Electronic Structure Changes As Source of Chemical and Interfacial Instability in LiCoPO4 As Positive Electrode for High Voltage Li-Ion Batteries

Abstract

The energy storage capability of a battery scales with the potential difference between its electrodes. Yet operation of positive electrode materials at high potentials introduces challenges of stabilization of charged states. As of today, no positive electrode material has been demonstrated to durably and safely operate above 4.5 V vs. Li+/Li0. LiCoPO4-based electrodes theoretically offer high specific capacity and high potentials of operation, around 4.8V vs. Li+/Li0, but these electrodes are prone to failure during cycling. Failure occurs through chemical and structural degradation in the bulk of the active material or at its interfaces with cell components, especially the electrolyte. The development of Li-ion battery electrodes operating at high potential is indispensable to meet the specific energy target of 250 kWh/kg at the packaged cell level. Changes in the electronic structure and chemical stability of olivine-type LiCoPO4 and Fe-substituted LiCoPO4 were explored as both a function of ion substitution and oxidation state. Soft Ex situX-Ray absorption spectroscopy (XAS) made it possible to compare the changes in chemical bonding between electrode bulk and surface as a function of lithium content. This technique can probe the density of states at the transition metal and O levels. The evolution of these levels revealed changes in the metal-oxide covalence when lithium was deintercalated from the structure, and, thus, the material was oxidized. An increase in covalence can lead to the destabilization of the anions. If this process takes place in the bulk of the material, this destabilization can lead to thermal degradation via oxygen loss. At the surface, even small degrees of destabilization are sufficient to produce oxidizing species that attack the electron-rich solvent molecules in the electrolyte, leading to irreversible capacity loss. Increased metal-oxygen covalence was universally observed in the spectroscopy in the form of a rising pre-O K-edge peak at ~530 eV as a function of lithium deintercalation in both bulk and surface. However, accompanying changes in the Co spectroscopy were only observed in the Fe-substituted sample. These changes are also indicative of increased hybridization between Co 3d and O 2p orbitals. Co K-edge XANES and EXAFS experiments further corroborated these findings, in which virtually no changes in the Co K-edge were observed upon oxidation of unsubstituted LiCoPO4. Fe doping appears to play a substantial role in getting Co to participate in redox chemistry, and the mechanism by which it occurs is currently being explored using Density Functional Theory. Fe-substituted LiCoPO4 is an exciting new positive electrode material that may prove useful in advancing Li-ion battery technology.