A New, Rapid Approach for the Synthesis of Germanium Nanowire Based Li-Ion Battery Anodes and Their Excellent Performance over Extended Cycles

Ge nanowires (NWs) have recently been highlighted as active materials for Li battery anodes.^1,2 Ge NWs (along with Si NWs^2,3) are well placed as anode materials for Li-ion storage due to their high theoretical capacity of 1384 mAhg^-1. The diffusion of Li within Ge is some 400 times faster than in Si¹ meaning that Ge is particularly suited to high power rate anodes. To maximize the capacity of NW based Li-ion anodes, they should be grown in high density directly on the metal current collector to allow for efficient current transport. Ancillary materials such as inactive catalyst seeds, binders and conductive additives should also be minimised as they diminish capacity by adding mass to the final device. Herein we present the development of a rapid pyrolysis route⁴ allowing the formation of high density Ge NW mats grown directly on stainless steel current collector substrates with reactions taking < 30 seconds (Figure 1 a). The growth system exploits the in-situ formation of discrete Cu₃Ge catalyst seeds from a thermally evaporated Cu layer producing centimetre scale NW coverage on device integrable substrates. Electrochemical testing of the NW covered substrates yielded capacity measurements greater than 1000 mAhr^-1g after 1000 cycles with a Coulombic efficiency of 99.7 % (Figure 1 b). The NW based anodes showed high-rate stability with discharge capacities of 800 mAhg^-1cycled at a rate of 10C. The method represents a rapid scalable approach for the formation of highly promising Li-ion anode materials.

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