Role of polar side chains in Li⁺coordination and transport properties of polyoxetane-based polymer electrolytes

Abstract

Lithium ion conducting polymer electrolytes (PEs) have been the subject of intense research for lithium metal battery applications. Here, we investigate the effects of polar side chains on Li coordination and ionic transport properties to gain insights for improving the insufficient conductivity of traditional ether-based solid PEs. Poly(trimethyleneoxide)-based (or polyoxetane-based) polymers with ether or nitrile groups were synthesized by ring-opening polymerization. The thermal, ionic transport, and electrochemical properties and the local structure of Li coordination were studied in the presence of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA). The glass transition temperature (T) of the PEs with ether side chains increased with increasing LiTFSA content, whereas the PEs with the nitrile functionality showed the opposite trend at higher salt concentrations. In addition to the unique trend for the T values of the PEs in the presence of LiTFSA, the nitrile groups played pivotal roles as coordination sites for Li ions in the first coordination shell and as a polar medium to increase the permittivity of the PEs. These characteristics of the nitrile groups can endow PEs with improved ionic transport properties.

Steric effect on Li+ coordination and transport properties in polyoxetane-based polymer electrolytes bearing nitrile groups

“…To clarify the contribution of the ethyl group to the electrolyte properties such as the ionic conductivity (s) and lithium transference number (t Li + ), PCHO complexed with lithium bis(triuoromethanesulfonyl)amide (LiTFSA) were evaluated and were compared with previously reported PCEO electrolytes. 17 Here, we veried that subtle changes in the chemical structure of the repeating monomer unit of the polyoxetane-based PEs (i.e., the presence or absence of the ethyl group on the main chain) greatly affected the thermal properties and ionic transport, which were well correlated with different Li + ion coordination structures studied by Raman spectroscopy. A Li/LiFePO 4 cell was fabricated and its charge/ discharge behavior was examined to demonstrate the applicability of the polyoxetane-based PEs to lithium secondary batteries.…”

“…The transference numbers of Li ions (t Li + ) in PCHO 5 LiTFSA and PCEO 5 LiTFSA (the sample reported in the previous report, 17 M n ¼ 4.56 Â 10 4 g mol À1 ) were determined using the electrochemical method proposed by Bruce and Vincent. 19 The required parameters were obtained using potentiostatic polarization and ac impedance spectroscopy with a potentio/ galvanostat (SP-150, BioLogic).…”

“…The Li|PCEO 5 LITFSA|LiFePO 4 cell was prepared using the previously reported PE based on PCEO. 17 Lithium metal foil (diameter: 10 mm, Honjo Metal Co., Ltd.) was used as the negative electrode. The positive electrode was composed of LiFePO 4 as the active material (theoretical capacity: 170 mA h g À1 ), acetylene black (AB) as the conductive supporting agent, polyvinylidene uoride (PVdF), and PCEO 5 LiTFSA as the binder at a weight ratio of LiFePO 4 : AB : PVdF : PCEO 5 LiTFSA ¼ 75 : 15 : 10 : 1.…”

“…We conrmed that Li + ion coordination with the nitrile groups in PCEO is responsible for improving the ionic transport properties of the PEs. 17 In this study, poly(3-(2-cyanoethoxymethyl)-oxetane) (PCHO) was newly synthesized. The only difference between PCEO and PCHO is the absence of an ethyl group on the polyoxetane main chain.…”

Poly-(3-ethyl-3-hydroxymethyl)oxetanes—Synthesis and Adhesive Interactions with Polar Substrates

“…Polyoxetanes are not paid sufficient attention, though their chemical properties like the ease of side chain functionalization or grafting make them versatile and valuable scaffolds in many fields of material science. Reports on the applicability of functionalized polyoxetanes cover, among others, liquid crystals [18], polymer electrolytes with lithium ion coordination abilities [19,20], fluorine-containing hydrophobic materials [21], amphiphilic block copolymers with poly(ethylene oxide) [22], hybrid networks of fluorine-bearing polyoxetane and inorganic phases [23], hybrid fluorine-polyoxetane-polyurethane coatings [24], and hyperbranched polyoxetanes in drug delivery systems [25][26][27]. Wynne and co-workers also used functionalized polyoxetanes to incorporate telechelics in polyurethanes [28,29].…”