Silicon/Graphene Nanosheets Composite Prepared By Plasma Assisted Milling As High Capacity Anode Materials for Li-Ion Batteries

Lithium ion batteries (LIBs) are very important power sources in a variety of applications, including portable electronic devices and home electronics, and are expanding to electric vehicles nowadays. The rapid development of these applications demands for making LIB smaller and lighter, namely, requires incorporation of higher gravimetric and volumetric capacity materials to replace the currently commercialized graphite anode[1]. Silicon (Si) is the most attractive anode material due to its high theoretical capacity of 3579 mAh g^-1 at room temperature, which is an order higher in magnitude than that of graphite (372 mAh g^-1)[2]. However, the poor cycleability, which is caused by structural failure and pulverization due to the huge volume change (>300%) of Si during the Li⁺ insertion and removal, hinders its practical application.

In order to enhance the cycling performance, one of the promising ways is to prepare composite with nano-Si particles/clusters uniformly dispersed in Graphene nanosheets (GNs) matrix, which could not only buffer the large volume expansion but also enhance the conductivity of Si during cycling[3]. However, in the cases of Si-based anode materials, there are still many difficulties in the utilization of graphene by the mechanical peel-off pristine graphite and CVD methods which appear to be too tedious and too expensive for mass production[4]. Here, we reported a one-step-method to produce efficiently nano-Si/GNs composite in large scale by P-milling, with the concept of using the rigid nano-Si particles as nanomillers to in-situ peel GNs from microsized graphite in the P-milling process.

Acted by synergy effect of rapid heating of plasma and mechanical grinding of ball milling together with nano-Si as nanomiller, the graphite powder was turned to GNs with nano-Si particles tightly integrated to the in-situ formed GNs gradually (Fig.1). With this composite structure, the agglomeration of nano-Si was inhibited and electronic conductivity was effectively improved. Consequently, the composite anode cloud deliver quite stble capacities in different current rates (Fig.2a). Furthermore, the practicality of the nano-Si/GNs composite anode was further investigated in a coin-type full cell using LiMn₂O₄ cathode (Fig.2b). The full cell cloud delivered a specific capacity about 600 mAhg^{-1 ­­­­}for the P-20h anode with good capacity retention among 30 cycles at a constant current rate of 400 mAg^-1 (based on anode, about 0.5C).

In summary, the nano-Si/GNs composite, which was treated by P-milling for 20h, processed a unique structure of nano-Si particles homegeneously embedded among the graphene nanosheets together with abundant nanosized free spaces, leading to much enhancement on conductivity and cycle performance for anode in lithium ion batteries. The synthetic route developed in this work was simple, low-cost, and pollution-free, enabling it to be adopted for large-scale production of  Si/graphene based composite anodes.

Acknowledgements:

This work was supported by the National Science Foundation of China (Projects 51201065 and 51231003), by the Guangdong National Science Foundation (Project S2012040008050) and by the Doctorate Foundation of the Ministry of Education (Projects 20120172120007 and 2014ZZ0002).

References

1. B. Wang, M. Liang, L. Zhi, Nano letters, 2013, 13, 5578.

2. W. J. Zhang, J. Power Sources 2011, 196, 13.

3. R. Z. Hu,W. Sun, M. Zhu, J. Mater. Chem. A, 2014, 2, 9118

4. W. Sun, R. Z. Hu, M. Zhu, J. Power Sources 2014, 268, 610.