Amorphous germanium-crystalline bismuth films as a promising anode for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are promising alternatives to lithium-ion batteries due to their safety and high theoretical specific capacity, and the abundance of magnesium reserves. However, their anodes and electrolytes severely restrict the development of MIBs, so alloy-type anodes provide an effective strategy to circumvent the surface passivation issue encountered with Mg metal in conventional electrolytes. Theoretically, a germanium anode can deliver a high specific capacity of 1476 mAh g−1, but hitherto, no experimental reports have described Ge in MIBs. Herein, we experimentally verified that Ge could reversibly react with Mg2+ ions through the design of dual-phase Ge–Bi film electrodes fabricated by magnetron co-sputtering. Notably, a Ge57Bi43 electrode delivered a high specific capacity of 847.5 mAh g−1, owing to the joint alloying reactions of Ge and Bi with Mg, which was much higher than the specific capacity of Bi (around 385 mAh g−1). Moreover, the Ge–Bi anode showed excellent rate performance, good cycling stability, and superior compatibility with conventional electrolytes such as Mg(TFSI)2. More importantly, the Mg storage mechanism of the Ge–Bi anode was unveiled by operando X-ray diffraction, and density functional theory calculations rationalized that the introduction of Bi to form Ge–Bi evidently decreased the defect formation energy and effectively boosted the electrochemical reactivity of Ge with Mg.