Research On The Preparation And Performance Of High Energy Density Sodium Ion Storage Material

In recent years,growing demand for large-scale energy storage technology brings fresh life and new vitality to the development of sodium-ion batteries.Sodium is a cheap,nontoxic and abundant element that is uniformly distributed around the world and therefore would be ideal as a choice for rechargeable batteries.Cathode materials and anode materials of sodium ion batteries are the key to determine performances of sodium ion batteries.In this paper,it is studied with test and modification for NaVPO₄F cathode materials and sodium titanate anode materials through preparation of them.NaVPO₄F with trivalent vanadium was successfully synthesized under air through controlling the appropriate heating/cooling rate and holding temperature after a hydrothermal process.The vanadium???in NaVPO₄F can be facilely and successfully maintained even under air.As produced NaVPO₄F had similar structure and cycle performance as the one prepared under argon protection.Even more excellent capability was exhibited when operating at high rate currents as cathode material for sodium-ion batteries.With increasing of the current rate from 0.1,1,2,4,8,12,16,to 20 C,the material prepared under air delivered the average capacity of 88.3,75.5,72.7,64.0,53.0,37.8,33.2,27.1 mAh/g respectively,and it decline by about 8.4% of their initial capacity to 80.9 mAh g-1 after deep cycling at high rates.The effects of heat treatment atmosphere?under air?on electrochemical performances were further evaluated by cyclic voltammetry and electrochemical impedance spectroscopy.NaVPO₄F synthesized under air could offer a wider range of applications for sodium-ion batteries due to its facile synthesis and low cost.Sodium titanate,an anode material for sodium ion batteries has been synthesized via a modified sol-gel method followed by heat treatment.Diffraction peaks in X-ray diffraction?XRD?were found obviously when rate of oxalic acid is 1.5,the materials as prepared were multiphase materials because part of diffraction peaks represented Na₂Ti₆O₁₃.The materials exhibited a discharge plateau at 0.3 V in the limited voltage range of 0.1-2 V.The batteries of sodium titanate were charged-discharged at a voltage range of 0.1-2 V,surface of material will form solid electrolyte interface?SEI?film in this voltage range,a lot of sodium ions will react in the first discharge process?process of sodium ions insetting?.Therefore,the first discharge capacity of the materials was higher than theory discharge capacity of sodium titanate,and this part of capacity was irreversible capacity.Although the materials as prepared were multiphase,the satisfactory electrochemical performance of the material was achieved with adjusting of oxalic acid content,and the discharge capacity is 148.4 m Ah/g after 30 cycles,which could offer promising choice as future anode material for sodium-ion batteries.The multiphase materials presented better electrochemical performances,which inspired the subsequent research.Sodium titanate(Na₂Ti₆O₁₃),an anode material for sodium-ion batteries,was synthesized via liquid phase method.Multiphase materials which had better electrochemical performances were obtained though controlling excessive Na because of enlightenment of preceding research.Different amounts of carbon coating were introduced to improve the electrochemical performance of multiphase materials.The structure of the resultant carbon coating modified anode materials were examined by X-ray diffraction.The effects of carbon coating on the electrochemical performance were evaluated by cyclic voltammetry,discharge curve,rate performance and electrochemical impedance spectroscopy,and the mechanism were further discussed.The carbon film can improve the conductivity of as prepared materials.In addition,the malignant interaction between electrolyte and the material were prevented effectively in low voltage range,then the polarization during the electrochemical reaction process were reduced,leading to better rate capability and cycle performance.Among them,7 wt% carbon coated material took on best electrochemical performance,which could offer promising choice as future anode material for sodium-ion batteries.Sodium titanate,Na_4+2xTi₁₂O_26+x?x=0,0.5,1,1.5,2?,anode materials of sodium ion batteries were synthesized through a crystal method after hydrolysis,performances of multiphase materials between Na₂Ti₆O₁₃ and Na₂Ti₃O₇ were researched detailedly.Characteristic peaks of Na_4+2xTi₁₂O_26+x transformed regularly with increasing of x in molecular formula through XRD,they reveal transformation process of crystal from Na₂Ti₆O₁₃ to Na₂Ti₃O₇.It is found that morphology of the materials was short rhabditiform and nothing changed with increasing of x through SEM.The length of a short rhabditiform was approximately 750 nm and its diameter was approximately 200 nm.Besides,Na₅Ti₁₂O_26.5 material was analyzed through AAS and ICP in order to ascertain content of sodium and titanium,it is confirmed that the real rate between sodium and titanium was 5:12 and was accordant with estimation before experiment.The voltage plateaus of discharge were changed regularly with increasing of x.The first discharge capacity of Na₅Ti₁₂O_26.5 materials is 257.89 mAh/g.With increasing of the electric current density from,50,100,200,500 and 1000 mA/g,the Na₅Ti₁₂O_26.5 material delivered the average capacity of 165.0,124.0,105.0,78.6 and 65.4 mAh/g respectively.And it also had a discharge capacity of 132.432 mAh/g after 135 cycles and delivered excellent electrochemical performances.