Probing the Release and Uptake of Water in a-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 a-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 a-MnO₂ structure during charge, and their reintroduction during discharge. In this investigation, hydrated a-MnO₂ was used, as a first step, to study the release and uptake of oxygen in a-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 a-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 a-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 a-MnO₂. Finally, DFT calculations revealed the likelihood of disordered water species in binding sites in a-MnO₂ tunnels and a facile diffusion process.