Poly(ethylene oxide)-co-poly(propylene oxide)-based gel electrolyte with high ionic conductivity and mechanical integrity for lithium-ion batteries

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.