Synthesis And Modification Of Prussian Blue Cathode Materials For Sodium Ion And Potassium Ion Secondary Batteries

Among the various energy storage systems currently developed,lithium-ion batteries(LIBs)have the advantages of high energy density and stable cycleling performance,therefore have been widely used in portable electronics and distributed energy systems.However,the disadvantages of the low abundance and high cost of lithium resources limit the sustainable development of LIBs.Ssodium ion and potassium ion batteries(SIBs/PIBs)are cheaper than LIBs in terms of the Na/K resources,which are expected to have broad application in the field of large-scale energy storage.However,compared to Li+(rLi+=0.76 A),Na+ and K+are larger in size(rNa+=1.02 A,rK+=1.38 A),so the diffusion of alkali ions is much slower in the battery and the structural damage of electrode materials is more obvious than in LIBs.Therefore,development of advanced electrode materials is the key to realize high-performance SIB s/PIB s.Prussian blue(AxM[M’(CN)6]1-y·□y·zH2O(0≤x≤2,0≤y≤0.25),A represents alkali metal or alkaline earth metal element,M and M’represent transition metal element,□ represents[M’(CN)6]4-defects,and H2O refers to crystal water)is a class of compounds with open framework structures,providing large number of interstitial spaces in the lattice structure for reversible(de)intercalation of alkali metal ions.The most widely used subclass is hexacyanoferrates(HCFs,M’=Fe),among which the MnHCFs(M=Mn)show higher capacity(dual active centers)and lower costs than others,which have attracted great interests as cathode materials of SIBs/PIBs.However,the mismatch of Na+(2.04 A in diameter)and the interstitial sites(>3.5 A)of the MnHCF framework is dominant,which leads to presence of significant interstitial water and lattice expansion.In order to inhibit the intrinsic irreversible changes of the NaMnHCF as SIB cathod,large alkali-metal ions were introduced into the battery system by two different ways(co-pricipitation and electrolyte addition)as a strategy to modify the structural stability and improve the stability of the cathode.In addition,concerning the promise of KMnHCF in PIBs,the synthesis and electrochemical performances of KMnHCF as a PIB cathode were also investigated as an additional part.(1)In the first part,K+ions were directly introduced in the process of co-precipitation preparation of NaMnHCF.Different ratios of Na+/K+ were added into the solution,obtaining mixed phases of NaMnHCF and KMnHCF in the scale of hundreds nanometers.Although the size of K+is larger than Na+,the distortion of the KMnHCF primitive cell is lower than that of NaMnHCF,and the volume of the unit cell is smaller.The more compact KMnHCF has higher thermodynamic stability,and the larger steric hindrance of K+makes the inherent crystal water content of KMnHCF lower.In the two-phase mixed material,K+can not only participate in reversible deintercalation,but also has a tendency to further diffuse in the crystal grains,which effectively stabilizes the frame structure of MnHCF and improves the stability of cycling.We also tried to introduce larger alkali-metal ion Rb+to synthesize Rb/Na mixed phases(RbNaMnHCF).The results further verifies that tuning the alkali-site is an effective method to improve the structural stability and cycling stability of MnFe-based Prussian blue in SIBs.(2)In the second part,K+ions were introduced into the electrolyte of the battery using as-synthesized NaMnHCF and metallic sodium as the positive and negative materials,respectively.Mixed organic solutions of K+/Na+ were adopted for the electrolyte and the introduction of K+into the active material was completed through the conditioning and cycling processes of the battery.Compared to the co-precipitation method,the as-formed KMnHCF phase in this way was more uniformly dispersed in the primary particles and the its proportion could be maintained for quite a long term,which further promoted the life span of the cathode.The capacity retention of the modified cathode after 500 cycles at 1 C reached over 90%in the mixed ion battery system.(3)In the third part,we studied the influence of different reaction conditions on the synthesis quality of KMnHCF cathode materials,and further tested the electrochemical performance of the materials in potassium ion batteries.The method of crystallization control was used with adjust of several precipitation parameters such as temperature,concentration and complexing agents in order to study the effects of the particle morphology and chemical composition on the electrochemical performances of KMnHCF in PIBs.It is revealed that the particle size and agglomeration of the materials have significant impact on the specific capacity of the cathode material because of the intrinsically poor electrical conductivity of KMnHCF.The samples with smaller particle size and good dispersivity tend to show better cycling and rating performance.