Polyethylene oxide-based solid-state polymer electrolyte hybridized with liquid catholyte for semi-solid-state rechargeable Mg–O2 batteries

Abstract

In this article, we report the fabrication of a free-standing non-porous polyethylene oxide (PEO)-based solid-state polymer electrolyte with the amalgamation of magnesium triflate (Mg(OTf)₂) and plasticizer succinonitrile (SN) for room-temperature secondary Mg–O₂ batteries. The polymer electrolyte comprising [EO] : Mg²⁺ = 20 : 1 and 30 wt% SN delivered the highest room-temperature (RT) conductivity of 3.9 × 10⁻⁵ S cm⁻¹, which reached 2.2 × 10⁻⁴ S cm⁻¹ at 60 °C. Ruthenium nanoparticles anchored multi-walled carbon nanotubes (Ru/CNT) on carbon paper were used as cathode catalysts for rechargeable Mg–O₂ batteries. The non-establishment of the cathode–solid-state polymer electrolyte interphase was alleviated by introducing a separator soaked with 50–60 μL of 1 M Mg(TFSI)₂ in diglyme (G2) between the polymer electrolyte and Ru/CNT cathode catalyst. The Mg–O₂ battery with this hybrid electrolyte configuration delivered a deep discharge capacity of 9489 mA h g⁻¹ at 100 mA g⁻¹ and a stable galvanostatic discharge–charge performance with the cycle number reaching 51 at 100 mA g⁻¹ with 500 mA h g⁻¹ curtailing capacity, demonstrating an average charging–discharging voltage hysteresis of 1.14 V at RT. The polymer electrolyte also participated in forming an MgF₂-rich stable solid-electrolyte interphase layer on the Mg anode, substantially protecting the anode surface from severe side reactions and corrosion. Raman spectroscopy and distribution of relaxation time studies shed light on the probable mechanism of the liquid catholyte in improving the interphase issue.