Alloying Germanium Nanowire Anodes Dramatically Outperform Graphite Anodes in Full-Cell Chemistries over a Wide Temperature Range

The electrochemical performance of Ge, an alloying anode in the form of directly grown nanowires (NWs), in Li-ion full cells (vs LiCoO₂) was analyzed over a wide temperature range (-40 to 40 °C). LiCoO₂||Ge cells in a standard electrolyte exhibited specific capacities 30× and 50× those of LiCoO₂||C cells at -20 and -40 °C, respectively. We further show that propylene carbonate addition further improved the low-temperature performance of LiCoO₂||Ge cells, achieving a specific capacity of 1091 mA h g^-1 after 400 cycles when charged/discharged at -20 °C. At 40 °C, an additive mixture of ethyl methyl carbonate and lithium bis(oxalato)borate stabilized the capacity fade from 0.22 to 0.07% cycle^-1. Similar electrolyte additives in LiCoO₂||C cells did not allow for any gains in performance. Interestingly, the capacity retention of LiCoO₂||Ge improved at low temperatures due to delayed amorphization of crystalline NWs, suppressing complete lithiation and high-order Li₁₅Ge₄ phase formation. The results show that alloying anodes in suitably configured electrolytes can deliver high performance at the extremes of temperature ranges where electric vehicles operate, conditions that are currently not viable for commercial batteries without energy-inefficient temperature regulation.