Advances and perspectives in understanding the structure-redox relationship of layered Li-Co-Ni-Mn oxide cathode materials

A comprehensive understanding of the relationship between the structure (electron/bulk/surface structures) and redox chemistry in the cathodes was discussed in this Review. First, the attention is given to the comparison of different layered Li-Co-Ni-Mn oxide cathodes, especially the bulk atomic configuration (Section 2.1). Second, corresponding to the distinct layered structure, the electronic structures, Fermi level energies of different redox couples are introduced (Section 2.2). The structural failures induced by the redox chemistry at the deep lithiation state, including bulk phase transition, surface structure degradation, as well as the resulting cracking, cation mixing, oxygen release, dissolution of metal cations, voltage fading and low initial Coulombic efficiency, are discussed (3.1 Co-rich cathode LiCoO, 3.1.1 Bulk phase transition, 3.1.2 Surface degradation, 3.2 Ni-rich LiNi, 3.2.1 Cation mixing, 3.2.2 Microcracks, 3.2.3 Reversible/irreversible oxygen redox, 3.3 Li-Mn-rich). Correspondingly, the strategies for stabilizing the structural stability by regulating the redox activity, including bulk atomic doping design, surface engineering, cations mixing, particle morphology, oxygen vacancy and oxygen stacking type, are summarized (4.1 Co-rich LiCoO, 4.1.1 Bulk doping elements, 4.1.2 Surface engineering, 4.2 Ni-rich LiNi, 4.2.1 Suppressing Li/Ni cations mixing, 4.2.2 Suppressing microcracking, 4.2.3 Single crystal, 4.2.4 Oxygen redox chemistry, 4.3 Li-Mn-rich). The advanced characterization techniques, such as X-ray, electron, neutron and nuclear magnetic resonance techniques, are summarized for detecting the cationic/anionic charge state (5.1 X-ray techniques, 5.2 Electron microscopy, 5.3 Neutron scattering, 5.4 Nuclear magnetic resonance). In the last section (Section 6), the promising strategies and future perspectives are highlighted to propel significant breakthroughs in developing high-energy-density LIBs.