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

Alpha-MnO 2 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 Ba 2+, K +, NH 4 +, and H 3 O + (or water, H 2 O). When stabilized by H 3 O + /H 2 O, the protons can be ion-exchanged with lithium to produce a Li 2 O-stabilized α-MnO 2 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 α-MnO 2 structure during charge, and their reintroduction during discharge. In this investigation, hydrated α-MnO 2 was used, as a first step, to study the release and uptake of oxygen in α-MnO 2 . 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-MnO 2 tunnels. The two stages correspond to H 2 O release from intercalated H 2 O species at lower temperatures and H 3 O + species at higher temperature. Thermogravimetric analysis confirmed the release of oxygen from α-MnO 2 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-MnO 2 . Finally, DFT calculations revealed the likelihood of disordered water species in binding sites in α-MnO 2 tunnels and a facile diffusion process.