Effect of mechanical activation process parameters on the properties of LiFePO4 cathode material

Pure, nano-sized LiFePO₄ and carbon-coated LiFePO₄ (LiFePO₄/C) positive electrode (cathode) materials are synthesized by a mechanical activation process that consists of high-energy ball milling and firing steps. The influence of the processing parameters such as firing temperature, firing time and ball-milling time on the structure, particle size, morphology and electrochemical performance of the active material is investigated. An increase in firing temperature causes a pronounced growth in particle size, especially above 600 °C. A firing time longer than 10 h at 600 °C results in particle agglomeration; whereas, a ball milling time longer than 15 h does not further reduce the particle size. The electrochemical properties also vary considerably depending on these parameters and the highest initial discharge capacity is obtained with a LiFePO₄/C sample prepared by ball milling for 15 h and firing for 10 h at 600 °C. Comparison of the cyclic voltammograms of LiFePO₄ and LiFePO₄/C shows enhanced reaction kinetics and reversibility for the carbon-coated sample. Good cycle performance is exhibited by LiFePO₄/C in lithium batteries cycled at room temperature. At the high current density of 2C, an initial discharge capacity of 125 mAh g⁻¹ (73.5% of theoretical capacity) is obtained with a low capacity fading of 0.18% per cycle over 55 cycles.     

Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte

La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) powder was prepared by glycine–nitrate combustion method. The electrochemical properties of porous LSCF cathodes and LSCF–Gd_0.1Ce_0.9O_1.95 (GDC) composite cathodes were evaluated at intermediate/low temperatures of 500–700 °C. The polarization resistance of pure LSCF cathode sintered at 975 °C for 2 h was 1.20 Ω cm² at 600 °C. The good performance of pure LSCF cathode is attributed to its unique microstructure—small grain size, high porosity and large surface area. The addition of GDC to LSCF cathode further reduced the polarization resistance. The lowest polarization resistance of 0.17 Ω cm² was achieved at 600 °C for LSCF–GDC (40:60 wt%) composite cathode. An anode-supported solid oxide fuel cell (SOFC) was prepared using LSCF–GDC (40:60 wt%) composite as cathode, GDC film (49-μm-thick) as electrolyte, and Ni–GDC (65:35 wt%) as anode. The total electrode polarization resistance was 0.27 Ω cm² at 600 °C, which implies that LSCF–GDC (40:60 wt%) composite cathode used in the anode-supported SOFC had a polarization resistance lower than 0.27 Ω cm² at 600 °C. The cell generated good performance with the maximum power density of 562, 422, 257 and 139 mW/cm² at 650, 600, 550 and 500 °C, respectively.     

Novel nano-network cathodes for solid oxide fuel cells

A novel nano-network of Sm_0.5Sr_0.5CoO_3−δ (SSC) is successfully fabricated as the cathodes for intermediate-temperature solid oxide fuel cells (SOFCs) operated at 500–600 °C. The cathode is composed of SSC nanowires formed from nanobeads of less than 50 nm thus exhibiting high surface area and porosity, forming straight path for oxygen ion and electron transportation, resulting in high three-phase boundaries, and consequently showing remarkably high electrode performance. An anode-supported cell with the nano-network cathode demonstrates a peak power density of 0.44 W cm⁻² at 500 °C and displays exceptional performance with cell operating time. The result suggests a new direction to significantly improve the SOFC performance.     

Novel polymer electrolytes based on triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO2

A novel polymer electrolyte based on triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO₂ inclusions has been designed. The electrolyte shows favorable features for ion migration such as low glass transition temperature and high concentration of amorphous phase. Combined with the effect of active SiO₂, its ionic conductivity is about 8.0 × 10⁻⁵ S cm⁻¹ at 30 °C, which exceeds that for the PEO-based systems. As applying them to cells with LiFePO₄-type cathodes, a capacity of about 147.0 mAh g⁻¹ is obtained at 60 °C, which is retained by more than 90% after 40 charge/discharge cycles. Moreover, about 100 mAh g⁻¹ could still be delivered as temperature decreases to 30 °C.     

Effect of synthesis temperature on the properties of LiFePO4/C composites prepared by carbothermal reduction

LiFePO₄/C composite cathode materials were synthesized by carbothermal reduction method using inexpensive FePO₄ as raw materials and glucose as conductive additive and reducing agent. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. The microstructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and particle size analysis. Cyclic voltammetry (CV) and charge/discharge cycling performance were used to characterize their electrochemical properties. The results showed that the LiFePO₄/C composite synthesized at 650 °C for 9 h exhibited the most homogeneous particle size distribution. Residual carbon during processing was coated on LiFePO₄, resulting in the enhancement of the material's electronic properties. Electrochemical measurements showed that the discharge capacity first increased and then decreased with the increase of synthesis temperature. The optimal sample synthesized at 650 °C for 9 h exhibited a highest initial discharge capacity of 151.2 mA h g⁻¹ at 0.2 C rate and 144.1 mA h g⁻¹ at 1 C rate with satisfactory capacity retention rate.     

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.     

High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3−δ-Ce0.8Sm0.2O2−δ composite cathode

A Sm_0.5Sr_0.5CoO_3−δ-Ce_0.8Sm_0.2O_2−δ (SSC-SDC) composite is employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs). BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) is used as the electrolyte, and the system exhibits a relatively high performance. An extremely low electrode polarization resistance of 0.066 Ω cm² is achieved at 700 °C. The maximum power densities are: 665, 504, 344, 214, and 118 mW cm⁻² at 700, 650, 600, 550, and 500 °C, respectively. Moreover, the SSC-SDC cathode shows an essentially stable performance for 25 h at 600 °C with a constant output voltage of 0.5 V. This excellent performance implies that SSC-SDC, which is a typical cathode material for SOFCs based on oxide ionic conductor, is also a promising alternative cathode for H-SOFCs.     

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.     

Optimized solid-state synthesis of LiFePO4 cathode materials using ball-milling

Olivine-type LiFePO₄ cathode materials were synthesized by a solid-state reaction method and ball-milling. The ball-milling time, heating time and heating temperature are optimized. A heating temperature higher than 700 °C resulted in the appearance of impurity phase Fe₂P and growth of large particle, which was shown by high resolution X-ray diffraction and field emission scanning electron microscopy. The impurity phase Fe₂P exhibited a considerable capacity loss at the 1st cycle and a gradual increase in discharge capacity upon cycling. Moreover, it exhibited an excellent high-rate capacity of 104 mAh g⁻¹ at 3 C in spite of the large particle size. The optimum synthesis conditions for LiFePO₄ were ball-milling for 24 h and heat-treatment at 600 °C for 3 h. LiFePO₄/Li cells showed an enhanced cycling performance and a high discharge capacity of 160 mAh g⁻¹ at 0.1 C.     

Novel synthesis of LiFePO4-Li3V2(PO4)3 composite cathode material by aqueous precipitation and lithiation

LiFePO₄-Li₃V₂(PO₄)₃ composite cathode material is synthesized by aqueous precipitation of FeVO₄·xH₂O from Fe(NO₃)₃ and NH₄VO₃, following chemical reduction and lithiation with oxalic acid as the reducer and carbon source. Samples are characterized by XRD, SEM and TEM. XRD pattern of the compound synthesized at 700 °C indicates olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ are co-existed. TEM image exhibits that LiFePO₄-Li₃V₂(PO₄)₃ particles are encapsulated with a carbon shell 5-10 nm in thickness. The LiFePO₄-Li₃V₂(PO₄)₃ compound cathode shows good electrochemical performance, and its discharge capacity is about 139.1 at 0.1 C, 135.5 at 1 C and 116 mA h g⁻¹ at 3 C after 30 cycles.     

Ba0.5Sr0.5Co0.8Fe0.2O3 − δ + LaCoO3 composite cathode for Sm0.2Ce0.8O1.9-electrolyte based intermediate-temperature solid-oxide fuel cells

A novel Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ + LaCoO₃ (BSCF + LC) composite oxide was investigated for the potential application as a cathode for intermediate-temperature solid-oxide fuel cells based on a Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. The LC oxide was added to BSCF cathode in order to improve its electrical conductivity. X-ray diffraction examination demonstrated that the solid-state reaction between LC and BSCF phases occurred at temperatures above 950 °C and formed the final product with the composition: La_0.316Ba_0.342Sr_0.342Co_0.863Fe_0.137O_3 − δ at 1100 °C. The inter-diffusion between BSCF and LC was identified by the environmental scanning electron microscopy and energy dispersive X-ray examination. The electrical conductivity of the BSCF + LC composite oxide increased with increasing calcination temperature, and reached a maximum value of ∼300 S cm⁻¹ at a calcination temperature of 1050 °C, while the electrical conductivity of the pure BSCF was only ∼40 S cm⁻¹. The improved conductivity resulted in attractive cathode performance. An area-specific resistance as low as 0.21 Ω cm² was achieved at 600 °C for the BSCF (70 vol.%) + LC (30 vol.%) composite cathode calcined at 950 °C for 5 h. Peak power densities as high as ∼700 mW cm⁻² at 650 °C and ∼525 mW cm⁻² at 600 °C were reached for the thin-film fuel cells with the optimized cathode composition and calcination temperatures.     

(La0.74Bi0.10Sr0.16)MnO3−δ–(Bi2O3)0.7(Er2O3)0.3 composite cathodes for intermediate temperature solid oxide fuel cells

(La_0.74Bi_0.10Sr_0.16)MnO_3−δ (LBSM)–(Bi₂O₃)_0.7(Er₂O₃)_0.3(ESB) composite cathodes were fabricated for intermediate-temperature solid oxide fuel cells with Sc-stabilized zirconia as the electrolyte. The performance of these cathodes was investigated at temperatures below 750 °C by AC impedance spectroscopy and the results indicated that LBSM–ESB had a better performance than traditional composite electrodes such as LSM–GDC and LSM–YSZ. At 750 °C, the lowest interfacial polarization resistance was only 0.11 Ω cm² for the LBSM–ESB cathode, 0.49 Ω cm² for the LSM–GDC cathode, and 1.31 Ω cm² for the LSM–YSZ cathode. The performance of the cathode was improved gradually by increasing the ESB content, and the performance was optimal when the amounts of LBSM and ESB were equal in composite cathodes. This study shows that the sintering temperature of the cathode affected performance, and the optimum sintering temperature for LBSM–ESB was 900 °C.     

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.     

A study on the electrochemical characteristics of LiFePO4 cathode for lithium polymer batteries by hydrothermal method

Phospho-olivine LiFePO₄ cathode materials were prepared by hydrothermal reaction at 150 °C. Carbon black was added to enhance the electrical conductivity of LiFePO₄. LiFePO₄-C powders (0, 3, 5 and 10 wt.%) were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). LiFePO₄-C/solid polymer electrolyte (SPE)/Li cells were characterized electrochemically by charge/discharge experiments at a constant current density of 0.1 mA cm⁻² in a range between 2.5 and 4.3 V vs. Li/Li⁺, cyclic voltammetry (CV) and ac impedance spectroscopy. The results showed that initial discharge capacity of LiFePO₄ was 104 mAh g⁻¹. The discharge capacity of LiFePO₄-C/SPE/Li cell with 5 wt.% carbon black was 128 mAh g⁻¹ at the first cycle and 127 mAh g⁻¹ after 30 cycles, respectively. It was demonstrated that cycling performance of LiFePO₄-C/SPE/Li cells was better than that of LiFePO₄/SPE/Li cells.     

Study on Sm1.8Ce0.2CuO4-Ce0.9Gd0.1O1.95 composite cathode materials for intermediate temperature solid oxide fuel cell

Sm_1.8Ce_0.2CuO₄-xCe_0.9Gd_0.1O_1.95 (SCC-xCGO, x = 0-12 vol.%) composite cathodes supported on Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte are studied for applications in IT-SOFCs. Results show that Sm_1.8Ce_0.2CuO₄ material is chemically compatible with Ce_0.9Gd_0.1O_1.95 at 1000 °C. The composite electrode exhibits optimum microstructure and forms good contact with the electrolyte after sintering at 1000 °C for 4 h. The polarization resistance (R_p) reduces to the minimum value of 0.17 Ω cm² at 750 °C in air for SCC-CGO06 composite cathode. The relationship between R_p and oxygen partial pressure indicates that the reaction rate-limiting step is the surface diffusion of the dissociative adsorbed oxygen on the composite cathode.     

LiCr0.2Ni0.4Mn1.4O4 spinels exhibiting huge rate capability at 25 and 55 °C: Analysis of the effect of the particle size

The comparison of the rate capability of LiCr_0.2Ni_0.4Mn_1.4O₄ spinels synthesized by the sucrose aided combustion method at 900, 950 and 1000 °C is presented. XRD and TEM studies show that the spinel cubic structure remains unchanged on heating but the particle size is notably modified. Indeed, it increases from 695 nm at 900 °C to 1465 nm at 1000 °C. The electrochemical properties have been evaluated by galvanostatic cycling at 25 and 55 °C between 1 C and 60 C discharge rates. At both temperatures, all samples exhibit high working voltage (∼4.7 V), elevated capacity (∼140 mAh g⁻¹) and high cyclability (capacity retention ∼99% after 50 cycles even at 55 °C). The samples also have huge rate capability. They retain more than 70% of their maximum capacity at the very fast rate of 60 C. The effect of the particle size on the rate capability at 25 and at 55 °C has been investigated. It was demonstrated that LiCr_0.2Ni_0.4Mn_1.4O₄ annealed at 900 °C, with the lowest particle size, has the best electrochemical performances. In fact, among the LiNi_0.5Mn_1.5O₄-based cathodes, SAC900 exhibits the highest rate capability ever published. This spinel, able to deliver 31,000 W kg⁻¹ at 25 °C and 27,500 W kg⁻¹ at 55 °C is a really promising cathode for high-power Li-ion battery.     

Effect of SDC-impregnated LSM cathodes on the performance of anode-supported YSZ films for SOFCs

Sm_0.2Ce_0.8O_1.9 (SDC)-impregnated La_0.7Sr_0.3MnO₃ (LSM) composite cathodes were fabricated on anode-supported yttria-stabilized zirconia (YSZ) thin films. Electrochemical performances of the solid oxide fuel cells (SOFCs) were investigated in the present study. Four single cells, i.e., Cell-1, Cell-2, Cell-3 and Cell-4 were obtained after the fabrication of four different cathodes, i.e., pure LSM and SDC/LSM composites in the weight ratios of 25/75, 36/64 and 42/58, respectively. Impedance spectra under open-circuit conditions showed that the cathode performance was gradually improved with the increasing SDC loading. Similarly, the maximum power densities (MPD) of the four cells were increased with the SDC amount below 700 °C. Whereas, the cell performance of Cell-4 was lower than that of Cell-3 at 800 °C, arising from the increased concentration polarization at high current densities. This was caused by the lowered porosity with the impregnation cycle. This disadvantage could be suppressed by lowering the operating temperature or by increasing the oxygen concentration at the cathode side. The ratio of electrode polarization loss in the total voltage drop versus current density showed that the cell performance was primarily determined by the electrode polarization. The contribution of the ohmic resistance was increased when the operating temperature was lowered. When a 100 ml min⁻¹ oxygen flow was introduced to the cathode side, Cell-3 produced MPDs of 1905, 1587 and 1179 mW cm⁻² at 800, 750 and 700 °C, respectively. The high cell outputs demonstrated the merits of the novel and effective SDC-impregnated LSM cathodes.     

Investigation of Sm0.5Sr0.5CoO3−δ/Co3O4 composite cathode for intermediate-temperature solid oxide fuel cells

The electrochemical properties of an Sm_0.5Sr_0.5CoO_3−δ/Co₃O₄ (SSC/Co₃O₄) composite cathode were investigated as a function of the cathode-firing temperature, SSC/Co₃O₄ composition, oxygen partial pressure and CO₂ treatment. The results showed that the composite cathodes had an optimal microstructure at a firing temperature of about 1100 °C, while the optimum Co₃O₄ content in the composite cathode was about 40 wt.%. A single cell with this optimized C₄₀-1100 cathode exhibited a very low polarization resistance of 0.058 Ω cm², and yielded a maximum power density of 1092 mW cm⁻² with humidified hydrogen fuel and air oxidant at 600 °C. The maximum power density reached 1452 mW cm⁻² when pure oxygen was used as the oxidant for a cell with a C₃₀-1100 cathode operating at 600 °C due to the enhanced open-circuit voltage and accelerated oxygen surface-exchange rate. X-ray diffraction and thermogravimetric analyses, as well as the electrochemical properties of a CO₂-treated cathode, also implied promising applications of such highly efficient SSC/Co₃O₄ composite cathodes in single-chamber fuel cells with direct hydrocarbon fuels operating at temperatures below 500 °C.     

Synthesis of LiFePO4/C composite with high-rate performance by starch sol assisted rheological phase method

LiFePO₄/C composite was synthesized at 600 °C in an Ar atmosphere by a soluble starch sol assisted rheological phase method using home-made amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/C composite has good crystallinity, ultrafine sphere-like particles of 100-200 nm size and in situ carbon. The synthesized LiFePO₄ could inherit the morphology of FePO₄ precursor. The electrochemical performance of the LiFePO₄ by galvanostatic cycling studies demonstrates excellent high-rate cycle stability. The Li/LiFePO₄ cell displays a high initial discharge capacity of more than 157 mAh g⁻¹ at 0.2C and a little discharge capacity decreases from the first to the 80th cycle (>98.3%). Remarkably, even at a high current density of 30C, the cell still presents good cycle retention.     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.