Study On Modification Of Manganese Based Polyanionic Cathode Materials For Sodium-ion Batteries

The deterioration of the global environment and the consumption of non-renewable energy sources have prompted people to explore new energy sources such as wind,solar and tidal energy,etc.In the practical application of these new energy sources,energy storage is the most critical part,so energy storage equipment is crucial,and secondary batteries as the most convenient and effective energy storage equipment on the market are undoubtedly the best choice.Sodium-ion battery is the hot spot of secondary battery research at this stage due to its abundant resources,low cost and high safety.As an important component of sodium-ion batteries,the cathode has a great influence on the battery performance,so the development of cathode materials with high energy density and long cycle life is the key to the research of sodium-ion batteries.Manganese-based polyanionic compound cathode materials with a three-dimensional framework structure can effectively increase the energy density of the materials due to their high working voltage based on Mn²⁺/Mn³⁺and Mn³⁺/Mn⁴⁺redox pairs and high specific capacity brought by the two-electron reaction.However,the lattice distortion of MnO₆ octahedra triggered by the Jahn-Teller effect of Mn³⁺during the cycling process,as well as the continuous rearrangement of the ordered structure and the occurrence of irreversible phase transitions,seriously affect the electrochemical properties of the material.In order to give full play to the high voltage and high specific capacity of manganese-based materials,the Na₄MnM（PO₄）₃（M=transition metal）composite manganese-based polyanionic materials are prepared by combining a metal element with high redox potential with manganese-based materials:the synergistic effect of the bimetallic element is beneficial to mitigate the Jahn-Teller effect caused by Mn³⁺,and can further increase the average working voltage of the positive electrode material.material,thus increasing the energy density.Meanwhile,to improve the intrinsic defects of the material,the material was optimized by using coating and carbon composite,respectively.The main studies and results are as follows:（1）The Na₄MnV（PO₄）₃（NMVP）cathode material was prepared by combining Mnand V elements（3.6 V）by sol-gel method.Meanwhile,in order to solve the problems of interfacial degradation and structural degradation during the electrochemical reaction of the material,Na_1.3Al_0.3Ti_1.7P₃O₁₂（NATP）was used to construct a coating layer with fast ionic conductor properties on the surface of NMVP material particles.The structural characterization showed that the NATP coating enabled the successful incorporation of some Ti⁴⁺and Al³⁺into the inner lattice of the native material.Compared with the unmodified sample,the NATP-coated modified material exhibited better rate performance(24.8 m Ah g^-1 specific discharge capacity at a current density of 30 C)and long cycle life（44.8%capacity retention after 1000 cycles at a current density of 0.1 C）.The improved electrochemical performance of the material may be attributed to the fact that the NATP coating layer effectively mitigates the occurrence of side reactions at the electrode/electrolyte interface and inhibits the dissolution of transition metal ions in the bulk phase material;at the same time,the ions with larger radii are incorporated into the interior of the material lattice,increasing the layer spacing of the material and thus improving the Na⁺de/embedding rate.Finally,the in situ XRD results show that the NATP coating layer can alleviate the accidental phase transition induced by the bulk material at high voltages,thus reducing the lattice deformation during charging and discharging,and thus enhancing the stability of the material structure during electrochemical cycling.（2）In order to fully activate the Mn³⁺/Mn⁴⁺redox reaction within the NASICON framework and give full play to the advantages of manganese-based materials,this work introduces Cr（4.2 V）elements with a higher redox reaction potential than Mnand successfully prepares NASICON-type Na₄MnCr（PO₄）₃ with a highly reversible Mn³⁺/Mn⁴⁺redox reaction（NMCP）cathode material with highly reversible Mn³⁺/Mn⁴⁺redox reaction.Meanwhile,for the disadvantage of poor inherent electronic conductivity of the material,the highly conductive carbon nanotubes（CNTs）were compounded with the native material by liquid-phase method to prepare a pair of NMCP@CNTs materials.The impedance test results yielded a transfer impedance of 144.3Ωfor the CNTs modified material after 50 cycles,which is much lower than 345.1Ωfor the original material,indicating that the addition of CNTs with good conductivity effectively reduces the internal resistance of electron transfer and greatly improves the electronic conductivity of the material.The modified sample can provide a specific capacity of 30.1 m Ah g^-1 at large multiplicity（15 C）,and the capacity is still41.4 m Ah g^-1 after 500 cycles at a current density of 3 C,corresponding to a capacity retention of 68.8%,which is due to the in situ composite CNTs interwoven into the NMCP,which promotes the electron conduction inside the material particles,while the NMCP is embedded into the mutually cross-linked carbon matrix which provides a good electron conduction framework and accelerates the electron transfer between the numerous NMCP particles,thus improving the multiplicative performance and long cycle life of the material.