Synergistic Li-Na co-alloying for high-capacity, long-life, dual-alkali ion batteries

High-capacity, alloying-mode sodium-ion battery (NIB) anode materials remain elusive, mostly due to poor Na ion diffusivity within attractive candidates such as Ge and Si. Herein, for the first time, increased cation activity is unlocked in a Ge nanowire active material, through the use of a dual ion Li/Na electrolyte. In comparison to low specific capacity (297 mAh g -1 ) in the Na-only electrolyte, the dual electrolyte enabled a 2x capacity increase (605 mAh g -1 ) via a dual-cation alloying mechanism which has never been reported before. Electrochemical data and material characterization demonstrates that the mechanism follows an amorphous path, with the formation of amorphous Na-rich and Li-rich Ge phases during electrochemical alloying reactions. Complex stoichiometries of Li-Na-Ge ternary phases were validated using ab-initio random structure searching (AIRSS) computational technique. This dual-cation mechanism led to exceptional specific capacity (80% capacity retention after 1000 cycles at 1 mA cm -2 ), with demonstrated full-cell compatibility using a sustainable FeS 2 cathode.