Fabrication And Electrochemical Performance Of Hierarchical Micro-Nano Structured Sodium Vanadium Fluoride Phosphate

Na₃V₂O₂（PO₄）₂F（NVOPF）possesses high operation voltage and theoretical capacity and is regarded as a promising cathode material for alkali-ion batteries.However,the poor intrinsic electronic conductivity leads to inferior electrochemical performance.To solve this problem,a series of three-dimensional（3D）hierarchical micro-nano structures were constructed through morphology control and graphene oxide modification within different methods,which effectively improved the reaction kinetics and electrochemical performance of NVOPF.A systematic study is carried out from material preparation,morphology characterization,growth mechanism,performance optimization to an ion storage mechanism for investigating the application of NVOPF in alkali-ion batteries.All the studies will lay the scientific foundation for the development of high-performance electrode materials for alkali-ion batteries.Some meaningful research results are as follows:（1）By controlling the reaction time during the solvothermal process,the NVOPF microspheres with a yolk-shell structure were synthesized.The yolk-shell structured NVPOF microspheres have thin outer shells（about 100 nm）composed of interwoven nanosheets and porous inner cores with a diameter of about 1.6μm.The yolk-shell structure can effectively improve the contact area of electrode-electrolyte,which is beneficial to the diffusion of electrons and Na ions.In addition,the void cavity in the structure can not only provide more active reaction sites for Na ion storage but also restrain the volume change during the charge and discharge process.Benefited from the favorable interwoven nanosheets shell,inner cavity and porous core structure,the resulting NVPOF microspheres exhibit a superior rate capability of63 m Ah g^-1 at 20 C as well as an outstanding long-cycling performance with the capacity retention of up to 92.1%over 1000 cycles at 5 C when employed as the cathode material for sodium-ion batteries.（2）Benefited from the high conductivity and high flexibility of r GO framework,we synthesized the r GO wrapped NVOPF nanoparticles through a spray-drying method and subsequent calcination process.With the introduction of 3D r GO network,the aggregation of NVOPF nanoparticles is avoided,and the ionic and electronic conductivity of the composite is enhanced greatly,both of the two aspects play an important role in the improvement of Na storage performance.When used as the cathode material for sodium-ion batteries,the obtained NVOPF@r GO displays a high capacity of 120.4 m Ah g^-1 at 0.5 C and an enhanced rate capability of 70.3 m Ah g^-1 at a high rate of 100 C.It also exhibits excellent long-cycling stability with the capacity retention of 83.4%after 2000 charge-discharge cycles at the high rate of 30C.The high reversible single-phase solid solution reaction mechanism of NVOPF@r GO during the insertion/extraction of Na ion was revealed by in situ X-ray diffraction and ex situ high-resolution transmission electron microscope techniques,and the high reversible reaction processes are beneficial to the Na storage properties.Combined with the experimental results and theoretical analysis,the crystal structure of NVOPF at each state during the charge and discharge process was confirmed,that is to say,from the structure of NVOPF to Na V₂O₂（PO₄）₂F and NVOPF.The mobility of Na ions in the structure of NVOPF was pointed out.（3）We synthesized the graphene decorated nanosheet-assembled flower-like NVOPF microspheres composite via a solvothermal method.The three-dimensional hierarchical structure not only retains the size effect of nanosheets,but also further effectively increases the contact area of the electrode with electrolyte.When used as the electrode material of lithium-ion batteries,the structure changes and ion storage performance of NVOPF/G based on the multi-electron reaction were investigated by controlling the operation voltage windows.The results of in situ X-ray diffraction demonstrated that the reversible single-phase solid solution reaction and the irreversible amorphization transition occur in the lithiation process at different voltage ranges.The valence change of vanadium from+5 to+2 was confirmed by X-ray photoelectron spectroscopy.Based on the DFT calculations,the latter inserted Li ions will induce the rearrangement of the former inserted Li ions and the residual Na ions.NVOPF/G displays an initial discharge capacity of 966.7 m Ah g^-1 due to these V²⁺/V³⁺,V³⁺/V⁴⁺and V⁴⁺/V⁵⁺redox couples and irreversible side reactions but suffers from sharp capacity decaying（within 0-4.8 V）.When employed as the cathode material to make full use of the V³⁺/V⁴⁺and V⁴⁺/V⁵⁺,NVOPF/G presents a one-phase solid solution reaction mechanism with a much higher energy density of753.2 Wh kg^-1.Besides,the deeply lithiated NVOPF/G electrode also exhibits an acceptable initial capacity(446.9 m Ah g^-1 at 0.05 A g^-1)and cycling stability(264.1m Ah g^-1 after 200 cycles)when tested in lower operating voltages（≤2.5 V）with the redox couples of V²⁺/V³⁺and V³⁺/V⁴⁺.Based on the above results,the achieved NVOPF/G shows potential as both cathode and anode material for high-performance lithium-ion batteries.（4）The unique morphology of NVOPF nanoplates was synthesized by adjusting the sodium source,organic solvent,and surfactants in the solvothermal reaction process.Similar to the morphology of the above works,constructing the three-dimensional hierarchical structure is an advantage for electrochemical performance.When decorated by graphene during the spray drying process,the graphene and NVOPF nanoplates overlap and interweave to form the NVOPF/r GO microspheres.Firstly,we deintercalated two Na ions from NVOPF in sodium-ion batteries,and the obtained sample was employed as the cathode material for potassium ion batteries.It displays a high specific capacity of 120.2 m Ah g^-1 at the current density of 50 m A g^-1,a high rate capability of 93.9 m Ah g^-1 at 800 m A g^-1and excellent long cycling performance with the capacity retention of 81.3%after1500 cycles.