(Invited) Oxygen Defects in Oxide-Based Lithium Ion Battery Cathodes: Formation and Influences on Electrochemical Properties

Abstract

IntroductionDemands on high energy density batteries increase to exploit renewable energies efficiently and develop zero-emission vehicles [1]. Among technological issues for next-generation batteries, we focus on the development of cathode active materials. So far, oxide-based cathode materials have been used in lithium ion batteries because of their excellent electrochemical properties. However, the formation of oxygen defects like oxygen vacancy is inevitable in oxides. There are two important motivations for the study of oxygen defect formation in oxide-based cathodes, one is the safety of batteries and the other is the battery performance. For the safety, it was reported that the reaction between organic liquid electrolytes and released oxygen from oxide-based cathodes is exothermic and cause abnormal heat generation [2]. To prevent catastrophic thermal runaway of battery systems, understanding oxygen release mechanism is important. For the influence on battery performance, it was reported that preliminary reduction treatments for Li-rich cathodes improved capacity, rate capability and capacity retention [3]. To establish the guideline for the oxygen defect engineering for advanced battery materials, it is necessary to understand influences of oxygen defects on electrochemical properties. In the presentation, oxygen vacancy formation behavior in oxide-based cathodes and influences of oxygen vacancy on battery performances are discussed.ExperimentalLi1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 were synthesized by solid state reaction of carbonate precursors and LiOH·H2O. The oxygen vacancy formation was evaluated by thermogravimetry and the coulometric titration using yttria stabilized zirconia. The absolute oxygen content of the sample was determined by iodometric titration. Samples with different oxygen vacancy concentration were also prepared by the coulometric titration method. Influences of oxygen vacancy on crystal and electronic structures were evaluated by X-ray diffraction (XRD) and soft X-ray absorption spectroscopy (XAS). Charge/discharge measurements were performed on the pristine and the oxygen-deficient cathodes.Results and discussionFigure 1 shows the variation of the oxygen content in Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2. In the figure, the vertical axis, O/M, represents the molar ratio of oxygen in and metal components (Li, Ni, Co and Mn) in the target material, meaning that O/M = 1 represents the oxygen stoichiometric composition for Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2, and the horizontal axis represents the equilibrium P(O2) for each oxygen content. With decreasing O/M, P(O2) decreased and reached a constant value regardless of the relative oxygen content. The oxygen nonstoichiometric data in Figure 1 can be roughly classified into two regions depending on the slope of the curve. In the higher P(O2) region where the O/M vs. log P(O2) curve showed a certain slope, meaning both oxygen content and P(O2) changes simultaneously. In the lower P(O2) region, on the other hand, P(O2) is almost invariant regardless of O/M. The high P(O2) region is the oxygen deficient nonstoichiometry region where the oxygen vacancies were randomly distributed in the oxide-based cathodes. On the contrary, the low P(O2) region is the multiphase co-existing region where the sample is decomposed by the electrochemical oxygen extraction and co-exist with the decomposition phases. In this case, the equilibrium P(O2) is fixed with that of the multiphase co-existing state regardless of apparent O/M. The reduction stability of Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 were also confirmed by XRD. The reduction stability limit of Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 were log(P(O2)/bar) = -17.7 and -20.7, respectively. From these evaluations, it can be concluded that Li1.2Mn0.6Ni0.2O2 and Li1.2Ni0.13Co0.13Mn0.54O2 can accept oxygen vacancy up to about 2 mol% and about 3 mol%, respectively. The results of XRD, XAS and battery tests will be shown in the presentation, and influences of oxygen defects on crystal and electronic structures and electrochemical performance will be discussed.