Amorphization and recrystallization study of lithium insertion into manganese dioxide

Abstract

Various polymorphs of MnO₂ are widely used as electrode materials in Li/MnO₂ batteries. Electrolytic manganese dioxide (EMD) is the most electrochemically active form of MnO₂ and is very difficult to characterize. Their structural details are still largely unknown owing to the poor quality of X-ray diffraction (XRD) patterns obtained from most MnO₂ samples. Simulated amorphisation and crystallization technique was used to derive microstructural models for Li–MnO₂ which included most microstructural details that one would expect to find in the real material. Specifically, pyrolusite-MnO₂, comprising about 25 000 atoms, was amorphised (strain-induced) under molecular dynamics (MD) and different concentrations of lithium ions were inserted. Each system was then crystallized under MD simulation. The resulting models conformed to the pyrolusite polymorph, with microstructural features including: extensive micro-twinning and more general grain-boundaries, stacking faults, dislocations and isolated point defects and defect clusters. Molecular graphical images, showing the atom positions for the microstructural features together with simulated XRD patterns they give rise to, are presented and compared with measured XRD. The calculated XRD are in accord with experiment thus validating the structural models.