| In recent years,sodium ion battery(SIBs)technology has become a hot topic in developing the large-scale energy storage systems(EES).The SIBs has many advantages,such as the cost-effective,resonable rate capability,and re-cycleability and reuse.While the bottleneck lies in the lack of qualified cathodes with high energy density,high stability,high rate capability to promote the practical application of SIBs.In this context,this thesis focuses on the improvement of the stability and rate capability of P2 Na-Ni-Mn-O transition metal oxide using doping method,and explores the underlying working mechanism of the doping element upon sodiation and desodiation of Na-Ni-Mn-O transition metal oxide cathodes for SIBs.Although it delivers promising theoretical capacity and voltage etc.,the Na0.66Ni0.33Mn0.67O2 electrode has poor cycling stability.Therefore,an A1 doped Na0.78Al0.05Ni0.33Mn0.60O2 material is synthesized as SIB cathode and a dual effect is revealed on the improvement of electrochemical/structural by etching XPS and TEM mapping.The first one is doping the Al into the bulk of Na0.66Ni0.33Mn0.67O2 electrode to stabilize the outmost surface region,which could suppress obviously the structural degradation especially at high desodiation level.The other one is coating the surface of Na0.66Ni0.33Mn0.67O2 electrode to largely reduce the side reaction in th electrolyte-eletrode interface.Both advantanges of the Al doing interact with each other to synergetically contribute to the superior electrochemical performance of Na0.78Al0.05Ni0.33Mn0.60O2 material,resulting in the capacity retention increase from the original 52.5%to doped electrode of 83.9%.Using in-situ XRD,this dual effect is partially evidenced from the sluggish transition from the P2 to O2 phases.Moreover,the volume expansion of P2 electrode is reduced from the original 21.8%to 15.4%after Al doping,which implies the decrease of particle pulverization at high voltage.Of special interest is that a clear atomic-scale 02 phase is observed at high desodiation level by spherical aberrated scanning tranmission electron microscopy(STEM),which is kept even in the fully discharged sample at 2.0 V.Hnece,the irreversible transition from 02 to P2 phase is confirmed from STEM.Upon the preparation of P2 type Na0.75Mn0.65Ni0.20Co0.15O2 material,it is found the glides of the transition metal octahedra is doable,which inspires us to design new P type cathodes for SIBs.By manipulating the stoichiometric ratios and temperature,a mew P3 type Na0.75Mn0.65Ni0.20Co0.15O2 material is synthesized by oxalic acid hydrothermal method.The long-term cycling test indicates that the pristine P3 type material shows the rapid capacity decay.Further ex-situ XRD demonstrates that a new phase with larger interplanar spacing is formed at high voltage region,and a large volume change with phase transition occurs,which contributes negratively to the structural stability.However,the electrochemical tests show that the P3 type’s rate capability is much better than that of the P2 analogue,which is ascribed to the higher Na ion diffusion coefficient and lower interface impedance in P3 type material than these of P2 type. |
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