Organic Solvothermal Synthesis Of Monoclinic Prussian Blue As Cathode Materials For Sodium Storage

The ever-increasing demands of large-scale energy storage makes it vital to develop advanced battery techniques featuring long lifespan,environmental friendless and low cost.Sodium-ion batteries（SIBs）have been regarded as prospective next-generation energy storage techniques with the rich-abundance reserves.Compared with Li⁺ion,Na⁺ion has larger ionic radius and heavier atomic mass,leading to the sluggish ion diffusion kinetics and severe crystal distortion during the Na⁺insertion/extraction process among crystal structures of cathode.Therefore,it is necessary to match suitable cathode and anode materials to achieve higher specific capacities and longer cycle life.To explore an ideal cathode material with open structure and suitable for large size cation de-intercalation is an important research direction for the future development of SIBs.Prussian blue and its analogues（PBAs）have emerged as a focal point linked to the rigid open structure,large interstitial spaces,and facile preparation.However,Prussian blue（PB）synthesized by conventional co-precipitation method shows low specific capacity and poor cyclic stability due to low sodium content,poor conductivity and large numbers of crystal water.Therefore,it is vital to design the preparation process of PB to make the electrode material exhibit excellent sodium storage properties.In this paper,monoclinic Prussian blue（M-PB）is prepared by organic solvothermal method.By exploring the reaction conditions in the synthesis process,a salt-in-water-in-oil environment is created.Compared to the traditional cubic Prussian blue（C-PB）,M-PB shows a superior electrochemical performance and the mechanism of sodium storage in the electrochemical process is analyzed.The main work is divided into the next aspects:1.PB was synthesized by ethylene glycol solvothermal method,in which a salt-in-water-in-oil environment was created.Ethylene glycol（EG）plays significantly important roles in the reaction process:（1）constructing micro-reaction space to isolate different particles and spatially restrains the rapid occurrence of the entire nucleation process;（2）providing the viscosity to slow down the transition metal ion and ferrous cyanide ion binding rate,ensuring the integrity of crystal structure;（3）preventing Fe²⁺from oxidation with the help of mild reducing agent.2.The effects of solvent composition,reaction temperature and reaction time during the preparation of PB were investigated in detail.Compared to the traditional co-precipitation method,M-PB was prepared by controllable synthesis process,which limits the formation of[Fe（CN）₆]^4-vacancies and contents of water molecules.3.The differences in crystal structure,physical and chemical properties and electrochemical properties of PB with different crystal phases were systematically investigated.Compared to C-PB,M-PB featuring stable monocline phase structure and limited water content exhibits an initial capacity of 120 mAh g^-1 at 50 mA g^-1 and a long cycle stability of 77.5%after 1000 cycles at 2 A g^-1.In addition,the structural evolution of M-PB was revealed by Raman,FT-IR and XRD techniques.