Temperature characterization analysis of LiFePO<Subscript>4</Subscript>/C power battery during charging and discharging

In order to study the surface temperature change and distribution during charging and discharging and in the simulation working condition of LiFePO₄/C power battery at normal temperature, the surface temperature is tested by placing the battery in the incubator and fixing 10 temperature probes on the battery surface. Results show that the temperature of the upper part is higher, and the temperature at the bottom is the lowest, while around the positive electrode is the highest during charging and discharging. The maximum temperature rising rate is reached at the moment of constant current charging transforming to the constant voltage charging during charging, and at the end moment during discharging. During charging in a certain range and discharging, the relations between the maximum temperature, the average temperature rising rate, and the maximum temperature difference of all the measurement points at the same time and the current are approximately linear, respectively. In the simulation working condition, the moment of the maximum temperature is consistent with the large current discharging instantaneous in each stage.     

Preparation of LiFePO<Subscript>4</Subscript>/graphene composites by microwave-assisted solvothermal method

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 present high specific capacity, an excellent high rate capability, and stable cycling performance.The short reaction times of just 10 min show the basis for an efficient and time-saving synthesis of LiFePO₄ρaphene composite.     

Particle size effects of carbon sources on electrochemical properties of LiFePO<Subscript>4</Subscript>/C composites

Carbon-coated LiFePO₄ cathode materials were prepared by a solid-state method incorporating different sizes of polystyrene (PS) spheres as carbon sources. In scanning electron microscope images, small PS spheres appear more effective at preventing aggregation of LiFePO₄ particles. From transmission electron microscopy images, it was found that the LiFePO₄ particles were completely uniformly coated with 5-nm carbon layer when the carbon source was 0.22 μm PS spheres. When the size of PS sphere was increased to 2.75 μm, a network of carbon was formed and wrapped around the LiFePO₄ to create a conductive web. Raman spectroscopy and four-point probe conductivity measurement showed that using larger sizes of PS spheres as carbon sources leads to greater conductivity of LiFePO₄/C. The LiFePO₄ precursor sintered with 0.22 μm PS spheres delivered an initial discharge capacity of 145 mAh g^?1 at a 0.2 C rate, but it only sustained 289 cycles at 80% capacity. When the diameter of PS spheres was increased to 2.75 μm, the discharge capacity of LiFePO₄/C decreased, but the cycle life reached 755 cycles, the highest number in this work probably due to the network formation of carbon wrapping around LiFePO₄ particles.     

Synthesis and characterization of macroporous Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C composites as cathode materials for Li-ion batteries

The macroporous Li₃V₂(PO₄)₃/C composite was synthesized by oxalic acid-assisted carbon thermal reaction, and the common Li₃V₂(PO₄)₃/C composite was also prepared for comparison. These samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and electrochemical performance tests. Based on XRD and SEM results, the sample has monoclinic structure and macroporous morphology when oxalic acid is introduced. Electrochemical tests show that the macroporous Li₃V₂(PO₄)₃/C sample has a high initial discharge capacity (130 mAh g⁻¹ at 0.1 C) and a reversible discharge capacity of 124.9 mAh g⁻¹ over 20 cycles. Moreover, the discharge capacity of the sample is still 91.5 mAh g⁻¹, even at a high rate of 2 C, which is better than that of the sample with common morphology. The improvement in electrochemical performance should be attributed to its improved lithium ion diffusion coefficient for the macroporous morphology, which was verfied by cyclic voltammetry and electrochemical impedance spectroscopy.     

LiFePO<Subscript>4</Subscript>/CA cathode nanocomposite with 3D conductive network structure for Li-ion battery

A novel LiFePO₄/Carbon aerogel (LFP/CA) nanocomposite with 3D conductive network structure was synthesized by using carbon aerogels as both template and conductive framework, and subsequently wet impregnating LiFePO₄ precursor inside. The LFP/CA nanocomposite was characterized by X-ray diffraction (XRD), TG, SEM, TEM, nitrogen sorption, electrochemical impedance spectra and charge/discharge test. It was found that the LFP/CA featured a 3D conductive network structure with LiFePO₄ nanoparticles ca. 10–30 nm coated on the inside wall of the pore of CA. The LFP/CA electrodes delivered discharge capacity for LiFePO₄ of 157.4, 147.2, 139.7, 116.3 and 91.8 mA h g⁻¹ at 1 °C, 5 °C, 10 °C, 20 °C and 40 °C, respectively. In addition, the LFP/CA electrode exhibited good cycling performance, which lost less than 1% of discharge capacity over 100 cycles at a rate of 10 °C. The good high rate performances of LiFePO₄ were attributed to the unique 3D conductive network structure of the nanocomposite.     

Aging property for LiFePO<Subscript>4</Subscript>/graphite cell with different temperature and DODs

In this paper, we fabricated 1.7 A h soft-packed cells using commercial-grade LiFePO₄ and manmade graphite as the active materials for the cathode and anode, respectively. It has been shown that the cycle performances of assembled soft-packed full-cell were still temperature-dependent. An accelerated mechanism of the operating temperature to reformation/repairing of SEI layer have been established, which greatly consumes active lithium during cycling, therefore causes fast capacity loss at elevated temperatures. At same time, cycle property for LiFePO₄/graphite cell with different depth of discharge (DOD) levels and ranges. It has been shown that DOD level has very little effect on capacity fade for cell lifecycle; but for DOD range, obvious influence was observed on capacity fade, which is due to the sensitivity of SOC during the storage of the cell.     

Kinetics of Li-ion transfer reaction at LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>, LiCoO<Subscript>2</Subscript>, and LiFePO<Subscript>4</Subscript> cathodes

Kinetics of LiFePO₄, LiMn₂O₄, and LiCoO₂ cathodes operating in 1 M LIPF₆ solution in a mixture of ethylene carbonate and dimethyl carbonate was deduced from impedance spectra taken at different temperatures. The most striking difference of electrochemical impedance spectroscopy (EIS) curves is the impedance magnitude: tens of ohms in the case of LiFePO₄, hundreds of ohms for LiMn₂O₄, and thousands of ohms for LiCoO₂. Charge transfer resistances (R _ct) for lithiation/delitiation processes estimated from the deconvolution procedure were 6.0 Ω (LiFePO₄), 55.4 Ω (LiCoO₂), and 88.5 Ω (LiMn₂O₄), respectively. Exchange current density for all the three tested cathodes was found to be comparable (0.55–1·10^?2 mAcm^?2, T = 298 K). Corresponding activation energies for the charge transfer process, \( {E}_{ct}^{\#} \), differed considerably: 66.3, 48.9, and 17.0 kJmol^?1 for LiMn₂O₄, LiCoO₂, and LiFePO₄, respectively. Consequently, temperature variation may have a substantial influence on exchange current densities (j _o) in the case of LiMn₂O₄ and LiCoO₂ cathodes.     

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.     

Nearly monodispersed LiFePO<Subscript>4</Subscript>F nanospheres as cathode material for lithium ion batteries

Tavorite-structured lithium transition metal fluorophosphates have been considered as a good alternative to olivine-type cathode for lithium-ion batteries due to its exceptional ionic conductivity and excellent thermal stability. In this work, nearly monodisperse LiFePO₄F nanospheres with high purity are successfully synthesized by a solid-state route associated with chemically induced precipitation method for the first time. The synthesized LiFePO₄F presents nearly monodisperse nanospheres particles with average particle size of ~?500 nm. Cyclic voltammetry data exhibit a clear indication of the Fe³⁺/Fe²⁺ redox couple that involves a two-phase transition. Its electrochemical behaviors are examined by galvanostatic charge-discharge. The results show that the initial discharge capacity is 110.2 mAh g^?1 at 0.5 C, after 200 cycles is still retained 104.0 mAh g^?1 with the retention rate of 94.4%. The excellent cycle performance is mainly attributed to the uniform nanospheres-like morphology which is not only beneficial to shorten the transport distance of ions and electrons, but also improve the interface area between electrode and electrolyte, and thus improve the kinetics of Li ions.     

The electrochemical properties of nano-LiFeBO<Subscript>3</Subscript>/C as cathode materials for Li-ion batteries

Carbon-coated LiFeBO₃ is prepared by sol-gel method, and polyethylene glycol 6000 (PEG-6000) is chosen as carbon source. Compared with conventional solid-state method, the LiFeBO₃/C nano-particles were obtained at a relatively lower temperature (500 °C). It was demonstrated that the prepared LiFeBO₃ is a highly pure monoclinic structure with space group of C2/c, the primary particle size of the carbon-coated LiFeBO₃ is about 20–50 nm, and the thickness of carbon coating is about 6.2 nm. In comparison with the LiFeBO₃/C with lower carbon content, the LiFeBO₃/C with higher carbon content exhibits better electrochemical properties. The specific capacities of LiFeBO₃/C are 206.9, 181.3, and 166.1 mAh g^?1 after 50 cycles in the range of 5, 40, and 100 mA g^?1 current density, respectively. Particularly, the LiFeBO₃/C with higher carbon content is demonstrated to own great structural stability, which can maintain its original crystal structure and Li storage properties even after 6 months of air exposure at room temperature.     

Electrochemical performance of LiFePO<Subscript>4</Subscript>/C doped with F synthesized by carbothermal reduction method using NH<Subscript>4</Subscript>F as dopant

The effect of fluorine doping on the electrochemical performance of LiFePO₄/C cathode material is investigated. The stoichiometric proportion of LiFe(PO₄)_1−x F_3x /C (x = 0.01, 0.05, 0.1, 0.2) materials was synthesized by a solid-state carbothermal reduction route at 650 °C using NH₄F as dopant. X-ray diffraction, scanning electron microscope, energy-dispersive X-ray, and X-ray photoelectron spectroscopy analyses demonstrate that fluorine can be incorporated into LiFePO₄/C without altering the olivine structure, but slightly changing the lattice parameters and having little effect on the particle sizes. However, heavy fluorine doping can bring in impurities. Fluorine doping in LiFePO₄/C results in good reversible capacity and rate capability. LiFe(PO₄)_0.95 F_0.15/C exhibits highest initial capacity and best rate performance. Its discharge capacities at 0.1 and 5 C rates are 156.1 and 119.1 mAh g⁻¹, respectively. LiFe(PO₄)_0.95 F_0.15/C also presents an obviously better cycle life than the other samples. We attribute the improvement of the electrochemical performance to the smaller charge transfer resistance (R _ct) and influence of fluorine on the PO₄³⁻ polyanion in LiFePO₄/C.     

Preparation and characterization of LiFePO<Subscript>4</Subscript> from NH<Subscript>4</Subscript>FePO<Subscript>4</Subscript>·H<Subscript>2</Subscript>O under different microwave heating conditions

Olivine-type LiFePO₄ is a very promising polyanion-type cathode material for lithium-ion batteries. In this work, LiFePO₄ with high specificity capacity is obtained from a novel precursor NH₄FePO₄·H₂O via microwave processing. The grains grow up in the duration of sintering until they reach the decomposition temperature. The apparent conductivity of the samples rises rapidly with the irradiation time and influences the electrochemical performance of the material greatly at high current density. As a result, the LiFePO₄ cathode material obtained with a sintering time of 15 min has good electrochemical performance. Between 2.5 and 4.2 V versus Li, a reversible capacity is as high as 156 mAh g⁻¹ at 0.05 C.     

Carbon nanotubes as a conductive additive in LiFePO<Subscript>4</Subscript> cathode material for lithium-ion batteries

The electrochemical properties and cyclic performances of commercial LiFePO₄ cathode material with different ratio of carbon black (CB) and carbon nanotubes (CNTs) as conductive material were tested in this study. Compared with other samples, the sample with 3 wt % CNTs exhibited the best electro-chemical and cyclic performances at various discharging rate at room temperature; and adhesion strength of electrode was improved by adding CNTs. The enhanced electrode performance may due to the unique natures of CNTs and the contact area of CNTs with active material or current collector.     

Facile synthesis of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> microsheets with porous micro-nanostructure as high-rate cathode materials for Li-ion batteries

Porous LiMn₂O₄ microsheets with micro-nanostructure have been successfully prepared through a simple carbon gel-combustion process with a microporous membrane as hard template. The crystal structure, morphology, chemical composition, and surface analysis of the as-obtained LiMn₂O₄ microsheets are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscope (XPS). It can be found that the as-prepared LiMn₂O₄ sample presents the two-dimensional (2-D) sheet structure with porous structure comprised with nano-scaled particles. As cathode materials for lithium-ion batteries, the obtained LiMn₂O₄ microsheets show superior rate capacities and cycling performance at various charge/discharge rates. The LiMn₂O₄ microsheets exhibit a higher charge and discharge capacity of 137.0 and 134.7 mAh g^?1 in the first cycle at 0.5 C, and it remains 127.6 mAh g^?1 after 50 cycles, which accounts for 94.7% discharge capacity retention. Even at 10 C rate, the electrode also delivers the discharge capacity of 91.0 mAh g^?1 after 300 cycles (93.5% capacity retention). The superior electrochemical properties of the LiMn₂O₄ microsheets could be attributed to the unique microsheets with porous micro-nanostructure, more active sites of the Li-ions insertion/deinsertion for the higher contact area between the LiMn₂O₄ nano-scaled particles and the electrolyte, and better kinetic properties, suggesting the applications of the sample in high-power lithium-ion batteries.     

Polyethylene Glycol for LiFePO<Subscript>4</Subscript>/C Composites Preparation: Large or Small Molecular Weight

Olivine LiFePO₄ is challenged by its poor electronic and ionic conductivities for lithium-ion batteries. Polyethylene glycol (PEG) has been applied for LiFePO₄ preparation by different research groups, but there is no consensus on the influence of the mean molecular weight of PEG on the structure and electrochemical performances of LiFePO₄/C composites. In this work, LiFePO₄/C composites were prepared by using micronsized FePO₄·2H₂O powder as starting material, PEG (mean molecular weight of 200, 400, 4000 or 10000) and citric acid as complex carbon source. The structure and electrochemical performances of LiFePO₄/C composites would be decided considerably by the mean molecular weight of PEG, and the sample using PEG200 exhibited the least inter-particle agglomeration, the smallest charge transfer resistance and the highest discharge capacity. A probable growth mechanism is also proposed based on SEM images and electrochemical results: with the assistance of citric acid, PEG molecule with small molecular weight tends to cover one or only a few micron-sized FePO₄·2H₂O particles, significantly suppress the agglomeration of primary LiFePO₄ particles and thus result in uniform particle-size distribution and carbon coating.     

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.     

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.     

Effect of carbon sources on the morphology of LiFePO<Subscript>4</Subscript> cathode materials for lithium ion batteries

The LiFePO₄ materials were prepared by incorporating conductive carbon from pyrolysising three different carbon sources (acetylene black, glucose and phonetic resin). The morphology LiFePO₄/C was investigated by the SEM. Results revealed that the carbon precursor has much effect on the morphology of the samples. The carbon coated LiFePO₄ showed much better performance in terms of the discharge capacity and rate capability than the bare LiFePO₄. Among the carbon coated LiFePO₄, the particles coated with phonetic resin residual carbon exhibited better electrochemical properties than others and a proper mechanism was proposed.     

Preparation and electrochemical properties of Li<Subscript>0.97</Subscript>Er<Subscript>0.01</Subscript>FePO<Subscript>4</Subscript>/C composite for lithium-ion batteries

Li_0.97Er_0.01FePO₄/C composite was prepared by solid-state reaction, using particle modification with amorphous carbon from the decomposition of glucose and lattice doping with supervalent cation Er³⁺. All samples were characterized by X-ray diffraction, scanning electron microscopy, multi-point Brunauer Emmett and Teller methodes. The electrochemical tests show Li_0.97Er_0.01FePO₄/C composite obtains the highest discharge specific capacity of 154 mAh g⁻¹ at C/10 rate and the best rate capability. Its specific capacity reaches 131 mAh g⁻¹ at 2C rate. Its capacity loss is only 14.9 % when the rate varies from C/10 to 2C.     

A novel method with low-cost Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> to synthesize small size LiFePO<Subscript>4</Subscript> by carbothermal reduction method

The phase-pure LiFePO₄/C cathode materials have been successfully fabricated by precursor containing Fe₂O₃ coated with polyaniline through carbothermal reduction method. The polyaniline coating at Fe₂O₃ could restrict the growth of crystal and subsequently become electric carbon at higher synthesis temperature. Compared with conventional carbothermal reduction method, the sample showed a better capacity and less than 3% capacity fading after 30 cycles at various charge and discharge rate.