Research On Prussian Blue Analogues As Cathode Materials For Sodium Ion Batteries

Among a variety of batteries,lithium ion batteries?LIBs?with a superior performance are often viewed as energy storage device.However,limited resource and high price of lithium salts restrict its application.Because of abundant reserves and higher redox potentials of sodium,sodium ion batteries?SIBs?have received considerable attention from the community.What's more,sodium ion batteries are low-cost,environmental friendly and safe.The biggest challenge for developing sodium ion battery is the bad performance of cathode materials,which is the critical aspect of high rate capability and cycling stability in the whole cell.Na-rich type Prussian blue and its analogues?PBAs?are ideal cathode materials for sodium ion battery because of the three-dimensional framework structure and large enough channels for insertion and extraction of Na⁺.This material was firstly reported by Goodenough et.al,and then was further investigated even for aqueous sodium ion batteries.Firstly,a novel titanium-containing Prussian blue analogues TiHCF was synthesized in this paper.To prevent hydrolysis of Ti³⁺,a certain amount of hydrochloric acid was added to control acidic conditions.The material have regular microstructure and the spatial structure consists of trivalent titanium ions,ferrous ions and cyano to form the basic framework.The initial discharge capacity is 79.4 mAh g^-1,and after the cycle stability,the capacity keeps 67 mAh g^-1.In the paper,we also choose two transition metals nickel and manganese elements to synthesize a kind of binary material with high specific capacity and cycling performance.The XRD pattern of sample reveals a well-defined crystal structure of face-centered cubic with Fm3m space group.The SEM results suggest that the s-NMPB material present structured morphology and uniform size with lower moisture.The initial discharge capacity can reach 118.8 mAh g^-1,and after 50 cycles,the electrode retains 73%of initial capacity.The charge-discharge curve shows mutually corresponding platform,among of which 3.83V/3.49V corresponds to the Mn^2+/3+redox couple and 3.58V/3.07V corresponds to the Fe?CN?₆^4-/3-redox couple.Ni²⁺ions are electrochemically inert,so there is no electrochemical reaction during the process of charge and discharge,but it can play a role of supporting the material structure.In order to promote the cycling stability of the electrode,we further synthetic the material with core-shell structure,which successfully increases the surface area of the particles and has a more stable cycle performance and rate capability.The initial discharge capacity is 119.4 mAh g^-1,and it can retain 83%after 100 cycles.At different current density of 50,100,200,400 and 800 mA g^-1,the capacity were maintained 119,106,92,75 and 48 mAh g^-1.Meanwhile,the material of Ni:Mn=1:3showed a better electrochemcial property campered with other ratio.