Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate

Operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) studies of Ge anodes are carried out to understand the effect of cycling rate on Ge phase transformation during charge/discharge process and to relate that effect to capacity. It is discovered that the formation of crystalline Li₁₅Ge₄ (c-Li₁₅Ge₄) during lithiation is suppressed beyond a certain cycling rate. A very stable and reversible high capacity of ≈1800 mAh g^-1 can be attained up to 100 cycles at a slow C-rate of C/21 when there is complete conversion of Ge anode into c-Li₁₅Ge₄. When the C-rate is increased to ≈C/10, the lithiation reaction is more heterogeneous and a relatively high capacity of ≈1000 mAh g^-1 is achieved with poorer electrochemical reversibility. An increase in C-rate to C/5 and higher reduces the capacity (≈500 mAh g^-1) due to an impeded transformation from amorphous Li_xGe to c-Li₁₅Ge₄, and yet improves the electrochemical reversibility. A proposed mechanism is presented to explain the C-rate dependent phase transformations and the relationship of these to capacity fading. The operando XRD and XAS results provide new insights into the relationship between structural changes in Ge and battery capacity, which are important for guiding better design of high-capacity anodes.