A Nonflammable Electrolyte Combining Phosphate and Fluorinated Ether for Li 4 Ti 5 O 12 /LiNi 0.5 Mn 1.5 O 4 Cells
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A Nonflammable Electrolyte Combining Phosphate and Fluorinated Ether for Li4Ti5O12/LiNi0.5Mn1.5O4 Cells Hao Zheng, Wei Fang, Yi Sun, Xin Liang and Hongfa Xiang*, School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009 Anhui, People’s Republic of China Lihua Jiang and Qingsong Wang, State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, People’s Republic of China Received: 30 August 2019/Accepted: 10 December 2019
Abstract. Nonflammable electrolytes are promising substitutes for the state-of-theart carbonate-based electrolytes in order to directly enhance the safety characteristics of lithium ion batteries. Combination of trimethyl phosphate (TMP) and a fluorinated ether of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (FEPE) is designed to formulate nonflammable electrolytes. The reformulated electrolytes of 1 mol/L LiPF6/TMP + FEPE (9:1, 8:2 and 7:3, w/w) are totally nonflammable. The FEPE solvent has higher oxidative stability and the FEPE-containing electrolytes have better separator wettability than the pure TMP electrolyte (1 mol/L LiPF6/ TMP). The improved oxidative stability and separator wettability can enhance electrochemical stability of electrolyte on high-voltage cathode LiNi0.5Mn1.5O4. In Li4Ti5O12/LiNi0.5Mn1.5O4 cells, the FEPE-containing electrolyte exhibits the better cycling stability than the pure TMP electrolyte, and the solvent composed of TMP + FEPE (8:2) is the optimal ratio. The Li4Ti5O12/LiNi0.5Mn1.5O4 cell with the optimal electrolyte exhibits better rate capability because of the reduced polarization and improved oxidation stability. This work reveals the effect of fluorinated ethers on separator wettability of TMP-based nonflammable electrolytes and high-voltage applications. Keywords: Lithium-ion batteries, Nonflammable electrolyte, Separator wettability, High voltage, Safety
1. Introduction Lithium-ion batteries as satisfying energy storage technology have been widely used in many fields including electronic devices and electric vehicles. However, lithium-ion batteries with high energy density face safety concern because they may encounter thermal runaway under abusive conditions and even bring fire * Correspondence should be addressed to: Hongfa Xiang, E-mail: [email protected]; Qingsong Wang, E-mail: [email protected]
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Fire Technology 2019 accidents [1]. The reasons for thermal runaway of lithium-ion batteries have been systematically investigated [2, 3]. Among these reasons, electrolyte has been considered to be the key factor, because it is involved in many key exothermic reactions and especially it is so flammable in nature that fire hazards could be directly triggered [4, 5]. In order to enhane the safety characteristics of lithium-ion batteries, replacing the state-of-the-art flammable electrolytes with flame-retarded or even nonflammable electrolytes is the most effective and promising solution [6]. The flame-retarded electrolytes with a limited amount of flame retardants, e.g., < 10%, are still easily
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