Synthesis and performance of lithium vanadium phosphate as cathode materials for lithium ion batteries by a sol–gel method

Monoclinic lithium vanadium phosphate, Li₃V₂(PO₄)₃, was synthesized by a sol–gel method under Ar/H₂ (8% H₂) atmosphere. The influence of sintering temperatures on the synthesis of Li₃V₂(PO₄)₃ has been investigated using X-ray diffraction (XRD), SEM and electrochemical methods. XRD patterns show that the Li₃V₂(PO₄)₃ crystallinity with monoclinic structure increases with the sintering temperature from 700 to 800 °C and then decreases from 800 to 900 °C. SEM results indicate that the particle size of as-prepared samples increases with the sintering temperature increase and there is minor carbon particles on the surface of the sample particles, which are very useful to enhance the conductivity of Li₃V₂(PO₄)₃. Charge–discharge tests show the 800 °C-sample exhibits the highest initial discharge capacity of 131.2 mAh g⁻¹ at 10 mA g⁻¹ in the voltage range of 3.0–4.2 V with good capacity retention. CV experiment exhibits that there are three anodic peaks at 3.61, 3.70 and 4.11 V on lithium extraction as well as three cathodic peaks at 3.53, 3.61 and 4.00 V on lithium reinsertion at 0.02 mV s⁻¹ between 3.0 and 4.3 V. It is suggested that the optimal sintering temperature is 800 °C in order to obtain pure monoclinic Li₃V₂(PO₄)₃ with good electrochemical performance by the sol–gel method, and the monoclinic Li₃V₂(PO₄)₃ can be used as candidate cathode materials for lithium ion batteries.     

Electrochemical properties of LiNi0.8Co0.2−xAlxO2 prepared by a sol–gel method

Prospective positive-electrode (cathode) materials for a lithium secondary battery, viz., LiNi_0.8Co_0.2−xAl_xO₂ (x = 0.00, 0.01, 0.03, and 0.05), were synthesized using a sol–gel method and the structural and electrochemical properties are examined by means of X-ray diffraction, cyclic voltammetry, and charge–discharge tests. The LiNi_0.8Co_0.2−xAl_xO₂ maintains the α-NaFeO₂-type layered structure regardless of the aluminium content in the range x ≤ 0.05. On the other hand, as the aluminium content is increased, the capacity retention of LiNi_0.8Co_0.2−xAl_xO₂ is improved while initial discharge capacity is slightly decreased. Results also show that the current peaks on the cyclic voltammograms are diminished and merged on aluminium addition. This suggests that the improved cycle stability of LiNi_0.8Co_0.2−xAl_xO₂ is due to suppression of the phase transition.     

Adipic acid assisted,sol–gel route for synthesis of LiCrxMn2−xO4 cathode material

Spinel LiMn₂O₄ and LiCr_xMn_2−xO₄ (x=0.00−0.20) have been synthesized by a soft chemistry method using adipic acid as the chelating agent. This technique offers better homogeneity, preferred surface morphology, reduced heat-treatment conditions, sub-micron sized particles, and better crystallinity. The synthesized spinel materials are characterized by X-ray diffraction, scanning electron microscopy, cyclic voltammetry, and charge–discharge testing. It is found that chromium substitution alleviates capacity fading in the 4-V region and improves the structural stability of LiMn₂O₄ spinel upon repeated cycling.     

Synthesis and physicochemical properties of LiAl0.05Mn1.95O4 cathode material by the ultrasonic-assisted sol–gel method

Spinel LiAl_0.05Mn_1.95O₄ has been successfully synthesized by a new ultrasonic-assisted sol–gel (UASG) method. The structure and physicochemical properties of this as-prepared powder compared with the pristine LiMn₂O₄ and LiAl_0.05Mn_1.95O₄ synthesized by the traditional sol–gel method were investigated by differential thermal analysis (DTA) and thermogravimetery (TG), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charge–discharge testing in detail. The results show that all samples have high phase purity, and ultrasonic process plays an important role in controlling morphology; LiAl_0.05Mn_1.95O₄ has higher Mn oxidation state, and the absorption peak of Mn(III)O and Mn(IV)O bonds has blue shift because of the doped Al. CV confirms that the LiAl_0.05Mn_1.95O₄ sample (UASG) has a good reversibility and its structure is very advantageous for the transportation of lithium ions. The charge–discharge tests indicate that LiAl_0.05Mn_1.95O₄ (UASG) has nearly equal initial capacity with LiMn₂O₄ (sol–gel) at 1C discharge rate, but LiAl_0.05Mn_1.95O₄ (UASG) has higher discharge potential than that of LiMn₂O₄ (sol–gel). In addition, LiAl_0.05Mn_1.95O₄ (UASG) has higher discharge potential and capacity than that of LiAl_0.05Mn_1.95O₄ (sol–gel) at 1C discharge rate, and LiAl_0.05Mn_1.95O₄ (UASG) has high capacity retention at C/3 and 1C discharge rate among three samples after 50 cycles, which reveals that the sample obtained via UASG method, has the best electrochemical performance among three samples.     

Efficient hydrogen production from glycerol photoreforming over Ag2OTiO2 synthesized by a sol–gel method

Photoreforming hydrogen production from glycerol aqueous solutions has been investigated over Ag₂OTiO₂ catalysts with variable compositions (0.72–6.75 wt.% of Ag₂O) synthesized by a sol–gel method. The structural and morphologic characteristics were examined by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and Brunauer–Emmett–Teller (BET) method etc. By comparing the results of FESEM and DLS, aggregations of all samples were found to occur in both dry powder and water suspension states. The TiO₂ based catalyst with 2.82 wt.% Ag₂O calcined under 500 °C showed less average particle size differences between in air and in aqueous state, suggesting better dispersion stability which could lead to a more efficient of using light. It was found that mixing certain amounts (0.72–2.82 wt.%) of Ag₂O to TiO₂ could reduce anatase grain size and increased the specific surface area of the catalysts. The hydrogen production results indicated that the photocatalytic activity and the reaction rate of TiO₂ were significantly improved by mixing Ag₂O. The optimal Ag₂O mixing amount and calcination temperature for H₂ production was found. With an appropriate catalyst composition, the heterostructures could be well generated to enhance the separation of electrons and holes and it was also found that the desirable heterostructure was likely to disappear after the calcination temperature of 700 °C.     

Preparation of Sn–Co alloy electrode for lithium ion batteries by pulse electrodeposition

Sn–Co alloy films for Li-ion batteries were prepared by pulse electrodeposition on the copper foils as current collectors. Nanocrystalline Sn–Co alloy electrodes produced by using a solution containing cobalt chloride and tin chloride at constant electrodeposition conditions (pulse on-time t_on at 5 ms and pulse off-time t_off at 5 ms) with varying peak current densities, Jp have been investigated. The structures of the electroplated Sn–Co alloy thin films were studied to reveal film morphology current density relationships and the effect of the current density parameters on the electrochemical properties. X-Ray Diffractometer (XRD), Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analyzer and Energy-Dispersive X-ray Spectroscopy (EDS) facilities were used for determination the relationships between structure and experimental parameters. Cyclic voltammetry (CV) tests were carried out to reveal reversible reactions between cobalt–tin and lithium. Galvanostatic charge/discharge (GC) measurements were performed in the cells formed by using anode composite materials produced by pulse electro co-deposition. The discharge capacities of these cells were cyclically tested by a battery tester at a constant current in the different voltage ranges between 0.02 V–1.5 V. The results have shown that Sn–Co alloy yielded promising reversible discharge capacities with a satisfactory cycle life for an alternative anode material to apply for the Li-ion batteries.     

Modifying the cathodes of intermediate-temperature solid oxide fuel cells with a Ce0.8Sm0.2O2 sol–gel coating

To increase the performance of solid oxide fuel cells operated at intermediate temperatures (<700 °C), we used the electronic conductor La_0.8Sr_0.2MnO₃ (LSM) and the mixed conductor La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) to modify the cathode in the electrode microstructure. For both cathode materials, we employed a Sm_0.2Ce_0.8O₂ (SDC) buffer layer as a diffusion barrier on the yttria-stabilized zirconia (YSZ) electrolyte to prevent the interlayer formation of SrZrO₃ and La₂Zr₂O₇, which have a poor ionic conductivity. These interfacial reaction products were formed only minimally at the electrolyte–cathode interlayer after sintering the SDC layer at high temperature; in addition, the degree of cathode polarization also decreased. Moreover to extend the triple phase boundary and improve cell performance at intermediate temperatures, we used sol–gel methods to coat an SDC layer on the cathode pore walls. The cathode resistance of the LSCF cathode cell featuring SDC modification reached as low as 0.11 Ω cm² in air when measured at 700 °C. The maximum power densities of the cells featuring the modified LSCF and LSM cathodes were 369 and 271 mW/cm², respectively, when using O₂ as the oxidant and H₂ as the fuel.     

Effect of various synthetic parameters on purity of LiMn2O4 spinel synthesized by a sol–gel method at low temperature

Spinel lithium manganese oxide, LiMn₂O₄, prepared by the sol–gel method using citric acid as a chelating agent under different (i) pH conditions, (ii) molar ratio of citric acid to total metal ion, (iii) amount of water, (iv) calcination temperature, and (vi) starting materials. The effects of various synthetic parameters on the purity of this oxide are analysed by means of X-ray diffraction measurements. The results show that pure LiMn₂O₄ can be prepared from nitrate salts as starting materials at a low temperature of 600°C. The optimum pH and molar ratio of chelating agent to total metal ions are 6.0 and 1.0, respectively.     

Study on durability of novel core-shell-structured La0.8Sr0.2Co0.2Fe0.8O3-δ@Gd0.2Ce0.8O1.9 composite materials for solid oxide fuel cell cathodes

Core-shell-structured La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ@Gd_0.2Ce_0.8O_1.9 (LSCF@GDC) composite materials are synthesized and sintered as the SOFC cathodes by screen-printing method. The durability of core-shell-structured LSCF@GDC composite cathodes are evaluated through constant current polarizations (CCP) process at 750 °C and the results indicate that the core-shell-structured LSCF@GDC composite cathode (nanorod, 0.6) possesses an excellent long-term stability. In addition, molecular dynamics (MD) model is developed and applied to simulate the interaction between LSCF and GDC particles. According to the simulation results, compressive stress is generated at the cathode-electrolyte interface by the coated GDC layer. Combining with the X-ray diffraction (XRD) refinement results, it's revealed that the lattice strains are introduced in LSCF lattices because of the compressive stress. Furthermore, XPS results show that the core-shell-structured LSCF@GDC composite cathode (nanorod, 0.6) possess a better inhibition ability for Sr surface segregation. This study provides a possible way to suppress Sr surface segregation.     

Nanocomposite BaZr0.7Sm0.1Y0.2O3−δ–La0.8Sr0.2Co0.2Fe0.8O3−δ materials for single layer fuel cell

Single layer fuel cell (SLFC) is a novel breakthrough in energy conversion technology. This study is to realize the physical-electrochemical co-driving mechanism of a single component device composed of mixed ionic and semiconductor material. This paper is focused on investigating the mechanism and characterization of synthesized nanocomposite BaZr_0.7Sm_0.1Y_0.2O_3?δ (BZSY)–La_0.8Sr_0.2Co_0.2Fe_0.8O_3?δ (LSCF) in proportion 1:1 and 3:7 for SLFC. The crystallographic structure and morphology is studied with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The nano-particles lie in the range of 100–210 nm. Ultraviolet (UV) and electrochemical impedance spectroscopy (EIS) is used to analyze the semiconducting nature of nanocomposite (BZSY–LSCF). The performance of SLFC was carried out at different temperatures ranging between 400 and 650 °C. The mixed conductivity of the synthesized material was about 2.3 S cm^?1. The synergic effect of junction and energy band gap towards charge separation as well as the promotion of ion transport by junction built in field contributes to the working principle and high power output in the SLFC.     

Ba1−xSrxCe0.8−yZryY0.2O3−δ protonic electrolytes synthesized by hetero-composition-exchange method for solid oxide fuel cells

This study reports the synthesis of proton-conducting Ba_0.8Sr_0.2Ce_0.6Zr_0.2Y_0.2O_3?δ oxides by using a combination of the sol–gel process and hetero-composition-exchange technique. The experimental results show that the sintered Ba_0.8Sr_0.2Ce_0.6Zr_0.2Y_0.2O_3?δ pellet synthesized by the hetero-composition-exchange method exhibits excellent sinterability, good relatively density, and high protonic conduction. Furthermore, the Pt/electrolyte/Pt single cell with such an electrolyte shows a significantly higher maximum power density as compared to those oxides prepared from conventional sol–gel powders. Based on the experimental results, we attempt to explain the improvement mechanism in terms of as-calcined particle characteristics and proton hopping distance. This work shows that the Ba_1?xSr_xCe_0.8?yZr_yY_0.2O_3?δ oxides synthesized by the sol–gel combined with hetero-composition-exchange method would be a promising electrolyte for H⁺-SOFC applications. More importantly, this new fabrication approach could be applied to other similar perovskite-type electrolyte systems.     

Synthesis and electrochemical characteristics of LiCrxNi0.5−xMn1.5O4 spinel as 5 V cathode materials for lithium secondary batteries

A series of electrochemical spinel compounds, LiCr_xNi_0.5−xMn_1.5O₄ (x=0, 0.1, 0.3), are synthesized by a sol–gel method and their electrochemical properties are characterized in the voltage range of 3.5–5.2 V. Electrochemical data for LiCr_xNi_0.5−xMn_1.5O₄ electrodes show two reversible plateaus at 4.9 and 4.7 V. The 4.9 V plateau is related to the oxidation of chromium while the 4.7 V plateau is ascribed to the oxidation of nickel. The LiCr_0.1Ni_0.4Mn_1.5O₄ electrode delivers a high initial capacity of 152 mAh g⁻¹ with excellent cycleability. The excellent capacity retention of the LiCr_0.1Ni_0.4Mn_1.5O₄ electrode is largely attributed to structural stabilization which results from co-doping (chromium and nickel) and increased theoretical capacity due to substitution of chromium.     

A La0.8Sr0.2MnO3/La0.6Sr0.4Co0.2Fe0.8O3−δ core–shell structured cathode by a rapid sintering process for solid oxide fuel cells

A La_0.8Sr_0.2MnO₃ (LSM)/La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) core–shell structured composite cathode of solid oxide fuel cells (SOFCs) has been fabricated by wet infiltration followed by a rapid sintering (RS) process. The RS is carried out by placing LSCF infiltrated LSM electrodes directly into a preheated furnace at 800 °C for 10 min and cooling down very quickly. The heating and cooling step takes about 20 s, substantially shorter than 10 h in the case of conventional sintering (CS) process. The results indicate the formation of a continuous and almost non-porous LSCF thin film on the LSM scaffold, forming a LSCF/LSM core–shell structure. Such RS-formed infiltrated LSCF–LSM cathodes show an electrode polarization resistance of 2.1 Ω cm² at 700 °C, substantially smaller than 88.2 Ω cm² of pristine LSM electrode. The core–shell structured LSCF–LSM electrodes also show good operating stability at 700 °C and 600 °C over 24–40 h.     

Preparation of CeO2 based nanocomposites for fuel cell electrolyte via sol–gel method

Abstract

A Ce_0·8Sm_0·2O_1·9 nanocomposite with potential for application as a fuel cell electrolyte was prepared via the sol–gel method. Nitrates and citric acid were respectively adopted as reactor and complexing agent to synthesise the composite. X-ray diffraction was used to investigate the influence of temperature and content of nitric acid on the purity and grain size of the CeO₂ phase. It was found that nanocrystals with 20 nm grain size can be obtained following heating for 4 h at 800°C and that addition of nitric acid improved the purity of the phase produced. A microscale model of the structural transformation from precursor gel to powder for the composite is proposed.     

Thermal simulation of large-scale lithium secondary batteries using a graphite–coke hybrid carbon negative electrode and LiNi0.7Co0.3O2 positive electrode

Thermal simulation was applied to 2 Wh-class cells (diameter 14.2 mm, height 50 mm) using LiNi_0.7Co_0.3O₂ or LiCoO₂ as the positive electrode material, in order to clarify the thermal behavior of the cells during charge and discharge. The thermal simulation results for the 2 Wh-class cells showed a good agreement with measured temperature values. The heat generation of a cell using LiNi_0.7Co_0.3O₂ was found to be much less than that using LiCoO₂ during discharge. This difference was considered to be caused by the difference in the change of entropy. A 250 Wh-class cell (diameter 64 mm, height 296 mm) was also constructed using LiNi_0.7Co_0.3O₂ and thermal simulation was applied. We confirmed that the results of the thermal simulation agreed with measured values and that this simulation model is effective for analyzing the thermal behavior of large-scale lithium secondary batteries.     

Synthesis and electrochemical characteristics of Li(Ni · M)O2 (M = Co,Mn) cathode for rechargeable lithium batteries

The synthesis and electrochemical characteristics of LiNiO₂ and Li(Ni · M)O₂ (M = Co or Mn) as the cathode materials for rechargeable lithium batteries were investigated. It was clarified from these investigations that LiNiO₂ has been produced from crystalline NiO, which was derived from Ni(OH)₂ and LiOH, and that the property of NiO had some influence on the LiNiO₂ preparation. It was assumed that the formation of the layered structure has been inhibited by the existence of the Ni vacancy and Ni³⁺ ion in NiO. The synthesis of a solid solution of Li(Ni · Co)O₂ suggested that a part of the Ni replacement by Co might inhibit the formation of the Ni vacancy of NiO and promote the formation of the layered structure. The capacity fading with increase in cycle number was suppressed by the replacement of a part of Ni with Co. We considered that the capacity fading was suppressed by the development of the layered structure wherein formation of Ni vacancy was suppressed by replacement with Co. LiNi_0.8Co_0.2O₂ prepared under the stream of oxygen gas showed a small irreversible capacity at first cycle and higher cycling capacity of ∼ 180 mA h g⁻¹.     

Electrochemical properties of Co3O4, Ni–Co3O4 mixture and Ni–Co3O4 composite as anode materials for Li ion secondary batteries

By varying the synthetic temperature and time, Co₃O₄ with highly optimized electrochemical properties was obtained from the solid state reaction of CoCO₃. As a result, Co₃O₄ showed a high capacity around 700 mAh/g and stable capacity retention during cycling (93.4% of initial capacity was retained after 100 cycles). However, its initial irreversible capacity reached about 30% of capacity. Several phenomenological examinations in our previous results told us that the main causes of low initial coulombic efficiency, that is, large initial irreversible capacity, were solid electrolyte interphase (SEI) film formation on surface and incomplete decomposition of Li₂O during the first discharge process. SEI film formation cannot be restrained without the development of a special electrolyte, and there has been little research on the proper electrolyte composition, whereas in our research, Ni had the catalytic activity to facilitate Li₂O decomposition. Thus, in order to improve the low initial coulombic efficiency of Co₃O₄ (69%), Ni was added to Co₃O₄ using two methods like physical mixing and mechanical milling. When adding the same amount of Ni, the mechanical milling showed the improvement in initial coulombic efficiency, 79%, but physical mixing had no effect. Finally, when the charge–discharge mechanism of Co₃O₄ was considered and the morphologies of Ni–Co₃O₄ mixture obtained by physical mixing and Ni–Co₃O₄ composite prepared by mechanical milling were compared, it was revealed that the initial coulombic efficiency of Ni–Co₃O₄ composite depends on the contact area between the Ni and the Co₃O₄.     

A study of the electrochemical lithium intercalation behavior of porous LiNiO2 electrodes prepared by solid-state reaction and sol–gel methods

The electrochemical lithium intercalation behavior of porous LiNiO₂ electrodes prepared by solid-state reaction and sol–gel methods is investigated by using X-ray diffractometry (XRD), a galvanostatic intermittent charge–discharge experiment, electrochemical impedance spectroscopy(EIS), and a charge–discharge cycling test. The ultrafine LiNiO₂ powder is prepared by the sol–gel method in order to overcome the disadvantage of the conventional solid-state reaction method. From the results of XRD, the layered LiNiO₂ phase proves to be stable above 400°C. The conventional oxide electrode suffers a larger capacity loss, a greater instantaneous IR drop during the first intermittent discharge, and a smaller chemical diffusivity than the gel-derived electrode. The results are discussed with respect to the marked cation mixing effect in the former electrode. Furthermore, the charge–discharge cycling test shows that the cell Li/organic electrolyte/gel-derived LiNiO₂ electrode displays improved performance, i.e., an initial specific capacity of 150 Ah kg⁻¹ and a specific energy density above 500 Wh kg⁻¹.     

Methanol steam reforming over highly efficient CuO–Al2O3 nanocatalysts synthesized by the solid-state mechanochemical method

CH₃OH steam reforming is an attractive way to produce hydrogen with high efficiency. In this study, CuO.xAl₂O₃ (x = 1, 2, 3, and 4) were fabricated based on the solid-state route, and the calcined samples were employed in methanol steam reforming at atmospheric pressure and in the temperature range of 200–450 °C. The results revealed that all samples have a high BET area (173–275 m² g⁻¹), and their crystallinity was reduced by increasing the alumina content in the catalyst formulation. The catalytic activity tests showed that the CH₃OH conversion and H₂ selectivity decreased by rising the Al₂O₃·CuO molar ratio. The methanol conversion enhanced from 13% to 85% by increasing the reaction temperature from 200 °C to 450 °C over the CuO·Al₂O₃ catalyst, due to the higher reducibility of this catalyst at lower temperatures compared to other prepared samples. The influence of calcination temperature (300–500 °C), GHSV (28,000–48000 ml h⁻¹. g⁻¹_cat), feed ratio (C:W = 1:1 to 1:9), and reduction temperature (250–450 °C) was also determined on the yield of the chosen sample. The results revealed that the maximum methanol conversion decreased from 90 to 79% by raising the calcination temperature from 300 to 500 °C due to the reduction of surface area and sintering of species at high calcination temperatures.