Kinetics of the Thermally-Induced Structural Rearrangement of γ-MnO 2

This work presents a temperature-dependent and time-resolved X-ray and neutron diffraction study of the thermally induced structural rearrangement of γ-MnO 2 . Here, we study electrochemically prepared γ-MnO 2, the manganese dioxide phase used in the majority of battery applications, which we find to be ∼64% ramsdellite [ a = 4.4351(6) Å, b = 9.486(2) Å, c = 2.8128(7) Å, and V = 118.33(3) Å 3 ] and ∼36% pyrolusite [ a = 4.718(3) Å, c = 2.795(2) Å, and V = 62.22(8) Å 3 ]. Taking a deeper look at the kinetics of the structural rearrangement, we find two steps: a fast transition occurring within 4–8 min with a temperature-dependent ramsdellite to pyrolusite transformation (rate constant 0.11–0.74 min –1 ) and a slow transition over 4 h that densifies (with changes in unit cell and volume) the ramsdellite and pyrolusite phases to give structures that appear to be temperature-independent. This effectively shows that γ/β-MnO 2 prepared in the range of 200–400 °C consists of temperature-independent structures of ramsdellite, unit cell a = 4.391(1) Å, b = 9.16(5) Å, c = 2.847(1) Å, and V = 114.5(6) Å 3, and pyrolusite, unit cell a = 4.410(2) Å, c = 2.869(2) Å, and V = 55.79(4) Å 3, with a temperature-dependent pyrolusite fraction between 0.45 and 0.77 and increasing with temperature. Therefore, we have linked the temperature and time of heat treatment to the structural evolution of γ-MnO 2, which will aid the optimization of γ/β-MnO 2 as used in Li-primary batteries.