Mass transport and charge transfer through an electrified interface between metallic lithium and solid-state electrolytes

All-solid-state Li-ion batteries are one of the most promising energy storage devices for future automotive applications as high energy density metallic Li anodes can be safely used. However, introducing solid-state electrolytes needs a better understanding of the forming electrified electrode/electrolyte interface to facilitate the charge and mass transport through it and design ever-high-performance batteries. This study investigates the interface between metallic lithium and solid-state electrolytes. Using spectroscopic ellipsometry, we detected the formation of the space charge depletion layers even in the presence of metallic Li. That is counterintuitive and has been a subject of intense debate in recent years. Using impedance measurements, we obtain key parameters characterizing these layers and, with the help of kinetic Monte Carlo simulations, construct a comprehensive model of the systems to gain insights into the mass transport and the underlying mechanisms of charge accumulation, which is crucial for developing high-performance solid-state batteries.

Li+ Conductivity of Space Charge Layers Formed at Electrified Interfaces Between a Model Solid-State Electrolyte and Blocking Au-Electrodes

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.

Characterization and Quantification of Depletion and Accumulation Layers in Solid-State Li+ -Conducting Electrolytes Using In Situ Spectroscopic Ellipsometry

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.

Properties of the Space Charge Layers Formed in Li-Ion Conducting Glass Ceramics

For years, the space charge layer formation in Li-conducting solid electrolytes and its relevance to so-called all solid-state batteries have been controversially discussed from experimental and theoretical perspectives. In this work, we observe the phenomenon of space charge layer formation using impedance spectroscopy at different electrode polarizations. We analyze the properties of these space charge layers using a physical equivalent circuit describing the response of the solid electrolytes and solid/solid electrified interfaces under blocking conditions. The elements corresponding to the interfacial layers are identified and analyzed with different electrode metals and applied biases. The effective thickness of the space charge layer was calculated to be ∼60 nm at a bias potential of 1 V. In addition, it was possible to estimate the relative permittivity of the electrolytes, specific resistance of the space charge layer, and the effective thickness of the electric double layer (∼0.7 nm).

Review on physical impedance models in modern battery research

Electrochemical impedance spectroscopy (EIS) is a versatile tool to understand complex processes in batteries. This technique can investigate the effects of battery components like the electrode and electrolyte, electrochemical reactions, interfaces, and interphases forming in the electrochemical systems. The interpretation of the EIS data is typically made using models expressed in terms of the so-called electrical equivalent circuits (EECs) to fit the impedance spectra. Therefore, the EECs must unambiguously represent the electrochemistry of the system. EEC models with a physical significance are more relevant than the empirical ones with their inherent imperfect description of the ongoing processes. This review aims to present the readers with the importance of physical EEC modeling within the context of battery research. A general introduction to EIS and EEC models along with a brief description of the mathematical formalism is provided, followed by showcasing the importance of physical EEC models for EIS on selected examples from the research on traditional, aqueous, and newer all-solid-state battery systems.