Physical and electrochemical properties of La-doped LiFePO4/C composites as cathode materials for lithium-ion batteries

Olivine-type LiFePO₄ is one of the most promising cathode materials for lithium-ion batteries, but its poor conductivity and low lithium-ion diffusion limit its practical application. The electronic conductivity of LiFePO₄ can be improved by carbon coating and metal doping. A small amount of La-ion was added via ball milling by a solid-state reaction method. The samples were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM)/mapping, differential scanning calorimetry (DSC), transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS), and total organic carbon (TOC). Their electrochemical properties were investigated by cyclic voltammetry, four-point probe conductivity measurements, and galvanostatic charge and discharge tests. The results indicate that these La-ion dopants do not affect the structure of the material but considerably improve its rate capacity performance and cyclic stability. Among the materials, the LiFe_0.99La_0.01PO₄/C composite presents the best electrochemical behavior, with a discharge capacity of 156 mAh g^?1 between 2.8 and 4.0 V at a 0.2 C-rate compared to 104 mAh g^?1 for undoped LiFePO₄. Its capacity retention is 80% after 497 cycles for LiFe_0.99La_0.01PO₄/C samples. Such a significant improvement in electrochemical performance should be partly related to the enhanced electronic conductivities (from 5.88?×?10^?6 to 2.82?×?10^?3 S cm^?1) and probably the mobility of Li⁺ ion in the doped samples. The LiFe_0.99La_0.01PO₄/C composite developed here could be used as a cathode material for lithium-ion batteries.     

Electrochemical performance of Yb-doped LiFePO4/C composites as cathode materials for lithium-ion batteries

LiFePO₄/C and LiYb_0.02Fe_0.98PO₄/C composite cathode materials were synthesized by simple solution technique. The samples were characterized by X-ray diffraction, scanning electron microscope, and thermogravimetric–differential thermal analysis. Their electrochemical properties were investigated by cyclic voltammetry, four-point probe conductivity measurements, and galvanostatic charge and discharge tests. The carbon-coated and Yb³⁺-doped LiFePO4 sample exhibited an enhanced electronic conductivity of 1.9 × 10^?3 Scm^?1, and a specific discharge capacity of 146 mAhg^?1 at 0.1 C. The results suggest that the improvement of the electrochemical performance can be attributed to the ytterbium doping, which facilitates the phase transformation between triphylite and heterosite during cycling, and the conductivity improvement by carbon coating.     

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。     

Li<Subscript>0.99</Subscript>Ti<Subscript>0.01</Subscript>FePO<Subscript>4</Subscript>/C composite as cathode material for lithium ion battery

Three kinds of LiFePO₄ materials, mixed with carbon (as LiFePO₄/C), doped with Ti (as Li_0.99Ti_0.01FePO₄), and treated both ways (as Li_0.99Ti_0.01FePO₄/C composite), were synthesized via ball milling by solid-state reaction method. The crystal structure and electrochemical behavior of the materials were investigated using X-ray diffraction, SEM, TEM, cyclic voltammetry, and charge/discharge cycle measurements. It was found that the electrochemical behavior of LiFePO₄ could be increased by carbon coating and Ti-doping methods. Among the materials, Li_0.99Ti_0.01FePO₄/C composite presents the best electrochemical behavior, with an initial discharge capacity of 154.5 mAh/g at a discharge rate of 0.2 C, and long charge/discharge cycle life. After 120 cycles, its capacity remains at 92% of the initial capacity. The Li_0.99Ti_0.01FePO₄/C composite developed here can be used as the cathode material for lithium ion batteries.     

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.     

Effect of different carbon conductive additives on electrochemical properties of LiFePO4-C/Li batteries

LiFePO₄-C nanoparticles were synthesized by a hydrothermal method and subsequent high-energy ball-milling. Different carbon conductive additives including nanosized acetylene black (AB) and multi-walled carbon nanotube (MWCNT) were used to enhance the electronic conductivity of LiFePO₄. The structural and morphological performance of LiFePO₄-C nanoparticles was investigated by X-ray diffraction (XRD) and scanning electron microscopy. The electrochemical properties of LiFePO₄-C/Li batteries were analyzed by cyclic voltammetry and charge/discharge tests. XRD results demonstrate that LiFePO₄-C nanoparticles have an orthorhombic olivine-type structure with a space group of Pnma. LiFePO₄-C/Li battery with 5 wt% MWCNT displays the best electrochemical properties with a discharge capacity of 142 mAh g⁻¹ at 0.25 C at room temperature.     

LiFePO4/C正极材料的液相合成及电化学性能研究

尝试对共沉淀法进行改进, 利用自制的加料装置合成了橄榄石型LiFePO₄/C复合正极材料. 应用X射线衍射(XRD)、扫描电镜(SEM)、X射线能谱(EDS)、循环伏安(CV)以及恒电流充放电测试等方法对目标材料进行了结构表征和电化学性能测试. 实验结果表明采用该法得到的样品具有单一的橄榄石结构, 样品形貌规则, 粒径细小均匀. 改性后的材料具有较高的首放容量及良好的循环稳定性能. 0.1C倍率下充放电测试表明, 其首次放电比容量超过145 mAh•g^－1, 50次循环后, 容量没有明显衰减. 0.2C和0.5C倍率下的平均放电容量分别为130及120 mAh•g^－1, 循环过程中样品表现出较好的循环稳定性.     

锂离子电池正极材料LiV3O8-xClx的低温固相法合成及电化学性能研究

采用低温固相法成功地合成了锂离子电池正极材料LiV3O8-xClx (x=0.00,0.05,0.10,0.15)。分别用XRD、SEM、充放电实验、循环伏安、交流阻抗等测试方法研究了Cl- 的掺入对LiV3O8结构、形貌及电化学性能的影响。结果表明, Cl-的掺入显著地提高了材料的充放电循环性能。当掺杂量 x=0.10时,材料的循环性能最好, 循环100周后放电容量仍为198.6 mAh/g。     

铌源和掺杂位置对LiFePO4/C电化学性能的影响

使用Nb2O5和Nb(OC6H5)5为铌源对LiFePO4/C中的锂位和铁位分别掺杂,采用碳热还原法合成掺杂Nb的磷酸铁锂系列材料。运用X射线衍射仪、扫描电镜、循环伏安、交流阻抗谱和恒电流充放电测试等对材料进行表征。结果表明:相比掺杂位置,铌源对材料的颗粒形貌和粒径分布影响更大,而颗粒大小对材料的电化学性能,尤其是大倍率性能的提高有重要作用;掺杂在Li位的Nb元素比在Fe位能更好的稳定晶体结构,从而有利于提高循环性能。     

EDTA辅助水热法制备性能优异的棒状LiFePO4/C材料

以乙二胺四乙酸为配位剂采用水热法制备了棒状LiFePO₄/C材料。采用X射线衍射、扫描电镜、透射电镜、循环伏安、交流阻抗和恒电流充放电测试等对材料进行表征。结果表明:乙二胺四乙酸对材料的形貌和电性能均有很大影响。通过加入乙二胺四乙酸, 材料的形貌由不规则的颗粒变为棒状的颗粒且颗粒的厚度由140~200 nm减少至40~90 nm, 材料的表面包覆约3.5 nm的均匀碳层, 且该材料极化较小且界面阻抗较低。0.1C放电比容量为167 mAh·g^-1(接近理论容量170 mAh·g^-1)。     

Carboxylic acid-assisted solid-state synthesis of LiFePO4/C composites and their electrochemical properties as cathode materials for lithium-ion batteries

To enhance the capability of LiFePO₄ materials, we attempted to coat carbon by incorporating various organic carboxylic acids as carbon sources. The purity of LiFePO₄ was confirmed by XRD analysis. Galvanostatic cycling, cyclic voltammetry, electric impedance spectroscopy, and conductivity measurements were used to evaluate the material’s electrochemical performance. The best cell performance was delivered by the sample coated with 60 wt.% malonic acid. Its first-cycle discharge capacity was 149 mA h g^?1 at a 0.2 C rate or 155 mA h g^?1 at a 0.1 C rate. The presence of carbon in the composite was verified by total organic carbon and Raman spectral analysis. The actual carbon content of LiFePO₄ was 1.90 wt.% with the addition of 60 wt.% malonic acid. The LiFePO₄/C samples sintered with 60 wt.% various carboxylic acids were measured by Raman spectral analysis. The intense broad bands at 1,350 and 1,580 cm^?1 are assigned to the D and G bands of residual carbon in LiFePO₄/C composites, respectively. The peak intensity (I _D/I _G) ratio of the synthesized powders is from 0.907 to 0.935. Carbon coatings of LiFePO₄ with low I _D/I _G ratios can be produced by incorporating carboxylic acid additives before the final calcining process. The use of carboxylic acid as a carbon source increases the overall conductivity (～10^?4 S cm^?1) of the material.     

Improvement of electrochemical properties of LiFePO4/C cathode materials by chlorine doping

The olivine-type cathode materials of LiFePO₄ were prepared via solid-state reaction under argon atmosphere and doped by chlorine to improve their electrochemical performances. The crystal structure, morphology, and electrochemical properties of the prepared samples were investigated using thermogravimetry–differential scanning calorimetry, X-ray diffraction, Fourier transform infrared, scanning electron microscopy, cyclic voltammetry, and charge–discharge cycle measurements. The result showed that the electrochemical performance of LiFePO₄ had been improved by chlorine doping, and the effect of chlorine in lattice was discussed. The heavily doped samples show better electrochemical performance in relative high rates.     

Physical and electrochemical characterizations of LiFePO4-incorporated Ag nanoparticles

Olivine-type LiFePO₄ composite materials for cathode material of the lithium-ion batteries were synthesized by using a sol-gel method and were coated by a chemical deposition of silver particles. As-obtained LiFePO₄/C-Ag (2.1 wt.%) composites were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), conductivity measurements, cyclic voltammetry, as well as galvanostatic measurements. The results revealed that the discharge capacity of the LiFePO₄/C-Ag electrode is 136.6 mAh/g, which is 7.6% higher than that of uncoated LiFePO₄/C electrode (126.9 mAh/g). The LiFePO₄/C coated by silver nanoparticles enhances the electrode conductivity and specific capacity at high discharge rates. The improved capacity at high discharge rates may be attributed to increased electrode conductivity and the synergistic effect on electron and Li⁺ transport after silver incorporation.     

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.     

流变相法制备倍率性能优异的LiFePO4/C复合材料

以月桂酸为碳源和表面活性剂,氢氧化锂、碳酸锂和醋酸锂为锂源,采用流变相法制备LiFePO4/C复合材料。运用X射线衍射(XRD)、扫描电子显微镜(SEM)、粒度分析、恒流充放电测试、循环伏安以及交流阻抗测试等方法对复合材料进行表征。结果表明,不同的锂源对LiFePO4/C复合材料的结构和电化学性能均有很大影响,以氢氧化锂为锂源合成的LiFePO4/C材料展示出最佳的循环性能和倍率性能。该材料在0.1C下放电比容量为153.4 mAh.g-1,在大倍率10 C下,容量保持率仍可达76%,甚至10C下循环800次后,容量衰减率仅有4%,SEM结果显示该材料具有较小的粒径(～200 nm),且分布集中,有效提高了电子迁移速率,从而改进了LiFePO4/C的倍率性能。     

Improving the rate and low-temperature performance of LiFePO<Subscript>4</Subscript> by tailoring the form of carbon coating from amorphous to graphene-like

A solid-state reaction process with poly(vinyl alcohol) as the carbon source is developed to synthesize LiFePO₄-based active powders with or without modification assistance of a small amount of Li₃V₂(PO₄)₃. The samples are analyzed by X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy. It is found that, in addition to the minor effect of a lattice doping in LiFePO₄ by substituting a tiny fraction of Fe²⁺ ions with V³⁺ ions, the change in the form of carbon coating on the surface of LiFePO₄ plays a more important role to improve the electrochemical properties. The carbon changes partially from sp³ to sp² hybridization and thus causes the significant rise in electronic conductivity in the Li₃V₂(PO₄)₃-modified LiFePO₄ samples. Compared with the carbon-coated baseline LiFePO₄, the composite material 0.9LiFePO₄·0.1Li₃V₂(PO₄)₃ shows totally different carbon morphology and much better electrochemical properties. It delivers specific capacities of 143.6 mAh g^?1 at 10 C rate and 119.2 mAh g^?1 at 20 C rate, respectively. Even at the low temperature of ?20 °C, it delivers a specific capacity of 118.4 mAh g^?1 at 0.2 C.     

Synthesis, characterization, and electrochemical performance of LiFePO4/C cathode materials for lithium ion batteries using various carbon sources: best results by using polystyrene nano-spheres

Olivine LiFePO₄/C cathode materials for lithium ion batteries were synthesized using monodisperse polystyrene (PS) nano-spheres and other carbon sources. The structure, morphology, and electrochemical performance of LiFePO₄/C were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge–discharge tests, electrochemical impedance spectroscopy (EIS) measurements, and Raman spectroscopy measurements. The results demonstrated that LiFePO₄/C materials have an ordered olivine-type structure with small particle sizes. Electrochemical analyses showed that the LiFePO₄/C cathode material synthesized from 7 wt.% PS nano-spheres delivers an initial discharge capacity of 167 mAh g^-1 (very close to the theoretical capacity of 170 mAh g^-1) at 0.1 C rate cycled between 2.5 and 4.1 V with excellent capacity retention after 50 cycles. According to Raman spectroscopy and EIS analysis, this composite had a lower I _D/I _G, sp ³/sp ² peak ratio, charge transfer resistance, and a higher exchange current density, indicating an improved electrochemical performance, due to the increased proportion of graphite-like carbon formed during pyrolysis of PS nano-spheres, containing functionalized aromatic groups.     

The improvement of the high-rate charge/discharge performances of LiFePO4 cathode material by Sn doping

Nanocrystalline LiFePO₄ and LiFe_0.97Sn_0.03PO₄ cathode materials were synthesized by an inorganic-based sol–gel route. The physicochemical properties of samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and elemental mapping. The doping effect of Sn on the electrochemical performance of LiFePO₄ cathode material was extensively investigated. The results showed that the doping of tin was beneficial to refine the particle size, increase the electrical conductivity, and facilitate the lithium-ion diffusion, which contributed to the improvement of the electrochemical properties of LiFePO₄, especially the high-rate charge/discharge performance. At the low discharge rate of 0.5 C, the LiFe_0.97Sn_0.03PO₄ sample delivered a specific capacity of 158 mAh g⁻¹, as compared with 147 mAh g⁻¹ of the pristine LiFePO₄. At higher C-rate, the doping sample exhibited more excellent discharge performance. LiFe_0.97Sn_0.03PO₄ delivered specific capacity of 146 and 128 mAh g⁻¹ at 5 C and 10 C, respectively, in comparison with 119 and 107 mAh g⁻¹ for LiFePO₄. Moreover, the doping of Sn did not influence the cycle capability, even at 10 C.     

LiFePO4的软化学合成及锂快离子导体修饰

采用溶剂热法制备正极材料LiFePO_4,采用溶胶凝胶法制备Li_(0.5)La_(0.5)TiO_3(LLTO)粉体,并通过酒精悬浮法对LiFePO_4进行修饰,修饰量为LiFePO_4质量的1%~4%,获得了薄壁蜂窝状自组装结构的LiFePO_4上修饰有球状LLTO纳米颗粒的复合正极材料。通过进行充放电测试、交流阻抗测试及循环伏安测试,研究了不同修饰量对电池的充放电比容量、循环性能及可逆性的影响,发现当LLTO含量为3%(w/w)时,以2C和5C倍率放电相对于没有修饰LLTO的LiFePO_4的比容量分别提高29.7%和31.6%,30次循环之后,容量损失率较未改性前减小4.13%,循环伏安曲线上氧化还原峰之间的电位差仅为0.117 V,以3%的LLTO修饰改性的LiFePO_4显著提高了电池的倍率性能、循环性能和低温性能。     

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.