Mechanism of Oxygen Release in Layered Ncm Cathode Materials for Li-Ion Batteries

Introduction Lithium-transition metal oxides with the layered rock-salt structure are widely used as cathodes for Li ion batteries (LIBs) which achieve high working voltage and high gravimetric/volumetric capacity. However, oxide-based cathodes may release oxygen during the battery operation, causing electrochemical degradation and side reactions between the cathode active material and organic liquid electrolytes. The side reactions are exothermic and can trigger thermal runaway in the worst case. To achieve both high energy density and safety, it is important to understand the mechanism of oxygen release from oxide-based cathode materials. In this work, we evaluated the oxygen release behavior in Li(Ni1/3Co1/3Mn1/3)O2 (NCM111) and Li(Ni0.5Co0.2Mn0.3)O2 (NCM523) by applying coulometric titration technique; crystal structure and charge compensation are discussed by analyzing X-ray diffraction and soft X-ray absorption spectroscopy. The highlight of this work is that the materials maintained the original layered structure with more than 3% oxygen loss, which is meaningful for providing complementary information to researchers who have interests on the thermochemical stability of NCM materials. Experimental NCM powders were synthesized by solid state reaction between carbonate precursors and LiOH·H2O. Oxygen release behavior was evaluated by coulometric titration using yttria stabilized zirconia as an oxide ion conductor. Samples with different oxygen deficiency were also prepared by coulometric titration method. X-ray diffraction (XRD) and Rietveld refinement were carried out on the pristine and oxygen deficient samples to evaluate the crystal structure variation. Soft X-ray absorption spectroscopy (SXAS) by the fluorescence yielding (FY) mode was performed to measure the transition metal L edge and investigate the charge compensation mechanism. Results and discussion Oxygen release behavior at 873 K of NCM111 is shown in Fig. (a). The vertical axis represents the molar ratio of the extracted oxygen from the sample and the sum of metals (Li, Ni, Co and Mn) in the structure. According to XRD, the sample accepted more than 3% oxygen deficiency without phase transition and decomposition. The XRD patterns and results of structure refinement would be shown in the presentation. The slope of the oxygen nonstoichiometric behavior is steep in the higher P(O2) region, moderate in the intermediate P(O2) region and becomes steep again in the lower P(O2) conditions. A plateau behavior at the middle P(O2) region was formed at around ΔO/M=-0.02. According to XAS, this compositional plateau is caused by the transition of redox species, Ni3+/Ni2+ redox in higher P(O2) region and Co3+/Co2+ transition in lower P(O2) region. Results of the crystal structure variation with oxygen deficiency was investigated by XRD and Rietveld refinement assuming structure. Fig. (b) and (c) are the lattice parameters and d-spacing of Li and transition metal slabs, respectively. Reduction expansion was clearly observed in NCM111. Although oxygen extraction induced the lattice expansion in all dimensions, the transition metal layers expanded and Li layers contracted by the introduction of oxygen deficiency. Structural changes due to oxygen deficiency narrows the space in Li slab, and may impede the Li diffusion. In the presentation, additional results of NCM523 will be shown and compared with NCM111 in detail, including cation mixing and charge compensation, as effects of oxygen deficiency concentration on the layered structure. Figure 1