Nanocolumnar germanium thin films as a high-rate sodium-ion battery anode material

Both nanocolumnar and dense germanium thin films, synthesized by evaporative deposition, were tested as a potential anode material for sodium-ion batteries. The reversible capacity of the nanocolumnar films was found to be 430 mAh/g, which is higher than the theoretical capacity of 369 mAh/g. The nanocolumnar films retained 88% of their initial capacity after 100 cycles at C/5, whereas the dense films began to deteriorate after ∼15 cycles. Additionally, the nanocolumnar films were stable at charge/discharge rates up to 27C (10 A/g). The diffusion coefficient for sodium in germanium was estimated, from impedance analysis of the dense films, to be ∼10^-13 cm ² s^-1. Modeling of diffusion in the sodium- germanium system predicts that sodium diffusion in the near-surface layers of the material is significantly faster than in the bulk. These results show that small feature sizes are critical for rapid, reversible electrochemical sodiation of germanium.