Li anodes represent a theoretical 10-fold upgrade over the capacity of graphite at comparable potential, and as such are key anode candidates for next-generation high energy density (> 1000 Wh/L) batteries. However, Li still falls below the 99.95-99.97% CE required for long-life cycling⁽¹⁾ and displays rate capability an order-of-magnitude lower than necessary for fast-charging (>2 C).⁽²⁾ Interestingly, intrinsic Li⁰/Li⁺ redox has been reported to be more facile (j₀ > 10 mA/cm²)⁽³⁾ than graphite intercalation (j₀ = 1-3 mA/cm²)⁽⁴⁾. In spite of this, the chemical exchange of Li⁺ through the native solid electrolyte interphase (SEI) on Li is typically slow (0.5-3 mA/cm²),⁽⁵⁾ and as such it can bottleneck Li⁺/Li⁰ redox⁽³⁾ and increase charge-transfer resistance.⁽⁶⁾ The SEI thus manifests itself by substantially decreasing the exchange current j₀ measured on Li down from its intrinsic kinetic value.⁽⁷⁾ While multiple recent studies see j₀ as a relevant property in determining Li reversibility,⁽¹⁾ measuring j₀ in the presence of an SEI is not straightforward. Consequently, current literature presents widely varying numerical values of j₀ in the presence of an SEI, making it challenging to discern the relationship between j₀ and CE.

To bridge this gap, Li⁺ exchange at the Li anode is here systematically quantified using cyclic voltammetry (CV) at slow scan rates and electrochemical impedance spectroscopy (EIS), both of which allow an SEI to develop natively. To avoid ambiguity with the intrinsic Li⁰/Li⁺ redox exchange current j₀, exchange rates are here interpreted in the framework of a “pseudo”-exchange current, j₀^p, that represents the total rate of Li⁺ exchange on the electrode. j₀^p was measured across a selection of historically-relevant and modern electrolytes, spanning low (78.0%) to high (99.3%) CE. In both methodologies, a strong dependence of j₀^p on electrolyte chemistry was identified. These differences reflect a strong correlation between CE and j₀^p, with electrolytes that display higher j₀^p typically also displaying higher CE. Upon cycling, a dynamic behavior of Li⁺ exchange on both Cu and Li were observed, with j₀^p typically increasing through cycling, attributed to morphological changes induced by non-uniform plating/stripping inherent to Li electrochemistry.⁽⁸⁾ We will discuss the implications of this dynamic behavior on both the formation cycle on Cu, as well as how j₀^p changes report on SEI evolution during cycling. Finally, it was found that cycling Li with current densities j beyond j₀^p leads to substantial capacity loss and low CE, whereas electrolytes that can sustain high j₀^p are insensitive to j. Altogether, our results indicate that Li⁺ exchange plays a dominant role in determining the rate capability and CE of Li anodes, with high-j₀^p electrolytes displaying higher CE and better rate capability than their low-j₀^p counterparts.

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