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 0.22 Co 0.12 Mn 0.66 ] 0.83 O 2 (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 Li 2 MnO 3 . For this so-called "activation", it has been hypothesized that the electrochemically inactive Li 2 MnO 3 would convert into MnO 2 while lattice oxide ions are oxidized and released as O 2 (or even CO 2 ) from the host structure. However, qualitative and quantitative examination of the O 2 and CO 2 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 Li 2 MnO 3 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 O 2 and CO 2 , 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.