Structural Modulation And Sodium/Lithium Storage Performance Of Prussian Blue And Its Derivatives

With global energy consumption growing,the usage of renewable and clean energy has become an irresistible trend.The development of advanced energy storage device is a necessary way to realize the effective conversion,storage and utilization of clean energy.As one of the most advanced energy storage systems,lithium-ion batteries(LIBs)have been widely used in portable electronic devices.However,they faced the problem of lithium resources shortage.Sodium locates in the same main group as lithium and with similar physical and chemical properties,but with abundant reserves and low price.Therefore,sodium-ion batteries(SIBs)are considered to have great potential to replace LIBs.However,due to the large ion radius and the slow electrochemical kinetics of sodium-ion,the energy density and power density of SIBs are unsatisfactory and are only suitable for application in large-scale energy storage devices.In recent years,lithium-sulfur batteries(LSBs)have received extensive attention due to their high theoretical energy density.However,lack of reasonable sulfur reservoir materials and low utilization of sulfur limited the practical application of LSBs to some extent.Prussian blue(PB)material,due to its unique 3-D channel structure,can allow rapid de-intercalation of sodium/lithium ions and achieve efficient electrochemical reaction.Because of its low cost,simple synthesis,high yield and controllable structure,it has been paid close attention by many researchers.In addition,there are abundant active metal sites on the PB lattice skeleton,which have excellent chemical adsorption on the LPSs during electrochemical reaction,which can effectively limit"shuttle effect"and enhance utilization of sulfur.PB are the research object in this paper.First of all,from two strategies of crystal water(interstitial water and coordination water)elimination and redox centers activation,the electrochemical performance of PB based SIBs positive electrodes were improved.Then,the channel structure of PB was optimized and the multifunctional mesoporous structure was constructed to enhance the loading and utilization of sulfur,thus improving the electrochemical performance of PB based LSBs.Finally,the structure of PB derivatives were regulated,and the electrochemical performance of the anode of LIBs were improved by coordinating carbon coating strategy.The main research contents of this paper are as follows:(1)In view of the crystal water caused voltage platform instability,rapid capacity decay and poor cycle performance,a step-by-step heating process under argon and ammonia atmosphere was developed to eliminate the crystal water in NH₄Fe₂(CN)₆(NH₄HCF).The experimental results show that the NH₄HCF under argon treatment still contains part of crystal water,and the electrode only has one pair of voltage platform.After 250 cycles at current density of 50 m A g^-1,the electrode has a reversible specific capacity of 48.5 m Ah g^-1,corresponding to a capacity retention of 61.5%.While with a further ammonia treatment,the crystal water in NH₄HCF was almost removed and the electrochemical inert low spin Fe²⁺turned to activated high spin Fe²⁺.Therefore,the electrode shows a stable double-voltage platform de-intercalation reaction.After 1000 cycles at the higher current density of 0.5 A g^-1,the reversible specific capacity of NH₄HCF electrode remains at 80.4 m Ah g^-1,corresponding to a capacity attenuation of 0.018%per cycle,and the Coulombic efficiency is close to 100%,which exhibiting good cycling performance.(2)On basis of the proposed ammonia elimination theory on crystal water,we selected Fe₄[Fe(CN)₆]₃(Fe HCF)as the research object,and carried out high-temperature(240?)ammonia treatment.The experimental results show that ammonia can not only etch the crystal water,but also continue to destroy the Fe atoms connected with the crystal water,leading to the loss of local coordination sites and the creation of Fe-H₂O vacancy.The result was that the locally microporous framework was damaged,and numerous Fe-H₂O vacancies were interconnected with each other to produce a local mesoporous structure.Meanwhile,Fe HCF under ammonia treatment(Fe HCF-A)exhibits excellent LPSs adsorption capability due to the local etching process,which initiates the formation of unsaturated Fe sites.The S/Fe HCF-A electrode shows excellent electrochemical performance.At the current density of 2 C(1 C=1675 m A g^-1),the electrode exhibits a reversible specific capacity of 600 m Ah g^-1,corresponding to a capacity attenuation of 0.024%per cycle,and the Coulomb efficiency is close to 99%.Moreover,due to the introduction of abundant mesoporous structure,the electrode also exhibits excellent performance of high sulfur loading.When sulfur loading reaches to 5.2 mg cm^-2,the electrode has a high area specific capacity of 4.5m A h cm^-2 after180 cycles at 0.2 C.(3)Considering the low cost of Prussian blue analogues(PBA),a cobalt based PBA,namely Co₃[Co(CN)₆]₂,with simple synthesis method and high yield as the precursor was selected to prepare Co O hollow nanocage structure with compact surface morphology under step-by-step heating treatment.The compacted particle stacking structure can effectively decrease excessive consumption of electrolyte and reduce side reactions,thus achieving a higher initial Coulombic efficiency(82.2%).Subsequently,nitrogen-doped carbon coated Co O complex(Co N@NC)was further prepared by means of polydopamine-coating and heat treatment.The composite electrode shows better cycle stability(755 m Ah g^-1 after 100 cycles at 0.1 A g^-1)and rate performance(510 mAh g^-1at 1 Ag^-1).