Beneficial vs. inhibiting passivation by the native lithium solid electrolyte interphase revealed by electrochemical Li + exchange

Despite being a leading candidate to meet stringent energy targets of Li-ion batteries, the lithium (Li) metal anode has yet to achieve Coulombic efficiency (CE) requirements for long cycle life (>99.9%). These limitations derive from the native solid electrolyte interphase (SEI) which, among multiple functions, stabilizes and protects deposited Li. The SEI also plays a critical role in regulating Li + exchange between the electrolyte and the electrode, but quantification of this effect has been non-straightforward, and a general relationship between Li + exchange and CE has not been clearly elucidated to date. Using electrochemical impedance spectroscopy and voltammetry, we report self-consistent Li + exchange values of native SEIs over a range of relevant electrolytes with CE spanning 78.0% to >99%. CE and its retention at high rates are found to be positively correlated with the rate of SEI Li + exchange. Additionally, SEI Li + exchange rates increased during cycling in high-CE electrolytes, in some cases by an order of magnitude to exceed 10 mA cm −2 , whereas for low-CE electrolytes they remained low (1 mA cm −2 ).