The role of oxygen release from Li- and Mn-Rich layered oxides during the first cycles investigated by on-line electrochemical mass spectrometry

In the present work, the extent and the role of oxygen release during the first charge of lithium- and manganese-rich Li_1.17[Ni_0.22Co_0.12Mn_0.66]_0.83O₂ (also referred to as HE-NCM) was investigated with on-line electrochemical mass spectrometry (OEMS). HE-NCM shows a unique voltage plateau at around 4.5 V in the first charge, which is often attributed to a decomposition reaction of the Li-rich component Li₂MnO₃. For this so-called “activation”, it has been hypothesized that the electrochemically inactive Li₂MnO₃ would convert into MnO₂ while lattice oxide ions are oxidized and released as O₂ (or even CO₂) from the host structure. However, qualitative and quantitative examination of the O₂ and CO₂ evolution during the first charge shows that the onset of both reactions is above the 4.5 V voltage plateau and that the amount of released oxygen is an order of magnitude too low to be consistent with the commonly assumed Li₂MnO₃ activation. Instead, the amount of released oxygen can be correlated to a structural rearrangement of the active material which occurs at the end of the first charge. In this process, oxygen depletion from the HE-NCM host structure leads to the formation of a spinel-like phase. This phase transformation is restricted to the near-surface region of the HE-NCM particles due to the poor mobility of oxide ions within the bulk. From the evolved amount of O₂ and CO₂, the thickness of the spinel-like surface layer was estimated to be on the order of ≈2–3 nm, in excellent agreement with previously reported (S)TEM data.