Ni-rich NCM-type layered oxide cathode materials are promising candidates to satisfy the increasing energy demand for 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 There are however mayor drawbacks in terms of instability issues and cycling stability. Bulk and surface modifications have been successfully applied to improve the cycle life. However, while aiming towards NCM-type layered oxide materials with more than 90 % nickel, which 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⁺³ is explored on NCM900505 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. Coated cathode materials were characterized via X-Ray diffraction (XRD) and low energy ion scattering (LEIS) to characterize the percentage of surface coverage of the coating. Electrochemical cycling in NCM||Graphite cells was combined with cross-sectional scanning electron microscopy (SEM) investigations of pristine and cycled electrodes to investigate morphological changes and micro-crack formation upon cycling. In addition, the thermal stability of delithiated materials were analyzed via differential scanning calorimetry (DSC) and in-situ high temperature synchrotron X-ray diffraction 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 contrast to that, there is still strong material degradation visible in the cross-section analysis of the electrodes. This leaves the possibility for further improvements such as bulk stabilization via doping or core-shell approaches and indicates the importance to combine both for the successful development of NCM-type layered oxide materials with more than 90 % nickel.
¹ R. Schmuch, R. Wagner, G. Hörpel, T. Placke and 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 and T. Placke, ACS Appl. Mater. Interfaces, 2019, 11, 18404–18414.
³ Z. Gan, G. Hu, Z. Peng, Y. Cao, H. Tong and K. Du, Appl. Surf. Sci., 2019, 481, 1228–1238.