Ion Conduction and Viscoelastic Response of Epoxy‐Based Solid Polymer Electrolytes Containing Solvating Plastic Crystal Plasticizer

Abstract

To develop a safe electrolyte for lithium‐ion batteries, solid polymer electrolytes (SPEs) are prepared using a commercially available cross‐linkable diglycidyl ether of bisphenol‐A (DGEBA) epoxy resin, a methyl tetrahydrophthalic anhydride (MeTHPA) curing agent, and a plastic crystal‐based electrolyte containing a mixture of succinonitrile (SN) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) by a simple thermal curing process. For the uncured mixture of DGEBA/MeTHPA/SN/LiTFSI, oscillatory and steady shear measurements are conducted to investigate microstructure and processability; the storage modulus (G′) exceeds the loss modulus (G″) at all frequencies studied (solid‐like behavior) and the viscosity decreases with an increase in the shear rate (shear‐thinning behavior), demonstrating that there is a network structure, and the connected structure changes at high shear rates. Fourier transform infrared spectroscopy confirms the physical interactions between Li+ of LiTFSI and anhydride group of MeTHPA, which are responsible for the network structure, and helps to rationalize the observed moduli and viscosity. For the cured epoxy‐based SPEs, their ionic conductivities follow the Arrhenius temperature dependence, and increasing SN/LiTFSI electrolyte content leads to higher conductivity and lower activation energy for conduction, resulting in ionic conductivity σDC ≈ 2 × 10−4 S cm−1 with a shear modulus approaching G′ ≈ 1 MPa at room temperature.

Thermal and Electrochemical Properties of Solid Polymer Electrolytes Prepared via Lithium Salt-Catalyzed Epoxide Ring Opening Polymerization

“…LiTFSI THF − Jeffamine D2000, SG80 LiClO 4 acetone −41~10 −7 n/a n/a Liang [29] Jeffamine D400, DGEPEG LiClO 4 acetone −20~10 −7 n/a n/a Kuo [30] Jeffamine D400, DGEBA, PEG, nano silica n/a n/a Matsumoto [37] DGEPEG, PACM LiTFSI, PC none 160 1.0 × 10 −3 n/a n/a Andrews [6] DGEBA, BDMA LiTFSI, SN none n/a 10 −2 n/a n/a Jang [38] YD-128 LiTFSI, SN none n/a 10 −4 n/a n/a Choi [39] DGEPEG, DGEBA LiTFSI, EMImFSI none n/a 4.3 × 10 −3 n/a n/a Matsumoto [40] Jaffamine D400, DGEBA…”

“…This generally results in more linear fits than with the Arrhenius equation as the pre-factor is related to charge carrier concentration [14,55]. Activation energies for ion transfer processes in polymer electrolytes tend to be between 10 and 40 kJ/mol with lower activation energies being linked to less hindered ion transport [10,17,27,38,54]. The ionic conductivity of the samples presented in Table 1 was measured using electrochemical impedance spectroscopy (EIS) at ambient temperature (25 or 30 • C in most publications) as this is the temperature range that is most relevant to the operation of personal electronic devices.…”

“…In addition to introducing various functional groups into epoxide-based solid polymer electrolytes to improve ionic conductivity, plasticizers can be added to these polymer frameworks for further enhancement of the ionic conductivity [36,38]. Plasticizers improve ionic conductivity in electrolytes as a result of solvating ions, decreasing their affinity for the polymer phase and therefore increasing ion mobility [88].…”

“…Jang et al prepared plasticizer-containing solid polymer electrolytes via the aminecatalyzed polymerization of an epoxide [38]. N-benzyldimethylamine (BDMA) was used to catalyze the polymerization of DGEBA [38].…”

“…Jang et al prepared plasticizer-containing solid polymer electrolytes via the aminecatalyzed polymerization of an epoxide [38]. N-benzyldimethylamine (BDMA) was used to catalyze the polymerization of DGEBA [38]. The resultant polymer was soaked in a mixture of succinonitrile (SN) and LiTFSI to produce samples with various weight percent of the plasticizer mixture.…”