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.
The improved ionic conductivity (1.64 × 10(-4) S cm(-1) at room temperature) and excellent electrochemical stability of nanoporous β-Li3PS4 make it one of the promising candidates for rechargeable all-solid-state lithium-ion battery electrolytes. Here, elastic properties, defect thermodynamics, phase diagram, and Li(+) migration mechanism of Li3PS4 (both γ and β phases) are examined via the first-principles calculations. Results indicate that both γ- and β-Li3PS4 phases are ductile while γ-Li3PS4 is harder under volume change and shear stress than β-Li3PS4. The electrochemical window of Li3PS4 ranges from 0.6 to 3.7 V, and thus the experimentally excellent stability (>5 V) is proposed due to the passivation phenomenon. The dominant diffusion carrier type in Li3PS4 is identified over its electrochemical window. In γ-Li3PS4 the direct-hopping of Lii(+) along the [001] is energetically more favorable than other diffusion processes, whereas in β-Li3PS4 the knock-off diffusion of Lii(+) along the [010] has the lowest migration barrier. The ionic conductivity is evaluated from the concentration and the mobility calculations using the Nernst-Einstein relationship and compared with the available experimental results. According to our calculated results, the Li(+) prefers to transport along the [010] direction. It is suggested that the enhanced ionic conductivity in nanostructured β-Li3PS4 is due to the larger possibility of contiguous (010) planes provided by larger nanoporous β-Li3PS4 particles. By a series of motivated and closely linked calculations, we try to provide a portable method, by which researchers could gain insights into the physicochemical properties of solid electrolyte.
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