2 (abbreviated as LiBOB), LiBF 2 (C 2 O 4 ) (abbreviated as LiDFOB), LiN(SO 2 R F ) 2 , LiN(SO 2 F)(SO 2 R F ) or Several.
  • the lithium salt may be selected from one of LiPF 6 , LiN(SO 2 F) 2 , LiN(CF 3 SO 2 ) 2 , LiB(C 2 O 4 ) 2 , LiBF 2 (C 2 O 4 ) Kind or several. Further preferably, the lithium salt may be selected from one or more of LiPF 6 , LiN(SO 2 F) 2 , LiBF 2 (C 2 O 4 ).
  • RF represents C n F 2n+1 , and n is an integer within 1 to 10, and preferably n is an integer within 1 to 3.
  • the additive may further include one or both of vinylene carbonate (VC), fluoroethylene carbonate (FEC).
  • VC vinylene carbonate
  • FEC fluoroethylene carbonate
  • the secondary battery according to the second aspect of the invention may be a lithium ion secondary battery or a lithium metal secondary battery.
  • the secondary battery according to the second aspect of the present invention may further include: a positive electrode sheet, a negative electrode sheet, a separator, and the like.
  • the positive electrode sheet includes a positive electrode current collector and a positive electrode active slurry layer disposed on the positive electrode current collector, and the positive electrode active slurry layer includes a positive electrode active material, a conductive agent, and a binder.
  • the negative electrode sheet includes a negative electrode current collector and a negative electrode active slurry layer disposed on the negative electrode current collector, the negative electrode active slurry layer including a negative electrode active material, a binder, and an optional conductive agent.
  • the cathode active material may be selected from lithium cobaltate (LiCoO 2 ), LiNi x A y B (1-xy) O 2 , One or more of LiMPO 4 , Li 1-x' (Q y' L z' C 1-y'-z' )O 2 .
  • a and B are each independently selected from one of Co, Al, and Mn, and A and B are different, and 0 ⁇ x ⁇ 1, 0 ⁇ y ⁇ 1, and x+y ⁇ 1.
  • LiMPO 4 has an olivine structure, and M is selected from one of Co, Ni, Fe, Mn, and V.
  • Q, L, and C are each independently selected from one of Co, Ni, Fe, and Mn, and 0 ⁇ x' ⁇ 1, 0 ⁇ y' ⁇ 1, 0 ⁇ z' ⁇ 1, and y'+z' ⁇ 1 .
  • the anode active material may be selected from metallic lithium.
  • the negative active material may also be selected from materials capable of intercalating lithium when the electrode potential is ⁇ 2 V with respect to the equilibrium potential of Li/Li + .
  • the negative active material may be selected from natural graphite, artificial graphite, mesophase micro carbon. Ball (abbreviated as MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO 2 , lithiated lithiated TiO 2 - One or more of Li 4 Ti 5 O 12 and Li-Al alloy.
  • the type of the separator is not particularly limited, and any conventional separator material such as polyethylene, polypropylene, polyvinylidene fluoride, and the above polyethylene, poly A multilayer composite film of propylene or polyvinylidene fluoride, but is not limited to the materials mentioned above.
  • the method for preparing a lithium ion secondary battery provided by the present invention is well known in the art, and the lithium ion secondary battery provided by the present invention can be produced by a conventional lithium ion secondary battery production method.
  • the lithium ion secondary batteries of Examples 1-13 and Comparative Examples 1-4 were each prepared in the following manner.
  • the positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O 2 ), the binder polyvinylidene fluoride, and the conductive agent conductive carbon black are mixed at a weight ratio of 98:1:1, and added to the solvent N- In the methylpyrrolidone (NMP), the mixture was stirred under the action of a vacuum mixer until the system showed uniform stability to obtain a positive electrode slurry; the positive electrode slurry was uniformly coated on a current collector aluminum foil having a thickness of 12 ⁇ m; and the aluminum foil was dried at room temperature. The mixture was transferred to an oven at 120 ° C for 1 hour, and then subjected to cold pressing and slitting to obtain a positive electrode sheet.
  • NMP methylpyrrolidone
  • the negative active material silicon-carbon composite, the conductive agent conductive carbon black, and the binder polyacrylate are mixed at a weight ratio of 98:1:1, added to the solvent deionized water, and stirred under a vacuum mixer until the system is presented. Uniformly stable, obtaining a negative electrode slurry; uniformly coating the negative electrode slurry on a current collector copper foil having a thickness of 8 ⁇ m; drying the copper foil at room temperature, transferring it to an oven at 120 ° C for 1 hour, and then subjecting it to cold pressing and slitting A negative electrode sheet was obtained.
  • EC, EMC, and DEC were mixed to form an organic solvent, wherein the weight ratio of EC, EMC, and DEC was 1:1:1.
  • the sufficiently dried lithium salt LiPF 6 was dissolved in the above organic solvent, wherein the content of LiPF 6 was 12.5% of the total weight of the electrolytic solution.
  • an additive is added to the organic solvent, and the mixture is uniformly mixed to obtain an electrolyte.
  • Table 1 the content of the additive is the weight calculated based on the total weight of the electrolyte. Sub-content.
  • the positive electrode sheet, the negative electrode sheet, and the separator (16 ⁇ m thick polypropylene film, model A273, supplied by Celgard) were stacked in this order, so that the separator was in isolation between the positive and negative sheets, and then rolled. After winding the battery core, put the battery core into the package, inject the electrolyte, and then seal. The preparation of the lithium ion secondary battery is completed by a process of standing, hot-cold pressing, chemical formation, exhaust, and test capacity.
  • the lithium ion secondary battery was allowed to stand at 25 ° C for 30 min, then charged to 4.2 V with a constant current of 1 C, then charged at a constant voltage of 4.2 V to a current of ⁇ 0.05 C, and allowed to stand for 5 min, and then kept at a constant current of 1 C.
  • the secondary battery was transferred to a low temperature environment of -20 ° C for more than 2 h to make the internal and external temperature of the lithium ion secondary battery uniform.
  • the current of the lithium ion secondary battery was discharged for 10 s with a current of 0.3 C, and the voltage difference before and after the discharge was recorded. Calculate the DC discharge resistance (DCR) at low temperature of the lithium ion secondary battery.
  • DCR DC discharge resistance
  • the lithium ion secondary battery was allowed to stand at 25 ° C for 30 min, then charged to 4.2 V with a constant current of 1 C, then charged at a constant voltage of 4.2 V to a current of ⁇ 0.05 C, allowed to stand for 5 min, and then stored at 60 ° C. After the day, the reversible capacity retention rate of the lithium ion secondary battery was measured.
  • the lithium ion secondary battery was charged to 4.2V at a constant current of 1C at 25 ° C and 45 ° C, respectively, and then charged at a constant voltage of 4.2 V until the current was 0.05 C, and then discharged with a constant current of 1 C to 2.8 V, which was the first time.
  • the lithium ion secondary battery was cycled several times in accordance with the above conditions until the discharge capacity after the cycle was ⁇ 80% of the first cycle discharge capacity, and the number of cycles of the lithium ion secondary battery was recorded.
  • the reason may be that the oxygen atoms on the two carbonyl groups of the ⁇ -ketoester coordinate with the transition metal ions (Ni, Co, Mn) on the surface of the positive electrode to form a stable protective film on the positive electrode, preventing the positive electrode active material from electrolysis. Oxidation of the liquid. It can be seen from the comparison between Example 7 and Example 8 that further increasing the content of ethyl trifluoropyruvate to 15% does not further reduce the DCR of the lithium ion secondary battery at -20 ° C, and the lithium ion II The cycle performance and storage performance of the secondary battery will not be further improved, probably due to the coordination of transition metal ions on the surface of the positive electrode. Saturated.
  • the content of ethyl trifluoropyruvate should preferably be controlled to 15% or less.
  • methyl trifluoropyruvate is used in combination with vinylene carbonate or fluoroethylene carbonate which is easily formed into a film on the negative electrode, thereby forming protection on the positive and negative surfaces of the lithium ion secondary battery.
  • the membranes, which cooperate with each other, can further reduce the side reaction of the electrode surface, so that the DCR of the lithium ion secondary battery at -20 ° C is further reduced, and the cycle performance and storage performance of the lithium ion secondary battery are further improved.
  • Comparative Example 4 Ethyl acetoacetate (i.e., ⁇ -butyric acid ethyl ester) was added to the electrolyte.
  • the two carbonyl groups were separated by a methylene group, and the structure itself was unstable, and The carbonyl groups are far apart and cannot form a stable coordination with the transition metal ions on the surface of the positive electrode. Therefore, the DCR of the lithium ion secondary battery cannot be lowered at -20 ° C, and the cycle performance and storage performance of the lithium ion secondary battery are also Can't be improved.

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Abstract

一种电解液及二次电池。所述电解液包括：有机溶剂；电解质盐，溶于有机溶剂中；以及添加剂。所述添加剂包括α-酮酸酯化合物。所述电解液包括α-酮酸酯化合物，其能够改善二次电池的循环性能和存储性能，并降低二次电池的内阻。

Description

电解液及二次电池 技术领域
本发明涉及电池技术领域，尤其涉及一种电解液及二次电池。
背景技术
锂离子二次电池具有能量密度高、无记忆效应、工作电压高等优点，因此成为汽车行业应对能源危机和环境压力的首选方案。汽车行业对续航里程的要求不断提高，意味着对锂离子二次电池高能量密度的需求不断提高，因而传统锂离子二次电池面临巨大挑战。
为达到高的能量密度，需要提高锂离子二次电池的工作电压。然而，高的工作电压意味着正极活性材料需要具有更高的电极电位，正极活性材料的氧化性更强，与电解液之间的副反应更多而且反应更剧烈，导致循环后期锂离子二次电池鼓胀，锂离子二次电池循环后的容量保持率下降而存储后的体积膨胀率增加，甚至导致锂离子二次电池失效。
为解决这一问题，目前常用的方法是引入正极添加剂，在正极活性材料表面形成保护层，从而减少正极活性材料与电解液的直接接触，减少正极活性材料与电解液之间的副反应的发生。但正极添加剂的使用通常会带来一些负面的影响，例如导致锂离子二次电池的功率性能的下降等。
发明内容
鉴于背景技术中存在的问题，本发明的目的在于提供一种电解液及二次电池，所述电解液能够改善二次电池的循环性能和存储性能，并降低二次电池的内阻。
为了达到上述目的，在本发明的一方面，本发明提供了一种电解液，其包括：有机溶剂；电解质盐，溶于有机溶剂中；以及添加剂。所述添加剂包括α-酮酸酯化合物。
在本发明的另一方面，本发明提供了一种二次电池，其包括根据本发明 一方面所述的电解液。
相对于现有技术，本发明的有益效果为：
本发明的电解液包括α-酮酸酯化合物，其能够改善二次电池的循环性能和存储性能，并降低二次电池的内阻。
具体实施方式
下面详细说明根据本发明的电解液及二次电池。
首先说明根据本发明第一方面的电解液。
根据本发明第一方面的电解液包括：有机溶剂；电解质盐，溶于有机溶剂中；以及添加剂。所述添加剂包括α-酮酸酯化合物。
在根据本发明第一方面所述的电解液中，α-酮酸酯的两个羰基上氧原子能与正极表面的过渡金属离子配位，在正极表面形成稳定的保护膜，阻止了正极活性材料对电解液的氧化，有效减少正极活性材料与电解液之间的副反应，降低了二次电池的内阻，同时能够改善二次电池的循环性能和存储性能。
在根据本发明第一方面所述的电解液中，所述α-酮酸酯化合物可选自式1所示的化合物中的一种或几种。在式1中，R1、R2各自独立地选自碳原子数为1～10的烷基、碳原子数为2～10的烯基、碳原子数为2～10的炔基、碳原子数为6～10的芳香基、碳原子数为5～10的杂芳基中的一种，其中，烷基、烯基、炔基、芳香基、杂芳基中的H可部分或全部被F、Cl、Br、氰基、羧基、羰基、磺酸基、芳香基、杂芳基中的一种或几种取代。
Figure PCTCN2017093378-appb-000001
在根据本发明第一方面所述的电解液中，所述α-酮酸酯化合物可选自下述化合物中的一种或几种；
Figure PCTCN2017093378-appb-000002
Figure PCTCN2017093378-appb-000003
Figure PCTCN2017093378-appb-000004
Figure PCTCN2017093378-appb-000005
在根据本发明第一方面所述的电解液中，所述α-酮酸酯化合物的含量可为所述电解液的总重量的0.01％～15％。当α-酮酸酯化合物的重量百分含量大于15％时，二次电池的内阻不会进一步降低，且对二次电池的循环性能和存储性能的改善效果不会进一步增加。优选地，所述α-酮酸酯化合物的含量可为所述电解液的总重量的0.1％～10％。进一步优选地，所述α-酮酸酯化合物的含量可为所述电解液的总重量的0.5％～8％。
在根据本发明第一方面所述的电解液中，所述有机溶剂的具体种类可根据实际需求进行选择。优选地，选用非水有机溶剂。所述有机溶剂可包括任意种类的碳酸酯或羧酸酯，例如环状碳酸酯、链状碳酸酯、环状羧酸酯或链状羧酸酯。所述有机溶剂还可包括碳酸酯的卤代化合物。具体地，所述有机溶剂可包括碳酸乙烯酯(EC)、碳酸丙烯酯、碳酸丁烯酯、碳酸戊烯酯、碳酸二甲酯、碳酸二乙酯(DEC)、碳酸二丙酯、碳酸甲乙酯(EMC)、碳酸甲丙酯、碳酸乙丙酯、四氢呋喃、1,4-丁内酯、甲酸甲酯、甲酸乙酯、甲酸丙酯、甲酸丁酯、乙酸乙酯、丙酸甲酯、丙酸乙酯、丙酸丙酯、丁酸甲酯、丁酸乙酯中的一种或几种。
在根据本发明第一方面所述的电解液中，所述电解质盐可选自锂盐、钠盐或锌盐，依据所述电解液应用的二次电池的不同而不同。
在根据本发明第一方面所述的电解液中，所述电解质盐的含量可为所述 电解液的总重量的6.25％～25％。优选地，所述电解质盐的含量可为所述电解液的总重量的6.25％～18.8％。
在根据本发明第一方面所述的电解液中，所述锂盐可选自LiPF6、LiBF4、LiN(SO2F)2(简写为LiFSI)、LiClO4、LiAsF6、LiB(C2O4)2(简写为LiBOB)、LiBF2(C2O4)(简写为LiDFOB)、LiN(SO2RF)2、LiN(SO2F)(SO2RF)中的一种或几种。优选地，所述锂盐可选自LiPF6、LiN(SO2F)2、LiN(CF3SO2)2、LiB(C2O4)2、LiBF2(C2O4)中的一种或几种。进一步优选地，所述锂盐可选自LiPF6、LiN(SO2F)2、LiBF2(C2O4)中的一种或几种。其中，RF表示CnF2n+1，n为1～10内的整数，优选地，n为1～3内的整数。
在根据本发明第一方面所述的电解液中，所述添加剂还可包括碳酸亚乙烯酯(VC)、氟代碳酸乙烯酯(FEC)中的一种或两种。
其次说明根据本发明第二方面的二次电池，其包括根据本发明第一方面所述的电解液。
根据本发明第二方面所述的二次电池可为锂离子二次电池或锂金属二次电池。
根据本发明第二方面所述的二次电池还可包括：正极片、负极片以及隔离膜等。所述正极片包括正极集流体和设置于该正极集流体上的正极活性浆料层，所述正极活性浆料层包括正极活性材料、导电剂以及粘结剂。所述负极片包括负极集流体和设置于该负极集流体上的负极活性浆料层，所述负极活性浆料层包括负极活性材料、粘结剂以及可选的导电剂。
当所述二次电池可为锂离子二次电池或锂金属二次电池时，所述正极活性材料可选自钴酸锂(LiCoO2)、LiNixAyB(1-x-y)O2、LiMPO4、Li1-x’(Qy’Lz’C1-y’-z’)O2中的一种或几种。其中，A、B各自独立地选自Co、Al、Mn中的一种，且A和B不相同，0<x<1、0<y<1且x+y<1。LiMPO4具有橄榄石型结构，M选自Co、Ni、Fe、Mn、V中的一种。Q、L、C各自独立地选自Co、Ni、Fe、Mn中的一种，0<x’<1、0≤y’<1、0≤z’<1且y’+z’<1。
当所述二次电池可为锂离子二次电池或锂金属二次电池时，所述负极活性材料可以选自金属锂。所述负极活性材料也可以选自相对于Li/Li+平衡电位的电极电位＜2V时可以嵌入锂的材料，具体地，所述负极活性材料可选 自天然石墨、人造石墨、中间相微碳球(简称为MCMB)、硬碳、软碳、硅、硅-碳复合物、Li-Sn合金、Li-Sn-O合金、Sn、SnO、SnO2、尖晶石结构的锂化TiO2-Li4Ti5O12、Li-Al合金中的一种或几种。
在本发明所述的锂离子二次电池中，所述隔离膜的种类并没有具体的限制，可选用任何常规隔离膜材料，例如聚乙烯、聚丙烯、聚偏氟乙烯以及上述聚乙烯、聚丙烯、聚偏氟乙烯的多层复合膜，但并不限于上述所举出的材料。
需要说明的是，本发明提供的锂离子二次电池的制备方法在本领域中是公知的，可以按现有的锂离子二次电池制备方法制造本发明所提供的锂离子二次电池。
下面结合具体实施实例，进一步阐述本申请。应说明，这些实施实例仅用于说明本申请但不对本申请的保护范围进行任何限制。在本发明所述实施例、对比例中，所使用到的试剂、材料以及仪器如没有特殊的说明，均可商购获得。
实施例1-13以及对比例1-4中的锂离子二次电池均按照下述方法进行制备。
(1)正极片的制备
将正极活性材料镍钴锰酸锂(LiNi0.8Co0.1Mn0.1O2)、粘结剂聚偏氟乙烯、导电剂导电碳黑按照重量比为98:1:1进行混合，加入到溶剂N-甲基吡咯烷酮(NMP)中，在真空搅拌机作用下搅拌至体系呈现均一稳定状，获得正极浆料；将正极浆料均匀涂覆于厚度为12μm的集流体铝箔上；将铝箔在室温晾干后转移至120℃烘箱干燥1h，然后经过冷压、分切得到正极片。
(2)负极片制备
将负极活性材料硅-碳复合物、导电剂导电碳黑、粘结剂聚丙烯酸酯按照重量比为98:1:1进行混合，加入到溶剂去离子水中，在真空搅拌机作用下搅拌至体系呈现均一稳定状，获得负极浆料；将负极浆料均匀涂覆在厚度为8μm的集流体铜箔上；将铜箔在室温晾干后转移至120℃烘箱干燥1h，然后经过冷压、分切得到负极片。
(3)电解液的制备
在干燥房中，将EC、EMC和DEC混合均匀形成有机溶剂，其中，EC、EMC和DEC的重量比为1:1:1。将充分干燥的锂盐LiPF6溶解于上述有机溶剂中，其中，LiPF6的含量为电解液的总重量的12.5％。然后在有机溶剂中加入添加剂，混合均匀，即获得电解液，其中，添加剂种类及其含量示出在表1中，在表1中，添加剂的含量为基于电解液的总重量计算得到的重量百分含量。
(4)锂离子二次电池的制备
将正极片、负极片以及隔离膜(16μm厚的聚丙烯膜，型号为A273，由Celgard公司提供)按顺序叠好，使隔离膜处于正、负极片之间起到隔离的作用，然后进行卷绕得到电芯，将电芯放入包装壳后，注入电解液，再进行封口。经静置、热冷压、化成、排气、测试容量等工序，完成锂离子二次电池的制备。
表1 实施例1-13以及对比例1-4的电解液参数
Figure PCTCN2017093378-appb-000006
Figure PCTCN2017093378-appb-000007
接下来说明锂离子二次电池的性能测试过程以及测试结果。
(1)锂离子二次电池-20℃直流放电电阻测试：
在25℃下，将锂离子二次电池静置30min，之后以1C恒流充电至4.2V，再在4.2V下恒压充电至电流≤0.05C，并静置5min，然后以1C电流恒流放电至2.8V，记录实际放电容量C0，然后以1C0电流对锂离子二次电池充电30min，调节锂离子二次电池的荷电状态为50％SOC；将调节好荷电状态的锂离子二次电池转移至-20℃低温环境下静置2h以上，使锂离子二次电池的内外部温度一致，最后以0.3C的电流对锂离子二次电池恒流放电10s，记录放电前后电压差，计算得出锂离子二次电池低温下的直流放电电阻(DCR)。
(2)锂离子二次电池的高温存储性能测试
在25℃下，将锂离子二次电池静置30min，之后以1C恒流充电至4.2V，再在4.2V下恒压充电至电流≤0.05C，静置5min，然后在60℃下储存30天后，测定锂离子二次电池的可逆容量保持率。
(3)锂离子二次电池的循环性能测试
分别在25℃和45℃下，将锂离子二次电池以1C恒流充电至4.2V，然后以4.2V恒压充电至电流为0.05C，再用1C恒流放电至2.8V，此为首次循环，按照上述条件将锂离子二次电池进行多次循环，直至循环后的放电容量≤首次循环放电容量的80％，记录锂离子二次电池的循环圈数。
表2 实施例1-13以及对比例1-4的性能测试结果
Figure PCTCN2017093378-appb-000008
Figure PCTCN2017093378-appb-000009
从表2的测试结果可以看出：相比对比例1-3，在实施例1-8制备的锂离子二次电池中，在电解液中加入重量百分含量为0.1％到15％的三氟丙酮酸乙酯后，锂离子二次电池在-20℃下的DCR得到了明显的降低，同时，在25℃和45℃下锂离子二次电池的放电容量衰减至首次循环的放电容量的80％时的循环圈数均明显提高，且锂离子二次电池在60℃下存储30天后的可逆容量保持率也得到了明显提升。其原因可能是由于α-酮酸酯的两个羰基上的氧原子与正极表面的过渡金属离子(Ni、Co、Mn)配位，在正极形成稳定的保护膜，阻止了正极活性材料对电解液的氧化。从实施例7和实施例8的比较中可以看出，进一步增加三氟丙酮酸乙酯的含量到15％，并不能使锂离子二次电池-20℃下的DCR进一步降低，且锂离子二次电池的循环性能和存储性能也不会进一步改善，可能是由于正极表面过渡金属离子的配位已 达到饱和。且当三氟丙酮酸乙酯的含量超过15％时，有可能会影响电解液的粘度等性质，使得锂离子二次电池的其它性能变差。因此三氟丙酮酸乙酯的含量宜控制在15％以下。
在实施例9-11中，加入重量百分含量为5％的三氟丙酮酸甲酯、丙酮酸乙酯、2-丁酮酸甲酯，也能够明显降低锂离子二次电池在-20℃下的DCR，提升锂离子二次电池的循环性能和存储性能。
在实施例12-13中，将三氟丙酮酸甲酯与容易在负极成膜的碳酸亚乙烯酯、氟代碳酸乙烯酯联用，从而在锂离子二次电池的正负极表面都会形成保护膜，二者相互配合能进一步减少电极表面的副反应，使得锂离子二次电池在-20℃下的DCR进一步降低，同时锂离子二次电池的循环性能和存储性能得到进一步提高。
对比例4在电解液中加入乙酰乙酸乙酯(即β-丁酮酸乙酯)，在该结构中，两个羰基中间间隔有一个亚甲基，其本身的结构不稳定，且由于两个羰基相隔较远，不能与正极表面的过渡金属离子形成稳定的配位作用，因此锂离子二次电池在-20℃下的DCR不能得到降低，且锂离子二次电池的循环性能和存储性能也不能得到提高。
根据上述说明书的揭示，本发明所属领域的技术人员还可以对上述实施方式进行适当的变更和修改。因此，本发明并不局限于上述具体实施方式，对本发明的一些修改和变更也应当纳入本发明权利要求的保护范围内。

Claims (10)

  1. 一种电解液，包括：
    有机溶剂；
    电解质盐，溶于有机溶剂中；以及
    添加剂；
    其特征在于，
    所述添加剂包括α-酮酸酯化合物。
  2. 根据权利要求1所述的电解液，其特征在于，所述α-酮酸酯化合物选自式1所示的化合物中的一种或几种；
    Figure PCTCN2017093378-appb-100001
    在式1中，
    R1、R2各自独立地选自碳原子数为1～10的烷基、碳原子数为2～10的烯基、碳原子数为2～10的炔基、碳原子数为6～10的芳香基、碳原子数为5～10的杂芳基中的一种，其中，烷基、烯基、炔基、芳香基、杂芳基中的H可部分或全部被F、Cl、Br、氰基、羧基、羰基、磺酸基、芳香基、杂芳基中的一种或几种取代。
  3. 根据权利要求2所述的电解液，其特征在于，所述α-酮酸酯化合物选自下述化合物中的一种或几种；
    Figure PCTCN2017093378-appb-100002
    Figure PCTCN2017093378-appb-100003
    Figure PCTCN2017093378-appb-100004
  4. 根据权利要求1所述的电解液，其特征在于，所述α-酮酸酯化合物的含量为所述电解液的总重量的0.01％～15％，优选为0.1％～10％，进一步优选为0.5％～8％。
  5. 根据权利要求1所述的电解液，其特征在于，所述电解质盐的含量为所述电解液的总重量的6.25％～25％，优选为6.25％～18.8％。
  6. 根据权利要求1所述的电解液，其特征在于，所述有机溶剂包括碳酸乙烯酯、碳酸丙烯酯、碳酸丁烯酯、碳酸戊烯酯、碳酸二甲酯、碳酸二乙酯、碳酸二丙酯、碳酸甲乙酯、碳酸甲丙酯、碳酸乙丙酯、四氢呋喃、1,4-丁内酯、甲酸甲酯、甲酸乙酯、甲酸丙酯、甲酸丁酯、乙酸乙酯、丙酸甲酯、丙酸乙酯、丙酸丙酯、丁酸甲酯、丁酸乙酯中的一种或几种。
  7. 根据权利要求1所述的电解液，其特征在于，所述电解质盐选自锂盐。
  8. 根据权利要求7所述的电解液，其特征在于，所述锂盐选自LiPF6、LiBF4、LiN(SO2F)2、LiClO4、LiAsF6、LiB(C2O4)2、LiBF2(C2O4)、LiN(SO2RF)2、LiN(SO2F)(SO2RF)中的一种或几种，其中，RF表示CnF2n+1，n为1～10内的整数。
  9. 根据权利要求1所述的电解液，其特征在于，所述添加剂还包括碳酸亚乙烯酯、氟代碳酸乙烯酯中的一种或两种。
  10. 一种二次电池，其特征在于，包括根据权利要求1-9中任一项所述的电解液。
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