First-Principles Study on the Cation-Dependent Electrochemical Stabilities in Li/Na/K Hydrate-Melt Electrolytes

Aqueous alkali-ion batteries, particularly earth-abundant sodium- or potassium-based systems, are potentially safe and low-cost alternatives to nonaqueous Li-ion batteries. Recently, concentrated aqueous electrolytes with Na and K salts as well as Li ones have been extensively studied to increase the voltage of aqueous batteries; however, the potential windows become narrower in the order of Li > Na > K. Here, we study the difference in the potential windows of Li-, Na-, and K-salt concentrated aqueous electrolytes (hydrate melts) by first-principles molecular dynamics. As the Lewis acidity of alkali cations decreases (Li+ > Na+ > K+), the sacrificial reduction of counter anions is less active and water molecules are more aggregated. This situation is unfavorable for achieving stable anion-derived passivation on negative electrodes as well as for being stabilized to oxidation on positive electrodes. Hence, the Lewis acidity of alkali cations is essential to dominate the potential windows of hydrate-melt electrolytes.