Impact of Electrolyte on the Cycling of Si-Based Materials

Alloy-based negative electrodes have been considered the “next-generation negative electrode” expected to replace graphite for many years. However, few exist in the marketplace. Manufacturing alloy-based cells with acceptable cycling poses a major challenge, indeed many components of the cell must be adapted in order to obtain successful cycling. 3M has a long history of alloy research for negative electrodes and has developed commercially viable Si-based alloys.

                Si-based negative electrode materials undergo massive volume changes during cycling. Pure Si expands by approximately 280% while 3M’s active/inactive Si alloys expand on the order of 135%. The expansion and contraction of the particles has important consequences on the surface stability of these materials. Furthermore, the choice of electrolyte has profound impacts on the cycling of cells containing Si-based materials. In this presentation we will show the impact of electrolyte reactivity on the physical and chemical characteristics of the particles, the composition of the electrolyte and the cycling performance of the cells.

Cross section SEM studies of cycled 3M Si alloy and pure Si show the dimensional changes occurring in Si-based materials with cycling due to electrolyte reactivity. After 100 cycles, an SEI is found on the 3M active/inactive alloy, while the pure nano Silicon dramatically increases in size. Figure 1 shows cross section images of the electrodes containing only binder and either 3M Si alloy or pure Si. While electrolyte reactivity can occur with minimal fade in half cells, capacity fade will occur in full cells.

Fluoroethylene carbonate (FEC) has long been known to be an important electrolyte for Si-based materials. Furthermore, full cells containing Si-based materials can sometimes undergo sudden failure with cycling. GC/MS studies of electrolytes from cycled full cells containing Si-based materials will be presented showing the relationship between electrolyte composition and sudden failure. These results highlight the importance of full cell design optimization for the implementation of Si-based materials in commercially-relevant full cells.