Co-modification by LiAlO2-coating and Al-doping for LiNi0.5Co0.2Mn0.3O2 as a high-performance cathode material for lithium-ion batteries with a high cutoff voltage

Highlights: • The LiAlO₂-coated and Al-doped LiNi_0.5Co_0.2Mn_0.3O₂ have been successfully synthesized. • The coating process is carried on the precursor making the coating layer uniform and continuous. • Cycling performance and rate capability of LiNi_0.5Co_0.2Mn_0.3O₂ has been improved by Al-modification. LiAlO₂-coated and Al-doped LiNi_0.5Co_0.2Mn_0.3O₂ (Al-modified NCM) materials are successfully synthesized via a facile coprecipitation method. The lattice parameters calculated from X-ray diffraction (XRD) patterns reveal that the slab spacing of the modified material is broadened after Al substitution. A continuous and uniform LiAlO₂ coating layer with a thickness of ∼4.3 nm on the surface of Al-doped LiNi_0.5Co_0.2Mn_0.3O₂ is confirmed by transmission electron microscopy (TEM). Al-modified NCM shows remarkable improvements in cycling and rate performance and possesses a high capacity retention of 82.06% at 0.5 C after 200 repeated cycles in the voltage range of 3.0–4.6 V, whereas the capacity retention of pristine NCM is only 60.04%. Even at a high current density and an elevated temperature, the Al-modified sample still shows better cycling performance than that of pristine NCM, with capacity retentions of 67.68% (25 °C) and 57.36% (85 °C) after 100 cycles at 10 C. Furthermore, Al-modified NCM delivers a high discharge specific capacity of 142.73 mAh g⁻¹ at a current density of 10 C. The improved electrochemical performance could be ascribed to the synergistic effect of the LiAlO₂ coating and Al doping. The enlarged slab spacing resulting from Al substitution and the Li⁺-conducting LiAlO₂-coating layer benefits lithium ion diffusion. Moreover, the LiAlO₂-coating layer acts as a protective shell and can suppress the increase in impedance during repeated cycling.