Comparison of the effects of FePO4 and FePO4·2H2O as precursors on the electrochemical performances of LiFePO4/C

Carbon-coated LiFePO₄/C composite as cathode materials is synthesized by solid-state method using anhydrous FePO₄ and hydrous FePO₄·2H₂O as precursors.The effects of sintering temperature and carbon content on the properties of LiFePO₄/C composite are compared by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and charging–discharging test. The crystallinity, morphology, and particle size distribution of these two precursors are compared to investigate their effect on the electrochemical performances of LiFePO₄/C composite. Compared with hydrous FePO₄·2H₂O, anhydrous FePO₄ has good crystallinity, uniform particle morphology and symmetrical size distribution, contributing to LiFePO₄/C composite have excellent electrochemical performances. Due to the dehydration of hydrous FePO₄·2H₂O during synthesis, uneven distribution of carbon content and carbon layer is coated on LiFePO₄ surface, deteriorating the electrochemical performance of LiFePO₄/C composite. When anhydrous FePO₄ was used as the precursor, the LiFePO₄/C composite sintered at 700 °C with carbon content of 0.4 by molar ratio show high discharge capacity and stable cycling performance, with discharge capacity of 106.3 mA h g⁻¹ at 10 C, and a capacity retention rate of 99.2% after 200 cycles at 1 C.     

Improved cyclability of lithium–sulfur battery cathode using encapsulated sulfur in hollow carbon nanofiber@nitrogen-doped porous carbon core–shell composite

Hollow carbon nanofiber@nitrogen-doped porous carbon (HCNF@NPC) core–shell composite, which was carbonized from HCNF@polyaniline, was prepared as an improved high conductive carbon matrix for encapsulating sulfur as a cathode composite material for lithium–sulfur batteries. The prepared HCNF@NPC-S composite with high sulfur content of 77.5 wt.% showed an obvious core–shell structure with an NPC layer coating on the surface of the HCNFs and sulfur homogeneously distributed in the coating layer. This material exhibited much better electrochemical performance than the HCNF-S composite, delivered initial discharge capacity of 1170 mAh g⁻¹, and maintains 590 mAh g⁻¹ after 200 cycles at the current density of 837.5 mA g⁻¹ (0.5 C). The significantly improved electrochemical performance of the HCNF@NPC-S composite was attributed to the synergetic effect between HCNF cores, which provided electronic conduction pathways and worked as mechanical support, and the NPC shells with relatively high surface area and pore volume, which could trap sulfur/polysulfides and provide Li⁺ conductive pathways.     

Microwave-assisted sintering of Al2O3-MWCNT nanocomposites

Alumina-MWCNT composite was densified by microwave sintering. CNTs were coated with boehmite nanoparticles to enhance their distribution in composite samples. Calcination temperature of composite powder was determined by TGA analysis (5 °C/min). Samples containing 0 and 1vol%CNT were produced by cold isostatic pressing at 180 MPa. Microwave sintering (1520 °C for 45 min) was conducted under the flow of argon. Phase analysis of the calcined composite powder showed complete transformation of boehmite into gamma-alumina. The relative densities were 99.3% and 98.1% for monolithic alumina and composite, respectively. CNT addition improved the fracture toughness of alumina by ~37%. SEM images showed that microwave sintering was successful. Also, coating CNTs improved their distribution in the alumina matrix.     

Facile synthesis of micro-spherical LiMn0.7Fe0.3PO4/C cathodes with advanced cycle life and rate performance for lithium-ion battery

A series of micro-spherical LiMn_0.7Fe_0.3PO₄/C (LMFP) cathode materials are synthesized via co-precipitation method combining spray drying and solid-state reaction. All as-prepared materials are well-characterized to determine their crystal structure, morphology and electrochemical performance. All as-obtained LMFP materials correspond to orthorhombic olivine structure with Pbnm space group and show uniform porous spherical structure with an average particle size of 3 µm and a carbon coating layer of about 3 nm. In particular, the resulting LMFP material prepared at 600 °C exhibits a high discharge capacity of 160 mAh g⁻¹ at 0.1 C. Even at a high rate of 10 C, it can still deliver 133 mAh g⁻¹ and maintain capacity retention of 84.9% after 200 cycles. The excellent electrochemical performance is ascribed to the synergetic effect of porous micro-spherical structure and uniform carbon coating layer.     

Preparation of graphene supported porous Si@C ternary composites and their electrochemical performance as high capacity anode materials for Li-ion batteries

Graphene supported porous Si@C ternary composites had been synthesized by various routes and their structural, morphological and electrochemical properties were investigated. Porous Si spheres coated with carbon layer and supported by graphene have been designed to form a 3D carbon conductive network. Used as anode materials for lithium ion batteries, graphene supported porous Si@C ternary composites demonstrate excellent electrochemical performance and cycling stability. The first discharge capacity is 2184.7 mA h/g at a high current density of 300 mA/g. After 50 cycles, the reversible capacity is 652.4 mA h/g at a current density of 300 mA/g and the coulomb efficiency reaches at 98.7%. Due to their excellent electrochemical properties, graphene supported porous Si@C ternary composites can be a kind of promising anode materials for lithium ion batteries.     

Fabrication and properties of mullite fiber matrix porous ceramics by a TBA-based gel-casting process

A bird nest-like structure was designed by using the mullite fiber as the matrix and SiO₂ as the high temperature binder. This special material was successfully prepared by a TBA-based gel-casting process. The randomly arranged fiber laps bonded by SiO₂ binder was the most important structure characteristic of this porous material. The effect of sintering temperature on the properties, i.e. porosity, bulk density, linear shrinkage, compressive strength, thermal conductivity and the microstructure was studied. The composite exhibited significant pseudoductility. The fracture mechanism of this composite under compression was discussed. The results indicated that the sintering temperature ranging from 1500 to 1600 °C was suitable for yielding mullite fiber matrix porous ceramics which had a low thermal conductivity (0.19–0.22 W/m K), a relatively high compressive strength (3–13 MPa) and a high resilience (66–70%) for applications in the thermal insulators and high-temperature elastic seal field.     

Reduced graphene oxide anchored with δ-MnO2 nanoscrolls as anode materials for enhanced Li-ion storage

We developed a one-pot in situ synthesis procedure to form nanocomposite of reduced graphene oxide (RGO) sheets anchored with 1D δ-MnO₂ nanoscrolls for Li-ion batteries. The as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The electrochemical performance of the δ-MnO₂ nanoscrolls/RGO composite was measured by galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy. The results show that the δ-MnO₂ nanoscrolls/RGO composite displays superior Li-ion battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1520 and 810 mAh g⁻¹, respectively. After 50 cycles, the reversible discharge capacity is still maintained at 528 mAh g⁻¹ at the current density of 100 mAh g⁻¹. The excellent electrochemical performance is attributed to the unique nanostructure of the δ-MnO₂ nanoscrolls/RGO composite, the high capacity of MnO₂ and superior electrical conductivity of RGO.     

A hierarchical carbon fiber/sulfur composite as cathode material for Li–S batteries

A novel hierarchical structure carbon/sulfur composite is presented based on carbon fiber matrices, which are synthesized by electrospinning. The fibers are constituted with hollow graphitized carbon spheres formed using catalytic Ni nano-particles as hard templates. Sulfur is loaded to the carbon substrates via thermal vaporization. The structure and composition of the hierarchical carbon fiber/S composite are characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and nitrogen adsorption isotherms. The electrochemical performance is evaluated by cyclic voltammetry and galvanostatic charge–discharge. The results exhibit an initial discharge capacity of 845 mA h g⁻¹ at 0.25 C (420 mA g⁻¹), with a retention of 77% after 100 cycles. A discharge capacity of 533 mA h g⁻¹ is still attainable when the rate is up to 1.0 C. The good cycling performance and rate capability are contributed to the uniform dispersion of sulfur, the conductive network of carbon fibers and hollow graphitized carbon spheres.     

Electrospun ZnO–SnO2 composite nanofibers with enhanced electrochemical performance as lithium-ion anodes

ZnO–SnO₂ composite nanofibers with different structures were synthesized by a simple electrospinning approach with subsequent calcination at three different temperatures using polyacrylonitrile as the polymer precursor. The electrochemical performance of the composites for use as anode materials in lithium-ion batteries were investigated. It was found that the ZnO–SnO₂ composite nanofibers calcined at 700 °C showed excellent lithium storage properties in terms of cycling stability and rate capability, compared to those calcined at 800 and 900 °C, respectively. ZnO–SnO₂ composite nanofibers calcined at 700 °C not only delivered high initial discharge and charge capacities of 1450 and 1101 mAh g⁻¹, respectively, with a 75.9% coulombic efficiency, but also maintained a high reversible capacity of 560 mAh g⁻¹ at a current density of 0.1 A g⁻¹ after 100 cycles. Additionally, a high reversible capacity of 591 mAh g⁻¹ was obtained when the current density returned to 0.1 A g⁻¹ after 50 cycling at a high current density of 2 A g⁻¹. The superior electrochemical performance of ZnO–SnO₂ composite nanofibers can be attributed to the unique nanofibrous structure, the smaller particle size and smaller fiber diameter as well as the porous structure and synergistic effect between ZnO and SnO₂.     

Effect of carbon contamination on the sintering of alumina ceramics

The present work aimed with the carbon contamination in alumina ceramics and its influence on sinterability of alumina in low vacuum and atmospheres of argon and nitrogen. The commercially available alumina was coated with carbon and sintered at different atmospheres to investigate the effect of carbon presence on alumina sintering behaviour. The sintering conditions were: heating/cooling rates 5 °C/min and 1.7 °C/min until the maximum temperature of 1400 °C and a dwell time of 2 h. The microstructure of the samples was investigated from fracture and surface, prior to polishing, chemical or thermal etching. The non-densified (porous) surface layer was found in the samples sintered in nitrogen and vacuum, however, sintering in argon atmosphere showed a negligible effect on the surface. The core of investigated specimens exposes a transgranular/intergranular fracture mode and is dense in all cases. In the case of vacuum sintering, the strong carbon diffusivity was also noticeable by the dark grey color of the samples. Interestingly, the formation of aluminium nitride took place during sintering of carbon coated alumina samples in a nitrogen atmosphere at 1400 °C. The thickness of the reactive porous layer was approximately 15 μm beneath the surface. Such a porous layer is inappropriate to the desired features of final ceramic products. Presented results lead to better understanding of the sintering behaviour of ceramic and to suitable selecting of the set-up by densification conditions.     

Polymer microsphere-assisted synthesis of lithium-rich cathode with improved electrochemical performance

The Li-rich layered cathode material Li_1.165Mn_0.501Ni_0.167Co_0.167O₂ with porous structure has been successfully synthesized through a facile co-precipitation approach followed with a high-temperature calcination treatment, adopting polymer microsphere (PSA) as a template and conductive agent. The PSA-assisted Li_1.165Mn_0.501Ni_0.167Co_0.167O₂ composite exhibits remarkably improved cycling stability and rate capability compared with the bare composite. It delivers a high initial discharge capacity of 267.0 mA h g⁻¹ at 0.1 C (1 C=250 mA g⁻¹) between 2.0 V and 4.65 V. A discharge capacity of 214.9 mA h g ⁻¹ is still obtained after 100 cycles. Furthermore, the diffusion coefficients of Li⁺ investigated by the cyclic voltammetry technique are approximately 10⁻¹⁵–10⁻¹⁴ cm² s⁻¹. Such outstanding performance is mainly ascribed to: on one hand, the carbon residue of PSA after being calcined at high temperature contributes to enhance the electronic conductivity of the electrode and alleviates the volume changes during the Li⁺-insertion/extraction, leading to an improved rate capability; on the other hand, the unique porous structure and small particle size are conductive to increase the exposed electrochemical active surface, shorten Li⁺ diffusion distance and thus enhance the lithium storage capacity.     

High temperature interfacial phase stability of a Mo/Ti3SiC2 laminated composite

A Mo/Ti₃SiC₂ laminated composite is prepared by spark plasma sintering at 1300 °C under a pressure of 50 MPa. Al powder is used as sintering aid to assist the formation of Ti₃SiC₂. The fabricated composites were annealed at 800, 1000 and 1150 °C under vacuum for 5, 10, 20 and 40 h to study the composite's interfacial phase stability at high temperature. Three interfacial layers, namely Mo₂C layer, AlMoSi layer and Ti₅Si₃ solid solution layer are formed during sintering. Experimental results show that the Mo/Ti₃SiC₂ layered composite prepared in this study has good interfacial phase stability up to at least 1000 °C and the growth of the interfacial layer does not show strong dependence on annealing time. However, after being exposed to 1150 °C for 10 h, cracks formed at the interface.     

Effect of Al addition on pressureless sintering of B4C

The effect of Al addition on pressureless sintering of B₄C ceramic was analyzed in the present work. Different amounts of Al, from 0 to 5 wt.% were added to the base material and pressureless sintering was conducted at 2050 and 2150 °C under argon atmosphere. Microstructure, crystal phases and density evolution were studied and correlated to Al additions and firing temperature. Density and grain size of sintered samples, increased significantly with Al load while less evidence is the effect of sintering temperature; 94% dense material was obtained by adding 4 wt.% Al regardless of the maximum firing temperature.     

Electrically conductive porous alumina/graphite composite synthesized by starch consolidation with reductive sintering

This paper reports a facile and environment-friendly process to synthesize electrically conductive porous alumina/graphite composites by starch consolidation technique followed by reductive sintering. Green ceramic composites were consolidated with different starches and sintered at different temperatures in an argon atmosphere. Electrical measurements, carbon contents and Raman analyses of carbon structures determined an optimal sintering temperature of 1700 °C, which lead to a uniform formation of conductive graphitic networks at an optimal concentration of about 3.8 vol% in the porous composites. These carbon networks resulted into porous composites having high electrical conductivities measured in the range from 3 to 7 S/cm, which depended on the starch types and their porous properties. Correspondingly, the bulk porosities of the sintered composites were measured from 42 to 46%, with rounded micropores having diameters ranging from 14 to 39 μm. These porous properties of the sintered composites offer promising applications for conductive membrane and porous electrode.     

Microwave heating synthesis of spindle-like LiMnPO4/C in a deep eutectic solvent

Orthorhombic structure LiMnPO₄/C with space group Pnmb was synthesized using a microwave heating process in a chloride/ethylene glycol-based deep eutectic solvent (DES) at 130 °C for 30 min under atmospheric pressure. The prepared sample was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and electrochemical tests. LiMnPO₄/C prepared in a DES has a nano-spindle form coated with a carbon layer (approximately 3 nm thick). This spindle-like LiMnPO₄/C material delivers a discharge capacity of 129 mA h g^–1 with a capacity retention ratio of approximately 96.1% after 100 cycles at 1 C. Even at 5 C, it still gives a discharge capacity of 106 mA h g^–1, exhibiting good rate performance and cycle stability. The results of this work show that the chloride/ethylene glycol-based DES can act as a crystal-face inhibitor to adjust the oriented growth and morphology of LiMnPO₄. Furthermore, deep eutectic solvents could find a wide application in the synthesis of electrode materials with special morphologies for lithium ion batteries.     

Synthesis and electrochemical properties of a composite LiMn0.63Fe0.37PO4 with Li3V2(PO4)3 for lithium-ion battery cathode materials

A composite materials LiMn_0.63Fe_0.37PO₄ with Li₃V₂(PO₄)₃ can be synthesized by a sol-gel method using N,N-dimethylformamide (DMF) as a dispersing agent. The structures, characteristics of the appearance, and electrochemical properties of the composites have been studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The composites contained LiMnPO₄/C (LMP/C), LiFePO₄/C (LFP/C), and Li₃V₂(PO₄)₃/C (LVP/C) phases with a nano-sized dispersion. The TEM images showed that the composites are crystalline with a grain size of 10–50 nm. The Mn2p, V2p, and Fe2p valence states were analyzed by X-ray photoelectron spectroscopy (XPS). The incorporation of LVP and LFP with LMP effectively enhanced the electrochemical kinetics of the LMP phase by a structural modification and shortened the lithium diffusion length in LMP. The capacity of the composite 0.79LiMn_0.63Fe_0.37PO₄·0.21Li₃V₂(PO₄)₃/C remained at 152.3 mAh g⁻¹ (94.7%) after 50 cycles at a 0.05 C rate. The composite exhibited excellent reversible capacities 159.4, 150, 140.1, 133.7 and 123.6 mAh g⁻¹ at charge-discharge rates of 0.05, 0.1, 0.2, 0.5 and 1 C, respectively.     

Effect of Fe-doping followed by C+SiO2 hybrid layer coating on Li3V2(PO4)3 cathode material for lithium-ion batteries

A novel Li₃V₂(PO₄)₃ composite modified with Fe-doping followed by C+SiO₂ hybrid layer coating (LVFP/C-Si) is successfully synthesized via an ultrasonic-assisted solid-state method, and characterized by XRD, XPS, TEM, galvanostatic charge/discharge measurements, CV and EIS. This LVFP/C-Si electrode shows a significantly improved electrochemical performance. It presents an initial discharge capacity as high as 170.8 mA h g⁻¹ at 1 C, and even delivers an excellent initial capacity of 153.6 mA h g⁻¹ with capacity retention of 82.3% after 100 cycles at 5 C. The results demonstrate that this novel modification with doping followed by hybrid layer coating is an ideal design to obtain both high capacity and long cycle performance for Li₃V₂(PO₄)₃ and other polyanion cathode materials in lithium ion batteries.     

Three-dimensional MnO/reduced graphite oxide composite films as anode materials for high performance lithium-ion batteries

MnO/reduced graphite oxide (MnO/RGO) composite films with three dimensionally porous structures have been synthesized by an improved electrostatic spray deposition setup and their microstructure and electrochemical properties have been characterized by X-ray diffraction, scanning electron microscopy, thermal gravimetric, Raman spectrometry and galvanostatic cell cycling. The results show that the structure and electrochemical performance of the electrode film are influenced significantly by the RGO content. The three dimensionally porous structure collapse does not occur in the MnO/RGO thin films for a RGO content lower than 16.58 wt%, the 16.58 wt% reduced graphite oxide content being optimal. Such an improvement in the cycling performance (772 mAh g⁻¹ after 100 cycles at 1 C) and rate capability (425 mAh g⁻¹ at 6 C) might be attributed to the excellent microstructure and electrical conductivity of MnO/reduced graphite oxide composite film electrodes.     

Phase structure and electrochemical performance of layered-spinel integrated LiNi0.5Mn0.5O2-LiMn1.9Al0.1O4 composite cathodes for lithium ion batteries

In recent years, multi-component integrated composite cathodes for lithium ion batteries have attracted considerable attention. In this work, novel layered-spinel integrated cathode materials of (1−x)LiNi_0.5Mn_0.5O₂-xLiMn_1.9Al_0.1O₄ were synthesized by a sol-gel method, and their phase structures, morphologies and electrochemical performance were investigated. The crystal structure of the (1−x)LiNi_0.5Mn_0.5O₂-xLiMn_1.9Al_0.1O₄ is changed from layered to spinel structure with increasing x. All the samples exhibit nanoscale grains with the minimum grain size of ~130 nm when x = 0.5. The composite electrode with x = 0.5 exhibits the optimal discharge capacity, presenting a large initial discharge capacity of 236 mAh g⁻¹ at the current density of 20 mA g⁻¹. Good rate capability is also obtained at the composite electrode with x = 0.5 where the electrode displays the relatively high discharge capacity of 64.9 mAh g⁻¹ at the high rate of 5 C. The improved electrochemical performance is related to the introduction of spinel structure into layered structure and small grain size. The spinel structure can stabilize the layered structure, which leads to the improvement in the electrochemical performance of the composites; and the small grain size in the sample with x = 0.5 provides short lithium ion diffusion way and thus enhances the electrochemical performance.     

Synthesis and electrochemical performance of Li4Ti5O12/Ag composite prepared by electroless plating

To improve the electrochemical and anti flatulence performance of Li₄Ti₅O₁₂, Ag modified Li₄Ti₅O₁₂ (LTO) with high electrochemical performance as anode materials for lithium-ion battery was synthesized successfully by two-step solid phase sintering and subsequent electroless plating process in the presence of silver. The effect of Ag modification on the physical and electrochemical properties were investigated by the extensive material characterization of X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM). The results showed that the samples possessed single spinel structure, it could be observed that the LTO/Ag composite and the pure LTO shared the same vibration frequencies, which indicated that the crystal structure of LTO didn’t change after electroless plating process, and the particles were uniformly and regularly shaped within 0.5–1.0 µm. Electrochemical performance of the samples were evaluated by the charging and discharging, cyclic voltammetry, electrochemical impedance spectroscopy, cycling and rate tests. It's obvious that the LTO/Ag composite prepared at the 10 min of electroless plating showed the highest performance with capacitance of 182.3 mA h/g at 0.2 C current rates. What's more, the LTO/Ag composites still maintained 92% of its initial capacity even after 50 charge/discharge cycles. Modification of appropriate Ag not only benefits the reversible intercalation and deintercalation of Li⁺, but also improves the diffusion coefficient of lithium ion. Besides, modification of appropriate Ag lower electrochemical polarization leads to higher conductivity and cycle performance of LTO, which is consistent with the results of the best reversible capacities.