Quasi-Solid-State Electrolytes of Polyethylene Oxide-Based Composite Electrolyte with Liquid Electrolytes

Zhichao Wang ¹, Daisuke Mori ¹, Sou Taminato ¹, Yasuo Takeda ¹, Osamu Yamamoto ^1,*, Nobuyuki Imanishi ^1,2

1. Graduate School of Engineering, Mie University, Tsu City, Mie Prefecture 514-8507, Japan

2. Research Institute for Integrated Materials and Interface for Sustainable Energy, Mie University, Tsu City, Mie Prefecture 514-8507, Japan

* Correspondence: Osamu Yamamoto

Academic Editor: Eugene S. Mananga

Special Issue: Advanced Materials Systems in Fuel Cells and Batteries

Received: November 18, 2025 | Accepted: February 18, 2026 | Published: February 26, 2026

Journal of Energy and Power Technology 2026, Volume 8, Issue 1, doi:10.21926/jept.2601004

Recommended citation: Wang Z, Mori D, Taminato S, Takeda Y, Yamamoto O, Imanishi N. Quasi-Solid-State Electrolytes of Polyethylene Oxide-Based Composite Electrolyte with Liquid Electrolytes. Journal of Energy and Power Technology 2026; 8(1): 004; doi:10.21926/jept.2601004.

© 2026 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Lithium polymer batteries are an attractive candidate for a large-scale high-energy rechargeable battery, because they are less flammable, preparation can be easily scaled up to large-sized batteries, and liquid electrolyte leaks are avoided [1,2]. Following publications on Na⁺ and K⁺ conducting polymer electrolytes by Fenton et al. [3] in 1973 and on Li⁺, Na⁺, and K⁺ conducting polymer electrolytes by Armand et al. [4] in 1979, many types of polymer electrolytes have been reported [5,6,7]. The ion conductivity of the first reported Li⁺ conducting polymer electrolyte of PEO₈LiCF₃SO₃ was as low as around 10^-5 S cm at 60°C [8]. Scrosatie and co-workers proposed the addition of nano-size ceramic fillers (TiO₂, Al₂O₃ or SiO₂) to a polymer electrolyte and observed high conductivity of 10^-5 S cm^-1 at 25°C [2]. Even higher Li⁺ conducting composite polymer electrolytes have been reported recently [9]. A high electrical conductivity of 2.4 × 10^-4 S cm^-1 was observed at 25°C in PEO-10 wt.% nano-powder (40 nm particle size prepared using high energy planetary ball milling), a lithium-ion conducting solid electrolyte of Li₇La₃Zr₂O₁₂ [10]. Song et al. also reported the high conductivity of 3.6 × 10^-4 S cm^-1 at room temperature for a composite polymer electrolyte of PEO₁₀LiClO₄-15 wt.% nano-sheet Li_6.5La₃Nb_0.5O₁₂ [11]. However, the stability of these high-conductivity PEO-based composite electrolytes in contact with a lithium electrode remains unclear. A Li/PEO-15 wt.% nano-sheet Li₇La₃Zr₂O₁₂(LLZ)/Li cell showed a high cell resistance of 12000 Ω cm² at 40°C [11]. Abe et al. reported a significant reduction of cell resistance for a Li/polymer electrolyte/Li by the co-addition of liquid plasticizers of tetraethylene glycol dimethyl ether (G4) and 1,2-dimethoxyethane (DME); a low cell resistance of 400 Ω cm² at 25°C was observed in a Li/PEO₁₈Li(CF₃SO₂)₂N (LiTFSI)-2G4-10 wt.% LLZ-10 wt.% DME/Li [12]. Wang et al. reported a reduction of cell resistance from 3180 Ω cm for a Li/PEO₁₈LiTFSI/Li to 666 Ω cm for a Li/PEO₁₈LiTFSI-2G4/Li at 25°C [13]; both the passivation film resistance and charge transport resistance were reduced by the addition of G4. We also reported previously [14] that the cell resistance of a Li/PEO₁₈LiTFSI-Li_1.4Al_0.4Ge_0.2Ti_1.4(PO₄)₃-LiCl·H₂O (LAGTP·LiCl)/Li cell was significantly reduced by the addition of G4 and DME liquid plasticizers [14]. In this study, a large-sized thin film of (PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl was prepared by the addition of DME. We have focused on the cell resistance and on the lithium deposition and stripping cyclic performance as a function of the DME content in PEO₁₈LiTFSI-5 wt.% LAGTP·LiCl with an aim of developing a flexible polymer electrolyte for rechargeable lithium batteries that is free from lithium dendrite formation.

2. Experimental

A super high lithium-ion conducting solid electrolyte of Li_1.4Al_0.4Ge_0.2Ti_1.4(PO₄)₃ LiCl·H₂O (LAGTP·LiCl) was prepared by the method reported by Takahashi et al. [15], where a more high lithium-ion conducting NASICON-type solid electrolyte of Li_1.4Al_0.4Ge0_0.2Ti_1.4(PO₄)₃ (LAGTP) [16] was used instead of Li_1.4Al_0.4Ti_1.6(PO₄)₃. The addition of LiCl·H₂O into the grain boundary of Li_1.4Al_0.4Ge_0.2Ti_1.4(PO₄)₃ was conducted by the immersion of out to immerse LAGTP pellet sintered at 950°C for 6 h into a saturated LiCl aqueous solution at 50°C for one week. The immersed LAGTP pelt with impregnated LiCl was washed with water and dried at 220°C for 12 h under vacuum. Some LiCl was observed in the grain boundary [15]. The obtained LAGTP·LiCl pelt was kept under open air, which resulted in the conversion of LiCl to LiCl·H₂O. The content of LiCl in LAGTP pellet was estimated to be 0.75 wt.% from the weight change of the pelt before and after immersion in the saturated LiCl aqueous solution.

Figure 1 shows the procedure for preparing the composite polymer electrolyte containing polyethylene oxide (PEO), LiTFSI, LAGTP, G4, and DME. PEO (Sigma Aldrich, M_w = 6 × 10⁵) and LiTFSI (Wako Chemical, Japan) were dried overnight under vacuum at 50°C and 160°C, respectively. G4 and DME (Kishida Chemicals, Japan) were immersed in activated 0.3 nm molecular sieves for 1 week. Composite polymer electrolytes of (PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl were prepared by a conventional casting method. Briefly, PEO, LiTFSI, G4, and LAGTPLiCl were added into acetonitrile (AN) and stirred for 24 h in an Ar-filed glove box, after which AN was evaporated slowly at room temperature. A small amount of DME was added dropwise onto the composite electrolyte sheet in an Ar-filed glove box.

Figure 1 Procedure for preparation of the composite of [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-x wt.% DME composite polymer electrolyte.

Figure 2 shows the photograph of the films of PEO₁₈LiTFSI, (PEO₁₈LiTFSI)-5 wt.% LAGTP·LiCl, and (c) [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-10 wt.% DME polymer electrolyte films. Flat films were obtained for the composite electrolyte of [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-10 wt.% DME. The thickness of the composite electrolyte was approximately 0.020 cm. The composite electrolyte used for the electrochemical tests had a diameter of 16 mm.

Figure 2 Surface photos of (a) PEO₁₈LiTFSI, (b) PEO₁₈LiTFSI-5 wt.% LAGTP·LiCl, and (c) [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-10 wt.% DME.

The cell impedance was measured using a frequency response analyzer (Solatron 1250) in the frequency range of 0.1 Hz to 1 MHz. The cell was kept at 80°C for 1 hour to achieve good contact with the electrode, and then cooled down to room temperature. Conductivity data were recorded on the second heating run. The stress vs. strain curves of the composite electrolyte were measured using a tensile meter (Japan Instrumentation System JSV-H1000). The lithium deposition and stripping cyclic performance of the Li/composite electrolyte/Li cell was determined at constant current at 25°C using a potentiostat (Biologic VMP X).

3. Results and Discussion

Figure 3a shows impedance profiles of the Li/[(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-x wt.% DME/Li cell at 25°C as a function of x. The cell resistances slightly increased with time, and steady cell resistances were observed after 10 days, as shown in Figure 4. In contrast with the composite electrolyte of PEO₁₈LiTFSI-LAGTP·LiCl without liquid plasticizer [14], the impedance profiles had a small semi-circle and a large semi-circle. The small semi-circle in the high frequency range is associated with grain boundary resistance, and the large semi-circles in the low frequency range are associated with the resistance of a solid electrolyte interface (SEI) formed on the lithium electrode surface and also the charge transfer resistance [14]. The grain boundary resistance observed in the composite electrolyte without liquid plasticizers may be reduced by the addition of the liquid plasticizer. The Li symmetrical cell resistance was considerably reduced from 10,000 Ω for Li/PEO₁₈LITFSI-5 wt.% LAGTP·LiCl to 1400 Ω by the addition of G4, and slightly increased by the further addition of DME in [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl] as shown in Figure 3a. Figures 3b and 3c show the grin boundary resistance and interface resistance as a function of x in [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-xDME, respectively. The reduction of the cell resistance by the addition of DME is due to the reduction of the grain boundary resistance; the grain boundary resistance was as low as 10 Ω. The FT-IR spectra and SEM images showed no change with the addition of 10 wt.% DME into (PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl as shown in Appendix A and Appendix B. The increase in SEI and charge-transfer resistances uponadding DME may be due to the mixing effect of the G4 and DME plasticizers. The bulk resistances of the composite electrolytes were estimated from the intercept of the high-frequency semicircle with the real axis. The bulk resistances were almost constant, regardless of DME content in the composites. Figure 3d shows the inverse temperature dependence of the electrical conductivity for various x. The estimated activation energy for conduction is 59 kJ mole^-1, which shows no dependence on x and is comparable to that of 52.6 kJ mole for LiI [17].

Figure 3 (a) Impedance profiles at 25°C, (b) grain boundary resistance, (c) interface resistance, and (d) temperature dependence of the electrical conductivity of Li/[(PEO18LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-x wt.% DME/Li cell as a function of x.

Figure 4 Change in the impedance profiles with the storage time at 25°C for the Li/(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-xDME/Li cell.

Figure 4 shows the change in the impedance profiles with the storage time at 25°C for the Li/(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-xDME/Li cell with various x. There was no significant increase of the cell resistance for over month. In the inset of Figure 4a, the equivalent circuit is shown. The intercept of the high-frequency semicircle with the real axis at high frequency is the bulk resistance (R₁). The semicircle in the high frequency range of 1 MHz to 2 kHz corresponds to the grain boundary resistance (R₂) with the parallel constant phase angel (CPE₂), and solid electrolyte interphase (SEI; R₃) with a parallel constant angel (CPE₃), and those in the low frequency range correspond to the charge transfer resistance (R₄) with a parallel constant phase angel (CPE₄). The parts of the SEI and the grain boundary phase were integrated in this composite electrolyte [14].

The lithium-ion transport numbers of PEO₁₈LiTFSI-5 wt.% LAGTP·LiCl without DME and with 10 wt.% DME was measured using the method reported by Evans et al. [18]. The transport number of the composite with 10 wt% DME was increased to 0.22 from that of 0.10 without DME, which is compared to those of 0.20 and 0.12 of PEO₁₈LiTFSI-2G4 without DME and with10 wt.% DME, respectively, reported by Abe [12]. The experimental results are shown in the additional materials of Appendix C.

Figure 5 shows the stress vs. strain curves of the [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-xDME electrolyte at 25°C for various x. The tensile strength (0.4 MPa) of (PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl-10 wt.% DME is comparable to that of PEO-10 wt.% LiTFSI (0.32 MPa) [19]. The tensile strengths of the composites were decreased with the DME content until 10 wt.%. The composite with 15 wt.% DME showed an increase in tensile strength. It may be due to the mixing effect of DME and G4.

Figure 5 Stress-strain curves for [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-xDME at 25°C as a function of x.

Figure 6 shows the cyclic performance of the [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-xDME at 0.2 mA cm^-2 for 1 h polarization and 10 min rest and 25°C as a function of x. These cells show an excellent cyclic performance at 25°C.

Figure 6 Cyclic performance of the Li/(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl-x wt.% DME cell at 25°C.

Figure 6 shows the cycle performance of the Li/[(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-x wt.% DME/Li cells with various x at 0.2 mA cm^-2 for 1 h polarization, 10 min rest, and 25°C. These cells exhibited excellent cyclic performance at room temperature and 0.2 mA cm^-2. No short circuit due to Li dendrite formation or increase in cell potential due to electrolyte decomposition during the lithium deposition and stripping process was observed [20]. No significant effect of the DME content was observed during this cycling period.

4. Conclusion

Flexible high electrical conductivity thin films of PEO-based polymer electrolytes of (PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl were prepared with various DME contents. The electrochemical and mechanical properties were examined. The electrical conductivities of [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-10 wt.% and 5 wt.% DME were as high as 1.2 × 10^-4 and 5.7 × 10^-5 S cm^-1 at 25°C, respectively. The cell resistances of 1400 Ω at 25°C for the Li/[(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl] with 0-15/Li cells were acceptable for the electrolyte in lithium batteries. The tensile strength was 0.4 MPa for [(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-10 wt.% DME electrolyte, which was comparable to that of PEO-based electrolytes. The lithium deposition and stripping cyclic performance of the Li/[(PEO₁₈LiTFSI-2G4)-5 wt.% LAGTP·LiCl]-x wt.% DME cells showed no significant difference with the change in the DME content. This PEO-based composite polymer electrolyte is thus an attractive candidate for thin-layer solid electrolyte batteries.

Author Contributions

Z.W. investigation, formal analysis; D.M. and S.T. formal analysis; Y.T. review and editing; O.Y. supervision, writing, & editing; N.I. supervision; project administration, review & editing.

Competing Interests

The authors have declared that no competing interests exist.

Additional Materials

The following additional materials are uploaded at the page of this paper.

1. Appendix A: SEM of the surfaces of PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl-10 wt.% DME.
2. Appendix B: FTIR spectra of PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl-10 wt.% DME and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl-10 wt.% DME.
3. Appendix C: The current decay curves upon application of a 10 mv DC bias and the impedance profile changes before and after polarization for PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl and PEO₁₈LiTFSI-0.5 wt.% LAGTP·LiCl-10 wt.% DME.