On the Origin of Reactive Oxygen Species in Lithium-Excess Layered Oxide Cathode Materials

Abstract

Lithium-excess layered oxide cathode materials (Li(1+x)TM(1-x)O2) for lithium-ion batteries achieve high specific capacities (~250 mAh/g) via redox participation of oxygen anions as well as the transition metals in the oxide cathode. While oxygen is initially present as O2- in the cathode, oxidized oxygen species such as peroxo-like oxygen (O2 2-) and oxygen gas (O2) are detected on charge. Possible sources of these oxygen species include both the lattice and cathode surface contaminants such as Li2CO3 and LiOH. In this work, differential electrochemical mass spectrometry (DEMS) is used to study the mechanisms by which these oxygen species form and react within the battery. DEMS results show that organic fragments containing diatomic oxygen are present on the cathode surface during the first charge starting around 4.2 V vs. Li/Li+. Since no O2 moieties exist in the carbonate electrolyte, these fragments must be a result of reactive oxygen attack on the electrolyte. In order to further understand the origin of these fragments, controlled amounts of 18O-enriched Li2CO3 are deposited on the cathode surface via reaction with CO2 gas before cycling. These results offer insight into the influence of surface contaminants and oxygen activity on interfacial reactivity in lithium-excess cathode materials.