Search

As an electrolyte solvent for lithium ion capacitors (LICs), tetrahydrofuran (THF) showed lots of advantages on increasing the discharge capacity, suppressing the potential drop at charged state, and improving the slope of discharge curve. Several electrochemical studies were conducted to verify these effects of THF on LICs. The open circuit potentials of AC/Li beaker-cells with THF were shifted to more positive potentials because of the lowered electrostatic potential of the electrolyte, the weakened solvation energy with LiBF4, and the easier adsorption of BF4− ions to AC electrode in aging step. Although the concentration of LiBF4 decreased with adding THF, the electrolyte conductivity increased by the improved mobility of ions more than enough to compensate to the electrolyte conductivity decreasing by the reduced salt concentration. The capacitive resistance of activated carbon (AC)/Li cell was little changed by adding THF in fully charged state but was much smaller in fully discharged state than the reference condition. These results imply that THF seldom disturbed the behavior of BF4− ions but accelerated the mobility of Li+ ions at AC electrode.

As one of obvious electrolyte design technologies of Li ion batteries (LIBs) to meet durable high-rate capability at elevated temperatures for battery electric vehicles, this study assesses the superiority of gel polymer electrolyte (GPE) based on experimental results supporting its working mechanism. Our previous study indicated that degradation of cathode electrode under high-rate cycling at elevated temperature was a major cause of the decrease in performance of LIBs, and the single full cells (SFCs) with a GPE designed from dipentaerythritol hexaacrylate and methyl ether methacrylate was re-verified to have superior 3.0C cycling performance at 80 °C. The superiority of the GPE is studied from comparing mid-voltages of discharge profiles of the SFCs, observing the cross-sectional morphology of the electrodes by field emission scanning electron microscopy, assessing the interacting force among the electrolyte components by thermogravimetric analysis, and examining each resistance component of the SFCs by electrochemical impedance spectroscopy. Gel polymerization of liquid electrolyte results in a significant increase of durable high-rate capability of LIBs due to the mechanisms of not only its buffering effect on solvating process of Li+ ions being extracted from the active materials during high rate operation, but also higher thermal stability of electrolyte components, lower susceptibility of the ionic conductivity of the electrolyte to a temperature change, and lower energy barrier to breakup of solvated structures during conducting of Li+ ions in gel polymer matrix.

This paper presents a method to enhance the rate performance of high voltage capacitors using an electrolyte additive, 1,3,5-trifluorobenzene (TFB). With increasing discharge rate, the capacity of the activated carbon (AC)/lithium (Li) cell decreases with increasing the slope of the discharge curve and its potential drop at 4.6 V. By adding TFB, the discharge slope improves to increase the rate performance of the cell, and EIS showed that the charge transfer resistance (Rc) of the AC cell decreases. These results suggest that TFB affects the distribution of the electrolyte components near the microporous AC and improves the rate performance of the AC cell.

The electronic spatial extent (ESE) of ions was defined as a major concept for assessing the cause of overpotential in the charging and discharging processes of a nanoporous activated carbon (AC) electrode. The performance degradation of AC/Li half-cells was caused by the overpotential, which was in discord with the electrolyte conductivity and ion size. Compared to the overpotential with the salt concentration, the AC/Li half-cell with a high concentration had a smaller overpotential, and its discharge patterns were similar to the curves obtained from the half-cells with a smaller ESE of BF 4 − ion. The ESE is a more realistic solution for determining the overpotential of the nanoporous capacitor, such as supercapacitor and Li ion capacitor, because its capacity is dependent on the electron density at the electric double layer of the capacitor electrode.

A new finite element approach is introduced for direct prediction of blank shapes and strain distributions from desired final shapes in sheet metal forming. The approach deals with the geometric compatibility of finite elements, plastic deformation theory, minimization of plastic work with constraints and a proper initial guess. The algorithm developed is applied to cylindrical cup drawing, square cup drawing and oil pan drawing in order to confirm its validity by demonstrating reasonably accurate numerical results for each problem. Rapid calculation with this algorithm enables easy determination of various process variables for design of a sheet metal forming process.