Frameworked electrolytes: Ionic transport behavior and high mobility for solid state batteries

Abstract

All solid-state batteries (ASSBs) are the holy grails of rechargeable batteries, where extensive searches are ongoing in the pursuit of ideal solid-state electrolytes. Nevertheless, there is still a long way off to the satisfactorily high (enough) ionic conductivity, long-term stability and especially being able to form compatible interfaces with the solid electrodes. Herein, we have explored ionic transport behavior and high mobility in the sub-nano pore networks in the framework structures. Macroscopically, the frameworked electrolyte behaves as a solid, and however in the (sub)-nano scales, the very limited number of solvent molecules in confinement makes them completely different from that in liquid electrolyte. Differentiated from a liquid-electrolyte counterpart, the interactions between the mobile ions and surrounding molecules are subject to dramatic changes, leading to a high ionic conductivity at room temperature with a low activation energy. Li⁺ ions in the sub-nano cages of the network structure are highly mobile and diffuse rather independently, where the rate-limiting step of ions crossing cages is driven by the local concentration gradient and the electrostatic interactions between Li⁺ ions. This new class of frameworked electrolytes (FEs) with both high ionic conductivity and desirable interface with solid electrodes are demonstrated to work with Li-ion batteries, where the ASSB with LiFePO₄ shows a highly stable electrochemical performance of over 450 cycles at 2°C at room temperature, with an almost negligible capacity fade of 0.03‰ each cycle. In addition, the FE shows outstanding flexibility and anti-flammability, which are among the key requirements of large-scale applications.