Quantifying Lithium Ion Exchange in Solid Electrolyte Interphase (SEI) on Graphite Anode Surfaces

Solid Electrolyte Interphase (SEI) has been identified as the most important and least understood component in lithium-ion batteries. Despite extensive studies in the past two decades, a few mysteries remain: what is the chemical form of and degree of mobility of Li+ in the interphase? What fraction of Li+ is permanently immobilized in the SEI, while the rest are still able to participate in the cell reactions via the ion-exchange process with Li+ in the electrolyte? This study attempted to answer, in part, these questions by using 6Li and 7Li-isotopes to label SEIs and electrolytes, and then quantifying the distribution of permanently immobilized and ion-exchangeable Li+ with solid-state NMR and ToF-SIMS. The results showed that the majority of Li+ were exchanged after one SEI formation cycle, and a complete exchange after 25 cycles. Ion exchange by diffusion based on concentration gradient in the absence of applied potential also occurred simultaneously. This knowledge will provide a foundation for not only understanding but also designing better SEIs for future battery chemistries