Scanning Electrochemical Microscopy Applied to the Investigation of Lithium (De-)Insertion in TiO2

Operando Investigation of Solid Electrolyte Interphase Formation, Dynamic Evolution, and Degradation During Lithium Plating/Stripping

The solid electrolyte interphase (SEI) dictates the stability and cycling performance of highly reactive battery electrodes. Characterization of the thin, dynamic, and environmentally sensitive nature of the SEI presents a formidable challenge, which calls for the use of microscopic, time-resolved operando methods. Herein, we employ scanning electrochemical microscopy (SECM) to directly probe the heterogeneous surface electronic conductivity during SEI formation and degradation. Complementary operando electrochemical quartz crystal microbalance (EQCM) and ex situ X-ray photoelectron spectroscopy (XPS) provide comprehensive analysis of the dynamic size and compositional evolution of the complex interfacial microstructure. We have found that stable anode passivation occurs at potentials of 0.5 V vs Li/Li+, even in cases where anion decomposition and interphase formation occur above 1.0 V. We investigated the bidirectional relationship between the SEI and lithium plating-stripping, finding that plating-stripping ruptures the SEI. The current efficiency of this reaction is correlated to the anodic stability of the SEI, highlighting the interdependent relationship between the two. We anticipate this work will provide critical insights on the rational design of stable and effective SEI layers for safe, fast-charging, and long-lifetime lithium metal batteries.

In Situ Observation of Interface Evolution on a Graphite Anode by Scanning Electrochemical Microscopy

A well-formed solid electrolyte interface (SEI) is critical for achieving long-term cycling stability in lithium-ion batteries (LIBs). However, the SEI remains the poorly understood component in LIBs especially under dynamic conditions. Here, scanning electrochemical microscopy (SECM) was applied to study the spontaneous reaction on a graphite electrode, SEI formation in the first cycle, SEI evolution during 10 cycles, and the stability of the as-formed SEI in the electrolyte. The conversion, dissolution, stabilization, and growth behaviors of the SEI were determined. Moreover, the SECM results were analyzed in combination with ex situ material characterization to understand the SEI on the graphite electrode comprehensively.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Solid Electrolyte Interphase (SEI) at TiO2 Electrodes in Li-Ion Batteries: Defining Apparent and Effective SEI Based on Evidence from X-ray Photoemission Spectroscopy and Scanning Electrochemical Microscopy

The high (de)lithiation potential of TiO₂ (ca. 1.7 V vs Li/Li⁺ in 1 M Li⁺) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO₂ electrodes at potentials above 1.0 V vs Li/Li⁺. We investigate the formation of a SEI at anatase TiO₂ electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li⁺, respectively. No SEI is formed when a completely dried and residues-free TiO₂ electrode is cycled between 3.0 and 2.0 V vs Li/Li⁺. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li⁺. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO₂ electrodes to differentiate it from the protecting and effective SEI formed at graphite electrodes.

Revealing the electronic character of the positive electrode/electrolyte interface in lithium-ion batteries

It is not blocking! A surface layer formed at the interface of high voltage cathode materials is an apparent SEI.

Lithium-Ion-Transfer Kinetics of Single LiMn2 O4 Particles

A stochastic investigation of lithium deinsertion from individual 200-nm-sized particles of LiMn₂ O₄ reveals the rate-determining step at high overpotentials to be the transfer of the cation across the particle-electrolyte interface. Measurement of the (electro)chemical behavior of the spinel is undertaken without forming a conductive composite electrode. The kinetics of the interfacial ion transfer defines a theoretical upper limit for the discharge rates of batteries using LiMn₂ O₄ in an aqueous environment.

Scanning electrochemical microscopy of Li-ion batteries

Li-ion batteries (LIBs) are receiving increasing attention over the past decade due to their high energy density. This energy storage technology is expected to continue improving the performance, especially for its large-scale deployment in plug-in hybrid electric vehicles (PHEVs) and full electric vehicles (EVs). Such improvement requires having a large variety of analytical techniques at scientists' disposal in order to understand and address the multiple mechanisms and processes occurring simultaneously in this complex system. This perspective article aims to highlight the strength and potential of scanning electrochemical microscopy (SECM) in this field. After a brief description of a LIB system and the most commonly used techniques in this field, the unique information provided by SECM is illustrated by discussing several recent examples from the literature.

Understanding surface reactivity of Si electrodes in Li-ion batteries by in operando scanning electrochemical microscopy

In operando SECM is employed to monitor the evolution of the electrically insulating character of a Si electrode surface during (de-)lithiation.

Determination of the formation and range of stability of the SEI on glassy carbon by local electrochemistry

The formation of the solid electrolyte interphase was detected in operando on glassy carbon electrodes in presence of different cations.

Solid electrolyte interphase in semi-solid flow batteries: a wolf in sheep's clothing

The new role of the electrically insulating solid electrolyte interphase (SEI) in semi-solid flow batteries hinders the use of classic negative electrode materials forcing the search for active materials operating within the ranges of 1.2–0.8 V vs. Li/Li⁺.