The future of mobility depends on the development of next‐generation battery technologies, such as all‐solid‐state batteries. As the ionic conductivity of solid Li+‐conductors can, in some cases, approach that of liquid electrolytes, a significant remaining barrier faced by solid‐state electrolytes (SSEs) is the interface formed at the anode and cathode materials, with chemical instability and physical resistances arising. The physical properties of space charge layers (SCLs), a widely discussed phenomenon in SSEs, are still unclear. In this work, spectroscopic ellipsometry is used to characterize the accumulation and depletion layers. An optical model is developed to quantify their thicknesses and corresponding concentration changes. It is shown that the Li+‐depleted layer (≈190 nm at 1 V) is thinner than the accumulation layer (≈320 nm at 1 V) in a glassy lithium‐ion‐conducting glass ceramic electrolyte (a trademark of Ohara Corporation). The in situ approach combining electrochemistry and optics resolves the ambiguities around SCL formation. It opens up a wide field of optical measurements on SSEs, allowing various experimental studies in the future.
The
formation of space charge layers in solid-state ion conductors
has been investigated as early as the 1980s. With the advent of all-solid-state
batteries as an alternative to traditional Li-ion batteries, possibly
improving performance and safety, the phenomenon of space charge formation
caught the attention of researchers as a possible origin for the observed
high interfacial resistance. Following classical space charge theory,
such high resistances result from the formation of the depletion layers.
These layers of up to hundreds of nanometers in thickness are almost
free of mobile cations. With the prediction of a Debye-like screening
effect, the thickness of the depletion layer is expected to scale
with the square root of the absolute temperature. In this work, we
studied the temperature dependence of the depletion layer properties
in model solid Ohara LICGC Li+ conducting electrolytes
using electrochemical impedance spectroscopy. We show that the activation
energy inside the depletion layer increases to ca 0.42 eV compared
to ca 0.39 eV in the bulk electrolyte. Moreover, the proportionality
between temperature and depletion layer thickness, correlating to
the Debye length, is tested and validated.
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