Enhanced Structural Stability of Single-Crystalline Ni-Rich Cathode Enables Improved Cyclability in Pouch Cells

Single-crystalline LiNi_0.9Co_0.05Mn_0.05O₂ (SCNCM90) cathode materials experience continuous capacity degradation during cycling, primarily due to irreversible structural transformations and oxygen loss. These alterations are driven by the local adjustment of in-layer and interlayer transition metal ions as a result of anionic and cationic redox reactions. In this study, selenium (Se) and titanium (Ti) were simultaneously incorporated into the SCNCM90 structure to enhance structure stability, inhibit the irreversible reactions of lattice oxygen, and mitigate the severe internal strain induced by phase transformations near the end of the charge. Moreover, Se/Ti structure regulation in the SCNCM90 cathode reduces the Li⁺ migration barrier, suppresses Li/Ni cation mixing during cycling, and further stabilizes the structure of SCNCM90. The formation of O-transition metal -Se bonds during deep charging can reduce the outward migration of O^α- (α < 2) and increase the oxygen vacancy formation energy, thereby improving the stability of anionic and cationic redox processes within SCNCM90. Ti⁴⁺ promotes the formation of a nanoscale cationic mixed-phase layer on the surface of SCNCM90, enhancing the reversibility of the H2-H3 phase transition. Additionally, the alleviation of internal strain and the enhanced stability of lattice oxygen significantly contribute to the long-term cyclic stability of SCNCM90 cathodes. Hence, the modification material achieves a capacity retention of 87.6% after 500 cycles at 1 C with 2.8-4.5 V, compared to only 61.4% for the undoped cathode. A 2.83 Ah pouch cell with SCNCM90-0.6ST||graphite electrodes demonstrates a long cycle life of over 500 cycles, with only a 3.1% capacity loss at 1 C within 3-4.25 V. This work reveals that the mitigation of particle cracking and the suppression of oxygen release by enhancing structural stability are crucial for further improvements in Ni-rich layered cathode materials.