Ion Accessibility Governs Electrochemically Effective Actuation in Carbon Nanotube Muscles

Electrochemical muscles convert charge-induced changes in conductive networks into macroscopic deformation, but stored charge does not necessarily produce mechanically effective actuation. Here, we identify spatial ion accessibility within hierarchical carbon nanotube (CNT) networks as a controlling factor in this electrochemical-mechanical coupling. We construct a solvent-tunable platform using EMIm·BF4 dissolved in propylene carbonate (PC), trimethyl phosphate (TMP), and their mixtures while retaining the same ionic species and CNT architecture. Although charge storage decreases in TMP-rich electrolytes, this change is far smaller than the more than 20-fold reduction in contraction, from 50.4% in PC to 2.1% in TMP at 2.5 V. Charge-normalized analysis shows that stored charge alone cannot predict mechanical output. In-situ Raman spectroscopy and depth-resolved TOF-SIMS and XPS analyses indicate that PC is associated with spatially extended ion access and electrochemical doping, whereas TMP predominantly supports surface-accessible charge storage. Molecular simulations further associate this contrast with differences in ion–solvent stabilization and solvation-shell reorganization. These findings establish spatial ion accessibility as a design principle for converting electrochemical charge into mechanical work in porous conductive networks.