Mechanistic and structural investigation of Li_xMnO₂ cathodes during cycling in Li-ion batteries

Increasingly there is demand for clean energy sources and suitable batteries to store this energy. Manganese dioxide and lithiated variants are a promising alternative to conventional Li-ion cathodes due to their cost, abundance, safety and electrochemical performance. Cathodes which operate by a single-phase lithium insertion/extraction process can offer some intrinsic advantages over those with two-phase processes. In this work, in-situ and ex-situ synchrotron X-ray diffraction (XRD) is used to investigate the structural evolution and lithium insertion/extraction mechanism of various Li_xMnO₂ cathodes that have been derived from γ-MnO₂. Li_0.30MnO₂ is found to cycle solely with a single-phase mechanism, in contrast to previous literature reports, with only subtle changes in the crystal structure. However, a better cycling discharge capacity is realised through a two-step lithiation synthesis, thermally lithiated Li_0.08MnO₂ which is then electrochemically lithiated to Li_0.33MnO₂. After an irreversible two-phase reaction early in the first discharge, this material cycles by a single-phase reaction with good structural reversibility and a stable unoptimised cycling capacity of 120 mAh/g. Comparing cathodes using a combination of in-situ and ex-situ synchrotron XRD data allows us to rationalise cathodic performance with structure and thereby directing research into promising candidates