Ion–solvent interactions play a crucial role in secondary battery systems: the desolvation of ions at an electrode/electrolyte interface can be the rate-determining step of a battery reaction, for instance. The present theoretical study investigates the interactions between K ions and organic electrolyte solvents for application in non-aqueous K-ion batteries, which have recently drawn interest as novel rechargeable batteries. Compared to Li, Na, and Mg ions, K ions display the lowest interaction energy, reflecting the large ionic radius and weak Lewis acidity of K. The weak interaction of K ions with solvents is consistent with the high rate capability exhibited by K-ion batteries and the relatively low solubility of K-ion salts observed experimentally.

De-solvation of a Li ion at an electrode/electrolyte interface can be the rate-determining step of the reaction in lithium-ion secondary batteries. The present study theoretically evaluates the de-solvation energies of Li, Na, and Mg ions to organic electrolyte solvents. The Na-ion complexes revealed commonly smaller de-solvation energies compared to the Li-ion complexes due to the weaker Lewis acidity, while the solvation structures were similar to each other. The Mg-ion complexes showed remarkably larger de-solvation energies because of the double positive charge. The increase of coordination number, which was associated with the change in the solvation structure, was observed for the Mg-ion complexes. Detailed analysis revealed good correlations between the de-solvation energies and the electrostatic potentials made by the solvents, as well as the chemical hardness of the solvents.

Aqueous Na‐ or K‐ion batteries could virtually eliminate the safety and cost concerns raised from Li‐ion batteries, but their widespread applications have generally suffered from narrow electrochemical potential window (ca. 1.23 V) of aqueous electrolytes that leads to low energy density. Herein, by exploring optimized eutectic systems of Na and K salts with asymmetric imide anions, we discovered, for the first time, room‐temperature hydrate melts for Na and K systems, which are the second and third alkali metal hydrate melts reported since the first discovery of Li hydrate melt by our group in 2016. The newly discovered Na‐ and K‐ hydrate melts could significantly extend the potential window up to 2.7 and 2.5 V (at Pt electrode), respectively, owing to the merit that almost all water molecules participate in the Na+ or K+ hydration shells. As a proof‐of‐concept, a prototype Na3V2(PO4)2F3|NaTi2(PO4)3 aqueous Na‐ion full‐cell with the Na‐hydrate‐melt electrolyte delivers an average discharge voltage of 1.75 V, that is among the highest value ever reported for all aqueous Na‐ion batteries.