Controllable Preparation And Energy Storage Mechanism Of Nickel-Cobalt Based Electrode Materials

Supercapacitor is a new type of electrochemical energy storage device,which not only has the characteristics of rapid charge and discharge of traditional capacitors,but also has the energy storage characteristics of batteries.Supercapacitor has the advantages of high power density,long cycle life,short charging time,green and environmental protection,and works normally in the temperature range of-40℃ to 80℃,so it can meet the working requirements under extremely harsh conditions.With the above advantages,supercapacitors show great application prospects in the low-carbon and environmentally friendly economic era,and have been widely used in power grid energy storage,rail transit,industrial energy conservation and military weapons.However,its inherent low electrical conductivity,poor rate capability,and volume shrinkage and expansion of electrode materials during cycling seriously hinder the rapid development of supercapacitors.Therefore,the development of supercapacitor electrode materials with high electrical conductivity,high energy density and long cycle life is still one of the main research directions in the future.As the core component of supercapacitors,electrode material has a vital impact on the charge storage capacity.Among the various electrode materials reported so far,nickel-cobalt-based oxides have attracted much attention due to their low cost,mixed oxidation states,and ultra-high theoretical specific capacitance.nickel-cobalt-based oxides have two main crystal structures:the spinel structure(e.g.NiCo2O4)and the rock salt structure(e.g.NiCoO2).So far,many studies on the synthesis and electrochemical evaluation of NiCo2O4 have been reported,but the research on NiCoO2 is still rare.Therefore,this paper mainly focuses on the low conductivity and weak cycling performance of rock salt structure NiCoO2,and attempts to improve its electrochemical performance by means of structural design,heteroatomic doping,surface modification and the introduction of oxygen vacancy.It is extended and applied to the field of aqueous zinc ion battery.The details are as follows:(1)Using dopamine hydrochloride as a suitable carbon and nitrogen sources,nitrogen-doped carbon layers coated on NiCoO2(NiCoO2@N-C)hollow nanospheres were successfully synthesized by facile hydrothermal method and annealing treatment.The morphology,structure and electrochemical properties of the samples before and after doping were systematically analyzed by different characterization methods.Prepared NiCoO2@N-C materials exhibited an admirable specific capacitance of 1028 F/g at a current density of 3 A/g,which is much higher than that of undoped NiCoO2(313 F/g).Even at high current densities of 20 A/g,the specific capacitance values remain at 625 F/g.Therefore,nitrogen and carbon elements doping can not only improve the conductivity of the electrode materials,but also introduce additional pseudocapacitance,thereby exhibiting more excellent electrochemical performance.(2)The mass loading of the N-doped carbon layer can be controlled facilely by changing the dopamine hydrochloride concentration,so as to analyze the effect of different thickness of nitrogen doped carbon layer on the electrochemical properties of NiCoO2 electrode materials.BET results indicated excessive dopamine loading will lead to the agglomeration of nanorods,thereby reducing the specific surface area and resulting in a substantial degradation of electrochemical performance.The electrochemical properties of NiCoO2@N-C electrode materials with different doping concentrations were investigated using a three-electrode system.At the same current density,NiCoO2@N-C-1 sample exhibited the highest discharge specific capacitance.Compared with the undoped NiCoO2 sample,the cycling stability performance increased from the pristine 60%to 91%after 5000 cycles at a current density of 10 A/g.Therefore,appropriate N-doped carbon layer acts as a protective layer to suppresses the volume change during the charging and discharging processes.In addition,the asymmetric supercapacitors were assembled by using the NiCoO2@N-C-1 as the positive material and AC as the negative material for practical applications.With an operating potential window range of 0～1.5 V,the device achieved a high energy density of 45.9 Wh/kg at a power density of 375 W/kg and maintained 35.2 Wh/kg even at a high power density of 3749 W/kg.To identify the real active sites of electrode materials,we used in-situ synchrotron X-ray near-edge absorption spectroscopy(XANES)to dynamically track the changes in the average oxidation state of metal ions during charge-discharge reactions.(3)Optimizing the annealing temperature can significantly adjust the electronic structure,generate oxygen vacancies,and reduce the adsorption energy of active species.Herein,we reported a very simple approach to modulate the surface defects of these as-synthesized electrode materials by controlling the calcination temperature.XRD analysis showed that the phase composition of the samples was different at different annealing temperatures.When the annealing temperature was 300℃,there was a phenomenon that two phases coexisted.This two-phase coexistence could form an efficient electron transport path and improve the electrical conductivity of the composite.Aqueous zinc-ion batteries were assembled using the prepared composites as cathode and metal zinc foils as anode.An admirable capacity of 323 mAh/g was achieved by assembled device at a current density of 0.5 A/g.In order to study the charge storage and conversion mechanism of electrode materials,we also used some advanced characterization methods such as in-situ Raman,ex-situ XRD,cx-situ XPS to detect intermediate products and their existing forms during the charging and discharging process.