Understanding of a Ni‐Rich O3‐Layered Cathode for Sodium‐Ion Batteries: Synthesis Mechanism and Al‐Gradient Doping

O3-type NaNi_0.8Mn_0.1Co_0.1O₂ (NaNMC811) cathode active materials for sodium-ion batteries (SIBs), with a theoretical high specific capacity (∼ 187 mAh g^-1), are in the preliminary exploration stage. This study comprehensively investigates NaNMC811 from multiple perspectives. For the first time, the phase evolution ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>P</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi> <mml:mn>1</mml:mn></mml:mrow> <mml:annotation>$P\overline{3}m1$</mml:annotation></mml:semantics> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>F</mml:mi> <mml:mi>m</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi></mml:mrow> <mml:annotation>$Fm\overline{3}m$</mml:annotation></mml:semantics> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>R</mml:mi> <mml:mover><mml:mn>3</mml:mn> <mml:mo>¯</mml:mo></mml:mover> <mml:mi>m</mml:mi></mml:mrow> <mml:annotation>$R\overline{3}m$</mml:annotation></mml:semantics> </mml:math> ) during the solid-state synthesis is systemically investigated, which elucidates in-depth the mechanisms of the thermal sodiation process. Furthermore, an Al-gradient doping of NaNMC811 was successfully implemented through Al₂O₃ coating on the cathode active material (CAM) precursor. The modified Al-NaNi_0.8Mn_0.1Co_0.1O₂ (Al-NaNMC811) exhibits excellent electrochemical dynamics and performance, maintaining a specific capacity above 100 mAh g^-1 after 100 cycles at 0.1 C (1.5-4.1 V) while providing a promising capacity retention of 63%. Additionally, the material demonstrates excellent rate capabilities, retaining a specific capacity of 107 mAh g^-1 at 5 C. Compared to pristine NaNMC811, the modified Al-NaNMC811 is proven to have improved electrochemical kinetics with a higher Na⁺ diffusion coefficient due to dilated (003) interplanar spacing, and a more stable structure during the electrochemical charge-discharge processes, which is attributed to stronger Al-O bond energy. Understanding phase formations during the synthesis and comprehensive insight in the gradient doping for O3-type NaNMC811 CAMs guides further development of next-generation SIBs materials.