Aligned Ion Conduction Pathway of Polyrotaxane-Based Electrolyte with Dispersed Hydrophobic Chains for Solid-State Lithium-Oxygen Batteries

Abstract

A critical challenge hindering the practical application of lithium-oxygen batteries (LOBs) is the inevitable problems associated with liquid electrolytes, such as evaporation and safety problems. Our study addresses these problems by proposing a modified polyrotaxane (mPR)-based solid polymer electrolyte (SPE) design that simultaneously mitigates solvent-related problems and improves conductivity. mPR-SPE exhibits high ion conductivity (2.8 × 10^-3 S cm^-1 at 25 °C) through aligned ion conduction pathways and provides electrode protection ability through hydrophobic chain dispersion. Integrating this mPR-SPE into solid-state LOBs resulted in stable potentials over 300 cycles. In situ Raman spectroscopy reveals the presence of an LiO₂ intermediate alongside Li₂O₂ during oxygen reactions. Ex situ X-ray diffraction confirm the ability of the SPE to hinder the permeation of oxygen and moisture, as demonstrated by the air permeability tests. The present study suggests that maintaining a low residual solvent while achieving high ionic conductivity is crucial for restricting the sub-reactions of solid-state LOBs.

Keywords: Hydrophobic chain; Lithium-oxygen batteries; Polyrotaxane ion conductivity; Solid polymer electrolyte.

Fig. 1

Schematic illustration of the design concept of modified polyrotaxane-based solid polymer electrolyte (mPR-SPE)

Fig. 2

a XRD pattern of pPR-SPE, pPR-SPE without salts, mPR-SPE and mPR-SPE without salts. Expected simplified schematic of b pPR-SPE and c mPR-SPE. d TGA data of pPR-SPE and mPR-SPE. e Temperature-dependent ionic conductivity of pPR-SPE and mPR-SPE. f Water contact angle of pPR-SPE and mPR-SPE. g Geometrically optimized three-dimensional (3D) structure of H₂O and glucose unit of pPR and mPR

Fig. 3

a Weight change ratio of each lithium metal over time. b Photo images of air permeability model experiment. c Schematic diagram of permeability model experiment and protection ability model experiment. d XRD results after air protection ability model experiment and expected. Expected interaction with water molecules of e mPR-SPE and f pPR-SPE

Fig. 4

a LSV profile of mPR-SPE. b Li⁺ transference number of mPR-SPE. c Temperature-dependent ionic conductivity of PDA-SPE, PCD-SPE and mPR-SPE. d ⁷Li NMR spectra of LiTFSI, PDA-SPE, PCD-SPE and mPR-SPE. e Cycling performance of the symmetric Li cells with mPR-SPE

Fig. 5

Galvanostatic cycling of solid-state LOBs with a mPR-SPE and b pPR-SPE at a current density of 100 mA g⁻¹ and a fixed capacity limit of 500 mAh g⁻¹. c Cycling performance of mPR-SPE and pPR-SPE cell associated with Figs. 5a, b and S10. d Comparison of the current density and cycle performance with those of previously reported polymer-based solid-state LOBs. e Galvanostatic first discharge–charge curves of mPR-SPE and pPR-SPE at a current density of 500 mA g⁻¹. f Galvanostatic first discharge–charge curves of mPR-SPE cell at different current densities. g Overpotential at end potential for each capacity- and current density-controlled cycling corresponding Fig. S12a, b. The controlled analyses of capacity and current density were measured for 5 cycles each, followed by subsequent cycling

Fig. 6

a First galvanostatic discharge–charge curves of mPR-SPE cell at a current rate of 500 mA g⁻¹. b In situ Raman spectra of mPR-SPE cell from the discharge to the charge process recorded at different stages corresponding to Fig. 6a. c Contour profile plot or the Raman spectra of the mPR-SPE cell from the discharge to the charge process. d Relative Raman peak intensities as a function of time during the discharging and charging process