Morphology and selectivity of hydrated alkali metal ions as depth of discharge in the 1T-MoS2 electrode with aqueous electrolytes

Abstract

Aqueous ion batteries have great commercial potential in green power and energy storage due to their green nature, safety and high ionic conductivities. Different from organic electrolytes, alkali ions (Li⁺, Na⁺, and K⁺) inevitably bring water molecules into the electrodes during the charging/discharging process due to the hydration of ions with water molecules. The selectivity of alkali ions and the mechanism of how water molecules are involved in the ion extraction/insertion process in the electrodes have not been clarified. In this study, we focus on the characteristics of the intra-layer distribution of different hydrated ions (Li⁺, Na⁺, and K⁺) and the quantitative analysis of the selectivity of hydrated cations in aqueous batteries. We found that the concentration of hydrated ions greatly affects their distribution within the 1T-MoS₂ layers, and the presence of hydrogen bonding and O–O repulsive forces between water molecules causes the hydrated ions to gradually form chains from the dispersed state under the effect of hydrogen bonding and ionic bonding, then further form strips, and ultimately be densely dispersed within the whole layer. In addition, the chemical potential difference of hydrated ions is the key to the competitive reaction, and we quantitatively analyze the selectivity relationship between hydrated cations throughout the charging and discharging process; hydrated sodium ions will have better performance than lithium and potassium ions in aqueous batteries.