Deciphering the paradox between the Co-intercalation of sodium-solvent into graphite and its irreversible capacity

While sodium ion (Na+) itself cannot intercalate into graphitic structure to form binary graphite-intercalation-compounds (GICs) as its alkaline brothers (lithium, potassium etc.) do, the intercalation of its solvated form [Na(solvent)(x)](+) can readily happen, leading to reversible formation of ternary GICs at stage 1, as long as used solvent molecules are stable against reduction, such as ethers. However, a mystery still remains: what causes the initial irreversible capacity loss given that co-intercalation requires an interphase-free surface? In this work, we answered this fundamental question by elaborately examining the solvation structures, desolvation process and the corresponding electrochemical reduction stability of ether-based solvated Na+. Based on the understanding, a simple and effective approach was proposed to significantly increase the initial coulombic efficiency of sodium-ether co-intercalation. Such precise control over solvation-sheath and interfacial structures provides molecular-level guidance to the designing of new electrolyte systems and graphite-intercalation chemistries.