Precise Control of the Nanostructure of Ge Films by High-Pressure Plasma Sputtering for Li-Ion Battery with Super-High Capacity

The automobile industry is currently shifting towards hybrid and electric vehicles powered by electrochemical energy storage systems, or batteries. However, these batteries are less fuel efficient than conventional gasoline systems, and it is therefore important to develop high-performance batteries that have a high energy density, high electromotive force, and a long charge/discharge cycle life. Recently, because of the capacity limit of carbon anodes in Li-ion batteries, the development of alternative anode materials that are reactive with Li has been actively promoted. Among these, Si and Ge are the most interesting materials because they have high theoretical capacities of 4,200 and 1,600 mAh/g, respectively, which are much higher than the value of 372 mAh/g for conventional carbon active material. However, high-capacity materials undergo large volume changes during Li alloying/de-alloying reactions and prolonged cycling leads to electrode pulverization from the current collector, resulting in capacity fading. In this study, we focus on Ge as a Li-ion battery anode material, for which the Li diffusion rate is 400 times higher than that for Si, and the electrical conductivity is 10⁴ times higher.<br/>The Ge films were fabricated on a copper substrate using 13.56-MHz radio-frequency (rf) magnetron sputtering. The sputtering target was a polycrystalline Ge disk (1 inch diameter) with a purity of 99.99 %. An rf power of 60 W was supplied to the sputtering target for plasma production. Ar gas was supplied from the direction from the target to the substrate holder at a flow rate of 80 sccm. The Ar gas pressure was set to a relatively high value of 0.5 Torr. The distance z between the target and the substrate holder was changed from a short distance of 5 to 20 mm. The substrate holder was not heated or cooled during film deposition. Surface morphology of films deposited at z = 5, 10, 20 mm was analyzed by using atomic force microscopy (AFM). The nanostructure of the Ge was clearly observed for all films. We found that the grain size changed with z: for z = 10 and 20 mm, grain size was 36 and 38 nm, respectively, while for a short distance z = 5 mm, grain size was 82 nm. In this study, the grain size was successfully controlled in the Ar high-pressure sputtering process without substrate heating. We measured the charge/discharge properties of Li-ion battery cells with Ge anodes deposited at z = 5, 10, and 20 mm. A high gravimetric capacity of more than 800 mAh/g was observed for all the test cells in the first cycle, which was much higher than 372 mAh/g of conventional carbon anode. The discharge gravimetric capacity after 100 cycles strongly depended on the distance z for film deposition. The discharge capacity decreased to almost 0 for the test cell with the Ge film deposited at z= 10 and 20 mm, while it kept a high value 509 mAh/g for the test cells with the Ge films deposited at z = 5 mm. Our experiment clearly showed that precise control of the nanostructure, which was realized in our Ar high-pressure plasma sputtering process, was important for stable cycling of high-capacity metal anodes.