Search

Capacity fading during long-term cycling (>1500×) is still a critical challenge for Li-ion batteries that use Ni-rich layered oxides, e.g. LiNi0.8Co0.15Al0.05O2 (NCA), as the cathode. Microcracks have been previously recognized as one of the primary reasons for the observed capacity fade. Although there exists a generally developed mechanical understanding of microcracks, the role of the electrolyte has not been clearly understood, especially after extended cycling and at the atomic scale. Here, we unveil the microstructural evolution of spherical NCA secondary particles after long-term cycling using scanning transmission electron microscopy accompanied with electron energy loss spectroscopy. We found that the microcracks initiated and grew through grain boundaries, which then serve as the pathway for electrolyte penetration into secondary NCA particles. Additionally, the rock-salt phase reconstruction is prone to occur at the (003) surfaces of the primary particles or the crack surfaces, largely due to electrolyte (LiPF6 EC/EMC) corrosion. Crack propagation within the NCA grains is primarily a joint consequence from electrolyte corrosion and mechanical strain during lithiation/delithiation. During extended cycling, due to the distinctive surface facets, the primary grains located in the center of the secondary particles experience more intensive electrolyte corrosion, leading to a reduced contact with nearby particles, impairing the overall capacity. These results establish the initiation and growth mechanism of microcracks and voids in NCA-based cathodes during cycling and point out the role of the electrolyte in affecting the degradation of NCA-based cathodes. [ABSTRACT FROM AUTHOR]

In this investigation, the improved electrochemical behavior in Si-doped Li-rich cathodes is studied with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Z-contrast images show a layered structure that develops a thin, spinel-like surface layer after the first charge cycle. Si-doping increases discharge capacity by ∼25% and appears to retard the surface phase transformation. Based on electron energy loss spectra, the surface layer in the doped material has an altered oxygen electronic environment, which supports the STEM findings. Furthermore, Si-doping changes the redox behavior during the activation cycle. Density functional theory calculations indicate that Si-doping can increase oxygen vacancy formation, and change the sequence of the redox couples by introducing more oxygen vacancies before or during the typical high voltage activation process. The results of this work indicate that the type of doping employed here is an effective strategy for controlling the complex charge compensation mechanisms in lithium-rich cathodes. [ABSTRACT FROM AUTHOR]

A three-dimensional all-in-one Sn–Co alloy anode is reported for the first time, which delivers a high capacity along with a stable coulombic efficiency as well as good temperature tolerance. The binder-free electrode eliminates the complexity of conventional slurry preparation while maintaining an integrated scaffold, which provides space to accommodate volume expansion, as confirmed by an in situ transmission electron microscopy study. [ABSTRACT FROM AUTHOR]

This work reports a new strategy toward the design of a new class of supported catalysts with intrinsic high-temperature stabilities through entropy maximization. The use of Pt, Ni, Mg, Cu, Zn, and Co not only enables the active sites to be highly dispersed for high catalytic activity in CO oxidation, but also results in extreme thermal stability (900 °C) owing to the entropy-stabilized behavior inside the metal oxide being able to survive harsh conditions. [ABSTRACT FROM AUTHOR]

A novel battery hybrid composite anode consisting of tin nanoparticles encapsulated by double nets of carbon matrixes is presented. The improved electron transfer and volume change accommodation are confirmed via density functional theory modeling and in situ transmission electron microscopy observations, respectively. [ABSTRACT FROM AUTHOR]

Due to their high specific capacity and negligible volume expansion during cycling, anatase titanium dioxide (a-TiO2) nanotubes have been considered as a prime candidate for anodes in lithium-ion batteries. However, their rate capability for electrochemical cycling is limited by the low electronic conductivity of a-TiO2 nanotubes. Here, we show that a desirable amount of nitrogen doping can significantly enhance the electronic conductivity in a-TiO2 nanotubes, resulting in improvements in both the capacity stability and the rate capability at fast charge–discharge rates. Electron energy loss spectroscopy revealed a high doping concentration of nitrogen (∼5%) by substituting for oxygen ions in a-TiO2 nanotubes. The lithiation mechanism of N-doped a-TiO2 nanotubes was further investigated using in situ transmission electron microscopy, where a three-step lithiation mechanism was revealed. Lithium ions initially intercalate into the a-TiO2 lattice structure. Further insertion of lithium ions triggers a phase transformation from a-TiO2 to orthorhombic Li0.5TiO2 and finally to polycrystalline tetragonal LiTiO2. Our results reveal that nitrogen doping significantly facilitates lithiation in TiO2 through enhanced electronic conductivity, while the structural and chemical evolutions during the lithiation process remain similar to those of undoped TiO2. [ABSTRACT FROM AUTHOR]

We created Immense Surface Area Carbons (ISACs) by a novel heat treatment that stabilized the micelle structure in a biological based precursor prior to high temperature combined activation – pyrolysis. While displaying a morphology akin to that of commercial activated carbon, ISACs contain an unparalleled combination of electrochemically active surface area and pore volume (up to 4051 m2 g−1, total pore volume 2.60 cm3 g−1, 76% small mesopores). The carbons also possess the benefit of being quite pure (combined O and N: 2.6–4.1 at%), thus allowing for a capacitive response that is primarily EDLC. Tested at commercial mass loadings (∼10 mg cm−2) ISACs demonstrate exceptional specific capacitance values throughout the entire relevant current density regime, with superior rate capability primarily due to the large fraction of mesopores. In the optimized ISAC, the specific capacitance (Cg) is 540 F g−1 at 0.2 A g−1, 409 F g−1 at 1 A g−1 and 226 F g−1 at a very high current density of 300 A g−1 (∼0.15 second charge time). At intermediate and high currents, such capacitance values have not been previously reported for any carbon. Tested with a stable 1.8 V window in a 1 M Li2SO4 electrolyte, a symmetric supercapacitor cell yields a flat energy–power profile that is fully competitive with those of organic electrolyte systems: 29 W h kg−1 at 442 W kg−1 and 17 W h kg−1 at 3940 W kg−1. The cyclability of symmetric ISAC cells is also exceptional due to the minimization of faradaic reactions on the carbon surface, with 80% capacitance retention over 100 000 cycles in 1 M Li2SO4 and 75 000 cycles in 6 M KOH. [ABSTRACT FROM AUTHOR]

We report a facile synthesis of Ag@Au core–sheath nanowires through the conformal deposition of Au atoms onto the surface of pre-synthesized Ag nanowires. The resulting Ag@Au nanowires showed morphology and optical properties almost identical to the pristine Ag nanowires, but with greatly improved stability under different corrosive environments. [ABSTRACT FROM AUTHOR]

Three new lithium salts, lithium difluoro-2-methyl-2-fluoromalonatoborate (LiDFMFMB), lithium difluoro-2-ethyl-2-fluoromalonatoborate (LiDFEFMB), and lithium difluoro-2-propyl-2-fluoromalonatoborate (LiDFPFMB), have been synthesized and evaluated for application in lithium ion batteries. These new salts are soluble in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1 : 2 by wt) and 1.0 M salt solutions can be easily prepared. The ionic conductivities of these new salts are close to those of LiBF4 and LiPF6. Cyclic voltammograms reveal that these new salt based electrolytes can passivate both natural graphite and high voltage spinel LiNi0.5Mn1.5O4 (LNMO) to form effective solid electrolyte interphases (SEIs). In addition, these new salt-based electrolytes exhibit good cycling stability with high coulombic efficiencies in both LiNi0.5Mn1.5O4 and graphite based half-cells and full cells. [ABSTRACT FROM AUTHOR]

We offer new insights into the roles played by surface capping in controlling the pattern of growth involving Pd cubic seeds and a HAuCl4 precursor. The final products can take different surface structures (concave vs. flat side faces) depending on the presence or absence of surface capping. [ABSTRACT FROM AUTHOR]

Li-rich layered oxides hold great promise for improving the energy density of present-day Li-ion batteries. Their application is, however, limited by the voltage decay upon cycling, and the origin of such a phenomenon is poorly understood. A major issue is determining the voltage range over which detrimental reactions originate. In the present study, a unique yet effective approach was employed to probe this issue. Instead of studying the materials during the first cycle, electrochemical behavior and evolution of the atomic structures were compared in extensively cycled specimens under varied charge/discharge voltages. With the upper cutoff voltage lowered from 4.8 to 4.4 V, the voltage decay ceased to occur even after 60 cycles. In the meantime, the material maintained its layered structure without any spinel phase emerging at the surface, which is unambiguously shown by the atomic-resolution Z-contrast imaging and electron energy loss spectroscopy. These results have conclusively demonstrated that structural/chemical changes responsible for the voltage decay began between 4.4 and 4.8 V, where the layered-to-spinel transition was the most dramatic structural change observed. This discovery lays important groundwork for the mechanistic understanding of the voltage decay in Li-rich layered cathode materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Monodispersed bimetallic PdAu nanoparticles with controlled composition are prepared by an emulsion-assisted synthetic strategy with ternary metal precursors in the surfactants of oleic acid and oleylamine. The PdAu nanoparticles are loaded on a carbon support, and their electrocatalytic activities are tested for methanol oxidation in alkaline media. The bimetallic PdAu nanoparticles show superior electrocatalytic activities for methanol oxidation compared with the pure Pd nanoparticles prepared by the same method. The most active Pd30Au70 nanoparticles, with significantly low Pd content, even show remarkably higher activities than the commercial Pt/C catalyst. Various characterization techniques such as TEM, XPS and UV-vis are applied to study the nature of the catalysts. It is concluded that the increased activity is dependant on the unique Pd-rich shell and Au-rich core structure of such bimetallic PdAu particles as well as the nature of the Pd species on the catalyst surface. [ABSTRACT FROM AUTHOR]

A detailed surface investigation of the lithium-excess nickel manganese layered oxide Li1.2Ni0.2Mn0.6O2 structure was carried out using X-ray photoelectron spectroscopy (XPS), total electron yield and transmission X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) during the first two electrochemical cycles. All spectroscopy techniques consistently showed the presence of Mn4+ in the pristine material and a surprising reduction of Mn at the voltage plateau during the first charge. The Mn reduction is accompanied by the oxygen loss revealed using EELS. Upon the first discharge, the Mn at the surface never fully recovers back to Mn4+. The electrode/electrolyte interface of this compound consists of the reduced Mn at the crystalline defect-spinel inner layer and an oxidized Mn species simultaneously with the presence of a superoxide species in the amorphous outer layer. This proposed model signifies that oxygen vacancy formation and lithium removal result in electrolyte decomposition and superoxide formation, leading to Mn activation/dissolution and surface layer-spinel phase transformation. The results also indicate that the role of oxygen is complex and significant in contributing to the extra capacity of this class of high energy density cathode materials. [ABSTRACT FROM AUTHOR]

Stable high-concentration suspensions (up to 0.95 mg mL−1) of non-oxidized few layer graphene (FLG), five or less sheets, with micrometre-long edges were obtained viadirect exfoliation of natural graphite flakes in ionic liquids, such as 1-butyl-3-methyl-imidazolium bis(trifluoro-methane-sulfonyl)imide ([Bmim]-[Tf2N]), by tip ultrasonication. [ABSTRACT FROM AUTHOR]

Correction for ‘New promising lithium malonatoborate salts for high voltage lithium ion batteries’ by Xiao-Guang Sun et al., J. Mater. Chem. A, 2017, 5, 1233–1241. [ABSTRACT FROM AUTHOR]

Supported noble metal catalysts, ubiquitous in chemical technology, often undergo dynamic transformations between reduced and oxidized states-which influence the metal nuclearities, oxidation states, and catalytic properties. In this investigation, we report the results of in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, and other physical characterization techniques, bolstered by density functional theory, to elucidate the structural transformations of a set of MgO-supported palladium catalysts under oxidative treatment conditions. As the calcination temperature increased, the as-synthesized supported metallic palladium nanoparticles underwent oxidation to form palladium oxides (at approximately 400 °C), which, at approximately 500 °C, were oxidatively fragmented to form mixtures of atomically dispersed palladium cations. The data indicate two distinct types of atomically dispersed species: palladium cations located at MgO steps and those embedded in the first subsurface layer of MgO. The former exhibit significantly higher (>500 times) catalytic activity for ethylene hydrogenation than the latter. The results pave the way for designing highly active and stable supported palladium hydrogenation catalysts with optimized metal utilization., Competing Interests: The authors declare no conflict of interests., (This journal is © The Royal Society of Chemistry.)

A detailed surface investigation of the lithium-excess nickel manganese layered oxide Li1.2Ni0.2Mn0.6O2 structure was carried out using X-ray photoelectron spectroscopy (XPS), total electron yield and transmission X-ray absorption spectroscopy (XAS), and electron energy loss spectroscopy (EELS) during the first two electrochemical cycles. All spectroscopy techniques consistently showed the presence of Mn(4+) in the pristine material and a surprising reduction of Mn at the voltage plateau during the first charge. The Mn reduction is accompanied by the oxygen loss revealed using EELS. Upon the first discharge, the Mn at the surface never fully recovers back to Mn(4+). The electrode/electrolyte interface of this compound consists of the reduced Mn at the crystalline defect-spinel inner layer and an oxidized Mn species simultaneously with the presence of a superoxide species in the amorphous outer layer. This proposed model signifies that oxygen vacancy formation and lithium removal result in electrolyte decomposition and superoxide formation, leading to Mn activation/dissolution and surface layer-spinel phase transformation. The results also indicate that the role of oxygen is complex and significant in contributing to the extra capacity of this class of high energy density cathode materials.

A strategy for stabilizing ultrasmall gold clusters under thermal treatment has been developed. The essence of this methodology lies in construction of heterostructured transition-metal oxide-mesoporous silica supports. The supported clusters have been demonstrated to be sintering resistant and highly active for catalytic CO oxidation.