Understanding Charge and Discharge Behavior in Graphite-SiOx Composite Electrodes

Abstract

Si has been developed for a promising anode material in the next generation lithium ion batteries due to its high gravimetric capacity density of 4200mAhg−1. However, the lithiation and delithiation reactions are accompanied by large volume changes, resulting in the fracture and pulverization of Si particles and leading to the failure of electrical contacts [1]. To overcome these problems, many kinds of Si-based materials (Si/C, Si nano-flakes, SiOx, etc.) have been proposed for relaxation of the Si volume changes [2-4]. In addition to the volume changes, numerous problems (i.e., reversible capacity, coulombic efficiency, charge–discharge rate capability) still remain to be solved for practical use. To reduce these effects, Guerfi et al.,[5] developed graphite-SiOx composite anodes, coupled with a flexible binder like polyimides, and showed its promising cycle life.In this study, we discuss electrochemical lithiation and delithiation (charge and discharge) behavior of a graphite-SiOx composite electrode. The graphite-SiOx electrode was composed of 88.1% graphite, 8.7% of SiOx, 1.0% of conductive agent, 1.0% of CMC and 1.2% of SBR binder. The discharge capacity and coulombic efficiency at 1/20C in 1M LiPF6/EC+DEC (1:1) was 450mAh/g and 85.5%, respectively.Fig.1 shows a cross-sectional FE-SEM image of a SiOx particle in the electrode after being charged to 0.01V at 2C. Dark stripes, probably cracks, were observed, which indicates that lithiation proceeded non-uniformly in a SiOx particle. Compared to graphite, SiOx consists of two phases (Si and SiO2 domains) and has a specific charge and discharge profile. Therefore, it is easy to divide the total capacity into those of graphite and SiOx using the discharge profile; we defined the capacity at potentials <0.24V as delithiation from graphite, while the capacity at potentials >0.24V as that from SiOx. SOC’s of graphite and SiOx at different potentials are shown in Fig.2. This result indicates that lithiation started from SiOx and the full lithiation was attained at a higher potential for SiOx.Fig.3 (a) shows the charge rate performance at different C rates (at a fixed discharge rate of 1/20 C). With increasing the charge rate, the discharge capacity from graphite (<0.24 V) decreased, while that from SiOx (>0.24 V) increased. Fig.3 (b) shows the discharge rate performance at different C rates (at a fixed charge rate at 1/20C). With an increase in the discharge rate, the overpotential for delithiation from graphite became high compared with that from SiOx.These results suggests that lithiation and delithiation reaction of SiOx is exceptionally fast in the graphite- SiOx composite anode and this intensive reaction of SiOx may cause the contact loss failure in graphite- SiOx composite electrode upon cycling. In other words, the control of the reaction rate (i.e. the charge and discharge transfer resistance) of graphite and SiOx is a key to use a large amount of SiOx for practical use.