Preparation And Electrochemical Performance Of Manganese-based Layered Solid Solution Cathode Materials For Sodium Ion Batteries

P2 layered manganese-based solid solution is one of the most promising cathode materials for sodium ion batteries.In this paper,the Ti⁴⁺doping,Ce⁴⁺doping and V₂O₅modification of Na_0.67Co_0.20Mn_0.80O₂were respectively carried out by solid phase method to study the influence of doping and modification on the structure morphology and electrochemical performance of the parent material.The Ce⁴⁺doping and V₂O₅modification of Na_0.67Co_0.20Mn_0.80O₂in this thesis has not been reported in the literature so far.The P2 phase structure and layered hexagonal morphology of the parent material are not changed by Ti⁴⁺doping and Ce⁴⁺doping,and the element distribution are uniform after doping.The introduction of Ti⁴⁺and Ce⁴⁺occupy the Mn ions site.The Ti–O bond and Ce–O bond with stronger bond energy cause the TMO₂layer to shrink,and enhance the stability of the crystal structure,and alleviate the phenomenon of Jahn-Teller distortion,and therefore the cycle performance of the doped electrode is improved.The improvement of the rate performance of the doped electrode is mainly due to the increase of the sodium layer spacing and the extension of the Na–O bond caused by the doping of Ti⁴⁺and Ce⁴⁺.The Na_0.67Co_0.20Mn_0.755Ti_0.045O₂electrode achieves a high discharge capacity of 99.8 m Ah g^–1at 1 C in the voltage range of 2.0–4.0 V,and it still maintains the capacity retention of92.4%after 300 cycles,while the parent electrode is only 77.3%.The Na_0.67Co_0.20Mn_0.79Ce_0.01O₂electrode releases 135.8 m Ah g^–1capacity at 0.1 C,and the capacity retention rate is still 92.3%after 100 cycles,which is far greater than the 83.4%of the parent electrode.The P2-type crystal structure of the parent material is not changed by V₂O₅modification.The V₂O₅modified layer is almost evenly distributed on the surface of the parent material.The elongation of the c-axis and the contraction of the TM–O bond prove that V⁵⁺has been incorporated into the lattice of the parent material.The shrinkage of the TM–O bond and the protection of the V₂O₅modified layer increase the stability of the crystal structure from the inside and the surface,and greatly enhance the cycle stability of the modified electrode during long-term charge and discharge.The increased thickness of the sodium layer and the extended Na–O bond reduces the diffusion barrier of sodium ions,and accelerates the diffusion and transfer speed of sodium ions,and further improves the kinetic performance of the modified electrode.The Na_0.67Co_0.20Mn_0.80O₂-1wt%V₂O₅modified electrode can release 81.3 m Ah g^–1capacity in the first lap at 5 C,and still has a high capacity retention rate of 93.9%after a long cycle of 900 laps.The parent electrode has attenuated to 77.1%.The results show a significant synergistic effect between the V₂O₅modified layer and the incorporation of V⁵⁺.The charging and discharging curves show that Ti⁴⁺,Ce⁴⁺doped electrodes and V₂O₅modified electrodes have similar voltage platform between 3.3～3.7 V.The ex-situ XRD test results prove that a highly reversible P2–P3–P2 phase change occurs in this voltage range.This has played a very important role in extending the cycle life of the electrode.The work in this thesis provides a new idea for the development of P2 layered manganese-based solid solution materials.