Probing and Resolving the Heterogeneous Degradation of Nickel-Rich Layered Oxide Cathodes across Multi-Length Scales

Ni-rich layered oxides (NRLO) are widely considered among the most promising cathode materials for high energy-density lithium ion batteries. However, the high proportion of Ni content accelerates the cycling degradation that restricts their large-scale applications. The origins of degradation are indeed heterogeneous and thus there are tremendous efforts devoted to understanding the underlying mechanisms at multi-length scales spanning atom/lattice, particle, porous electrode, solid-electrolyte interface, and cell levels and mitigating the degradation of the NRLO. This review combines various advanced in situ/ex situ analysis techniques developed for resolving NRLO degradation at multi-length scales and aims to convey a comprehensive picture of its heterogeneous degradation mechanism. This contribution starts with discussing various factors influencing NRLO stability and proceeds to elaborate the multi-scale characterization, including synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, X-ray imaging, Raman spectroscopy, electron microscopy, online-electrochemical mass spectrometry, and secondary ion mass spectrometry. Further, the detailed degradation mechanisms at each length scale are analyzed, and corresponding strategies to alleviate the degradation are evaluated. By conveying the progress (mainly between the years 2015 and 2020), methods, insights, and perspectives, this review contributes significantly to the understanding and tackling of the cycling degradation of NRLO.