Poly(ethylene oxide)-co-Poly(propylene oxide)-Based Gel Electrolyte with High Ionic Conductivity and Mechanical Integrity for Lithium-Ion Batteries

TLDR

Gel polymer electrolytes for lithium‑ion batteries often suffer from poor mechanical integrity. The authors develop a GPE by swelling a Celgard membrane with a poly(ethylene oxide)-co-poly(propylene oxide) copolymer and 1 M LiPF6 in carbonate solvents to achieve high performance. The resulting GPE retains three times the liquid electrolyte of the Celgard composite, shows ionic conductivities of 2.8 × 10⁻³ S cm⁻¹ at 30 °C and 5.1 × 10⁻⁴ S cm⁻¹ at –20 °C, a 5 V electrochemical window, a Li⁺ transference number of 0.5, and in LiFePO₄ cells delivers 5 % higher capacity, 44 % lower IR drop, and only 1.2 % decay over 150 cycles, confirming its high stability, conductivity, and mechanical integrity.

Abstract

Using gel polymer electrolytes (GPEs) for lithium-ion batteries usually encounters the drawback of poor mechanical integrity of the GPEs. This study demonstrates the outstanding performance of a GPE consisting of a commercial membrane (Celgard) incorporated with a poly(ethylene oxide)-co-poly(propylene oxide) copolymer (P(EO-co-PO)) swelled by a liquid electrolyte (LE) of 1 M LiPF6 in carbonate solvents. The proposed GPE stably holds LE with an amount that is three times that of the Celgard-P(EO-co-PO) composite. This GPE has a higher ionic conductivity (2.8 × 10–3 and 5.1 × 10–4 S cm–1 at 30 and −20 °C, respectively) and a wider electrochemical voltage range (5.1 V) than the LE-swelled Celgard because of the strong ion-solvation power of P(EO-co-PO). The active ion-solvation role of P(EO-co-PO) also suppresses the formation of the solid–electrolyte interphase layer. When assembling the GPE in a Li/LiFePO4 battery, the P(EO-co-PO) network hinders anionic transport, producing a high Li+ transference number of 0.5 and decreased the polarization overpotential. The Li/GPE/LiFePO4 battery delivers a discharge capacity of 156–135 mAh g–1 between 0.1 and 1 C-rates, which is approximately 5% higher than that of the Li/LE/LiFePO4 battery. The IR drop of the Li/GPE/LiFePO4 battery was 44% smaller than that of the Li/LE/LiFePO4. The Li/GPE/LiFePO4 battery is more stable, with only a 1.2% capacity decay for 150 galvanostatic charge–discharge cycles. The advantages of the proposed GPE are its high stability, conductivity, Li+ transference number, and mechanical integrity, which allow for the assembly of GPE-based batteries readily scalable to industrial levels.