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.     

Preparation of Li3V2 (PO4)3/LiFePO4 composite cathode material for lithium ion batteries

A novel process was attempted for synthesis of Li₃V₂ (PO₄)₃/LiFePO₄ composite cathode material via loading nano-LiFePO₄ (LFP) powders onto the outside of micrometer-size spherical Li₃V₂ (PO₄)₃ (LVP). The precursor of nano-LFP and LVP were synthesized via “controlled crystallization” and “spray drying” techniques, respectively. The X-ray diffraction characterization, scanning electron microscopy, and electrochemical performance measurements were studied. The results indicated that the prepared Li₃V₂(PO₄)₃/LiFePO₄ (LVP/LFP) composite material exhibited better discharging capacity at high C rate and at low temperature than that of LFP and bulk LVP/LFP. This can pave an effective way to improve the performance of LFP at high C rate and at low temperature.     

Preparation and characterization of Li3V2(PO4)3/MWCNTs cathode material for lithium-ion batteries

The Li₃V₂(PO₄)₃/multiwalled carbon nanotubes (LVP/MWCNTs) composite is successfully synthesized by a sol?Cgel route using oxalic acid as the chelating reagent. Its structure and physicochemical properties are investigated using X-ray diffraction, field-emission scanning electron microscopy, and electrochemical methods. LVP particles are well mixed with MWCNTs, and most of them are around 100?nm. The galvanostatic charge?Cdischarge tests show that LVP/MWCNTs electrode owns an initial discharge capacity of 126?mAh?g^?1 at 0.5 C with capacity retention of 94% during the 100th cycle in the voltage range of 3.0?C4.3?V. A superior rate capability is also achieved, e.g., exhibiting discharge capacities of 75 and 58?mAh?g^?1 at high C rates of 10 and 15 C, respectively.     

Synthesis of flower-like Li3V2(PO4)3/C cathode with mixed morphology for advanced lithium-ion batteries

A rheological phase-assisted ball milling method was developed to synthesize of flower-like Li₃V₂(PO₄)₃/C composites consisting of nanofibers and nanoplate porous microstructure. The flower-like Li₃V₂(PO₄)₃/C composite delivered specific capacities of 120 and 108 mAh g^?1 at 0.5 and 10 C rates, respectively. A capacity retention of 99.5 % was sustained after 100 cycles at a 10-C cycling rate. The remarkable performance was attributed to the porous nanostructures that provide short electron/ion diffusion distance and large electrode/electrolyte contact area.     

Synthesis of NiCo2S4 nanospheres/reduced graphene oxide composite as electrode material for supercapacitor

The NiCo₂S₄ nanospheres arrayed on the surface of reduced graphene oxide (rGO) was fabricated via one-step hydrothermal method. The effect of initial feeding mass of Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O to rGO on the microstructure and electrochemical performance of the as-prepared composites was studied. The results indicated that the specific capacitances of the composites were first increased and then reduced due to the aggregation of NiCo₂S₄ nanospheres. NiCo₂S₄ nanospheres/rGO composites exhibited a remarkable specific capacitance of 1406 F/g and excellent cyclic stability of 82.36% at the current density of 1 A/g, which were better than those of individual NiCo₂S₄ (792 F/g and 64.77%) counterpart. These results showed that the as-prepared NiCo₂S₄ nanospheres/rGO composites were outstanding candidate for electrode material of supercapacitors.     

Effects of synthetic route on the structural,physical and electrochemical properties of Li3V2(PO4)3 cathode materials

A series of monoclinic Li₃V₂(PO₄)₃ cathode materials were prepared by H₂ reduction (LVP-H₂) and carbothermal reduction (LVP-CTR) methods. LVP-H₂ showed a primary particle size of about 1 μm, which was much larger than the LVP-CTR samples. A uniform surface carbon layer was observed for the LVP-CTR samples by transmission electron microscope. This carbon layer not only limited the particle growth of the materials but also enhanced the material's electronic conductivity by five orders of magnitude. The LVP-CTR samples exhibited much better electrochemical performance than LVP-H₂. The good electrochemical performance of LVP-CTR was attributed to its nano particle size, high electronic conductivity, as well as the surface carbon layer which limited the vanadium dissolution in electrolyte.     

Ionic conductivity of isostructural crystals of the superionic conductors Li3Fe2(PO4)4 and Li3Sc2(PO4)3

The results of measurements of the ionic conductivity σ in Li₃M₂(PO₄)₃ (M=Fe, Sc) single crystals along various crystallographic directions are analyzed. Possible causes of the different behavior of σ in the isostructural crystals are discussed: a jump of the conductivity in the transition to the superionic phase in Li₃Sc₂(PO₄)₃ and its absence in Li₃Fe₂(PO₄)₃; the existence of a conductivity maximum in different crystallographic directions (along the c axis in Li₃Sc₂(PO₄)₃ and along the a axis in Li₃Fe₂(PO₄)₃). Fiz. Tverd. Tela (St. Petersburg) 39, 83–86 (January 1997)     

Structural and electrical characterization of the NASICON-type Li2FeZr(PO4)3 and Li2FeTi(PO4)3 compounds

In order to develop new electrolytes for all-solid-state rocking chair lithium batteries, the NASICON-type compounds Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ were investigated by powder X-ray diffraction technique and impedance spectroscopy. Li₂FeZr(PO₄)₃ is orthorhombic Pbna (a=8.706(3), b=8.786(2), c=12.220(5) Å) and Li₂FeTi(PO₄)₃ is orthorhombic Pbca (a=8.557(3), b=8.624(3), c=23.919(6) Å). They show no phase transitions from RT to 800 °C. In the same temperature range logσT vs. 1/T show no slope variations. The activation energies for the ionic conductivity were 0.62 and 0.64 eV for Li₂FeTi(PO₄)₃ and Li₂FeTi(PO₄)₃, respectively. In order to better evaluate the present results they were compared with those of α and β-LiZr₂(PO₄)₃ phases, which were also prepared and characterised. A change of activation energy from 0.47 eV to 1.03 eV was observed in the case of β phase, at about 300 °C; attributed to the β (orthorhombic) ? β′ (monoclinic) phase transition. In the α phase the activation energy 0.47 eV in the temperature range 150 – 850 °C. The Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ compounds can be interesting for applications as solid electrolytes in high temperature (>300 °C) lithium batteries.     

Enhanced rate capability of LiMn0.9Mg0.1PO4 nanoplates by reduced graphene oxide/carbon double coating for Li-ion batteries

The reduced graphene oxide (RGO)/carbon double-coated LiMn_0.9Mg_0.1PO₄ (LMP) nanoplates are introduced as a cathode material for Li-ion batteries with excellent rate capability. The double coating of RGO and carbon simultaneously brings the unique advantages of conformal carbon layer on each particle surface, and soft RGO sheets connecting the nanoplates to each other, thereby provides easy conduction pathways for the whole LMP aggregates. In particular, the simple self-assembly process driven by the electrostatic interactions enables conducting RGO sheets effectively to wrap the carbon-coated LMP, establishing three-dimensional RGO network. The RGO/C/LMP nanocomposites exhibit remarkably enhanced rate capability compared to the only C- or RGO-coated LMP, which is well explained by the reduced charge-transfer resistance achieved from electrochemical impedance spectroscopy.     

Glass Transition (T g ) and Dielectric Constant (e'r) of Li3PO4-Pb3(PO4)2-BiPO4 (Li2O-PbO-Bi2O3-P2O5) Glasses

Thermal and dielectric properties of heavy metal oxide glasses, Li₃PO₄-Pb₃(PO₄)₂-BiPO₄ (Li₂O-PbO-Bi₂O₃-P₂O₅), were studied from ambient temperature to 500°C by differential thermal analysis (DTA) and dielectric constant (?′_r) measurements. Experiment results show a strong influence of lithium, lead and bismuth ions on T _g and ?′_r.     

High power nano-LiMn2O4 cathode materials with high-rate pulse discharge capability for lithium-ion batteries

Nano-LiMn 2 O 4 cathode materials with nano-sized particles are synthesized via a citric acid assisted sol-gel route.The structure,the morphology and the electrochemical properties of the nano-LiMn 2 O 4 are investigated.Compared with the micro-sized LiMn 2 O 4,the nano-LiMn 2 O 4 possesses a high initial capacity (120 mAh/g) at a discharge rate of 0.2 C (29.6 mA/g).The nano-LiMn 2 O 4 also has a good high-rate discharge capability,retaining 91% of its capacity at a discharge rate of 10 C and 73% at a discharge rate of 40 C.In particular,the nano-LiMn 2 O 4 shows an excellent high-rate pulse discharge capability.The cut-off voltage at the end of 50-ms pulse discharge with a discharge rate of 80 C is above 3.40 V,and the voltage returns to over 4.10 V after the pulse discharge.These results show that the prepared nano-LiMn 2 O 4 could be a potential cathode material for the power sources with the capability to deliver very high-rate pulse currents.     

Synthesis and characterization of nanocomposites films with graphene oxide and reduced graphene oxide nanosheets

Graphene oxide (GO) and reduced graphene oxide (CRGO), as a graphene derivatives, possess unique properties and a high aspect ratio, indicating great potential in nanocomposite fields. The present work reports the fabrication of the nanocomposite films by a simple and environmentally friendly process using aqueous solution and optimized time sonication for better exfoliation of the graphene sheets within Poly(Vinyl alcohol) (PVA) as matrix. The films were characterized using high-resolution TEM (HRTEM), X-ray diffraction (XRD), Microtensile testing, Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA). The TEM images revealed a successfully exfoliation of the GO/CRGO nanosheets. XRD combined with TGA and DSC measurements showed an improvement in the thermal stability and tunable thermal properties. In addition, the Young's modulus and tensile yield strength of the composite films containing 1 wt% GO were obtained to be 4.92 GPa and 66 MPa respectively. These excellent reinforcement effects were achieved by the strong interaction between the components.     

Synthesis and characterization of graphene oxide,reduced graphene oxide and their nanocomposites with polyethylene oxide

This work describes the synthesis of GO, rGO and their nanocomposites with PEO. GO and rGO were prepared by the modified Hummers method and in-situ reduction of GO utilizing green reductant L (+) Ascorbic acid. The nanocomposites were characterized by Fourier-transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Thermogravimetric Analysis (TGA), and Universal Testing Machine (UTM). FT-IR and XRD confirmed the synthesis of GO and rGO. FE-SEM confirmed the uniformly exfoliated GO and rGO nanosheets in the polymer matrix. Hydrogen bonding was the main interaction mechanism for GO with PEO while no interaction was detected by FT-IR for rGO. Enhanced thermal stability was observed for both GO/PEO and rGO/PEO nanocomposites. The mechanical analysis showed an increase in Young's modulus, tensile strength, and elongation at break for GO/PEO nanocomposites, which is attributed to the homogeneous dispersion and hydrophilic hydrogen bonding interaction of GO with PEO.     

Synthesis and electrochemical performance of LiNi0.6Co0.2Mn0.2O2/reduced graphene oxide cathode materials for lithium-ion batteries

A LiNi_0.6Co_0.2Mn_0.2O₂/reduced graphene oxide (RGO) composite with RGO content of 1.2 % was prepared by a simple spray-drying method instead of high-energy ball milling method. The composite has been characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, energy dispersive spectroscopy, and charge/discharge test. The X-ray diffractometry result showed that composite possessed a typical hexagonal structure. The RGO sheets served as efficient electronically conductive frameworks benefitting from its 2D structure and outstanding electronic conductivity. The scanning electron microscope and transmission electron microscopy verified that LiNi_0.6Co_0.2Mn_0.2O₂ particles were wrapped with RGO sheets, which facilitated electronic conductivity between particles. The electrochemical results indicated that composite delivered a higher discharge capacity at various discharge rates. The cycling performance was also evaluated. The composite exhibited better cycling performance than pristine sample. Electrochemical impedance spectroscopy showed that the RGO can greatly reduce the charge transfer resistance. The results here gave clear evidence of RGO to improve electrochemical performance.     

Mn5(PO3(OH))2(PO4)2(H2O)4的水热合成和光谱研究

在水热反应条件下合成出具有红磷锰矿结构的Mn5(PO3(OH) ) 2 (PO4 ) 2 (H2 O) 4单晶 ,在X ray单晶结构分析的基础上 ,对其固体紫外可见漫反射光谱、红外光谱、荧光光谱和热重光谱进行了研究。结果表明 ,构成该化合物的PO4 四面体及MnO6 八面体通过共顶点或共棱方式相连接 ,与P ,Mn配位的氧分为 3类 :即端基氧 (Od)、二桥氧 (Ob)和三桥氧 (Oc)。因而在 2 10和 2 5 0nm左右出现了Od→Mn和Ob ,c→Mn的荷移跃迁吸收谱带 ;在 10 0 0～ 110 0cm- 1 处 ,P—O的伸缩振动峰分裂为 3个 ;70 0～ 980cm- 1 处存在 3类Mn—O的伸缩振动。对标题化合物分别采用 2 18和 310nm的光激发 ,分别在 35 4和 4 13nm产生强而尖锐的荧光光谱发射峰 ,表现了很强的光学效应。热重分析表明该化合物在 2 70℃以下结构保持稳定 ,在 2 70～36 0℃范围内失去配位水。量化计算得单点能为 - 4 5 5 8 6 5 95 5 5 1a u ;前线轨道能量HOMO(Alpha) =- 0 2 80 80a u ,LOMO(Alpha) =0 0 15 2 7a u ,能隙为 0 2 96 0 7a u ;HOMO(Beta) =- 0 2 5 919a u ,LOMO(Beta)=0 0 0 10 8a u ,能隙为 0 2 6 0 72a u ;偶极矩为 4 2 0 82Debye。     

Synthesis and properties of thermally reduced graphene oxide/polyacrylonitrile composites

Graphene oxide (GO) was prepared from graphite using a modified Hummers method. The GO dispersed in dimethyl sulfoxide (DMSO) was incorporated into polyacrylonitrile (PAN) dissolved in DMSO to prepare composite films using a conventional solution-casting method. DSC results showed a significant decrease in the cyclization temperature of polyacrylonitrile (PAN) in the presence of GO because the functional groups of GO initiated cyclization at lower temperature by an ionic mechanism. Heat treatment in air at 250 °C for 3 h led to stabilization of PAN and a simultaneous partial reduction of GO. A significant decrease in the electrical resistivity of the GO/PAN composite films was observed because the partially reduced GO acted as a conducting filler.     

Technical graphene (reduced graphene oxide) and its natural analog (shungite)

The wide structure and chemical-composition spectrum of the main technological material of molecular graphenics—reduced graphene oxide (RGO)—is explained from a quantum-chemical standpoint. The proposed concept is used to consider the results of experimental investigations of a natural analog of RGO, namely, shungite carbon, by high-resolution electron microscopy and nanopoint energy dispersive spectral analysis. The results obtained are used to propose an atomic-microscopic model for the structure of shungite carbon.