Improvement of the rate capability of LiMn2O4 by surface coating with LiCoO2

In order to use LiMn₂O₄ as a cathode material of lithium-secondary battery for an electric vehicle (EV), its rate capability should be improved. To enhance the rate capability of LiMn₂O₄ in this work, the surface of LiMn₂O₄ particle was coated with LiCoO₂ by a sol–gel method. Because LiCoO₂ has a higher electric conductivity than LiMn₂O₄, it is possible to improve the rate capability of LiMn₂O₄. After the surface coating, LiCoO₂-coated LiMn₂O₄ showed a higher discharge capacity of 120 mAh/g than as-received LiMn₂O₄ (115 mAh/g) because LiCoO₂ has a higher capacity than LiMn₂O₄. The rate capability of the coated LiMn₂O₄ improved significantly. While as-received LiMn₂O₄ maintained only 50% of its maximum capacity at a 20C rate (2400 mA/g), the LiCoO₂-coated LiMn₂O₄ maintained more than 80% of maximum capacity. LiCoO₂-coated LiMn₂O₄ with 3 wt.% conducting agent (acetylene black) showed the higher rate capability than as-received LiMn₂O₄ with 20 wt.% conducting agent. From electrochemical impedance spectroscopy (EIS) result that the first and second semicircles of coated LiMn₂O₄ were reduced, the improvement of rate capability is attributed to a decrease of passivation film that acts as an electronic insulating layer and a reduced inter-particle contact resistance. Accordingly, It is proposed that the surface coating of LiMn₂O₄ with LiCoO₂ improve the rate capability as well as the specific and volumetric energy density due to the decrease of conducting agent.     

Improvement of 12 V overcharge behavior of LiCoO2 cathode material by LiNi0.8Co0.1Mn0.1O2 addition in a Li-ion cell

The 12 V overcharge instability of the LiCoO₂ cathode material was improved by the physical blending it with LiNi_0.8Co_0.1Mn_0.1O₂. Even though a Li-ion cell containing a LiCoO₂ cathode did not exhibit thermal runaway at 12 V at the 1 C overcharging rate, it showed thermal runaway at the 2 C overcharging rate, and the cell surface temperature reached more than 400 °C. However, the LiCoO₂ cell containing 40, 50, and 60 wt.% LiNi_0.8Co_0.1Mn_0.1O₂ did not exhibit thermal runaway at the 2 C overcharging rate. In conclusion, 60 wt.% LiNi_0.8Co_0.1Mn_0.1O₂ in the LiCoO₂ cathode showed the lowest cell surface temperature of <90 °C even at a 3 C overcharging rate.     

Electrochemical performance of LiCoO2 cathodes by surface modification using lanthanum aluminum garnet

LiCoO₂ particles were coated with various wt.% of lanthanum aluminum garnets (3LaAlO₃:Al₂O₃) by an in situ sol–gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirmed the formation of a 3LaAlO₃:Al₂O₃ compound and the in situ sol–gel process synthesized 3LaAlO₃:Al₂O₃-coated LiCoO₂ was a single-phase hexagonal α-NaFeO₂-type structure of the core material without any modification. Scanning electron microscope (SEM) images revealed a modification of the surface of the cathode particles. Transmission electron microscope (TEM) images exposed that the surface of the core material was coated with a uniform compact layer of 3LaAlO₃:Al₂O₃, which had an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt.% 3LaAlO₃:Al₂O₃-coated LiCoO₂ cathode showed excellent cycle stability of 182 cycles, which was much higher than the 38 cycles sustained by the pristine LiCoO₂ cathode material when it was charged at 4.4 V.     

Subcooled flow boiling CHF enhancement with porous surface coatings

The effect of micro/nanoporous inside surface coated vertical tubes on CHF was determined during water flow boiling at atmospheric pressure. CHF was measured for smooth and three different coated tubes, at mass fluxes (100–300 kg/m² s) and two inlet subcooling temperatures (50 °C and 75 °C). Greater CHF enhancement was found with microporous coatings than with nanoporous coatings. Al₂O₃ microporous coatings with particle size <10 μm and coatings thickness of 50 μm showed the best CHF enhancement. Maximum increase in CHF was about 25% for microporous Al₂O₃. A wettability test was performed to study an increase of CHF with microporous coated surfaces.     

A stabilized NiO cathode prepared by sol-impregnation of LiCoO2 precursors for molten carbonate fuel cells

Layers of LiCoO₂ were formed on the internal surface of a porous NiO cathode to reduce the rate of NiO dissolution into the molten carbonate. A sol-impregnation technique assisted by acrylic acid (AA) was used to deposit gel precursors of LiCoO₂ on the pore surface of the Ni plate. Thermal treatment of the gel-coated cathode above 400 °C produced LiCoO₂ layers on the porous cathode. A number of bench-scale single cells were fabricated with LiCoO₂-coated cathodes and the cell performance was examined at atmospheric pressure for 1000 h. With the increase in the LiCoO₂ content in the cathode, the initial cell voltage decreased, but the cell performance gradually improved during the cell test. It was found from symmetric cathode cell test that the cathode was initially flooded with electrolyte, but redistribution of the electrolyte took place during the test and cell performance became comparable to that of a conventional NiO cathode. The amount of Ni precipitated in the matrix during the cell operation for 1000 h was significantly reduced by the LiCoO₂ coating. For instance, coating 5 mol% of LiCoO₂ in the cathode led to a 56% reduction of Ni precipitation in the matrix. The results obtained in this study strongly suggest that LiCoO₂ layers formed on the internal surface of the porous NiO cathode effectively suppress the rate of NiO dissolution for 1000 h.     

Gel-type polymer electrolytes with different types of ceramic fillers and lithium salts for lithium-ion polymer batteries

Gel-type polymer electrolytes are prepared using PVdF/PEGDA/PMMA, LiPF₆/LiCF₃SO₃ mixed lithium salts and ceramic fillers such as Al₂O₃, BaTiO₃ and TiO₂. The electrochemical properties of these electrolytes, such as electrochemical stability, ionic conductivity and compatibility with electrodes are investigated in addition to the physical properties. The charge–discharge performances of lithium-ion polymer batteries using these get-type polymer electrolytes are investigated. The gel-type polymer electrolytes containing a mixed lithium salt of LiPF₆/LiCF₃SO₃ (10/1, wt.%) exhibit more stable ionic conductivity and lower interfacial resistance than those containing only LiPF₆. In addition, an Al₂O₃ filler improves interfacial stability between the electrode and the polymer electrolyte. Stacking cells (MCMB 1028/LiCoO₂, 8 cm × 13 cm × 7 ea) composed of gel-type polymer electrolytes based on PVdF/PEGDA/PMMA, LiPF₆/LiCF₃SO₃ (10/1, wt.%) and Al₂O₃ filler maintain 95% of initial capacity after 100 cycles at a C/2 rate.     

Ethanol oxidative steam reforming over Ni-based catalysts

Oxidative steam reforming of ethanol for hydrogen production in order to feed a solid polymer fuel cell (SPFC) has been studied over several catalysts at on board conditions (a molar ratio of H₂O/EtOH and of O₂/EtOH equal to 1.6 and 0.68 respectively) and a reforming temperature between 923 and 1073 K. Two Ni (11 and 20 wt.%)/Al₂O₃ catalysts and five bimetallic catalysts, all of them supported on Al₂O₃, were tested. The bimetallic catalysts were Ni (approximately 20 wt.%) based catalysts doped with Cr (0.65 wt.%), Fe (0.6 wt.%), Zn (0.7 wt.%) or Cu (0.6 and 3.1 wt.%). The results in terms of H₂ production and CO₂/CO_x ratio obtained over Ni-based catalysts supported on Al₂O₃ are compared with those obtained over Ni–Cu/SiO₂ and Rh/Al₂O₃ catalysts reported in our previous works. Tendencies of the product selectivities are analyzed in the light of the reaction network proposed.     

PVDF-g-PSSA and Al2O3 composite proton exchange membranes

Poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) membranes doped with different amount of Al₂O₃ (PVDF/Al₂O₃-g-PSSA) were prepared based on the solution-grafting technique. The microstructure of the membranes was characterized by IR-spectra and scanning electron microscope (SEM). The thermal stability was measured by thermal gravity analysis (TGA). The degree of grafting, water-uptake, proton conductivity and methanol permeability were measured. The results show that the PVDF-g-PSSA membrane doped with 10% Al₂O₃ has a lower methanol permeability of 6.6 × 10⁻⁸ cm² s⁻¹, which is almost one-fortieth of that of Nafion-117, and this membrane has moderate proton conductivity of 4.5 × 10⁻² S cm⁻¹. Tests on cells show that a DMFC with the PVDF/10%Al₂O₃-g-PSSA has a better performance than Nafion-117. Although Al₂O₃ has some influence on the stability of the membrane, it can still be used in direct methanol fuel cells in the moderate temperature.     

Effect of (Al,Mg) substitution in LiNiO2 electrode for lithium batteries

Stabilized lithium nickelate is receiving increased attention as a low-cost alternative to the LiCoO₂ cathode now used in rechargeable lithium batteries. Layered LiNi_1−x−yM_xM_yO₂ samples (M_x = Al³⁺ and M_y = Mg²⁺, where x = 0.05, 0.10 and y = 0.02, 0.05) are prepared by the refluxing method using acetic acid at 750 °C under an oxygen stream, and are subsequently subjected to powder X-ray diffraction analysis and coin-cell tests. The co-doped LiNi_1−x−yAl_xMg_yO₂ samples show good structural stability and electrochemical performance. The LiNiAl_0.05Mg_0.05O₂, cathode material exhibits a reversible capacity of 180 mA h g⁻¹ after extended cycling. These results suggest that the threshold concentration for aluminum and magnesium substitution is of the order of 5%. The co-substitution of magnesium and aluminium into lithium nickelate is considered to yield a promising cathode material.     

Preparation of LiCoO2 from spent lithium-ion batteries

A recycling process involving mechanical, thermal, hydrometallurgical and sol–gel steps has been applied to recover cobalt and lithium from spent lithium-ion batteries and to synthesize LiCoO₂ from leach liquor as cathodic active materials. Electrode materials containing lithium and cobalt can be concentrated with a two-step thermal and mechanical treatment. The leaching behavior of lithium and cobalt in nitric acid media is investigated in terms of reaction variables. Hydrogen peroxide in 1 M HNO₃ solution is found to be an effective reducing agent by enhancing the leaching efficiency. Of the many possible processes to produce LiCoO₂, the amorphous citrate precursor process (ACP) has been applied to synthesize powders with a large specific surface area and an exact stoichiometry. After leaching used LiCoO₂ with nitric acid, the molar ratio of Li to Co in the leach liquor is adjusted to 1.1 by adding a fresh LiNO₃ solution. Then, 1 M citric acid solution at a 100% stoichiometry is added to prepare a gelatinous precursor. When the precursor is calcined at 950 °C for 24 h, purely crystalline LiCoO₂ is successfully obtained. The particle size and specific surface-area of the resulting crystalline powders are 20 μm and 30 cm² g⁻¹, respectively. The LiCoO₂ powder is found to have good characteristics as a cathode active material in terms of charge–discharge capacity and cycling performance.     

Lithium cobalt oxide cathode film prepared by rf sputtering

Thin films of LiCoO₂ prepared by radio frequency magnetron sputtering on Pt-coated silicon are investigated under various deposited parameters such as working pressure, gas flow rate of Ar to O₂, and heat-treatment temperature. The as-deposited film was a nanocrystalline structure with (1 0 4) preferred orientation. After annealing at 500–700 °C, single-phase LiCoO₂ is obtained when the film is originally deposited under an oxygen partial pressure (P_O2) from 5 to 10 mTorr. When the sputtering process is performed outside these P_O2 values, a second phase of Co₃O₄ is formed in addition to the HT-LiCoO₂ phase. The degree of crystallization of the LiCoO₂ films is strongly affected by the annealing temperature; a higher temperature enhances the crystallization of the deposited LiCoO₂ film. The grain sizes of LiCoO₂ films annealed at 500, 600 and 700 °C are about 60, 95, and 125 nm, respectively. Cyclic voltammograms display well-defined redox peaks. LiCoO₂ films deposited by rf sputtering are electrochemically active. The first discharge capacity of thin LiCoO₂ films annealed at 500, 600 and 700 °C is about 41.77, 50.62 and 61.16 μAh/(cm² μm), respectively. The corresponding 50th discharge capacities are 58.1, 72.2 and 74.9% of the first discharge capacity.     

Characterization of a LiCoO2 thick film by screen-printing for a lithium ion micro-battery

A thick film cathode has been fabricated by a screen-printing technique using LiCoO₂ paste to improve the discharge capacity in lithium ion micro-batteries. The LiCoO₂ thick film (about 6 μm) was obtained by screen-printing, but high discharge capacity and a suitable surface roughness of printed LiCoO₂ film cathodes could not be obtained by adding carbon black only to the LiCoO₂ paste. On the other hand, the printed cathode which was prepared using the mixture of carbon-coated LiCoO₂ powders and carbon black showed a typical discharge curve of a LiCoO₂ cathode with a high discharge capacity (179 μAh cm⁻²).     

Creep behavior of Ni-12 wt.% Al anodes for molten carbonate fuel cells

Ni-12 wt.% Al anodes are fabricated for use in molten carbon fuel cells by tape casting and sintering. Sintering is performed in three steps, first at 1200 °C for 10 min in argon, then at 700 °C for 2.5 h in a partial oxidation atmosphere (P_H2/P_H2O=10⁻²), and finally at 950 °C for 5 min, 30 min or 1.5 h in hydrogen. Three anodes with different phases or microstructures are produced at different reduction times. One anode contains three phases, namely Ni–Al solid solution, Ni₃Al, and Al₂O₃. The amount of Al₂O₃ is extremely small at 5 min. A second anode also contains the three phases with the amount of Al₂O₃ comparable with that of Ni₃Al at 30 min. Third anode contains two phases, i.e. Ni–Al solid solution and Al₂O₃ formed at 1.5 h. The creep strains measured for the three anodes after a 100-h creep test are practically the same with an average value of 0.85%.     

Boiling enhancement by TiO2 nanoparticle deposition

TiO₂ nanoparticle-coated nickel wires were produced by electrical heating in various nanofluid concentrations ranging from 0.01 to 1 wt.% with various processing heat fluxes from 0 to 1000 kW/m². The experimental results demonstrated up to 82.7% enhancement on critical heat flux (CHF) in condition of coated nickel wire (processed in 1 wt.% with 1000 kW/m²) boiling in pure water. The contact angle measurement revealed that the hydrophilic porous coating formed by vigorous vaporization of TiO₂ nanofluid in nucleate boiling regime enormously modified the wettability of heating surface consequently improving the CHF. Besides, it is evident that the coverage of nanoparticle deposition tended to become more complete as concentration and processing heat flux increased based on SEM and EDS analysis. The nanoparticles dispersed in base fluid exhibited little effect on CHF enhancement and could even hinder the percentage of CHF augmentation from boosting, which demonstrated that one could enhance CHF by using only small amount of nanoparticles just adequate to form surface coatings instead of preparing working fluid with great bulk. However, according to the boiling curves in all cases of coated nickel wires, it is supposed that the nucleate boiling heat transfer coefficient deteriorates as a result of thermal resistance resulted from the occurrence of nanoparticle deposition. In summary, the coated porous structure of nanoparticles leads to enhance CHF and to decrease boiling heat transfer coefficient.     

Synthetic optimization of spherical LiCoO2 and precursor via uniform-phase precipitation

LiCoO₂ had been successfully prepared from spherical basic cobalt carbonate via a simple uniform-phase precipitation method at normal pressure, using cobalt sulfate and urea as the reactants. The preparation of spherical basic cobalt carbonate was significantly dependant on synthetic condition, such as the reactant concentration, reaction temperature and impeller speed, etc. The optimized condition resulted in spherical basic cobalt carbonate with uniform particle size distribution, as observed by scanning electron microscopy. Calcination of the uniform basic cobalt carbonate with lithium carbonate at high temperature led to a well-ordered layer-structured LiCoO₂ without shape change, as confirmed by X-ray diffraction. Due to the homogeneity of the basic cobalt carbonate, the final product, LiCoO₂, was also significantly uniform, i.e., the average particle size was of about 10 μm in diameter and the distribution was relatively narrow. As a result, the corresponding tap-density was also high approximately 2.60 g cm⁻³, of which the value is higher than that of commercialized LiCoO₂ of Hunan Ruixing, co. In the voltage range 2.8–4.2, 2.8–4.3, and 2.8–4.4 V, the discharge capacities of LiCoO₂ electrode were 153, 159, and 168 mAh g⁻¹, respectively, with better cyclability.     

Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2Ce0.8O2 for an SOFC running on methane fuel

A Ni/scandia-stabilized zirconia (ScSZ) cermet anode was modified by coating with nano-sized gadolinium-doped ceria (GDC, Gd_0.2Ce_0.8O₂) prepared using a simple combustion process within the pores of the anode for a solid oxide fuel cell (SOFC) running on methane fuel. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the anode characterizations. Then, the short-term stability for the cells with the Ni/ScSZ and 2.0 wt.%GDC-coated Ni/ScSZ anodes in 97%CH₄/3%H₂O at 700 °C was checked over a relative long period of operation. Open circuit voltages (OCVs) increased from 1.098 to 1.179 V, and power densities increased from 224 to 848 mW cm⁻², as the operating temperature of an SOFC with 2.0 wt.%GDC-coated Ni/ScSZ anode was increased from 700 to 850 °C in humidified methane. The coating of nano-sized Gd_0.2Ce_0.8O₂ particle within the pores of the porous Ni/ScSZ anode significantly improved the performance of anode supported cells. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibited far greater impedances than the cell with 2.0 wt.%GDC-coated Ni/ScSZ anode. Introduction of nano-sized GDC particles into the pores of porous Ni/ScSZ anode will result in a substantial increase in the ionic conductivity of the anode and increase the triple phase boundary region expanding the number of sites available for electrochemical activity. No significant degradation in performance has been observed after 84 h of cell testing when 2.0 wt.%GDC-coated Ni/ScSZ anode was exposed to 97%CH₄/3%H₂O at 700 °C. Very little carbon was detected on the anodes, suggesting that carbon deposition was limited during cell operation. Consequently, the GDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition.     

An experimental study on CHF enhancement in flow boiling using Al2O3 nano-fluid

The critical heat flux (CHF) is one of the most important thermal hydraulic parameters in heat transfer system design and safety analyses. CHF enhancement allows higher limits of operation conditions such that heat transfer equipment can be operated safely with greater margins and better economy. The application of nano-fluids is thought to have strong potential for enhancing the CHF. In this study, zeta potentials of Al₂O₃ nano-fluids were measured and flow boiling CHF enhancement experiments using Al₂O₃ nano-fluids were conducted under atmospheric pressure. The CHFs of Al₂O₃ nano-fluids were enhanced up to ～70% in flow boiling for all experimental conditions. Maximum CHF enhancement (70.24%) was shown at 0.01 vol% concentration, 50 °C inlet subcooling, and a mass flux of 100 kg/m² s. Inner surfaces of the test section tube were observed by FE–SEM and the zeta potentials of Al₂O₃ nano-fluids were measured before and after the CHF experiments.     

Electrochemical properties of the Li-ion polymer batteries with P(VdF-co-HFP)-based gel polymer electrolyte

Gel polymer electrolytes consisting of 25 wt.% P(VdF-co-HFP), 65 wt.% ethylene carbonate + propylene carbonate and 10 wt.% LiN(CF₃SO₂)₂ are prepared using by a solvent-casting technique. The electrodes are for use in lithium-ion polymer batteries. The electrochemical characteristics of the gel polymer electrolytes are evaluated by means of ac impedance and cyclic voltammetry. The charge–discharge performance of lithium polymer and lithium-ion polymer batteries is examined. A LiCoO₂ | gel polymer electrolyte (GPE) | mesocarbon microbeads (MCMB) cell delivers a discharge capacity of 146.8 and 144.5 mAh g⁻¹ on the first and the 20th cycle, respectively. The specific discharge capacity is greater than 140 mAh g⁻¹ for up to 20 cycle at all the current densities examined.     

The elevated temperature performance of LiMn2O4 coated with LiNi1−XCoXO2 (X = 0.2 and 1)

The surface coating of LiMn₂O₄ using a gel precursor of LiNi_1−XCo_XO₂ (X=0.2 and 1) prepared from a solution-based chemical process was attempted in order to enhance the electrochemical performances of LiMn₂O₄ at elevated temperature. After the surface of LiMn₂O₄ was coated with LiNi_1−XCo_XO₂ (X=0.2 and 1) coating solution and heated at 750 °C, the surface of LiMn₂O₄ was covered with fine LiNi_1−XCo_XO₂ (X=0.2 and 1) particles. LiNi_1−XCo_XO₂ (X=0.2 and 1)-coated LiMn₂O₄ showed an excellent capacity retention at 65 °C compared to pure LiMn₂O₄. While pure LiMn₂O₄ retained 81% of the initial capacity after storage in the discharged state at 65 °C for 300 h, LiCoO₂-coated LiMn₂O₄ showed no capacity loss. The improvement of storage performance at 65 °C is attributed to the suppression of electrolyte decomposition and the reduction of Mn dissolution resulting from encapsulating the surface of LiMn₂O₄ with LiCoO₂. The surface coating with LiNi_0.8Co_0.2O₂ also enhanced the high temperature cycle performance of LiMn₂O₄. Consequently, It is proposed that the surface encapsulation of LiMn₂O₄ with fine LiNi_1−XCo_XO₂ (X=0.2 and 1) particles improve its high temperature performance.     

Synthesis and electrochemical properties of LiNi0.80(Co0.20−xAlx)O2 (x = 0.0 and 0.05) cathodes for Li ion rechargeable batteries

The effect of simultaneous cobalt as well as aluminum doping was studied to understand their effect on the phase formation behavior and electrochemical properties of solution derived lithium nickel oxide cathode materials for rechargeable batteries. The discharge capacities of LiNi_0.80Co_0.20O₂ and LiNi_0.80Co_0.15Al_0.05O₂ cathodes, measured at constant current densities of 0.45 mA cm⁻² in the cut-off voltage range of 4.3–3.2 V, were 100 and 136 mAh g⁻¹, respectively. LiNi_0.80Co_0.15Al_0.05O₂ had better cycleability than the LiNi_0.80Co_0.20O₂ cathodes. The retention of undesirable Li₂CO₃ phase both in LiNi_0.80Co_0.20O₂ and LiNi_0.80Co_0.15Al_0.05O₂ cathodes was argued to be responsible for the relatively lower discharge capacity of these materials.