Influence of surface modification by vanadium oxide and carbon on the electrochemical performance of LiFePO4/C

Surface modification with metal oxides is an efficient method to improve the performance of LiFePO₄. Carbon and V₂O₃ co-coated LiFePO₄ is synthesized by carbothermal reduction method combined with star-balling technique, and vanadium oxide is produced in situ. The structure and pattern of LiFePO₄/C modified with different amounts of vanadium oxide (0–5 mol%) were studied by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy. The electrochemical performance of material electrodes was analyzed by constant current charge–discharge and electrochemical impedance spectra (EIS). Electrochemical test results show that sample B (1.0 mol%) exhibits the best electrochemical performance, whose discharge capacity is up to 160.1, 127.2, and 88.4 mAh?g^?1 at 1, 5, and 10 °C, respectively. It indicates that V₂O₃ modification efficiently improves specific capacity and rate capability. The EIS experiment demonstrates that catalytic activity and reversibility of the cathode electrode are obviously increased by the surface modification of vanadium oxide.     

Local structure of vanadium in doped LiFePO4

LiFePO₄ composites with 5 at.% vanadium doping are prepared by solid state reactions. X‐ray absorption fine‐structure spectroscopy is used as a novel technique to identify vanadium sites. Both experimental analyses and theoretical simulations show that vanadium does not enter into the LiFePO₄ crystal lattice. When the vanadium concentration is lower then 1 at.%, the dopant remains insoluble. Thus, a single‐phase vanadium‐doped LiFePO₄ cannot be formed and the improved electrochemical properties of vanadium‐doped LiFePO₄ previously reported cannot be associated with crystal structure changes of the LiFePO₄via vanadium doping.     

A GGA+U study of lithium diffusion in vanadium doped LiFePO4

Recent experiments showed beneficial influence of vanadium doping on the electrochemical performance of lithium iron phosphate (LiFePO₄) cathode materials. First-principles calculations have been performed to investigate the stability, electronic structure and lithium diffusivity of vanadium-doped LiFePO₄ and to elucidate the origin of such improvement. It is found that vanadium prefers occupying Fe sites and leads to additional density of states within the intrinsic bandgap. By the nudged elastic band method, we show that the barrier of Li ions diffusion along the one dimensional channel in both LiFePO₄ and FePO₄ phases can be effectively reduced by vanadium doping. Structural analysis shows the lower diffusion barrier ties closely to a volumetric expansion of the diffusion channel.     

Propylene oxide-assisted fast sol–gel synthesis of mesoporous and nano-structured LiFePO4/C cathode materials

LiFePO₄/C nanocomposites are synthesized by a propylene oxide-assisted fast sol–gel method using FeCl₃, LiNO₃, NH₄H₂PO₄, and sucrose as the starting materials. It was found that after adding propylene oxide into the solution containing the starting materials, a monolithic jelly-like FePO₄ gel containing lithium and carbon source is generated in a few minutes without controlling the pH value of the solution and a time-consuming heating process. Propylene oxide plays a key role in the fast generation of the precursor gel. The final products of LiFePO₄/C are obtained by sintering the dry precursor gel. The structures, micro-morphologies, and electrochemical properties of the LiFePO₄/C composites are investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption analysis, electrochemical impedance spectrum, and charge–discharge cycling tests. The results indicate that the LiFePO₄/C composite prepared by sintering the precursor gel at 680 °C for 5 h is about 30 nm in size with a meso-porous structure (the main pore size distribution is around 3.4 nm). It delivers 166.7 and 105.8 mAh g^?1 at 0.2 and 30 C, respectively. The discharge specific capacity is 97.8 mAh g^?1 even at 40 C. The cycling performance of the prepared LiFePO₄/C composite is stable. The excellent electrochemical performance of the LiFePO₄/C composite is attributed to the nano-sized and mesoporous structure of LiFePO₄/C and the in-situ surface coating of the carbon. It was also found that propylene oxide is crucial for the generation of mesoporous and nano-structured LiFePO₄/C.     

A piperidinium-based ionic liquid electrolyte to enhance the electrochemical properties of LiFePO<Subscript>4</Subscript> battery

A piperidinium-based ionic liquid, N-methylpiperidinium-N-acetate bis(trifluoromethylsulfonyl)imide ([MMEPip][TFSI]), was synthesized and used as an additive to the electrolyte of LiFePO₄ battery. The electrochemical performance of the electrolytes based on different contents of [MMEPip][TFSI] has been investigated. It was found that the [MMEPip][TFSI] significantly improved the high-rate performance and cyclability of the LiFePO₄ cells. In the optimized electrolyte with 3 wt% [MMEPip][TFSI], 70 % capacity can be retained with an increase in rate to 3.5 C, which was 8 % higher than that of electrolyte without [MMEPip][TFSI]. For the Li/LiFePO₄ half-cells, after 100 cycles at 0.1 C, the discharge capacity retention was 78 % in the electrolyte without ionic liquid. However, in the electrolyte with 3 wt% [MMEPip][TFSI], it displayed a high capacity retention of 91 %. The good electrochemical performances indicated that the [MMEPip][TFSI] additive would positively enhance the electrochemical performance of LiFePO₄ battery.     

碳热还原法制备V2O3-C双层包覆磷酸铁锂及其电化学性能

通过V₂O₅的碳热还原反应制备了具有优异倍率性能和循环稳定性的V₂O₃-C双层包覆的磷酸铁锂正极材料. 粉末X射线衍射、元素分析、高分辨投射电镜和拉曼光谱研究表明V2O3相与碳层共包覆于磷酸铁锂颗粒表面. 在V₂O₅的碳热还原反应后,碳含量明显降低,但石墨化程度未发生明显改变. 电化学测试结果表明少量V₂O₃显著改善了磷酸铁锂正极材料的倍率性能和高温循环性能,包含1%氧化钒的复合正极材料在0.2 C放电容量为167 mAh/g,5 C时放电容量为129 mAh/g,并且循环稳定性优异;在55 oC和1 C时放电容量为151 mAh/g,循环100次后无明显容量衰减.     

LiFePO4在不同pH值水溶液中电化学性能表现及其衰减机理

LiFePO₄在含Li⁺水溶液中的电化学性能稳定性与水溶液的pH值密切相关,当溶液的pH值达到11后LiFePO₄在充放电循环过程中的容量衰减十分明显. 通过循环伏安测试、交流阻抗测试、电极充放电性能测试、非原位X射线衍射测试以及化学分析的方式对其容量衰减机理进行了研究. 结果表明LiFePO₄在pH=7的LiNO₃水溶液中具有相对最高的电化学稳定性,但是LiFePO₄材料在水溶液中较之其在有机电解液中依然会有较差的电化学性能表现. 认为LiFePO₄在水介质中的容量衰减现象归因于其在持续充放电过程中的Li、Fe、P溶解,同时电极表面也会附着一层沉淀物. 这些最终导致了材料晶体结构的破坏、电极极化的增大以及电极容量的衰减.     

CdSe–Au nanorod networks welded by gold domains: a promising structure for nano-optoelectronic components

LiFePO_4?x F _x /C nanorods were prepared by room-temperature solid-state reaction and microwave heating. The structure and morphology of the as-prepared materials were analyzed by X-ray diffractometry and transmission electron microscopy. The results shows that the LiFePO_4?x F _x /C were well crystallized and consisted of nanoparticles with an average diameter of ten to several tens of nanometers, many round grains constituted solid rod-like structure. The length of the rods can be up to several hundreds of nanometers, and their diameters are around 100?nm. The results of electrochemical testing show that the initial discharge capacity of LiFePO_3.85F_0.15/C is 124.7?mAh?g^?1, with a negligible fading after 50 cycles at a constant current density of 1 C at room temperature. The capacity retention rate is 99.5?%, which is higher than that of LiFePO₄/C prepared by the same method. The doping of F helps improve electrical conductivity and Li⁺ diffusion of LiFePO₄/C. This study may provide new insights and understanding on the effect of F-doping on the electrochemical performance of LiFePO₄/C.     

Particle size distribution and electrochemical properties of LiFePO4 prepared by a freeze-drying method

The electrochemical performance of carbon-coated nanocrystalline LiFePO₄ prepared by a freeze-drying method is examined. This method is based on the thermal decomposition of homogeneous phosphate-formate precursors. Structural and morphological characterization of LiFePO₄ is carried out by powder XRD, BET measurements, SEM and XPS analyses. The electrochemical behaviour is tested in model lithium cells using galvanostatic mode. By changing the solution concentration, the freeze-drying method allows preparing LiFePO₄ with mean particle sizes between 60 and 100 nm and different particle size distributions. The content of carbon appearing mainly on the particle surface depends on both the solution concentration and the annealing temperature. The effect of particle size distribution on the voltage profile of LiFePO₄ is also demonstrated. The specific capacity is mainly determined by the amount of carbon deposited on the particle surfaces.     

Effects of yttrium ion doping on electrochemical performance of LiFePO<Subscript>4</Subscript>/C cathodes for lithium-ion battery

The olivine-type LiFe_1-x Y _x PO₄/C (x?=?0, 0.01, 0.02, 0.03, 0.04, 0.05) products were prepared through liquid-phase precipitation reaction combined with the high-temperature solid-state method. The structure, morphology, and electrochemical performance of the samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy-dispersive spectroscopy (EDS), galvanostatic charge-discharge, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). We found that the small amount of Y³⁺ ion-doped can keep the microstructure of LiFePO₄, modify the particle morphology, decrease charge transfer resistance, and enhance exchange current density, thus enhance the electrochemical performance of the LiFePO₄/C. However, the large doping content of Y³⁺ ion cannot be completely doped into LiFePO₄ lattice, but existing partly in the form of YPO₄. The electrochemical performance of LiFePO₄/C was restricted owing to YPO₄. Among all the doped samples, LiFe_0.98Y_0.02PO₄/C showed the best electrochemical performance. The LiFe_0.98Y_0.02PO₄/C sample exhibited the initial discharge capacity of 166.7, 155.8, 148.2, 139.8, and 121.1 mAh g^?1 at a rate of 0.2, 0.5, 1, 2, and 5 C, respectively. And, the discharge capacity of the material was 119.6 mAh g^?1 after 100 cycles at 5 C rates.     

Kinetic study on LiFePO<Subscript>4</Subscript>-positive electrode material of lithium-ion battery

LiFePO₄-positive electrode material was successfully synthesized by a solid-state method, and the effect of storage temperatures on kinetics of lithium-ion insertion for LiFePO₄-positive electrode material was investigated by electrochemical impedance spectroscopy. The charge-transfer resistance of LiFePO₄ electrode decreases with increasing the storage temperatures. This suggests that it has a high electrochemical activity at high temperature. The diffusion coefficient of lithium ion is greatly increased with increasing the storage temperatures, indicating that the kinetics of Li⁺ and electron transfer into the electrodes were much fast at high storage temperature.     

Electrochemical behavior of Mg-doped 7LiFePO4–Li3V2(PO4)3 composite cathode material for lithium-ion batteries

Mg-doping effects on the electrochemical property of LiFePO₄–Li₃V₂(PO₄)₃ composite materials, a mutual-doping system, are investigated. X-ray diffraction study indicates that Mg doping decreases the cell volume of LiFePO₄ in the composite. The cyclic voltammograms reveal that the reversibility of the electrode reaction and the diffusion of lithium ion is enhanced by Mg doping. Mg doping also improves the conductivity and rate capacity of 7LiFePO₄–Li₃V₂(PO₄)₃ composite material and decreases the polarization of the electrode reaction. The discharge capacity of the Mg-doped composite was 93 mAh?g^?1 at the current density of 1,500 mA?g^?1, and Mg-doped composite has better discharge performance than the original 7LiFePO₄–Li₃V₂(PO₄)₃ composite at low temperature, too. At ?30 °C, the discharge capacity of Mg-doped LFVP is 89 mAh?g^?1, higher than that of the original composite. Electrochemical impedance spectroscopy study shows that Mg²⁺ doping could enhance the electrochemical activity of 7LiFePO₄–Li₃V₂(PO₄)₃ composite. Mg doping has a positive influence on the electrochemical performance of the LiFePO₄–Li₃V₂(PO₄)₃ composite material.     

Li3V2(PO4)3 modified LiFePO4/C cathode materials with improved high-rate and low-temperature properties

LiFePO₄/C surface modified with Li₃V₂(PO₄)₃ is prepared with a sol–gel combustion method. The structure and electrochemical behavior of the material are studied using a wide range of techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. It is found that LiFePO₄/C surface modified with Li₃V₂(PO₄)₃ has the better electrochemical performance. The discharge capacity of the as-prepared material can reach up to 153.1, 137.7, 113.6, and 93.3 mAh g^?1 at 1, 2, 5, and 10 C, respectively. The capacitance of the LiFePO₄/C modified by Li₃V₂(PO₄)₃ is higher under lower discharging rate at ?20 °C, and the initial discharge capacity of 0.2 C is 131.4 mAh g^?1. It is also demonstrated that the presence of Li₃V₂(PO₄)₃ in the sample can reduce the charge transfer resistance in the range of ?20 to 25 °C, resulting in the enhanced electrochemical catalytic activity.     

Improved of cathode performance of LiFePO<Subscript>4</Subscript>/C composite using different carboxylic acids as carbon sources for lithium-ion batteries

Among the several materials under development for use as a cathodes in lithium-ion batteries olivine-type LiFePO₄ is one of the most promising cathode material. However, its poor conductivity and low lithium-ion diffusion limits its practical application. In this study, we report seven different carboxylic acids used to synthesize LiFePO₄/C composite, and influences of carbon sources on electrochemical performance were intensively studied. The structure and electrochemical properties of the LiFePO₄/C were characterized by X-ray diffraction, scanning electron microscopy, electrical conductivity, and galvanostatic charge–discharge measurements. Among the materials studied, the sample E with tartaric acid as carbon source exhibited the best cell performance with a maximum discharge capacity of 160 mAh g⁻¹ at a 0.1 C-rate. The improved electrochemical properties were attributed to the reduced particle size and enhanced electrical contacts by carbon.     

Electrochemical characterization of ionic liquid based gel polymer electrolyte for lithium battery application

High molecular weight polymer poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP), ionic liquid 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIMFSI), and salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-based free-standing and conducting ionic liquid-based gel polymer electrolytes (ILGPE) have been prepared by solution cast method. Thermal, electrical, and electrochemical properties of 80 wt% IL containing gel polymer electrolyte (GPE) are investigated by thermogravimetric (TGA), impedance spectroscopy, linear sweep voltammetry (LSV), and cyclic voltammetry (CV). The 80 wt% IL containing GPE shows good thermal stability (~?200 °C), ionic conductivity (6.42?×?10^?4 S cm^?1), lithium ion conductivity (1.40?×?10^?4 S cm^?1 at 30 °C), and wide electrochemical stability window (~?4.10 V versus Li/Li⁺ at 30 °C). Furthermore, the surface of LiFePO₄ cathode material was modified by graphene oxide, with smooth and uniform coating layer, as confirmed by scanning electron microscopy (SEM), and with element content, as confirmed by energy dispersive X-ray (EDX) spectrum. The graphene oxide-coated LiFePO₄ cathode shows improved electrochemical performance with a good charge-discharge capacity and cyclic stability up to 50 cycles at 1C rate, as compared with the without coated LiFePO₄. At 30 °C, the discharge capacity reaches a maximum value of 104.50 and 95.0 mAh g^?1 for graphene oxide-coated LiFePO₄ and without coated LiFePO₄ at 1C rate respectively. These results indicated improved electrochemical performance of pristine LiFePO₄ cathode after coating with graphene oxide.     

A facile hydrothermal method to prepare LiFePO4/C submicron rod with core–shell structure

Submicron rod LiFePO₄/C has been synthesized via a facile hydrothermal process. The morphology, crystal structure, and charge–discharge performance of the prepared samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and galvanostatic charge–discharge testing. The SEM and TEM illustrate that submicron rods with a width of about 140 nm and a length of up to 400 nm have been obtained. The TEM test also indicates a “core–shell” structure with a 1.5–2 nm carbon shell on the LiFePO₄ core. Even though the separate carbon-coated procedure is not used in this method, the electrochemical behavior results are satisfied. It displays that LiFePO₄/C has highly crystalline and a desirable core–shell structure with uniform carbon film. Galvanostatic battery testing shows that LiFePO₄/C delivers 104 mAh g^?1 at 5 C rates. The highest specific capacity of 166 mAh g^?1 is achieved at 0.1 C rate, and 99.8 % of the initial specific capacitance remained after 30 cycles.     

Recent developments in the doping and surface modification of LiFePO4 as cathode material for power lithium ion battery

Lithium ion batteries have become attractive for portable devices due to their higher energy density compared to other systems. With a growing interest to develop rechargeable batteries for electric vehicles, lithium iron phosphate (LiFePO₄) is considered to replace the currently used LiCoO₂ cathodes in lithium ion cells. LiFePO₄ is a technically important cathode material for new-generation power lithium ion battery applications because of its abundance in raw materials, environmental friendliness, perfect cycling performance, and safety characteristics. However, the commercial use of LiFePO₄ cathode material has been hindered to date by their low electronic conductivity. This review highlights the recent progress in improving and understanding the electrochemical performance like the rate ability and cycling performance of LiFePO₄ cathode. This review sums up some important researches related to LiFePO₄ cathode material, including doping and coating on surface. Doping elements with coating conductive film is an effective way to improve its rate ability.     

A novel carbon source coated on C-LiFePO<Subscript>4</Subscript> as a cathode material for lithium-ion batteries

The poor electronic conductivity and low lithium-ion diffusion are the two major obstacles to the largely commercial application of LiFePO₄ cathode material in power batteries. In order to improve the defects of LiFePO₄, a novel carbon source polyacrylonitrile (PAN), which would form the hierarchical porous structure after carbonization, is fabricated and used. This work comes up with a simple and facile carbothermal reduction method to prepare porous-carbon-coated LiFePO₄ (C-LiFePO₄-PC) composite and to study the effect of carbon-coated temperature on ameliorating the electrochemical performance. The obtained C-LiFePO₄-PC composite shows a high initial discharge capacity of 164.1 mA h g^?1 at 0.1 C and good cycling stability as well as excellent rate capacity (49.0 mA h g^?1 at 50 C). The most possible factors that improve the electrochemical performance could be related to the enhancement of electronic conductivity and the existence of porous carbon layers. In a word, the C-LiFePO₄-PC material would become an excellent candidate for application in the fields of lithium-ion batteries.     

Effect of slurry preparation and dispersion on electrochemical performances of LiFePO<Subscript>4</Subscript> composite electrode

A study is attempted on how the dispersion of solid ingredients in solvent affects the electrochemical performance of LiFePO₄ composite electrode. The slurries comprising LiFePO₄ powder, carbon black and polymeric binder in solvent NMP (N-methyl-2-pyrrolidone) are prepared by two different processes, and results in different dispersion qualities. With better dispersion, the surfactant (Triton-100) is added into slurry as dispersing agent, whereas no dispersing agent in the other sample. The former process leads to more uniform dispersion of solid ingredients as compared to the latter. In the composite electrode prepared from the former process, the LiFePO₄ and carbon black particles are homogeneously distributed without obviously reunion. Indebted to this favourable feature, this electrode exhibits a better high C-rate electrochemical performance for cycling and capacity than those without the surfactant.     

Charge storage performance of lithiated iron phosphate/activated carbon composite as symmetrical electrode for electrochemical capacitor

In this study, a symmetric electrochemical capacitor was fabricated by adopting a lithium iron phosphate (LiFePO₄)-activated carbon (AC) composite as the core electrode material in 1.0 M Na₂SO₃ and 1.0 M Li₂SO₄ aqueous electrolyte solutions. The composite electrodes were prepared via a facile mechanical mixing process. The structural properties of the nanocomposite electrodes were characterised by scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) analysis. The electrochemical performances of the prepared composite electrode were studied using cyclic voltammetry (CV), galvanostatic charge–discharge (CD) and electrochemical impedance spectroscopy (EIS). The experimental results reveal that a maximum specific capacitance of 112.41 F/g was obtained a 40 wt% LiFePO₄ loading on an AC electrode compared with that of a pure AC electrode (76.24 F/g) in 1 M Na₂SO₃. The improvement in the capacitive performance of the 40 wt% LiFePO₄–AC composite electrode is believed to be attributed to the contribution of the synergistic effect of the electric double layer capacitance (EDLC) of the AC electrode and pseudocapacitance via the intercalation/extraction of H⁺, OH⁻, Na⁺ and SO₃²⁻ and Li⁺ ions in LiFePO₄ lattices. In contrast, it appears that the incorporation of LiFePO₄ into AC electrodes does not increase the charge storage capability when Li₂SO₄ is used as the electrolyte. This behaviour can be explained by the fact that the electrolyte system containing SO₄²⁻ only exhibits EDLC in the Fe-based electrodes. Additionally, Li⁺ ions that have lower conductivity and mobility may lead to poorer charge storage capability compared to Na⁺ ions. Overall, the results reveal that the AC composite electrodes with 40 wt% LiFePO₄ loading on a Na₂SO₃ neutral electrolyte exhibit high cycling stability and reversibility and thus display great potential for electrochemical capacitor applications.