Enhanced Cycling Performance of LiNiO₂ in Li-Ion Batteries via Nb-Based Surface Coating

Li-ion batteries (LIBs) have emerged as a leading choice for electric vehicles, with key advancements focusing on increasing energy density and rate capability while reducing costs, particularly in the positive electrode (cathode). LiNiO₂ (LNO), one of the end members of the LiNi_xCo_yMn_zO₂ (NCM or NMC) family of layered oxides, offers a high specific capacity but suffers from accelerated degradation during battery operation. To enhance its bulk and surface stability, common strategies include doping and protective coating. Niobium oxides, such as LiNbO₃, Li₃NbO₄, and Nb₂O₅, have been highlighted in numerous studies for their benefits as coatings to increase cycling stability and rate capability by reducing surface/interface degradation. Therefore, in this study, we investigate Nb-based coatings on LNO, using techniques that minimize or completely eliminate solvent use. Two methods were explored: dry-coating (DRC) and incipient wetness impregnation (IWI). In the dry-coating method, LNO was ball-milled with hydrated niobium oxide (Nb₂O₅.nH₂O), chosen for its amorphous nature, which typically exhibits a higher surface area and reactivity compared to crystalline Nb₂O₅. The second method, incipient wetness impregnation (using an ethanolic solution of Nb(C₂H₅O)₅) was selected since it requires minimal solvent to achieve a uniform distribution through capillary action. Nb-coated samples (1 mol.% of Nb) underwent treatment at two different temperatures (400 °C and 700 °C under O₂ atmosphere) to explore their impact on the material’s properties and electrochemical performance. In principle, from XRD analysis, it was found that low concentrations of niobium did not significantly alter the LNO structure. However, higher coating contents indicated the formation of a mixed-rock salt phase of Li₃Ni₂NbO₆ (Fm−3m) at 700 °C. In addition, more detailed characterization revealed distinct outcomes based on the coating method: DRC resulted in Nb-based agglomerates, whereas IWI provided a more uniform coverage. At high temperature, Nb incorporation extended into subsurface regions, possibly contributing to the formation of the mixed-disordered rock-salt phase near the surface. The electrochemical performance of the bare and coated LNO samples was assessed in half cells (3.0–4.3 V vs. Li⁺/Li). Charge/discharge curves for all samples showed similar voltage profiles, with surface coating having minimal impact on the pristine LNO. However, DRY400 and DRY700 cathodes had higher overpotentials and delivered lower initial capacities (210 and 206 mAh/g, respectively) compared to the bare LNO (215 mAh/g). The agglomerates on DRY700 led to impedance buildup, while DRY400, with amorphous phase formation, showed improved cycling stability over bare LNO despite poorer surface coverage. In the latter case, acid titration measurements showed that applying the DRC process with post-annealing at 400 °C decreases the LiOH content in LNO by a factor of about two. In contrast, IMP400 and IMP700 cathodes, with thinner and more uniform coatings, achieved higher initial capacities (218 and 220 mAh/g), but IMP700 showed inferior cycling performance likely due to increased resistance over time. In summary, our results demonstrate that effective coating involves thoroughly protecting the cathode particles, consuming surface impurities, and forming amorphous species of (ideally) high ionic conductivity.