Enhanced electrochemical performance and thermal stability of La2O3-coated LiCoO2

An enhanced electrochemical performance LiCoO₂ cathode was synthesized by coating with various wt.% of La₂O₃ to the LiCoO₂ particle surfaces by a polymeric method, followed by calcination at 923 K for 4 h in air. The surface-coated materials were characterized by XRD, TGA, SEM, TEM, BET and XPS/ESCA techniques. XRD patterns of La₂O₃-coated LiCoO₂ revealed that the coating did not affect the crystal structure, α-NaFeO₂, of the cathode material compared to pristine LiCoO₂. TEM images showed a compact coating layer on the surface of the core material that had an average thickness of about ∼15 nm. XPS data illustrated that the presence of two different environmental O 1s ions corresponds to the surface-coated La₂O₃ and core material. The electrochemical performance of the coated materials by galvanostatic cycling studies suggest that 2.0 wt.% coated La₂O₃ on LiCoO₂ improved cycle stability (284 cycles) by a factor of ∼7 times over the pristine LiCoO₂ cathode material and also demonstrated excellent cell cycle stability when charged at high voltages (4.4, 4.5 and 4.6 V). Impedance spectroscopy demonstrated that the enhanced performance of the coated materials is attributed to slower impedance growth during the charge-discharge processes. The DSC curve revealed that the exothermic peak corresponding to the release of oxygen at ∼464 K was significantly smaller for the La₂O₃-coated cathode material and recognized its high thermal stability.     

Effect of Cr dopant on the cathodic behavior of LiCoO2

Compounds of the formula LiCo_1−yCr_yO₂ (0.0≤y≤0.20 and y=1.0) have been synthesized by high temperature solid-state reaction and were characterized by XRD and FT-IR. Hexagonal a and c lattice parameters increase with increasing y as expected from ionic size effects. Cyclic voltammograms reveal that the phase transformation occurring at x=0.5 in Li_1−x(Co_1−yCr_y)O₂ is suppressed for y=0.05 and 0.10. Low-current (0.01 C; 1 C=140 mA g⁻¹) galvanostatic charging curves show that the deintercalation voltage for y=0.05 and 0.10 decrease for a given x as compared to LiCoO₂. Galvanostatic charge-discharge cycling of the Li(Co_1−yCr_y)O₂ cathodes at 0.14 C and 2.7-4.3 V (vs. Li) show that increasing amount of chromium content in the LiCoO₂ lattice drastically reduces the amount of Li that can be reversibly cycled. Ex-situ XRD of the cycled cathodes show that slight cation-mixing occurs in the layered structure for y=0.05 and 0.10 and could be the reason for their poor electrochemical performance. Reversible Li intercalation/deintercalation is not possible in LiCrO₂ in the voltage range 2.7-4.3 V.     

Improved phase stability and electrochemical performance of (Y,In,Ca)BaCo3ZnO7+δ cathodes for intermediate temperature solid oxide fuel cells

With an aim to combine the performance-enhancing properties of Ca with the stability-promoting properties of In in the swedenborgite YBaCo₄O_7+δ-based cathodes for solid oxide fuel cells (SOFC), cation-substituted Y_1−x−yIn_xCa_yBaCo₃ZnO_7+δ (0.2 ≤ (x + y) ≤ 0.5) oxides have been explored. All samples presented in this work are stable in air after 120 h exposure to 600, 700, and 800 °C. Increasing In content shows a negligible impact on polarization resistances (R_p), but causes an increase in the activation energies (E_a) of (Y,In,Ca)BaCo₃ZnO_7+δ + Gd_0.2Ce_0.8O_1.9 (GDC) composite cathodes on 8 mol% yttria-stabilized zirconia (8YSZ) electrolyte supported symmetric cells. Increasing Ca content shows a decrease in R_p and an increase in E_a on similar electrochemical cells. All (Y,In,Ca)BaCo₃ZnO_7+δ samples investigated here show superior performance compared to the unsubstituted YBaCo₃ZnO_7+δ + GDC cathode in the range of 400–800 °C. Especially, the Y_0.5In_0.1Ca_0.4BaCo₃ZnO_7+δ + GDC composite cathode exhibits good performance on GDC electrolytes in the range of 400–600 °C. With superior phase stability and electrochemical performance, the (Y,In,Ca)BaCo₃ZnO_7+δ series of oxides are attractive cathode candidates for intermediate temperature SOFCs.     

Effect of Fe substitution on the structure and properties of LnBaCo2−xFexO5+δ (Ln = Nd and Gd) cathodes

The effect of Fe substitution for Co on the crystal chemistry, thermal and electrical properties, and catalytic activity for oxygen reduction reaction of the layered LnBaCo_2−xFe_xO_5+δ (Ln = Nd and Gd) perovskite has been investigated. The air-synthesized LnBaCo_2−xFe_xO_5+δ samples exhibit structural change with increasing Fe content from tetragonal (0 ≤ x ≤ 1) to cubic (1.5 ≤ x ≤ 2) for the Ln = Nd system and from orthorhombic (x = 0) to tetragonal (0.5 ≤ x ≤ 1) for the Ln = Gd system. The thermal expansion coefficient (TEC) and electrical conductivity decrease with increasing Fe content in LnBaCo_2−xFe_xO_5+δ. While the substitution of a small amount of Fe (x = 0.5) for Co leads to slightly improved performance in solid oxide fuel cells (SOFC), larger Fe contents (x ≥ 1.0) deteriorate the fuel cell performance. In the Ln = Gd system, the better performance of the x = 0.5 sample is partly due to the improved chemical stability with the LSGM electrolyte at high temperatures. With an acceptable electrical conductivity of >100 S cm⁻¹ at 800 °C, the x = 0.5 sample in the LnBaCo_2−xFe_xO_5+δ (Ln = Nd and Gd) system offers promising mixed oxide-ion and electronic conducting (MIEC) properties.     

Effect of composition of (La0.8Sr0.2MnO3-Y2O3-stabilized ZrO2) cathodes: Correlating three-dimensional microstructure and polarization resistance

Composite La_0.8Sr_0.2MnO₃ (LSM)-Y₂O₃-stabilized ZrO₂ (YSZ) cathodes with compositions ranging from 30:70 to 70:30 wt.% LSM:YSZ were studied both electrochemically and microstructurally. Polarization resistance was lowest for the 50 wt.% YSZ composition, and increased symmetrically as the composition deviated from this value. Serial-sectioning using focused ion beam-scanning electron microscopy was implemented to reconstruct the three-dimensional cathode microstructure. Various averaged structural parameters were determined versus composition, including phase volume fractions, surface area densities, total triple-phase boundary (TPB) densities, interfacial curvatures, phase tortuosities, and the levels of phase connectivity. Typically >90% of the pore and YSZ networks were found to be intra-connected to the surrounding phase, but the LSM networks showed lower connected fractions, as low as 37.5% for a LSM weight fraction of 30%. The composition dependences of the total TPB density and electrochemically-active TPB density (i.e., TPB's on three fully intra-connected phases) were shown to agree reasonably well with simple “sphere-packing” structural models. An electrochemical model that accounted for the linear-specific resistance of TPB's, phase intra-connectivity, and oxygen ion transport in the YSZ as influenced by its tortuosity, was found to provide reasonable agreement with the measured polarization resistance versus composition.     

Synthesis, characterization and electrochemical properties of 4.8 V LiNi0.5Mn1.5O4 cathode material in lithium-ion batteries

In this work the synthesis of a nickel doped cubic manganese spinel has been studied for application as cathode material in secondary lithium batteries. Six different experimental approaches have been tested in order to carry out a screening of the various possible synthetic routes. The used synthetic strategies were wet chemistry (WC), solid state (SS), combustion synthesis (CS), cellulose-based sol-gel synthesis (SG-C), ascorbic acid-based sol-gel synthesis (SG-AA) and resorcinol/formaldehyde-based sol-gel synthesis (SG-RF). The goal of our study is to obtain insights about how the synthesis conditions can be modified in order to achieve a material with improved electrochemical performances in such devices, especially in high current operating regimes. The synthesized materials have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), atomic absorption, inductively coupled plasma (ICP-MS) atomic emission spectroscopy, surface area measurements and tested as high voltage cathodes in Li-ion electrochemical devices.     

Inhibition of solid electrolyte interface formation on cathode particles for lithium-ion batteries

Thermal reactions between cathode particles (LiNi_0.8Co_0.2O₂, LiCoO₂, LiMn₂O₄ and LiFePO₄) and ternary electrolyte (1.0 M LiPF₆ in 1:1:1 diethyl carbonate/dimethyl carbonate/ethylene carbonate) with or without the thermal stabilizing additive dimethyl acetamide (DMAc) have been investigated. Ternary electrolyte reacts with the surface of lithiated metal oxides (LiNi_0.8Co_0.2O₂, LiCoO₂ and LiMn₂O₄) upon storage to corrode the surface and generate a complex mixture of organic and inorganic surface species, but the bulk ternary electrolyte does not decompose. There is little evidence for reaction between the surface of carbon coated LiFePO₄ and ternary electrolyte upon storage at elevated temperature (>60 °C), but the bulk ternary electrolyte decomposes. Addition of DMAc to ternary electrolyte reduces the surface corrosion of the lithiated metal oxides and stabilizes the electrolyte in the presence of LiFePO₄.     

Nanostructured La0.5Ba0.5CoO3 as cathode for solid oxide fuel cells

A simple method has been used to synthesize nanostructured La_0.5Ba_0.5CoO₃ (LBCO) powders, by confining chemical precursors into the pores of polycarbonate filters. The proposed method allows us to obtain powders formed by crystallites of different sizes, it is scalable and does not involve the use of sophisticated deposition techniques.The area specific polarization resistance of symmetrical cells was studied to analyze the electrochemical behavior of the LBCO nanostructures as cathodes for Solid-Oxide Fuel Cells.We show that the performance is improved by reducing the size of the crystallites, obtaining area specific resistance values of 0.2 Ωcm² at 700 °C, comparable with newly developed cathodes using novel deposition techniques.     

Fabrication and electrochemical characteristics of electrospun LiFePO4/carbon composite fibers for lithium-ion batteries

LiFePO₄/C composite fibers were synthesized by using a combination of electrospinning and sol-gel techniques. Polyacrylonitrile (PAN) was used as an electrospinning media and a carbon source. LiFePO₄ precursor materials and PAN were dissolved in N,N-dimethylformamide separately and they were mixed before electrospinning. LiFePO₄ precursor/PAN fibers were heat treated, during which LiFePO₄ precursor transformed to energy-storage LiFePO₄ material and PAN was converted to carbon. The surface morphology and microstructure of the obtained LiFePO₄/C composite fibers were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and elemental dispersive spectroscopy (EDS). XRD measurements were also carried out in order to determine the structure of LiFePO₄/C composite fibers. Electrochemical performance of LiFePO₄/carbon composite fibers was evaluated in coin-type cells. Carbon content and heat treatment conditions (such as stabilization temperature, calcination/carbonization temperature, calcination/carbonization time, etc.) were optimized in terms of electrochemical performance.