Study On Energy Storage Of Zinc Oxide Derived From Metal–Organic Framework For Secondary Battery

Lithium-ion batteries（LIBs）have been widely used in various fields because of their excellent characteristics.Graphite,as a commercial electrode material of LIBs,has gradually been unable to meet the needs of society,because its theoretical capacity is only372 mAh g^-1.Zinc oxide（ZnO）is regarded as a promising electrode material because of its high theoretical capacity(978 mAh g^-1),good conductivity and controllable morphology.However,the volume of ZnO nanoparticles is easy to expand and agglomerate in the electrochemical process,which leads to its poor electrochemical performance in energy storage.Therefore,a new method is proposed to address the issues of ZnO in electrochemical system in this paper.Firstly,ZnO is combined with the metal-organic frameworks（MOF）by hydrothermal carbonization,since MOF has stable structure,large specific surface area and well-defined ion transport channel,However,the temperature of the preparation process is low（200 ^oC）,leading to the synthesized materials with no obvious crystal shape,and associated with a large amount of organic matters.As a result,there is almost no apparent oxidation reduction peak in cyclic voltammetry,and the electrochemical reaction rate of synthetic materials is low.The reversible capacity in the long cycle test process is only13 mAh g^-1,which cannot meet the requirement of commercial demand.Therefore,in order to solve the problem of poor conductivity of ZnO materials,the synthetic method of ZnO materials by high temperature carbonization was induced.It found that the ZnO materials synthesized at 650 ^oC in N₂ atmosphere had regular hexagonal stable mesoporous structure.At the current density of 0.1C,the reversible capacitance is close to that of 100 mAh g^-1.To improve the conductivity and further increase the capacity of ZnO material,the carbonation temperature was increased to 750^oC,850 ^oC.As a result,it found that the structure of ZnO collapses at high temperature and almost melts together,resulting in an extremely unstable electrochemical performance and a large attenuation of reversible capacity in the long cycles.Finally,graphene oxide（GO）and carbon were introduced into the synthesis process to modify the ZnO material at 650 ^oC.ZnO/rGO/C with unique structure was synthesized by high temperature carbonization method.It exhibited excellent electrochemical performance in lithium ion battery system.After 100 cycles,the capacity remains at 850mAh g^-1,approximately 85%of its theoretical capacity.Notably,ZnO/rGO/C was the first time in using in sodium ion batteries,and explored the electrochemical reaction mechanism between ZnO and Na.That the ZnO/rGO/C also showed an outperformance reversible capacity(300 mAh g^-1,after 100 cycles),which is much higher than that of most electrode materials.This simple method can be further extended to other energy storage applications.