Nanosponge-Based Composite Gel Polymer Electrolyte for Safer Li-O2 Batteries

Abstract

Li-O₂ batteries represent a promising rechargeable battery candidate to answer the energy challenges our world is facing, thanks to their ultrahigh theoretical energy density. However, the poor cycling stability of the Li-O₂ system and, overall, important safety issues due to the formation of Li dendrites, combined with the use of organic liquid electrolytes and O₂ cross-over, inhibit their practical applications. As a solution to these various issues, we propose a composite gel polymer electrolyte consisting of a highly cross-linked polymer matrix, containing a dextrin-based nanosponge and activated with a liquid electrolyte. The polymer matrix, easily obtained by thermally activated one pot free radical polymerization in bulk, allows to limit dendrite nucleation and growth thanks to its cross-linked structure. At the same time, the nanosponge limits the O₂ cross-over and avoids the formation of crystalline domains in the polymer matrix, which, combined with the liquid electrolyte, allows a good ionic conductivity at room temperature. Such a composite gel polymer electrolyte, tested in a cell containing Li metal as anode and a simple commercial gas diffusion layer, without any catalyst, as cathode demonstrates a full capacity of 5.05 mAh cm^-2 as well as improved reversibility upon cycling, compared to a cell containing liquid electrolyte.

Keywords: Li-O2 cell; O2 cross-over; composite gel polymer electrolyte; nanosponge.

Figure 1

Schematic representation of the polymerization process.

Figure 2

Pictures of the different CGPEs: without NS (a), with 5 wt% NS (b), with 10 wt% NS (c). FESEM micrographs of the surface of CGPE without NS (d), CGPE with 5 wt% NS (e), CGPE with 10 wt% NS (f). FESEM micrographs of the cross-section of CGPE without NS (g), CGPE with 5 wt% NS (h), CGPE with 10 wt% NS (i).

Figure 3

TGA traces of the CGPE precursors and the different CGPEs prepared (a). XRD patterns of NS, CGPE without NS, with 5 wt% NS, and 10 wt% NS (b).

Figure 4

Ionic conductivity vs. temperature plot of the different CGPEs (a), LSV profile of the different CGPEs compared to a commercial glass fiber separator impregnated with liquid electrolyte (LiTFSI 0.5 M in DMSO) (b). Potentiostatic polarization analysis of a symmetric cell containing the glass fiber separator impregnated with the liquid electrolyte (c) and a symmetric cell containing the CGPE with 5 wt% NS (d).

Figure 5

Voltage profile of the Li plating/stripping galvanostatic cycling at a current density of 0.5 mA cm⁻² (specific capacity: 0.5 mAh cm⁻²) on symmetric Li|LE|Li cell (black) and Li|5 wt% NS CGPE|Li cell (red).

Figure 6

Full discharge profile of the STD cell (solid black line) and 5 wt% NS CGPE cell (dotted red line) (a). The 2nd cycle profile of the STD cell (solid black line) and 5 wt% NS CGPE cell (dotted red line) (b). Discharge/charge capacities and Coulombic efficiency vs. cycle number of the STD cell (c) and 5 wt% NS CGPE cell (d).

Figure 7

FESEM micrographs of the cathode surfaces after 10 cycles for the STD cell (a) and the 5 wt% NS CGPE cell (b). XRD patterns of a pristine cathode, the STD cathode and the 5 wt% NS CGPE cathode after 10 cycles (ICCD database: Li₂O₂ [00-009-0355] and LiOH [00-004-0708]) (c).