Tuning the Sodium Electrode Potential by Solvent Molecular Framework Engineering

抄録

Sodium-ion batteries (SIBs) are promising next-generation energy-storage systems, yet their low energy density remains a major challenge. An effective strategy to utilizing low-potential anodes is tuning their electrode potentials with respective to an electrolyte potential window. Herein we propose a molecular framework design of electrolyte solvents to rationally tune sodium electrode potential (E_Na). Extending the alkyl chains of phosphate ester solvents upshifted E_Na by up to 0.32 V (tripropyl phosphate vs. trimethyl phosphate) despite similar donor numbers, indicating a dominant steric effect to weaken the Na⁺ solvation. In contrast, cyclic crown ether 15-crown-5 downshifted E_Na by 0.16 V compared to its linear counterpart (tetraglyme) via the formation of a more stable chelate coordination. Furthermore, the magnitude of these steric/structural effects depended strongly on the cation species, leading to large variations in the Na-Li electrode potential difference (ΔE_Na–Li), from 0.53 V in tripropyl phosphate to 0.10 V in 18-crown-6. Machine-learning-based molecular dynamics simulations revealed that the changes in the cation solvation structure, induced by the solvent molecular framework engineering, are responsible for the observed potential shifts. These findings establish solvent molecular framework engineering as a versatile strategy to tune electrode potentials in battery electrolytes.