Commercial ternary cathode materials have their theoretical performance limits. Consequently, enhancing energy density, reducing costs, and improving cycling stability without compromising safety are critical challenges for both academia and industry. Among the existing candidates, LiNi_0.9Mn_0.1O₂(NM91) offers a significant potential due to its high specific capacity, cost-effectiveness, and low-cobalt (or cobalt-free) nature. However, ahigh nickel content induces severe structural instability. Elevated Ni levels exacerbate Li⁺/Ni²⁺ cation disordering, thus disrupting the α-NaFeO₂-type layered structure and impeding lithium-ion de/intercalation kinetics, which degrades a rate capability. Furthermore, during high-voltage cycling (>4.3 V), detrimental interfacial reactions occur, i.e., electrolyte decomposition, transition metal dissolution, and the formation of Ni-rich rock-salt or amorphous phases. These degradation mechanisms destabilize the electrode-electrolyte interface, accelerate capacity fade, exacerbate electrode polarization, and severely compromise cycling longevity and battery safety. To address these challenges, this study was to propose an innovative Mg/Nb/Ti co-modification strategy to enhance the structural stability and electrochemical performance of single-crystal LiNi_0.9Mn_0.1O₂ cathodes.

The Ni_0.9Mn_0.1(OH)₂ precursor was first homogeneously blended with LiOH·H₂O and flux under strictly controlled stoichiometric conditions (i.e., at a Li : (Ni+Mn) molar ratio of 1.05∶1.00 and a (LiCl+KCl) ∶ (Ni+Mn) mass ratio of 0.3 ∶ 1.0). The mixture was then calcined in a flowing oxygen atmosphere, heated to 550 ℃ at 3 ℃/min (300 min hold), ramped to 900 ℃ at the same rate for 60 min, cooled controllably at 5 ℃/min to 820 ℃ for 840 min, and finally cooled to ambient temperature, yielding a phase-pure single-crystal LiNi_0.9Mn_0.1O₂ (designated SC-NM91).

For the doped samples, Ni_0.9Mn_0.1(OH)₂ precursor was homogenized with LiOH·H₂O, Mg(NO₃)₂, and C₁₀H₅NbO₂₀·xH₂O in anhydrous ethanol under vigorous agitation. C₁₆H₃₆O₄Ti was added in dropwise, followed by continuous stirring and drying at 60 ℃ until complete solvent evaporation. The resultant mixture had a controlled stoichiometry of Li ∶ (Ni+Mn) ∶ Mg ∶ Nb ∶ Ti = 1.05 : (1-x-y-z) ∶ x ∶ y ∶ z (x=0.005, 0.010, 0.020; y=0.002 5, 0.005 0, 0.010 0; z=0.01).

The identical solid-state sintering protocol used for SC-NM91 was applied to synthesize Mg/Nb/Ti tri-doped LiNi_0.9-x-y-zMn_0.1Mg_xNb_yTi_zO₂ cathodes with controlled dopant gradients. The compositions of SC-NM91-0.5Mg@0.25Nb@1Ti, SC-NM91-1Mg@0.5Nb@1Ti, and SC-NM91-2Mg@1Nb@1Ti were produced via unified thermal processing.

Among the modified cathodes, SC-NM91-0.5Mg@0.25Nb@1Ti demonstrates significantly enhanced cycling stability and superior overall electrochemical performance, compared to SC-NM91-1Mg@0.5Nb@1Ti and SC-NM91-2Mg@1Nb@1Ti.

The XRD patterns reveal a systematic peak shift toward lower angles in the modified samples relative to SC-NM91, indicating a lattice expansion along the c-axis induced by elemental doping. SC-NM91-0.5Mg@0.25Nb@1Ti exhibits enhanced cationic ordering with an R-factor of 1.749 0 versus 1.641 8 for pristine SC-NM91, demonstrating that optimized Mg/Nb/Ti co-doping effectively mitigates cationic disordering and stabilizes the layered structure. The SEM images indicate that SC-NM91 comprises irregular particles (the diameter of ~1 µm) with a non-uniform distribution and a broad size range, including oversized particulates. Increasing Mg/Nb/Ti doping concentration yields a significant morphological evolution. The particle distribution becomes more homogeneous, particle size gradually decreases, and the EDS mapping confirms spatially uniform elemental dispersion, verifying an effective dopant incorporation. The XPS analysis reveals that Mg/Nb/Ti doping effectively modulates the material surface electronic structure and valence state distribution, enhances structural stability, increases the Ni³⁺ proportion, consequently improving the electrochemical performance.

SC-NM91-0.5Mg@0.25Nb@1Ti has a superior rate capability across multiple current densities, significantly outperforming other specimens. This indicates that optimal doping enhances a structural stability, while maintaining high lithium-ion transport kinetics. When cycled at 1 C under different cut-off voltages (i.e., 4.3 V and 4.4 V), SC-NM91-0.5Mg@0.25Nb@1Ti electrode exhibits the maximum capacity retention rate, showing a remarkable improvement over counterparts. These results verify that multi-element doping combined with surface modification effectively enhances electrochemical properties. Furthermore, its differential capacity curves exhibit more symmetric phase transition peaks, significantly reduced voltage hysteresis, and compact/stable peak profiles, indicating enhanced phase transition reversibility and substantially improved structural stability. The electrochemical impedance spectroscopy (EIS) primarily reveal two key parameters, i.e., series resistance (R_s) and charge transfer resistance (R_ct), reflecting intrinsic material resistance and the electron transfer barrier at the electrode-electrolyte interface, respectively. After 100 cycles, SC-NM91-0.5Mg@0.25Nb@1Ti exhibits significantly lower resistance values rather than pristine SC-NM91. This demonstrates that Mg/Nb/Ti co-modification effectively mitigates electrolyte corrosion, stabilizes the interfacial structure, and facilitates the formation of arobust cathode electrolyte interphase film. Consequently, the modified electrode maintains efficient charge transfer kinetics during prolonged cycling, while retarding R_ct degradation. Finally, the cyclic voltammetry (CV) analysis reveals that the modified sample has a distinct hysteretic shift in the oxidation peak and reduces peak intensities, compared to the unmodified sample. Nevertheless, the high degree of peak overlap across cycles indicates that the synergistic doping-coating modification significantly suppresses electrochemical polarization and enhances interfacial reaction kinetics, thereby contributing to superior electrochemical performance.

This study systematically explored a synergistic co-modification strategy combining Mg/Nb bulk doping and Ti surface coating for SC-NM91 high-nickel cathodes. The optimally designed SC-NM91-0.5Mg@0.25Nb@1Ti sample that was fabricated via high-temperature solid-state synthesis could exhibit the superior structural stability and electrochemical performance. It demonstrated a capacity retention of 89.2% at 4.3 V after 100 cycles, significantly exceeding the unmodified counterpart (i.e., 73.3%), while maintaining a retention of 66.5% at 4.4 V, indicating the exceptional high-voltage stability. The mechanistic investigations revealed that Mg doping primarily suppressed Li/Ni cation mixing and inhibited the H1→M phase transition at low voltages, while Nb doping stabilized oxygen frameworks, mitigated oxygen release under high-voltage conditions, and enhanced H2→H3 phase transition reversibility. Ti⁴⁺doping reinforced transition metal layer stability and suppressed Ni⁴⁺-induced lattice oxygen release at high voltages, extending cycle life. The concomitant formation of TiO₂/LiTi₂O₄ coating layers during thermal processing effectively blocked parasitic reactions and boosted electronic conductivity, collectively improving interfacial integrity and rate capability. The electrochemical characterization confirmed reduced R_ct post-cycling and diminished redox peak potential separation in CV, evidencing enhanced reaction kinetics and suppressed polarization. This co-modification approach could elevate the comprehensive performance of SC-NM91 cathodes, establishing an effective paradigm for developing high-energy-density lithium-ion battery cathodes with a robust stability.