Clarifying Anionic Redox Chemistry in LiCoO2 By Direct Detection of O-O Bond Length and First-Principle Investigations

Abstract

Developing high energy density lithium ion battery cathode materials requires exploring novel chemistries that go beyond the conventional redox couple which is solely based on transition metal. Anionic redox reaction that activates lattice oxygen anions during charging and discharging can almost double the capacity of conventional cathode and thereby significantly increases the energy density. The nature of chemical bonding, in particular whether or not oxygen anions can bond in forming dimers or peroxo-like species, is critical to the reversibility of anionic redox couple and consequent cyclability of the material. Unfortunately, because oxygen scatters X-ray weakly and dimers may form locally rather than globally, it is very challenging to characterize the oxygen-oxygen bond unambiguously using conventional structural study tools like XRD. Here it is shown that neutron pair distribution function (NPDF) analysis is an effective tool in probing the existence of oxygen dimers without limitation to long range order. This technique is applied to LiCoO2 which still attracts lots of attention due to its high packing density and potential high energy density (delivers around 220 mAh/g when charged to 4.6 V). Through combining experimental results with theoretical calculations, the nature of anionic redox reaction is unraveled and it is shown that the structure of LiCoO2 can be fairly robust against oxygen release, suggesting the relative stability of the bulk structure and the possibility of fulfilling its potential of high energy density. Acknowledgement The work done at Brookhaven National Laboratory were supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program, including Battery500 Consortium under contract DE-SC0012704. Research conducted at ORNL's Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.