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Charge Heterogeneity and Surface Chemistry in Polycrystalline Cathode Materials

  • Author(s): Tian, C
  • Xu, Y
  • Nordlund, D
  • Lin, F
  • Liu, J
  • Sun, Z
  • Liu, Y
  • Doeff, M
  • et al.
Abstract

© 2018 Elsevier Inc. Nanoscale full-field (FF) transmission X-ray microscopy (TXM) and ensemble-averaged soft X-ray absorption spectroscopy (soft XAS) were used to investigate state-of-charge (SOC) heterogeneities in electrochemically charged or discharged and chemically oxidized samples of LiNi0.6Mn0.2Co0.2O2 cathode materials. We observed considerable and similar non-uniformities in terms of Ni oxidation states (and, by proxy, lithium distributions) for all the samples in the bulk. Therefore, the chemically delithiated samples are similar to the electrochemically charged samples in terms of mesoscale charge heterogeneity in large polycrystalline particle ensembles. However, the gradient oxidation states of transition metals on the surface, which is partly responsible for the electrode degradation mechanism known as surface reconstruction, is much less apparent in chemically delithiated samples. Chemical delithiation is extensively used in preparation of charged cathodes free of inactive components for fundamental battery studies. The surface chemistry and bulk lithium distribution of chemically delithiated samples may not accurately represent the electrochemically charged materials in a real battery. In our work, bulk and surface-sensitive characterization techniques were used to investigate the differences between chemically and electrochemically delithiated layered cathode materials. The two types of materials show similar bulk lithium distributions but distinctly different surface chemistries. The present study not only strongly implies that the electrolytic solution is an active participant in the reactions that lead to the surface reconstruction to a rock salt structure observed in layered oxide cathode materials, but also develops a chemical basis for understanding and improving charge homogeneity across the length scale of secondary particles. Both chemical and electrochemical delithiation methods lead to state-of-charge non-uniformity in the bulk of layered transition metal oxide cathode particles. Less surface reconstruction to rock salt is observed for chemically delithiated samples compared with electrochemically delithiated ones. This indicates that electrolytic solution plays a significant role in the surface reconstruction phenomenon.

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