Investigation of the Influence of a WO₃ Coating on High-Ni NCM-Type Layered Oxide Cathode Materials

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Ni-rich Li(Ni,Co,Mn)O₂ (NCM)-type layered oxide cathode materials are promising candidates to satisfy the increasing energy demand for lithium ion batteries for automotive applications. The main advantages of increasing the nickel content lies on an increased energy density on the material level and the reduction of cobalt as critical raw material.¹ However, there are major drawbacks in terms of instability issues and cycling stability. Bulk and surface modifications have been successfully applied to improve the cycle life. Nevertheless, while aiming towards NCM-type layered oxide materials with more than 90 % nickel is crucial to achieve sufficient energy density to enable extensive market penetration of electric vehicles, further understanding of the challenges remaining after solely one modification method is lacking.
In this work, the effect of WO₃ as promising surface coating²⁻⁴ is explored on LiNi₀̣ ₉₀Co₀̣ ₀₅Mn₀̣ ₀₅O₂ and its effects on bulk and thermal stability are thoroughly investigated. To ensure understanding of the improvements that solely result from the post processing treatment, a heat treated sample is also compared. All cathode materials are characterized 𝑣𝑖𝑎 X-Ray diffraction (XRD) and low energy ion scattering to characterize the percentage of surface coverage of the coating. Electrochemical cycling in NCM||graphite cells is combined with cross-sectional scanning electron microscopy investigations of pristine and cycled electrodes to investigate morphological changes and micro-crack formation upon cycling. In addition, the thermal stability of delithiated materials is analyzed 𝑣𝑖𝑎 differential scanning calorimetry and in-situ high temperature synchrotron XRD measurements to investigate the mitigation of possible safety issues.
The modified samples show improved cycling stabilities and smaller overall heat flows of the delithiated materials without electrolyte contact. The WO₃-coated material not only shows high discharge capacities and the best cycling stability, but also a slightly higher degradation onset temperature in contact with the electrolyte. In addition, the phase transformation of delithiated materials from a layered oxide structure to a spinel-type structure which is accompanied by O₂ loss⁵ is shifted to higher temperatures indicating improvements in the safety aspect. However, there are bulk instabilities remaining which leaves the possibility for further improvements such as bulk stabilization 𝑣𝑖𝑎 doping or particle design approaches and indicates the importance to combine both for the successful development of NCM-type layered oxide materials with more than 90 % nickel in the future.

¹ R. Schmuch, R. Wagner, G. Hörpel, T. Placke, M. Winter, Nat. Energy 2018, 3, 267–278.
² D. Becker, M. Börner, R. Nölle, M. Diehl, S. Klein, U. Rodehorst, R. Schmuch, M. Winter, T. Placke, ACS Appl. Mater. Interfaces 2019, 11, 18404–18414.
³ Z. Gan, G. Hu, Z. Peng, Y. Cao, H. Tong, K. Du, Appl. Surf. Sci. 2019, 481, 1228–1238.
⁴ F. Reissig, M. A. Lange, L. Haneke, T. Placke, W. G. Zeier, M. Winter, R. Schmuch, A. Gomez‐Martin, ChemSusChem 2021, DOI 10.1002/cssc.202102220.
⁵ S.-M. Bak, E. Hu, Y. Zhou, X. Yu, S. D. Senanayake, S.-J. Cho, K.-B. Kim, K. Y. Chung, X.-Q. Yang, K.-W. Nam, ACS Appl. Mater. Interfaces 2014, 6, 22594–22601.

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