A New Geometric Method Based on Two-Dimensional Transmission Electron Microscopy for Analysis of Interior versus Exterior Pd Loading on Hollow Carbon Nanofibers

Hollow carbon nanofibers (CNFs) are being explored as catalyst supports because of their unique properties. Internal versus external loading of metal nanoparticles impacts catalytic performance; we developed a fast and accurate geometric analysis method based on two-dimensional transmission electron microscopy (2D TEM) images to estimate Pd internal versus external loading percentages. Three different Pd-loaded CNF catalysts were prepared using methods reported in the literature to yield different amounts of Pd inside loading. Results indicate the percentage of inside-loaded Pd increases as expected in the three samples (from 22.7 ± 17.8%, to 47.2 ± 22.8%, to 71.4 ± 19.7%, based on Pd nanoparticle number). We compared percent inside loading values for one segment of a Pd-loaded CNF using our method and three-dimensional scanning transmission electron microscopy (3D STEM), and observed adequate agreement (27.8% vs 32.7%). Our geometric analysis method is proposed as a more straightforward and fast way to evaluate metal nanoparticles on tubular supports

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

Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core–shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core–shell” structure in cycled primary particles with a formula of LiNi_0.80Co_0.15Al_0.05O₂, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core–shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core–shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes

Formation of an Anti-Core–Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries

The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core–shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core–shell” structure in cycled primary particles with a formula of LiNi_0.80Co_0.15Al_0.05O₂, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core–shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core–shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes

In Situ TEM Observations of Sn-Containing Silicon Nanowires Undergoing Reversible Pore Formation Due to Fast Lithiation/Delithiation Kinetics

In situ transmission electron microscopy (TEM) studies were carried out to observe directly in real time the lithiation and delithiation of silicon (Si) nanowires with significant amounts of tin (Sn). The incorporation of Sn significantly enhances the lithiation rate compared to typical Si nanowires: surface diffusion is enhanced by 2 orders of magnitude and the bulk lithiation rate by 1 order of magnitude, resulting in a sequential surface-then-core lithiation mechanism. Pore formation was observed in the nanowires during delithiation as a result of the fast lithiation/delithiation kinetics of the nanowires. Pore formation was found to be associated with nonlithiated crystalline domains in the nanowire, which prevent uniform structural changes of the nanowire, and the resulting pores increase in size after each cycle. When an amorphous Si shell was applied to the nanowires, pore formation was not observed during the in situ TEM experiments. Ex situ TEM analysis of Sn-containing Si nanowires cycled in coin cell batteries also showed that the application of an amorphous Si shell slows pore formation in these nanowires, while fast lithiation/delithiation kinetics is retained

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

Tracking the Oxidation of Silicon Anodes Using Cryo-EELS upon Battery Cycling

Silicon is a high-capacity material for the anode of a rechargeable lithium-ion battery. One of the fundamental challenges for using Si in anodes is capacity fading, which has been revealed to be partially associated with the interfacial instability between the Si and liquid electrolyte due to the large volume swing of Si upon charging and discharging. Smart nanoscale design concepts, either presynthesized or formed in situ, have led to the mitigation of the detrimental factors associated with the volume swing of Si. However, it has never been clear how the chemical state of Si evolves and contributes to the capacity fading upon battery cycling. Here, we use cryo-electron energy loss spectroscopy to directly monitor, at a subnanometer scale, the chemical evolution of Si upon battery cycling. We discover that during the cycling process Si particles are progressively oxidized to form SiO2, which is initiated from the particle surface and gradually penetrates toward the interior of the particle, directly contributing to the capacity fading. Possible mechanisms of Si oxidation are postulated. We further show how the cycling stability can be improved by an electrolyte additive to form an effective passivation layer, representatively, even a small concentration of fluoroethylene carbonate causes the formation of an LiF layer on the Si nanoparticle surface that prevents Si oxidation and improves cycling stability. The present work unveils Si oxidation as a previously unrecognized factor that contributes to capacity fading, therefore providing insight into the design of anodes with Si-based materials

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