Probing the Release and Uptake of Water in α-MnO2·xH2O

Alpha-MnO₂ is of interest as a cathode material for 3 V lithium batteries and as an electrode/electrocatalyst for higher energy, hybrid Li-ion/Li–O₂ systems. It has a structure with large tunnels that contain stabilizing cations such as Ba²⁺, K⁺ , NH₄⁺ , and H3O⁺ (or water, H₂O). When stabilized by H₃O⁺ /H₂O, the protons can be ion-exchanged with lithium to produce a Li₂O-stabilized α-MnO₂ structure. It has been speculated that the electrocatalytic process in Li–O₂ cells may be linked to the removal of lithium and oxygen from the host α-MnO₂ structure during charge, and their reintroduction during discharge. In this investigation, hydrated α-MnO₂ was used, as a first step, to study the release and uptake of oxygen in α-MnO₂. Temperature-resolved in situ synchrotron X-ray diffraction (XRD) revealed a nonlinear, two-stage, volume change profile, which with the aide of X-ray absorption near-edge spectroscopy (XANES), redox titration, and density functional theory (DFT) calculations, is interpreted as the release of water from the α-MnO₂ tunnels. The two stages correspond to H₂O release from intercalated H₂O species at lower temperatures and H₃O⁺ species at higher temperature. Thermogravimetric analysis confirmed the release of oxygen from α-MnO₂ in several stages during heating–including surface water, occluded water, and structural oxygen–and in situ UV resonance Raman spectroscopy corroborated the uptake and release of tunnel water by revealing small shifts in frequencies during the heating and cooling of α-MnO₂. Lastly, DFT calculations revealed the likelihood of disordered water species in binding sites in α-MnO₂ tunnels and a facile diffusion process.