A novel oxygen vacancy assisted transition metal (TM) diffusion mechanism is proposed for the first time to explain the near-surface phase transformation in lithium excess transition metal layered oxides. Oxygen vacancies and TM migration have been observed at nm scale spatial resolution by Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy. Formation of (dilute) oxygen vacancies and their roles in assisting transition metal ion diffusion were further investigated using first principles calculations. The activation barriers of TM diffusion in the presence of oxygen vacancies are drastically reduced and consistently in a reasonable range for room temperature diffusion.

Lithium-ion batteries (LIBs) commercially dominate portable energy storage and have been extended to hybrid/electric vehicles by utilizing electrode materials with enhanced energy density. However, the energy density and cycling life of LIBs must extend beyond the current reach of commercial electrodes to meet the performance requirements for transportation applications. Carbon-based anodes, serving as the main negative electrodes in LIBs, have an intrinsic capacity limitation due to the intercalation mechanism. Some nanostructured carbon materials offer very interesting reversible capacities and can be considered as future anode materials. However, their fabrication processes are often complicated and expensive. Theoretically, using a lithium metal anode is the best way of delivering high energy density due to its largest theoretical capacity of more than 3800 mAh g^-1; however, lithium metal is highly reactive with liquid electrolytes. Alternative anodes are being explored, including other lithium-reactive metals, such as Si, Ge, Zn, V, and so forth. These metals react reversibly with a large amount of Li per formula unit to form lithium-metal alloys, rendering these materials promising candidates for next-generation LIBs with high energy density. Though, most of these pure metallic anodes experience large volume changes during lithiation and delithiation processes that often results in cracking of the anode material and a loss electrical contact between the particles. Nanosized metal sulfides were recently found to possess better cycling stability and larger reversible capacities over pure metals. Further improvements and developments of metal sulfide-based anodes rely on a fundamental understanding of their electrochemical cycling mechanisms. Not only must the specific electrochemical reactions be correctly identified, but also the microstructural evolution upon electrochemical cycling, which often dictates the cyclability and stability of nanomaterials in batteries, must be clearly understood. Probing these dynamic evolution processes, i.e. the lithiation reactions and morphology evolutions, are often challenging. It requires both high-resolution chemical analysis and microstructural identification. In situ transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) has recently been raised as one of the most powerful techniques for monitoring electrochemical processes in anode materials for LIBs. In this work, we focus on elucidating the origin of the structural stability of SnS₂ during electrochemical cycling by revealing the microstructural evolution of SnS₂ upon lithiation using in situ TEM. Crystalline SnS₂ was observed to undergo a two-step reaction after the initial lithium intercalation: (1) irreversible formation of metallic tin and amorphous lithium sulfide and (2) reversible transformation of metallic tin to Li-Sn alloys, which is determined to be the rate-determining step. More interestingly, it was discovered that a self-assembled composite framework formed during the irreversible conversion reaction, which has not been previously reported. Crystalline Sn nanoparticles are well arranged within an amorphous Li₂S "matrix" in this self-assembled framework. This nanoscale framework confines the locations of individual Sn nanoparticles and prevents particle agglomeration during the subsequent cycling processes, therefore providing desired structural tolerance and warranting a sufficientelectron pathway. Our results not only explain the outstanding cycling stability of SnS₂ over metallic tin anodes, but also provide important mechanistic insights into the design of high-performance electrodes for next-generation LIBs through the integration of a unique nanoframework.

Lithium-ion batteries are a leading candidate for electric vehicle and smart grid applications. However, further optimizations of the energy/power density, coulombic efficiency and cycle life are still needed, and this requires a thorough understanding of the dynamic evolution of each component and their synergistic behaviors during battery operation. With the capability of resolving the structure and chemistry at an atomic resolution, advanced analytical transmission electron microscopy (AEM) is an ideal technique for this task. The present review paper focuses on recent contributions of this important technique to the fundamental understanding of the electrochemical processes of battery materials. A detailed review of both static (ex situ) and real-time (in situ) studies will be given, and issues that still need to be addressed will be discussed.

Improved methods for the flux growth of single crystals of the important battery material LiFePO4 have been developed, allowing the facile preparation of single crystals up to 1 cm across with well-developed facets at relatively low temperatures. The structural characterization of these samples by both powder X-ray diffraction and single crystal diffraction (X-ray and neutron) indicates that the samples are typically stoichiometric with a very low concentration of Fe defects on the Li site, though crystals with larger concentrations of defects can be specifically grown using Fe-rich fluxes. These defects occur through the formation of a Fe-rich (Li1-2xFe x)FePO4 partial solid solution, in contrast to the antisite defects more commonly discussed in the literature which would preserve the ideal LiFePO4 stoichiometry. The LiFePO4 defects are shown to be sarcopside-like (2 Li+ → Fe2+ + vacancy) based on compositions refined from single crystal diffraction data, the observed dependence of unit cell parameters on defect concentration, and their observed phase behavior (defects only appear in growths from fluxes which are Fe-rich relative to stoichiometric LiFePO4). The distribution of defects has been studied by aberration corrected scanning transmission electron microscopy and was found to be highly inhomogenous, suggesting that defect-containing crystals may consist of endotaxial intergrowths of olivine LiFePO4 and sarcopside Fe3(PO4)2 in a manner that minimizes the detrimental influence of FeLi defects on the rate of Li-ion transport within crystallites. © 2013 American Chemical Society.

Using a combination of vertical transport measurements across and lateral transport measurements along the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface, we demonstrate that significant potential barrier lowering and band bending are the cause of interfacial metallicity. Barrier lowering and enhanced band bending extends over 2.5 nm into LaAlO$_{3}$ as well as SrTiO$_{3}$. We explain origins of high-temperature carrier saturation, lower carrier concentration, and higher mobility in the sample with the thinnest LaAlO$_{3}$ film on a SrTiO$_{3}$ substrate. Lateral transport results suggest that parasitic interface scattering centers limit the low-temperature lateral electron mobility of the metallic channel.

The atomic and electronic structures at interfaces in thin films are typically different from the bulk and are vitally important in determining the physical properties of thin films. The interface between SrVO3, chosen as a prototype for vanadium-based perovskite materials in this work, and LaAlO3 substrate is investigated by scanning transmission electron microscopy, electron energy-loss spectroscopy, and theoretical multi-electron calculations. Extra electrons have been detected on the interface layer by comparing the energy-loss near-edge structures of V-L3,2 edges to those from the film far from the interface. Monochromated EELS and theoretical calculations for SrVO3, VO2, and V2O 3 support this conclusion. The extra electrons appear to originate from a change in the local bonding configuration of V at the La-O terminated substrate surface as determined by Z-contrast imaging. © 2011 American Institute of Physics.

The catalytic performance of nanoparticles is primarily determined by the precise nature of the surface and near-surface atomic configurations, which can be tailored by post-synthesis annealing effectively and straightforwardly. Understanding the complete dynamic response of surface structure and chemistry to thermal treatments at the atomic scale is imperative for the rational design of catalyst nanoparticles. Here, by tracking the same individual Pt3Co nanoparticles during in situ annealing in a scanning transmission electron microscope, we directly discern five distinct stages of surface elemental rearrangements in Pt3Co nanoparticles at the atomic scale: initial random (alloy) elemental distribution; surface platinum-skin-layer formation; nucleation of structurally ordered domains; ordered framework development and, finally, initiation of amorphization. Furthermore, a comprehensive interplay among phase evolution, surface faceting and elemental inter-diffusion is revealed, and supported by atomistic simulations. This work may pave the way towards designing catalysts through post-synthesis annealing for optimized catalytic performance.

Recent calculations and experimental data suggest that understanding the local ordering behavior of Ni/Mn will be critical to optimize the electrochemical properties of LiNi0.5Mn1.5O4 (LNMO) high voltage spinel. In this study, we systematically controlled the evolution of Ni and Mn ordering in LNMO samples by annealing them at 700 °C in air for different dwelling times, followed by quenching to room temperature. The long- and short-range ordering behavior of Ni and Mn were analyzed by combining neutron powder diffraction, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) data. The results show that the fraction of ordered phase increases rapidly during initial annealing at 700 °C for 6 h, and accompanied by decreasing amounts of secondary phases. Annealing longer than 6 h led to the growth in size of ordered domains (i.e., increased segregation of ordered and disordered domains) along with a slow increase in the fraction of ordered phase. The dependence of open circuit voltages (OCVs) of the LNMO on the degree of ordering agrees well with recent calculations using the density functional theory. The increase in the degree of ordering increases the open circuit voltage (OCV) and the initial capacity but reduces cycle life and rate capability. The LNMO delivered optimal battery performances (capacity, cycle life, and rate capability) after annealing at 700 °C for 2 h. This partially ordered sample showed the respective advantages from both disordered and ordered spinels: better spinel-phase purity (thus, higher initial capacity) from the ordered LNMO and better cycle life and rate capability from the disordered LNMO. © 2014 American Chemical Society.

Despite the large number of reports on colloidal nanocrystals, very little is known about the mechanistic details in terms of nucleation and growth at the atomistic level. Taking bimetallic core-shell nanocrystals as an example, here we integrate in situ liquid-cell transmission electron microscopy with first-principles calculations to shed light on the atomistic details involved in the nucleation and growth of Pt on Pd cubic seeds. We elucidate the roles played by key synthesis parameters, including capping agent and precursor concentration, in controlling the nucleation site, diffusion path, and growth pattern of the Pt atoms. When the faces of a cubic seed are capped by Br^-, Pt atoms preferentially nucleate from corners and then diffuse to edges and faces for the creation of a uniform shell. The diffusion does not occur until the Pt deposited at the corner has reached a threshold thickness. At a high concentration of the precursor, self-nucleation takes place and the Pt clusters then randomly attach to the surface of a seed for the formation of a non-uniform shell. These atomistic insights offer a general guideline for the rational synthesis of nanocrystals with diverse compositions, structures, shapes, and related properties.