Carbon Hybrid As Anode Material For Super Lithium Ion Capacitor

Super lithium ion capacitor (SLIC) is a new kind of energy storage device possessing high energy density as well as high power density. It storages energy mainly through electric double layer and lithium ion intercalate-deintercalate. It is significant to exploit "dual-functional" anode for SLIC. In this paper, porous carbons which can storage energy through electric double layer were firstly blended into graphitic carbons which can storage energy through lithium ion intercalate-deintercalate. Various of carbon composites were prepared for SLIC through different methods. Their physical characteristics and electrochemical performance were investigated. On the basis of the results, a "dual-functional" carbon composite named as PGCC was obtianed, a working model of anode was established and the energy storage mechanism was explained. The main conclusions are as follows:(1) Series of carbon hybrids as anode materials were prepared for SLIC for the first time by ultrasonic mixing using graphite (G) and active carbon (AC) as raw materials. The influence of G/AC ratio on the electrochemical behaviors of anode both in battery and capacitor were investigated by constant current charge-discharge test. The results showed that the compound anode had good capacitive performance as well as Li-ion battery performance. When AC content was less than 10%, the battery maintained high performance while the capacitor possessed capacitance of 10 F/g. This showed it was possible to obtain an anode material that have both capacitor characteristic and battery characteristic from G and AC. But when AC content increased, the cycle performance of anode became worse.(2) Two series of carbon composites named as SCSC and HCSC were prepared by in-situ method using carbonaceous mesophase spherules (CMS) as core material and pitch/sucrose as the active carbon shell precursor. The results showed that a layer of AC-shell had been produced on the CMS surface and the AC-shell formed a cross-linked matric. The materials were optimized through changing the precursor and controlling the shell thickness. The optimized compound material exhibited both typical electric double layer performance in Et4NBF4/AN electrolyte with capacitance of 27.3F/g (0.5A/g) in capacitor and typical Li ion intercalate-deintercalate behavior in LiPF6/EC+DMC electrolyte with high reversible capacity of 342.2mAh/g (0.2C) in half-cell. The compound showed better rate performance and cycle ability than CMS. However the disadvantage of this compound was low capacitance resulting from the low mesoporosity.(3) A series carbon composites named as PGC were prepared by sol-gel template method using silica sol as template and sucrose as carbon source. Transmission electron microscope (TEM) and pore structure analysis results showed that mesoporous carbon had been prepared with average pore diameter of 15-20nm. Electrochemical test resluts showed the PGC series materials were better than HCSC series that the mesoporosity were bigger and capacitance were higher. The PGC material exhibited good performance in both capacitor and battery with capacitance of 54.8F/g in the capacitor and high reversible capacity of 379.4mAh/g in the half-cell when optimum silica sol/sucrose ratio and pyrolysis temperature were adopted. The potential window of the optimum PGC-2 could be as high as 3.5V vs Li/Li+ and its cycle performance was the best among the PGC series. However the PGC suffered from large irreversible capacity at the first cycle and large lagging voltage between charge and discharge caused by sucrose hard carbon.(4) Based on the characteristics of SLIC and the results above, an optimum dual-functional anode material PGCC used for SLIC was designed as follows:it had a core/shell structure, CMS which has the optimum lithium ion intercalate-deintercalate characteristics was choosed as core material and PGC as shell material. The PGCC was prepared by generation in-situ method combined template method. The optimum condition of PGCC preparation was CMS/sucrose ratio 10:4, sucrose/silica sol template ratio 1:2, and pyrolysis temperature 1000℃. Then PGCC was employed into capacitor, half-cell and eventually LiFePO4-AC/AG-AC type SLIC. Capacitance of 34.4F/g and capacity of 400mAh/g were obtained. It also possessed good rate performance and cycle performance. The charge and discharge capacity retention of 5C/0.5C were 92.3%and 85%, the capacity retention after 50 cycles was 93.5%.(5) Charge and discharge curves of LiFePO4-AC/AG-AC type SLIC showed that in the initial and platform stage, the curves showed a certain degree of capacitor characteristics. It indicated that the energy storage occured not only in the lithium ions intercalate-deintercalate process but also in the double-layer adsorption process. Then a model of SLIC was established. The model included three-part, including the ideal compound cathode, the ideal multi-ion electrolyte and the compound anode studied in this work. According to the model, the energy storage reactions of compound anode occuring in the lithium ions intercalate-deintercalate and double-layer adsorption process in SLIC were described. The double-layer adsorption energy storage mechanism, lithium ions intercalate-deintercalate energy storage mechanism and synergy between the two energy storage mechanisms were described through the interaction between electrolyte and porous carbon shell and interaction between CMS core and porous carbon shell interaction.