Enhancement of capacity at high charge/discharge rate and cyclic stability of LiFePO4/C by nickel doping

By introducing nickel chemical into the precursor sol of LiFePO₄, a series of Ni-doped LiFePO₄ composite cathode materials, denoted as LiFe_1???x Ni _x PO₄/C (x?=?0, 0.01, 0.03, 0.05 and 0.10) were prepared by a spray drying–carbothermal approach. The materials were characterized with X-ray diffraction (XRD), scanning electron microscope (SEM), and electrochemical impedance spectrum etc. It is found that the doping of nickel with appropriate amount caused a slight shift of diffraction peaks towards higher angles and enhanced the dispersion of nanoprimary particles, which could be observed from their XRD patterns and SEM images. For the sample with 3 mol% Ni doing, the charge transfer resistance reduced from 52.4?Ω of LiFePO₄ to 18.7?Ω of LiFe_0.97Ni_0.3PO₄/C, and the potential interval of the redox peaks reduced from 0.51 to 0.40 V, indicating the better reversible of Ni-doped materials. For the sample LiFe_0.97Ni_0.03PO₄/C, its initial discharge capacities at various rates are 169.2 (0.2 C), 156.2 (1.0 C), 147.9 (2.0 C), 135.5 (5.0 C), and 94.0 (10.0 C)?mAh g^?1, respectively, enhanced by 55.2 % (at 5.0 C) and 82.1 % (at 10.0 C) compared with LiFePO₄. Furthermore, after 200 cycles of charge/discharge at 0.5 C, the capacity of LiFe_0.97Ni_0.03PO₄/C only decreased 8.8 %, but over 25 % decrease was observed for LiFePO₄/C.     

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.     

Structural and electrochemical properties of LiFe1 − 3x/2Bi x PO4/C synthesized by sol–gel

In this study, LiFe_1???3x/2Bi _x PO₄/C cathode material was synthesized by sol–gel method. From XRD patterns, it was found that the Bi-doped LiFePO₄/C cathode material had the same olivine structure with LiFePO₄/C. SEM studies revealed that Bi doping can effectively decrease the particle sizes. It shortened Li⁺ diffusion distance between LiFePO₄ phase and FePO₄ phase. The LiFe_0.94Bi_0.04PO₄/C powder exhibited a specific initial discharge capacity of about 149.6 mAh g^?1 at 0.1 rate as compared to 123.5 mAh g^?1 of LiFePO₄/C. EIS results indicated that the charge-transfer resistance of LiFePO₄/C decreased greatly after Bi doping.     

Synthesis and lithium-ion storage performances of LiFe<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>PO<Subscript>4</Subscript>/C nanoplatelets and nanorods

Carbon-coated olivine-structured LiFe_0.5Co_0.5PO₄ solid solution was synthesized by a facile rheological phase method and applied as cathode materials of lithium-ion batteries. The nanostructure’s properties, such as morphology, component, and crystal structure for the samples, characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer, Emmett, and Teller (BET) determination, X-ray photoelectron spectroscopy (XPS), and the electrochemical performances were evaluated using constant current charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results indicate that nanoplatelet- and nanorod-structured LiFe_0.5Co_0.5PO₄/C composites were separately obtained using stearic acid or polyethylene glycol 400 (PEG400) as carbon source, and the surfaces of particles for the two samples are ideally covered by full and uniform carbon layer, which is beneficial to improving the electrochemical behaviors. Electrochemical tests verify that the nanoplatelet LiFe_0.5Co_0.5PO₄/C shows a better capacity capability, delivering a discharge specific capacity of 133.8, 112.1, 98.3, and 74.4 mAh g^?1 at 0.1, 0.5, 1, and 5 C rate (1 C?=?150 mA g^?1); the corresponding cycle number is 5th, 11th, 15th, 20th, and 30th, respectively, whereas the nanorod one possesses more excellent cycling ability, with a discharge capacity of 83.3 mAh g^?1 and capacity retention of 86.9% still maintained after cycling for 100 cycles at 0.5 C. Results from the present study demonstrate that the LiFe_0.5Co_0.5PO₄ solid solution nanomaterials with favorable carbon coating effect combine the characteristics and advantage of LiFePO₄ and LiCoPO₄, thus displaying a tremendous potential as cathode of lithium-ion battery.     

Hydrothermal synthesis and properties of manganese-doped LiFePO4

A series of carbon-coated LiFe_1???x Mn _x PO₄ compounds are prepared by a hydrothermal method at 170 °C for 12 h. The structure and morphology of the prepared composites are characterized to examine the effects of Mn²⁺ substitution. All LiFe_1???x Mn _x PO₄ compositions are found to have an ordered olivine-type structure with homogeneous Fe²⁺ and Mn²⁺ distributions. The substitution leads to grain refinement from ~500 to ~150 nm, as well as to increased initial capacity and improved electronic conductivity. The amount of carbon coating varies with increased doping amount. The discharge curves of the LiFe_1???x Mn _x PO₄/C materials reveal a high discharge plateau corresponding to Fe^2+/3+ and no obvious plateau assigned to Mn^2+/3+, although a slight contribution of manganese is detected. However, the electrochemical performance, including the discharge capacity and cyclic performance, deteriorates with increased Mn content in the composite.     

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.     

Enhanced electrochemical performance of LiFePO4/C prepared by sol–gel synthesis with dry ball-milling

LiFePO₄/C was prepared by a modified aqueous sol–gel route developed by incorporating an additional ball-milling step where the dry gel was milled with the additives of synthetic graphite and carbon black. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and elemental analysis. Results showed that the LiFePO₄/C synthesized by suitable ball-milling process had pure, fine and homogenous LiFePO₄ particles. Results of cyclic voltammetry and charge/discharge plateaus demonstrated that the LiFePO₄/C composite synthesized by milling for 2 h had much better electrochemical kinetics. High performances were achieved with its discharge capacities of 157 mA h g^?1 at 0.1?C and 133 mA h g^?1 at 1?C between 2.5 and 4.2 V (1?C?=?170 mA g^?1). And no obvious capacity fading was observed upon cycling. The simple and convenient synthesis route is promising for large-scale production of LiFePO₄/C.     

Electrochemical comparison of LiFe<Subscript>0.4</Subscript>Mn<Subscript>0.595</Subscript>Cr<Subscript>0.005</Subscript>PO<Subscript>4</Subscript>/C and LiMnPO<Subscript>4</Subscript>/C cathode materials

A comparison of electrochemical performance between LiFe_0.4Mn_0.595Cr_0.005PO₄/C and LiMnPO₄/C cathode materials was conducted in this paper. The cathode samples were synthesized by a nano-milling-assisted solid-state process using caramel as carbon sources. The prepared samples were investigated by XRD, SEM, TEM, energy-dispersive X-ray spectroscopy (EDAX), powder conductivity test (PCT), carbon-sulfur analysis, electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge cycling. The results showed that LiFe_0.4Mn_0.595Cr_0.005PO₄/C exhibited high specific capacity and high energy density. The initial discharge capacity of LiFe_0.4Mn_0.595Cr_0.005PO₄/C was 163.6 mAh g^?1 at 0.1C (1C = 160 mA g^?1), compared to 112.3 mAh g^?1 for LiMnPO₄/C. Moreover, the Fe/Cr-substituted sample showed good cycle stability and rate performance. The capacity retention of LiFe_0.4Mn_0.595Cr_0.005PO₄/C was 98.84 % over 100 charge-discharge cycles, while it was only 86.64 % for the pristine LiMnPO₄/C. These results indicated that Fe/Cr substitution enhanced the electronic conductivity for the prepared sample and facilitated the Li⁺ diffusion in the structure. Furthermore, LiFe_0.4Mn_0.595Cr_0.005PO₄/C composite presented high energy density (606 Wh kg^?1) and high power density (574 W kg^?1), thus suggested great potential application in lithium ion batteries (LIBs).     

通过掺杂V和F提高碳包覆的磷酸铁锂锂离子电池的电化学性能

在原位聚合合成方法的基础上,结合两步烧结过程制得LiFe_1-xV_x(PO₄)_(3-y)/₃F_y/C.V和F掺杂对碳包覆的磷酸铁锂材料的结构、形貌和电化学性能有影响.通过XRD、FTIR、SEM、充/放电测试和电化学阻抗谱对材料的结构、形貌和电化学性能进行了表征.结果表明,V和F的掺杂并没有破坏橄榄石结构中的LiFePO₄/C,但可以提高晶体结构的稳定,降低电荷的转移阻抗,提高锂离子扩散速度,改善了LiFePO₄/C材料的循环性能和高倍率性能.     

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.     

First principles study on electronic properties and occupancy sites of molybdenum doped into LiFePO4

The electronic properties of Mo-doped LiFePO₄ and occupancy sites of Mo are investigated by employing the density functional theory plane-wave pseudopotential method. The calculated results show that Mo doping at Fe site has lower formation energy, which implies that Mo dopants prefer to occupy Fe sites within the LiFePO₄ lattice. Furthermore, the LiFe_1?3/12Mo_1/12PO₄ has wider lithium ion migration channels than Li_1?6/12Mo_1/12FePO₄. For the case of LiFe_1?3/12Mo_1/12PO₄, the calculated narrow band gap (0.18 eV) indicates that the electronic conductivity of LiFePO₄ could be enhanced by doping Mo at the Fe sites. The density of states and charge densities of LiFe_1?3/12Mo_1/12PO₄ demonstrate that the Mo-4d states and MoO bonding play important roles in band gap reduction of LiFe_1?3/12Mo_1/12PO₄.     

Synthesis of Cr-doped LiMnPO4/C cathode materials by sol–gel combined ball milling method and its electrochemical properties

LiMn_1-x Cr _x PO₄/C (x?=?0, 0.01, 0.03, and 0.05) compounds are synthesized by a sol–gel combined ball milling method. The effects of Cr doping on the structure, morphology, and electrochemical performance of LiMnPO₄ are investigated. XRD analysis results indicate that all the samples exhibit the single LiMnPO₄ phase and Cr ions substitute on Mn site (x?≤?0.03), with charge compensating vacancies on Li site. The vacancies are of benefit to improving the electronic conductivity of LiMnPO₄. SEM studies reveal that Cr doping can effectively inhibit the aggregation of LiMnPO₄ particles. Electrochemical tests show that the Cr-doped samples exhibit higher initial capacities and better cycling performance than the undoped one. LiMn_0.97Cr_0.03PO₄/C exhibit the best electrochemical performance that the first specific discharge capacity is 132.4 mAh g^?1 at 0.1 C rate, and the capacity retention is 94.8 % after 30 cycles.     

Study on LiFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript>/C used as cathode materials for lithium-ion batteries with low Sm component

LiFe_1???x Sm _x PO₄/C cathode materials were synthesized though a facile hydrothermal method. Compared with high-temperature solid-phase sintering, the method can allow for the fabrication of low Sm content (2 %), a scarce and expensive rare earth element, while the presence of an optimized carbon coating with large amount of sp²-type carbon sharply increases the material’s electrochemical performance. The high-rate dischargeability at 5 C, as well as the exchange current density, can be increased by 21 and 86 %, respectively, which were attributed to the fine size and the large cell parameter a/c as much. It should be pointed out that the a/c value will be increased for the LiFePO₄ Sm-doped papered by both of the two methods, while the mechanism is different: The value c is increased for the front and the value a is decreased for the latter, respectively.     

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.     

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.     

LiFexMn1-xPO4固溶体的密度泛函理论与电化学研究

本文用DFT计算方法研究了LiFe_xMn_1-xPO₄的热力学稳定性和嵌/脱锂电位. 结果表明,LiFe_xMn_1-xPO₄固溶体的自由能比相分离的LiFePO₄/LiMnPO₄混合物略高,这两种形式可能在实际LiFe_xMn_1-xPO₄材料中共存. 计算表明,LiFe_xMn_1-xPO₄固溶体的嵌/脱锂电位随锰/铁比以及过渡金属离子的空间排列而变化,并用计算结果解释了放电曲线的形状. 采用固相反应法合成了LiFe_xMn_1-xPO₄材料并研究了其电化学性质,实验中观察到附加的放电平台,其出现可能与LiFe_xMn_1-xPO₄固溶体的存在有关.     

Modified carbothermal synthesis and electrochemical performance of LiFePO4/C composite as cathode materials for lithium-ion batteries

LiFePO₄/C active materials were synthesized via a modified carbothermal method, with a low raw material cost and comparatively simple synthesis process. Rheological phase technology was introduced to synthesize the precursor, which effectively decreased the calcination temperature and time. The LiFePO₄/C composite synthesized at 700 °C for 12 h exhibited an optimal performance, with a specific capacity about 130 mAh g^?1 at 0.2C, and 70 mAh g^?1 at 20C, respectively. It also showed an excellent capacity retention ratio of 96 % after 30 times charge–discharge cycles at 20C. EIS was applied to further analyze the effect of the synthesis process parameters. The as-synthesized LiFePO₄/C composite exhibited better high-rate performance as compared to the commercial LiFePO₄ product, which implied that the as-synthesized LiFePO₄/C composite was a promising candidate used in the batteries for applications in EVs and HEVs.     

Synthesis of carbon microtubules core structure LiFePO4 via a template-assisted method

The carbon microtubules core structure LiFePO₄ is synthesized using a cotton fiber template-assisted method. The crystalline structure and morphology of the product is characterized by X-ray diffraction and field emission scanning electron microscopy. The charge–discharge kinetics of the LiFePO₄ electrode is investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The result shows that the well-crystallized carbon microtubules core structure LiFePO₄ is successfully synthesized. The as-synthesized material exhibits a high initial discharge capacity of 167 mAh g^?1 at 0.2 C rate. The material also shows good high-rate discharge performance and cycling stability, about 127 mAh g^?1 and 94.7 % capacity retention after 100 cycles even at 5.0 C rate.     

Synthesis and electrochemical characterization of graphene nanoflakes and LiFe<Subscript>0.97</Subscript>Ni<Subscript>0.03</Subscript>PO<Subscript>4</Subscript>/C for lithium-ion battery

The graphene nanoflakes and olivine-type LiFe_0.97Ni_0.03PO₄/C (LFNP3/C) samples have been synthesized as anode and cathode materials, respectively. Physicochemical characterization of the graphene nanoflakes and LFNP3/C material were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM). The XRD patterns reveal the formation of the pure phase of both the synthesized samples. SEM micrographs disclose the formation of spherically shaped nanosized particles for LFNP3/C while graphene shows flake-type morphology. CR2032 half and full coin cells were assembled for electrochemical testing of the synthesized samples. Cyclic voltammetry (CV) results indicate that the graphene-based half-cells, i.e., GN1H and GN2H, possess reduction peak/plateau around 0.17 V while LFNP3/C cathode shows discharging voltage plateau at 3.4 V vs. Li/Li⁺. The discharge capacities were found to be 700, 900, and 153 mAhg^?1 for GN1H, GN2H, and LFNP3/C half-cells vs. Li/Li⁺, respectively. Among full cells, LFPGN1F with γ = 0.75 (mass/capacity balancing factor) shows better charging/discharging profile at each C-rate as compared to LFPGN2F with γ = 0.55. LFPGN1F delivered an initial discharge capacity of around 154 mAhg^?1 at 0.1C and even at a high discharge rate of 1C, it retained ~97% of the discharge capacity as compared to the initial cycle at the same rate.     

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.