Capacity Fade and Its Mitigation in Li-Ion Cells with Silicon-Graphite Electrodes

4 years ago

Capacity Fade and Its Mitigation in Li-Ion Cells with Silicon-Graphite Electrodes

Daniel P. Abraham, Matilda Klett, James A. Gilbert, Ilya A. Shkrob, Javier Bareño

Silicon-graphite (Si-Gr) electrodes typically contain lithiated carboxylates as polymer binders that are introduced through aqueous processing. Li-ion cells with such electrodes show significantly faster capacity fade than cells with graphite (Gr) electrodes. Here we examine the causes for capacity loss in Si-Gr cells containing LiPF₆-based electrolytes. The presence of SiO_xF_y in the Si-Gr electrode, fluorophosphate species in the electrolyte, and silica on the positive electrode indicates the crucial role of the hydrolytic cycle. In particular, HF acid that is generated through LiPF₆ hydrolysis corrodes Si particles. As it reacts, the released water re-enters the cycle. We trace the moisture initiating this detrimental cycle to the hydration water in the lithiated binders that cannot be fully removed by thermal treatment. The rate of HF corrosion can be reduced through the use of electrolyte additives. For the fluoroethylene carbonate (FEC) additive, the improved performance arises from changes to the solid electrolyte interphase (SEI) that serves as a barrier against HF attack. We propose that the greater extent of polymer cross-linking, that gives FEC-derived SEI elastomer properties, slows down HF percolation through this SEI membrane and inhibits the formation of deep cracks through which HF can access and degrade the Si surface.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b06118

DOI: 10.1021/acs.jpcc.7b06118