Wang Chengyang

Electrochemical Performance of Lithium Ion Capacitors Using Li + -Intercalated Mesocarbon Microbeads as the Negative Electrode

Lithium ion capacitors (LICs) were fabricated using Li -intercalated mesocarbon microbeads (LMCMBs) as the negative electrode and commercial activated carbon (AC) as the positive electrode. The phase structure of LMCMB electrodes was characterized by X-ray diffraction (XRD). LMCMB electrodes retain their original graphite crystal structure when the capacity induced by initial Li + intercalation is less than 200 mAh·g-1. The charge-discharge performances of positive and negative electrodes and LICs were studied using a three-electrode cell. Using an LMCMB electrode as an anode, a stable working potential is obtained at lower voltage than using other electrodes, and the potential range of the positive electrode is extended to a lower range. The electrochemical performance of LMCMB/AC capacitors, including capacitance, cycle life, and efficiency, is improved compared with that of an MCMB/AC capacitor. The efficiency increases from less than 95% to nearly 100%, and the capacity retention is improved from 74.8% to nearly 100% for 100 cycles in a voltage range of 2.0 to 3.8 V. The stable capacity of LMCMB/AC capacitors with cycling is directly correlated to less polarization of AC during the charge storage process, which is caused in turn by the LMCMB negative electrode. Gravimetric energy densities as high as 85.6 1733 Acta Phys. ⁃Chim. Sin. 2012 Vol.28 and 97.9 Wh·kg-1 are obtained in voltage ranges of 2.0 to 3.8 V and 1.5 to 3.8 V, respectively.

Structure and Electrochemical Capacitive Behavior of Novel Crystallite Carbon

A novel crystallite carbon (NCC) was prepared by KOH activation of carbonized petroleum coke precursors. The porous structure, crystallite structure, and microstructure were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), and high resolution transmission electron microscopy (HRTEM) measurements, respectively. The capacitive behavior of the NCC electrodes was also examined with 1 mol·L-1 tetraethylammonium tetrafluoroborate (Et4NBF4) solution in propylene carbonate (PC). It was found that the NCC was composed of much well-grown graphite-like crystallite with average interlayer spacing (d002) of 0.377 nm. The NCC had lower surface area (130.7 m2·g-1) and more distinct crystal character than activated carbon (AC). The energy storage for the NCC depended on the intercalation of electrolyte ions into graphite-like crystallite layers. The specific capacitance of the NCC was 110.6 F·g-1, showing specific energy of 27.5 Wh·kg-1, specific power of over 1.2 kW·kg-1 and excellent cycle stability. Therefore, the crystallite carbon is a promising candidate for novel electrode material.