A Class of Solid-like Electrolytes for Rechargeable Batteries Based on Metal-Organic Frameworks Infiltrated with Liquid Electrolytes
Energy storage and conversion are key technologies in modern society, and they are becoming more and more important. This is mainly due to the severe future impact of fossil fuels on the world’s economy and ecology. So, there is an urgent need for alternative energy sources to address the depletion of fossil fuels and the environmental impact of their continued use. However, the large-scale development of renewable energy resources such as wind, solar, geothermal, biomass and hydropower, that are unpredictable and intermittent. Thus, these technologies require highly reliable electrical energy storage (EES) devices, which can store the excess produced electricity and release it on demand.
Rechargeable batteries such as lithium-ion batteries, store energy through electrochemical reactions that typically occur throughout the bulk active materials, allowing comparatively large amount of energy to be stored compared with electric capacitors. The last decade has witnessed a tremendous growth in lithium-ion batteries for applications such as microelectronics and electric vehicles. However, the development of battery energy density has seriously lagged behind the demand growth of Li-ion batteries. Thus, high energy density electrode materials are extremely demanded in next-generation cutting-edge electronic devices. Parallel to this development, rechargeable batteries based on Na+, K+, Mg2+ and Al3+ ions have also attracted great interests due to their abundance and low cost.
Such batteries generally employ flammable liquid electrolytes, which bring severe safety concerns. In this case, solid electrolytes are believed to be able to suppress Li dendrite growth because of their high mechanical strength and high Li+ transference number. In order to allow the implementation of high-specific-energy Li-metal batteries, both inorganic and organic solid electrolytes have been explored. Inorganic electrolytes may exhibit high ionic conductivity (e.g., > 10–4 S cm–1), whereas scale fabrication of solid batteries remains challenging. Polymeric electrolytes are less difficult to be integrated, whereas their ionic conductivity remains low at ambient temperature (e.g., < 10–5 S cm–1). Solid-like electrolytes, which are generally made by encapsulating liquid electrolytes within solid porous scaffolds, represent another direction with the merits of both liquid electrolyte and solid electrolyte.
In this dissertation, we developed a novel family of solid-like electrolytes, which are made by infiltrating MIL-100(Al), a MOF with high porosity and excellent thermal, chemical and electrochemical stabilities, with a series of liquid electrolytes that contain cations from the 3rd period (Na+, Mg2+ and Al3+) and the 1st group (Li+, Na+, K+ and Cs+). Particularly, the Mg2+ solid-like electrolyte exhibits superionic conductivity (>10–3 S cm–1) with a low activation energy of 0.20 eV. From Li+, Na+, K+ to Cs+ with reducing Stokes radii and ionic solvation shell thickness, both the liquid electrolytes and solid-like electrolytes show a similar trend of increasing conductivity. This work investigates the ion-conduction mechanism of MOFs based solid-like electrolytes, providing reliable principles to the design of fast-conducting solid-like electrolytes for alkali or multivalent metal ions.
Furthermore, we successfully employed MOF-based solid-like electrolytes in Na-metal batteries. Both MOF/polymer composite electrolytes on GF served as functional separator or directly as gel polymer electrolytes show advantages compared with commercial separators. The cell using solid-like electrolyte notably surpasses the cell using liquid electrolyte in terms of cycle stability and Coulombic efficiency. This work expands the application of MOF-based solid-like electrolytes from Li to Na metal batteries, offering the possibility for further applications in high energy density rechargeable batteries.