Fine microstructure tuning via titanium doping in ultrahigh-nickel cathodes to enhance the reversibility of the H2/H3 phase transition and reduce micro-cracks

Abstract

Due to increasing demands for electric vehicles with high energy density and low costs, ultrahigh-nickel LiNixCoyM1-x-yO2 (x≥0.9, M=Mn/Al) cathodes have attracted increasing attention. However, the preparation of ultrahigh-nickel cathode materials with satisfactory cycle stability remains challenging because of their structural and interface instability. In this study, the effects of Ti-doping on the performance of Li(Ni0.95Co0.05Mn0.01)1-xTixO2 (NCMT) were systematically investigated. Titanium doping enlarged the spacing between the lithium layers and increased the c/a ratio, thereby improving the structural stability of the cathode materials. In situ X-ray diffraction showed that the anisotropy change was significantly reduced after Ti doping. The reversibility of the H2/H3 transition was enhanced and the initiation of microcracks caused by the anisotropy change was restrained. Moreover, the emission structure induced by Ti doping effectively mitigated stress accumulation and restrained the propagation of microcracks. Therefore, the optimal NCMT cathode (with 1 mol.% of Ti doping) demonstrated an outstanding rate performance and cycle stability without capacity loss. Compared with the retentions of the pristine cathode without Ti doping (89.73 % in a coin cell and 51.16 % in a pouch cell), the NCMT cathode with 1 mol.% Ti maintained a capacity retention of 92.45 % in a coin cell after 100 cycles at 0.5 C and 70.49 % in a pouch cell after 1000 cycles at 1 C. This study provides an effective solution to realize stable cyclability without capacity loss in ultrahigh-nickel cathode materials.