Preparation And Electrochemical Performances Of Novel Two-dimentional Carbon Nanosheets

Lithium-Ion Batteries (LIBs) and Supercapacitors (SCs), as a new generation ofenergy storage devices, have broad application prospects in potable devices, electricautomobiles and hybrid electric vehicles due to their long cycle life, environmentalfriendly, safety and stable properties. Electrode materials are the key component ofthe LIBs and SCs, their properties are the important factors determining theperformances of the devices. Therefore, developing new type of electrode materialshas become a focus topic for researchers.Two-dimensional carbon nano-materials, represented by graphene, are a newtype of carbon materials. They are very suitable for applications in LIBs and SCs dueto their unique structure, such as high surface area, excellent mechanical property andsuperior conductivity. Up to now, the two-dimensional carbon nano-materials can besynthesized by templated method, chemical vapor deposition (CVD) and reduction ofgraphite oxide methods. However, the templated and CVD methods suffer fromcomplex procedures, high costs and low efficiency, leading to limited yield. It can beprepared in large scale using reduction of graphite oxide method, but the materialsprepared by this method contain much defects, meanwhile, the degree ofgraphitization, mechanical property and conductivity of these materials are rather low,resulting in low rate performance. Hence, searching for simple and low-cost methodfor synthesizing two-dimensional carbon nano-materials with high properties remainsa great challenge.This work focused on preparing two-dimensional carbon nanosheets orin situ-activated two-dimensional carbon nanosheets by one-step pyrolysis method orKOH in situ-activating method using NaCl as a template and a dispersing agent. Theenergy storage properties and mechanism of these two-dimensional carbon nanosheetswere investigated.The precursor was prepared using glucose as carbon source, ferric nitrate ascatalyst precursor and sodium chloride as dispersing agent and template through adissolving-drying method. Then the precursor was calcined at a certain temperatureand the two-dimensional carbon nanosheets were obtained. The effects of thepyrolysis technical parameters on the morphology and microstructure of carbonnanosheets were studied, and then these carbon nanosheets were used for LIB anode.The results showed that calcining temperature has great effect on the size andcrystallinity degree of carbon nanosheets. Higher calcining temperature leads tobigger size of the nanosheets and better degree of crystallinity and higher content ofNaCl in the precursor results in thinner nanosheets and lower degree of crystallinity.The sample of1-700was selected for testing its lithium storage properties due to that it possesses high crystallinity and suitable specific surface area. It was found that avery high reversible capacity of up to722mAh g-1at a current density of100mA g-1was achieved. Moreover, the material has a high rate capability of535,485,380,200and115mAh g-1at1C,2C,10C,20C and30C, respectively (1C=372mA g-1).Another precursor was prepared using glucose as a carbon source, ferric nitrateas a catalyst precursor, sodium chloride as a dispersing agent&template andpotassium hydroxide as an activating agent through a dissolving-freezing-freezingdrying method, which was calcined at a certain temperature to obtain the KOHin situ-activated two-dimensional carbon nanosheets were obtained. The effects ofvarious physical chemistry conditions on the morphology and microstructure ofcarbon nanosheets were systematically studied, and then the as-prepared carbonnanosheets were used for supercapacitors. The results showed that the content ofKOH has great influence on the pore size of the nanosheets, higher content of KOHleads to lower ratio of micropores. The calcining temperature can influence the poresize distribution and the degree of crystallinity of carbon in the nanosheets. Withincreasing the calcining temperature under a certain range, the ratio of micropores wasdecreased and the ratio of mesopores and macropores was increased as well as thedegree of crystallinity was also increased. The electrochemical test results showedthat in the solution of6mol/L KOH, using three-electrode system, the capacity of thecarbon nanosheets can reach183F g-1at a current density of0.5A g-1. The bestsample possesses a capacity of130F g-1at a current density of0.5A g-1. When thecurrent density increases to20A g-1, the capacity of carbon nanosheets still remains95F g-1.