Composition And Architecture Design Of ZnMn₂（PO₄）₂ Crystals And Zinc Storage Performance

Aqueous zinc ion batteries（ZIBs）have the advantages of affordability,safety,pro-environment and resource-rich resource.It can effectively avoid the problems of poor safety and serious environmental pollution of many organic electrolyte batteries,and it becomes the key research direction of energy storage system in recent years.However,the low energy density of ZIBs greatly impedes its practical application.The key to breaking through the bottleneck of ZIB technology is to develop novel positive electrode materials with large reversible specific capacity and outstanding cycle life.Thus,the exploiting for appropriate cathode materials is a vital matter for developing high-powered ZIBs.In this work,ZnMn₂（PO₄）₂ materials have been systematically researched for searching the novel cathode materials for ZIBs.It is expected to enhance the structural stability and cycle life of ZnMn₂（PO₄）₂ compound through optimizing the synthesis process,adjusting the electrolyte composition,regulating the composition and designed the architecture for ZnMn₂（PO₄）₂materials.This work primarily explored as the following three parts:1.ZnMn₂（PO₄）₂ hollow sphere was prepared by a coprecipitation method and pioneeringly employed as the cathode materials for ZIBs.So as to optimize the preparation process,the effects of solvent,additive,Zn-Mn ratio,calcination temperature and calcination time on the composition,structure and zinc ion storage properties of the electrode materials have been systematically studied.According to the research results,the ZnMn₂（PO₄）₂ material synthesized with the conditions of using water as solvent,employing PVP as additive,utilizing the molar ratio of Zn:Mn=1:1.5,and designing the calcination temperature and time for 700 ^oC and4 h has optimal structural integrity and hollow configuration,which owns the more reactive active sites and better structural stability,providing the superior cycling stability and rate property.2.In order to reduce the generation of zinc dendrite and improve the cycle life of the materials,the electrolyte composition has been controlled in this part.To this end,we investigated the effect of three different electrolytes（3M Zn SO₄,3M Zn SO₄+0.05M Mn SO₄ and 3M Zn SO₄+0.05M Mn SO₄+0.05M SDS）on the electrochemical property of ZnMn₂（PO₄）₂ materials.The results shown that the internal short circuit phenomenon of ZnMn₂（PO₄）₂ material in Zn²⁺storage process can be effectively restrained by adding SDS in electrolyte,and the specific capacity has been improved with the cycling time of 2000 h.It is concluded that the introduction of SDS can efficiently inhibit the generation of zinc dendrites,and enhance the cycling life and capacity retention.3.By studying the mechanism of zinc storage,it is found that the phase of the material changes from ZnMn₂（PO₄）₂ phase to ZnMn₂（PO₄）₂·4H₂O phase during the cycle.Therefore,we designed ZnMn₂（PO₄）₂·4H₂O materials transformed from ZnMn₂（PO₄）₂ via a facile water treatment strategy.It was found that the morphology of ZnMn₂（PO₄）₂ material gradually changed from hollow sphere to ultra-thin micro-sheets structure with a higher specific surface after water treatment for 5 days.The results of electrochemical performance shown that the modified cathode materials delivered excellent capacity retention and stable electrochemical phase transition process at high rate of 200 m A g^-1.This research provides an instructive guide for the synthesis of ZnMn₂（PO₄）₂·4H₂O materials for ZIBs cathode materials.