Modification Of Manganese Prussian Blue Cathode Materials By Regulation Of Alkali Metal Site

In recent years,lithium-ion batteries have been widely used in daily life,bringing great convenience to us.However,there are not abundant lithium resources in the crust,which will limit the use of lithium-ion batteries on tremendous scale.Sodium and potassium are rich in nature,and as the main group elements,they have similar chemical and electrochemical properties to lithium.Therefore,sodium-ion batteries（SIBs）and potassium-ion batteries（PIBs）have great potential in the application of energy storage systems in the future.Compared with the radiu of lithium ion（0.76?）,sodium and potassium ions have larger radius(r _Na+=1.02?,r _K+=1.38?),the intercalation cathode materials for SIBs or PIBs are required to offer larger interstitial sites to host Na⁺and K⁺.Prussian blue and its analogues（PBAs）are promising candidates as cathodes for SIBs and PIBs due to their open frameworks and large interstitial sites,providing accommodating sites and diffusing channels for Na⁺and K⁺.Prussian blue and its analogues can be described as A_xM[M’（CN）₆]1-y·□_y·z H₂O,where A represents alkali-metal site,M and M’are transition-metal sites,□is the[M’（CN）₆]vacany,and H₂O is crystal water.while M=Mn,M’=Fe,the PB is manganese fexacyanoferrate（AMnHCF）.Owing to high voltage platform and capacity,AMnHCF has attracted much attention.AMnHCF,however,has a high content of defects and water,which are introduced in the synthesis process,and the Jahn-Teller effect originated from Mn will result in fast structural degradation.both of which will further erode the electrochemical performance of AMnHCF.The size of the interstitial voids in AMnHCF exceeds 3.5?,which can not only accommodate Na⁺and K⁺but also Cs⁺.In view of the above,the thesis carries out the following works:The first part of this thesis is to modify the surface of NaMnHCF by ion-exchange.In aqueous solution,Cs⁺can spontaneously exchange Na⁺from the NaMnHCF particle surface.Hence the pristine NaMnHCF particle was coated by Cs MnHCF and formed core-shell structure（Cs MnHCF@NaMnHCF）.It shows that the Cs-rich surface plays a positive role in the stability of NaMnHCF structure by prohibiting the leakage of crystal water,stabilizing the solid-liquid interfaces,and solidifying crystal structure.In addition,the coating layer is conducive to Na⁺transport.The electrochemical performance of the core-shell NaMnHCF is dramatically improved with a discharge capacity of 76.3m Ah/g after 1000 cycles at 1.0 C corresponding to a capacity retention of 69.7%,and a reversible capacity of 87.0 m Ah/g at 10 C.The second part of this thesis is to modify the bulk of NaMnHCF by co-precipitation.In the co-precipitation process,different moles of Cs⁺were added into reaction solution to synthesize samples with different Cs content.Because of different precipitation rate of Cs MnHCF from NaMnHCF,the samples show separated phase and Cs MnHCF is evenly distributed in the NaMnHCF particle.Cs MnHCF can effectively inhibit the Jahn-Teller effect of Mn,protect the redox centers of Mn^Ⅲ/Mn^Ⅱand Fe^Ⅲ/Fe^Ⅱand improve the structural stability.In addition,Cs MnHCF in particles can enhance the kinetic of Na⁺.All the modified samples show improved electrochemical performance,the best one can deliver 69.8 m Ah/g after 1000 cycles at 1 C with a capacity retention of71.7%,and a reversible capacity of 79.3 m Ah/g at 10 C.The third part of this thesis is to modify the KMnHCF by Cs,the surface and bulk modification of KMnHCF were carried out by ion-exchange and co-precipitation,respectively.The two kinds of samples were characterized and electrochemically tested as cathodes for potassium-ion batteries and sodium-ion batteries.Because the structural similarity of Cs MnHCF to KMnHCF is lower than to NaMnHCF,the improvement in the electrochemical performance of Cs-modified KMnHCF is inferior to that of Cs-modified NaMnHCF.