Reactions of graphene supported Co3O4 nanocubes with lithium and magnesium studied by in situ transmission electron microscopy

Visualizing the Interfacial Chemistry in Multivalent Metal Anodes by Transmission Electron Microscopy

Multivalent metal batteries (MMBs) have been considered potentially high-energy and low-cost alternatives to commercial Li-ion batteries, thus attracting tremendous research interest for energy-storage applications. However, the plating and stripping of multivalent metals (i.e., Zn, Ca, Mg) suffer from low Coulombic efficiencies and short cycle life, which are largely rooted in the unstable solid electrolyte interphase. Apart from exploring new electrolytes or artificial layers for robust interphases, fundamental works on deciphering interfacial chemistry have also been conducted. This work is dedicated to summarizing the state-of-the-art advances in understanding the interphases for multivalent metal anodes revealed by transmission electron microscopy (TEM) methods. Operando and cryogenic TEM with high spatial and temporal resolutions realize the dynamic visualization of the vulnerable chemical structures in interphase layers. Following a scrutinization of the interphases on different metal anodes, we elucidate their features for appealing multivalent metal anodes. Finally, perspectives are proposed for the remaining issues on analyzing and regulating interphases for practical MMBs.

Understanding Conversion-Type Electrodes for Lithium Rechargeable Batteries

The need/desire to lower the consumption of fossil fuels and its environmental consequences has reached unprecedented levels in recent years. A global effort has been undertaken to develop advanced renewable energy generation and especially energy storage technologies, as they would enable a dramatic increase in the effective and efficient use of renewable (and often intermittent) energy sources. The development of electrical energy storage (EES) technologies with high energy and power densities, long life, low cost, and safe use represents a challenge from both the fundamental science and technological application points of view. While the advent and broad deployment of lithium-ion batteries (LIBs) has dramatically changed the EES landscape, their performance metrics need to be greatly enhanced to keep pace with the ever-increasing demands imposed by modern consumer electronics and especially the emerging automotive markets. Current battery technologies are mostly based on the use of a transition metal oxide cathode (e.g., LiCoO₂, LiFePO₄, or LiNiMnCoO₂) and a graphite anode, both of which depend on intercalation/insertion of lithium ions for operation. While the cathode material currently limits the battery capacity and overall energy density, there is a great deal of interest in the development of high-capacity cathode materials as well as anode materials. Conversion reaction materials have been identified/proposed as potentially high-energy-density alternatives to intercalation-based materials. However, conversion reaction materials react during lithiation to form entirely new products, often with dramatically changed structure and chemistry, by reaction mechanisms that are still not completely understood. This makes it difficult to clearly distinguish the limitations imposed by the mechanism and practical losses from initial particle morphology, synthetic approaches, and electrode preparations. Transition metal compounds such as transition metal oxides, sulfides, fluorides, phosphides, and nitrides can undergo conversion reactions yielding materials with high theoretical capacity (generally from 500 to 1500 mA h g^-1). In particular, a number of transition metal oxides and sulfides have shown excellent electrochemical properties as high-capacity anode materials. In addition, some transition metal fluorides have shown great potential as cathode materials for Li rechargeable batteries. In this Account we present mechanistic studies, with emphasis on the use of operando methods, of selected examples of conversion-type materials as both potentially high-energy-density anodes and cathodes in EES applications. We also include examples of the conceptually similar conversion-type reactions involving chalcogens and halogens, with emphasis on the Li-S system. In this case we focus on the problems arising from the low electrical conductivities of elemental sulfur and Li₂S and the "redox shuttle" phenomena of polysulfides. In addition to mechanistic insights from the use of operando methods, we also cover several key strategies in electrode materials design such as controlling the size, morphology, composition, and architecture.

In situ analytical techniques for battery interface analysis

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

Understanding materials challenges for rechargeable ion batteries with in situ transmission electron microscopy

An in-depth understanding of material behaviours under complex electrochemical environment is critical for the development of advanced materials for the next-generation rechargeable ion batteries. The dynamic conditions inside a working battery had not been intensively explored until the advent of various in situ characterization techniques. Real-time transmission electron microscopy of electrochemical reactions is one of the most significant breakthroughs poised to enable radical shift in our knowledge on how materials behave in the electrochemical environment. This review, therefore, summarizes the scientific discoveries enabled by in situ transmission electron microscopy, and specifically emphasizes the applicability of this technique to address the critical challenges in the rechargeable ion battery electrodes, electrolyte and their interfaces. New electrochemical systems such as lithium–oxygen, lithium–sulfur and sodium ion batteries are included, considering the rapidly increasing application of in situ transmission electron microscopy in these areas. A systematic comparison between lithium ion-based electrochemistry and sodium ion-based electrochemistry is also given in terms of their thermodynamic and kinetic differences. The effect of the electron beam on the validity of in situ observation is also covered. This review concludes by providing a renewed perspective for the future directions of in situ transmission electron microscopy in rechargeable ion batteries.

In situ TEM is a powerful tool that helps to understand energy storage behaviors of various materials. This review summarizes the critical discoveries, enabled by in situ TEM, in rechargeable ion batteries, and foresees its bright future for extensive applications.

Kinetic Phase Evolution of Spinel Cobalt Oxide during Lithiation

Spinel cobalt oxide has been proposed to undergo a multiple-step reaction during the electrochemical lithiation process. Understanding the kinetics of the lithiation process in this compound is crucial to optimize its performance and cyclability. In this work, we have utilized a low-angle annular dark-field scanning transmission electron microscopy method to visualize the dynamic reaction process in real time and study the reaction kinetics at different rates. We show that the particles undergo a two-step reaction at the single-particle level, which includes an initial intercalation reaction followed by a conversion reaction. At low rates, the conversion reaction starts after the intercalation reaction has fully finished, consistent with the prediction of density functional theoretical calculations. At high rates, the intercalation reaction is overwhelmed by the subsequently nucleated conversion reaction, and the reaction speeds of both the intercalation and conversion reactions are increased. Phase-field simulations show the crucial role of surface diffusion rates of lithium ions in controlling this process. This work provides microscopic insights into the reaction dynamics in non-equilibrium conditions and highlights the effect of lithium diffusion rates on the overall reaction homogeneity as well as the performance.

Lithium intercalation and diffusion in TiO2 nanotubes: a first-principles investigation

The effective diffusion barriers between the outer and inner surfaces of the O_2c–TiO₂ NT can be as low as 0.53 eV.