Lithiation/delithiation of the electrode materials in Li-ion batteries (LIBs) induces large strains in the host material leading to plasticity and fracture. Lithiation is also often accompanied by phase transformations, such as electrochemically-driven solid-state amorphization (ESA). These electrochemical reaction-induced microstructural events limit the energy capacity and cycle lifetime of LIBs. It was recently reported that lithium-ion anode materials composed of nanowires can offer improved performance and lifetime compared to those of micron-scale or larger materials. The improvements are often attributed to the nanowire's unique geometry and enhanced accommodation of the transformation strains that occur during cycling. However, the detailed mechanisms of strain-induced plasticity and strain accommodation in nanowires during electrochemical charging are largely unknown. We report the creation of a nanoscale electrochemical device inside a transmission electron microscopeconsisting of a single SnO 2 nanowire anode, an ionic liquid electrolyte and a bulk LiCoO 2 cathode -and the in-situ observation of the lithiation of the SnO 2 nanowire during electrochemical charging [1]. Upon charging, a reaction front propagated progressively along the nanowire, causing the nanowire to swell, elongate, and spiral (Fig. 1). The reaction front is a "Medusa zone" containing a high density of mobile dislocations, which are continuously nucleated and absorbed at the moving front (Fig. 2). This dislocation cloud indicates large in-plane misfit stresses and is a structural precursor to electrochemically-driven solid-state amorphization. Because lithiation induced volume expansion, plasticity and pulverization of electrode materials are the major mechanical effects that plague the performance and lifetime of high capacity anodes in lithium-ion batteries, our observations provide important mechanistic insight for the design of advanced batteries.

Silicon is one of the most promising candidates for high-energy anodes in a lithium ion battery (LIB), but its rate performance is far below satisfaction due to the poor conductivity. In this talk, I will present our in-situ observation of the record-high charging rate, ~ 37600 A/g, or 10500 C with 1C = 3579 mA/g, of the Si nanowire doped with phosphorus and coated with carbon [1]. An open nano-LIB consisting of an individual Si nanowire as the anode, bulk LiCoO 2 as the cathode, and an ionic liquid (LiTFSI) electrolyte was mounted onto a transmission electron microscopy -scanning probe microscopy (TEM-SPM) platform to enable real-time recording the microstructure evolution during operation [2]. First, we compared the effects of doping and carbon coating on the Si lithiation rates with respect to the intrinsic Si nanowires (Fig. 1). The carbon coating and phosphorus doping each resulted in a two to three orders of magnitude increase in electrical conductivity of the nanowires that, in turn, resulted in a one order of magnitude increase in charging rate. Ultrafast lithiation of Si nanowires was only achieved by combining both phosphorus doping and carbon coating, i.e., charging a Si nanowire in one second in a flooded electrolyte. This demonstrated an effective way to enhance the rate performance of Si-based anodes. Second, electrochemical solid-state amorphization (ESA) and inverse ESA were directly observed and characterized as a two-step phase transformation process during lithiation (Fig. 2): crystalline silicon (Si) transforming to amorphous lithiumsilicon (Li x Si) which transforms to crystalline Li 15 Si 4 (capacity 3579 mAh/g). This confirms some earlier reports of Li 15 Si 4 as the fully lithiated state in a LIB operated at room temperature [3], not the widely believed Li 22 Si 5 . Third, the integrated shape of the Si nanowires was maintained even at the highest charging rate, without pulverization at the largest volume expansion in the first charging process. This fundamental study shows great promise in engineering Si as the high-energy and high-power anode for advanced LIBs used in electric vehicles and high power tools.

Double perovskite ferroelectric thin films are completely new material systems derived from single perovskite. Their diversity of composition and structure and the tendency for spontaneous atomic ordering broaden the path for the development of ferroelectric thin films. The ordered double perovskite ferroelectric thin films lead to excellent ferroelectric, dielectric, magnetic, and optical properties, promising further applications in photovoltaic cells, information memory, and spintronic and photoelectric devices, where the intrinsic coupling and tuning of multiple properties could also push it into multifunctional intersecting devices. However, complex internal physical mechanisms and difficult preparation conditions have prevented its further development. Based on ordered/disordered ferroelectric thin films of double perovskites, this paper first discusses ordered characterization methods such as superstructure reflection/diffraction peaks, especially for epitaxial thin films, saturation magnetization (macroscopic), and scanning transmission electron microscopy (microscopic). In response to the generally poor ordering of present systems, the paper also reviews the internal structure of the material and the external synthesis conditions that affect the ordering, including the valence and radii of the cations, preparation methods, element substitution and strain engineering, in the hope of triggering further research into ordered double perovskite ferroelectrics. Combined with the current state of research on existing double perovskite ferroelectricity thin film systems, advances in the fields of ferroelectric photovoltaics, magnetoelectric coupling, dielectric tunability, resistive switching, and photoelectric coupling have been presented. Finally, the challenges facing the material system are discussed and an outlook is provided for the development of the field.

Lithium ion batteries (LIBs) are attracting attention for energy storage device for electric vehicles where high power, energy density, and cyclability are required. Nanowire (NW) electrode [1] has advantage over conventional electrodes due to its unique geometry that enhances the electron and Li + transport. In addition, NWs can accommodate large volume change during charge/discharge cycles [2], leading to the improved cyclability and stability. In this study, the lithiation processes of individual ZnO NW electrodes in a LIB configuration were observed by in-situ transmission electron microscopy (TEM) using a unique nano-battery setup inside the TEM [3] developed recently for observing the electrochemistry processes in real time.The nano-battery consisted of a single ZnO NW as an anode, an ionic-liquid electrolyte (ILE), and LiCoO 2 particles as cathode. Figure 1 shows the lithiation process of the ZnO NW. The initial NW was straight and had smooth surface (Fig. 1A). After contact with ILE, a potential of -4.0 V with respect to the LiCoO 2 counter electrode was applied to the ZnO NW (Fig.1B). The solid-state reaction front propagated along the longitudinal direction of the NW away from the ILE (Fig. 1B-K). As the reaction front propagated, the diameter and the length of the NW increased, causing the NW to bend. Figure 1L-O shows detailed view of the reaction front propagation. Interestingly, the reaction front did not move progressively along the NW. Instead, it advanced by initiating discrete cracks about 80 nm before the reaction front (Fig. 1M, red arrowheads). The lithiation then propagated laterally along its two side (Fig. 1 N-O). The cracks divided the NW into smaller segments. The new crack grew in a similar fashion to the old crack, and this process repeated until the entire nanowire was lithiated. From the above observations, the lithiation of the ZnO NW consists of three steps. 1) The Li + adsorbs on the NW surface initiating the lithiation.2) The reaction leads to crack formation in the NW making path from the surface inward the bulk. 3) Li + penetrates into the NW from the crack and reacts with the NW. The lithiation process is schematically illustrated in Fig. 2A. The crack formation during the lithiation process caused the ZnO NW to break into multiple segments (Fig. 2B).The fracture of the NW is considered to cause poor cyclability of the battery when ZnO is used as the LIB electrode. Our observations provide important insight for developing battery with higher performance and longer lifetime by providing the failure mechanism of the electrode material [4].