Electrochemistry (Nov 2020)
Novel Electrochemical Reactions in Molten Salts and Ionic Liquids and Their Applications
Abstract
In this paper, the author describes several new electrochemical reactions in molten salts (MSs) and ionic liquids (ILs), which includes the development of new ILs, and their applications. First, the electrochemical reduction of solid SiO2 to Si in molten CaCl2 at 1123 K is mentioned. The author explains the mechanism how the insulating SiO2 is electrochemically reduced to silicon. A new production method of solar grade silicon (SOG-Si) is introduced as one of the promising applications of the electrochemical reduction of SiO2. The author also mentions another new production method of SOG-Si which utilizes a liquid Si–Zn cathode in molten CaCl2. Secondly, the author describes the electrodeposition of silicon and titanium from water-soluble KF–KCl MSs. Compact crystalline Si films are electrodeposited from molten KF–KCl–K2SiF6 at 923 K. When K2TiF6 and Ti are added in molten KF–KCl, Ti(III) ions are produced by the proportionation reaction between Ti(IV) and Ti(0), in which compact Ti films can be electrodeposited. Thirdly, the author describes the binary and ternary mixtures of M[TFSA] (M = Li, Na, K, Rb, and Cs; TFSA = bis(trifluoromethylsulfonyl)amide) ILs as a new class of electrolytes for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) operating in the intermediate temperature range. Fourthly, the development of binary and ternary mixtures of M[FSA] (M = Li, Na, K, Rb, and Cs; FSA = bis(fluorosulfonyl)amide) ILs and their applications for NIBs are mentioned; the binary and ternary mixtures of M[FSA] are unique in the points that they are inorganic compounds and have low melting temperatures. Finally, the author explains the development of several binary mixtures of M[FSA]–[Ocat][FSA] (M = Li, Na, and K; Ocat = organic cations) ILs which can be used for LIBs, NIBs, and KIBs (K-ion batteries). As a typical application of the M[FSA]–[Ocat][FSA] ILs, the development of practical NIB with a capacity of 27 Ah is described.
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