Unravelling Gas Evolution and Solid Electrolyte Interphase Formation on Hard Carbon Negative Electrodes in Sodium-ion Cells

Abstract

Sodium-ion cells are an attractive alternative chemistry to lithium-ion cells due to their low cost, suitable energy density, and the widespread crustal abundance of sodium. Any concerted research effort into prolonging their cycle life must consider parasitic reactions between the hard carbon negative electrode and electrolyte. Uncontrolled electrolyte reduction on hard carbon leads to solid electrolyte interphase growth (SEI), loss of sodium inventory, and gas evolution, ultimately resulting in cell failure. This thesis uncovers a number of unique gas evolution and consumption phenomena that have a profound impact on the SEI formation process in sodium-ion cells. In particular, it was found that gas adsorption within hard carbon nanopores can conceal reduction pathways that result in poor quality SEI. By uncovering these gas adsorption and desorption pathways, and determining the true onset potential of electrolyte reduction and SEI formation, new formation protocols were developed which preferentially formed a stable, insoluble SEI.