Preparation Of Anthraquinone And Prussian Blue Materials And Their Performance As Cathode For Potassium Ion Batteries

With the advancement of industrialization and human society,the expensive and scarce lithium resources cannot meet the large-scale demand for lithium-ion batteries（LIBs）in human production and life.Therefore,it is crucial to develop new type metal-ion batteries as a substitute for LIBs.As a rapidly developing new type of metal-ion batteries,low-cost potassium-ion batteries（KIBs）are expected to be developed as substitutes for LIBs.Research on KIBs electrode materials emerges in an endless stream,and the study on cathode materials is the focus of current KIBs research.Among the many KIBs cathode materials,organic materials and Prussian blue materials have been widely studied due to their various advantages.Organic materials are mainly composed of C,H,O,and N elements,and do not contain heavy metal elements that pollute the environment.Organic materials are environmentally friendly,cheap,easy to obtain,and high theoretical capacity.The small organic molecules anthraquinone material achieves the purpose of energy storage through the redox reaction of C=O and the combination and separation of K⁺,which has the advantage of high theoretical mass specific capacity.However,due to the characteristics of similar compatibility,anthraquinones are easily dissolved in organic electrolytes,and it is difficult to perform stable charge-discharge reactions in KIBs,resulting in poor cycle stability.Therefore,solving the problem of the dissolution of anthraquinone materials in organic electrolytes is the focus of current research.Prussian blue material has the advantages of high potential,simple preparation,cheap and easy availability,and is an ideal cathode material for KIBs.The Prussian blue electrode material is mainly prepared by co-precipitation reaction.The fast co-precipitation reaction leads to the rapid crystallization of the Prussian blue material,thereby introducing a large amount of defects and crystal water into the framework of the material.The presence of these defects and crystal water reduces the electrical conductivity and the stability of the framework of the Prussian blue,which exhibits lower rate capability and cycling stability in KIBs.In response to a series of problems existing in anthraquinone materials and Prussian blue materials,we modified these two materials.First,1,5-dihydroxyanthraquinone potassium was prepared by reaction between 1,5-dihydroxyanthraquinone and strong alkali.The similar compatibility of organic materials in organic electrolytes was weakened by introducing-OK group,and cycling performance of 1,5-dihydroxyanthraquinone was improved.Second,we prepared the Prussian blue composites in situ coated with polypyrrole by the reducibility of pyrrole and the oxidative property of Fe³⁺,which reduced the content of defects and crystal water in the Prussian blue framework,and the polypyrrole coating also improved the conductivity of the composites,thereby improving the electrochemical performance of the Prussian blue material.The specific research results are as follows:（1）The study on properties of Potassium 1,5-dihydroxyanthraquinone as cathode in potassium-ion batteries.Potassium 1,5-dihydroxyanthraquinone was prepared by treating 1,5-dihydroxyanthraquinone with KOH-ethanol solution to convert-OH to-OK.This method introduces-OK on anthraquinone,which reduces the solubility of this organic material in organic electrolytes.Potassium 1,5-dihydroxyanthraquinone was used as a cathode material in KIBs and obtained a high initial capacity of 226.1m Ah g^-1 at the current density of 100 m A g^-1 and the rate capacity of 143.1 g^-1at the current density of 500 m A g^-1.According to the cyclic voltammetry curve and the galvanostatic charge-discharge curve,the charging process of potassium 1,5-dihydroxyanthraquinone is a one-step two-electron reaction,where two K⁺are simultaneously separated from the carbonyl group of the electrode material,and the discharge process is a two-step continuous one-electron reaction process,where two K⁺are combined with two carbonyl groups on the electrode material,respectively.（2）The study on properties of Prussian blue-polypyrrole composites as cathode in potassium-ion batteries.As a strong reducing agent,pyrrole reduces Fe³⁺to Fe²⁺,which provides a precipitant for the Prussian blue co-precipitation reaction.As a polymerization initiator,Fe³⁺promotes the in-situ polymerization of pyrrole on the surface of the Prussian blue material and form a coating layer.Hence,Prussian blue-polypyrrole composites were prepared.The continuous reduction of Fe³⁺to Fe²⁺by pyrrole slows down the direct co-precipitation reaction rate,and reduces the content of crystal water and defects in the Prussian blue,The crystallinity of Prussian blue of the composite material is improved.The PB-PPY composite obtained a high initial capacity of 108.6 m Ah g^-1 at a current density of 50 m A g^-1 as cathode in KIBs,and was able to cycle stably for 900 cycles.On this basis,we also explored the energy storage mechanism of PB-PPY composites in KIBs,and proved that part of K⁺in Prussian blue did not participate in the charge-discharge cycle,but remained in the framework to play a supporting role.