Study On Polyanion Surface Modification Of VO₂（B） For Aqueous Energy Storage

With the gradual deterioration of environmental problems and increasing energy demand,the development of green and efficient energy storage devices is imminent.Among many energy storage devices,aqueous sodium ion batteries have been widely studied in the field of electrochemical energy storage because of their high theoretical energy density,low cost,and high safety.However,the lower voltage window of aqueous sodium-ion batteries and the lower energy density of aqueous electrolytes compared with organic electrolytes further limit the wider application of aqueous sodium-ion batteries.Therefore,researching high-performance electrode materials is the key to improving the energy density of aqueous sodium ion batteries.The unique layered structure,low cost,easy availability,wide potential window,and high energy density of monoclinic VO₂（B）make it very suitable for use as electrode materials for water-based and non-aqueous rechargeable batteries.However,due to the poor electronic and ion conductivity properties of VO₂（B）itself,the formation of soluble V ions in the aqueous electrolyte and the structural transformation during the cycle process,VO₂（B）has poor rate performance and cycle performance.In this paper,a simple method of annealing in a tube furnace containing sodium hypophosphite in an argon atmosphere is used to prepare a symmetrical electrode of VO₂（B）（called PVO）with modulated valence and surface phosphate ion modification.Further,by immersing PVO in potassium ferrocyanide solution based on ion exchange method,VO₂（B）（called HVO-K₄）symmetrical electrodes with surface branched Fe（CN）₆^4-are prepared,which are used as anode and cathode materials for aqueous sodium ion batteries shows further improved electrochemical performance.Using XRD,SEM,XPS,Raman,TEM,BET,FTIR infrared spectroscopy,UV-vis and UPS analysis to analyze the phase structure and morphology,valence state and energy band structure of PVO and HVO-K₄show that:（1）The valence state of PVO is significantly reduced,The introduction of V³⁺and surface phosphate ions can effectively improve the electron transport and ion conductivity of VO₂（B）,and can effectively broaden the voltage window of VO₂（B）in the negative range.（2）in HVO-K₄,Fe（CN）₆^4-is branched to the surface of VO₂（B）through the V-N-C-Fe bond.There is no obvious electron transfer during the branching process.Compared with PVO,the valence state of V will not change significantly.（3）In HVO-K₄,Fe（CN）₆^4-is bonded to the surface of VO₂（B）through V-N-C-Fe,the work function is further significantly reduced,which greatly improves the redox activity of the VO₂surface.The significantly improved electron transport and ion diffusion performance of HVO-K₄also promotes the insertion and extraction of sodium ions during electrochemical reactions and promotes the storage of sodium ions in a open tunnel.（4）In terms of electrochemical performance,When PVO and HVO-K₄are in the positive potential range,at a current density of 1m A cm^-2,the capacity of PVO is 680 C g^-1,and the capacity of HVO-K₄is 1479 C g^-1,both of which are obvious higher than VO(384 C g^-1).When PVO and HVO-K₄are in the negative potential range,under the potential window of-0.8～0,HVO-K₄shows that the capacity of 1651C g^-1is significantly higher than that of PVO(941 C g^-1)and the capacity of VO(162 C g^-1).Under the voltage windows of-1.0～0 V and-1.1～0 V,PVO shows high capacity of 1290 C g^-1and 1559 C g^-1,respectively.While,HVO-K₄shows the further improved capacity is 2347 C g^-1and 2542 C g^-1,respectively.In the high voltage range of-1.1～0 V,PVO and HVO-K₄show excellent cycle performance（capacity retention rates of 82.9%and 86%respectively after 10000 cycles）.In addition,this paper found that the preparation of HVO-K₃by soaking PVO in potassium ferricyanide solution has an ion exchange process different from that of HVO-K₄.When used as a cathode material for aqueous sodium ion batteries,it shows significantly improved electrochemical performance compared with VO and PVO.Furthermore,the HVO-K₄//HVO-K₄symmetrical aqueous sodium ion battery is designed and constructed to work stably under a voltage window of 1.8 V,showing a high capacity of 465 C g^-1at a current density of 1A g^-1and a coulombic efficiency close to 100%.It delivers an high energy density of 116 Wh kg^-1at a power density of 900 W kg^-1,and an excellent power density of8908 W kg^-1at an energy density of 24 Wh kg^-1and an excellent cycle life of the whole battery（2000 cycles of 92%capacity retention）,It has a very broad commercial prospect.