Preparation And Electrochemical Investigations Of Polyanion-type Sodium Ion Batteries Cathode Materials

The polyanionic electrode material with stable structure,high theoretical capacity and good safety is supposed to the most promising cathode material.However,the intrinsic electronic conductivity of such materials is low,severely limiting their electrochemical performance.Starting from Na₂FePO₄F,Na₂MnPO₄F and Na₃V₂（PO₄）₃,the materials were improved by optimizing synthesis methods,constructing special structures and surface composite modification to maximize the electrochemical performance,including the following contents:Carbon coated sodium iron fluorophosphate（Na₂FePO₄F/C）powder was prepared by two-step reaction method（solvothermal reaction combined with high temperature calcination）and one-step solvothermal reaction method,separately.The effect of pH value of the precursor solution in the two-step reaction method and the solvothermal reaction time in the one-step reaction method on the crystallinity,morphology and electrochemical properties of Na₂FePO₄F/C material were investigated in detail.When the concentration of SDS was 5 mmol,Na₂FePO₄F/C changed from urchin-like hollow sphere structure to spheroidal hollow sphere structure and finally changed into double-shelled hollow sphere structure with the reaction time extending from 8 h to 16 h and 24 h.According to the experimental results,a reasonable nucleation-dissolution-recrystallization mechanism was proposed to explain the formation mechanism of double-shelled hollow Na₂FePO₄F/C spheres.The reversible specific capacity of the double-shelled hollow Na₂FePO₄F/C sphere at1 C was 120.1 mAh g^-1.The Na₂MnPO₄F/C@rGO composite was synthesized by hydrothermal method by combining double carbon（carbon coating and graphene）with Na₂MnPO₄F/C nanoparticles.The glucose was cleaved into amorphous carbon deposited on the surface of Na₂MnPO₄F/C particles between Na₂MnPO₄F/C and graphene,which acts as a bridge to form a two-dimensional"nanoparticle-conducting network",significantly improves the continuity of electron and ionic conductivity and electrochemical reaction.The Na₂MnPO₄F/C@rGO composites deliverd a reversible discharge specific capacity of 122.3 mAh g^-1 and 45 mAh g^-1at 0.05 C and 2 C,respectively.The capacity of Na₂MnPO₄F/C@rGO composites mainly comes from tantalum capacitor behavior.As the scanning speed increases,the contribution of tantalum capacitor charge storage in Na₂MnPO₄F/C@rGO electrodes increases.Three-dimensional hierarchical porous structured Na₃V₂（PO₄）₃ with micropores,mesopores and macropores（NVP/C-T）was synthesized with SDS as a surfactant and a part of carbon source by simple hydrothermal method.The three-dimensional hierarchical porous structure provide NVP/C-T composite with a large specific surface area and porosity,which can increase the reactive area,beneficial to the storage of electrolyte and the rapid transmission of sodium ions.The ultra-thin NVP/C-T nanosheet shortens the ion and the electron transfer path,improve the electronic conductivity and the whole continuity of the electrochemical reaction.NVP/C-T composites exhibit good rate performance(even at ultra-high magnifications of 80 C,NVP/C-T maintains a high specific capacity of 73.2 mAh g^-1)and good cycle stability(93.3 mAh g^-1 was maintained after an ultralong cycle of8,000 cycles at 20 C).A nitrogen-doped three-dimensional graphene aerogel-loaded core-shell structure of Na₃V₂（PO₄）₃ composite（NVP@C/N-GA）was prepared by vacuum impregnation combined with heat treatment.The surface carbon layer and N-GA construct a 3D conductive model,which not only provides 3D effective conductive network,but also facilitates rapid charge transfer,and can effectively alleviate the volume change during Na⁺insertion and extraction.The NVP@C/N-GA composite still has a specific capacity of 51.2 mAh g^-1 at a high rate of 200 C and still maintains 81.4 mAh g^-1after a long cycle of 10,000 cycles at a higher current density of 20 C.