Study On Sodium Storage Properties And Modification Of Manganese Oxide Cathode Materials For Sodium Ion Batteries

Sodium-ion batteries are considered the most promising next-generation batteries due to their large storage capacity and low cost.P2 layered sodium transition metal oxides（P2-Na_xTMO₂）have received much attention due to their high rates and cycling properties.However,P2-Na_xTMO₂ often undergoes structural transformation when exposed to ambient air,which limits its practical application.In addition,the P2-Nax TMO₂ sodium content is low,so the specific capacity of the matched full battery is low,and the electrochemical performance needs to be improved.In this paper,the typical P2 structure layered oxide P2-Na_0.67Mn_0.67Ni_0.33O₂ was selected for modification.The first was to realize surface regulation through Nb doping,so as to improve the air stability and electrochemical stability of the material.Second,O3 phase was introduced by adjusting the sodium content and the proportion of transition elements,and P2/O3 heterostructure was constructed to improve the electrochemical performance of the material.The specific contents were as follows:By studying P2-Na_0.67Mn_0.67Ni_0.33O₂ doping,the C-axis spacing was increased and the Nb-O bond with strong bond energy(753 k J mol^-1)was introduced,which stabilized the structural stability of the whole transition metal layer.The performance of P2-Na_0.67Mn_0.67Ni_0.33O₂ before and after Nb doping in different humid environments was characterized by XRD,SEM and other characterization methods,which confirmed that effective doping of Nb can improve air stability.We demonstrated that the Nb-induced surface reconfiguration layer inhibited the surface dissolution of P2 material in the electrochemical reaction,resulting in a stable and thin（cathode-electrolyte interface phase,2-3 nm）CEI film,which reduced the internal charge transfer resistance and prevented water molecules from entering the P2-Na_xTMO₂ lattice.In RH93%humid environment exposure after 20 days of Na_0.67Mn_0.67Ni_0.33Nb_0.03O₂ half-cell,has excellent performance ratio(20 C reversible capacity of 72.5 m Ah g^-1),Moreover,it has excellent cycle retention rate（84.43%after 1000 cycles at 5 C）.A novel P2/O3 heterostructure Na_0.85Mn_0.6Ni_0.4O₂ cathode material was designed by adjusting the transition element ratio of sodium content.Due to the synergistic effect of the P2-O3 composite structure,the P2-O3 electrode has high initial charge-discharge capacity and excellent rate performance.The heterogeneous structure of P2/O3 was confirmed by X-ray diffraction and high-resolution transmission electron microscopy.The optimized heterostructured cathode P2/O3-Na_0.85Mn_0.6Ni_0.4O₂ half-cell yielded a high reversible capacity of 124.8 m Ah g^-1 at 0.2 C and a good magnification-capacity of 85.2 m Ah g^-1 at 20 C.The above experimental results can prove the excellent electrochemical performance of bipolar cathode materials and provide a new idea for the design of SIBs composite structural materials.