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[en] This study reports the onset of the Jahn-Teller distortion in 4 V LiMn2O4 thin film electrodes that was investigated using an in situ bending beam method (BBM). The phase transformation during lithium insertion/extraction could be detected using the BBM technique. The phase transformation between the cubic and tetragonal phases was depicted by the larger value of the compressive or tensile differential strain, which is consistent with a well-known phase transformation between those phases in 3 V LiMn2O4. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously. The potential ranges responsible for the Jahn-Teller distortion in 4 V range, which takes place at the electrode surface, was determined by the charge versus. differential strain (dε/dQ) curve. The onset of the Jahn-Teller distortion was observed at the end of the cathodic scan, and the relaxation of the Jahn-Teller distortion was observed at the beginning of anodic scan. Furthermore, the onset of the Jahn-Teller distortion was found to be dependent on the lithium ion insertion rate, which was controlled by the scan rate
[en] A new solution combustion synthesis of layered LiNi0.5Mn0.5O2 involving the reactions of LiNO3, Mn(NO3)2, NiNO3, and glycine as starting materials is reported. TG/DTA studies were performed on the gel-precursor and suggest the formation of the layered LiNi0.5Mn0.5O2 at low temperatures. The synthesized material was annealed at various temperatures, viz., 250, 400, 600, and 850 deg. C, characterized by means of X-ray diffraction (XRD) and reveals the formation of single phase crystalline LiNi0.5Mn0.5O2 at 850 deg. C. The morphology of the synthesized material has been investigated by means of scanning electron microscopy (SEM) and suggests the formation of sub-micron particles. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) studies on the synthesized LiNi0.5Mn0.5O2 powders indicate that the oxidation states of nickel and manganese are +2 and +4, respectively. Electrochemical galvanostatic charge-discharge cycling behavior of Li//LiNi0.5Mn0.5O2 cell using 1 M LiPF6 in EC/DMC as electrolyte exhibited stable capacities of ∼125 mAh/g in the voltage ranges 2.8-4.3 V and 3.0-4.6 V and is comparable to literature reports using high temperature synthesis route. The capacity remains stable even after 20 cycles. The layered LiNi0.5Mn0.5O2 powders synthesized by this novel route have several advantages as compared to its conventional synthesis techniques
[en] This study determined the potential range where the dissolution of a surface film on a thin film LiMn2O4 electrode, which forms during electrode synthesis, and the formation of a new surface film during cycling at room temperature using an in situ bending beam method (BBM) and an in situ electrochemical quartz crystal microbalance (EQCM) technique with cyclic voltammetry and a galvanostatic charge/discharge cycle. The electrolytes used were LiClO4/EC-DEC, LiClO4/PC and LiPF6/EC-DMC. The deflectogram and massogram showed large peaks during the anodic scan only in the first cycle. These phenomena, were observed regardless of the electrolytes and scan rate used. The tensile strain and the mass reduction in the early stage of the strain peak and the mass peak are related to the dissolution of the initial surface film. In addition, the compressive strain and the mass increase are related to the formation of a new surface film during cycling. The potential ranges where the formation of the new surface film begins ranged from 4.03 to 4.1 V, which appears to terminate at the end of the first anodic scan, and was also observed during the galvanostatic charge/discharge cycle in the same potential range
[en] We utilize transmission electron microscopy in conjunction with electron energy loss spectroscopy to investigate local degradation that occurs in LixNi0.8Co0.15Al0.05O2 cathode materials (NCA) after 30 cycles with cutoff voltages of 4.3 V and 4.8 V at 55 °C. NCA has a homogeneous crystallographic structure before electrochemical reactions; however, we observed that 30 cycles of charge/discharge reactions induced inhomogeneity in the crystallographic and electronic structures and also introduced porosity particularly at surface area. These changes were more noticeable in samples cycled with higher cutoff voltage of 4.8 V. Effect of operating temperature was further examined by comparing electronic structures of oxygen of the NCA particles cycled at both room temperature and 55 °C. The working temperature has a greater impact on the NCA cathode materials at a cutoff voltage of 4.3 V that is the practical the upper limit voltage in most applications, while a cutoff voltage of 4.8 V is high enough to cause surface degradation even at room temperature.
[en] Carbon-coated Li4Ti5O12 anode materials for the lithium ion battery were synthesized by using sucrose to improve the electrochemical properties of Li4Ti5O12, and the carbon content was then tested. X-ray diffraction (XRD) showed that the coating of carbon does not influence the formation of Li4Ti5O12. Transmission electron microscopy (TEM) and Raman spectroscopy confirmed that carbon content exists on the surface of Li4Ti5O12. Electronic conductivity measurement indicated that the electronic conductivity of the carbon-coated Li4Ti5O12 material was 3.8x10-4 S cm-1, which is higher than that for the primary Li4Ti5O12 material (4.3x10-7 S cm-1). CV results show that carbon-coated Li4Ti5O12 shows a larger diffusion coefficient. Charge and discharge tests show that rate capability and cycle performance were improved because of the carbon coating.
[en] Ni-rich lithium transition metal oxides have received significant attention due to their high capacities and rate capabilities determined via theoretical calculations. Although the structural properties of these materials are strongly correlated with the electrochemical performance, their structural stability during the high-rate electrochemical reactions has not been fully evaluated yet. In this work, transmission electron microscopy is used to investigate the crystallographic and electronic structural modifications of Ni-based cathode materials at a high charge/discharge rate of 10 C. It is found that the high-rate electrochemical reactions induce structural inhomogeneity near the surface of Ni-rich cathode materials, which limits Li transport and reduces their capacities. Furthermore, this study establishes a correlation between the high-rate electrochemical performance of the Ni-based materials and their structural evolution, which can provide profound insights for designing novel cathode materials having both high energy and power densities.
[en] Highlights: • Cobalt-doped FeF_3·0.5H_2O is synthesized and its electrochemical properties revealed. • Co-doping facilitates electron transport and stabilizes the structure of FeF_3·0.5H_2O. • This electrode delivers a discharge capacity of 227 mAh g"−"1 at a rate of 0.1C. • Steady cyclability of the electrode is demonstrated up to 200 cycles. • Lithium diffusion coefficient of pre- and post-cycled electrodes were measured. - Abstract: In the search of high-performance cathodes for next-generation Li-ion batteries (LIBs), iron fluorides are among the most promising materials because of their extremely high theoretical capacity. This study reports on the synthesis, structural and electrochemical characterizations of cobalt doped iron fluoride hydrate as a high-performance cathode material for LIBs. A simple non-aqueous precipitation method is used to synthesize cobalt doped iron fluoride (Fe_0_._9Co_0_._1F_3·0.5H_2O) while its structural and electrochemical properties are also evaluated. The thermogravimetric analysis reveals that the structure of the as-prepared material remains stable up to 243 °C. The structure was then found to collapse beyond this temperature due to the removal of water contents from the crystal structure. The material delivers a high discharge capacity of 227 mAh g"−"1 at 0.1C in the potential range of 1.8-4.5 V versus Li/Li"+. The electrode retains a high reversible capacity of 150 mAh g"−"1 at a rate of 0.1C after 200 cycles, indicating high reversibility and stability of the material. The electrode also shows a superior rate capability of up to 10C, showing its potential use as a cathode material for LIBs.
[en] Silicon (Si) has a large theoretical capacity of 4200 mAhg−1 and has great potential as a high-performance anode material for Li ion batteries (LIBs). Meanwhile, nanostructures can exploit the potential of Si and, accordingly, many zero-dimensional (0D) and one-dimensional (1D) Si nanostructures have been studied. Herein, we report on two-dimensional (2D) Si nanostructures, Si nanosheets (SiNSs), as anodes for LIBs. These 2D Si nanostructures, with a thickness as low 5 nm and widths of several micrometers, show reversible crystalline–amorphous phase transformations with the lithi-/delithiation by the dimensionality of morphology and large surface area. The reversible crystalline–amorphous phase transformation provides a structural stability of Li+ insertions and makes SiNSs promising candidates for reliable high-performance LIBs anode materials. (paper)
[en] Highlights: • A nanocomposite of SnF2 and acetylene black is prepared using ball-milling method. • The nanocomposite delivers exceptional electrochemical properties. • GITT results indicate higher values of DNa in the nanocomposite. • In-situ XRD results suggest a solid solution during sodiation process. • The reaction mechanism involves conversion and alloying reaction. Sn-based materials have drawn great attention as anodes for rechargeable batteries because of their extremely high theoretical energy storage capacities. Herein, a nanocomposite based on SnF2 and acetylene black is proposed as a high-performance anode material for sodium-ion batteries and their electrochemical performances, as well as related energy storage mechanism, are investigated. The nanocomposite electrode delivered a high reversible capacity of 563 mAh g−1 which is considerably improved compared to a reversible capacity of 323 mAh g−1 of the micron-sized bare SnF2 electrode. The nanocomposite electrode shows superior rate capability and delivers a reversible capacity of 191 mAh g−1 at a high current density of 1 C, while the bare electrode delivers negligible capacities. The changes in crystallographic structure are observed using in-situ XRD and the results reveal the existence of a solid solution of two or more species during dis/charging. The electronic and atomic configurations depending on the state of dis/charging are systematically investigated using ex-situ X-ray absorption spectroscopy. The results reveal that the valence change of Sn follows the conversion (SnF2 + 2Na → Sn + 2NaF) and alloying (Sn + XNa → SnNaX) reaction upon sodium insertion into a composite.
[en] We demonstrate the formation of a highly conductive, Fe0/Fe3O4 nanocomposite electrode by the hydrogen reduction process. Fe2O3 nanobundles composed of one-dimensional nanowires were initially prepared through thermal dehydrogenation of hydrothermally synthesized FeOOH. The systematic phase and morphological evolutions from Fe2O3 to Fe2O3/Fe3O4, Fe3O4, and finally to Fe/Fe3O4 by the controlled thermochemical reduction at 300 0C in H2 were characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM). The Fe/Fe3O4 nanocomposite electrode shows excellent capacity retention (∼540 mA h g-1 after 100 cycles at a rate of 185 mA g-1), compared to that of Fe2O3 nanobundles. This enhanced electrochemical performance in Fe/Fe3O4 composites was attributed to the formation of unique, core-shell nanostructures offering an efficient electron transport path to the current collector.