Xifei Li

Effect of Doping Ions on Electrochemical Capacitance Properties of Polypyrrole Films

Abstract Conducting polypyrrole (PPy) films doped with p-toluenesulfonate (TOS−), ClO4−, and Cl− were electrochemically prepared, respectively. The electrochemical capacitance properties of the PPy films were investigated with cyclic voltammetry (CV), galvanostatical charge/discharge, and electrochemical impedance spectroscope (EIS) techniques. The morphology observation and structure analysis of PPy films were performed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that PPy-Cl and PPy-TOS were characterized with a highly porous and ordered structure, which led to their fast ion switch processes. Moreover, they exhibited a rectangle-like shape of voltammetry characteristics even at a scanning rate of 50 mV·s−1, a linear variation of the voltage with respect to time in the charge/discharge process, and almost ideal capacitance behavior in low frequency, even in deeply charged/discharged states in 1 mol·L−1 KCl solution. Furthermore, specific capacitance of PPy-Cl (polymerization charge of 2 mAh·cm−2) could reach 270 F·g−1 (scanning rate of 5 mV·s−1) or 175 F·g−1 (scanning rate of 200 mV·s−1) and its specific energy could reach 35.3 mWh·g−1. Moreover, with heavier doping ion (TOS−), PPy-TOS (polymerization charge of 2 mAh·cm−2) had a slightly smaller specific capacitance (146 F·g−1, scanning rate of 5 mV·s−1), but a very rapidly charge/discharge ability (specific capacitance of 123.6 F·g−1 at scanning rate of 200 mV·s−1) and its specific power could reach 10 W·g−1. In addition, both PPy-TOS and PPy-Cl had good cycleability. All of the above implied that the PPy-Cl and PPy-TOS were two kinds of promising electrode material for supercapacitors.

A Novel Method to Improve Cycling Performance of LiMn2O4 Cathodes

Polypyrrole-film was formed successfully onto LiMn2O4 electrode by electrochemical polymerization, as a new method to improve cycling performance of LiMn2O4 cathode, which was quite different from cation doping and surface coating of LiMn2O4. Both of pure electrode and modified electrode were characterized by X-ray diffraction and electrochemical techniques. The XRD patterns demonstrated that crystallized polypyrrole was deposited onto the LiMn2O4 electrode. The charge-discharge tests revealed that modified electrode by polypyrrole-film had an excellent rechargeability, 98.2% of the initial discharge capacity and 90.1% of the 13th discharge capacity at C/3 rate in the voltage range of 3.5-4.3V after 150 cycles, but for pure electrode, only 53.4% of the initial discharge capacity. The CV curves showed that the new method was very efficient for restraining capacity fade of LiMn2O4, in fair agreement with the charge-discharge test. The EIS measurements in a frequency range from 100kHz to 1mHz were conducted on the electrodes. It was exhibited that the presence of polypyrrole-film decreased the charge transfer resistance and facilitated the charge-transfer reaction in the LiMn2O4 material, which is favorable for improving performance of LiMn2O4 cathode. 1 Corresponding author: ylxu@mail.xjtu.edu.cn, Tel:0086-29-82665161, mp:0086-13991815083

Preparation of LiMn2O4 cathode with excellent cycling performance

In order to enhance the rate capability as well as the cycleability at room temperature and elevated temperature，an especial method to modify pure spinel LiMn2O4 was investigated. Both of pristine LiMn2O4 and modified-LiMn2O4 were characterized by XRD, SEM, EDAX and electrochemical techniques. XRD results showed that modified-LiMn2O4 did not change the basic LiMn2O4 structure. SEM and EDAX indicated that the thin LiNixMn2-xO4 solid solution layer was covered uniformly on the surface of modified-LiMn2O4. CV and charge-discharge tests at room temperature revealed that modified-LiMn2O4 had excellent cycle stability compared to pristine LiMn2O4, maintained 97.6% of the maximal discharge capacity after the 190 cycles, however, only 68.3% after 170 cycles for pristine LiMn2O4. In addition, modified-LiMn2O4 showed superior cycling performance at the elevated temperature and better rate capability than pristine LiMn2O4. The improving remarkably is attributed to that the solid solution layer reduces the dissolution of Mn cations and suppresses the Jahn-Teller distortion.