Silicon is considered as the most promising future active material on the anode side of lithium-ion batteries (LIBs), because of its high capacity, high abundance and low costs. However, silicon suffers from two main drawbacks, a large volume change during lithiation [1, 2] and its poor electrical conductivity [3]. In order to address these drawbacks, in this study, nano-silicon/graphite composites (Si@Gr) have been produced by a scalable process route and the influence of different structural and compositional characteristics have been investigated. At first, nano-silicon was prepared by nano-grinding in a stirred media mill. In the next step, the nano-silicon suspension was sprayed onto coarser graphite particles to form the core-shell structured Si@Gr composite (about 10 wt.% silicon) using a pilot scale fluidized bed granulator [4]. A pitch derived carbon coating on Si@Gr composites (Si@Gr/C) not only stabilized the particulate structure but also decreased the specific surface area which is both favorable with regard to the irreversible solid electrolyte interface (SEI) build up in the cell formation. The resulting Si@Gr and Si@Gr/C particles were then continuously coated on copper foil to make electrodes and tested in full cells (with NCM622 as cathode) as well as in half cells. Even though the capacity for the Si@Gr and Si@Gr/C electrodes are almost same, 600 mAh g-1,but the electrochemical performance is improved by the carbon coating enabling a capacity retention of 92 % compared to 82 % for the uncoated composite after 125 cycles at 0.3 C. It could also be shown that the softening point and particle size of the used pitch affected the structural characteristics of the composite. Higher softening point pitches enabled higher electrical conductivities and better cycling stability while the carbon coating derived from lower softening point pitches collapsed after a certain amount of cycles. A similar behaviour could be observed for the used pitch particle sizes for carbon coating, whereby larger particles enabled a more stable cycling behaviour. Additionally, a study of the influence of the silicon particle sizes (120, 160 and 250 nm) and silicon concentrations are conducted to evaluate their influence on the performance of Si@Gr particles, electrodes and cell characteristics. As expected, with increasing silicon content and silicon particle size worse mechanical stability of composite particles, a lower electrical conductivity and worse cell performance are observed.
References
[1] Hatchard, T. D. and Dahn, J. R. 2004. J. Electrochem. Soc. 151, 6, A838.
[2] Obrovac, M. N. and Christensen, L. 2004. Electrochem. Solid-State Lett. 7, 5, A93.
[3] Pollak, E., Salitra, G., Baranchugov, V., and Aurbach, D. 2007. J. Phys. Chem. C 111, 30, 11437–11444.
[4] Müller, J., Abdollahifar, M., Vinograd, A., Nöske, M., Nowak, C., Chang, S.-J., Placke, T., Haselrieder, W., Winter, M., Kwade, A., and Wu, N.-L. 2021. Chemical Engineering Journal 407, 126603.