Highly Conductive PEO-like Polymer Electrolytes

Abstract

Polymer electrolyte membranes comprising poly(vinylidene fluoride)−hexafluoropropene (PVdF−HFP) copolymer plasticized with a solution of LiSO3CF3, LiN(SO2CF3)2, or LiPF6 in oligomeric poly(ethylene glycol) dimethyl ethers (PEGDME, M w = 250, 400, and 500) were prepared by hot-melt-rolling or solvent-casting techniques. Since the electrolytes containing PEGDME400 and PEGDME500 are “dry” with essentially no volatile components up to 150 °C, we have dubbed them PEO-like. Their thermal stability, mechanical strength, conductivity, electrochemical stability window, and Li/electrolyte interface stability were characterized. Plasticizing PVdF−HFP with the PEGDME/LiX solutions disordered the polymer structure leading to polymer electrolytes having lower crystallinity than the polymer host itself. The mechanical strength of the electrolyte membranes varied depending on the PVdF content. Tensile strength (stress) as high as 420 psi at an elongation-at-break value (strain) of 75% was observed. The conductivities of the electrolytes correlated with the molecular weights of PEGDME as well as the concentration of the Li salt, and most of the electrolytes prepared showed room-temperature conductivities of greater than 10-4 S/cm. The high room-temperature conductivity of these electrolytes compared to PEO-based electrolytes is attributed to the high mobility of the ionic charge carriers. The Li/electrolyte interface stability under open-circuit conditions was found to be good as assessed from the small change in the interfacial impedance for the measured case of the PVdF−PEGDME500−LiN(SO2CF3)2 electrolyte. This electrolyte also showed oxidation stability up to 4.5V versus Li+/Li on Al, Ni, and stainless steel (SS) and reduction stability down to 0.0V versus Li+/Li on both Ni and SS. The applicability of these electrolytes in batteries was demonstrated by the fabrication and testing of Li/oxygen and Li/LiMn2O4 cells.

Studies on structural, thermal and AC conductivity scaling of PEO-LiPF6 polymer electrolyte with added ionic liquid [BMIMPF6]

“…36 These crystalline peaks are riding over a broad halo which is the indication of the amorphous phase present in the polymer PEO. It may also be observed that the "halo" is less prominent in pure PEO than PEO+LiPF 6 or PEO+LiPF 6 +BMIMPF 6 membranes. The intensity of these PEO peaks is found to decrease while broadening in the peaks increases with increasing amount of BMIMPF 6 in the PEO-LiPF 6 polymer electrolyte membranes.…”

“…In this method, polymer PEO and lithium salt LiPF 6 were weighed in desired ratio and dissolved in distilled methanol and stirred for 5-6 hours at 50 • C for complete dissociation of salt. Subsequently, desired amount of BMIMPF 6 was added and stirring continued until a viscous solution was obtained. This viscous solution was poured in polypropylene Petri dishes and solvent was allowed to evaporate at ambient condition.…”

“…The XRD patterns of pure PEO and (PEO+10 wt.% LiPF 6 )+xwt.% BMIMPF 6 PEO. 36 These crystalline peaks are riding over a broad halo which is the indication of the amorphous phase present in the polymer PEO.…”

“…with the help of common dipolar solvent and PEO-lithium salt based polymer electrolytes have been studied much. [4][5][6][7] In SPEs, ionic conductivity is one of the key parameters that needs to be optimized for the better performance of the devices. Generally, the ionic conductivity of PEO based polymer electrolytes is poor.…”

“…In the present paper, studies on changes in the degree of crystallinity/amorphicity, phase transition temperatures (glass transition and melting temperature), complexation behaviour, ionic conductivity and frequency dependent ionic conductivity of PEO+LiPF 6 polymer electrolyte upon inclusion of varying amounts of ionic liquid BMIMPF 6 have been carried out. Such studies could be helpful for optimizing the general properties of ionic liquid based polymer electrolytes and hence the performance of the solid state electrochemical devices.…”