3Mg/Mg2Sn anodes with unprecedented electrochemical performance towards viable magnesium-ion batteries

Abstract

The development of high-performance Mg-alloyable anodes operable in conventional electrolytes could be a critical breakthrough for the viability of magnesium-ion batteries (MIBs). Herein, we show that 3Mg/Mg₂Sn can be used as an anode with unprecedented high performance in conventional electrolytes. The as-made 3Mg/Mg₂Sn consists of crystalline Mg-rich (c-Mg), amorphous Mg-rich (a-Mg), and intermetallic Mg₂Sn phases. During the 1^st de-magnesiation of 3Mg/Mg₂Sn in Mg(HMDS)₂ : MgCl₂ (HMDS = hexamethyldisilazide), c-Mg is first dealloyed. After the complete dissolution of c-Mg, a-Mg and intermetallic Mg₂Sn are then de-magnesiated at slightly higher voltages, resulting in a capacity of 1243 mA h g⁻¹ (3.9 Mg²⁺) at 100 mA g⁻¹. Subsequent charge/discharge reversibly delivers ca. 805 mA h g⁻¹ (2.5 Mg²⁺) with no re-formation of c-Mg, which indicates that the reversibility comes mostly from Mg₂Sn (Mg₂Sn ⇆ Sn) with an additional contribution from a-Mg (0.5 Mg²⁺). Though not involved in the reversible process, the dissolution of c-Mg generates micro-to-macropores, which contribute to the excellent rate-capability and cyclability of 3Mg/Mg₂Sn. The Mg²⁺ ions, irreversibly released from 3Mg/Mg₂Sn, also compensate for a decrease in the Mg²⁺ concentration, which can occur when a full-cell is constructed with a Mg²⁺-trapping Mo₆S₈ cathode. Furthermore, once dealloyed in Mg(HMDS)₂ : MgCl₂, the 3Mg/Mg₂Sn becomes compatible with various conventional electrolytes.