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-O-2 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 alpha-MnO2 structure. It has been speculated that the electrocatalytic process in Li-O-2 cells may be linked to the removal of lithium and oxygen from the host alpha-MnO2 structure during charge, and their reintroduction during discharge. In this investigation, hydrated alpha-MnO2 was used, as a first step, to study the release and uptake of oxygen in alpha-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 alpha-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 alpha-MnO2 in several stages during heating-in cluding 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 alpha-MnO2. Finally, DFT calculations revealed the likelihood of disordered water species in binding sites in alpha-MnO2 tunnels and a facile diffusion process.