Uncovering the Mysterious Transformations during Cycling in Li-Ni-Mn-O Cathode Materials

Abstract

Layered transition metal oxides, specifically LiNixMnyCozO2, lead the market for Li-ion battery cathode materials. Cobalt is a problematic metal to include in such a high-volume product like Li-ion batteries, as it is expensive, and it is mined using unethical practices. Contemporary electrodes use metal compositions with < 0.33 Co. However, even as the least abundant element in the material, eliminating Co entirely remains an important objective, but is so far necessary as it increases material stability leading to improved cycle life. Thus, research has been active in the Li-Ni-Mn-O system, looking for potential replacements for Co-containing layered oxides. The Li-Ni-Mn-O pseudo ternary system has been investigated in the past decade, revealing some interesting materials. Several compositions (including Li0.568Ni0.073Mn0.359O2 studied here) have been uncovered that show anomalous increasing capacity while cycling over 100 cycles, with some materials reaching 300 % capacity compared to their first cycle.1 It has been hypothesized that this is due to a phase transition occurring in the material, but to date the precise mechanism remains elusive. Other materials (including Li0.586Ni0.071Mn0.343O2 studied here), though very near to these compositions, show relatively stable capacities but dramatic voltage fade.2 In both materials, structural transformations are clearly at play during extended cycling. This work leverages X-ray absorption spectroscopy (XANES/EXAFS) and X-ray photoemission spectroscopy (XPS) to uncover the subtleties in the local environments of Ni and Mn before, during, and after cycling. XANES/XPS reveal to what extent there is a contrast between the oxidation states of the Ni/Mn cations at the surface vs. the bulk of the particles, which has been shown to be important in Ni containing cathodes.3 Interestingly, in both materials, no important changes are seen in the X-ray diffraction patterns, while the pair distribution functions from EXAFS evolve dramatically as shown in the figure here for the Ni environment. Interestingly, the material that shows capacity growth shows a less reversible , while there is no significant difference in the Mn site between the materials before and after charging. Analysis of the final structures shows conversion towards spinel-like structures in both cases, despite the dramatic differences in progression of the capacities, hinting that the spinel-like phases can behave very differently depending on composition. Figure 1