Crystal structure and lithium electrochemical extraction properties of olivine type LiFePO4

Lithium iron phosphate (LiFePO₄) cathode material has been synthesized by a solid-state reaction. The XRD patterns and SEM images of the samples show that the LiFePO₄ compounds prepared at 650 °C by using carbon gel in reaction have a single-phase, small grain-size and regular shapes. By using Rietveld refinement method, we calculated the Li–O interatomic distance in LiO₆ octahedra and the cross section area of the lithium ion one-dimension tunnel, and analyze the reason of the improvement of the Lithium ion diffusion. The electrochemical test results of the sample show the LiFePO₄ prepared by using carbon gel exhibits excellent electrochemical properties. Such a significant improvement in electrochemical performance should be partly related to the enhanced Lithium ion diffusion and electric conductivity due to the use of carbon gel.     

Nonstoichiometric LiFePO4/C nanofibers by electrospinning as cathode materials for lithium-ion battery

Nonstoichiometric lithium iron phosphate/carbon (LiFePO₄/C) composite nanofibers are prepared by electrospinning and subsequent calcination. The ratio of raw materials exerts great effects on the morphology and electrochemical performance of LiFePO₄/C nanofibers, and the sample prepared using the LiH₂PO₄/FeC₆H₅O₇ ratio of 1.3 has good fibrous morphology, porous structure and high purity, thus exhibiting high capacity and stability for lithium-ion battery.     

A new approach to LiFePO4/C synthesis: The use of complex carbon source without ball milling

A new approach to synthesize LiFePO₄/C composite without the need of ball milling was developed by using complex carbon source. The esterification reaction between citric acid and sucrose during the drying process fosters the well-mixing of precursor materials, which contributes to the small particle size, porous network, large surface area and good electrochemical performances of LiFePO₄/C composite. The ratio of sucrose to citric acid affects the morphology and electrochemical performance of LiFePO₄/C. This study offers a new method to optimize the morphology of LiFePO₄/C composite by controlling the interaction between different precursors during precursor treatment, such as the esterification reaction between carboxylic acid and sugar. The method developed is simple, requires inexpensive starting materials and the final product gives good electrochemical performance; therefore, it may be of great interest in the mass production of LiFePO₄/C cathode material.     

Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO₄ powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H₂O, FeCl₂·4H₂O and H₃PO₄ stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N₂ + 3% H₂ gas atmosphere. Raman spectroscopy revealed that α-Fe₂O₃ thin layers were formed on the surface of as-prepared LiFePO₄ powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO₄ powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g⁻¹ at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO₄ powders, LiFePO₄/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO₄/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g⁻¹ at 0.1 C and 84 mAh g⁻¹ at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO₄/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO₄ powders were also discussed.     

用碳热还原法制备LiFePO4/C复合正极材料

以Fe2O3为铁源,以葡萄糖为碳添加剂,利用碳热还原法成功地制备了LiFePO4/C复合材料.研究了不同焙烧温度对样品性能的影响.利用X射线衍射仪、扫描电镜和碳硫(质量分数)分析方法对所得样品的晶体结构、表面形貌、含碳量进行分析研究.研究结果表明,样品中碳含量(质量分数)为10%的LiFePO4/C复合材料为单一的橄榄石型晶体结构, 碳的加入使LiFePO4 颗粒粒径减小.碳分散于晶体颗粒之间,增强了颗粒之间的导电性.电化学性能测试结果表明,LiFePO4/C充放电性能和循环性能都得到显著改善.其中,碳含量为10%在700℃下焙烧8h合成出的样品电化学性能最佳,在0.1、0.5和1C倍率下放电,LiFePO4/C首次放电比容量达159.3、137.0、130.6mAh/g,充放电循环30次,容量只衰减了2.2%、5.3%、7.6%.其表现出良好的循环性能.     

Intermittent microwave heating synthesized high performance spherical LiFePO4/C for Li-ion batteries

An intermittent microwave heating method was used to synthesize spherical LiFePO₄/C in the presence of glucose as reductive agent and carbon source without the use of the inert gas in the oven processes. The FePO₄ was used as iron precursor to reduce the cost and three lithium salts of Li₂CO₃, LiOH and CH₃COOLi were chosen for comparison of the resulting materials. The materials can be alternatively heated by this method at a temperature controllable mode for crystallization and phase transformation and to provide relaxation time for protecting particles growth. The X-ray diffraction and scanning electron microscope measurements confirmed that the LiFePO₄/C is olivine structured with the average particle size of 50-100 nm. The spherical LiFePO₄/C as cathode material showed better electrochemical performance in terms of the specific capacity and the cycling stability, which might be attributed to the highly crystallized phase, small particle distribution and improved conductivity by carbon connection.     

Synthesis,crystal structure and electrochemical properties of the manganese-doped LiNaFe[PO4]F materials

The new compounds LiNaFe_1−xMn_x[PO₄]F (x ≤ 1/4) were synthesized by a solid state reaction route. The crystal structure of LiNaFe_3/4Mn_1/4[PO₄]F was determined from single crystal X-ray diffraction data. LiNaFe_3/4Mn_1/4[PO₄]F crystallizes with the Li₂Ni[PO₄]F-type structure, space group Pnma, a = 10.9719(13), b = 6.3528(7), c = 11.4532(13) Å, V = 798.31(16) Å³, and Z = 8. The structure consists of edge-sharing (Fe_3/4Mn_1/4)O₄F₂ octahedra forming (Fe_3/4Mn_1/4)FO₃ chains running along the b-axis. These chains are interlinked by PO₄ tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The manganese-doped phases show poor electrochemical behavior comparing to the iron pure phase LiNaFe[PO₄]F.     

两步碳热还原方法制备LiFePO4/C阴极复合材料

橄榄石型的LiFePO4材料是一种具有良好发展潜力的锂离子电池阴极材料。应用一种两步烧结的碳热还原方法制备出LiFePO4阴极材料,该法缩短了高温烧结阶段的时间,从而达到抑制晶粒长大的目的,并对LiFePO4进行原位碳包覆,制得LiFePO4/C复合阴极材料。对制得的材料进行0.1C恒电流充放电测试,首次放电容量为149.4mAh/g,首次放电效率可以达到93.5%。而用作对比的一步法烧结碳热还原样品在0.1C恒流充放电试验中首次容量只有99.1mAh/g,放电效率是81.4%,并对制备反应及充放电结果的机理进行了探讨。     

Effect of pH value on particle morphology and electrochemical properties of LiFePO4 by hydrothermal method

Lithium iron phosphate was prepared by hydrothermal synthesis using LiOH·H₂O, FeSO₄·7H₂O and H₃PO₄ as raw materials. The effects of pH value of reaction solution on particle morphology and electrochemical property were investigated. The pH value of the reaction solution was adjusted in the range of 2.5-8.8 by dilute sulfuric acid and ammonia water. The samples were characterized by field-emission scanning electronic microscope (FE-SEM), X-ray powder diffraction (XRD), constant-current charge/discharge cycling tests and chemical analysis. The results indicated that the particles exhibited acute angle diamond flake-like morphology at pH = 2.5, and as the pH value increased, the particle became hexagon flake-like, round flake-like and irregular flake-like morphology gradually. The optimal sample synthesized at pH = 6.4 exhibited discharge capacities of 151.8 mAh g⁻¹ at 0.2 C rate and 129.3 mAh g⁻¹ at 3 C rate. It was found that pH value affected the morphologies and properties of the product by means of different crystal growth rates.     

High rate electrochemical performances of nanosized ZnO and carbon co-coated LiFePO4 cathode

The high rate electrochemical performances of ZnO and carbon co-coated LiFePO₄ have been studied by X-ray diffraction, high-resolution transmission electron microscope, electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic measurements. The carbon coated LiFePO₄ material was prepared by a freeze-drying method, and the diffusion coefficient and exchange current of these materials were calculated from their electrochemical impedance spectroscopy. The electrode delivered a reversible capacity of about 90% of the theoretical capacity when cycled between 2.5 and 4.2 V and showed stable cycle performance at high charge/discharge rates. This study showed that the co-coating process and freeze-drying method can effectively improve the electrochemical performances of LiFePO₄ materials.     

Electrochemical properties of facile emulsified LiV3O8 materials

LiV₃O₈ cathode materials are post-treated by a special emulsion method (termed “EM”) and then calcinated at different temperatures. The experimental results show that the structure of these oxides is different from LiV₃O₈ prepared by the solid-state reaction (acronym “STATE”) route, although their starting materials are identical. The EM product prepared at 500 °C exhibits a better electrochemical behavior than its counterpart prepared by traditional methods (STATE) or by EM at other temperatures. Its initial discharge capacity is 305 mAh g⁻¹, and it still maintains 250.2 mAh g⁻¹ after 100 cycles at 0.2 C at the voltage range of 1.8–4.0 V vs. Li/Li⁺.     

Preparation and characterization of Cr-doped LiMnO2 cathode materials by Pechini's method for lithium ion batteries

o-LiMnO₂ and LiMn_1−xCr_xO₂ (x = 0.05 and 0.1) powders are synthesized via Pechini's method. Crystalline structure, surface morphology, and electrochemical properties of the prepared powders are investigated by XRD, SEM, cyclic voltammetric and capacity retention studies. From results, it is found that a small amount of Cr-doping into LiMnO₂ cathode can not only lead to a topographic change from orthorhombic to monoclinic geometry and particle size reduction, but also improve cycling performance and reversible capacity.     

Synthesis of LiV3O8 by an improved citric acid assisted sol–gel method at low temperature

LiV₃O₈ was synthesized by the normal citric acid assisted sol–gel method and an improved citric acid assisted sol–gel method. The improved method compares with the normal method in detail by thermogravimetry (TG), FTIR, X-ray diffraction (XRD), scanning electron microscopy (SEM), charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Results show that the improved method can synthesize LiV₃O₈ successfully at much lower temperature than normal method.     

Fabrication of microspherical LiMnPO4 cathode material by a facile one-step solvothermal process

The microspherical LiMnPO₄ cathode material was successfully prepared for the first time by a simple one-step solvothermal process in the presence of critic acid. The reaction conditions (reactant concentration, reaction temperature) were used further to fabricate the size, surface coarseness and morphology of the microspherical LiMnPO₄. The as-prepared microspherical LiMnPO₄ at variant conditions exhibited remarkably different discharge capacity and rate capacity, indicating the potential of the suggested method in tuning the morphology and the structure of LiMnPO₄ to improve its electrochemical performance.     

A PEG assisted sol-gel synthesis of LiFePO4 as cathodic material for lithium ion cells

In order to obtain fine-particle LiFePO₄ with excellent electrochemical performance, LiFePO₄/C powders were synthesized by a poly(ethylene glycol) (PEG) assisted sol-gel method. All samples were characterized by X-ray powder diffraction and scanning electron microscopy, and their electrochemical properties were investigated by cycle voltammograms and charge-discharge tests. The sample, synthesized with the n_PEG/n_LFP = 1:1 under sintering temperature of 600 °C, possesses the global morphology and particle size of about 100 nm. This sample delivers the first discharge capacity of 162 mAh g⁻¹, i.e. 95.3% of the theoretical capacity, at the 15 mA g⁻¹ discharge current between 2.5 and 4.0 V (versus Li/Li⁺). The sample also displays a robust rate capability and stable cycle-life. The improved electrochemical performance originates mainly from the fine particle of nanometric dimension, regular global morphology and uniform dispersing in the product as well as the increased electronic conductivity by carbon coating.     

高倍率性能磷酸亚铁锂/碳正极材料的制备与表征

以水溶性酚醛树脂为碳源, Li₂CO₃为锂源, 纳米FePO₄前躯体为铁源和磷源, 以水为介质, 采用湿法研磨混合均匀, 然后通过高温固相法制备出纳米磷酸亚铁锂/碳(LiFePO₄/C)复合材料。采用XRD、SEM、TEM、TG和拉曼光谱对该复合材料进行了表征, 并研究了其电化学性能。结果表明, 制备的LiFePO₄/C纳米颗粒为类球形, 表面均匀地包覆了一层约5 nm厚的碳层, 作为锂离子电池正极材料表现出良好的倍率性能和循环性能, 在0.2 C(1 C=170 mAh·g^-1)、0.5 C、1 C、2 C、5 C、10 C下首次放电容量分别为151、150、146、142、132、119 mAh·g^-1, 20 C下的首次放电容量也达105 mAh·g^-1, 且循环50次几乎无衰减。     

Preparation and electrochemical performance of LiFePO4/C composite with carbon core structure

LiFePO₄/C composite with carbon core structure was successfully prepared by using araldite as carbon source. The microstructure and morphology of LiFePO₄/C composite were confirmed by X-ray diffraction and transmission electron microscopic observation. The experimental results show that this structure is entirely different from carbon coating. The LiFePO₄/C composite forms a common core structure in which carbon is used as line core and carbon core is covered by nano-LiFePO₄ grains. Moreover, the LiFePO₄/C composite exhibits higher tap density of 1.66 g cm^− 3, shows higher capacity about 162 mAhg^− 1 applied 30 mAg^− 1 current, excellent cyclic ability and rate capability about 139 mAhg^− 1 applied 700 mAg^− 1 current at room temperature.     

Enhanced cycling stability of microsized LiCoO2 cathode by Li4Ti5O12 coating for lithium ion battery

The effect of Li₄Ti₅O₁₂ (LTO) coating amount on the electrochemical cycling behavior of the LiCoO₂ cathode was investigated at the high upper voltage limit of 4.5 V. Li₄Ti₅O₁₂ (≤5 wt.%) is not incorporated into the host structure and leads to formation of uniform coating. The cycling performance of LiCoO₂ cathode is related with the amount of Li₄Ti₅O₁₂ coating. The initial capacity of the LTO-coated LiCoO₂ decreased with increasing Li₄Ti₅O₁₂ coating amount but showed enhanced cycling properties, compared to those of pristine material. The 3 wt.% LTO-coated LiCoO₂ has the best electrochemical performance, showing capacity retention of 97.3% between 2.5 V and 4.3 V and 85.1% between 2.5 V and 4.5 V after 40 cycles. The coulomb efficiency shows that the surface coating of Li₄Ti₅O₁₂ is beneficial to the reversible intercalation/de-intercalation of Li⁺. LTO-coated LiCoO₂ provides good prospects for practical application of lithium secondary batteries free from safety issues.     

Synthesis and electrochemical behavior of nanosized LiNi1−xCaxO2 cathode materials for high voltage secondary lithium-ion cells

A new class of LiNi_1−xCa_xO₂ (x = 0.0, 0.1, 0.2, 0.3 and 0.5) layered oxide materials has been synthesized by a simple low temperature solid-state route with mixed nitrates/urea with glycerol as the starting materials. First we have taken TG/DTA for observing the phase transformations of LiNi_0.9Ca_0.1O₂. The structure of the synthesized oxides was analyzed using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) to identify the crystal structure and cation environment, respectively. The synthesized ceramic oxide battery materials were examined by using transmission electron microscope (TEM), scanning electron microscope (SEM) analysis to determine the particle size, nature and morphological structure. SEM with energy dispersive X-ray spectroscopic analysis (EDAX) analysis was carried out to explore the composition of the prepared materials. The electrochemical performance of LiNi_1−xCa_xO₂ electrodes was analyzed using cyclic voltammetry (CV) and galvanostatic charge-discharge cycling studies in the voltage range 3.0-4.5 V. Electrode made with cathode active material, acetylene black and poly(vinylidene difluoride) yield a discharge capacity of 178.1 mAh g⁻¹ (x = 0.2) with good specific capacity over several charge-discharge cycles. These results have been also supported by cyclic voltammograms.     

Electrochemical lithium insertion behavior of FeNbO4: Structural relations and in situ conversion into FeNb2O6 during carbon coating

Electrochemical properties of FeNbO₄ as a lithium insertion anode material were studied with a view to understand structure–property relationships. Orthorhombic and monoclinic polymorphs of FeNbO₄ were synthesized and characterized by powder X-ray diffraction and laser Raman spectroscopy. Possible redox reactions, as deciphered from cyclic voltammograms, suggest the structural similarity between orthorhombic and monoclinic polymorphs upon lithium insertion. A coating of carbon led to a remarkable improvement in the electrochemical performance of monoclinic FeNbO₄. The coated material exhibited an average reversible capacity of 125.5 mAh g⁻¹. The material also sustained hundreds of charge/discharge cycles and exhibited good rate capability. Upon coating with carbon, the monoclinic FeNbO₄ transformed into FeNb₂O₆. The conversion and stability were confirmed by powder XRD and laser Raman studies of carbon-coated material before and after 450 cycles. The in situ conversion of FeNbO₄ into FeNb₂O₆ during carbon coating was further supported by EPR studies in which the absence of signal for the carbon-coated material indicated conversion of Fe³⁺ to Fe²⁺. Our study reveals the possibility of exploring potential materials in the Fe–Nb–O system and enhancing their performance as anode materials for lithium-ion batteries.