Germanium Nanocrystals As Anode Materials for Lithium Ion Batteries

Abstract

For the last two decades rechargeable Li-ion batteries (LIBs) have been widely used in portable electronic devices owing to their peculiar characteristics such as high gravimetric and volumetric energy. In fact, LIBs are considered the most suitable candidates for hybrid (HEV) and electric vehicles (EV) [1-3]. However, towards the employment of Li-ion batteries into HEV/EV, a substantial improvement in energy and power density of the electrode materials is needed. In particular, at the anode side, the present Li-ion battery technology is mostly based on graphite anodes, providing a 372 mAh/g lithium storage capacity.In this regard, both Silicon and Germanium are excellent candidates as active anodes for LIBs owing to their high lithium storage capability, that is 4200 and 1620 mAh g-1, respectively [1,4]. Although Ge, owing to the drawback of its expensiveness, was in the recent past attracting considerably less attention than Si, it is now undergoing an increase in popularity because of its desirable advantages such as high intrinsic electrical conductivity (104 times higher than silicon) and higher capacity than graphite anodes. Furthermore, it has been reported that the lithium diffusion into Ge is 15 and 400 times faster than in Si at 360o C and at room temperature, respectively [1,5]. This ensures higher rate capability and more efficient charge transport than in silicon and graphite as well, fundamental properties in advanced high power density applications such as hybrid electric vehicles. However, similarly to silicon, the lithiation of Ge anode during cycling undergoes dramatic volume changes (~ 300%), which leads to pulverization and capacity fading in bulk. One of the most successful techniques to overcome the aforementioned problem and to improve the performance of LIBs, is the reduction of Ge size to the nanoscale.Here, we report about a novel synthesis method to produce Ge nanocrystals through solvothermal approach (200-300 oC), with size in 5-10nm range, used as anode in lithium ion batteries. The novelty lies in the simplicity of the proposed approach in fact, differently from standard processes [6], non-hazardous reducing agents were used and, at the same time, there is no longer need of maintaining inert atmosphere conditions. In particular, our approach starts from Germanium halogen GeX (X= Cl2, I2) salts. The Ge nanocrystals have been characterized with X-ray diffraction, X-ray photoelectron spectroscopy, scanning (SEM) and transmission (TEM) electron microscopies. Figure 1 shows the X-ray diffraction pattern of Ge nanocrystals with JCPDS card No. 03-065-0333 together with the TEM image of Ge nanocrystals. The latter demonstrates a good uniformity in the nanocrystals dimensions, falling in the range between 5nm and 10nm. Finally, the electrochemical investigation of lithium insertion/de-insertion of these nanocrystals have been studied with cyclic voltammetry, A.C impedance and galvanostatic charge-discharge measurements.