Role of Anions in Lithium-Ion Batteries Studied Using Core-Hole X-Ray Spectroscopy

Understanding the role of individual elements present in electrode materials used in lithium-ion batteries (LIB), particularly the significance of polyhedral bonds, helps us optimize them to improve capacity and cycling stability. While much research has focused on the contribution of cations, often the redox centers, the role of anions has been less explored. Core-hole spectroscopy, a technique that uses x-rays to create electron core-holes, is well suited for investigating this due to its element-selectivity and ability to probe materials at sufficient depths for in-situ and operando measurements. X-ray absorption spectroscopy (XAS), one such core-hole technique, measures the process of generating electron core-holes providing insights into electronic and local atomic structure of specific elements. Complementarily, x-ray emission spectroscopy (XES), measures the process of filling generated electron core-holes, offering sensitivity to chemical state, spin state, and ligand environment, making it a valuable counterpart to XAS.

In this work, the role of anions in layered and polyanion-based cathode materials is studied using core-hole methods. Role of oxygen in LiCoO₂ in altering the magnetic properties in the first 10% of lithium removal is examined. It is found that a new phase emerges at approximately 6% of lithium removal, driven by oxygen activity. This phase transition leads to anomalous change in the 3d spin state of cobalt, highlighting the role oxygen is playing in modifying the electronic and magnetic properties of the material. Additionally, the roles of both oxygen and phosphorus in LiFePO₄ (LFP, a polyanion-based cathode material) are investigated using core-hole spectroscopy. Phosphorous is found to be active in the redox process, showing signs of reduction during delithiation. This behavior results from the redistribution of electron density around oxygen atoms shared between iron and phosphorous during charging. The consequence is a reduced capacity, which can be mitigated by altering covalent Fe-O interactions. Furthermore, the effect of transition metal composition in LiNi_xMn_yCo_zO₂ (NMC, x + y + z = 1) on covalent TM-O interactions is studied. It is found that increasing the nickel content reduces TM-O covalency, making the structure less stable against oxygen release, which in turn can negatively impact material’s stability and performance. These findings highlight the critical role anions play in shaping electrochemical behavior in layered and polyanion-based cathode materials.