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 performance of LiFePO4 cathode material coated with ZrO2 nanolayer

ZrO₂ nanolayer coated LiFePO₄ particles were successfully prepared by a chemical precipitation method. Nanolayer structured ZrO₂ was found on the surface of LiFePO₄ particles by high resolution transmission electron microscopy (HRTEM). The coating does not affect the crystal structure of the LiFePO₄ core, as determined by X-ray diffraction (XRD) and selected area electron diffraction (SAED) on individual particles. The ZrO₂ coating can remarkably improve the electrochemical performance at high charge/discharge rate. This improvement may be due to the amelioration of the electrochemical dynamics on the LiFePO₄ electrode/electrolyte interface resulting from the effects of the ZrO₂ nanolayer coating.     

Synthesis of nano-LiFePO4 particles with excellent electrochemical performance by electrospinning-assisted method

Nanoscale LiFePO₄/C particles are synthesized using a combination of electrospinning and annealing. The important advantages of electrospinning technique are the production of separated nanofiber precursor, enabling the precursor particles arrangement to be changed, impeding the growth and agglomeration of the LiFePO₄ particles during the heat treatment, and contributing to the formation of nanosized LiFePO₄ particles. In this study, polyvinylpyrrolidone (PVP) is used as the fiber-forming agent in the electrospinning method, and also provides a reducing agent and carbon source. In situ carbon-coated LiFePO₄ particles are obtained by the pyrolysis of PVP during the thermal treatment. The LiFePO₄ particles are coated with and connected by interlaced carbons, and are uniformly distributed in the size range 50–80?nm. It is found that the as-prepared nanoscale LiFePO₄/C composite has a desirable electrochemical performance. It has discharge capacities of 163.5?mA?h g^?1 and 110.7?mA?h g^?1 at rates of 0.1?C and 10?C, respectively. In addition, this cathode has excellent cyclability with a capacity loss of less than 3?% at 0.1?C and 5?% at 5?C after 500 cycles. An effective synthesis and processing method is presented for obtaining nanosized LiFePO₄ with high electrochemical performance.     

Study of hydrothermally synthesized LiFePO4 with different morphology

Monodispersed LiFePO₄ particles with different morphology have been successfully obtained through hydrothermal method using same starting materials. The morphology of the as-synthesized LiFePO₄ particles were dependent on feeding sequence of starting materials. Different dissolution rate of solid phase in precursor prepared by reversing feeding sequence lead to different morphology. The half-cell using as-obtained LiFePO₄ with different morphology were assembled to investigate electrochemical performance. The results show that LiFePO₄ with nanorods morphology displays a higher initial discharge capacity and excellent cycle retention than the sample with microplates morphology.

     

Surface treatment of LiFePO4 cathode material with PPy/PEG conductive layer

In this work, we studied LiFePO₄ particles coated with thin films of highly conductive polypyrrole (PPy) and their electrochemical performance in cathode layers of lithium cells. Carbon-free LiFePO₄ particles were synthesized by a solvothermal method. Besides this, a part of the experiments were carried out on commercial carbon-coated LiFePO₄ for comparison. Polypyrrole coated LiFePO₄ particles (PPy-LiFePO₄) were obtained by a straightforward oxidative polymerization of dissolved pyrrole on LiFePO₄ particles dispersed in water. The use of polyethylene glycol (PEG) as an additive during the polymerization was decisive to achieve high electronic conductivities in the final cathode layers. The carbon-free and carbon-coated LiFePO₄ particles were prepared with PPy and with PPy/PEG coating. The obtained PPy-LiFePO₄ and PPy/PEG-LiFePO₄ powders were characterized by SEM, EIS, cyclic voltammetry, and galvanostatic charge/discharge measurements in lithium-ion cells with lithium metal as counter and reference electrode. Carbon-free LiFePO₄ coated with PPy/PEG hybrid films exhibited very good electrode kinetics and a stable discharge capacity of 156 mAh/g at a rate of C/10. Impedance measurements showed that the PPy/PEG coating decreases the charge-transfer resistance of the corresponding LiFePO₄ cathode material very effectively, which was attributed to a favorable mixed ionic and electronic conductivity of the PPy/PEG coatings.     

Facile low-temperature polyol process for LiFePO4 nanoplate and carbon nanotube composite

Crystalline LiFePO₄ nanoplates were incorporated with 5 wt.% multi-walled carbon nanotubes (CNTs) via a facile low temperature polyol process, in one single step without any post heat treatment. The CNTs were embedded into the LiFePO₄ particles to form a network to enhance the electrochemical performance of LiFePO₄ electrode for lithium-ion battery applications. The structural and morphological characters of the LiFePO₄–CNT composites were investigated by X-ray diffraction, Fourier Transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were analyzed by cyclic voltammetry, electrochemical impedance spectroscopy and charge/discharge tests. Primary results showed that well crystallized olivine-type structure without any impurity phases was developed, and the LiFePO₄–CNT composites exhibited good electrochemical performance, with a reversible specific capacity of 155 mAh g⁻¹ at the current rate of 10 mA g⁻¹, and a capacity retention ratio close to 100% after 100 cycles.     

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.     

Improved lithium exchange at LiFePO4 cathode particles by coating with composite polypyrrole–polyethylene glycol layers

The polypyrrole–LiFePO₄ composites were synthesized by simple chemical oxidative polymerization of pyrrole (Py) monomer directly on the surface of LiFePO₄ particles. Properties of resulting polypyrrole–LiFePO₄ (PPy-LiFePO₄) samples (especially conductivity) are strongly affected by the preparation technique, polymer additives, and conditions during synthesis. For increasing of PPy-LiFePO₄ conductivity, we used polyethylene glycol (PEG) as additive during polymerization. The electrochemical behavior of the samples was examined by cyclic voltammetry and electrochemical impedance spectroscopy. It was found that PPy/PEG composite polymer decreased the particle to particle contact resistance. Impedance measurements showed that the coating of PPy/PEG significantly decreases the charge transfer resistance of LiFePO₄ electrodes.     

多元醇法合成具有不同长径比的棒状LiFePO4/C材料

以三价铁盐为铁源,采用多元醇还原法在低温下制备出了具有不同长径比的棒状LiFePO₄材料. 通过X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、循环伏安(CV)、交流阻抗谱(EIS)和恒电流充放电测试等手段分析了不同回流反应时间下制备出的前驱体和最终的LiFePO₄/C 样品. 结果表明：回流反应时间对LiFePO₄的形貌和特性有明显的影响. 通过把回流反应时间从4 h延长至16 h,材料的形貌由不规则的短棒状颗粒变为规则的长棒状颗粒,且棒的直径明显变小. 当回流反应时间为10 h 时,样品复合了多种形貌,有利于电子的传输,在低倍率下具有优秀的性能,0.1C放电比容量为163 mAh·g^-1;当回流反应时间为16 h 时,样品具有最大的长径比,有利于锂离子的扩散,在高倍率下具有良好的性能,1C、3C、5C、10C、20C倍率下放电比容量分别为135、125、118、110、98 mAh·g^-1,循环性能良好,几乎无衰减.     

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.     

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.     

Preparation of nano-structured LiFePO4/graphene composites by co-precipitation method

Graphene materials with superior electrical conductivities and high surface area would be advantageous for application in energy storage. And LiFePO₄ has been a promising electrode material however its poor conductivity limits its practical application. To improve the electronic conductivity, we prepare LiFePO₄/graphene composites in a co-precipitation method, in which graphene nanosheets are used as additives. The composites were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), and their electrochemical properties were investigated by galvanostatic charge and discharge tests. The experimental results show that the capacity delivery and cycle performances of LiFePO₄ could be improved considerably by adding graphene. Therefore, LiFePO₄/graphene composites are a promising candidate for lithium secondary batteries.     

Biosynthesis and electrochemical characteristics of LiFePO4/C by microwave processing

The yeast cells are adopted as a template and cementation agent to prepare LiFePO₄/C with high surface area by co-precipitation and microwave processing. The electrochemical properties of the resultant products are investigated. The synthesized LiFePO₄/C is characterized by means of X-ray diffraction, transmission electron microscopy (TEM), Brunauer–Emmett–Teller method, and battery test instrument. The LiFePO₄/C particles with average size of 35-100 nm coated by porous carbon are observed by TEM. The LiFePO₄/C, with the specific surface area of 98.3 m²/g, exhibits initial discharge specific capacity of 147 mAh/g and good cycle ability. The yeast cells as a template are used to synthesize the precursor LiFePO₄/cells compounds. In microwave heating process, the use of yeast cells as reducing matter and cementation agent results in the enhancement of the electrochemical properties.     

Study of LiFePO4 synthesized using a molten method with varying stoichiometries

The synthesis of LiFePO₄ from a melt cast of Li₂CO₃, FePO₄, and carbon precursors at 1,000 °C was recently reported. The results indicate that it could be a competitive technique for the large-scale production of LiFePO₄. This paper focuses on particle size reduction and non-stoichiometric synthesis of LiFePO₄. The particles size was reduced using a planetary mill that is available in most research laboratories where the milling time and milling media were varied to obtain the best electrochemical results. In addition, the electrochemical performance of LiFePO₄ products synthesized from FePO₄ + x/2 Li₂CO₃ + C {\hbox{FeP}}{{\hbox{O}}_4} + x{/2}\,{\hbox{L}}{{\hbox{i}}_2}{\hbox{C}}{{\hbox{O}}_3} + {\hbox{C}} (sequentially varying the amount of x) at 1,000 °C has also been measured. The stoichiometric LiFePO₄ product shows the best capacity, while the off-stoichiometric material demonstrates different levels of impurities that have an effect on the performance of the material. The results indicate that adequate capacity at low discharge rates can be obtained using standard milling techniques, but, in order to obtain material from a melt cast synthesis that provides higher performance at faster discharge rates, a further particle size reduction will be required.     

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.     

A novel network composite cathode of LiFePO4/multiwalled carbon nanotubes with high rate capability for lithium ion batteries

A novel network composite cathode was prepared by mixing LiFePO₄ particles with multiwalled carbon nanotubes for high rate capability. LiFePO₄ particles were connected by multiwalled carbon nanotubes to form a three-dimensional network wiring. The web structure can improve electron transport and electrochemical activity effectively. The initial discharge capacity was improved to be 155 mA h/g at C/10 rate (0.05 mA/cm²) and 146 mA h/g at 1C rate. The comparative investigation on MWCNTs and acetylene black as a conducting additive in LiFePO₄ proved that MWCNTs addition was an effective way to increase rate capability and cycle efficiency.     

Application of 7Li NMR to characterize the evolution of intercalated and non-intercalated lithium in LiFePO4-based materials for Li-ion batteries

Different synthesis batches of LiFePO₄/C materials were prepared, and their electrochemical properties as positive cathodes for lithium-ion batteries were evaluated. Using standard solid-state NMR conditions, such as a 7-mm magic-angle-spinning probe performing at low spinning rates, information on both intercalated and non-intercalated (stored on the grain boundaries) lithium was obtained. A sharp signal assigned to non-intercalated lithium could be observed by diluting the active material in silica. Correlations could be, thus, obtained between the amount of each type of lithium and the electrochemical history and state of the material, revealing that the relative amount of surface lithium in a pristine LiFePO₄/C material is rather constant and cannot be used as a criterion for its further specification. However, a drastic increase of this surface lithium was observed in the cathode materials of out-of-order batteries. As the cathode material recovered from the batteries after electrochemical testing was carefully washed before analysis, we can conclude that the non-intercalated lithium is strongly bound to the active material probably inside the so-called solid electrolyte interface layer at the surfaces of LiFePO₄ particles. This work illustrates that solid-state lithium NMR can allow rapid characterization and testing of LiFePO₄/C cathode materials.     

水热法制备树叶状LiFePO4/C复合正极材料

采用柠檬酸辅助水热法合成了高分散性树叶状LiFePO₄/C复合正极材料。利用X射线衍射、傅里叶红外光谱、扫描电镜、高分辨率透射电镜和选区电子衍射分析了材料的形貌结构。结果表明,柠檬酸对树叶状LiFePO₄/C复合材料的形成具有促进作用。该材料的最大暴露晶面为(010)晶面,且分散性较好。与颗粒状LiFePO₄/C材料相比,该材料呈现出更高的放电比容量和更好的倍率性能,在0.1C和5C倍率下,放电比容量分别为158和126mAh·g^-1,其原因是由于锂离子沿[010]方向的扩散距离缩短,从而使锂离子扩散系数显著增大。     

Effects of carbon sources and carbon contents on the electrochemical properties of LiFePO4/C cathode material

LiFePO₄/C composites are prepared by using two types of carbon source: one using polymer (PAALi) and the other using sucrose. The physical characteristics of LiFePO₄/C composites are investigated by X-ray diffraction), scanning electron microscopy, BET, laser particle analyzer, and Raman spectroscopy. Their electrochemical properties are characterized by cyclic voltammograms, constant current charge–discharge, and electrochemical impedance spectra. These analyses indicate that the carbon source and carbon content have a great effect on the physical and electrochemical performances of LiFePO₄/C composites. An ideal carbon source and appropriate carbon content can effectively increase the lithium-ion diffusion coefficient and exchange current density, decrease the charge transfer resistance (R _ct), and enhance the electrochemical performances of LiFePO₄/C composite. The results show that PAALi is a better carbon source for the synthesis of LiFePO₄/C composites. When the carbon content is 4.11 wt.% (the molar ratio of PAALi/Li₂C₂O₄ was 2:1), as-prepared LiFePO₄/C composite shows the best combination between electrochemical performances and tap density.     

用于可充锂电池的高倍率磷酸铁(II)锂正极材料

本文以聚氧化乙烯为碳源,用柠檬酸辅助湿化学法合成了高倍率的碳包覆的LiFePO₄。使用热重、粉末X射线衍射、扫描电子显微镜、透射电子显微镜、循环伏安、电化学阻抗和恒流充放电表征材料的结构和电化学性质。结果表明,该材料组成为5 wt%疏松多孔的碳包覆相纯度很高的小的LiFePO₄颗粒。该材料适用于高倍率充放电,在5 C、10 C和20C的放电倍率下可以分别得到120、90和60 mAh·g^-1的稳定容量。