RESUMO
Low current drain driven by the low ionic conductivity of a solid polymer electrolyte is one of the major obstacles of solid-state battery. In an effort to improve the ionic conductivity of a solid polymer electrolyte membrane (PEM), polyethylene glycol diacrylate (PEGDA) and monofunctional polyethylene glycol methyl ether acrylate (PEGMEA) were copolymerized via photopolymerization to afford a PEGDA network with dangling PEGMEA side chains. By attaching PEGMEA side branches to the PEGDA network backbone, the glass transition temperature (Tg) was found to decrease, which may be controlled by relative amounts of PEGMEA and PEGDA. Concurrently, the ionic conductivity of a co-PEM consisting of lithium bis(trifluoromethane)sulfonylimide (LiTFSI) salt and a succinonitrile plasticizer in the PEGMEA-co-PEGDA copolymer network was enhanced with increasing PEGMEA side branching. The relationship between the network Tg and ionic conductivity of the branched co-PEM was analyzed in the context of the Vogel-Tammann-Fulcher equation. The plasticized branched co-PEM network exhibited room-temperature ionic conductivity at a superionic conductor level of 10-3 S/cm. Of particular importance is the fact that excellent capacity retention at a high current rate (2 C) in charge/discharge cyclings of Li4Ti5O12/co-PEM/Li and LiFePO4/co-PEM/Li half-cells was achieved. This improved charge retention may be attributed to lower frictional surfaces of the electrodes afforded by side brushes, which probably alleviates formation of irreversible reaction byproducts at the electrode/electrolyte interface.
RESUMO
We demonstrate a simple route to directionally grow crystals of oligothiophenes, based on 2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene with degrees of polymerization of 2 (BTTT-2) and 4 (BTTT-4) via zone annealing (ZA) of preseeded films. ZA of spun-cast films of BTTT-2 does not yield highly aligned crystals. However, if the film is oven-annealed briefly prior to ZA, highly aligned crystals that are millimeters in length can be grown, whose length depends on the velocity of the ZA front. The precrystallized region provides existing nuclei that promote crystal growth and limit nucleation of new crystals in the melted region. Aligned crystals of BTTT-2 can be obtained even when the moving velocity for ZA is an order of magnitude greater than the crystal growth rate. The relative nucleation rate to the crystallization rate for BTTT-4 is greater than that for BTTT-2, which decreases the length over which BTTT-4 can be aligned to â¼500 µm for the conditions examined. The temperature gradient and moving velocity of ZA enable control of the length of the aligned crystalline structure at the macroscale.
