Mitigating the Large-Volume Phase Transition of P2-Type Cathodes by Synergetic Effect of Multiple Ions for Improved Sodium-Ion Batteries

Layered transition metal oxide P2-Na_2/3Ni_1/3Mn_2/3O₂ usually suffers from large-volume phase transitions and different Na-vacancy ordering during sodium (de)intercalation, incurring rapid capacity decline and poor rate capability. Herein, an effective strategy based on synergetic effect of selected multiple metal ions is designed for P2-type cathodes with improved performance. The role of tetravalent titanium provides high redox potential, inactive divalent magnesium stabilizes the structure, and the monovalent lithium smooths the electrochemical curves. The combined analysis of in operando X-ray diffraction, in operando X-ray absorption spectroscopy and density functional theory calculations demonstrates the contribution of multi-metal ions converts the unfavorable and large-volume P2 to O2 transition into a moderate “Z”-intergrowth structure by increasing the energy barrier of transition metal slab gliding. As a consequence, the resultant P2-Na_0.7Li_0.03Mg_0.03Ni_0.27Mn_0.6Ti_0.07O₂ electrode delivers a reversible capacity of 134 mAh g⁻¹, a working voltage of 3.57 V, excellent cycling stability (82% of capacity retention after 200 cycles), and superior rate performance (110 mAh g⁻¹ at 4 C). Full cells fabricated with a hard carbon anode achieve an energy density of 296 Wh kg⁻¹. This study presents a route to rationally design cathode materials with this functionalization to improve the cell performance for sodium-ion batteries.