Structure of δ‑Alumina: Toward the Atomic Level Understanding of Transition Alumina Phases

Transition Al₂O₃ derived from thermal decomposition of AlOOH Boehmite have complex structures and to a large extent remain poorly understood. Here, we report a detailed atomic level analysis of δ-Al₂O₃ for the first time using a combination of high-angle annular dark field electron microscopy imaging, X-ray diffraction refinement, and density functional theory calculations. We show that the structure of δ-Al₂O₃ represents a complex structural intergrowth from two main crystallographic variants, which are identified as δ₁-Al₂O₃ and δ₂-Al₂O₃. The two main variants are fully structurally described, and in addition, we also derive their energy of formation. On the basis of comparison with other relevant transition Al₂O₃ phases, it is shown how energetic degeneracy leads to the structural disorder and complex intergrowths among several transition Al₂O₃. The results of the work have important implications for understanding thermodynamic stability and transformation processes in transition alumina

Structure of δ‑Alumina: Toward the Atomic Level Understanding of Transition Alumina Phases

Transition Al₂O₃ derived from thermal decomposition of AlOOH Boehmite have complex structures and to a large extent remain poorly understood. Here, we report a detailed atomic level analysis of δ-Al₂O₃ for the first time using a combination of high-angle annular dark field electron microscopy imaging, X-ray diffraction refinement, and density functional theory calculations. We show that the structure of δ-Al₂O₃ represents a complex structural intergrowth from two main crystallographic variants, which are identified as δ₁-Al₂O₃ and δ₂-Al₂O₃. The two main variants are fully structurally described, and in addition, we also derive their energy of formation. On the basis of comparison with other relevant transition Al₂O₃ phases, it is shown how energetic degeneracy leads to the structural disorder and complex intergrowths among several transition Al₂O₃. The results of the work have important implications for understanding thermodynamic stability and transformation processes in transition alumina

Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts

In this Environmental Transmission Electron Microscopy (ETEM) study we examined the growth patterns of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO₂ was heated at 550 °C in 560 Pa of O₂ while Pd/carbon was heated in vacuum at 500 °C and in 300 Pa of 5%H₂ in Argon at temperatures up to 600 °C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log-normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/carbon under H₂. This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism

Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts

In this Environmental Transmission Electron Microscopy (ETEM) study we examined the growth patterns of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO₂ was heated at 550 °C in 560 Pa of O₂ while Pd/carbon was heated in vacuum at 500 °C and in 300 Pa of 5%H₂ in Argon at temperatures up to 600 °C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log-normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/carbon under H₂. This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism

Mitigating Voltage Fade in Cathode Materials by Improving the Atomic Level Uniformity of Elemental Distribution

Lithium- and manganese-rich (LMR) layered-structure materials are very promising cathodes for high energy density lithium-ion batteries. However, their voltage fading mechanism and its relationships with fundamental structural changes are far from being well understood. Here we report for the first time the mitigation of voltage and energy fade of LMR cathodes by improving the atomic level spatial uniformity of the chemical species. The results reveal that LMR cathodes (Li­[Li_0.2Ni_0.2M_0.6]­O₂) prepared by coprecipitation and sol–gel methods, which are dominated by a LiMO₂ type <i>R</i>3̅<i>m</i> structure, show significant nonuniform Ni distribution at particle surfaces. In contrast, the LMR cathode prepared by a hydrothermal assisted method is dominated by a Li₂MO₃ type <i>C</i>2/<i>m</i> structure with minimal Ni-rich surfaces. The samples with uniform atomic level spatial distribution demonstrate much better capacity retention and much smaller voltage fade as compared to those with significant nonuniform Ni distribution. The fundamental findings on the direct correlation between the atomic level spatial distribution of the chemical species and the functional stability of the materials may also guide the design of other energy storage materials with enhanced stabilities

Revealing the Atomic Restructuring of Pt–Co Nanoparticles

We studied Pt–Co bimetallic nanoparticles during oxidation in O₂ and reduction in H₂ atmospheres using an aberration corrected environmental transmission electron microscope. During oxidation Co migrates to the nanoparticle surface forming a strained epitaxial CoO film. It subsequently forms islands via strain relaxation. The atomic restructuring is captured as a function of time. During reduction cobalt migrates back to the bulk, leaving a monolayer of platinum on the surface

Nanoscale Phase Separation, Cation Ordering, and Surface Chemistry in Pristine Li_1.2Ni_0.2Mn_0.6O₂ for Li-Ion Batteries

Li-rich layered material Li_1.2Ni_0.2Mn_0.6O₂ possesses high voltage and high specific capacity, which makes it an attractive candidate for the transportation industry and sustainable energy storage systems. The rechargeable capacity of the Li-ion battery is linked largely to the structural stability of the cathode materials during the charge–discharge cycles. However, the structure and cation distribution in pristine Li_1.2Ni_0.2Mn_0.6O₂ have not yet been fully characterized. Using a combination of aberration-corrected scanning transmission electron microscopy, X-ray energy-dispersive spectroscopy (XEDS), electron energy loss spectroscopy (EELS), and complementary multislice image simulation, we have probed the crystal structure, cation/anion distribution, and electronic structure of the Li_1.2Ni_0.2Mn_0.6O₂ nanoparticle. The electronic structure and valence state of transition-metal ions show significant variations, which have been identified to be attributed to the oxygen deficiency near certain particle surfaces. Characterization of the nanoscale phase separation and cation ordering in the pristine material are critical for understanding the capacity and voltage fading of this material for battery application

Revealing the Atomic Restructuring of Pt–Co Nanoparticles

We studied Pt–Co bimetallic nanoparticles during oxidation in O₂ and reduction in H₂ atmospheres using an aberration corrected environmental transmission electron microscope. During oxidation Co migrates to the nanoparticle surface forming a strained epitaxial CoO film. It subsequently forms islands via strain relaxation. The atomic restructuring is captured as a function of time. During reduction cobalt migrates back to the bulk, leaving a monolayer of platinum on the surface

Conflicting Roles of Nickel in Controlling Cathode Performance in Lithium Ion Batteries

A variety of approaches are being made to enhance the performance of lithium ion batteries. Incorporating multivalence transition-metal ions into metal oxide cathodes has been identified as an essential approach to achieve the necessary high voltage and high capacity. However, the fundamental mechanism that limits their power rate and cycling stability remains unclear. The power rate strongly depends on the lithium ion drift speed in the cathode. Crystallographically, these transition-metal-based cathodes frequently have a layered structure. In the classic wisdom, it is accepted that lithium ion travels swiftly within the layers moving out/in of the cathode during the charge/discharge. Here, we report the unexpected discovery of a thermodynamically driven, yet kinetically controlled, surface modification in the widely explored lithium nickel manganese oxide cathode material, which may inhibit the battery charge/discharge rate. We found that during cathode synthesis and processing before electrochemical cycling in the cell nickel can preferentially move along the fast diffusion channels and selectively segregate at the surface facets terminated with a mix of anions and cations. This segregation essentially can lead to a higher lithium diffusion barrier near the surface region of the particle. Therefore, it appears that the transition-metal dopant may help to provide high capacity and/or high voltage but can be located in a “wrong” location that may slow down lithium diffusion, limiting battery performance. In this circumstance, limitations in the properties of lithium ion batteries using these cathode materials can be determined more by the materials synthesis issues than by the operation within the battery itself