Operando high-Energy Synchrotron X-Ray Diffraction and Modeling of Alpha-MnO2 Battery Materials upon Heat Treatment

Hybrid Li-ion/Lithium-O₂ batteries offer extremely attractive theoretical energy densities and therefore represent a rapidly emerging area of research. Our research has focused on developing dual-functioning metal oxides electrode/electrocatalyst materials that can lower the charge hysteresis and potentially impact the nature of the discharge products. Specifically we have synthesized and characterized the transition metal oxide, α–MnO₂, which has a hollandite-type structure with relatively large, one-dimensional (1-D) ‘2x2’ tunnels formed by the connection of octahedral [MnO₆] units, operates as a superior electrode/catalyst in hybrid Li-ion/Li-O₂ cells. α–MnO₂ generally contains 2~3% of lattice water in the crystal structure, which stabilizes the structure and dramatically influences not only electrochemical properties but also other properties such as density and electronic conductivity. The water can be removed from α-MnO₂ framework by heat treatment without degradation of the structure, and then partially replaced by Li₂O. The Li₂O-doped alpha-MnO₂ electrodes, stabilize the structure and provide higher capacities on cycling than the parent material. However, the structure changes of hydrated and dehydrated MnO2 related with electrochemical properties has not been clarified yet.  Herein we report operando temperature-resolved high energy X-ray diffraction experiments performed on alpha-MnO₂ electrodes. The data provides evidence that the crystal structure of α-MnO₂ undergoes reversible changes in lattice parameters and strain during heating and cooling. Insights into reversible changes of tunnel structure of MnO₂ with H₂O/H₃O⁺, as determined by first principles density functional theory calculations, are used to provide a possible explanation for some of the observed results. With the help of the proposed technique, we are expecting to understand the water removal and insertion into MnO­₂ and generate knowledge on how to improve the sustainability in the practical usage of α-MnO2 as Li and oxides capture medium for hybrid Li-ion/Li-O2 cells.