Quantifying the unusual anion redox activity in lithium intercalation compounds*

This work provides novel insights into the oxygen activity and its correlation with the chemical environment of transition metals at the surfaces and sub-surfaces of layered transition metal oxides in lithium ion and sodium ion batteries. The oxygen activity in battery materials are historically challenging to be analyzed due to the lack of proper techniques that can simultaneously probe the unoccupied oxygen 2p and transition metal 3d orbitals. The energy range of soft X-ray covers both the oxygen K-edge and transition metal L-edges, the combination of which can provide precise information on the local transition metal-oxygen (TM-O) octahedral crystal field. We take advantage of unique features of soft X-ray absorption spectroscopy (s-XAS) and electron energy loss spectroscopy (EELS) to investigate the differences in the oxygen activity between the classical layered oxides and Li rich layered oxides and the impact of such difference on the surrounding TM-O environment, during the first cycle and after a number of high voltage cycles. The experimental data will be carefully interpreted with the help of first principles computation. With a quantitative comparison between the classical layered oxides and lithium rich layered oxides, we hope to provide a strategy to effectively control the oxygen activities in layered oxides, especially when guest ion (Li+ and Na+) concentrations are low (high voltage range). Last but not least, we will demonstrate the important role of defects in anion redox active materials.