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Li metal batteries are a promising technology for satisfying the emerging demands of highenergy-density storage systems. However, their pragmatic utilisation encounters a low Coulombic efficiency (CE) with the unceasing reductive decomposition of an electrolyte on Li metal with strong reducing ability. By improving the CE based on the chemistry of passivation films (i.e. solid electrolyte interphase, SEI), suppression of reductive decomposition has been achieved in a kinetic manner.However, the vague correlation between the CE and SEI has hampered further electrolyte development. Here, we report that in diverse electrolytes, the large shift (>0.6 V) in the Li electrode potential and its correlation with the Li + coordination state are 'hidden factors' that dominate the CE.Vibrational spectroscopy and machine learning hierarchal analysis revealed that the formation of ion pairs is essential for upshifting the Li electrode potential, that is, for weakening the reducing ability of Li, which would lead to a high CE with diminished electrolyte decomposition. Based on these criteria, various electrolytes enabling a significantly improved CE (>99%) were easily discovered. The findings of this study provide insights for the development of next-generation electrolytes for Li metal batteries.

Nonbonding oxygen 2p orbitals during oxygen-redox reaction are monitored using resonant inelastic X-ray scattering (RIXS).

MXenes are emerging electrode materials intended for electric double-layer capacitors because of their large specific capacitance of more than 300 F/g. Recent advances in synthesis methods have enabled a decrease in surface functional groups and chemical control of their design, but the influence of surface functional groups on capacitive properties is still unclear. Here, we applied density functional theory combined with effective screening medium and reference interaction site model calculations to systematically investigate the atomic-scale double-layer structure of Ti3C2T2 MXene electrodes depending on their terminated halogen elements. The termination with halogen atoms having larger atomic numbers (I > Br > Cl > F) increased the electric double-layer capacitance. The increased capacitance originates from the smaller valence electron numbers of the terminating atoms with lower electronegativity that facilitate the electrostatic accumulation of electrons at the electrode surface. Such a solid trend provides a basis for consideration in designing MXene surfaces with larger capacitance.

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