Preparation Of Chitosan-based Composite Carbon Material And Its Sodium Storage Performance

Sodium-ion batteries have abundant carrier Na in the earth’s crust,have huge cost advantages in the field of electrochemical energy storage,and have similar working principles with the successfully commercialized lithium-ion batteries,and become the next generation with promising applications.Energy storage technology.However,the large Na+radius results in slow sodium intercalation kinetics.In addition to the high standard electrode potential,the energy density of sodium ion batteries is still low.The key to the development of high performance SIB is to research and develop suitable electrode materials.Hard carbon materials stand out among the anode materials due to their structural stability,physical and chemical stability and excellent cost advantages.However,their low energy density and lack of rapid charge and discharge capabilities are currently urgent problems in the field of carbon materials for sodium storage.Generally,the limited interlayer spacing of carbon materials will cause the problem of poor sodium diffusion kinetics.Therefore,the surface adsorption and storage of sodium with excellent kinetics can effectively improve the rapid charge and discharge capabilities of carbon materials.Based on the above problems,this paper uses chitosan-based carbon materials as a three-dimensional conductive framework,introduces different proportions and types of carbon materials,and modifies the chitosan-based carbon materials to improve the rapid charge and discharge capacity and Na storage performance,and further optimize the material structure to improve the energy density of the chitosan-based carbon material.Combined with various test and characterization,the storage pattern of Na+in chitosan-based composite carbon materials was studied,and the contribution of capacitive sodium storage was digitally evaluated by CV calculations at different scanning speeds.In the preliminary test,we found that the directly carbonized chitosan-based carbon material has fewer defect sites and poor sodium storage performance;the three-dimensional conductive framework of chitosan-based carbon material prepared by freeze-drying can improve the electrochemical reaction on the surface of the material.However,there is still room for improvement in surface sodium storage sites and performance.We have carried out research on this and reached the following conclusions:(1)Three kinds of carbon-carbon composite materials were prepared by introducing coal-based carbon particles of different qualities into the chitosan gel,freeze-drying and carbonization.The introduction of coal-based carbon particles causes changes in the microstructure of chitosan-based carbon materials,and realizes the transformation of"sheet-network-fiber" structure.Coal-based carbon particles carry a lot of negative charges,and the chitosan gel,which carries a lot of positive charges due to the protonation of-NH2,is uniformly dispersed and closely adhered in the gel system.The surface is rich in a lot of defects,which can provide Na storage sites and improve the Na storage performance of the material.At the same time,the three-dimensional chitosan-based carbon material skeleton prevents coal-based carbon particles from agglomerating.The synergy of the two makes the chitosan/coal-based carbon particle composite with carbon particles accounting for 70%of the total mass excellent in electrochemical performance.After a long cycle of 200 cycles at a small rate and 3000 cycles at a large rate,it has excellent stability,non-destructive capacity,high efficiency,and demonstrated high-quality specific capacities of 312.5 mAh g"1 and 170 mAh g-1.Through the CV test and the charge-discharge curve,it is proved that the material mainly stores sodium through surface adsorption,and has good rapid charge-discharge ability.(2)A thick electrode of chitosan-based carbon material with low tortuosity was prepared by directional freeze-drying.This binder-free stable electrode has a unique vertical pore structure,and its charge transfer kinetics hardly attenuates as the load increases.But its sodium storage performance is almost zero at a large rate.We introduced two kinds of nano-carbon materials as conductive additives to improve the electronic conductivity of the framework,and prepared two kinds of chitosan/conductive nano-carbon composite low tortuosity thick electrodes.Conductive nano-carbon significantly improves the conductivity of thick electrodes.In addition,a large number of defects are created on the surface of conductive nano-carbon through mixed acid treatment,so that its sodium storage performance can be improved.Among them,the composite thick electrode compounded with graphene exhibits excellent sodium storage performance,and exhibits an area specific capacity of 2.61 mAh cm-2 at a rate of 0.1 C.Subsequent CV tests with different sweep speeds showed that its pseudocapacitance energy storage contribution reached 86.3%under 1.0 mV s"1,which proved that the sodium storage mechanism of this type of electrode is mainly surface adsorption,and the introduction of carbon materials can affect chitosan-based carbon materials.Rapid sodium storage on the surface has a positive effect.In addition,through the improvement of the device,the output can be expanded and the cost can be reduced under the premise of maintaining the vertical channel.