Electrochemical Deposition of Alkali Metal in Low-Melting Alkali Metal Perfluorosulfonylamides

Abstract

Alkali metal molten salts have been studied as electrolytes for various application, such as battery[1] due to their non-flammability, high thermal resistance and high specific conductivity. However, the conventional molten salt, such as alkali halides, needs high working temperature above 300 ºC due to their high melting point (T m) (ex. eutectic LiCl-KCl: 352 ºC). Therefore, its application was limited to specific fields. Low-melting molten salts, such as nitrate and chlorate, have oxidative reactivity causes an exothermic reaction. Low melting point and chemical stability are significant problems for molten salt.Recently, we have studied about low-melting alkali metal molten salt as an electrolyte of lithium secondary battery. We reported that a lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide, denoted as Li[fTfN], can be used as a molten salt electrolyte for the lithium secondary battery without dissolving it in organic solvents due to its specifically low melting point (100 ºC) and wide electrochemical window (5.0 V).[2] Melting points of the other alkali metal fTfN salts are also around 100 ºC, and those of their mixtures are near room temperature. In order to apply the novel low-melting molten salts, it is significant to investigate their physicochemical and electrochemical properties. In this study, the electrochemical deposition of alkali metals was measured in the single fTfN salts and their mixtures in order to investigate their cathodic stability.The alkali metal fTfN salts were prepared as our previous study.[3] Then, depositions of alkali metal in these molten salts, which correspond to their cathode limits, were investigated cyclic voltammetry (CV). A two-electrode electrochemical cell for CV, which contents the molten salt impregnated in a glass separator, each electrode and a stainless-steel holder, were constructed in an argon filled glove box in order to prevent absorption of moisture and oxygen. For a cyclic voltammogram of the single Li[fTfN], a pair of reduction and oxidation currents on the nickel electrode was observed at 0 V vs. Li/Li+. These reactions were interpreted as the deposition and redissolution of lithium metal because the Li[fTfN] contents only Li+ and fTfN–. On the other hand, another low reduction and oxidation peaks were also observed at +1.5 and +2.0 V vs. Li/Li+, respectively. These peaks may be attributed to the under-potential deposition of lithium on the nickel electrode.[3] The same lithium metal deposition was observed in Li[fTfN]-K[fTfN] and Li[fTfN]-Cs[fTfN] mixture. Thus, the order of deposition is Li > (K, Cs), which corresponds to high-melting alkali halides.[4] The results of the other alkali metal fTfN salts will be presented.