A Cost Efficient Silicon-Carbon Based Anode Material for Lithium-Ion Batteries

Silicon anodes have been proposed as anodes for Li-ion batteries due to their very high theoretical capacity. It has the potential to store ten times more lithium than graphite, the material commonly used in lithium-ion anodes [1]. However, great performance in one parameter comes with great costs in other parameters. Due to the extreme volume changes involved during lithiation and delithiation of silicon, cyclability becomes a challenge. The silicon-based anodes typically encounter massive cracking and degradation during electrochemical cycling, accompanied with a steady growth of SEI and reduction in conductivities, blocking of channels for electrolyte transport and irreversible consumption of Li-ions and electrolyte solvents. Typical mitigation methods involves using nanostructured silicon [2], building of hierarchical structured silicon composites [3], optimisation of binders [4] and adding appropriate SEI forming additives [5]. Silicon as anode material has now grown to a mature field, and many degradation phenomena has been explored in detail, in particular on nanostructured silicon using powerful in-situ TEM studies [6]. Some of the degradation mechanisms are well documented in the literature, while others are not so well understood. In this presentation we show indications of migration of silicon, electrochemical sintering, and the formation of a dendritic network on the surface of the silicon after long cycling of commercially relevant anodes, using post mortem FIB-SEM and TEM. We also show how these effects are mitigated by methods known to increase anode life-time, achieving >1200 cycles of 600 mAh/g total anode. The composite silicon-carbon anodes are based on industrial battery grade silicon produced at Elkem, one of the world’s leading companies for environment-friendly production of metals and materials. The anodes typically consist of nano-sized silicon crystallites induced by a milling pre-treatment, embedded in graphite and conductive carbon additives. The electrochemical performance was tested in half cells where typically the working electrode was made by mixing a silicon-carbon composite powder with an organic binder in an aqueous slurry and coated on a Cu-foil. Lithium metal was used as the counter electrode. Structural properties and degradation mechanisms were examined by electron microscopy (SEM, FIB-SEM, TEM) and XRD. The next stage in the development of the silicon-carbon based anode is to prepare full cells with a commercial cathode and industry-relevant loading on the anode. Work on Si/C full cells will be presented at the conference which will provide useful information about degradation mechanisms of silicon in a full cell, and tests its relevance as a commercial anode material. [1] D. Larcher, S. Beattie, M. Morcrette, K. Edström, J.-C. Jumas, and J.-M. Tarascon, J. Mater. Chem., vol. 17, no. 36, p. 3759, 2007. [2] U. Kasavajjula, C. Wang and A. J. Appleby, J. Power Sources, 2007,163, 1003; X. Su et al., Adv. Energy Mater., 4 (2014), p. 1300882. [3] Liu, N., Z.D. Lu, J. Zhao, M.T. McDowell, H.W. Lee, W.T. Zhao, and Y. Cui, Nature Nanotechnology, 2014. 9(3): p. 187-192. [4] D. Mazouzi et al. Journal of Power Sources, 280 (2015), pp. 533-549. [5] L. Chen, K. Wang, X. Xie, J. Xie . Journal of Power Sources, 174 (2007), pp. 538–543. [6] M.T. McDowell et al., Adv. Mater. 2013, 25, 4966 Figure 1