Fast ion conduction in solid-state
matrices constitutes the foundation
for a wide spectrum of electrochemical systems that use solid electrolytes
(SEs), examples of which include solid-state batteries (SSBs), solid
oxide fuel cells (SOFCs), and diversified gas sensors. Mixing different
solid conductors to form composite solid electrolytes (CSEs) introduces
unique opportunities for SEs to possess exceptional overall performance
far superior to their individual parental solids, thanks to the abundant
chemistry and physics at the new interfaces thus created. In this
review, we provide a comprehensive and in-depth examination of the
development and understanding of CSEs for SSBs, with special focus
on their physiochemical properties and mechanisms of ion transport
therein. The origin of the enhanced ionic conductivity in CSEs relative
to their single-phase parents is discussed in the context of defect
chemistry and interfacial reactions. The models/theories for ion movement
in diversified composites are critically reviewed to interrogate a
general strategy to the design of novel CSEs, while properties such
as mechanical strength and electrochemical stability are discussed
in view of their perspective applications in lithium metal batteries
and beyond. As an integral component of understanding how ions interact
with their composite environments, characterization techniques to
probe the ion transport kinetics across different temporal and spatial
time scales are also summarized.
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