Preparation and characterization of electrospun LiFePO4/carbon complex improving rate performance at high C-rate

LiFePO₄/carbon complexes were prepared by electrospinning to improve rate performance at high C-rate and their electrochemical properties were investigated to be used as a cathode active material for lithium ion battery. The LiFePO₄/carbon complexes were prepared by the electrospinning method. The prepared samples were characterized by SEM, EDS, XRD, TGA, electrometer, and electrochemical analysis. The LiFePO₄/carbon complexes prepared have a continuous structure with carbon-coated LiFePO₄ and the LiFePO₄ in LiFePO₄/carbon complex has improved thermal stability from carbon coating. The conductivity of LiFePO₄/carbon complex heat-treated at 800 °C is measured as 2.23 × 10^?2 S cm^?1, which is about 10⁶–10⁷ times more than that of raw LiFePO₄. The capacity ratio of coin cell manufactured from raw LiFePO₄ is 40%, whereas the capacity ratio of coin cell manufactured from LiFePO₄/carbon complex heat-treated at 800 °C is 61% (10 C/0.1 C). The improved rate performance of LiFePO₄/carbon complex heat-treated at 800 °C is due to the carbon coating and good electrical connection.     

Enhanced electrochemical performance of LiFePO<Subscript>4</Subscript>/C nanocomposites due to in situ formation of Fe<Subscript>2</Subscript>P impurities

We have studied LiFePO₄/C nanocomposites prepared by sol-gel method using lauric acid as a surfactant and calcined at different temperatures between 600 and 900 °C. In addition to the major LiFePO₄ phase, all the samples show a varying amount of in situ Fe₂P impurity phase characterized by x-ray diffraction, magnetic measurements, and Mössbauer spectroscopy. The amount of Fe₂P impurity phase increases with increasing calcination temperature. Of all the samples studied, the LiFePO₄/C sample calcined at 700 °C which contains ～15 wt% Fe₂P shows the least charge transfer resistance and a better electrochemical performance with a discharge capacity of 136 mA h g^?1 at a rate of 1 C, 121 mA h g^?1 at 10 C (～70 % of the theoretical capacity of LiFePO₄), and excellent cycleability. Although further increase in the amount of Fe₂P reduces the overall capacity, frequency-dependent Warburg impedance analyses show that all samples calcined at temperatures ≥700 °C have an order of magnitude higher Li⁺ diffusion coefficient (～1.3?×?10^?13 cm² s^?1) compared to the one calcined at 600 °C, as well as the values reported in literature. This work suggests that controlling the reduction environment and the temperature during the synthesis process can be used to optimize the amount of conducting Fe₂P for obtaining the best capacity for the high power batteries.     

LiFePO4/C composites from carbothermal reduction method

Using the cheap raw materials lithium carbonate, iron phosphate, and carbon, LiFePO₄/C composite can be obtained from the carbothermal reduction method. X-ray diffraction (XRD) and scanning electronic microscope (SEM) observations were used to investigate the structure and morphology of LiFePO₄/C. The LiFePO₄ particles were coated by smaller carbon particles. LiFePO₄/C obtained at 750 °C presents good electrochemical performance with an initial discharge capacity of 133 mAh/g, capacity retention of 128 mAh/g after 20 cycles, and a diffusion coefficient of lithium ions in the LiFePO₄/C of 8.80?×?10^?13 cm²/s, which is just a little lower than that of LiFePO₄/C obtained from the solid-state reaction (9.20?×?10^?13 cm²/s) by using FeC₂O₄ as a precursor.     

Low-temperature performance of LiFePO4/C cathode in a quaternary carbonate-based electrolyte

The low-temperature performance of LiFePO₄/C cathode in a quaternary carbonate-based electrolyte (1.0 M LiPF₆/EC+DMC+DEC+EMC (1:1:1:3, v/v)) was studied. The discharge capacities of the LiFePO₄/C cathode were about 134.5 mAh/g (20 °C), 114 mAh/g (0 °C), 90 mAh/g (−20 °C) and 69 mAh/g (−40 °C) using a 1C charge–discharge rate. Cyclic voltammetry measurements show obviously sluggish of the lithium insertion–extraction process of the LiFePO₄/C cathode as the operation temperature falls below −20 °C. Electrochemical impedance analyses demonstrate that the sluggish of charge-transfer reaction on the electrolyte/LiFePO₄/C interface and the decrease of lithium diffusion capability in the bulk LiFePO₄ was the main performance limiting factors at low-temperature.     

Low temperature nanostructured lithium titanates: controlling the phase composition, crystal structure and surface area

Low temperature lithium titanate compounds (i.e., Li₄Ti₅O₁₂ and Li₂TiO₃) with nanocrystalline and mesoporous structure were prepared by a straightforward aqueous particulate sol–gel route. The effect of Li:Ti molar ratio was studied on crystallisation behaviour of lithium titanates. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that the powders were crystallised at the low temperature of 500 °C and the short annealing time of 1 h. Moreover, it was found that Li:Ti molar ratio and annealing temperature influence the preferable orientation growth of the lithium titanate compounds. Transmission electron microscope (TEM) images showed that the average crystallite size of the powders annealed at 400 °C was in the range 2–4 nm and a gradual increase occurred up to 10 nm by heat treatment at 800 °C. Field emission scanning electron microscope (FE-SEM) analysis revealed that the deposited thin films had mesoporous and nanocrystalline structure with the average grain size of 21–28 nm at 600 °C and 49–62 nm at 800 °C depending upon the Li:Ti molar ratio. Moreover, atomic force microscope (AFM) images confirmed that the lithium titanate films had columnar like morphology at 600 °C, whereas they showed hill-valley like morphology at 800 °C. Based on Brunauer–Emmett–Taylor (BET) analysis, the synthesized powders showed mesoporous structure containing pores with needle and plate shapes. The surface area of the powders was enhanced by increasing Li:Ti molar ratio and reached as high as 77 m²/g for the ratio of Li:Ti = 75:25 at 500 °C. This is one of the smallest crystallite size and the highest surface areas reported in the literature, and the materials could be used in many applications such as rechargeable lithium batteries and tritium breeding materials.     

A rapid polyol combustion strategy towards scalable synthesis of nanostructured LiFePO4/C cathodes for Li-ion batteries

In the present study, carbon-coated lithium iron phosphate (LiFePO₄/C) is prepared directly by a polyol-assisted pyro-synthesis performed under reaction times of a few seconds in open-air conditions. The polyol solvent, tetraethylene glycol (TTEG), acts as a low-cost fuel to facilitate combustion and the released exothermic energy promotes the nucleation and growth processes of the olivine nanoparticles. In addition, phosphoric acid (used as the phosphorous source) acts as a catalyst to accelerate polyol carbonization. The structure analysis of the as-prepared LiFePO₄/C using X-ray, neutron diffraction and ⁷Li NMR studies suggested the efficacy of the rapid technique to produce highly crystalline phase-pure olivine nanocrystals. The electron microscopy and particle-size distribution studies revealed that the average particle diameters lie below 100 nm and confirmed the presence of a surface carbon layer of 2–3 nm thickness. The thermal and elemental studies indicated that the carbon content in the sample was approximately 5 %. The prepared LiFePO₄/C cathode delivered capacities of 162 mA h g^-1 at 0.1 °C rates with impressive capacity retention for extended cycling. The polyol-assisted pyro-synthesis, which evades the use of external energy sources, is not only a straightforward, simple and timely approach but also offers opportunities for large-scale LiFePO₄/C production.     

Electrochemical and magnetic characterization of LiFePO<Subscript>4</Subscript> and Li<Subscript>0.95</Subscript>Mg<Subscript>0.05</Subscript>FePO<Subscript>4</Subscript> cathode materials

A series of lithium iron phosphates was synthesized via the sol–gel route. Iron phosphides, which are electronic conductors, were formed when sintered at 850°C. Magnetic susceptibility measurements on the samples show antiferromagnetic behaviour with T _N=50±2 K for LiFePO₄ and Li_0.95Mg_0.05PO₄ sintered at temperatures below 850°C. The LiFePO₄ and Li_0.95Mg_0.05FePO4 cathodes show a stable electrochemical capacity in the range of 150–160 mA h/g on cycling. The cyclability deteriorates with increasing sample sintering temperature due to the increased crystal size and impurities.     

Storage Stability of LiFePO4/C in Humid‐hot Air

Storage stabilities of LiFePO₄/C composite at different conditions are investigated in terms of structural and electrochemical evolutions. The results from different aging tests indicate that moisture and temperature are the key factors that have the most profound effects on the structure homogeneity which in turn influences the electrochemical performance of LiFePO₄/C. Although the storage in a humid‐hot environment, such as saturated humidity air at 50°C, does not greatly influence the discharging capacity of LiFePO₄/C, it does reduce the initial charging capacity, thus the amount of reversible Li⁺ ions in a practical LiFePO₄/graphite cell decreases. This impact is explained by the lithium extraction during the storage, forming olivine FePO₄ and associated Li₃PO₄. Elevated storage temperature also favors the delithiation process. The degree of delithiation increases from about 6% at 50°C to 18% at 80°C. It is also found that re‐calcination at 650°C effectively resolves the problem of the structural heterogeneity of the stored LiFePO₄/C. Therefore both the initial charging capacity and coulombic efficiency of the stored sample in the first cycle revert to the original value of the fresh one.     

Synthesis and electrochemical characterization of LiFePO<Subscript>4</Subscript>/C-polypyrrole composite prepared by a simple chemical vapor deposition method

A LiFePO₄/C-polypyrrole (LiFePO₄/C-PPy) composite as a high-performance cathode material is successfully prepared through a simple chemical vapor deposition (CVD) method. According to the transmission electron microscope (TEM) analysis, the surface of the LiFePO₄/C is surrounded with PPy in the LiFePO₄/C-PPy composite. The as-prepared LiFePO₄/C-PPy material shows outstanding rate capability at 20°C and good cycle performance at 55°C in comparison with those of the bare LiFePO₄/C material against Li anode. After 700 cycles, the discharge capacity of LiFePO₄/C-PPy could still remain 110 mA h g⁻¹ with the retention of 82% at 5 C rate at 55°C. This could be ascribed to the fact that PPy coating on LiFePO₄/C could significantly improve the ionic conductivity of the LiFePO₄/C-PPy composite and could greatly reduce the electrode resistance. Furthermore, the PPy coating on LiFePO₄/C could effectively decrease the dissolution of Fe in the LiPF₆ electrolyte and subsequently suppress the reduction of Fe ions on anode.     

Effects of molecular weight,heating rate,synthetic temperature and sintering duration on electrochemical properties of a LiFePO<Subscript>4</Subscript>/C cathode material pyrolyzed from lithium polyacrylate

A LiFePO₄/C composite was obtained by a polymer pyrolysis reduction method, using lithium polyacrylate (LiPAA) as carbon source and fractional lithium source, and FePO₄·2H₂O as iron and phosphorus source. The structure of the LiFePO₄/C composites was investigated by X-ray diffraction (XRD). The micromorphology of the precursors and LiFePO₄/C powders was observed using scanning electron microscopy (SEM). Laser particle analyzer and BET were also used to characterize the materials. It was found that the micromorphology, particle size distribution and specific surface area of LiFePO₄/C composites were greatly influenced by the molecular weight of LiPAA. The electrochemical properties of the LiFePO₄/C composites were evaluated by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and constant current charge/discharge cycling tests. The results showed that the molecular weight of LiPAA, heating rate, synthetic temperature and sintering duration directly affected the electrochemical properties of LiFePO₄/C composites. The sample with the optimized electrochemical properties were obtained in the following conditions, i.e., LiPAA with the molecular weight of 20,000, heating rate of 10 °C min⁻¹, synthetic temperature of 700 °C and sintering duration of 15 h.     

Effect of synthesis temperature and molar ratio of organic lithium salts on the properties and electrochemical performance of LiFePO4/C composites

LiFePO₄/C cathode materials were synthesized through in situ solid-state reaction route using Fe₂O₃, NH₄H₂PO₄, Li₂C₂O₄, and lithium polyacrylate as raw materials. The precursor of LiFePO₄/C was investigated by thermogravimetric/differential thermal analysis. The effects of synthesis temperature and molar ratio of organic lithium salts on the performance of samples were characterized by X-ray diffraction, scanning electron microscopy, electrochemical impedance spectra, cyclic voltammogram, and constant current charge/discharge test. The sample prepared at optimized conditions of synthesis temperature at 700 °C and molar ratio with 1.17:1 exhibits excellent rate performance and cycling stability at room temperature.     

Graphite and Solvothermally Synthesized LiFePO<Subscript>4</Subscript>/Graphene Electrode for Soft-Packed Cell

Well-crystallized and nano-sized LiFePO₄/graphene composite have been successfully synthesized by in-situ disperse graphene oxide (GO) in precursor via a rapid microwave-solvothermal process at 200°C within 10 min. In spite of the low synthesis temperature, the structural and morphological properties of as-prepared composites are of high specific capacity, an excellent high rate capability, and stable cycling performance. In comparison with LiFePO₄/grahite soft-packed full-cell, the assembled soft-packed full-cell with solvothermally synthesized LiFePO₄/graphene composite and graphite electrode show better cycle performances prepared at higher temperature.     

Development and implementation of a high temperature electrochemical cell for lithium batteries

An electrochemical cell designed to perform high temperature lithium battery tests has been developed adapting a typical Swagelok^® cell. The high temperature cell is intended to work in a wide temperature range, namely from room temperature up to 300 °C. It has been successfully tested at 250 °C using LiFePO₄ as cathode, LiTFSI as molten salt electrolyte and metallic lithium as anode.     

Optimising organic ionic plastic crystal electrolyte for all solid-state and higher than ambient temperature lithium batteries

Organic ionic plastic crystal (OIPC) electrolytes are among the key enabling materials for solid-state and higher than ambient temperature lithium batteries. This work overviews some of the parameter studies on the Li|OIPC interface using lithium symmetrical cells as well as the optimisation and performance of Li|OIPC|LiFePO₄ cells. The effects of temperature and electrolyte thickness on the cycle performance of the lithium symmetrical cell, particularly with respect to the interfacial and bulk resistances, are demonstrated. Whilst temperature change substantially alters both the interfacial and bulk resistance, changing the electrolyte thickness predominantly changes the bulk resistance only. In addition, an upper limit of the current density is demonstrated, above which irreversible processes related to electrolyte decomposition take place. Here, we demonstrate an excellent discharge capacity attained on LiFePO₄|10 mol% LiNTf₂-doped [C₂mpyr][NTf₂]|Li cell, reaching 126 mAh g^-1 at 50 °C (when the electrolyte is in its solid form) and 153 mAh g^-1 at 80 °C (when the electrolyte is in its liquid form). Most remarkably, at high temperature operation, the capacity retention at long cycles and high current is excellent with only a slight (3%) drop in discharge capacity upon increasing the current from 0.2 C to 0.5 C. These results highlight the real prospects for developing a lithium battery with high temperature performance that easily surpasses that achievable with even the best contemporary lithium-ion technology.     

LiFePO4在饱和LiNO3溶液中的锂化行为

锂离子电池是目前应用最广泛的二次电池,均利用有机电解液。然而,有机体系锂离子电池存在易燃、易爆的安全隐患,限制了其使用范围。水溶液锂离子电池作为一类新型的二次电池[1￣10],使用水溶液电解液代替有机电解液,消除了因有机电解液与电极材料反应形成枝晶可能造成的燃烧、爆炸等安全隐患,使其在低电压电池如铅酸电池、碱锰电池等领域的应用有很大的竞争潜力[10]。目前,大量研究集中在选择合适的电极材料来组装水溶液锂离子电池,文献报道的水溶液锂离子电池正极材料主要有LiMnO4[1￣9]、LiNi1-xCoO2[10],但是LiMnO4在循环约20次后容…     

Cycling a Lithium Metal Anode at 90 °C in a Liquid Electrolyte

Stable operation at elevated temperature is necessary for lithium metal anode. However, Li metal anode generally has poor performance and safety concerns at high temperature (>55 °C) owing to the thermal instability of the electrolyte and solid electrolyte interphase in a routine liquid electrolyte. Herein a Li metal anode working at an elevated temperature (90 °C) is demonstrated in a thermotolerant electrolyte. In a Li|LiFePO₄ battery working at 90 °C, the anode undergoes 100 cycles compared with 10 cycles in a practical carbonate electrolyte. During the formation of the solid electrolyte interphase, independent and incomplete decomposition of Li salts and solvents aggravate. Some unstable intermediates emerge at 90 °C, degenerating the uniformity of Li deposition. This work not only demonstrates a working Li metal anode at 90 °C, but also provides fundamental understanding of solid electrolyte interphase and Li deposition at elevated temperature for rechargeable batteries.     

Cycling a Lithium Metal Anode at 90 °C in a Liquid Electrolyte

Stable operation at elevated temperature is necessary for lithium metal anode. However, Li metal anode generally has poor performance and safety concerns at high temperature (>55 °C) owing to the thermal instability of the electrolyte and solid electrolyte interphase in a routine liquid electrolyte. Herein a Li metal anode working at an elevated temperature (90 °C) is demonstrated in a thermotolerant electrolyte. In a Li|LiFePO₄ battery working at 90 °C, the anode undergoes 100 cycles compared with 10 cycles in a practical carbonate electrolyte. During the formation of the solid electrolyte interphase, independent and incomplete decomposition of Li salts and solvents aggravate. Some unstable intermediates emerge at 90 °C, degenerating the uniformity of Li deposition. This work not only demonstrates a working Li metal anode at 90 °C, but also provides fundamental understanding of solid electrolyte interphase and Li deposition at elevated temperature for rechargeable batteries.     

Enhanced carbon dioxide adsorption performance and kinetic study of K and Al co-doped Li4SiO4

Effective K and Al incorporation in Li₄SiO₄ leads to the broadened adsorption temperature range and enhanced carbon dioxide adsorption performance.     

Optimization of preparing LiFePO<Subscript>4</Subscript> for lithium-ion batteries via carbothermal reduction route

Lithium iron phosphate olivines (LiFePO₄) have been considered as very promising cathode for lithium-ion batteries due to their energy storage capacity combined with electrochemical and thermal stability. A key issue in synthesizing this materials is to optimize the synthetic conditions for obtain materials with excellent electrochemical properties. Here, we report full studies that investigate the synthesis of the LiFePO₄ by promising carbothermal reduction methods to prepare LiFePO₄ coated with pyrolytic carbon. Variation of the synthesis parameters showed that the materials synthesized at 700°C for 12 h have appropriate particles size and electronically conductive carbon. This makes it have better performances than others prepared at different temperature.     

Low temperature synthesis of Li5La3Nb2O12 with cubic garnet-type structure by sol–gel process

A cubic Li₅La₃Nb₂O₁₂ phase with a garnet framework was synthesized by the sol–gel process, in which lithium hydroxide, niobium oxide and acetic lanthanum were used as starting materials, while water was used as solvent. Pure garnet-like Li₅La₃Nb₂O₁₂ powders were obtained after heating the gel precursor at 700 °C for 6 h with 10 % excess lithium salt. The calcination temperature is nearly 250 °C lower than that by the solid state reaction. The phase transforms from cubic to tetragonal symmetry with loss of lithium at 717 °C, but the garnet framework remains stable to above 900 °C. A pellet annealed at 900 °C for 6 h had a room-temperature Li⁺-ion conductivity σ_Li (22 °C) = 1.0 × 10^?5 S cm^?1, a little higher than that attained by solid-state synthesis. The Li₅La₃Nb₂O₁₂ compound was chemically stable against two commonly used cathode materials, LiMn₂O₄ and LiCoO₂, up to 900 °C and against metallic lithium.