Achieving multifunctional MOF/polymer-based quasi-solid electrolytes via functional molecule encapsulation in MOFs

Abstract

The simultaneous realization of high safety and high electrochemical performance of quasi-solid-state electrolytes (QSSEs) has been challenging to accomplish since decades. Herein, a flame retardant-encapsulated metal–organic framework (MOFs) was incorporated as a filler into a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) QSSE, addressing its safety concerns and enhancing the electrochemical performance. Notably, the mechanical properties and electrochemical performance of PVDF-HFP QSSE were improved by the introduction of UiO-66 fillers. As the flame retardant was trapped in the channels of MOFs, this approach effectively prevented side reactions arising from it. Moreover, the introduction of the flame retardant hexachlorocyclotriphosphazene (HCCP) modulated the electronic distribution characteristics within the channels of UiO-66 and reduced the zeta potential of UiO-66, thereby further enhancing the performance of the QSSE. The resultant QSSEs (PHU-QSSE) remained intact at 180 °C and exhibited excellent self-quenching characteristics. The flame retardant-encapsulated UiO-66 filler boosted the ionic conductivity of the PVDF-HFP electrolyte from 3.1 × 10⁻⁴ S cm⁻¹ to 6.9 × 10⁻⁴ S cm⁻¹ and elevated the Li⁺ transfer number of the electrolyte from 0.27 to 0.59. At a current density of 0.5 mA cm⁻², a lithium symmetric battery based on PHU-QSSE maintained a stable cycling for over 2500 hours, 15 times longer than those of PVDF-HFP electrolytes. When PHU-QSSE was paired with a LiFePO₄ cathode, it showed a high capacity of 156 mA h g⁻¹ at 1 C, exhibiting outstanding rate performance and maintaining 84.6% capacity after 500 cycles. This work not only provides a new pathway to solve the dilemma between safety and high electrochemical performance of QSSEs but also proves that the modification of MOF channels could provide more possibilities for future solid battery designs.