Application Of Graphite/AC Composite Electrodes In Organic Asymmetric Capacitors

Supercapacitors is a new type of power energy-storage device. Comparing with rechargeable batteries, supercapacitors feature high specific power, broad operating temperature range, long cycle life and fast charge-discharge rate. The promising application areas of supercapacitors include solar energy, mobile telecommunication, information technology, consumer electronics, electric vehicles, military and aviation & aerospace. Majority of the commercial supercapacitors today use activated carbon (AC) material as electrodes, since AC has high surface area typically in the 1000 m2/g - 2000 m2/g range. However, the performance of such supercapacitros is limited by the poor conductivity, low specific capacitance and surface irregularity, non-uniform pore size distribution and carbon impurity. Graphite with high conductivity, high reversible specific capacity and unique layer structure, as the negative electrode in lithium-ion battery, has been studied recently as the electrode material of asymmetric capacitors, and its outstanding electrochemical performance have been reported in many research works. In this dissertation, research work has been carried out to investigate the composite electrodes, which are composed of the mixure of high surface graphite (G300, with 280m² g^-1 surface area) and low surface graphite (AGP, 20—30m²g^-1 surface area) with three activated carbons, namely, 100-nmAC (China), YP-17D (Japan), SUPRA-30 (Holland), respectively. The electrochemical performance of the composite electrodes has been studied in asymmetric capacitor, where commercial AC electrode and composite electrodes are used as positive and negative electrodes, respectively. Through a series of experiments, the following conclusions were drawn:1. The CV results of samples S1, S2, S3, S3, S4, S5 (contains 5wt%G300, 20wt%, 30wt%, 50wt%, 65wt%, respectively), grahite(G300) and AC (100nmAC) in a three-electrode using 1.3M TEMABF4/PC,1.0M Et4NBF4/PC and 1.0M LiPF6/ EC+ DMC electrolytes show that: the electrochemical potential window of the composite electrodes is wider with adding G300, and the specific capacitance of S3 is larger than that of other composite electrodes, but less than that of purified 100nmAC meatrial. The ESR of S3 is 4.5Ω(the data of actual axis in 1KHz), which is larger than that of G300 (0.46Ω), but less than that of 100nmAC (5.3Ω). Additionally, symmetric triangular charge-discharge curves and nearly rectangular CV pofiles of composite electrodes showed only electric double-layer capacitance was displayed in composite electrodes.2. The optimum composite electrode S3 was prepared by using G300 and YP-17D, SUPRA 30, respectively. The parallel results were obtained in CV and EIS tests using 1.3M TEMABF4/PC and 1.0M LiPF6/ EC+ DMC.3. The results for graphite electrode (AGP) in two-electrode lithium intercalation experiment and three-electrode CV test using 1.0M LiPF6/ EC+ DMC electrolyte show that the SEI (Solid Electrolyte Interface) film formation potential range on this graphite in the first cathodic process is 0.8V-0.2V, intercalation and deintercalation of Li+ potential range is 0V-0.2V, and the SEI mostly formed in the first cathodic process. The specific capacity of AGP/AC asymmetric capacitor from 1.5V to 4.5 V is 92.5mAh g^-1 at a current density of 25mA g^-1, and 68.5mAh g^-1 at 40mA g^-1.4. The specific capacitance for S3 prepared by AGP and SUPRA 30 is 189.3 mAh g^-1 from 0V up to 3.0V in the 3th discharge process, which is larger than AGP electrode (132.6mAh g^-1) at the same process; the specific capacity for S2 asymmetric device is 144mAh g^-1 at a current density of 50mA g^-1 from 1.5V to 4.5V, and 135 mAh g^-1 at 200 mA g^-1. The cycle life for the C1/S2 hybrid cell (retaining 70.4% after 1000 cycles at 200 mA g^-1) are better than that of C1/AGP hybrid cell (retaining 87.6% after 1000 cycles at 40 mA g^-1). As a result, this composite material combine the advantages of graphite and AC (a good cyclability for AC and large reversible specific capacity for graphite), further work in cycle life testing will be carried out in the future.