Currently, lithium ion batteries have been extensively used on electric vehicles and other electronic devices to help the decarbonization strategy. To improve their electrochemical performance, more research has been performed on optimization of their cathode materials1. Lithium transition metal oxides, including Li1+xTM1-xO2, Li2TMO3 and LiTM2O4 materials, are common cathode materials for lithium ion batteries. During cycling, their capacity mainly comes from the oxidation and reduction of TM atoms, accompanied by the extraction and insertion of Li atoms, respectively. For some Li1+xTM1-xO2 and Li2TMO3 materials, such as LiNiO2 and Li2MnO3, their O atoms show electrochemical activity with the construction of the Li-O-□ (□ = vacancy) configuration at high voltage2-4. This is followed by oxygen loss, which would accelerate the degradation of these cathode materials through particle cracking5. Some of these O2 molecular would be first trapped in the lattice and released as gas after cycling, resulting in their votlage decay6. However, it is still unclear whether those O atoms in LiTM2O4 materials show oxygen activity during cycling.
Amongst the reported LiTM2O4 materials, Co-free spinel LiMn1.5Ni0.5O4 (LMNO) material is an attractive lithium cathode material that can be operated at high voltage (> 4.7 V) and deliver a comparable energy density of 690 Wh·kg−1. Interestingly, LMNO undergoes oxygen loss at calcination temperatures above 700 °C, while these oxygen vacancies in LMNO can be occupied again after post-annealing7-10. Inspired by these chemical oxygen reactions during preparation of LMNO, resonant inelastic X-ray scattering and differential electrochemical mass spectroscopy techniques are used to explore the electrochemical oxygen activity of LMNO during cycling. Besides, ex-situ X-ray absorption spectroscopy and operando X-ray diffraction measurements were performed to investigate the redox mechanisms of its TM atoms and its phase transitions during cycling. The results show that LMNO contains some peroxo-like dimers in the pristine material. The redox reactions between peroxo-like and superoxo-like dimers during cycling of LMNO would affect its redox reactions of TM atoms. However, no obvious gaseous and molecular O2 is observed during charge and discharge of LMNO. These observations lead to a better understanding of electrochemical oxygen reactions in LiTM2O4 materials.