But, as always, 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 huge 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].
The full theoretical capacity of silicon is not required for lithium ion batteries in the near term future since the cathode materials available today becomes too limiting for the overall capacity. Thus, composite anodes containing silicon and a conventional anode material such as graphite will likely be sufficient to meet the short-term targets for anode materials. However, degradation phenomena specific to composite silicon-carbon electrodes has not been studied in same detail as for pure nano-silicon structures. In addition, in situ TEM often suffer from the fact that only a single cycle, or a limited amount of cycles, has been observed. In our work we have performed post-mortem FIB-SEM and TEM studies to investigate degradation occurring over multiple cycles examining the effect and interplay between the graphite and the silicon in composite anodes and in this presentation we will explore different degradation phenomena observed in silicon-carbon composite anodes including:
- Electrode thickening
- Silicon migration
- Electrochemical sintering
- Dendritic surface formation
- Inhomogeneous lithiation
- Strong dependence on lithiation level and electrode thickness
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.
References
[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.