Abstract
Si has emerged as the most promising successor to graphite as an anode material in Lithium-ion batteries. Known to have nearly ten times the capacity of graphite, Si is increasingly being used to partially replace traditional graphite to raise the overall energy density of a cell. High nickel cathode chemistries such as NMC811 are also of particular interest due to its high specific capacity and low cobalt content, which drives down cost. Pairing these active materials is an obvious design choice, however it results in combining their respective drawbacks. Relative to lower nickel NMCs, NMC811 has been shown to be more reactive with carbonate electrolyte components and is known to be more violently reactive upon thermal decomposition. Si based materials swell dramatically upon lithiation, causing the SEI layer to break and reform with repeated cycling, consuming the electrolyte in the process. In combination, high energy density also implies increased safety concerns. To overcome these problems new electrolyte additives are needed.
We report a new family of electrolyte additives designed to improve the robustness of the SEI and overall stability of the electrolyte system, which carries significance in improving cycle life and abuse tolerance in higher energy density cells. This is a tunable, drop-in solution to addressing the need for improving battery performance as well as safety. We demonstrate the effect of these additives on the SEI via analytical characterizations (SEM, XPS) and how it relates to improved long-term cycle life. The effect of additives on thermal decomposition at the material level is investigated via DSC and on the cell level via ARC, presenting a relationship between electrolyte design and inhibited electrode-electrolyte decomposition reactions. Novel electrolyte additives are thus shown to be a promising approach to resolving future high energy demand without compromising safety.