Open-circuit voltage study on LiFePO4 olivine cathode

Olivine LiFePO₄ particles were prepared by solid-state reaction using Li₂CO₃, (NH₄)₂HPO₄ and FeC₂O₄2H₂O as raw materials, and they were coated with an appropriate amount of carbon through thermal decomposition of C₁₆H₁₀ pyrene. Cathodes using the olivine particles were subjected to an open-circuit voltage measurement under the relaxation condition of 24 h at each SOC and DOD states. The electrochemical reaction in the LiFePO₄ cathode was composed of a large plateau around 3.45 V with sloped regions nearby for both the fully charged and discharged states. It was found that the sloped region widths exhibited a hysteresis, that is, they depend on the direction of the redox reaction. Furthermore, both sloped regions became narrower when the operating temperature was raised from 30 °C to 60 °C. These facts implied that the obtained profiles were not in an equilibrium state with a quasi-OCV profile than the real one, and that the potential relaxation in the sloped regions took an extremely long time (more than 24 h).     

Electrochemical performance of nanostructured spinel LiMn2O4 in different aqueous electrolytes

A nanostructured spinel LiMn₂O₄ electrode material was prepared via a room-temperature solid-state grinding reaction route starting with hydrated lithium acetate (LiAc·2H₂O), manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, followed by calcination of the precursor at 500 °C. The material was characterized by X-ray diffraction (XRD) and transmission electron microscope techniques. The electrochemical performance of the LiMn₂O₄ electrodes in 2 M Li₂SO₄, 1 M LiNO₃, 5 M LiNO₃ and 9 M LiNO₃ aqueous electrolytes was studied using cyclic voltammetry, ac impedance and galvanostatic charge/discharge methods. The LiMn₂O₄ electrode in 5 M LiNO₃ electrolyte exhibited good electrochemical performance in terms of specific capacity, rate dischargeability and charge/discharge cyclability, as evidenced by the charge/discharge results.     

Electrochemical activity of Li2FeTiO4 and Li2MnTiO4 as potential active materials for Li ion batteries: A comparison with Li2NiTiO4

We demonstrate, for the first time, a considerable electrochemical activity of two members of lithium transition element titanates: Li₂FeTiO₄ and Li₂MnTiO₄. Both materials consist of 10–20 nm particles embedded in a conductive carbon coating. We show that not the coating but the small particle size is decisive for materials’ activity. Li₂FeTiO₄ shows a stable reversible capacity of up to 123 mA hg⁻¹ at C/20 and 60 °C which is 83% of the theoretical value for exchange of 1 electron (148 mA hg⁻¹). Li₂MnTiO₄ could only be prepared in a nanosized form that contained about 30% of impurities. The capacity of the whole material (including impurities) is comparable to that of Li₂FeTiO₄ but the cycling stability is much poorer. In contrast to the Fe and Mn analogues, the third member of the titanate family, Li₂NiTiO₄, shows a good electrochemistry even when the particle size is much larger (about 100 nm). During initial cycles at C/10 and 60 °C, exchange of more than 1 electron per compound formula has been observed. The cycling stability at high temperatures, however, is poor.     

Lithium ion intercalation in partially crystalline TiO2 electrodeposited on platinum from aqueous solution of titanium(IV) oxalate complexes

Starting from the aqueous solution of titanium(IV) oxalate complexes and controlling electrochemical conditions using a cyclic voltammetry (CV) method, the thin layers of TiO₂ on platinum were obtained, which after additional heat treatment, at 450 °C, were still of amorphous nature. The amorphous state of the samples, containing an admixture of crystalline anatase, was confirmed by Raman spectroscopy and by a variety of electrochemical techniques. The new electrochemical procedure allows preparing the oxide with different morphologies. By the comparison with the peroxotitanium route, the oxalate precursor method offers the possibility of the synthesis of amorphous TiO₂ at higher temperatures that is the essential key for the cycling stability of the oxide if one is used as an anode material in lithium ion batteries. The results from cycling voltammetry revealed that electrodeposited TiO₂ reversibly and fast intercalates lithium ions due to its high internal surface area. Therefore, the nanostructural morphology facilitates lithium ion intercalation which was monitored and confirmed in all electrochemical testing. The specific capacity of the TiO₂ approaches the value of 145 mAh g⁻¹ at 8 C-rate in the best case. From the electrochemical impedance spectroscopy (EIS) measurements in connection with SEM investigations, it was concluded that Li⁺ diffusion is the finite space process and its rate is depending on the size of the crystallites building the oxide films. Evaluated values of the D-coefficients are of the order of 10⁻¹⁴ cm² s⁻¹.     

Electrochemical properties of LiFePO4 prepared via ball-milling

LiFePO₄ cathode materials with distinct particle sizes were prepared by a planetary ball-milling method. The effects of particle size on the morphology, thermal stability and electrochemical performance of LiFePO₄ cathode materials were investigated. The ball-milling method decreased particle size, thereby reducing the length of diffusion and improving the reversibility of the lithium ion intercalation/deintercalation. It is worth noting that the small particle sample prepared using malonic acid as a carbon source achieved a high capacity of 161 mAh g⁻¹ at a 0.1 C rate and had a very flat capacity curve during the early 50 cycles. However, the big particle samples (∼400 nm) decayed more dramatically in capacity than the small particle size samples (∼200 nm) at high current densities. The improvement in electrode performance was mainly due to the fine particles, the small size distribution, and the increase in electronic conductivity as a result of carbon coating. The structure and morphology of the ground LiFePO₄ samples were characterized with XRD, FE-SEM, TEM, EDS, and DSC techniques.     

Synthesis and electrochemical properties of olivine LiFePO4 prepared by a carbothermal reduction method

LiFePO₄/C composite cathode material was prepared by carbothermal reduction method, which uses NH₄H₂PO₄, Li₂CO₃ and cheap Fe₂O₃ as starting materials, acetylene black and glucose as carbon sources. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. X-ray diffraction (XRD), scanning electron microscopy (SEM) micrographs showed that the LiFePO₄/C is olivine-type phase, and the addition of the carbon reduced the LiFePO₄ grain size. The carbon is dispersed between the grains, ensuring a good electronic contact. The products sintered at 700 °C for 8 h with glucose as carbon source possessed excellent electrochemical performance. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 159.3 mAh g⁻¹ at 0.1 C rate, and the capacity fading is only 2.2% after 30 cycles.     

Electrochemical properties of LiFePO4 prepared via hydrothermal route

LiFePO₄ as a cathode material for rechargeable lithium batteries was prepared by hydrothermal process at 170 °C under inert atmosphere. The starting materials were LiOH, FeSO₄, and (NH₄)₂HPO₄. The particle size of the obtained LiFePO₄ was 0.5 μm. The electrochemical properties of LiFePO₄ were characterized in a mixed solvent of ethylene carbonate and diethyl carbonate (1:1 in volume) containing 1.0 mol dm⁻³ LiClO₄. The hydrothermally synthesized LiFePO₄ exhibited a discharge capacity of 130 mA h g⁻¹, which was smaller than theoretical capacity (170 mA h g⁻¹). The annealing of LiFePO₄ at 400 °C in argon atmosphere was effective in increasing the discharge capacity. The discharge capacity of the annealed LiFePO₄ was 150 mA h g⁻¹.     

Cathode performance of olivine-type LiFePO4 synthesized by chemical lithiation

Chemical lithiation with LiI in acetonitrile was performed for amorphous FePO₄ synthesized from an equimolar aqueous suspension of iron powder and an aqueous solution of P₂O₅. An orthorhombic LiFePO₄ olivine structure was obtained by annealing a chemically lithiated sample at 550 °C for 5 h in Ar atmosphere. The average particle size remained at approximately 250 nm even after annealing. The lithium content in the sample was quantitatively confirmed by Li atomic absorption analysis and ⁵⁷Fe Mössbauer spectroscopy. While an amorphous FePO₄/carbon composite cathode has a monotonously decreasing charge–discharge profile with a reversible capacity of more than 140 mAh g⁻¹, the crystallized LiFePO₄/carbon composite shows a 3.4 V plateau corresponding to a two-phase reaction. This means that the lithium in the chemically lithiated sample is electrochemically active. Both amorphous FePO₄ and the chemically lithiated and annealed crystalline LiFePO₄ cathode materials showed good cyclability (more than 140 mAh g⁻¹ at the 40th cycle) and good discharge rate capability (more than 100 mAh g⁻¹ at 5.0 mA cm⁻²). In addition, the fast-charge performance was found to be comparable to that with LiCoO₂.     

More on the reactivity of olivine LiFePO4 nano-particles with atmosphere at moderate temperature

The changes appearing for LiFePO₄-C nano-composites exposed to atmosphere at 120 °C have been structurally and chemically examined by the use of TGA, XRD, XPS, Mössbauer, ⁷Li MAS NMR and electrochemical methods. The results conclude that a highly disordered phase resulting from the aging of LiFePO₄ appears on the surface of the grains of the material, is assigned to a phosphate phase and can insert lithium around 2.6 V with poor reversibility. The essential role of water has been investigated and clearly demonstrated. Thus, the aging mechanism occurring in hot humid air is completely different from a simple oxidation as well as from the aging process observed above 150 °C and involves the incorporation of hydroxyl groups. In addition, Fe₂O₃ formation has not been observed for such an aging in mild conditions.     

Experimental and modeling kinetic study of the CO2 absorption by Li4SiO4

Nowadays, research aims to produce H₂ efficiently through modifying conventional processes by removing CO₂ at high temperature (T ≥ 500 °C). The sorption enhanced reforming (SER) is an example of such a process where CO₂ capture offers significant energy savings (≈23%). Besides, feedstock to this process may include different sources of biofuels. An essential part of this new reaction system is the use of a solid CO₂ absorbent. Among absorbents stands lithium orthosilicate (Li₄SiO₄) for its high absorption capacity and thermal stability. Therefore, the present research aims to study and model the kinetics of CO₂ absorption by Li₄SiO₄ in a temperature range of 550–650 °C. Results were consistent with a first order reaction dependence with respect to CO₂ concentration. Apparent activation energy of the gas–solid reaction (22.5 kcal/mol) is approximately equal the intrinsic activation energy (28.6 kcal/mol), suggesting that the surface reaction resistance determines the overall reaction rate.     

In situ Raman spectroscopy of the electrochemical reduction of WO3 thin films in various electrolytes

With in situ micro-Raman measurements during the electrochemical reduction of WO₃ thin films, the influence of the intercalated cation (H⁺/Li⁺) and an addition of water to the aprotic lithium electrolyte was investigated. The Raman spectra of lithium bronzes Li_xWO₃ show two main results: (i) the intercalation of hydrogen can be clearly distinguished in situ from the intercalation of lithium with this technique and (ii) even with an addition of 500 ppm of water to the lithium electrolyte no hydrogen intercalation was observed.     

Electrochemical profile of an asymmetric supercapacitor using carbon-coated LiTi2(PO4)3 and active carbon electrodes

In this work, we reported an asymmetric supercapacitor in which active carbon (AC) was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode in 1 M Li₂SO₄ aqueous electrolyte. The LiTi₂(PO₄)₃/AC hybrid supercapacitor showed a sloping voltage profile from 0.3 to 1.5 V, at an average voltage near 0.9 V, and delivered a capacity of 30 mAh g⁻¹ and an energy density of 27 Wh kg⁻¹ based on the total weight of the active electrode materials. It exhibited a desirable profile and maintained over 85% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1000 W kg⁻¹, it had a specific energy 15 Wh kg⁻¹ compared with 24 Wh kg⁻¹ at the power density about 200 W kg⁻¹.     

Aging of LiFePO4 upon exposure to H2O

The effect of H₂O on carbon-coated LiFePO₄ particles was investigated by chemical analysis, structural analysis (X-ray diffraction, SEM, TEM), optical spectroscopy (FTIR, Raman) and magnetic measurements. Upon immersion in water, part of the product floats while the main part sinks. Both the floating and the sinking part have been analyzed. We find that the floating and sinking part only differ by the amount of carbon that partly detaches from the particles upon immersion in water. Exposure to H₂O results in rapid attack, within minutes, of the surface layer of the particles, because the particles are no longer protected by carbon. The deterioration of the carbon coat is dependent on the synthesis process, either hydrothermal or solid-state reaction. In both cases, however, the carbon coat is permeable to water and fails to protect the surface of the LiFePO₄ particles. The consequence is that this immersion results in the chemical attack of LiFePO₄, but is restricted to the surface layer of the particles (few nanometers-thick). In case the particles are simply exposed to humid air, the carbon coat protects the particles more efficiently. In this case, the exposure to H₂O mainly results in the delithiation of the surface layer, due to the hydrophilic nature of Li, and only the surface layer is affected, at least for a reasonable time of exposure to humid air (weeks). In addition, within this timescale, the surface layer can be chemically lithiated again, and the samples can be dried to remove the moisture, restoring the reversible electrochemical properties.     

Electrochemical performance of MnFe2O4 nano-ferrites synthesized using thermal decomposition method

Manganese nanoferrites (MnFe₂O₄) were synthesized via thermal decomposition method using metal organic precursors in organic solvent. The structural and optical properties of the nanoparticles were investigated using X-ray diffraction (XRD), Field emission gun scanning electron microscopy (FEG-SEM), Fourier transform infrared (FTIR) spectroscopy and Ultraviolet visible spectrophotometer (UV–Vis). Electrochemical performance of MnFe₂O₄ was evaluated using cyclic voltammetry (CV). The voltammograms were obtained using Pt as the working electrode within a potential range of ?0.5 to +0.5 V at a scan rate of 50 mVs^?1. The electrochemical behavior of the nanoferrites were studied in different electrolytes such as acidic, basic and phosphate buffer (PBS). The CV profile clearly indicates that the capacitive behavior of nanoferrites changes with change in electrolyte media. The results obtained from this study will be useful for the further applications such as magnetic materials, semiconductors, sensors and energy storage devices.     

Effects of Fe2P and Li3PO4 additives on the cycling performance of LiFePO4/C composite cathode materials

In this study, a solution method was employed to synthesize LiFePO₄-based powders with Li₃PO₄ and Fe₂P additives. The composition, crystalline structure, and morphology of the synthesized powders were investigated by using ICP-OES, XRD, TEM, and SEM, respectively. The electrochemical properties of the powders were investigated with cyclic voltammetric and capacity retention studies. The capacity retention studies were carried out with LiFePO₄/Li cells and LiFePO₄/MCMB cells comprised LiFePO₄-based materials prepared at various temperatures from a stoichiometric precursor. Among all of the synthesized powders, the samples synthesized at 750 and 775 °C demonstrate the most promising cycling performance with C/10, C/5, C/2, and 1C rates. The sample synthesized at 775 °C shows initial discharge capacity of 155 mAh g⁻¹ at 30 °C with C/10 rate. From the results of the cycling performance of LiFePO₄/MCMB cells, it is found that 800 °C sample exhibited higher polarization growth rate than 700 °C sample, though it shows lower capacity fading rate than 700 °C sample. For Fe₂P containing samples, the diffusion coefficient of Li⁺ ion increases with increasing amount of Fe₂P, however, the sample synthesized at 900 °C shows much lower Li⁺ ion diffusion coefficient due to the hindrance of Fe₂P layer on the surface of LiFePO₄ particles.     

Thermal decomposition of (NH4)2SO4 in presence of Mn3O4

The main objective of this work is to develop a hybrid water-splitting cycle that employs the photon component of sunlight for production of H₂ and its thermal (i.e. IR) component for generating oxygen. In this paper, (NH₄)₂SO₄ thermal decomposition in the presence of Mn₃O₄, as an oxygen evolving step, was systematically investigated using thermogravimetric/differential thermal analyses (TG/DTA), temperature programmed desorption (TPD) coupled with a mass spectrometer (MS), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) techniques. Furthermore, thermolysis of ammonium sulfate, (NH₄)₂SO₄, in the presence of Mn₃O₄ was also investigated by conducting flow reactor experiments. The experimental results obtained indicate that at 200-450 °C, (NH₄)₂SO₄ decomposes forming NH₃ and H₂O and sulfur trioxide that in the presence of manganese oxide react to form manganese sulfate, MnSO₄. At still higher temperatures (800∼900 °C), MnSO₄ further decomposed forming SO₂ and O₂.     

Study of the Li-insertion/extraction process in LiFePO4/FePO4

The structural properties of LiFePO₄ prepared by the hydrothermal route and chemically delithiated have been studied using analytical electron microscopy and Raman spectroscopy. High-resolution transmission electron microscopy and selected area electron diffraction measurements indicate that the partially delithiated particles include LiFePO₄ regions with cross-sections of finite size along the ac-plane, as a result of tilt grain boundary in the bc-plane, and dislocations in other directions. Only the boundary along the bc-plane is accompanied by a disorder over about 2 nm on each side of the boundary. The Raman spectrum shows the existence of both LiFePO₄ and FePO₄ phases in the shell of the particles at a delithiation degree of 50%, which invalidates the core–shell model. This result also invalidates the recent model according to which each particle would be single-domain, i.e. either a LiFePO₄ particle or a FePO₄ particle. On the other hand, our results, like prior ones, can be understood within the framework of a model similar to the spinodal decomposition of a two-phase system, which is discussed within the framework of morphogenesis of patterns in systems at equilibrium. Both end-members, however, are well crystallized, suggesting a recovery similar to that observed in superplastic alloys, with dynamics that are due to the motion of nucleation fronts and dislocations, and not due to a diffusion phenomenon associated with a concentration gradient.     

Development of high power lithium-ion batteries: Layer Li[Ni0.4Co0.2Mn0.4]O2 and spinel Li[Li0.1Al0.05Mn1.85]O4

Layer Li[Ni_0.4Co_0.2Mn_0.4]O₂ and lithium excess spinel Li[Li_0.1Al_0.05Mn_1.85]O₄ were compared as positive electrode materials for high power lithium-ion batteries. Physical properties were examined by Rietveld refinement of X-ray diffraction pattern and scanning electron microscopic studies. From continuous charge and discharge tests at higher currents and different temperature environments using 3Ah class lithium-ion batteries, it was found that both materials presented plausible battery performances such as rate capability, cyclability at 60 °C at elevated temperature, and low temperature properties as well.     

New freeze-drying method for LiFePO4 synthesis

The freeze-drying method is proposed as an effective synthesis process for the obtaining of LiFePO₄/C composites. The citric acid is used as a complexing agent and carbon source. After the low temperature annealing, the freeze-dried solution leads to a homogeneous carbon covered LiFePO₄ sample. The chemical characterization of the material included ICP and elemental analysis, infrared spectroscopy, X-ray diffraction, magnetic measurements and thermal analysis. SEM and TEM microscopies indicate an aggregate morphology with tiny particles of lithium iron phosphate inside a carbon matrix. Impedance spectroscopy showed a 8.0 × 10⁻⁷ S cm⁻¹ conductivity value. Cyclic voltammetry graphics displayed the two peaks corresponding to the Fe(II)/Fe(III) reaction and demonstrated the good reversibility of the material. The specific capacity value obtained at C/40 rate was 164 mAh g⁻¹, with a slight decrease on greater C-rates reaching 146 mAh g⁻¹ at C/1. The capacity retention study has evidenced good properties, with retention over 97% of the maximum values in the first 50 cycles, which allows an effective performance of the freeze-dried sample as cathodic material in lithium-ion batteries.