Ex situ and in situ Raman microscopic investigation of the differences between stoichiometric LiMO2 and high-energy xLi2MnO3·(1–x)LiMO2 (M = Ni, Co, Mn)

Layered mixed transition metal oxides of the type LiMO2 (M = Ni, Co, Mn), known as stoichiometric NCM for equal amounts of transition metals, have been proposed as positive electrode materials for lithium-ion batteries. If overlithiated, these compounds can be written as xLi2MnO3·(1–x)LiMO2, are referred to as high-energy NCM and show higher specific charges and improved cycling stabilities. For the first time, the Raman spectroscopic changes occurring in stoichiometric and high-energy NCM upon electrochemical cycling under identical conditions are compared. The former was synthesised via the sol-gel method while the latter was obtained commercially. The compounds were characterised by scanning electron microscopy and X-ray diffraction. Ex situ Raman measurements of NCMs with increasing overlithiation revealed a shift of the A1g band towards the Ag band of Li2MnO3, whereas no such shift was observed after electrochemical cycling, thus lending support to the Li2MnO3 domain model and the irreversibility of Li2MnO3 activation, respectively. Finally, in situ Raman spectra of stoichiometric and high-energy NCM exhibited the emergence of a new reversible band at ∼545 cm−1, which was stable over a larger potential range in the latter compound.