Synthesis And Performance Of Prussian Blue Analogues As Cathod Materials For Sodium Ion Batteries

Sodium ion battery has the same working principle as lithium ion battery,and with more adundant raw material resource and low cost.Therefore,it is more economical to use sodium ion battery in large scale energy storage systems.However,due to the larger radius of sodium ion,it is particularly important to develop electrode materials with suitable sodium ion intercalation and deintercalation channels.Among all those cathode materials for sodiom ion battery,Prussian blue and its analogues with open frame structure for sodium ions to migrate quickly have good industrial application prospects due to their low price,simple preparation process and excellent electrochemical performance.The purpose of this thesis is to improve the electrochemical performance and promote the commercial application of Prussian blue analogues by exploring the cheap and effective synthesis routes and optimizing the lattice structure.The main contents of this thesis are as follows:（1）Na_xMnFe（CN）₆（PBM）,a manganese-based Prussian blue with low price and excellent electrochemical performance,has broad commercial development potential.We used a simple coprecipitation method to synthesize PBM materials,and explored different raw materials and preparation temperature,and developed a new route for preparing high-performance PBM materials with high yield.PBM-Ⅰ,PBM-Ⅱand PBM-Ⅲwere prepared by coprecipitation method using MnCl₂ solution,Mn（ClO₄）₂ solution and MnSO₄ solution as Mn²⁺precusor,respectively.There is no obvious difference in the micro-morphology of the three obtained PBM products,which all belong to monoclinic crystal system with good crystallinity.PBM-Ⅱhas a high specific charge-discharge capacity for the first cycle,which were 123.9 mAh g^-1 and 115.7 mAh g^-1,respectively,at a current density of 10 mA/g（0.1C）.It also has the best cycling performance.After200 cycles at 100 mA/g（1C）,the capacity retention rate is 53.81%.When using Mn（ClO₄）₂ to prepare PBM material,the by-product was only NaClO₄,which was commonly used as electrolyte salt,thus,the product does not need cleaning,and the yield is high.Therefore,we then chose Mn（ClO₄）₂ as the raw material to synthesize PBM materials at 60℃,25℃and 0℃,respectively,in order to find the optimal preparation temperature.The material synthesized at 60℃has higher sodium content,larger particle size and more regular morphology,which belongs to monoclinic crystal system in lattice structure.The charge/discharge specific capacity of the as-prepared material at 60℃was141.7 mAh g^-1 and 123.7 mAh g^-1 at 10 mA/g（0.1C）,respectively.Thus,we used Mn（ClO₄）₂ as raw material to synthesize PBM material by aqueous coprecipitation method at 60℃to obtain high yield and excellent product performance.（2）Prussian blue（PB）materials synthesized by traditional hydrothermal or coprecipitation method always have a certain amount of coordinated water,which seriously affects the properties of the materials.In order to solve the coordinated water lattice defect of PB,we prepared the iron-based Prussian blue Na_xFeFe（CN）₆ under water free environment by liquid-solid ball milling reaction of Na₄Fe（CN）₆ and glycol solution of FeCl₂.The material has a specific capacity of 121.3 mAh g^-1 at a current density of0.05C.When the current density is increased to 100 mA/g（1C）,the specific capacity is88.2 mAh g^-1.The capacity retention rate of the as-prepared PB reached 72.14%after cycling for 200 cycles at 100 mA/g（1C）in the voltage range of 2.0 V to 4.0 V,which showed the long-term cycle stability was significantly improved.（3）The subsequent cleaning process of the liquid-solid ball milling product is complicated,so we further develolp a completely solid reaction method by using dehydrated Na₄Fe（CN）₆ and Fe₄[Fe（CN）₆]₃ as precursor materials.A NaFeFe（CN）₆cathode material was synthesized by ball milling the Fe₄[Fe（CN）₆]₃/Na₄Fe（CN）₆ mixture.The obtained NaFeFe（CN）₆ demonstrated a single cubic phase indexed to Fm3m space group similar to Fe₄[Fe（CN）₆]₃ but with larger lattice parameter due to the existence of Na⁺in the lattice framework.The NaFeFe（CN）₆ electrode delivered first desodiation capacity of 119.4 mAh g^-1 and first sodiation capacity of 153.6 mAh g^-1 at 0.05C rate.The NaFeFe（CN）₆ showed excellent cycling stability with reversible capacities of 118.2mAh g^-1 and 96.8 mAh g^-1 at 0.1C and 1C rate,respectively.After cycling at 10 mA/g（0.1C）for 200 cycles,the specific discharge capacity is 94.9 mAh g^-1,and the capacity retention rate reaches 80.3%.After cycling at 100 mA/g（1C）for 1000 cycles,the capacity retention rate reaches 61.3%.The results of TGA-MS and In-situ XRD also show that its excellent cycle stability and rate performance are due to the low water content of the as-prepared product and the stable single-phase process during its charge and discharge process.The product prepared by the solid reaction has low coordinated water content,high charge and discharge specific capacity,and good cycling and rate performance.Meanwhile,the as-prepared product does not need complicated post-treatment with high yield and good commercial prospect.