Construction Of Carbon Matrix Transiton Metal Oxide/Sulfide For Sodium Ion Capacitors

As energy demand continues to grow,efficient energy storage technologies and devices are receiving increasing attention.Sodium ion capacitors（Na-HSCs）are emerging as an electrochemical energy storage device with high energy/power density and long cycle life.Na-HSCs are geneally assembled by capacitive and battery-type electrdes.The capacitive electrodes as cathodes have the advantages of high power density and long life due to the fast physical adsorption/desorption processes at the electrode/electrolyte interface,while the battery-type electrodes as anodes exhibt high energy density owing to the sluggish redox reaction in the electrode materials.Numerous efforts have been made over the past decades to find suitable electrode materials to overcome the kinetic imbalance between battery-type and capacitive electrodes.Among all the anode materials,transition metal oxy/sulphides have considered as promising anode materials for Na-HSCs.However,the large volumetric variation during the sodiation/desodiation process limits their application.The introduction of carbonaceous materials is an effient strategy to improve the structural stability and electrical conductivity of electrode materials.In order to improve the electrochemical properties of transition metal chalcogen compounds,carbon hollow sphere and carbon box with good electrical conductivity and structural stability have been introduced to prepare CoMn₂O₄/C hollow spheres and cobalt sulfide nanoparticles/carbon nanobox（CoS/MCNBs）.The main research elements of this thesis are as follows:First,sandwich-shell structured CoMn₂O₄/C hollow spheres have been synthesized by a facile hydrothermal reaction and subsequent calcination,where mesoporous carbon hollow spheres（CHSs）serve as non-sacrificial hard templates.CHSs with numerous mesoporous channels are beneficial for the penetration of reactant ions.Therefore,CoMn₂O₄ nanosheets were successfully deposited on the inner and outer surfaces of CHSs,generating sandwich-shell structured CoMn₂O₄/C hollow spheres.Due to its unique design,CoMn₂O₄/C HS has excellent sodium storage performance with a specific capacity of up to 289 m Ah g^-1 at 0.1 A g^-1 and excellent long cycle performance.A Na-HSC assembled by CoMn₂O₄/C HSs anode and activated carbon cathode exhibits a high energy density(265 Wh kg^-1)and a wide operating voltage range（0.01–4.0 V）.Second,CoS/MCNBs have been desiged and fabricated by the etching-ligand and in-situ convertion treatment of ZIF-67,involving the etching of ZIF-67 with tannic acid and subsequent calcination reduction together with sulphation treatment.CoS nanoparticles（below 20 nm）werte uniformly embedded in the mesoporous walls of carbon nanoboxes.The unique structure edowed it some arresting advantages,including more exposed interfacial area with electrochemical activity,abundant spaces for accommodating the volumetric variation,sufficient and stable electrical contacts,and accessible nanochannels of electrolyte ions.When served as electrode material for Na-HSC,CoS/MCNBs displayed excellent sodium storage capability,including outstanding rate performance and pro-longed cycling durability.After mass balance between the anode and the cathode,a Na-HSC was assembled by using CoS/MCNBs as negative electrode and activated carbon（AC）as positive electrode.It delivered a high energy density of 199 Wh kg^-1 at a current density of 0.1 A g^-1,and a high power density of 7113 W kg^-1.After 1000 cycles,the capacitance retention was still 70%.