(Invited) Design of Lithium Battery Electrolytes Based on Electrode Potentials

Abstract

Lithium (Li) metal is an ideal battery anode with low electrode potential (-3.045 V vs. SHE) and high theoretical capacity (3860 mAh g-1, 2060 mAh cm-3). However, its critical problem is a low Coulombic efficiency (CE), which is caused the thermodynamic instability of a Li/electrolyte interface because of the strong reducing ability of Li metal. Since Li electrode potential (E Li) is located far below the cathodic limit of the electrolyte's potential window, the electrolyte components are inevitably reduced and decomposed on the Li metal anode (Fig. 1). Some of the decomposition products function as solid electrolyte interphase (SEI) to suppress further electrolyte decomposition. Various electrolytes and additives have been proposed to form a stable SEI on Li metal, which leads to higher CEs. However, the correlation between CE and SEI is still unclear.Here we report E Li as a quantitative and thermodynamic descriptor of the CEs of Li metal anodes.1 We measured E Li in various electrolytes with reference to 1 mM ferrocene as an internal standard. The obtained E Li varied significantly (up to 0.6 V) depending on electrolyte formulations. We also evaluated the CEs of Li plating/stripping in various electrolytes without ferrocene. Based on these results, we found that the CEs are correlated with E Li (Fig. 2). The CE increased with increasing E Li, suggesting that the reductive decomposition of electrolytes was suppressed at high E Li (lower reducing ability of Li).Theoretically, E Li is directly determined by the chemical potential of Li+ (μ Li+) in the electrolyte. Hence, E Li, that is the reducing ability of Li, can be controlled by designing an electrolyte focusing on μ Li+. Raman Spectroscopy revealed that the formation of ion pairs is essential for increasing the μ Li+ and upshifting the E Li. This finding provides fundamental science that can tune the μ Li+ and E Li to increase the CE of Li metal anodes.1. Seongjae Ko, Tomohiro Obukata, Tatau Shimada, Norio Takenaka, Masanobu Nakayama, Atsuo Yamada, Yuki Yamada, Nat. Energy, 7, 1217-1224 (2022). Figure 1