Electrochemical stability of thin film LiMn2O4 cathode in organic electrolyte solutions with different compositions at 55 °C

A survey of the electrochemical stability of electrostatic spray deposited thin film of LiMn₂O₄ was performed in LiClO₄-EC-PC, LiBF₄-EC-PC, and LiPF₆-EC-PC solutions at 55 °C. The solution resistance, the surface film resistance, and the charge-transfer resistance were all found to depend on the electrolyte composition. Among the LiX-salts studied, the lowest charge transfer-resistance, and surface layer resistance were obtained in LiBF₄-EC-PC solution. There is no major influence of the electrolyte solution compositions upon lithium ion transport in the LiMn₂O₄ bulk at 55 °C. The diffusion coefficient of lithium in the solid phase varied within 10⁻¹⁰-10⁻⁸ cm² s⁻¹ in the three solutions. In general, it seems that in LiBF₄ solutions, the surface chemistry is the most stable in the three solutions examined, and hence the electrode impedance in LiBF₄ solutions was the lowest. In LiPF₆ solutions, HF seems to play an important role, and thus, the electrode impedance is relatively high due to the precipitation of surface LiF.     

Characterization of electrode/electrolyte interface using in situ X-ray reflectometry and LiNi0.8Co0.2O2 epitaxial film electrode synthesized by pulsed laser deposition method

An in situ experimental technique was developed for detecting structure changes at the electrode/electrolyte interface of lithium cell using synchrotron X-ray reflectometry and two-dimensional model electrodes with a restricted lattice plane. The electrode was constructed with an epitaxial film of LiNi_0.8Co_0.2O₂ synthesized by the pulsed laser deposition method. The orientation of the epitaxial film depends on the substrate plane; the 2D layer of LiNi_0.8Co_0.2O₂ is parallel to the SrTiO₃ (1 1 1) substrate ((0 0 3)_{LiCo0.2Ni0.8O2}//(1 1 1)SrTiO₃), while the 2D layer is perpendicular to the SrTiO₃ (1 1 0) substrate ((1 1 0)_{LiCo0.2Ni0.8O2}//(1 1 0)SrTiO₃). These films provided an ideal reaction field suitable for detecting structure changes at the electrode/electrolyte interface during the electrochemical reaction. The X-ray reflectometry indicated a formation of a thin-film layer at the LiNi_0.8Co_0.2O₂ (1 1 0)/electrolyte interface during the first charge-discharge cycle, while the LiNi_0.8Co_0.2O₂ (0 0 3) surface showed an increase in the surface roughness without forming the surface thin-film layer. The reaction mechanism at the electrode/electrolyte interface is discussed based on our new experimental technique for lithium batteries.     

Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolyte filled with TiO2 nanoparticles

Nanoscale TiO₂ particle filled poly(vinylidenefluoride-co-hexafluoropropylene) film is characterized by investigating some properties such as surface morphology, thermal and crystalline properties, swelling behavior after absorbing electrolyte solution, chemical and electrochemical stabilities, ionic conductivity, and compatibility with lithium electrode. Decent self-supporting polymer electrolyte film can be obtained at the range of <50 wt% TiO₂. Different optimal TiO₂ contents showing maximum liquid uptake may exist by adopting other electrolyte solution. Room temperature ionic conductivity of the polymer electrolyte placed surely on the region of >10⁻³ S/cm, and thus the film is very applicable to rechargeable lithium batteries. An emphasis is also be paid on that much lower interfacial resistance between the polymer electrolyte and lithium metal electrode can be obtained by the solid-solvent role of nanoscale TiO₂ filler.     

Effects of Al2O3 addition on the sintering behavior and microwave dielectric properties of CaSiO3 ceramics

The effects of Al₂O₃ addition on the densification, structure and microwave dielectric properties of CaSiO₃ ceramics have been investigated. The Al₂O₃ addition results in the presence of two distinct phases, e.g. Ca₂Al₂SiO₇ and CaAl₂Si₂O₈, which can restrict the growth of CaSiO₃ grains by surrounding their boundaries and also improve the bulk density of CaSiO₃-Al₂O₃ ceramics. However, excessive addition (≥2 wt%) of Al₂O₃ undermines the microwave dielectric properties of the title ceramics since the derived phases of Ca₂Al₂SiO₇ and CaAl₂Si₂O₈ have poor quality factor. The optimum amount of Al₂O₃ addition is found to be 1 wt%, and the derived CaSiO₃-Al₂O₃ ceramic sintered at 1250 °C presents improved microwave dielectric properties of ?_r = 6.66 and Q × f = 24,626 GHz, which is much better than those of pure CaSiO₃ ceramic sintered at 1340 °C (Q × f = 13,109 GHz).     

Effect of Al2O3 nanoparticles on the electrochemical characteristics of P(VDF-HFP)-based polymer electrolyte

Alumina (Al₂O₃) nanoparticles have been used as fillers in the preparation of poly(vinylidenefluoride-co-hexafluorpropylene) (P(VDF-HFP))-based porous polymer electrolyte. The degree of crystallization of polymer film filled with Al₂O₃ nanoparticles decreases with increase of the mass fraction of Al₂O₃ nanoparticles and the amorphous phases of polymer film expand accordingly. The Al₂O₃ nanoparticles play the role of solid plasticizer for polymer matrix. Nevertheless that excessive Al₂O₃ nanoparticles existing in polymer matrix leads to micro-phase separation between polymer matrix and fillers. As a result, both ionic conductivity and lithium ions transference number reduces whereas the activation energy for ions transport increases. When the polymer film is filled with 10% of the mass fraction of Al₂O₃ nanoparticles, polymer electrolyte possesses the ionic conductivity up to 1.95 × 10⁻³ S cm⁻¹ and the lithium ions transference number to 0.73 while the activation energy for ions transport of them falls to 5.6 kJ mol⁻¹. Effect of Al₂O₃ on the electrochemical properties of polymer electrolyte has been investigated in this paper. Analysis of FTIR spectra shows that there is the interaction between Al₂O₃ nanoparticles and polymer chains.     

Spark plasma sintering technique for reaction sintering of Al2O3/Ni nanocomposite and its mechanical properties

Al₂O₃/Ni nanocomposites were prepared by spark plasma sintering (SPS) using reaction sintering method and the mechanical properties of the obtained nanocomposites are reported. The starting materials of Al₂O₃–NiO solid solution were synthesized from aluminum sulfate and nickel sulfate. These Al₂O₃–NiO powders were changed into Al₂O₃ and Ni phases during sintering process. The obtained nanocomposites showed high relative densities (>98%). SEM micrographs showed homogeneously dispersed Ni grains in the matrix. The 3-point strength and the fracture toughness of the composites significantly improved from 450 MPa in the monolithic α-Al₂O₃ to 766 MPa in the 10 mol% (2.8 vol.%) Ni nanocomposite and from 3.7 to 5.6 MPa m^1/2 in 13 mol% (3.7 vol.%) Ni nanocomposite. On the other hand, Young's modulus and Vickers hardness of the nanocomposites were mostly same as those of the monolithic α-Al₂O₃.     

Synthesis of high surface area Al2O3-CeO2 composite nanopowder via inverse co-precipitation method

In the present work, Al₂O₃-CeO₂ composite nanopowder was synthesized by inverse co-precipitation method using metal chlorides, aluminum powder and NH₄OH as precipitant agent. The thermal decomposition of the precipitate and subsequent formation of Al₂O₃-CeO₂ were investigated by X-ray diffractometery, scanning electron microscopy, thermogravimetric and differential thermal analysis, Brunauer-Emmett-Teller surface area measurement and Fourier transform infrared spectroscopy. The results showed that the presence of ceria suppressed the formation of α-Al₂O₃. The BET-specific surface area was 173 m²/g for powders calcined at 800 °C. The particle size examined by using scanning electron microscopy was in the range 30-70 nm. The activation energy of Al₂O₃-15 wt.% CeO₂ nanocrystallite growth during calcination was measured to be 32.4 kJ/mol whereas that of Al₂O₃ was about 23.8 kJ/mol.     

Fabrication and mechanical properties of Al2O3-Si3N4/ZrO2-Al2O3 laminated composites

In this paper, Al₂O₃-Si₃N₄/ZrO₂-Al₂O₃ laminated composites were fabricated by tape casting and hot press sintering, and the relationships between the process, microstructure, and mechanical properties of Al₂O₃-Si₃N₄/ZrO₂-Al₂O₃ laminated composites were determined. The SiAlON phase was found in the Al₂O₃-Si₃N₄ layer, and liquid-phase sintering was proposed. Nano-scratch tests were carried out to investigate the interface bonding strength of the laminates. The distribution of residual stresses, generated due to the different coefficients of thermal expansion between the different layers, was estimated according to lamination theory and confirmed using Vickers indentation. When the sintering temperature was 1550 °C, the sintered laminated ceramics had good mechanical properties, with a maximum strength and toughness of 413 MPa and 6.2 MPa m^1/2, respectively. The main toughness mechanics of laminated composites was residual stress.     

Synthesis and characterization of Al2O3 nanopowders by a simple chitosan-polymer complex solution route

Al₂O₃ nanopowders were synthesized by a simple chitosan-polymer complex solution route. The precursors were calcined at 800–1200 °C for 2 h in air. The prepared samples were characterized by XRD, FTIR and TEM. The results showed that for the precursors prepared with pH 3–9 γ-Al₂O₃ and δ-Al₂O₃ are the two main phases formed after calcination at 800–1000 °C. Interestingly, when the precursor prepared with pH 2 was used, α-Al₂O₃ was formed after calcination at 1000 °C, and pure α-Al₂O₃ was obtained after calcination at 1200 °C. The crystallite sizes of the prepared powders were found to be in the range of 4–49 nm, as evaluated by the XRD line broadening method. TEM investigation revealed that the Al₂O₃ nanopowders consisted of rod-like shaped particles and nanospheres with particle sizes in the range of 10–300 nm. The corresponding selected-area electron diffraction (SAED) analysis confirmed the formation of γ- and α-Al₂O₃ phases in the samples.     

Polyterthiophene/CNT composite as a cathode material for lithium batteries employing an ionic liquid electrolyte

A polyterthiophene (PTTh)/multi-walled carbon nanotube (CNT) composite was synthesised by in situ chemical polymerisation and used as an active cathode material in lithium cells assembled with an ionic liquid (IL) or conventional liquid electrolyte, LiBF₄/EC-DMC-DEC. The IL electrolyte consisted of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄) containing LiBF₄ and a small amount of vinylene carbonate (VC). The lithium cells were characterised by cyclic voltammetry (CV) and galvanostatic charge/discharge cycling. The specific capacity of the cells with IL and conventional liquid electrolytes after the 1st cycle was 50 and 47 mAh g⁻¹ (based on PTTh weight), respectively at the C/5 rate. The capacity retention after the 100th cycle was 78% and 53%, respectively. The lithium cell assembled with a PTTh/CNT composite cathode and a non-flammable IL electrolyte exhibited a mean discharge voltage of 3.8 V vs Li⁺/Li and is a promising candidate for high-voltage power sources with enhanced safety.     

Preparation and characterization of ramsdellite Li2Ti3O7 as an anode material for asymmetric supercapacitors

Ramsdellite Li₂Ti₃O₇ was first synthesized via sol-gel process with good crystallity of an average particle size of 0.175 μm. The product was thoroughly investigated as a lithium intercalation compound, and as an active anode material in asymmetric supercapacitors coupling with activated carbon as cathode. Lithium intercalation reactions were found occurring at 1.32 and 1.62 V versus Li/Li⁺, respectively. A reversible specific capacity of 150 mA h g⁻¹ at 1C was obtained on Li₂Ti₃O₇ electrode in a nonaqueous electrolyte. The charge current was found to strongly influence the anodic discharge capacity in the asymmetric cell. The capacity retention at 10C charge-discharge rate was found to be 75.9% in comparison with that at 1C.     

Impact of “Super Acid” like filler on the properties of a PEGDME/LiClO4 system

Composite polymer electrolyte based on a new class of filler added to a PEGDME/LiClO₄ model system has been investigated. “Ceramic super acids” used consist of grafted SO₄²⁻ groups on Al₂O₃ particles surface obtained by calcinations route. Conductivity, DSC and FT-IR measurements performed on such composite electrolytes, when compared to the model PEGDME/LiClO₄ electrolyte, showed only slight improvement of their inner characteristics. In contrary, Li/Li symmetric cells study, by means of impedance spectroscopy, has presented a spectacular decrease of the interfacial resistance compare to the model electrolyte. This result opens a new pathway of investigation to master the lithium metal/polymer electrolyte interface.     

Influence of sintering temperature and holding time on the densification, phase transformation, microstructure and properties of hot pressing WC-40 vol.%Al2O3 composites

WC-40 vol.%Al₂O₃ composites were prepared by high energy ball milling followed by hot pressing. The tungsten carbide (WC) and commercial alumina (Al₂O₃) powders composed of amorphous Al₂O₃, boehmite (AlOOH) and χ-Al₂O₃ were used as the starting materials. The phase transformation during sintering, the influence of sintering temperature and holding time on the densification, microstructure, Vickers hardness and fracture toughness and the toughening effects of WC-40 vol.%Al₂O₃ composites were investigated. The results showed that the amorphous Al₂O₃, AlOOH and χ-Al₂O₃ were transformed to α-Al₂O₃ completely during the sintering process. With the increasing sintering temperature and holding time, the relative density increased and both the Vickers hardness and fracture toughness increased initially to the maximum values and then decreased. When the as milled powders were hot pressed at 1540 °C for 90 min, a relative density of 97.98% and a maximum hardness of 18.65 GPa with an excellent fracture toughness of 10.43 MPa m^1/2 of WC-40 vol.%Al₂O₃ composites were obtained.     

Electrochemical investigation of LiMn2O4 cathodes in gel electrolyte at various temperatures

A composite lithium battery electrode of LiMn₂O₄ in combination with a gel electrolyte (1 M LiBF₄/24 wt% PMMA/1:1 EC:DEC) has been investigated by galvanostatic cycling experiments and electrochemical impedance spectroscopy (EIS) at various temperatures, i.e. −3<T<56 °C. For analysis of EIS data, a mathematical model taking into account local kinetics and potential distribution in the liquid phase within the porous electrode structure was used. Reasonable values of the double-layer capacitance, the exchange-current density and the solid phase diffusion were found as a function of temperature. The apparent activation energy of the charge-transfer (∼65 kJ mol⁻¹), the solid phase transfer (∼45 kJ mol⁻¹) and of the ionic bulk and effective conductance in the gel phase (∼34 kJ mol⁻¹), respectively, were also determined. The kinetic results related to ambient temperature were compared to those obtained in the corresponding liquid electrolyte. The incorporated PMMA was found to reduce the ionic conductivity of the free electrolyte, and it was concluded that the presence of 24 wt% PMMA does not have a significant influence on the kinetic properties of LiMn₂O₄.     

Lithium/V6O13 cells using silica nanoparticle-based composite electrolyte

The electrochemical behavior of Li/V₆O₁₃ cells is investigated at room temperature (22 °C) both in liquid electrolyte consisting of oligomeric poly(ethyleneglycol)dimethylether+lithium bis(trifluoromethylsulfonylimide) and composite electrolytes formed by blending the liquid electrolyte with silica nanoparticles (fumed silica). The addition of fumed silica yields a gel-like electrolyte that demonstrates the desirable property of suppressing lithium dendrite growth due to the rigidity and immobility of the electrolyte structure. The lithium/electrolyte interfacial resistance for composite gel electrolytes is less than that for the corresponding base-liquid electrolyte, and the charge-discharge cycle performance and electrochemical efficiency for the Li/V₆O₁₃ cell is significantly improved. The effect of fumed silica surface group on the electrochemical performance is discussed; the native hydrophilic silanol surface group appears better than fumed silica that is modified with a hydrophobic octyl surface moiety.     

LiF对Na3AlF6-Al2O3熔盐电解阴极过程的影响

电解铝工业中铝在阴极上析出,铝析出反应的机理对电解铝生产具有理论指导意义。在运用循环伏安法研究的基础上,通过理论计算,对Na₃AlF₆-Al₂O₃和Na₃AlF₆-Al₂O₃-LiF体系中金属铝在钨电极上的电化学沉积行为以及铝钨金属间化合物的形成机理进行了研究。结果表明：两体系中铝钨金属间化合物在50 mV·s^-1≤ν≤150 mV·s^-1扫描速率下的形成过程是受扩散控制的准可逆过程。在化合物形成的过程中,两体系中Al³⁺的扩散系数从4.54×10^-9 cm²·s^-1增长到5.71×10^-9 cm²·s^-1,Al³⁺反应的活化能分别为11.14 kJ·mol^-1和10.47 kJ·mol^-1。在Na₃AlF₆-Al₂O₃-LiF体系的还原过程中,Li并没有还原析出,而在氧化过程中Al在金属间化合物中的氧化电流增大;在恒电流电解时,Al-W金属间化合物并不溶于熔盐中,会附着在工作电极表面,LiF的加入会使电极表面的WAl₄量变小,取而代之的是Al₂O₃的增加,说明LiF的加入使电解更加稳定,抑制了电极表面WAl₄的生长。     

Spark plasma sintering of Al2O3-cBN composites facilitated by Ni nanoparticle precipitation on cBN powder by rotary chemical vapor deposition

Al₂O₃-cBN/Ni composites were consolidated by spark plasma sintering (SPS) using α-Al₂O₃ and Ni nanoparticle precipitated cBN (cBN/Ni) powders. The Ni nanoparticles, 10-100 nm in diameter and 0.5-2.2 mass% in content, were precipitated on cBN powder by rotary chemical vapor deposition. The effect of sintering temperature (T_SPS) and Ni content (C_Ni) on the densification, phase transformation, microstructure and hardness of the Al₂O₃-cBN/Ni composites were investigated. The highest relative density of Al₂O₃-30 vol% cBN composite was 99% at T_SPS = 1573 K and C_Ni = 1.7 mass%. At T_SPS = 1673 K, the relative density decreased due to the phase transformation of cBN to hBN. The Vickers hardness of Al₂O₃-30 vol% cBN/Ni at T_SPS = 1573 K and C_Ni = 1.7 mass% showed the highest value of 27 GPa.     

The effects of decomposition products of electrolytes on the thermal stability of bare and TiO2-coated delithiated Li1−xNi0.8Co0.2O2 cathode materials

In this work, the effects of decomposition products of electrolytes on the thermal stability of bare and TiO₂-coated Li_1−xNi_0.8Co_0.2O₂ (1 > x ≥ 0) cathode material have been investigated by means of thermoanalytical, thermokinetic and temperature-programmed desorption-mass spectroscopy (TPD-MS) techniques. It is shown clearly that the decomposition products of the electrolytes such as carboxylates have distinctive effects on the thermal stability of the electrode materials. Firstly, the thermoanalytical and TPD-MS results indicate that surface coating can suppress the amount of oxygen release from the delithiated cathode material. The thermokinetic analytical results show that the reaction of oxygen release (i.e. oxygen loss) from delithiated Li_1−xNi_0.8Co_0.2O₂ material can be promoted by carboxylate salts supported on the electrode surface due to the decrease of initiated activation energy E_a of the reaction. Finally, the amount of carboxylate salts and length of carbon chains in carboxylates have different promotional effects on the thermal properties of the electrode materials.     

Aging characteristics of high-power lithium-ion cells with LiNi0.8Co0.15Al0.05O2 and Li4/3Ti5/3O4 electrodes

The impedance rise that results from the accelerated aging of high-power lithium-ion cells containing LiNi_0.8Co_0.15Al_0.05O₂-based positive and graphite-based negative electrodes is dominated by contributions from the positive electrode. Data from various diagnostic experiments have indicated that a general degradation of the ionic pathway, apparently caused by surface film formation on the oxide particles, produces the positive electrode interface rise. One mechanistic hypothesis postulates that these surface films are components of the negative electrode solid electrolyte interphase (SEI) layer that migrate through the electrolyte and separator and subsequently coat the positive electrode. This hypothesis is examined in this article by subjecting cells with LiNi_0.8Co_0.15Al_0.05O₂-based positive and Li_4/3Ti_5/3O₄-based negative electrodes to accelerated aging. The impedance rise in these cells was observed to be almost entirely from the positive electrode. Because reduction products are not expected on the 1.55 V Li_4/3Ti_5/3O₄ electrode, the positive electrode impedance cannot be attributed to the migration of SEI-type fragments from the negative electrode. It follows then that the impedance rise results from mechanisms that are “intrinsic” to the positive electrode.