Tunnel Structured α-MnO2 with Different Tunnel Cations (H+, K+, Ag+) as Cathode Materials in Rechargeable Lithium Batteries: The Role of Tunnel Cation on Electrochemistry

Abstract

α-MnO2 type manganese dioxide is an interesting prospective cathode material for reversible lithium insertion owing to its cation accessible tunnels (0.46 nm × 0.46 nm), high voltage, and low cost. The tunneled structure is synthetically formed by the assistance of cations acting as structure directing agents where the cations may remain in the tunnel. The electrochemistry of this family of materials is strongly dependent on the morphological and physicochemical (i.e. surface area, crystallite size, and average manganese oxidation state) properties as well as tunnel occupancy. In this work, we prepared a set of materials Mn8O16·0.81H2O, K0.81Mn8O16·0.78H2O and Ag1.33Mn8O16·0.95H2O with similar nanorod morphology, crystallite size, surface area, and tunnel water content. This set of samples allowed us to investigate the role of tunnel cations in the electrochemistry of α-MnO2 type manganese dioxide in a lithium based environment while minimizing the effects of the other parameters. The electrochemistry was evaluated using cyclic voltammetry, galvanostatic cycling, rate capability, and galvanostatic intermittent titration type testing. Mn8O16·0.81H2O showed higher loaded voltages, improved capacity retention, and higher specific energy relative to K0.81Mn8O16·0.78H2O and Ag1.33Mn8O16·0.95H2O. After 100 cycles, Mn8O16·0.81H2O delivered ∼200% more capacity than Ag1.33Mn8O16·0.95H2O (64 vs. 129 mAh/g) and ∼35% more capacity than K0.81Mn8O16·0.78H2O (85 vs. 129 mAh/g). Mn8O16·0.81H2O also showed higher effective lithium diffusion coefficients (DLi+) and higher rate capability compared to K0.81Mn8O16·0.78H2O and Ag1.33Mn8O16·0.95H2O suggesting faster Li+ ion diffusion in the absence of large metal tunnel cations.