Understanding Charge and Discharge Behavior in Graphite-SiOx Composite Electrodes

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, SiO_x, 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-SiO_x 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-SiO_x composite electrode. The graphite-SiO_x electrode was composed of 88.1% graphite, 8.7% of SiO_x, 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 SiO_x particle.  

Compared to graphite, SiO_x 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 SiO_x 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 SiO_x. SOC’s of graphite and SiO_x at different potentials are shown in Fig.2. This result indicates that lithiation started from SiO_x and the full lithiation was attained at a higher potential for SiO_x.

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 SiO_x (>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 SiO_x is exceptionally fast in the graphite- SiO_x composite anode and this intensive reaction of SiO_x may cause the contact loss failure in graphite- SiO_x composite electrode upon cycling. In other words, the control of the reaction rate (i.e. the charge and discharge transfer resistance) of graphite and SiO_x is a key to use a large amount of SiO_x for practical use.

 

References: [1]B. A. Boukamp et al., J. Electrochem. Soc., 128, 725 (1981).; [2]I. Kim et al., J.Power Sources, 136. 145 (2004).; [3]M. Saito et al., Solid State Ionics, 225, 506 (2012).; [4]Y. Nagao et al., J. Electrochem. Soc., 151, A1572 (2004).; [5]A. Guerfi et al., J.Power Sources, 196. 5667 (2011)