Potassium-Containing α-MnO₂Nanotubes: The Impact of Hollow Regions on Electrochemistry

α-MnO2 type materials have been studied as electrode materials in rechargeable batteries and electrocatalysts due to their 2 × 2 tunneled crystal structures capable of accommodating cations and their tunable physiochemical properties. In this study, we deliberately synthesized K+ containing α-MnO2 (K0.9Mn8O16) hollow nanotubes varying the dimensions of the hollow regions and level of surface defects. The K0.9Mn8O16 nanotube material samples have similar crystallinity, thermal stability, and average Mn oxidation state. Oxygen surface defects in the hollow regions were revealed through detailed studies using electron energy loss spectroscopy. The impact of the hollow regions and associated surface defects on the electrochemistry of KxMn8O16 were investigated using cyclic voltammetry, galvanostatic intermittent titration technique, and galvanostatic cycling. The K0.9Mn8O16 nanotubes with a large hollow region (∼30 nm) and higher level of surface defects show higher apparent lithium ion diffusion coefficients and lower polarization compared to the nanotubes with a small hollow region (∼10 nm). In-situ lithiation demonstrated that the dimensions of the nanotube walls expanded, but the hollow region did not change in size as result of lithiation. This research demonstrates that tuning particle architecture and surface defects can positively impact functional behavior of electrochemical storage materials.