Understanding the Correlation between Electrochemical Performance and Operating Mechanism of a Co-free Layered-Spinel Composite Cathode for Na-Ion Batteries

Compositing different crystal structures of layered transition metal oxides (LTMOs) is an emerging strategy to improve the electrochemical performance of LTMOs in sodium-ion batteries. Herein, a cobalt-free P2/P3-layered spinel composite, P2/P3-LS-Na_1/2Mn_2/3Ni_1/6Fe_1/6O₂ (LS-NMNF), is synthesized, and the synergistic effects from the P2/P3 and spinel phases were investigated. The material delivers an initial discharge capacity of 143 mAh g^-1 in the voltage range of 1.5-4.0 V and displays a capacity retention of 73% at the 50^th cycle. The material shows a discharge capacity of 72 mAh g^-1 at 5C. This superior rate performance by the material could be by virtue of the increased electronic conductivity contribution of the incorporated spinel phase. The charge compensation mechanism of the material is investigated by in operando X-ray absorption spectroscopy (in a voltage range of 1.5-4.5 V vs Na⁺/Na), which revealed the contribution of all transition metals toward the generated capacity. The crystal structure evolution of each phase during electrochemical cycling was analyzed by in operando X-ray diffraction. Unlike in the case of many reported P2/P3 composite cathode materials and spinel-incorporated cobalt-containing P2/P3 composites, the formation of a P'2 phase at the end of discharge is absent here.