Meso-porous amorphous Ge: Synthesis and mechanism of an anode material for Na and K storage

Crystalline Ge is a highly active anode material for Li storage but inactive for Na storage because of high diffusion barrier. By in-situ Raman spectrum, we explore that the Na could reversibly alloy/dealloy with the amorphous Ge, but does not with the crystalline Ge. Herein, the amorphous Ge is fabricated by an acid-etching Zintl phase Mg₂Ge route at room temperature, which shows a mesoporous architecture with a Brunauer-Emmett-Teller (BET) surface area of 29.9 m²·g^-1 and a Barrett-Joyner-Halenda (BJH) average pore diameter of 7.6 nm. This mesoporous architecture would enhance the Na-ion/electron diffusion rate and buffer the volume expansion. As a result, the as-prepared amorphous Ge shows superior Na-ion storage performance including high reversible capacity over 550 mA·h·g^-1 at 0.2 C after 50 cycles, good rate capability with a capacity of 273 mA·h·g^-1 maintained at 5.0 C, and long-term cycling stability with capacities of 450 mA·h·g^-1 at 0.4 C after 200 cycles. For the K-ion storage, the amorphous Ge is also more active than the crystalline counter and maintains a capacity of 210 mA·h·g^-1 after 100 cycles at 0.2 C.