WO2019196417A1 - 一种锂离子电池电解液 - Google Patents

一种锂离子电池电解液 Download PDF

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WO2019196417A1
WO2019196417A1 PCT/CN2018/117557 CN2018117557W WO2019196417A1 WO 2019196417 A1 WO2019196417 A1 WO 2019196417A1 CN 2018117557 W CN2018117557 W CN 2018117557W WO 2019196417 A1 WO2019196417 A1 WO 2019196417A1
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electrolyte
lithium
ion battery
lithium ion
carbonate
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English (en)
French (fr)
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殷俊
葛晓军
郝敬磊
丁祥欢
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南通新宙邦电子材料有限公司
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Publication of WO2019196417A1 publication Critical patent/WO2019196417A1/zh

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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
    • H01M10/052—Li-accumulators
    • 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
    • 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
    • H01M10/42—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
    • 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
    • H01M10/0568—Liquid materials characterised by the solutes
    • 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
    • H01M10/0569—Liquid materials characterised by the solvents
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0025—Organic electrolyte
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0025—Organic electrolyte
    • H01M2300/0028—Organic electrolyte characterised by the solvent
    • 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 invention belongs to the technical field of lithium ion battery electrolytes, and in particular relates to a lithium ion battery electrolyte containing two flame retardant additives.
  • the lithium ion battery is composed of a positive electrode, a negative electrode, a separator and an electrolyte.
  • the lithium ion battery electrolyte functions to accommodate and conduct ions between the positive and negative electrodes of the battery, and the lithium ion battery obtains high voltage and high specific energy.
  • the lithium ion battery electrolyte is generally prepared from a solvent, a lithium salt, and an additive in a certain ratio. At present, the most mature lithium salt used in the production of commercial lithium-ion batteries is lithium hexafluorophosphate (LiPF 6 ).
  • the solvents are mainly carbonates and small amounts of carboxylates such as ethylene carbonate (EC) and propylene carbonate (PC).
  • DMC Dimethyl carbonate
  • EMC ethyl methyl carbonate
  • DEC diethyl carbonate
  • additives are mainly used to improve the overall performance of the electrolyte, such as: improve the cycle efficiency of the battery, improve the electrode Film formation performance, reducing the internal resistance of the battery, improving the safety of the battery, and the like.
  • the traditional main solvent in the lithium ion battery electrolyte is carbonate
  • the linear carbonate such as DMC, EMC, DEC is a flammable solvent
  • the cyclic carbonate EC and PC are flammable solvents. Therefore, the lithium ion battery electrolyte in which the lithium salt and the conventional additive are dissolved can be burned, and in some cases, even flammable.
  • the technical problem to be solved by the present invention is to provide an improved lithium ion battery electrolyte solution against the problem that the flame retardant electrolyte in the prior art is insufficient in cycle performance.
  • a lithium ion battery electrolyte including a lithium salt, an organic solvent, and an additive, including a phosphazene compound and perfluorohexanone.
  • perfluorohexanone theoretically contributes to the improvement of the flame retardant properties of the electrolyte.
  • the solubility of perfluorohexanone is extremely low, and the concentration allowed is not more than 1%. Under the added amount, the addition of perfluorohexanone has no obvious effect on the flame retardant performance of the electrolyte.
  • the solubility of perfluorohexanone can be improved by adding a surfactant or a solubilizing agent to the electrolyte.
  • the addition of the above surfactant or solubilizing agent is bound to seriously affect the electrochemical performance of the electrolyte.
  • the invention surprisingly finds that the addition of the phosphazene compound and the perfluorohexanone to the electrolyte, with the addition of the phosphazene compound, the solubility of the perfluorohexanone is correspondingly improved, and the synergistic effect of the two, the perfluoro
  • concentration of pentanone which can be used in the electrolyte is far more than 1%, and can even reach 6%, so that perfluorohexanone can fully exert its effect in the electrolyte.
  • the electrolyte after adding 0.5-10% of perfluorohexanone to the electrolyte containing the phosphazene compound, preferably 1-6% of perfluorohexanone, the electrolyte cannot be ignited by an open flame.
  • 1% to 20% of the phosphazene compound is added to the perfluorohexanone-containing electrolyte, it is preferably 5-15%, which can promote the dissolution of perfluorohexanone in the electrolytic solution.
  • the phosphazene compound since the phosphazene compound has a high oxidation potential and flame retardancy, it can help inhibit the oxidative decomposition of the electrolyte on the electrode surface, thereby further improving the high temperature performance and safety performance of the battery.
  • the electrolyte does not suffer from problems such as deterioration in rate performance, decrease in electrical conductivity, and shortened cycle life, etc., and electrical conductivity, wettability, and flame retardancy of the electrolyte are all Significant improvement, so that the electrolyte has better cycle performance and flame retardant safety.
  • the normal temperature and high temperature performance of the battery are significantly improved.
  • the lithium ion battery using the electrolytic solution of the present invention has stable cycle performance at normal temperature and 45 ° C, is not inflated in a 60 ° C hot box, has small internal resistance change, and maintains good storage performance and cycle performance.
  • the lithium ion battery electrolyte provided by the present invention includes a lithium salt, an organic solvent, and an additive, and the additive includes a phosphazene compound and perfluorohexanone.
  • phosphazene compound contains a compound having the structure shown in Formula 1:
  • R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are each independently selected from a fluorine atom, a fluorine-containing or fluorine-free phenoxy group, and a fluorine-containing or fluorine-free carbon atom is 1- Alkoxy groups of 3, and R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 may be the same or different.
  • the phosphazene compound is selected from the group consisting of perfluorocyclotriphosphazene, methoxypentafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene, n-propoxypentafluorocyclotriphosphazene, and different Propoxy pentafluorocyclotriphosphazene, phenoxy (pentafluoro)cyclotriphosphazene, pentafluoroethoxy pentafluorocyclotriphosphazene, trifluoromethoxypentafluorocyclotriphosphazene, trifluoroethoxy One or more of pentafluorocyclotriphosphazene and bis(trifluoromethoxy)tetrafluorocyclotriphosphazene.
  • preferred substituents R 1 , R 2 , R 3 , R 4 , R 5 , R 6 are a fluorine atom or a group of not more than 6 carbon atoms, such as an ethoxy-substituted ethoxy group. Pentafluorocyclotriphosphazene.
  • the flame retardant effect of the phosphazene compound in the electrolyte is related to its amount and also to the volatility of the solvent.
  • volatility (or vapor pressure) of the phosphazene compound is higher than the volatility of the solvent, it is easy to form a high concentration of the flame-retardant air mass, which is advantageous for achieving a good flame retardant effect.
  • the phosphazene compound, especially the cyclic phosphazene compound is not sufficiently volatile due to its structure. Therefore, the flame retardant effect in the electrolyte is still necessary to be improved.
  • the amount of the phosphazene compound which is too low is not sufficient for the improvement of the flame retardant effect.
  • the concentration of less than 1% has made it difficult to observe the phosphazene compound for electrolysis.
  • the excessively high content of the phosphazene compound may bring disadvantages to the solubility of the lithium salt, and the cost is too high.
  • perfluorohexanone i.e., perfluoro-2-methyl-3-pentanone.
  • perfluorohexanone and phosphazene compounds in the electrolyte can effectively improve the volatility of the phosphazene compound, promote the formation of flame-retardant air mass, thereby improving the flame retardancy and reducing the addition of phosphazene compounds. the amount.
  • it can improve the infiltration of the electrolyte and the positive and negative pole pieces, the electrolyte and the separator, and is advantageous for the improvement of the capacity and the cycle performance.
  • the principle is presumed to be related to the adsorption of perfluorohexanone on the surface of the positive or negative electrode, which inhibits the decomposition of the electrolyte on the electrode surface.
  • the lithium ion battery electrolyte provided by the invention has a content of perfluorohexanone ranging from 0.5% to 6%, and when the content is less than 0.5%, the concentration is low, and the perfluorohexanone contributes to the formation of the flame-retardant air mass. Smaller, the partial pressure of steam is lower, and the improvement effect on the flame retardant performance of the electrolyte is not obvious; when it exceeds 6%, the phenomenon that perfluorohexanone cannot be completely dissolved may occur, and delamination and turbidity appear in the electrolyte. , causing poor capacity or cycle performance.
  • a preferred range of content is from 1% to 3%. Within this range, perfluorohexanone has a good solubility stability and can have an effect of improving the flame retarding effect, improving the wettability of the electrolyte, and improving the cycle ability of the battery.
  • the lithium ion battery electrolyte provided by the invention further comprises one or more of vinylene carbonate, 1,3-propane sultone, vinyl sulfate and lithium difluorophosphate.
  • the above various additives may be selectively added according to actual conditions, and the content of the vinylene carbonate is from 0% to 3%, preferably from 0.1% to 3%, based on the total weight of the electrolyte.
  • the content of the 1,3-propane sultone is from 0% to 3%, preferably from 0.1% to 3%.
  • the content of the vinyl sulfate is from 0% to 3%, preferably from 0.1% to 3%.
  • the content of the lithium difluorophosphate is from 0% to 3%, preferably from 0.1% to 3%.
  • an organic solvent may be used.
  • the organic solvent includes one or more of a chain carbonate, a cyclic carbonate, and a carboxylate.
  • the chain carbonate is selected from one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate.
  • the cyclic carbonate is selected from one or more of ethylene carbonate, propylene carbonate, and fluoroethylene carbonate.
  • the carboxylic acid ester is selected from the group consisting of ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, propionic acid formic acid, ethyl propionate, propionic acid propionic acid, isopropyl propionate, butyrate One or more of an ester and ethyl butyrate.
  • a solvent having a lower vapor pressure is preferred, and a vapor pressure of the cyclic carbonate is low, which is advantageous for flame retardancy of the electrolyte, and ethylene carbonate, propylene carbonate, and fluoroethylene carbonate are all Preferred.
  • chain carbonates diethyl carbonate is preferred.
  • carboxylic acid esters n-propyl propionate, isopropyl propionate and ethyl butyrate are preferred.
  • the content of a single solvent is adjustable from 5% to 50% based on the total weight of the electrolyte, and the content of the different solvents may be the same or different.
  • the lithium salt is one or more selected from the group consisting of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorooxalate borate, lithium bis(oxalate)borate, lithium bisfluorosulfonyl amide, and lithium bis(trifluoromethanesulfonyl)imide.
  • the concentration of the lithium salt is from 0.5 M to 2.5 M, preferably from 0.8 M to 1.5 M, based on the total weight of the electrolyte. The most commonly used is 1.0-1.3M lithium hexafluorophosphate.
  • the salt can also be used as an auxiliary lithium salt, the properties of which correspond to additives, in amounts ranging from 0.1 to 5%, more commonly from 0.5 to 3%.
  • Example 2 Under the same conditions as in Example 1, the components of the respective examples specified in Table 1 were blended to obtain the lithium ion battery electrolyte solutions of Examples 2 to 5, respectively.
  • Example 2 Under the same conditions as in Example 1, the components of the respective comparative examples specified in Table 1 were blended to obtain lithium ion battery electrolyte solutions of Comparative Examples 1 to 3, respectively.
  • the lithium battery electrolyte prepared in Examples 1-3 and Comparative Example 1 was respectively taken into a turbidity colorimetric tube with 50 mL of an electrolyte solution, and the turbidity of each electrolyte solution and the delamination of the electrolyte after standing were observed. The degree of turbidity and the stratification are used to determine the compatibility between the solvents in the electrolyte.
  • Test item 2 Flame retardant performance test
  • Example 4 The lithium battery electrolytes prepared in Example 4, Example 5 and Comparative Example 2 were each placed in a watch glass with 50 mL of an electrolyte, and then an open flame was used to ignite the corresponding electrolyte. The flame retardancy of the electrolyte is discriminated by whether the electrolyte can be ignited by an open flame.
  • the electrolytes of Examples 4 and 5 were not ignitable by an open flame, while the electrolyte of Comparative Example 2 was ignitable and ignited (although 5% of perfluorohexanone was added to Comparative Example 2, it actually dissolved in The amount of perfluorohexanone in the electrolyte is very small, the flame retardancy of the electrolyte is poor, and therefore, the electrolyte can be ignited, indicating that the flame retardancy of Example 4 and Example 5 is superior to that of Comparative Example 2, which proves that When the phosphazene additive is mixed with perfluorohexanone, the phosphazene-containing additive can compensate for the flame retardancy of the perfluorohexanone and further improve the flame retardancy of the electrolyte.
  • the lithium battery electrolyte prepared in the above Comparative Example 3 and the lithium ion battery electrolyte prepared in Example 4-5 were respectively injected into a soft pack battery in which the positive electrode was LiNi 0.5 Co 0.2 Mn 0.3 O 2 ternary material and the negative electrode was artificial graphite.
  • the battery is tested at a rated capacity of 1000 mAh.
  • Test item 3 Normal temperature cycle performance test
  • the battery was placed in an incubator at a constant temperature of 25 ° C, and charged at a constant current and constant voltage of 1 C to 4.4 V, an off current of 0.03 C, and then discharged at a constant current of 1 C to 3.0 V.
  • the cycle was repeated for 300 weeks, and the discharge capacity at the first week and the discharge capacity at the 300th week were recorded, and the capacity retention ratio was calculated by the following formula.
  • Capacity retention rate (%) (300th cycle discharge capacity / 1st cycle discharge capacity) ⁇ 100%
  • Test item 4 High temperature cycle performance test
  • Test Item 3 The test conditions were the same as Test Item 3 except that the oven temperature was 45 °C. The test results are shown in Table 4.
  • the lithium ion battery electrolyte of Example 4-5 When the prepared lithium ion battery is circulated under normal temperature conditions (25 ° C), the capacity decay rate is obviously slowed down, the battery capacity retention rate is significantly improved, and it is obviously superior to the comparative example 3, which proves that the phosphazene compound is mixed with the perfluorohexanone. The cycle performance is significantly better than that of the phosphazene compound additive alone, and the capacity retention rate of the battery is significantly improved.
  • Test Item 5 High Temperature Storage Test
  • Example 4 The fully charged lithium ion batteries of Example 4, Example 5 and Comparative Example 3 were placed in an oven at 60 ° C for 72 h to test the capacity, internal resistance and thickness variation of the battery.

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Abstract

为克服现有技术中电解液难以兼顾阻燃性能和循环等电化学性能的问题，提供了一种锂离子电池电解液，包括锂盐、有机溶剂和添加剂，所述添加剂包括磷腈化合物和全氟己酮。该锂离子电池电解液具有良好的循环性能和阻燃安全性，使用它的锂离子电池在常温和45℃下循环性能稳定，在60℃热箱中贮存不气胀，内阻变化小，并保持良好的存储性能和循环性能。

Description

一种锂离子电池电解液 技术领域
本发明属于锂离子电池电解液的技术领域，具体涉及一种包含两种阻燃性添加剂的锂离子电池电解液。
背景技术
锂离子电池由正极、负极、隔膜和电解液等部分组成，其中锂离子电池电解液在电池的正负极之间起到容纳和传导离子的作用，是锂离子电池获得高电压、高比能等优点的保证。锂离子电池电解液一般由溶剂、锂盐、添加剂按照一定的比例配制而成。目前商品化锂离子电池生产中运用最成熟的锂盐是六氟磷酸锂(LiPF 6)，溶剂主要是碳酸酯类和少量羧酸酯类，如：碳酸乙烯酯(EC)、碳酸丙烯酯(PC)、碳酸二甲酯(DMC)、碳酸甲乙酯(EMC)、碳酸二乙酯(DEC)等，而少量的添加剂主要是用来提升电解液的整体性能，例如：提高电池的循环效率、改善电极的成膜性能、降低电池的内阻、提高电池的安全性等。
为了满足锂离子电池的日益提高的性能要求，添加剂目前是必不可少的重要组成成分。特别是针对阻燃电解液，由于锂离子电池电解液中，传统的主要溶剂都是碳酸酯类，线性碳酸酯如DMC、EMC、DEC属于易燃溶剂，环状碳酸酯EC、PC属于可燃溶剂，因此溶解了锂盐和常规添加剂的锂离子电池电解液是可以燃烧的，有些情况下甚至是易燃的。为了实现电解液的阻燃，在使用碳酸酯类作为电解液主要溶剂的情况下，必须依赖加入一定含量的阻燃剂，才能实现电解液的阻燃化，这对于提高锂离子电池的安全性能至关重要。但是，目前通常采用的阻燃剂必须在较高的添加量下才能实现良好的阻燃效果，比如含磷的酯类磷酸三甲酯(TMP)、磷酸三乙酯(TEP)、磷酸三苯酯(TPP)、甲基膦酸二甲酯(DMMP)、乙基膦酸二乙酯(DEEP)等、以及同时含磷、氮元素的阻燃剂磷腈等。含磷的酯类主要缺陷是在负极上面容易分解，电池循环性能差，因此在产业界没有得到广泛的应用。
发明内容
本发明所要解决的技术问题是针对现有技术中的阻燃电解液在循环性能上不足的问题，提供一种改善的锂离子电池电解液。
本发明解决上述技术问题所采用的技术方案如下：
提供一种锂离子电池电解液，包括锂盐、有机溶剂和添加剂，所述添加剂包括磷腈化合物和全氟己酮。
发明人发现，全氟己酮理论上对提高电解液的阻燃性能有一定的帮助。但是，通过实验发现，在以碳酸酯类为主要溶剂的电解液中，全氟己酮的溶解度非常低，允许使用的浓度不超过1％。而在此添加量下，全氟己酮的加入对电解液的阻燃性能改善效果并不明显。理论上可通过向电解液中添加表面活性剂或增溶剂等提高全氟己酮的溶解度，但是，上述表面活性剂或增溶剂的加入势必将严重影响电解液的电化学性能。
本发明向电解液中同时添加磷腈化合物和全氟己酮后惊讶的发现，随着磷腈化合物的加入，全氟己酮的溶解度得到了相应的提高，通过二者的协同作用，全氟己酮在电解液中可以使用的浓度远远超过1％，甚至可以达到6％，从而可以使全氟己酮能在电解液中充分发挥其效果。详而言之，在含磷腈化合物的电解液中加入0.5-10％的全氟己酮，优选加入1-6％的全氟己酮后，该电解液无法用明火点燃。在含全氟己酮的电解液中加入1％-20％的磷腈类化合物时，优选5-15％，可以促进全氟己酮在电解液中的溶解。另一方面，由于磷腈化合物有较高的氧化电位和阻燃性，可以帮助抑制电解液在电极表面氧化分解，从而进一步改善电池的高温性能和安全性能。
并且，通过磷腈化合物和全氟己酮的配合使用，电解液也不会出现倍率性能变差、电导率下降以及循环寿命缩短等问题，电解液的电导率、浸润性及阻燃性都有明显的提高，从而使得该电解液具有更好的循环性能和阻燃安全性。该电解液注入电池后，电池的常温和高温性能都得到明显的提高。
使用本发明的电解液的锂离子电池，在常温和45℃下循环性能稳定，在60℃热箱中贮存不气胀，内阻变化小，并保持良好的存储性能和循环性能。
具体实施方式
为了使本发明所解决的技术问题、技术方案及有益效果更加清楚明白，以下结合实施例，对本发明进行进一步详细说明。应当理解，此处所描述的具体实施例仅仅用以解释本发明，并不用于限定本发明。
本发明提供的锂离子电池电解液包括锂盐、有机溶剂和添加剂，所述添加剂包括磷腈化合物和全氟己酮。
其中，磷腈化合物为现有技术中已知的，例如，所述磷腈化合物包含如式1所示结构的化合物：
式1：
Figure PCTCN2018117557-appb-000001
其中R 1、R 2、R 3、R 4、R 5、R 6各自独立的选自氟原子、含氟或不含氟的苯氧基、含氟或不含氟的碳原子数为1-3的烷氧基，且R 1、R 2、R 3、R 4、R 5、R 6可以相同或不同。
优选情况下，所述磷腈化合物选自全氟环三磷腈、甲氧基五氟环三磷腈、乙氧基五氟环三磷腈、正丙氧基五氟环三磷腈、异丙氧基五氟环三磷腈、苯氧基(五氟)环三磷腈、五氟乙氧基五氟环三磷腈、三氟甲氧基五氟环三磷腈、三氟乙氧基五氟环三磷腈、双(三氟甲氧基)四氟环三磷腈中的一种或多种。
当磷腈化合物上的取代基体积随取代基的碳原子数上升而变大之后，磷腈化合物的粘度和沸点都随之上升，会给电解液的导电性带来不良影响，其阻燃效果也会因为挥发性的下降而受到损害。对于上述磷腈化合物，优选的取代基R 1、R 2、R 3、R 4、R 5、R 6是氟原子或不超过6个碳原子的基团，如乙氧基取代的乙氧基五氟环三磷腈。
磷腈化合物在电解液中的阻燃效果与其用量有关，同时也和溶剂的挥发性相关。当磷腈化合物的挥发性(或称蒸汽压)高于溶剂的挥发性时，容易形成高浓度的阻燃性气团，有利于实现较好的阻燃效果。但磷腈化合物特别是环状磷腈化合物，由于自身结构原因，其挥发性仍不够理想。因此在电解液中其阻燃效果仍有改善的必要，过低的磷腈化合物用量对于阻燃效果的改善也不充分，一般而言，浓度低于1％已经难以观察到磷腈化合物对于电解液的阻燃性的改善效果。因此，其用量范围在1％-20％，优选5-15％。但是过高的磷腈化合物含量又会带来对锂盐的溶解性不利，成本过高等优点。
本发明人找到了一种新型添加剂全氟己酮(即全氟-2-甲基-3-戊酮)。实验证 明，在电解液同时使用全氟己酮和磷腈化合物，可以有效改善磷腈化合物的挥发性问题，促进阻燃性气团的形成，从而提高阻燃能力，并且可降低磷腈化合物的添加量。同时，可以改善电解液与正负极极片、电解液与隔膜的浸润，有利于容量和循环性能的提升。其原理推测可能与全氟己酮在正极或负极表面的吸附抑制了电解液在电极表面的分解有关。
本发明提供的锂离子电池电解液中，全氟己酮的含量范围为0.5％-6％，当含量为0.5％以下时，浓度较低，全氟己酮在阻燃性气团的形成中贡献较小，其蒸汽分压较低，对电解液阻燃性能的改善效果不明显；而当超过6％时，可能会产生全氟己酮不能完全溶解的现象，电解液中出现分层和混浊，从而引发容量或循环性能不良。优选的含量范围是1％-3％。在此范围内，全氟己酮具有较好的溶解稳定性并且能够起到提高阻燃效果、改善电解液的浸润能力以及提高电池循环能力的效果。
为提高电解液的综合性能，本发明提供的锂离子电池电解液中还包括碳酸亚乙烯酯、1,3-丙烷磺酸内酯、硫酸乙烯酯、二氟磷酸锂中的一种或多种。上述各种添加剂可根据实际情况选择性添加，所述电解液中，以电解液总重量为基准，所述碳酸亚乙烯酯的含量为0％-3％，优选为0.1％-3％。所述1,3-丙烷磺酸内酯的含量为0％-3％，优选为0.1％-3％。所述硫酸乙烯酯的含量为0％-3％，优选为0.1％-3％。所述二氟磷酸锂的含量为0％-3％，优选为0.1％-3％。
本发明中，有机溶剂可以采用现有的，优选情况下，所述有机溶剂包括链状碳酸酯、环状碳酸酯、羧酸酯中的一种或多种。
所述链状碳酸酯选自碳酸二甲酯、碳酸二乙酯、碳酸甲乙酯、碳酸甲丙酯中的一种或多种。
所述环状碳酸酯选自碳酸乙烯酯、碳酸丙烯酯、氟代碳酸乙烯酯中的一种或多种。
所述羧酸酯选自乙酸乙酯、乙酸正丙酯、乙酸异丙酯、乙酸正丁酯、丙酸甲酸、丙酸乙酯、丙酸正丙酸、丙酸异丙酯、丁酸甲酯、丁酸乙酯中的一种或多种。
为了保证较好的阻燃效果，蒸汽压较低的溶剂是优选的，环状碳酸酯的蒸汽压低，有利于电解液的阻燃，碳酸乙烯酯、碳酸丙烯酯、氟代碳酸乙烯酯都是优选的。链状碳酸酯中，碳酸二乙酯是优选的。羧酸酯中，丙酸正丙酯、丙酸异丙酯、丁酸乙酯是优选的。以电解液总重量为基准，单个溶剂其含量从 5％-50％范围内可调，不同溶剂之间其含量可以相同或不相同。
所述锂盐选自六氟磷酸锂、四氟硼酸锂、二氟草酸硼酸锂、双草酸硼酸锂、双氟磺氟酰亚胺锂、双(三氟甲磺酰)亚胺锂中的一种或多种；所述电解液中，以电解液总重量为基准，所述锂盐的浓度为0.5M-2.5M，优选为0.8M-1.5M。最为常用的是1.0-1.3M的六氟磷酸锂。当然，所述盐也可以作为辅助性的锂盐使用，其性质相当于添加剂，其用量在0.1-5％范围内，比较常见的用量是0.5-3％。
以下通过实施例对本发明进行进一步的说明。
表1
Figure PCTCN2018117557-appb-000002
实施例1
在氮气保护的手套箱内(水分<1ppm，氧分<1ppm)，将碳酸甲乙酯(EMC)、碳酸乙烯酯(EC)、碳酸二乙酯(DEC)等按照表格1指定的质量比进行混合，加入质量百分比为12.5％的六氟磷酸锂(LiPF 6)，再加入表格1规定品种和含量的其它添加剂和阻燃添加剂，搅拌均匀后得到实施例1的锂离子电池电解液。
实施例2～5
在实施例1相同的条件下，按表格1规定的各实施例的成分进行调配，分别得到实施例2～5的锂离子电池电解液。
对比例1～3
在实施例1相同的条件下，按表格1规定的各对比例的成分进行调配，分别得到对比例1～3的锂离子电池电解液。
测试项目1：溶解性测试
将实施例1-3和对比例1制备的锂电池电解液分别取50mL的电解液于浊度比色管中，观察各自电解液的浊度和电解液静置后是否出现分层情况，通过浊度的大小以及分层情况，来判断电解液中溶剂间的相溶性。
测试结果如表2所示。
表2
Figure PCTCN2018117557-appb-000003
由表2的数据可得，实施例1-3的电解液的浊度均小于对比例1，并且随着加入的乙氧基五氟环三膦腈的含量增多，电解液的浊度在逐步减小，电解液长时间静置后溶剂间的分层现象也逐步不明显，进一步表明磷腈类添加剂能够促进全氟己酮在碳酸酯溶剂中的溶解性，使得全氟己酮在电解液中的溶解更加均匀。虽然电解液中加入了6％的全氟己酮，但是，大部分并未实际溶解于电解液中。
测试项目2：阻燃性能测试
将实施例4、实施例5和对比例2制备的锂电池电解液分别取50mL的电解液置于表面皿中，然后用明火去点燃相应的电解液。通过电解液是否能被明火点燃 冒烟，来判别电解液的阻燃性。
测试结果如表3所示。
表3
Figure PCTCN2018117557-appb-000004
实施例4和实施例5的电解液是无法用明火点燃的，而对比例2的电解液是可以点燃着火的(虽然对比例2中添加了5％的全氟己酮，但事实上溶解于电解液中的全氟己酮量非常少，电解液阻燃性能差，因此，电解液可以被点燃)，说明实施例4和实施例5的阻燃性能优于对比例2，这证明了含磷腈类的添加剂与全氟己酮混合使用时，含磷腈类添加剂可以弥补全氟己酮阻燃性的不足，进一步提高电解液的阻燃性能。
电池性能测试
将上述对比例3制备的锂电池电解液和实施例4-5制备的锂离子电池电解液分别注入正极为LiNi 0.5Co 0.2Mn 0.3O 2三元材料，负极为人造石墨的软包电池中，电池额定容量为1000mAh，对电池进行测试。
测试项目3：常温循环性能测试
将电池置于恒温25℃的恒温箱中，以1C的电流恒流恒压充电至4.4V，截止电流为0.03C，然后以1C的电流恒流放电至3.0V。如此循环300周，记录第1周的放电容量和第300周的放电容量，按下式计算容量保持率。
容量保持率(％)＝(第300次循环放电容量/第1次循环放电容量)×100％
测试结果如表4所示。
测试项目4：高温循环性能测试
测试条件除恒温箱温度为45℃外，其它与测试项目3相同。测试结果见表4。
表4
Figure PCTCN2018117557-appb-000005
由表4的数据可以看出，在以镍钴锰Ni xCo yMn (1-x-y)三元材料作为正极，石墨作为负极的锂离子电池中，实施例4-5的锂离子电池电解液制备的锂离子电池在常温条件(25℃)循环时，其容量衰减速度明显变缓，电池容量保持率显著提升，且明显优于对比例3，证明磷腈化合物与全氟己酮一起混合使用时循环性能明显优于单独使用磷腈化合物添加剂的效果，电池的容量保持率得到显著提升。
从测试数据可以看出：在高温(45℃)循环后，无论是对比例还是实施例，其电池的容量保持率都高于常温循环时的数据，这可能与温度升高时电解液的电导率上升，浸润能力提升，导致的循环性能变好有关。实施例4和实施例5的容量保持率仍高于对比例3，进一步表明磷腈化合物和全氟己酮的协同使用有利于进一步提升电池的高温循环性能。
测试项目5：高温存储测试
将实施例4、实施例5和对比例3的满电态的锂离子电池置于60℃的烘箱中存储72h，测试电池的容量、内阻及厚度变化。
测试结果如表5所示。
表5
  容量保持率 厚度增加率 内阻增加率
实施例4 97.4％ 0.09％ 24％
实施例5 95.9％ 0.24％ 26％
对比例3 93.7％ 0.4％ 32％
由表5的数据可得，经过60℃高温存储72h后，使用实施例4和实施例5 的电解液制备的锂离子电池在高温存储后，容量保持率均优于对比例3，厚度增加率和内阻增加率均小于对比例3。这进一步表明磷腈化合物与全氟己酮的协同使用有利于电池容量的存储性能。
以上所述仅为本发明的较佳实施例而已，并不用以限制本发明，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明的保护范围之内。

Claims (9)

  1. 一种锂离子电池电解液，其特征在于，包括锂盐、有机溶剂和添加剂，所述添加剂包括磷腈化合物和全氟己酮。
  2. 根据权利要求1所述的锂离子电池电解液，其特征在于，所述磷腈化合物包含如式1所示结构的化合物：
    Figure PCTCN2018117557-appb-100001
    其中R 1、R 2、R 3、R 4、R 5、R 6各自独立的选自氟原子、含氟或不含氟的苯氧基、含氟或不含氟的碳原子数为1-3的烷氧基，且R 1、R 2、R 3、R 4、R 5、R 6可以相同或不同。
  3. 根据权利要求2所述的锂离子电池电解液，其特征在于，所述磷腈化合物选自全氟环三磷腈、甲氧基五氟环三磷腈、乙氧基五氟环三磷腈、正丙氧基五氟环三磷腈、异丙氧基五氟环三磷腈、苯氧基(五氟)环三磷腈、五氟乙氧基五氟环三磷腈、三氟甲氧基五氟环三磷腈、三氟乙氧基五氟环三磷腈、双(三氟甲氧基)四氟环三磷腈中的一种或多种。
  4. 根据权利要求1-3中任意一项所述的锂离子电池电解液，其特征在于，所述电解液中，以电解液总重量为基准，所述磷腈化合物的含量为1％-20％；所述全氟己酮的含量为0.5％-10％。
  5. 根据权利要求4所述的锂离子电池电解液，其特征在于，所述电解液中，以电解液总重量为基准，所述磷腈化合物的含量为5％-15％；所述全氟己酮的含量为1％-6％。
  6. 根据权利要求1-3、5中任意一项所述的锂离子电池电解液，其特征在于， 所述添加剂还包括碳酸亚乙烯酯、1,3-丙烷磺酸内酯、硫酸乙烯酯、二氟磷酸锂中的一种或多种。
  7. 根据权利要求6所述的锂离子电池电解液，其特征在于，所述电解液中，以电解液总重量为基准，所述碳酸亚乙烯酯的含量为0％-3％；所述1,3-丙烷磺酸内酯的含量为0％-3％；所述硫酸乙烯酯的含量为0％-3％；所述二氟磷酸锂的含量为0％-3％。
  8. 根据权利要求1所述的锂离子电池电解液，其特征在于，所述有机溶剂包括链状碳酸酯、环状碳酸酯、羧酸酯中的一种或多种；
    所述链状碳酸酯选自碳酸二甲酯、碳酸二乙酯、碳酸甲乙酯、碳酸甲丙酯中的一种或多种；
    所述环状碳酸酯选自碳酸乙烯酯、碳酸丙烯酯、氟代碳酸乙烯酯中的一种或多种；
    所述羧酸酯选自乙酸乙酯、乙酸正丙酯、乙酸异丙酯、乙酸正丁酯、丙酸甲酸、丙酸乙酯、丙酸正丙酸、丙酸异丙酯、丁酸甲酯、丁酸乙酯中的一种或多种。
  9. 根据权利要求1所述的锂离子电池电解液，其特征在于，所述锂盐选自六氟磷酸锂、四氟硼酸锂、二氟草酸硼酸锂、双草酸硼酸锂、双氟磺氟酰亚胺锂、双(三氟甲磺酰)亚胺锂中的一种或多种；所述电解液中，以电解液总重量为基准，所述锂盐的浓度为0.5M-2.5M，优选为0.8M-1.5M。
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