Mg metal has long been considered the most promising anode material for the energy storage systems beyond Li-ion batteries. Its merits come from the nearly 2 times higher high volumetric energy density than Li metal, significantly improved safety promise and earth abundance. However, the realization of Mg metal battery has been limited by the lack of proper intercalation cathode materials, due to the sluggish Mg2+ diffusion in most solids. To address this challenge, Associate Professor Dunwei Wang and his team broke the constraint of the solid cathode and demonstrate conversion chemistry of Br2/Br- in solution with fast kinetics and high energy density.