High-rate aqueous magnesium ion battery enabled by Li/Mg hybrid superconcentrated electrolyte

Rechargeable aqueous magnesium ion batteries (AMIBs) are considered a promising energy storage system due to the relatively high energy density, excellent rate performance and reversibility, and absence of dendrite formation during cycling. However, the high surface charge density of Mg²⁺ ions results in slow diffusion kinetics, and high voltage leads to excessive electrolyte decomposition at the electrode/aqueous electrolyte interface, thereby limiting the development of high-energy–density MIBs. In this work, we extend the electrochemical stability window of the aqueous electrolyte to 2.1 V by using a hybrid ion superconcentrated electrolyte at concentrations not achievable with Mg²⁺ electrolytes alone. The AMIB with a water-in-salt (WIS) electrolyte delivers discharge capacities of up to 200.6 and 84.4 mAh g⁻¹, and energy densities of 170.1 and 68.1 Wh Kg⁻¹ at 0.5 and 20 A g⁻¹, respectively, representing excellent rate performance, while retaining 85 % of the specific capacity after 2000 cycles. Additionally, we explore the corresponding electrochemical reaction mechanism, revealing the synergistic effect of Mg²⁺ and Li⁺ ions in the charge/discharge process, which supports the excellent electrochemical performance of AMIB.