Magnesium-sulfur batteries are one of the most promising next-generation battery systems due to their high energy density, low cost, and high level of safety. However, the reaction mechanisms are not well understood, and in particular, the discharge reaction products have not yet been identified. Here we show that zinc blende magnesium sulfide is observed as a reaction product after discharging in magnesium-sulfur batteries. When magnesium reacts electrochemically with sulfur in a sulfone-based magnesium electrolyte, sulfur becomes amorphous consisting of magnesium and sulfur in the cathode. In this study, it has been found that the amorphous material has an unusual local structure, which is not related to the most stable rock salt phase of magnesium sulfide but rather the metastable zinc blende phase. It was indicated that this material realizes the reversibility of magnesium-sulfur batteries.

Two boron-based additives, tris(pentafluorophenyl)borane (TPFPB) and tris(1H,1H-heptafluorobutyl)borate (THFBuBO), are studied for their effects on the cycling performance of the FeF 3 /Li battery. When dissolved in the electrolyte consisting of LiPF 6 -ethylene carbonate/1,2-dimethyl carbonate, these compounds act as F − receptors to improve the electrochemical properties of the FeF 3 cathode and cycling performance. The initial discharge capacity and voltage profile of FeF 3 /Li battery displayed little change with the additives. However, TPFPB and THFBuBO improved the capacity retention, with respective retention rates of 85.2% and 86.5% at the 10th cycle, while this value was 68.6% when no additive was used. Inductively coupled plasma atomic emission spectroscopy analysis showed that LiF was dissociated by TPFPB or THFBuBO in the electrolyte, and its solubility was higher with TPFPB. THFBuBO has the weak affinity for F − ions to allow reversible formation/dissociation of the [THFBuBO-F] − complex. Therefore, THFBuBO maintained the capacity of the FeF 3 /Li battery better than TPFPB.The lithium ion battery (LIB), with its superior cycling performance and moderately high energy density, has been widely used for mobile devices and greatly contributed to their prevalence. 1,2 The currently available commercial LIBs are based on the intercalation/deintercalation reactions of Li + ions with the cathode material (e.g. LiCoO 2 and LiFePO 4 ), and the anode material (e.g. graphite). During the reversible intercalation/deintercalation reaction, the volume change in the crystal lattice of cathode materials is quite small. This structural stability contributes to the extremely high cycle stability, high rate capability, and long lifetime of LIB. However, the number of the Li + ions that can be inserted per site is intrinsically limited by the electrode material. Meanwhile, impending larger devices such as electric vehicles, drones, wearing-type assist-robots require ever-higher energy density. Therefore, cathode active materials with larger capacities are being developed to increase the energy density of LIB. Cathode materials based on the conversion reactions have been proposed, 3,4 which possess theoretical capacities higher than that of intercalation-type materials. Sulfur 5 (theoretical capacity 1675 mAh g −1 ) and metal halides 6-8 (MF x and MCl y , M = Bi, Co, Cu, Fe, Ni, etc.) have been researched extensively for this purpose.Metal fluoride is one of the more promising candidates, having high energy density 3 and high stability. Iron trifluoride (FeF 3 ) has attracted considerable attention as an active material, because of its low cost, non-toxicity, and a high theoretical capacity (712 mAh g −1 for threeelectron Fe 3+ →Fe 0 reaction). 6 The electrochemical performance of FeF 3 in the cathode of LIB was first reported by Arai and co-workers. 9 The discharge reaction of FeF 3 in LIB can be divided into two steps: Insertion : FeF 3 + Li + + e − → LiFeF 3[1]Conversion : LiFeF 3 + 2Li + + 2e − → Fe 0 + 3LiFReaction 1...

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