Kinetics of the thermally-induced structural rearrangement of γ-MnO2

Abstract

This work presents a temperature-dependent and time-resolved X-ray and neutron diffraction study of the thermally induced structural rearrangement of γ-MnO2. Here, we study electrochemically prepared γ-MnO2, the manganese dioxide phase used in the majority of battery applications, which we find to be ∼64% ramsdellite [a = 4.4351(6) Å, 9.486(2) Å, c = 2.8128(7) Å, and V = 118.33(3) Å3] and ∼36% pyrolusite [a = 4.718(3) Å, c = 2.795(2) Å, and V = 62.22(8) Å3]. 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-MnO2 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) Å3, and pyrolusite, unit cell a = 4.410(2) Å, c = 2.869(2) Å, and V = 55.79(4) Å3, 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-MnO2, which will aid the optimization of γ/β-MnO2 as used in Li-primary batteries. © 2014, American Chemical Society.