Improved high rate performance and cycle stability for LiNi0.8Co0.2O2 by doping of the high valence state ion Nb5+ into Li+ sites

Highlights: • Nb⁵⁺ doping of LiNi_0.8Co_0.2O₂ has been achieved using Nb₂O₅ as dopant. • All samples have a well layered structure. • Nb-doped LiNi_0.8Co₀₂O₂ has high thermal stability at the charged state. • The Li/Ni mixing and polarization are mitigated by doping. • Nb⁵⁺ doping of LiNi_0.8Co_0.2O₂ enhances high rate capacity. High rate performance has been a challenging issue for LiNi_0.8Co_0.2O₂ material. Elemental doping is a very effective method that has been used to maintain the structure of cathode materials with high stability and improve the high rate performance. Encouraged by previous research and considering the shortcomings of LiNi_0.8Co_0.2O₂, materials with a composition of Li_1-xNb_xNi_0.8Co_0.2O₂ (x = 0, 0.01, 0.03) were prepared by co-precipitation and the solid phase sintering method. The structure and electrochemical performance were studied in detail. The results from structural analysis suggested that the doping element was successfully doped into LiNi_0.8Co_0.2O₂. Electrochemical measurements suggested that high rate capacities led to distinct improvements for a moderate Nb-doping content. Specifically, the initial capacities delivered by LiNi_0.8Co_0.2O₂ and Li_0.99Nb_0.01Ni_0.8Co_0.2O₂ increased from 97 to 156 mAh/g at 25 °C and 62.1 to 144.7 mAh/g at 50 °C at a rate of 5 C. In addition, the results from differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis demonstrated that the Nb-doped LiNi_0.8Co₀₂O₂ had a higher thermal stability in the charged state compared to the un-doped material. Therefore, the Li⁺ sites in LiNi_0.8Co_0.2O₂ were partially substituted by the high valence element Nb, which can lower Li/Ni mixing and polarization, accelerate the migration rate of Li⁺ and stabilize the structure of the cathode material, thus improving the high rate performance and cycling stability.