Germanium Nanowire Based Lithium-Ion Anode Exhibiting a Stable and High Capacity Over 1000 Cycles

Ge nanowire (NW) based materials have emerged as viable candidates for next generation rechargeable lithium-ion battery anodes with energy and power densities that are multiples of current graphitic based electrodes.^1-3  The key advance is the capability of NWs to overcome the well-known pulverization problem that is detrimental to the cycle life and hence viability of their bulk counterparts. NWs also provide good electrical conductivity along their length, have a high interfacial area in contact with the electrolyte, have an optimal short diffusion distance for Li-ion transport and can be grown directly from current collectors, eliminating the need for binders and conductive additives ^4,5  Here we demonstrate stable cycling over 1100 cycles of Sn seeded Ge NWs grown directly from a current collector via a solvent vapour growth system. We show by ex-situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy studies that the NW array transforms into a robust, porous network structure within the first 100 cycles.⁶ Once this network is formed it is highly stable, maintaining a capacity of   ̴ 900 mAh/g over the following 1000 cycles.   The electrolyte additive, vinylene carbonate (VC), was found to play an important role, facilitating the formation of this stable network morphology. The electrode material described here has several advantages as it is formed in a low energy, wet-chemical process with Ge NWs nucleating and growing from an evaporated Sn layer on stainless steel. Sn also has a high maximum theoretical capacity and we show both physically (HRTEM) and electrochemically (differential capacity plots) that the Sn seeds at the end of the NWs reversibly alloy with lithium and contribute to the electrodes overall specific capacity. The NW electrode architecture performed exceptionally well in rate capability tests achieving a discharge capacity of 435 mAh/g after 80 cycles at a discharge rate of 100C. Considering the low cost and low energy required, especially when compared with CVD systems, we believe our synthetic protocol to be an attractive and scalable synthesis approach for high-performance group IV nanowire based electrodes.

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