以Fe2O3为原料合成FePO4及其在正极材料LiFePO4合成中的应用

以Fe2O3、NH4H2PO4和H2NCONH2为原料,采用流变相法合成FePO4,探究了温度对其合成过程的影响。以此FePO4为铁源,再采用流变相法制备出正极材料LiFePO4。产物FePO4和LiFePO4的晶型、形貌分别通过XRD和SEM表征,XRD结果表明,FePO4属三斜晶系,具有α-石英结构,而LiFePO4属于斜方晶系,具有橄榄石结构;SEM结果表明,FePO4样品颗粒呈层状分布,LiFePO4晶体形貌为类球形,两者分散度良好,平均粒径均为2μm左右;对所得正极材料LiFePO4进行恒电流充放电测试,其初始放电容量达163.4 mAh/g。采用循环伏安、交流阻抗等对LiFePO4电化学性能表征,结果均显示其电化学性能良好。表明以Fe2O3为原料采用流变相法所得磷酸铁是合成锂离子电池正极材料磷酸铁锂的较理想的铁源。     

锂离子电池正极材料磷酸铁锂的碳热还原法制备及电化学性能的研究

橄榄石型磷酸铁锂(LiFePO₄)由于具有良好的优点,受到社会各界的广泛关注。由于磷酸铁锂自身结构存在的一些缺点,因此导致电子传导率低和锂离子扩散系数小,不仅影响放电倍率,还阻碍工业化的应用。该文采用碳热还原法制备Li FePO₄/C正极材料,研究不同三价铁源合成磷酸铁锂材料的电化学性能状况,通过XRD、SEM等手段表征所得材料,并通过恒流充放电等测试了解其电化学性能,从而找到一种最佳的低成本三价铁源,优化固相碳热还原工艺。     

溶胶-凝胶法合成磷酸铁锂正极材料

以FePO4·4H2O、LiOH·H2O、草酸为原料,以葡萄糖为碳源,采用溶胶-凝胶法合成磷酸铁锂正极材料。利用X射线衍射(XRD)、扫描电镜(SEM)等方法进行表征,并将材料组装成电池研究其电化学性能。结果表明:以葡萄糖为碳源,采用溶胶-凝胶法合成磷酸铁锂正极材料具有单一的橄榄石型晶体结构,充放电平台平稳。葡萄糖添加量为5.9%时,材料的充放电比容量和循环性能较高,室温下,0.1C和0.2C首次放电比容量达143.3mA·h/g和133.7mA·h/g,循环50次后仍保持在134.2mA·h/g和124.5mA·h/g。     

片层纳米结构磷酸铁制备及对磷酸铁锂电性能的影响

采用液相反应结晶法,在磷源及铁源中添加NaAlO_2,利用在结晶过程中生成的Al(OH)3胶体对结晶面的作用,合成出具有片层纳米结构纺锤体状磷酸铁前驱体,并通过高温固相法进一步制备成磷酸铁锂。采用XRD、FT-IR、SEM、TEM、比表面及孔隙率分析、激光粒度分析和电化学性能测试等手段对样品进行表征分析。结果表明,由具有片层纳米结构的磷酸铁前驱体制备的磷酸铁锂比由无片层纳米结构的磷酸铁前驱体制备的磷酸铁锂在0.1C下的首次放电容量提升了20%,达到151.48mAh/g,电极电荷转移电阻降低了约75%,仅为27.23Ω;0.1C倍率下循环50次后容量保持率可达96%。同时,对Al(OH)3胶体影响片层纳米结构磷酸铁生成机制进行了分析和讨论。     

纳米磷酸铁包覆锂离子电池正极材料LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2的制备及其电化学性能

为改善LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2正极材料的电化学性能,采用自制的磷酸铁纳米悬浮液,通过共沉淀法在LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2正极材料表面包覆纳米磷酸铁。应用XRD,TG-DTA,TEM等手段表征制备的磷酸铁的结构,形貌和液相状态;通过XRD,SEM,EDS,TEM,ICP,恒流充放电、循环伏安、交流阻抗表征制备的包覆材料的结构、形貌及电化学性能。研究烧结温度和包覆量对LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2正极材料电化学性能的影响。结果表明,热处理温度为400℃,2%(质量分数,下同)磷酸铁包覆能显著地改善LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2正极材料的循环性能和倍率性能。循环伏安和交流阻抗结果显示,包覆磷酸铁后改善了LiNi_(0.5)Co_(0.2)Mn_(0.3)O_2正极材料的可逆性和动力学性能。ICP测试结果表明,磷酸铁包覆层能够有效地降低电解液对正极材料的溶解与侵蚀,稳定其层状结构,从而提高正极材料的电化学性能。     

电化学合成磷酸铁锂正极材料的前驱体

以NH4H2PO4、锂盐和纯铁为主要原料,采用电化学法合成磷酸锂铁前驱体,再通过磷酸锂铁前驱体合成锂离子电池正极材料LiFePO4/C。通过X射线衍射(XRD)、扫描电镜(SEM)及充放电性能测试等方法对其晶体结构、微观形貌和电化学性能进行分析研究。结果表明,LiFePO4/C具有单一的橄榄石型晶体结构。其中在无水乙醇溶剂中合成的LiFePO4/C正极材料粒径细小且分布均匀,具有最好的电化学性能,在0.2C的放电电流下,首次放电比容量达到142.3mAh/g,充放电循环30次后放电比容量仍保持在141.2mAh/g。     

炭包覆LiFePO4纳米片的制备及电化学性能研究

为改善磷酸铁锂正极材料的倍率性能, 以乙二醇为溶剂, 采用一步溶剂热法制备磷酸铁锂纳米片。再以葡萄糖为碳前驱体, 对磷酸铁锂纳米片进行炭包覆。通过X射线衍射, N₂吸脱附曲线、扫描电子显微镜、透射电子显微镜和循环伏安法等测试方法考察了炭包覆量对磷酸铁锂纳米片结构与电化学性能的影响。结果表明, 制备的磷酸铁锂为具有较短b轴的纳米片状结构, 尺寸约为150 nm×100 nm×60 nm。磷酸铁锂纳米片的倍率性能随炭包覆量的增加而增强, 当炭包覆量为6.4wt%时具有最佳的倍率性能, 在0.2C和10C的电流密度下放电容量分别为157.3和132.6 mAh/g。同时循环稳定性良好, 在5C电流密度下循环500次后容量保持率达到了80.2%。     

锰掺杂对于磷酸铁锂高倍率性能影响的研究

以氢氧化锂、磷酸、硫酸亚铁和硫酸锰为原料,葡萄糖作为碳源,利用水热合成法制备了锰掺杂的磷酸铁锂,利用正交试验研究了掺杂浓度、水热反应温度和水热反应时间对于材料高倍率性能的影响。结果表明:三因素对试验指标的影响大小依次为水热反应温度、掺杂浓度、水热反应时间。最优的试验条件:掺杂浓度2%、水热反应温度180℃、水热反应时间3h,得到的材料充放电倍率10C下容量为127mAh·g-1。     

水热法制备磷酸铁／碳纳米电缆

以抗坏血酸、氯化亚铁和次亚磷酸钠为原料，采用一步水热法制备了以磷酸铁为芯、含碳化合物为壳的磷酸铁/碳纳米电缆，通过XRD、FTIR、SEM和TEM等手段对产物的成分和结构进行了表征，对选定条件下磷酸铁/碳纳米电缆的生长机理进行了探讨，获得了水热制备磷酸铁/碳纳米电缆的最佳实验条件。     

不同锂源水热法合成LiFePO4的研究进展

LiFePO4具有较高的能量密度、较好电化学性能和热力学稳定性而成为最为热门、应用最为广泛的锂离子电池正极材料。其合成方法有很多,其中的水热法具有反应快、操作简单、产品形貌易于控制且颗粒粒径小等优点而备受关注。用不同的锂源合成出来的产品的形貌和性能也不一样。现分别以磷酸锂、碳酸锂、醋酸锂、氢氧化锂等化合物为锂源对水热法合成LiFePO4进行了一定探讨,并综述了各自利用水热法合成磷酸铁锂的研究进展情况。     

A modified mechanical activation synthesis for carbon-coated LiFePO4 cathode in lithium batteries

An efficient synthesis based on mechanical activation (MA) was developed for carbon-coated lithium iron phosphate (LiFePO₄/C). The conventional MA process was modified by introducing two initial steps of slurry phase blending of the ingredients and solvent removal by rotary evaporation, so as to get an intimate mixing and homogenous dispersion of conductive carbon in the sample. Phase-pure, nanometer-sized particles of the active material covered with a porous, nanometer-sized web of carbon were obtained. LiFePO₄/C exhibited remarkably good electrochemical properties when evaluated as cathodes in room temperature lithium cells. An initial discharge capacity of 166 mAh/g (corresponding to 97.6% of theoretical capacity) was achieved at 0.1 C-rate. A very stable cycle performance was also realized; good capacity retention up to 100 cycles was achieved at different current densities.     

Temperature-dependent properties of FePO4 cathode materials

The temperature of formation and crystallinity of iron phosphate, FePO₄, is critical in determining its electrochemical behavior in lithium cells. Amorphous FePO₄ formed by heating amorphous iron phosphate dihydrate was found to have good capacity both in chemical and electrochemical lithiation. Initial capacities of up to 140 mAh/g were obtained at a current density of 0.2 mA/cm². Material formed from crystalline iron phosphate dihydrate was found to be much less active, due to both the crystallinity of the product and to the iron being in tetrahedral coordination. The trigonal quartz phase of FePO₄ loses almost all capacity after heating to 700°C, which might be due to glass formation on the surface.     

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.     

Optimized synthesis of LiFePO4 cathode material and its reaction mechanism during solvothermal

LiFePO₄ has been widely considered as a promising cathode material for lithium ion batteries because of its nontoxicity, high specific capacity, good safety characteristics and low cost. However, the actual fabrication of LiFePO₄ typically uses LiH₂PO₄, NH₄H₂PO₄ or H₃PO₄ as phosphorus sources, possibly leading to the corrosion of the experimental facilities and release of toxic gas during the synthesis process. Hence, we use phytic acid (PhyA) as a new eco-friendly and sustainable phosphorus source to synthesize LiFePO₄. Results show that the reaction time and temperature have significant effects on the morphology. LiFePO₄ prepared at 180 °C for 4 h (LFP-4) shows unique hierarchical structure and exhibits best electrochemical performance over a wide test temperature (25–55 °C). Through time-dependent experiments to explore the reaction mechanism of LiFePO₄, it is found that an intermediate Fe₃(PO₄)₂ is produced that acts as the substrate for the subsequent preparation of LiFePO₄. After carbon coating, LFP/C-4 (after carbon coating, LFP-4 labeled as LFP/C-4) shows an outstanding initial discharge capacity (156.9 mAh g^?1, 1C at 25 °C) and high temperature behavior (147.1 and 126.8 mAh g^?1 at 1C and 2C under 55 °C). This result is highly important for the controllable synthesis and improvement of electrochemical characteristics of LiFePO₄ cathode material.     

Suppressing Li3PO4 impurity formation in LiFePO4/Fe2P by a nonstoichiometry synthesis and its effect on electrochemical properties

Lithium iron phosphate (LiFePO₄) was synthesized by a solid-state reaction from a nonstoichiometric mixture of starting materials with an iron: phosphorus excess ratio of 2:1 at a high temperature. The nonstoichiometry synthesis did not affect conductive Fe₂P formation, lattice constants of LiFePO₄ and materials morphology, but could effectively suppress insulating Li₃PO₄ impurity formation which was clearly observed in the stoichiometric sample. Our results demonstrate that the positive effect of the conductive Fe₂P could be masked by the insulating Li₃PO₄ impurity presence, and the creation of Fe₂P without Li₃PO₄ formation from carbothermal reduction could be successfully achieved by our nonstoichiometry synthesis.     

Effect of magnetic field on the zero valent iron induced oxidation reaction

The magnetic field (MF) effect on the zero valent iron (ZVI) induced oxidative reaction was investigated for the first time. The degradation of 4-chlorophenol (4-CP) in the ZVI system was employed as the test oxidative reaction. MF markedly enhanced the degradation of 4-CP with the concurrent production of chlorides. The consumption of dissolved O₂ by ZVI reaction was also enhanced in the presence of MF whereas the competing reaction of H₂ production from proton reduction was retarded. Since the ZVI-induced oxidation is mainly driven by the in situ generated hydroxyl radicals, the production of OH radicals was monitored by the spin trap method using electron spin resonance (ESR) spectroscopy. It was confirmed that the concentration of trapped OH radicals was enhanced in the presence of MF. Since both O₂ and Fe⁰ are paramagnetic, the diffusion of O₂ onto the iron surface might be accelerated under MF. The magnetized iron can attract oxygen on itself, which makes the mass transfer process faster. As a result, the surface electrochemical reaction between Fe⁰ and O₂ can be accelerated with the enhanced production of OH radicals. MF might retard the recombination of OH radicals as well.     

Photoactive Earth‐Abundant Iron Pyrite Catalysts for Electrocatalytic Nitrogen Reduction Reaction

The generation of ammonia, hydrogen production, and nitrogen purification are considered as energy intensive processes accompanied with large amounts of CO₂ emission. An electrochemical method assisted by photoenergy is widely utilized for the chemical energy conversion. In this work, earth‐abundant iron pyrite (FeS₂) nanocrystals grown on carbon fiber paper (FeS₂/CFP) are found to be an electrochemical and photoactive catalyst for nitrogen reduction reaction under ambient temperature and pressure. The electrochemical results reveal that FeS₂/CFP achieves a high Faradaic efficiency (FE) of ≈14.14% and NH₃ yield rate of ≈0.096 µg min^?1 at ?0.6 V versus RHE electrode in 0.25 m LiClO₄. During the electrochemical catalytic reaction, the crystal structure of FeS₂/CFP remains in the cubic pyrite phase, as analyzed by in situ X‐ray diffraction measurements. With near‐infrared laser irradiation (808 nm), the NH₃ yield rate of the FeS₂/CFP catalyst can be slightly improved to 0.1 µg min^?1 with high FE of 14.57%. Furthermore, density functional theory calculations demonstrate that the N₂ molecule has strong chemical adsorption energy on the iron atom of FeS₂. Overall, iron pyrite‐based materials have proven to be a potential electrocatalyst with photoactive behavior for ammonia production in practical applications.     

Electrochemical performance of LiFePO4/Si composites as cathode material for lithium ion batteries

LiFePO₄/Si composites are synthesized via a simple milling isopropanol mixtures process, and the effects of Si-modification on the electrochemical performances of LiFePO₄ are investigated systematically by charge/discharge testing, cyclic voltammograms and AC impedance spectroscopy, respectively. In comparison with the pristine LiFePO₄, LiFePO₄/Si-nanoparticle shows better cyclability and higher rate capability, especially at elevated temperature. An analysis of the electrochemical measurements indicates that Si incorporation could significantly improve the electrochemical performance at high charge/discharge rate and elevated temperature. Among the investigated samples, (LiFePO₄)₉₈/(Si)₂ sample shows the best electrochemical performance with 150 mAhg⁻¹ at 0.5C at 60 °C. The enhancement could be mainly attributed to the lower charge-transfer resistance and higher lithium diffusion coefficients. In addition, the dangling bonds of Si and fluorosilica compounds are responsible for suppressing the dissolution of Fe²⁺ from olivine phase and preventing the rise of the surface resistance and charge transfer resistance.     

Magnetic properties of Ni-Zn ferrite powders formed by self-propagating high temperature synthesis reaction

The ferrite compositions of Ni_xZn_xFe₂O₄ were synthesized by self-propagating high temperature synthesis reaction with various contents of iron, iron oxide, nickel oxide and zinc oxide at oxygen partial pressures varying between 0.05 to 5.0 MPa. The oxygen pressure promoted the combustion reaction, while the compacting pressure retarded the reaction. The rate equation of ferrite formation is shown to be v = 14.5 exp (T_c1380 - 1) P _O2 ^0.2 . Phase identification of the final products by X-ray diffraction (XRD) revealed that the enhanced combustion reaction with oxygen pressure and iron content in the reactants resulted in increasing the spinel content in the combustion product. As the oxygen pressure changed from 0.1 to 5 MPa, the coercive force and residual magnetization decreased by about 73% and 66%, respectively, whereas, the maximum magnetization, susceptibility and Curie temperature increased by about 70%, 60%, and 32%, respectively. The improved magnetic properties are accounted for by the enhanced iron oxidation at a given combustion condition. Compared to the magnetic properties and productivity of the Ni-Zn ferrites prepared by wet chemical method, the self-propagating high temperature synthesis method at high oxygen pressure is one of the useful methods to fabricate improved ferrite powders.     

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.