Size-Controlled Intercalation-to-Conversion Transition in Lithiation of Transition-Metal ChalcogenidesNbSe₃

Transition-metal chalcogenides (TMCs) can be used either as intercalation cathodes or as conversion-type anodes for lithium ion batteries, for which two distinctively different lithiation reaction mechanisms govern the electrochemical performance of TMCs. However, the factors that control the transition of lithiation mechanisms remain elusive. In this work, we investigated the lithiation process of NbSe₃ ribbons using <i>in situ</i> transmission electron microscopy and observed a size-dependent transition from intercalation to the conversion reaction. Large NbSe₃ ribbons can accommodate high concentrations of Li⁺ through intercalation by relaxing their internal spacing, while lithiation of small NbSe₃ ribbons proceeds readily to full conversion. We found that the size-dependent variation of the lithiation mechanism is associated with both Li⁺ diffusion in NbSe₃ and the accommodation of newly formed phases. For large NbSe₃ ribbons, the intercalation-to-conversion transition is impeded by both long-range Li⁺ diffusion and large-scale accommodation of volume expansion induced by the formation of new phases. These results demonstrate the inherent structural instability of NbSe₃ as an intercalation cathode and its high lithiation rate as a promising conversion-type anode

Atomic Resolution Study of Reversible Conversion Reaction in Metal Oxide Electrodes for Lithium-Ion Battery

Electrode materials based on conversion reactions with lithium ions have shown much higher energy density than those based on intercalation reactions. Here, nanocubes of a typical metal oxide (Co₃O₄) were grown on few-layer graphene, and their electrochemical lithiation and delithiation were investigated at atomic resolution by <i>in situ</i> transmission electron microscopy to reveal the mechanism of the reversible conversion reaction. During lithiation, a lithium-inserted Co₃O₄ phase and a phase consisting of nanosized Co–Li–O clusters are identified as the intermediate products prior to the subsequent formation of Li₂O crystals. In delithiation, the reduced metal nanoparticles form a network and breakdown into even smaller clusters that act as catalysts to prompt reduction of Li₂O, and CoO nanoparticles are identified as the product of the deconversion reaction. Such direct real-space, real-time atomic-scale observations shed light on the phenomena and mechanisms in reaction-based electrochemical energy conversion and provide impetus for further development in electrochemical charge storage devices

Revealing the Dynamics of Platinum Nanoparticle Catalysts on Carbon in Oxygen and Water Using Environmental TEM

Deactivation of supported metal nanoparticle catalysts, especially under relevant gas conditions, is a critical challenge for many technological applications, including heterogeneous catalysis, electrocatalysis, and fuel cells. It has been commonly realized that deactivation of catalysts stems from surface area loss due to particle coarsening; however, the mechanism for this remains largely unclear. Herein, we use aberration-corrected environmental transmission electron microscopy, at an atomic level, to observe in situ the dynamics of Pt catalysts under fuel cell relevant gas and temperature conditions. Particle migration and coalescence is observed to be the dominant coarsening process. In comparison with the case of H₂O, O₂ promotes Pt nanoparticle migration on the carbon surface. Surprisingly, coating Pt/carbon with a nanofilm of electrolyte (Nafion ionomer) leads to a faster migration of Pt in H₂O than in O₂, a consequence of a Nafion–carbon interface water “lubrication” effect. Atomically, the particle coalescence features reorientation of particles toward lattice matching, a process driven by orientation-dependent van der Waals forces. These results provide direct observations of the dynamics of metal nanoparticles at the critical surface/interface under relevant conditions and yield significant insights into the multiphase interaction in related technological processes

Revealing the Reaction Mechanism of Na–O₂ Batteries using Environmental Transmission Electron Microscopy

Sodium–oxygen (Na–O₂) batteries are being extensively studied because of their higher energy efficiency compared to that of lithium oxygen (Li–O₂) batteries. The critical challenges in the development of Na–O₂ batteries include the elucidation of the reaction mechanism, reaction products, and the structural and chemical evolution of the reaction products and their correlation with battery performance. For the first time, in situ transmission electron microscopy was employed to probe the reaction mechanism and structural evolution of the discharge products in Na–O₂ batteries. The discharge product was featured by the formation of both cubic and conformal NaO₂. It was noticed that the impingement of the reaction product (NaO₂) led to particle coarsening through coalescence. We investigated the stability of the discharge product and observed that the reaction product NaO₂ was stable in the case of the solid electrolyte. The present work provides unprecedented insight into the development of Na–O₂ batteries

Revealing the Reaction Mechanism of Na–O₂ Batteries using Environmental Transmission Electron Microscopy

Sodium–oxygen (Na–O₂) batteries are being extensively studied because of their higher energy efficiency compared to that of lithium oxygen (Li–O₂) batteries. The critical challenges in the development of Na–O₂ batteries include the elucidation of the reaction mechanism, reaction products, and the structural and chemical evolution of the reaction products and their correlation with battery performance. For the first time, in situ transmission electron microscopy was employed to probe the reaction mechanism and structural evolution of the discharge products in Na–O₂ batteries. The discharge product was featured by the formation of both cubic and conformal NaO₂. It was noticed that the impingement of the reaction product (NaO₂) led to particle coarsening through coalescence. We investigated the stability of the discharge product and observed that the reaction product NaO₂ was stable in the case of the solid electrolyte. The present work provides unprecedented insight into the development of Na–O₂ batteries

Revealing the Reaction Mechanism of Na–O₂ Batteries using Environmental Transmission Electron Microscopy

Sodium–oxygen (Na–O₂) batteries are being extensively studied because of their higher energy efficiency compared to that of lithium oxygen (Li–O₂) batteries. The critical challenges in the development of Na–O₂ batteries include the elucidation of the reaction mechanism, reaction products, and the structural and chemical evolution of the reaction products and their correlation with battery performance. For the first time, in situ transmission electron microscopy was employed to probe the reaction mechanism and structural evolution of the discharge products in Na–O₂ batteries. The discharge product was featured by the formation of both cubic and conformal NaO₂. It was noticed that the impingement of the reaction product (NaO₂) led to particle coarsening through coalescence. We investigated the stability of the discharge product and observed that the reaction product NaO₂ was stable in the case of the solid electrolyte. The present work provides unprecedented insight into the development of Na–O₂ batteries

Rock-Salt Growth-Induced (003) Cracking in a Layered Positive Electrode for Li-Ion Batteries

For the first time, (003) cracking is observed and determined to be the major cracking mechanism for the primary particles of Ni-rich layered dioxides as the positive electrode for Li-ion batteries. Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase along the (003) plane of the layered oxide are the leading cause for the cracking of primary particles. The fracturing of the rock-salt platelet is induced by the stress discontinuity between the parent layered oxide and the rock-salt phase. The high nickel content is considered to be the key factor for the formation of the rock-salt platelet and thus the (003) cracking. The (003)-type cracking can be a major factor for the structural degradation and associated capacity fade of the layered positive electrode

Insights on the Mechanism of Na-Ion Storage in Soft Carbon Anode

Graphite is the commercial anode for lithium-ion batteries; however, it fails to extend its success to sodium-ion batteries. Recently, we demonstrated that a low-cost amorphous carbonsoft carbon exhibits remarkable rate performance and stable cycling life of Na-ion storage. However, its Na-ion storage mechanism has remained elusive, which has plagued further development of such carbon anodes. Here, we remedy this shortfall by presenting the results from an integrated set of experimental and computational studies that, for the first time, reveal the storage mechanism for soft carbon. We find that sodium ions intercalate into graphenic layers, leading to an irreversible quasi-plateau at ∼0.5 V versus Na⁺/Na as well as an irreversible expansion seen by in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD). Such a high-potential plateau is correlated to the defective local structure inside the turbostratic stacking of soft carbon and the associated high-binding energies with Na ions, suggesting a trapping mechanism. On the other hand, soft carbon exhibits long sloping regions above and below the quasi-plateau during the first sodiation, where the sloping regions present highly reversible behavior. It is attributed to the more defects contained by soft carbon revealed by neutron total scattering and the associated pair distribution function studies

Insights on the Mechanism of Na-Ion Storage in Soft Carbon Anode

Graphite is the commercial anode for lithium-ion batteries; however, it fails to extend its success to sodium-ion batteries. Recently, we demonstrated that a low-cost amorphous carbonsoft carbon exhibits remarkable rate performance and stable cycling life of Na-ion storage. However, its Na-ion storage mechanism has remained elusive, which has plagued further development of such carbon anodes. Here, we remedy this shortfall by presenting the results from an integrated set of experimental and computational studies that, for the first time, reveal the storage mechanism for soft carbon. We find that sodium ions intercalate into graphenic layers, leading to an irreversible quasi-plateau at ∼0.5 V versus Na⁺/Na as well as an irreversible expansion seen by in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD). Such a high-potential plateau is correlated to the defective local structure inside the turbostratic stacking of soft carbon and the associated high-binding energies with Na ions, suggesting a trapping mechanism. On the other hand, soft carbon exhibits long sloping regions above and below the quasi-plateau during the first sodiation, where the sloping regions present highly reversible behavior. It is attributed to the more defects contained by soft carbon revealed by neutron total scattering and the associated pair distribution function studies

Li⁺‑Desolvation Dictating Lithium-Ion Battery’s Low-Temperature Performances

Lithium (Li) ion battery has penetrated almost every aspect of human life, from portable electronics, vehicles, to grids, and its operation stability in extreme environments is becoming increasingly important. Among these, subzero temperature presents a kinetic challenge to the electrochemical reactions required to deliver the stored energy. In this work, we attempted to identify the rate-determining process for Li⁺ migration under such low temperatures, so that an optimum electrolyte formulation could be designed to maximize the energy output. Substantial increase in the available capacities from graphite∥LiNi_0.80Co_0.15Al_0.05O₂ chemistry down to −40 °C is achieved by reducing the solvent molecule that more tightly binds to Li⁺ and thus constitutes a high desolvation energy barrier. The fundamental understanding is applicable universally to a wide spectrum of electrochemical devices that have to operate in similar environments