Ionic Conductivity Enhancement in UHMW PEO Gel Electrolytes Based on Room-Temperature Ionic Liquids and Deep Eutectic Solvents

Abstract

Physical gels made of poly(ethylene oxide) (PEO) and deep eutectic solvents urea-Li bis(trifluoromethanesulfonyl)imide (TFSI) and ethylene glycol/LiTFSI, or pyrrolidinium ionic liquid solutions PYR13TFSI-LiTFSI and PYR14TFSI-LiTFSI, are prepared by a fast, single-step process, which involves no auxiliary solvents or intermediates and is reproducible and scalable. The properties of these gels are studied as a function of the PEO content and its molecular weight and the nature of the liquid electrolyte. The gels prepared with a low concentration (1–5 wt %) of ultrahigh molecular weight (UHMW) PEO are tough, stretchable materials which resemble soft elastomers and are also self-healing and transparent. Their rheology shows the conventional behavior of physical polymer gels, so that the higher the molecular weight of PEO, the lower the polymer concentration needed to produce the gel. However, the ion conductivities and diffusivities of the gels are striking, in many cases being equal to or significantly higher than those of pure liquid electrolytes. This ion conductivity enhancement is the highest for the lowest PEO concentration with the highest molecular weight. This unprecedented molecular weight dependence of conductivity and diffusivity is the result of two combined effects: the liquid electrolyte chemical structure modification as a consequence of the addition of PEO and the development of elastic networks, where ion mobility and rheology are uncoupled when the PEO added is of UHMW.

Electrolytes for Zn Batteries: Deep Eutectic Solvents in Polymer Gels

“…The ACE 4 PEO 0.2 Zn(TFSI) 2 ‐1 and ACE 4 PEO 0.2 Zn(TFSI) 2 ‐9 samples have very similar conductivity values that are higher than the original ACE 4 Zn(TFSI) 2 . This phenomenon has been previously reported [37] and is attributed to the combination of the chemical and the rheological effects of the polymer on the DES. The chemical effects depend on the PEO concentration but not on its MW, whereas the rheological effects depend also on the MW of the polymer.…”

“…[29] These results suggest the potential benefits of incorporating UHMW polymer gel electrolytes in Zn batteries as they can help to mitigate the dendrite growth without reducing the ionic conductivity. UHMW PEO is soluble under certain conditions in a wide variety of metal cation electrolytes, [36,37] including Al, Na and Li DESs and RTILs, making it possible to prepare polymer gels with very low PEO concentrations without introducing any auxiliary solvents.…”

Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes

“…Attempts to improve the performance of PEO primarily focus on reducing its crystallinity, further reducing the T g , and improving its salt solvation ability. The crystallinity of PEO can be reduced by designing nonlinear architectures, ,− addition of additives that inhibit crystallization, − cross-linking, − and using copolymerization to reduce the ordering of the PEO. , Small molecule additives can play various roles including as plasticizers − and dielectricizers. , In general, PEO is poorly selective ( t + < 0.2), and the addition of anion traps can modestly increase selectively. − A variety of other amorphous hosts based on other coordinating groups have also been reported; however, none of these candidates reliably surpass the benchmarks set by PEO. , Indeed, it appears in most cases that as conductivity is increased, selectivity is sacrificed resulting in a practical upper bound in performance …”

“…Attempts to improve the performance of PEO primarily focus on reducing its crystallinity, further reducing the T g , and improving its salt solvation ability. The crystallinity of PEO can be reduced by designing nonlinear architectures, 43 , 56 − 58 addition of additives that inhibit crystallization, 59 − 61 cross-linking, 62 − 64 and using copolymerization to reduce the ordering of the PEO. 65 , 66 Small molecule additives can play various roles including as plasticizers 67 − 69 and dielectricizers.…”

The Critical Role of Fillers in Composite Polymer Electrolytes for Lithium Battery

“…Fig.1Chemical structure of commonly polymers[45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] …”