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)), revealing a chemistry-dependent picture of SEI evolution with often-complex dynamics. The evolution in Li+ exchange unique to high-CE electrolytes also provides insights into the role and effectiveness of the formation cycle on Cu current collectors upon the first plating step. Altogether, these findings indicate that Li+ exchange governs several key processes related to Li deposition and cycling efficiency. Consequently, its quantification can help to guide future high-CE electrolyte design, particularly targeting high rates (>1 mA cm(-2)).

Automated summary

Despite its potential as a candidate for meeting energy targets of Li-ion batteries, the lithium metal anode has not achieved the necessary Coulombic efficiency for long cycle life. The limitations are due to the solid electrolyte interphase (SEI), which stabilizes and protects the deposited lithium. The study quantifies the effect of Li+ exchange with the SEI on Coulombic efficiency and reveals a chemistry-dependent picture of SEI evolution. These findings have implications for high-CE electrolyte design targeting high rates.