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.     

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.     

Multistep sintering preparation and electrochemical performances of LiFe0.7 V0.2PO4/C cathode material for lithium-ion batteries

Cathode material LiFe_0.7?V_0.2PO₄/C is successfully synthesized by multistep sintering through carbon thermal reaction including 650 °C for 10 h and 750 °C for 6 h. The crystal structure and surface morphology of the synthesized materials are characterized by X-ray diffractometer and scanning electron microscope, respectively. Cycle voltammetry, electrochemical impedance spectroscopy, and charge–discharge test are used to investigate the electrochemical performances of these samples. The results revealed that the synthesized LiFe_0.7?V_0.2PO₄/C material simultaneously contains olivine structure LiFePO₄ and monoclinic structure Li₃V₂(PO₄)₃. It shows improved conductivity, Li-ion diffusion coefficient, excellent charge/discharge performance, and reversibility due to both the incorporation of Li₃V₂(PO₄)₃ fast ion conductor and the employed multistep sintering. The initial discharge specific capacities of LiFe_0.7?V_0.2PO₄/C by multistep sintering are 167.8, 154.7, and 140.8 mAh g^?1 at 0.5, 1, and 2 C, respectively. After a total of 230 cycles at different rates, the sample still shows good performances. After 100 cycles at 2 C, the capacity retention is 99.1 %, and the capacity is 139.6 mAh g^?1. The LiFe_0.7?V_0.2PO₄/C material synthesized by this method can be used as a cathode material for advanced lithium-ion batteries.     

Synthesis of LiFe(1−x)V x PO4/C composite cathode materials with high performance via an aqueous solution–evaporation method

The modification techniques of applying carbon coating on particle surface and doping vanadium at Fe site were applied to make the LiFePO₄ cathode materials achieve high rate performance in lithium ion batteries. To design and synthesize these LiFe_(1?x)V _x PO₄/C (x?=?0, 0.02, 0.05, or 0.08) composites, an aqueous solution–evaporation method was taken, in which every kind of raw material was distributed at a high degree of uniformity. The LiFe_0.95V_0.05PO₄/2.57 wt% C composite displayed the best electrochemical performances. At rates of 0.1, 0.5, 2, 5, and 10 C (1 C?=?170 mAg^?1), it delivered a discharge capacity of 157.8, 156.9, 149, 139.6, and 130.1 mAh g^?1, respectively. The composite exhibited perfect cycle stabilities as well, maintaining 100 % (0.5 C), 99.7 % (2 C), 98.9 % (5 C), and 96.6 % (10 C) of the first discharge capacity after 100 cycles at different rates, respectively.     

Enhancement of electrochemical properties of niobium‐doped LiFePO4/C synthesized by sol–gel method

LiFePO₄/C and LiFe_1–xNb _xPO₄/C composites were synthesized using a sol–gel method. The influence of niobium doping on the constitution, morphology, and electrochemical properties of the samples was studied in detail. X‐Ray diffraction patterns indicate that appropriate Nb doping does not alter seriously the structure of LiFePO₄. Electrochemical characterization of the electrodes showed that the Li‐ion batteries based on LiFe_1–xNb _xPO₄/C electrode exhibited better charge/discharge performance than those based on LiFePO₄/C. The LiFe_0.95Nb_0.05PO₄/C‐based cell had the specific capacity of 157, 121, and 85 mAh/g at 0.2, 2, and 5 C, respectively, in comparison with 126, 94, and 52 mAh/g for the LiFePO₄/C cell. The results show that the addition of niobium promotes the electrochemical performance of the materials especially at high charge/discharge rates of the battery.     

Characteristics of nanosized LiNi x Fe1−x PO4/C (x = 0.00–0.20) composite material prepared via sol–gel-assisted carbothermal reduction method

Pure LiFePO₄ and LiNi _x Fe_1?x PO₄/C (x?=?0.00–0.20) nanocomposite cathode materials have been synthesized by cheap and convenient sol–gel-assisted carbothermal reduction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy, and inductively coupled plasma have been used to study the phase, morphology, and chemical composition of un-doped and Ni-doped materials. XRD patterns display the slight shrinkage in crystal lattice of LiFePO₄ after Ni²⁺ doping. The SEM images have revealed that Ni-doped particles are not agglomerated and the particle sizes are practically homogeneously distributed. The particle size is found between 50 and 100 nm for LiNi_0.20Fe_0.80PO₄/C sample. The discharge capacity at 0.2 C rate has increased up to 155 mAh g^?1 for the LiNi_0.05Fe_0.95PO₄/C sample and good capacity retention of 99.1 % over 100 cycles, while that of the unsubstituted LiFePO₄/C and pure LiFePO₄ has showed only 122 and 89 mAh g^?1, respectively. Doping with Ni has a noticeable effect on improving its electrical conductivity. However, serious electrochemical declension will occur when its doping density is beyond 0.05 mol LiNi_0.20Fe_0.80PO₄/C electrode shows only 118 mAh g^?1, which is less than un-doped LiFePO₄/C sample at 0.2 C. The cycling voltammogram demonstrates that Ni-doped LiNi_0.05Fe_0.95PO₄/C electrode has more stable lattice structure, enhanced conductivity, and diffusion coefficient of Li⁺ ions, in which Ni²⁺ is regarded to act as a column in crystal lattice structure to prevent the collapse during cycling process.     

两步固相法合成LiFe1-xNbxPO4/C复合正极材料及其电化学性能

以LiH₂PO₄和FeC₂O₄·2H₂O为原料, 采用分步添加聚乙烯醇和葡萄糖两种碳源的方式, 通过两步固相法合成了碳包覆的LiFePO₄材料. 700℃下处理的产物结晶良好, 颗粒分布均匀, 具有良好的电化学性能, 0.1C和1C倍率下放电比容量分别为157.3 和138.3 mAh·g^-1. 在碳包覆的基础上, 选择高价Nb⁵⁺进行铁位取代获得了复合改性的LiFe_1-xNb_xPO₄/C (x=0.005, 0.01, 0.015, 0.02)材料. 优化的LiFe_0.99Nb_0.01PO₄/C 材料显示了良好的倍率充放电能力和循环稳定性, 0.1C和5C倍率下放电比容量分别为160.5 和136.0 mAh·g^-1, 5C倍率下循环50 次后比容量保持在134.8 mAh·g^-1, 容量保持率为99.1%. 循环伏安测试结果表明, Nb⁵⁺离子掺杂减少了锂离子扩散阻力, 降低了充放电过程中的动力学限制, 提高了电极的可逆性.     

Synthesis of Li3Ni x V2−x (PO4)3/C cathode materials and their electrochemical performance for lithium ion batteries

Li₃Ni _x V_2?x (PO₄)₃/C (x?=?0, 0.02, 0.04 and 0.06) samples have been synthesized via an improved sol–gel method. X-ray diffraction patterns indicate that the structure of the prepared samples retains monoclinic, and the single phase has not been changed with Ni doping. From the analysis of electrochemical performance, the Li₃Ni_0.04?V_1.96(PO₄)₃/C sample exhibits the best electrochemical property. It delivers a discharge capacity of 112.1 mAh?g^?1 with capacity retention of 95.2 % over 300 cycles at 10 C rate in the range of 3.0–4.8 V; cyclic voltammetry and electrochemical impedance spectra testing further prove that the electrochemical reversibility and lithium ion diffusion behavior of Li₃V₂(PO₄)₃ have also been effectively improved through Ni doping.     

Studies of xLiFePO4·yLi3V2(PO4)3/C composite cathode materials with high tap density and high performance prepared by sol spray drying method

The xLiFePO₄·yLi₃V₂(PO₄)₃/C cathode materials are synthesized by a sol spray drying method. X-ray diffraction results reveal that the xLiFePO₄·yLi₃V₂(PO₄)₃/C (x,y?≠?0) composites are composed of LiFePO₄ and Li₃V₂(PO₄)₃ phases, and no impurities are detected. The samples show spherical particles with the size of 0.5–5 μm, and the tap densities of all the samples are higher than 1.5 g cm^?3. Electrochemical tests show that the xLiFePO₄·Li₃V₂(PO₄)₃/C (x,y?≠?0) composites exhibit much better performance than the single LiFePO₄/C or Li₃V₂(PO₄)₃/C. Among all the samples, 3LiFePO₄·Li₃V₂(PO₄)₃/C possesses the best comprehensive performance in terms of the discharge capacity, average working voltage, and rate capability. At 1, 5, and 10 C rates, the sample shows first discharge capacities of 152.0, 134.3, and 116.8 mAh g^?1 and capacity retentions of 99.2, 98.2, and 97.7 % after 100 cycles, respectively. The excellent electrochemical performance of micron-sized xLiFePO₄·Li₃V₂(PO₄)₃/C (x,y?≠?0) powders is owing to the homogeneous mixing of reactants at a molecular level by sol spray drying, the incorporation of fast ion conductor Li₃V₂(PO₄)₃, and the mutual doping in LiFePO₄ and Li₃V₂(PO₄)₃.     

Effect of vanadium doping on electrochemical performance of LiMnPO4 for lithium-ion batteries

A series of LiMn_1-x V _x PO₄ samples have been synthesized successfully via a conventional solid-state reaction method. The active materials are characterized by x-ray diffraction, x-ray photoelectron spectroscopy, and scanning electron microscopy. The electrochemical performances of the samples are tested using cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge measurement techniques. It is confirmed that the samples are in single phase when the content of vanadium (x) is lower than 0.05. If that content is higher than 0.1, the samples are shown to contain an additional conductive phase of Li₃V₂(PO₄)₃. The vanadium doping significantly enhances the electrochemical properties of LiMnPO₄. It is underlined that the optimal ratio for a low-vanadium doping with the best electrochemical performance is 0.1 and this material exhibits a corresponding initial charge and discharge capacity of 98.9 and 98.1 mAh g^?1 at 0.1 C under 50 °C. The capacity retention is higher than 99 % after 30 cycles. The dramatic electrochemical improvement of the LiMnPO₄ samples is ascribed to the strengthened ability of lithium-ion diffusion and enhanced electronic conductivity for the V-doped samples.     

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.     

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.     

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.     

Construction of the nano-carbon connections among the carbon-coated LiFe0.8Mn0.2PO4 grains by sol-gel technique

Nano-carbon connections among carbon-coated LiFe_0.8Mn_0.2PO₄ grains are successfully constructed using polyacrylic acid and sucrose as carbon sources by sol-gel method, which can improve the electrochemical performance of LiFe_0.8Mn_0.2PO₄. Samples were characterised by X-ray diffraction, scanning electron microscopy and electrochemical tests. The electrochemical tests show LiFe_0.8Mn_0.2PO₄ grains connected by nano-carbon networks obtain the discharge specific capacity of 165 mA h g^?1 at 0.1 C rate and excellent rate capability. Its specific capacity reaches 122 mA h g^?1 at 5 C rate. Its capacity retention at 5 C rate attains 97% cycled 100 cycles. Therefore the construction of nano-carbon networks offers an effective and convenient technique to improve the specific capacities and rate capabilities of electrode materials of low electronic conductivity.     

Synthesis and characterization of LiFe0.99Mn0.01 (PO4)2.99/3F0.01/C as a cathode material for lithium-ion battery

The olivine-typed cathode materials of LiFePO₄were prepared via solid-state reaction under argon atmosphere and co-doped by manganese and fluorine to improve their electrochemical performances. The crystal structure, morphology, and electrochemical properties of the prepared samples were investigated using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, cyclic voltammetry, and charge–discharge cycle measurements. The result showed that the electrochemical performance of LiFePO₄ had been improved dramatically by Mn–F co-doping. The initial discharge capacity of LiFe_0.99Mn_0.01 (PO₄)_2.99/3F_0.01/C samples reached 140.2 mAh/g at 1C rate and only had a small amount of fading in 50 cycles.     

Structural and electrochemical characterization of LiFePO4/C prepared by a sol–gel route with long- and short-chain carbon sources

A fast and convenient sol–gel route was developed to synthesize LiFePO₄/C composite cathode material, and the sol–gel process can be finished in less than an hour. Polyethyleneglycol (PEG), d-fructose, 1-hexadecanol, and cinnamic acid were firstly introduced to non-aqueous sol–gel system as structure modifiers and carbon sources. The samples were characterized by X-ray powder diffraction, field emission scanning electron microscopy, and elemental analysis measurements. Electrochemical performances of LiFePO₄/C composite cathode materials were characterized by galvanostatic charge/discharge and AC impedance measurements. The material obtained using compound additives of PEG and d-fructose presented good electrochemical performance with a specific capacity of 157.7 mAh g⁻¹ at discharge rate 0.2 C, and the discharge capacity remained about 153.6 mAh g⁻¹ after 50 cycles. The results indicated that the improved electrochemical performance originated mainly from the microporous network structure, well crystalline particles, and the increased electronic conductivity by proper carbon coating (3.11%).     

Effect of Dy3+‐doping on Electrochemical Properties of LiFePO4/C Cathode for Lithium‐Ion Batteries

Dy doping and carbon coating are adopted to synthesize a LiFePO₄ cathode material in a simple solution environment. The samples were characterized by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). Their electrochemical properties were investigated by cyclic voltammetry (CV) and galvanostatic charge‐discharge tests. An initial discharge capacity of 153 mAh/g was achieved for the LiDy_0.02Fe_0.98PO₄/C composite cathode with a rate of 0.1 C. In addition the electronic conductivity of Dy doped LiFePO₄/C was enhanced to 1.9 × 10^?2 Scm^?1. The results suggest that the improvement of the electrochemical properties are attributed to the dysprosium doping and carbon coating which facilitates the phase transformation between triphylite and heterosite during cycling. XRD data indicate that doping did not destroy the lattice structure of LiFePO₄. To evaluate the effect of Dy substitution, cyclic voltammetry was used at room temperature. prepared. From Cv measurement a more symmetric curve with smaller interval between the cathodic and anodic peak current was obtained by Dy substitution. This denoted a decreasing of polarization with Dy substitution, which illustrated an enhancement of electrochemical performances.     

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.     

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.     

Comparative investigations of LiVPO4F/C and Li3V2(PO4)3/C synthesized in similar soft chemical route

Triclinic LiVPO₄F and monoclinic Li₃V₂(PO₄)₃ are synthesized through a soft chemical process with mechanical activation assist, followed by annealing. In this process, ascorbic acid is used as reducing agent as well as carbon source. The as-prepared samples are coated with amorphous carbon. XPS analysis results show the expected valency states of ions in LiVPO₄F and Li₃V₂(PO₄)₃. The electrochemical properties of the prepared LiVPO₄F/C and Li₃V₂(PO₄)₃/C cathodes are evaluated. The as-prepared LiVPO₄F/C cathode shows an initial discharge specific capacity of 140?±?3 mAh?g^?1 at 30 mA?g^?1 in the voltage range of 3.0~4.4 V, compared with that of 138?±?3 mAh?g^?1 possessed by Li₃V₂(PO₄)₃/C. Both samples exhibit good cycle performance at different current densities. The capacity delivered by LiVPO₄F remains 95.5 and 91.7 % of its initial discharge capacity after 50 cycles at 150 and 750 mA?g^?1, respectively, while 97.4 and 90.6 % for Li₃V₂(PO₄)₃/C. But the rate capability of LiVPO₄F/C is not so good compared with as-prepared Li₃V₂(PO₄)₃/C.