Li-metal batteries are considered an ultimate energy-storage device since a Li metal anode presents high capacity and low redox potential.¹ However, a poor Coulombic efficiency (CE) during a plating/stripping of Li metal, has hampered their commercialization. To overcome this issue, a diverse type of electrolytes, which forms a robust solid electrolyte interphase (SEI) on the Li-metal surface, has been introduced over the past decade.² However, a correlation between the CE and SEI chemistry is still vague since it is mostly established on the basis of indirect experimental methodologies. Indeed, a huge change in CEs has been reported even in the presence of similar functional-SEIs, indicating that there is another essential factor that governs the CE.³

Fundamentally, the redox potential of Li metal is located far outside the thermodynamic potential window of organic electrolytes, thus inevitably inducing reductive degradation of the electrolyte. Importantly, the Li metal is known as a quasi-reference electrode, which indicates its redox potential strongly depends on the chemical potential of Li⁺ in the electrolyte. Thus, if the redox potential of Li metal is upshifted to narrow the gap from the potential window of the electrolyte, the CE should be increased with thermodynamically mitigated electrolyte decomposition.

Based on this assumption, the Li redox potential (V vs. electrolyte-independent redox system, ferrocene Fc/Fc⁺) was evaluated and plotted versus the average CEs of Li plating/stripping reactions in 74 different electrolytes. The result presents that the average CE is increased as an increase of the redox potential of Li metal that varies over 0.6 V depending on the electrolytes, demonstrating that the degree of the reductive degradation of electrolyte is governed by the redox potential of Li metal in the given electrolyte. The mechanism behind the redox potential variation of Li metal was studied with both machine-learning analysis and Raman spectroscopy. Obviously, the coordination states of Li⁺ and FSI^- shows a high relevance for the Li redox potential. Specifically, as an increase of Li⁺-FSI^- ion-pairing, the redox potential of Li metal is upshifted gradually.

In conclusion, we postulate that a large variation of Li electrode potential (>0.6 V), which is deeply linked to the coordination condition of Li⁺ and FSI^- in the electrolyte, is a simple yet overlooked ‘hidden factor’ that dominates Li-metal battery performance. This rationale will provide a new chance to design a stable Li-metal batteries.

Reference
1. D. Lin et al., Nat. Nanotechnol., 12, 194–206 (2017).
2. S.C. Kim et al., J. Am. Chem. Soc., 143, 18703−18713 (2021).
3. G. M. Hobold et al., Nat. Energy, 6, 951–960 (2021).