Investigation on electrochemical performance of LiNi0.8Co0.15Al0.05O2 coated by heterogeneous layer of TiO2

Highlights: • Bi-functional Ti modification on NCA is introduced for the first time. • Stability of NCA has been improved by the synergies of coating and doping. • Not TiO₂ but Ti-rich and Ni-poor heterogeneous coating layers forms on the surface. Ni-rich cathode materials always suffer from serious side reaction and irreversible phase transition leading to capacity fading and thermal instability, which could be improved by surface coating and elemental doping. However, it is difficult and cumbersome to carry on the coating and doping at the same time. Herein, a facile method of bi-functional Ti modification has been employed on LiNi_0.8Co_0.15Al_0.05O₂ to enhance surface and structural stability via heterogeneous layer coating and bulk doping. The mechanism and synergistic effect of Ti modification has been investigated by XRD, XPS, SEM, TEM and the half-cell test in details. The existence of Ti occupancy in Ni site of the transition metal layer has been confirmed. Besides, a 22 nm heterogeneous layer has been detected on the particle surface and the composition has been analyzed. Ti bulk doping can reduce the cation mixing degree, and stabilize the lattice due to the pillar effect and charge compensation. Moreover, the heterogeneous coating layer could protect the cathode particles from hydrofluoric acid attack and reduce the decomposition of electrolyte during cycling. With the synergistic effects of heterogeneous layer coating and bulk doping, NCA-T2 exhibits the highest initial capacities of 162.9 and 182.4 mAh·g⁻¹ at 1C and 0.1C, and the discharge capacity retentions of 1C cycling reach 85.0% after 200 cycles.