Recent breakthroughs and perspectives of high-energy layered oxide cathode materials for lithium ion batteries

Ni-rich layered oxides (NRLOs) and Li-rich layered oxides (LRLOs) have been considered as promising next-generation cathode materials for lithium ion batteries (LIBs) due to their high energy density, low cost, and environmental friendliness. However, these two layered oxides suffer from similar problems like capacity fading and different obstacles such as thermal runaway for NRLOs and voltage decay for LRLOs. Understanding the similarities and differences of their challenges and strategies at multiple scales plays a paramount role in the cathode development of advanced LIBs. Herein, we provide a comprehensive review of state-of-the-art progress made in NRLOs and LRLOs based on multi-scale insights into electrons/ions, crystals, particles, electrodes and cells. For NRLOs, issues like structure disorder, cracks, interfacial degradation and thermal runaway are elaborately discussed. Superexchange interaction and magnetic frustration are blamed for structure disorder while strains induced by universal structural collapse result in issues like cracks. For LRLOs, we present an overview of the origin of high capacity followed by local crystal structure, and the root of voltage hysteresis/decay, which are ascribed to reduced valence of transition metal ions, phase transformation, strains, and microstructure degradation. We then discuss failure mechanism in full cells with NRLO cathode and commercial challenges of LRLOs. Moreover, strategies to improve the performance of NRLOs and LRLOs from different scales such as ion-doping, microstructure designs, particle modifications, and electrode/electrolyte interface engineering are summarized. Dopants like Na, Mg and Zr, delicate gradient concentration design, coatings like spinel LiNi0.5Mn1.5O4 or Li3PO4 and novel electrolyte formulas are highly desired. Developing single crystals for NRLOs and new crystallographic structure or heterostructure for LRLOs are also emphasized. Finally, remaining challenges and perspectives are outlined for the development of NRLOs and LRLOs. This review offers fundamental understanding and future perspectives towards high-performance cathodes for next-generation LIBs.