Correlating Local Structure with Electrochemical Activity in Li₂MnO₃

Li₂MnO₃ is believed to be a critical component of the high capacity Li-rich-manganese-rich oxide materials; however, the mechanism of its electrochemical activity remains controversial. Here, Raman spectroscopy and mapping are used to follow the chemical and structural changes that occur in Li₂MnO₃ during electrochemical cycling. Conventional composite electrodes cast from a slurry and thin films are studied as a function of the state of charge (voltage) and cycle number. Thin films have similar electrochemical properties as electrodes prepared from slurries but enable spectroscopy of uniform samples without carbon additives and binder. First-principles density functional theory is used to calculate the phonon spectra and identify the Raman-active modes. On the basis of the calculations of phonon spectra for pristine Li₂MnO₃ and structures with Li vacancies, we discuss the origin of Raman-active peaks observed during the electrochemical cycling. The spectral changes correlate well with the electrochemical behavior and support a mechanism whereby capacity is lost upon extended cycling due to the formation of new manganese oxide phases.