Preparation,Modification And Electrochemical Performance Of Manganese Based Cathode Materials For Aqueous Zinc-Ion Batteries

Aqueous Zinc-ion Batteries（AZIBs）have the advantages of low cost,high safety and environmental protection,so it has a broad application prospect in the field of large-scale energy storage.The development of high-performance cathode materials is one of the key factors to realize the industrialization of AZIBs.However,the most promising manganese oxide are favored by researchers due to their low cost,rich raw materials,little environmental harm and high theoretical specific capacity.There are still several problems in the study of manganese based oxides:Low conductivity,manganese dissolution,electrostatic repulsion effect,by-products and oxygen evolution reaction.Aim at above problems,in this paper,the manganese-based cathode materials were modified by structural design,surface modification,biphasic composite and metal ion doping in order to achieve better electrochemical performance.（1）Using electrospinning and microemulsion method to synthesize fibrous and microspheric Zn Mn₂O₄respectively,they then are coated with reduced graphene oxide（r GO）,and r GO coated Zn Mn₂O₄composites were obtaine.The results show that although the morphology of the fibrous Zn Mn₂O₄is destroyed during the coating process,and the maximum reversible specific capacity was 141.8 m Ah g^-1at0.1 A g^-1current density,which can still maintain 50.7 m Ah g^-1after 250 cycles;however,the microspheric Zn Mn₂O₄keeps its original morphology during the coating process,and the maximum reversible specific capacity was 168.13 m Ah g^-1at 0.1 A g^-1current density,which can still maintain 93.8 m Ah g^-1after 250 cycles.Comparing the two,it is found that the composite prepared by microemulsion has advantages in morphology and electrochemical performance.Therefore,the combination of structural design and surface modification is a good strategy to improve the electrochemical performance of manganese based positive materials.（2）Carbon nanotubes（CNT）coated Zn Mn₂O₄/Mn₂O₃composites were successfully synthesized from zinc nitrate,manganese acetate and CNT via microemulsion method.The XRD and SEM analysis showed that CNT was uniformly coated on the surface of Zn Mn₂O₄/Mn₂O₃composite.The results of electrochemical test show that the maximum reversible specific capacity of CNT coated Zn Mn₂O₄/Mn₂O₃composite was 112.8 m Ah g^-1at 0.1 A g^-1current density,which can still maintain 49.6 m Ah g^-1after 300 cycles.The kinetic characteristics of the electrode were investigated by CV and EIS,and it was proved that the diffusion rate of Zn²⁺was increased by the biphasic composite,and the charge transfer process was improved by the coating of CNT.Therefore,the biphasic composite modification method provides an effective modification idea for the research of Zn Mn₂O₄cathode material with low capacity.（3）Al dopedδ-Mn O₂micron materials（Al-KMO）were prepared by hydrothermal method using potassium permanganate and aluminum nitrate as raw materials.The effects of Al doping on the morphology,phase,crystal water content,electrochemical performance,electrode process kinetics and energy storage mechanism of cathode materials were discussed.The experimental results show that the morphology of Al-KMO is micron flower composed of nanosheets;there is about 10.75%crystal water in Al-KMO,and the high content of crystal water is not only conducive to the stability of the material structure,but also can improve the ion diffusion rate in the material;the maximum reversible specific capacity of Al-KMO reached 269.5 m Ah g^-1at 0.5 A g^-1current density,which can still maintain 205.8m Ah g^-1after 300 cycles;the results of CV and EIS show that Al doping can improve the conductivity of the material and increase the diffusion rate of Zn²⁺in the material;the energy storage mechanism of Al-KMO is the joint action of H⁺and Zn²⁺co-insertion/co-extraction and dissolution/deposition mechanism through the analysis of in-situ XRD,HRTEM and XPS.Therefore,Al doping can improve the cyclic stability ofδ-Mn O₂,enhance the conductivity and Zn²⁺diffusion coefficient,etc.（4）Na-containing manganese-based composite of Na_0.44Mn O₂/Mn₂O₃and Na_2/3Fe_1/2Mn_1/2O₂are synthesized correspondingly by sol-gel methods combined with freeze-drying technique.The results of morphology characterization showed that the Na_0.44Mn O₂/Mn₂O₃composite with a cubic box morphology are composed of primary nanoparticles with a diameter of about 150～300 nm;the morphology of O3-Na_2/3Fe_1/2Mn_1/2O₂is uniform 3D solid block structure and platelet-shaped particles after calcined at 850°C;while the morphology of P2-Na_2/3Fe_1/2Mn_1/2O₂prepared by poly（vinyl alcohol）dispersion and calcination at 850°C is amorphous carbon interconnected spherical core-shell structure.The results of electrochemical test show that the reversible specific capacity of 68.3 m Ah g^-1and 42 m Ah g^-1can be maintained after 200 cycles of Na_0.44Mn O₂/Mn₂O₃and P2-Na_2/3Fe_1/2Mn_1/2O₂at 0.4 A g^-1current density;the specific capacity of Na_0.44Mn O₂/Mn₂O₃composite increases due to the co embedding of Na⁺and Zn²⁺in Na_0.44Mn O₂and Zn²⁺in Mn₂O₃,which indicates that Na_0.44Mn O₂and Mn₂O₃have synergistic energy storage effect;and because of Fe doping and carbon coating are beneficial to stabilize the structure of the composite,the cycling performance of Na_2/3Fe_1/2Mn_1/2O₂composite is greatly improved.