Silicon-Germanium Heterostructure Nanowires As High Capacity Lithium-Ion Battery Anodes - an Electrochemical and Structural Investigation

Abstract

Li-alloying nanowires (NWs) are promising anode materials for next generation lithium-ion batteries due to their large maximum theoretical capacity when compared with conventional graphitic based materials (372 mAh/g). Ge NWs, in particular, have shown great promise, exhibiting stable capacities of ̴ 900 mAh/g over 1000s of cycles and excellent rate capability due to the high rate of diffusivity of Li in Ge and its inherent high conductivity.1, 2 The superior capacity retention of Ge is attributed to the complete restructuring of the NWs that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust. The possibility of incorporating Si into this stable network of Ge is very appealing, resulting in electrodes that combine the very high capacity of Si with the excellent rate characteristics and capacity retention of Ge. Here we report the formation of high-performance and high-capacity lithium-ion battery anodes from high-density Si-Ge heterostructure nanowire arrays grown directly from the current collector. The unique morphology of the active material consists of Si NW branches grown from a Ge NW backbone. By tuning the ratio of Si to Ge in the material it is possible to design electrodes for either high-capacity applications (high Si : Ge mass ratio) or high rate capability applications (high Ge : Si mass ratio). We show by ex-situ high-resolution transmission electron microscopy (HRTEM) and high-resolution scanning electron microscopy (HRSEM) studies that the NW array transforms into a mechanically robust, porous network of Si and Ge ligaments. Once this network is formed it is highly stable, maintaining capacities of 1700 mAh/g over 200 cycles. The electrode material described here also has the advantage of being formed in a low energy, rapid synthetic protocol. Using a very simple reaction protocol, NW growth occurs by thermolytic decomposition of an organometallic precursor onto a preheated substrate in an inert atmosphere. 1. T. Kennedy, E. Mullane, H. Geaney, M. Osiak, C. O’Dwyer, K. M. Ryan, Nano Letters 14, 716-723(2014), 2. E. Mullane, T. Kennedy, H. Geaney, K. M. Ryan, ACS Applied Materials & Interfaces , 6, 18800-18807 (2014).