Abstract:

With the increasing demand for safe high energy density energy storage systems, solid-state lithium metal batteries have attracted extensive attention. The solid electrolyte, which is expected to replace the traditional liquid organic electrolyte core in solid-state lithium metal batteries because of its excellent mechanical properties and non-flammability. Lithium-ion solid-state electrolytes can be categorized into two broad types: inorganic electrolytes and polymer electrolytes. Inorganic solid electrolytes have the advantages of high room-temperature ionic conductivity, wide electrochemical window, and high mechanical strength. However, their high brittleness, high solid-solid interface contact resistance, complex preparation process, and high cost make future development and practical applications challenging. In contrast to inorganic electrolytes, polymer electrolytes are easy to process and exhibit better flexibility and easy formation of a good, stable interface with lithium metal. However, solid polymer electrolytes still exhibit insufficient ionic conductivity at room temperatures compared with polymer solid electrolytes. Therefore, neither the inorganic electrolytes nor the polymer electrolytes alone can meet the requirements of high-performance solid-state lithium metal batteries. Recently, dispersing ceramic fillers (especially fast lithium-ion conductors) in a polymer matrix to integrate with composite polymer electrolytes has been developed as an effective strategy for enhancing room-temperature ionic conductivity, mechanical properties, and thermal stability of solid polymer electrolytes. Inorganic fillers do not only reduce the polymer matrix crystallization but also improve the lithium-ion conductivity by promoting the dissociation of lithium salts. The Lewis acid-base groups and oxygen vacancy at the surface of inorganic fillers can increase the migration number of lithium ions. Nevertheless, the effect of the percolation structure of inorganic fillers on the conductivity of organic-inorganic composite electrolytes should be discussed. It is believed that the organic-inorganic interface is the main reason for the significantly enhanced lithium-ion conductivity of composite electrolytes based on the percolation theory. In this paper, from the perspective of percolation structure design, we summarize the progress on high lithium-ion conductive organic-inorganic composite electrolytes with different dimensional-structured inorganic fillers. From one-dimensional filler to three-dimensional filler, the ionic conductivity of a composite electrolyte can be significantly influenced by the rational design and optimization of the percolation structure and orientation of the inorganic filler. Vertically aligned inorganic fillers provide optimal ion transport pathways in the polymer matrix, significantly improving the lithium-ion conductivity of the composite electrolytes. Furthermore, the advantages and disadvantages of the different percolation structures are compared and discussed objectively. Finally, future development trends of organic-inorganic composite electrolytes are discussed.

Key words: Inorganic nano-filler, Polymer electrolyte, Composite electrolyte, High lithium-ion conductivity