Li_1-xCoO₂ is the first commercialized and the most common active material in Li-ion cathodes, owing to its stable electrochemical performance in terms of cycle and rate capability. However, despite its high theoretical energy density (1070 Wh kg^-1), LiCoO₂ has only achieved relatively low gravimetric energy density (< 630 Wh kg^-1) due to an irreversible phase transition after Li extraction of more than x = 0.6 (into Li_1-xCoO₂). On the other hand, Ni-rich layered materials (LiNi_xM_1-xO₂, M = Co or Mn, x > 0.6) have higher gravimetric energy density (> 720 Wh kg^-1) than LiCoO₂, in spite of the same Rm structure and similar theoretical capacity, because of their reversible Li extraction up to more than x = 0.7. However, the Ni-rich materials still have similar volumetric energy densities (< 2300 Wh L^-1) with LiCoO₂, due to their lower electrode densities (< 3.0 g cm^-3, compared to < 4.0 g cm^-3 for LiCoO₂). Therefore, a better way to achieve high volumetric energy density is increasing the electrochemical reversibility of LiCoO₂ to levels comparable to that of Ni-rich materials.

Previous studies about layered-structured materials mainly focused on improving their electrochemical reversibility, but fundamental investigation regarding different electrochemical reversibility between LiCoO₂ and Ni-rich material was not a main issue and related studies are conducted base upon only chemical de-lithiation and simulation data.

Herein, we directly examine the two materials' structural changes and different irreversible deterioration mechanisms during cycling. In-situ X-ray diffraction patterns and theoretical calculation suggest that the phase transition of LiCoO₂ from O3 oxygen stacking to O1 or O2 oxygen stacking is caused by increased oxygen-oxygen electrostatic repulsion during the charge process. Furthermore, we investigated a critical role of cations in lithium sites in layered-structured cathode materials. In case of Ni-rich materials, migration of Ni ions into the Li vacancies in Ni-rich materials during charge, a process called as 'cation mixing', screens the oxygen-oxygen repulsion that arises from lithium vacancies. This suppresses lattice parameter expansion along the c-axis, and therefore delays the phase transition although the cation mixing is cause of structural failure in Ni-rich material upon further Li extraction. As a result, LiCoO₂ with 10% nickel-doped (LiCo_0.9Ni_0.1O₂) displayed much enhanced electrochemical reversibility, which was increased by 0.1 mol lithium ion extraction with maintaining its electrode density of ~ 4.0 g cm^-3.