(Invited) Structure and Properties of Electrolyte Solutions Based on Fluorinated Alkylphosphates As Nonflammable Solvents for Lithium-Ion Batteries

Abstract

It has become more important to establish safety and reliability in lithium-ion battery (LIB) technology, especially for developing large-scale and higher energy density systems. Nonflammable organic solvents are considered to be safer components in LIBs [1]. Fluorinated ethers, carbonates and phosphates have been examined as possible candidates that consist of nonflammable electrolyte solutions. Among them, fluorinated alkylphosphates (FAPs) show excellent nonflammability as well as good compatibility with materials used in LIBs [2, 3]. In the present paper, ionic structure and basic characteristics of the electrolyte solutions composed of FAPs dissolving lithium salts are reported.Tris(2,2,2-trifluoroethyl) phosphate (TFEP) and bis(2,2,2-trifluoroethyl)-ethyl phosphate (BTEP) as FAP were used as the nonflammable solvent. Triethylphosphate was also used for comparison. Ionic conductivity and viscosity were measured for the electrolytic solutions dissolving lithium salts with different anions. The Walden’s product (conductivity ´ viscosity) of the electrolytic solution was analyzed to discuss the ionic structure depending on the combination of the solvent and the salt (anion). Although the most of lithium salts dissolve with considerable concentration, eg. ~0.6 mol dm-3 (M) for LiPF6 in TFEP and > 0.6 M for other LiX/FAP combinations, the conductance behavior of the solution suggested that rather strong ionic interactions, or ion-association, will exist over wide salt-concentration range. The ionic structure of each electrolyte system, including the coordination of solvent and anion to Li+ cation (solvation), was further examined by spectroscopic measurements (ATR-IR and Raman). The analyses of the spectroscopic data gave different solvation structures that are strongly dependent on the combination of the solvent and the salt. Thermal analyses of the electrolyte systems consisting of FAP solvents were conducted by DSC. Conventional alkylcarbonate-based electrolyte solution showed highly exothermic behavior above ca. 200 °C under the contact with charged (oxidized) positive electrode, whereas the TFEP-based electrolyte solution behaved stably up to 310 °C and gave smaller quantity of total exothermic heat than alkylcarbonate solution did. The above experimental results proved that FAP-based solvents give safer electrolyte system for larger-scale LIBs. [1] M. Ue, et al., in “Modern Aspects of Electrochemistry, Vol 58; Electrolytes for Lithium and Lithium-ion Batteries” (T. R. Jow, et al., editors), Ch. 2, p.93, Springer (2014). [2] T. R. Jow, et al., J. Electrochem. Soc., 149, A1489 (2002); 150, A161 (2003); 150, A171 (2003).[3] H. Nakagawa, et al., J. Electrochem. Soc., 161, A480 (2014). Figure 1