Study On Preparation And Electrochemical Performance Of Spinel Cathode Materials For Aqueous Zinc Ion Batteries

Rechargeable aqueous zinc ion batteries（ZIBs）is an important electrochemical energy storage device due to their unique advantages such as high safety,low cost,and abundant materials,which may be one of the alternatives to large energy storage devices in the future.Firstly,the use of neutral,cheap water-based electrolytes in ZIBs is safer and greener.Secondly,the electrodes and electrolyte materials used in ZIBs are cheap and have high reserves.They are stable in the air and do not react with water or oxygen.Among the candidate materials for cathode of zinc ion batteries,manganese oxide is one of the materials with potential due to its rich valence states(Mn²⁺,Mn³⁺,Mn⁴⁺,Mn⁷⁺).Among them,ZnMn₂O₄ has a spinel structure similar to that of LiMn₂O₄,Zn²⁺ occupies a tetrahedral position,and Mn³⁺ occupies an octahedral position in the spinel structure.This mixed metal oxide enhances the synergy of its components.In this paper,ZnMn₂O₄ is taken as the researchVIobject,and its inherent poor electrical conductivity and large volume changes caused by dezincification are modified to study its electrochemical performance and energy storage mechanism as cathode material of ZIBs.The specific research content of the paper is as follows:（1）The ZnMn₂O₄ composite electrode material with nickel and cobalt co-substituted and nitrogen-doped graphene modification was synthesized by a one-step hydrothermal method.A suitable amount of nickel and cobalt co-substituted spinel ZnMn₂O₄ nanoparticles were uniformly loaded onto nitrogen-doped reduced graphene oxide by a one-step hydrothermal method.The as-prepared cathode material(ZnNi_xCoyMn_2-x-yO₄@N-rGO)for accommodating the intercalation of zinc ions displayed excellent electrochemical performance,with a reversible capacity of 95.4 m A h g^–1,achieved at 1000 m A g^–1 after 900 cycles,and a capacity retention ratio of 79%.When the current density increased from 10 m A g^–1 to 1500 m A g^–1,high capacity(200.5 m A h g^–1 to 93.5 m A h g^–1)was achieved,which was much higher than that of ZMO@N-r GO without nickel and cobalt co-substituting(184 m A h g^–1 to 59.2 m A h g^–1),demonstrating excellent rate performance.XRD tests on Zn Nix Coy Mn2-x-y O4@N-r GO found that Ni and Co co-substituting ZnMn₂O₄ can increase its unit cell volume,which is an effective approach to promote Zn2+ de-intercalation and to stabilize the spinel structure in order to suppress the Jahn-Teller distortion of Mn3+.Isalso the reason for these excellent electrochemical performance.In addition,the modification of graphene effectively improved the electronic conductivity of the material.Therefore,nickel and cobalt co-substituting of spinel ZnMn₂O₄@N-r GO with a stable structure opens up new possibilities for large-scale application of rechargeable,aqueous ZIBs.（2）The composite electrode material with ZnMn₂O₄ and carbon nanotubes positioned coaxially and grown was designed and prepared to improve the performance of zinc ion batteries.ZnMn₂O₄/CNT composites were prepared by hydrothermal method,and the effects of different carbon nanotube content on the structure and electrochemical performance of the composites were discussed.Hydrothermally produced nanoparticles are uniformly dispersed on multi-walled carbon nanotubes,and the average particle size of ZnMn₂O₄ is about 50 nm.The high conductivity and strong skeleton of carbon nanotubes not only improve the electronic conductivity of the material,but also enhance the structural stability of the material during cycling,so it has more excellent electrochemical performance.Performance tests on it revealed that the discharge capacities at current densities of 10,50,100,200,500,1000,1500 and 2000 m A g^–1 were 217,183,164.3,142.7,129.1,114.4,106.2 and 104.1m A h g^–1,respectively.When the current density returns to 10 m A g^–1,the discharge specific capacity can still reach 228 m A h g^–1,showing excellent rateperformance.The ultra-high initial specific capacity of 134 m A h g^–1 is displayed when the current density is 2000 m A g^–1,the discharge capacity of 76 m A h g^–1 can be maintained after 600 cycles.