Synthesis Of Fe-Mn-based Prussian Blue Analogues And Their Alkali Metal Ion Storage Properties

Lithium-ion battery（LIBs）has the highest energy density among practical rechargeable batteries;novel sodium-ion and potassium-ion batteries（NIBs and KIBs）are rich in resources,which are considered as low-budget alternatives to LIBs,as well as potential secondary batteries for large-scale energy storage.Prussian blue analogues（PBAs）are very attractive as electrode materials for LIBs,NIBs and KIBs,due to PBAs’advantages of open rigid frame structure,multi-active centers,strong structure controllability,easy preparation,non-toxicity and low-cost.Nevertheless,at this stage,the unsatisfactory cycle and rate performance of PBAs limits their practical applications.This work focus on the research on Fe Mn-based PBAs with low-cost and environmental friendliness,aiming at exploring high-performance Fe Mn-PBAs electrode materials.On the one hand,via synthesis method optimization,surface modification,structure and composition regulation,impore the Li/Na/K storage performances of PBAs;on the other hand,the crystallization mechanisms of PBAs in the synthesis process,and the reaction mechanisms of Li⁺/Na⁺/K⁺with PBAs during the charge-discharge process are discussed in depth,to provide theoretical basis and technical support for the design and development of host materials for the next generation of advanced alkali-ion batteries.The main contents of this paper are as follows:（1）Aiming at the[Fe（CN）₆]defects and interstitial H₂O that seriously hinder the transportation of electrons and Na⁺migration,resulting in the poor rate performance of the Na_xFe Fe（CN）₆（Na Fe HCF）,the Na Fe HCF@r GO composite electrode is constructed by surface modification.A new simple one-step in-situ hydrothermal reaction route is designed,and GO with a large number of negative functional groups on the surface is introduced as the nucleation site to make the growth of Na Fe HCF more uniform and orderly,so as to improve the lattice integrity of Na Fe HCF.And GO is reduced to r GO after hydrothermal treatment.The results demonstrate that the short Na⁺diffusion path provided by Na Fe HCF nanoparticles,collaborating with the high electronic conductivity of r GO,could effectively improve the electrode rate performances.In addition,GO can also inhibit the aggregation of Na Fe HCF nanoparticles,and reduce the side reaction of Na Fe HCF nanoparticles with electrolyte,and block the formation of excessively thick SEI film during cycling.Consequently,the Na Fe HCF@r GO composite electrode could deliver a reversible capacity of 75 mAhg^-1at a high rate of 2000 m A g^-1,which is obviously better than that of pure Na Fe HCF(30 mAhg^-1),evincing that the power density of the battery is improved.（2）In view of the blindness of the preparation and modification of Na Fe HCF materials currently,a new reaction path is designed to investigate the formation and growth characteristics of Na Fe HCF crystals.It is found that the nucleation rate is the key to regulate the crystallization of Na Fe HCF.When ferric salts with lower stability constant,such as Fe Cl₃and Fe（CH₃COO）₂,are used as reactants,the nucleation rate is fast,and only spherical or quasi spherical Na Fe HCF nanoparticles with similar composition,structure and morphology could be obtained,which is a kinetically controlled crystallization process.However,when ferric salts with higher stability constant,such as Fe C₆H₅O₇and Fe₂（C₂O₄）₃,are used as reactants,Na Fe HCF crystals have slow nucleation and growth rates,and the crystals grow up in layer-by-layer assembly according to the non-classical crystallization theory,and the products are mostly micron-scale cubic morphology controlled by thermodynamics.Na Fe HCF products with high Na contents,low defects and low crystal water contents can be adjusted in the process of slow nucleation rate,so as to showing high reversible capacity and excellent cycling performance upon Na⁺insertion and extraction.This study can predict and guide the regulation and design of high-performance PBAs crystals.（3）The discharge voltage of Na_xMn Fe（CN）₆cathode is higher than that of Na_xFe Fe（CN）₆,which has the potential to achieve higher energy density.However,its cycling performance is limited by the multiple phase transition during Na⁺insertion/extraction.In this work,a facile strategy is developed to synthesize cubic and monoclinic structured Na_xMn Fe（CN）₆,and their structure evolution are investigated through in-situ XRD,ex-situ Raman characterizations.It is revealed that the monoclinic phase undergoes undesirable multiple two-phase reactions（monoclinic?cubic?tetragonal）due to the large lattice distortions caused by the Jahn-Teller effects of Mn³⁺,leading to poor cycling performances with 38%capacity retention.While the cubic Na_xMn Fe（CN）₆（C-Mn HCF）with high structural symmetry keeps the structural stability during the repeated Na⁺insertion/extraction process,demonstrating impressive electrochemical performances with specific capacity of～120 mAhg^-1at 3.5 V（vs.Na/Na⁺）,capacity retention of～70%over 500 cycles at 200 m A g^-1.In addition,the Ti O₂//C-Mn HCF full battery is fabricated with an energy density of 111 Wh kg^-1（based on the mass of C-Mn HCF）,suggesting the great potential of cubic Na_xMn Fe（CN）₆for practical energy storage applications.（4）In order to guide the further application of PBAs in alkali-ion batteries,electrochemical characterizations combined with theoretical calculations are carried out to study the intercalation properties of Li⁺,Na⁺,K⁺in Mn Fe（CN）₆framework.The micron-sized K_xMn Fe（CN）₆（KMn HCF-L）electrode with low interstitial H₂O and Fe（CN）₆defects contents achieves reversible capacities of 120.9,117.1 and 104.8 mAhg^-1in Li,Na,and K-ion cells,respectively,with average voltage of 3.7 V versus Li/Li⁺,3.5 V versus Na/Na⁺,and 3.8 V versus K/K⁺.Density functional theory（DFT）suggests that with the increase of ionic size,the most stable interstitial site changes from face-centered site to body-centered site（Li⁺:24d;Na⁺:48g;K⁺:8c）.It is also confirmed that Na⁺encounter the lowest energy barriers in the Mn Fe（CN）₆framework,corresponding to the fast migration kinetics,thus resulting in the best rate performances for Na⁺storage.And it is found that the potential to inset K⁺is higher than that of Li⁺and Na⁺,which can be attributed to the stronger bonding between the inserted K⁺and the Mn Fe（CN）₆framework anions.In addition,novel K-ion full cells based on all-PBAs electrodes（K_xMn Fe（CN）₆and Co₃[Co（CN）₆]₂）are assembled,delivering a discharge capacity of 84.9 mAhg^-1with a high average voltage of 2.8 V.This work provides theoretical guidance for the development of high performance and sustainable PBAs materials for alkali-ion batteries.（5）For the sake of expanding the application fields of PBAs electrodes,considering that transition metals（Fe,Mn,etc.）could also realize multi-electron redox reaction at low potential,the electrochemical behavior and storage mechanism of K_xMn Fe（CN）₆（KMn HCF）anodes were further discussed completely.KMn HCF as LIBs anodes with different H₂O content,structure and composition are obtained by controlling the crystallization rate.The results reveal that interstitial H₂O is a key factor to enhance the cycling performance of K_xMn Fe（CN）₆electrode,since H₂O can improve the diffusion and interface transfer of Li⁺in K_xMn Fe（CN）₆structure,and weaken the electrostatic repulsion force between multiple Li⁺ions.Importantly,the K_xMn Fe（CN）₆undergoes conversion reaction during the lithium process,which involves the fracture and recombination of Mn-N bond.Therefore,the KMn HCF electrode with weaker Mn-N bond synthesized at a faster nucleation rate exhibits better cycling performance.And the KMn HCF-25℃-0g electrode shows a reversible capacity of 475.1 mAhg^-1at a current density of 100 m A g^-1,and can stably cycle with～300 mAhg^-1for 1000 cycles,which is the most excellent cycle performance reported so far;even at the high current density of 2000 m A g^-1,it can also deliver a high capacity of 154.0 mAhg^-1.