Achieving Rich Mixed-Valence Polysulfide/Carbon Nanotube Films toward Ultrahigh Volume Energy Density and Largely Deformable Pseudocapacitors

In this work, new insights into dependence of electrochemical performance enhancement on transition metals' rich mixed valence and their atomic ratio as well as redox active polysulfides are proposed. Especially, the influence of atomic ratio is further demonstrated by both experiments and density functional theoretical calculation where increasing Co/S leads to the enlargement of both interatom distance and hole diameter in a Mn_xCo_yS_z cell. We rationally designed and prepared novel flexible electrodes of a rich mixed-valence polysulfide Mn_xCo_yS_z/carbon nanotube film (CNTF) through acid activation of a dense CNTF into a hydrogel-like conductive matrix, growth of the Mn_xCo_y(CO₃)_0.5OH precursor on each CNT, and controlled sulfidation. Nanostructure control allows us to obtain fast electron/ion transfer and increased availability of active sites/interfaces. The optimal MnCo₉S₁₀/CNTF shows a specific capacitance reaching 450 F cm^-3 at 10 mA cm^-2, much higher than reported values for CNT-based electrodes. Also, it exhibits remarkable cycling stability with only 1.6% capacity loss after 10 000 cycles at a high current density of 80 mA cm^-2. An all-solid-state asymmetric supercapacitor (ASC) applying MnCo₉S₁₀/CNTF delivers an exceptionally high volumetric energy density of 67 mW h cm^-3 (at 10 W cm^-3). Particularly, integrated electric sources with adjustable output voltages can be obtained by connecting several ASCs in series, and there are no structural failure and capacity loss during repeated large-angle twisting and vigorous hammering. This work provides a general route to energy storage devices with ultrahigh volumetric energy density and outstanding reliability for wearable electronics.