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.