Germanium-Based Electrode Materials for Lithium-Ion Batteries

Advanced energy-storage systems are critically important for meeting the ever-increasing demand for applications from portable electronics to all-electric vehicles, and recently for applications in the grid for storing energy from fluctuating renewable sources, such as wind or solar energy. Lithium-ion batteries (LIBs) have received worldwide attention as a top performing energy-storage system. Currently, graphite is being used as the commercial anode material in LIBs. However, graphite has a relatively low theoretical capacity of 372 mAhg , which significantly limits the fast-growing demand in energy storage. Other group-IV elements, such as Si, Ge, and Sn, are being considered as substitutes of graphite in LIBs because of their much higher theoretical capacities. Among these candidate anode materials, Si is the most popular one and has been extensively studied. This is because Si possesses the highest theoretical capacity, 4200 mAhg 1 for Li22Si5 at high temperature [2,3] or 3579 mAhg 1 for Li15Si4 at room temperature, which is about one order of magnitude higher than that of the graphite anode material. Compared to Si, Ge has received much less attention, despite its high volumetric capacity (7366 AhL 1 for Li15Ge4), second only to silicon (8334 AhL 1 for Li15Si4), as well as its high gravimetric capacity (1384 mAhg 1 for Li15Ge4). [1, 6] Although Ge is more costly than Si, which could be the main reason for the lack of attention in the past, Ge has several outstanding features as a promising high-capacity anode material : 1) High electronic conductivity : since the bandgap of Ge (Eg= 0.66 eV at 300 K) is smaller than that of Si (Eg=1.12 eV at 300 K), Ge has a much higher intrinsic electronic conductivity than Si. 2) High lithium (Li) ion diffusivity: the diffusivity of the Li ion in Ge is about two orders of magnitude larger than that in Si at room temperature. With these favorable transport properties, it is expected that Ge will have a better rate capability than Si. Fast transport of both electrons and Li ions is highly desired for achieving a high charging/discharging rate in LIBs. Thus, a battery design that balances optimal energy and power densities could be achieved through Ge-based LIBs. With technical improvements to produce Ge, which is abundant in the Earth’s crust, it is anticipated that the price of Ge could be reduced in the near future. In recent years, there has been a drastic increase in scientific research on Ge and Gebased materials for applications in LIBs. 10–44] A major drawback of high-capacity electrode materials, such as Si and Ge, is the huge volume change upon full lithiation/ delithiation (about 281% for Li15Si4 and 246% for Li15Ge4), [45]