Our new paper in Adv Sci "Medium-entropy O3-type Layered Oxide as a High-Performance and Air-Stable Cathode Material for Na-Ion Batteries: Investigation of Sn Doping to Enhance Cycling Stability"

O3-type layered oxide materials are attractive materials for cathode fabrication for commercial sodium-ion batteries (SIBs). However, their electrochemical performance and practical viability are hindered by rapid capacity decay—driven by severe phase transitions and air/moisture sensitivity. Herein, multielement codoped O3-type layered oxide materials NaLi_0.05Ni_0.3Mn_0.5₋xCu_0.1Mg_0.05Sn_xO₂ (x = 0, 0.01, 0.03, 0.05) were synthesized using a simple spray-drying method. These materials were used for fabricating cathodes for SIBs. The incorporation of Sn formed strong Sn–O bonds, effectively enhancing the structural stability. Enhanced cycling stability and rate performance were achieved, benefiting from the medium entropy–derived stabilization, disordered elemental distribution, and large interlayer spacing after Sn doping. The optimized NaLi_0.05Ni_0.3Mn_0.47Cu_0.1Mg_0.05Sn_0.03O₂ (x = 0.03; Sn0.03) cathode demonstrated a reversible capacity reaching 142 mAh g⁻¹ within a potential window of 2.0–4.2 V Na/Na⁺, while exhibiting excellent cycling endurance (retaining 83% of the initial capacity after 500 cycles at 1C) and rate capability (98 mAh g⁻¹ at 30 C). The underlying mechanism was investigated via operando synchrotron X-ray diffraction and X-ray absorption spectroscopy to monitor the lattice evolution and charge compensation behavior during cycling. Sn doping facilitated a solid solution–dominated phase transition, effectively mitigating lattice distortion and complex phase transition, and enhancing the cycling performance. The reversible redox reactions of Ni²⁺/Ni⁴⁺ and Cu²⁺/Cu³⁺ compensated the charges, while Mn⁴⁺ and Sn⁴⁺ remained electrochemically inactive, further stabilizing the structure. Sn doping improved the structural integrity, maintaining electrochemical performance after water immersion and air exposure. A Sn0.03//hard carbon full cell exhibited a competitively high energy density of 261.89 Wh kg⁻¹. Density functional theory results corroborated delayed phase transitions and enhanced air/moisture stability with Sn incorporation. These findings underscore the effects of multielement codoping medium entropy and Sn incorporation in O3-type layered oxides, establishing Sn0.03 as an efficient candidate cathode material for practical SIB applications.