•Carbon-coated Na2Mn3(P2O7)2 was first successfully synthesized via sol-gel method.•Rietveld refinement confirmed existence of Na2Mn3-xFex(P2O7)2 (x = 0–2) solid solution.•Fe-doped can significantly improve the electrochemical property of Na2Mn3(P2O7)2.•Fe-doped can increase Na-ion diffusion coefficient by two orders of magnitude.A series of Fe-doped Na2Mn3-xFex(P2O7)2 (x = 0.0, 0.5, 1.0, 1.5 and 2.0) compounds have been successfully prepared by using sol-gel method. Rietveld refinement results indicate that single phase Na2Mn3-xFex(P2O7)2 with triclinic structure can be obtained within 0 ≤ x ≤ 2 although no Na2Fe3(P2O7)2 existing under our experimental conditions, and the cell parameters (including a, b, c and V) are decreasing with the increasing of x. Our results reveal that Na2Mn3(P2O7)2 exhibits an electrochemical activity in the voltage range of 1.5 V–4.5 V vs. Na+/Na when using as the cathode material for SIBs although it gives a limited rate capability and poor capacity retention. However, the electrochemical performance of Fe-doped Na2Mn3-xFex(P2O7)2 (0 ≤ x ≤ 2) can be improved significantly where cycle performance and rate capability can be improved significantly than that of the pristine one. Sodium ion diffusion coefficient can be increased by about two orders of magnitude with the Fe-doping content higher than x = 0.5.Download high-res image (308KB)Download full-size image

High-quality single crystalline gadolinium hexaboride (GdB6) nanowires have been successfully prepared at very low temperatures of 200–240 °C by a high pressure solid state (HPSS) method in an autoclave with a new chemical reaction route, where Gd, H3BO3, Mg and I2 were used as raw materials. The crystal structure, morphology, valence, magnetic and optical absorption properties were investigated using XRD, FESEM, HRTEM, XPS, SQUID magnetometry and optical measurements. HRTEM images and SAED patterns reveal that the GdB6 nanowires are single crystalline with a preferred growth direction along [001]. The XPS spectrum suggests that the valence of Gd ion in GdB6 is trivalent. The effective magnetic momentum per Gd3+ in GdB6 is about 6.26 μB. The optical properties exhibit weak absorption in the visible light range, but relatively strong absorbance in the NIR and UV range. Low work function and high NIR absorption can make GdB6 nanowires a potential solar radiation shielding material for solar cells or other NIR blocking applications.High-quality single crystalline gadolinium hexaboride (GdB6) nanowires have been successfully prepared at very low temperatures of 200–240 °C by a high pressure solid state (HPSS) method in a autoclave.Download high-res image (166KB)Download full-size image

•2,3-Dichloro-6-hydroxy-5-cyano-1,4-benzoquinone (DHCQ) was synthesized and examined as a novel organic electrode for LIBs.•The DHCQ electrode showed a capacity of 534 mA h g−1 at 0.5 A g−1 after 400 cycles.•DHCQ presented better rate performance and cyclability than the raw material (DDQ).•Appropriate control of Super P in DHCQ composite could also realize good stability.Lithium ion batteries (LIBs) with organic electrode materials have drawn considerable attentions with advantages of their flexibility, low cost, and green sustainability, but been limited by high solubility in aprotic electrolytes, leading to capacity decay and short cycling life. Herein, we focus on the co-effect of structural modification and the increase of conductive carbon in electrode composites to accommodate soluble quinonic materials and achieve a long cycling life. Here, 2,3-dichloro-5-hydroxy-6-cyano-1,4-benzoquinone (DHCQ) is synthesized through a simple chemical hydrolysis of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The increase of conductive additive not only enhances the electronic conductivity but also suppresses the dissolution of DHCQ. Benefiting from reasonable molecular design and appropriate control of conductive carbon, the as-prepared material demonstrates a high reversible capacity (921 mA h g−1 at 50 mA g−1), good rate capability (207 mA h g−1 at 5 A g−1) and long-term cycling stability (remaining retention of around 78% after 400 cycles at 500 mA g−1).Download high-res image (274KB)Download full-size image

•A general solid state reaction for REB6 1-D nanostructures growth is introduced.•Vapor phase derived growth of the nanowires is depicted.•The optical absorption properties of 1-D REB6 are firstly investigated.•Tunable NIR absorption and VL transmittance of LaxSm1−xB6 nanostructures upon x variation are presented.We developed a general solid state reaction to synthesize both single and dual rare earth hexaborides (REB6) under autogenic pressure. Through a series of nearly similar experiments, one-dimensional (1-D) single crystalline REB6 (RE=La, Ce, Pr, Nd and Sm) nanostructures can be formed. Detailed structures, morphology and chemical composition of the synthesized samples were studied by XRD, SEM, TEM, EDS and elemental mapping techniques. As a rarely reported solid state reaction for especially 1-D nanostructures growth, the formation mechanism was explored and discussed here. Gaseous species containing boron and rare earth under high pressure was deemed to be important. Optical absorption tests were carried out for all the synthesized samples at the first time. Results indicate a tunable VL transmittance can be achieved in LaxSm1-xB6 nanostructures upon x variation.

•The carbon-coated Li9V3(P2O7)3(PO4)2 (LVPP/C) composite is prepared via carbon thermal reduction.•The Li-storage properties of LVPP/C are investigated, both as cathode and anode for Li-ion batteries.•An intercalation reaction mechanism for LVPP/C anode is firstly confirmed by in-situ XRD.•The LVPP(cathode)//LVPP(anode) symmetric batteries is designed and investigated.In this paper, we have prepared the carbon-coated Li9V3(P2O7)3(PO4)2 (LVPP/C) composite via a simple carbon thermal reduction and its lithium inserted-extracted mechanism are systematically investigated. LVPP/C cathode can keep a reversible capacity of 100.6 mAhg−1, 98.7% of the initial discharge capacity, after 50 cycles within 2.5 V–4.6 V, while an obviously capacity-fading is observed within 2.5 V–4.8 V. Meanwhile, the LVPP/C anode delivers a well rate capability and, a reversible capacity of 94.5 mAhg−1 (1.0–2.5 V) can still be retained after 60 cycles, ranging from 0.1C to 2C and finally in 0.1C succession. A good preservation of the LVPP structure is observed during the discharge-charge process by in-situ XRD and an intercalation reaction mechanism is also confirmed for LVPP/C anode here for the first time. At last, a novel symmetrical full battery by using LVPP/C as both cathode and anode simultaneously has been successfully assembled. The investigation shows that this symmetrical cell, with an average voltage of 2.5 V, can deliver a reversible capacity of 59.8 mAhg−1 after 30 cycles at 0.1C.

Three kinds of one-dimensional (1D) neodymium hexaboride (NdB6) nanostructures, including nanobelts, nanoawls, and nanotubes, have been synthesized through a chemical vapor deposition (CVD) process with a self-catalyzed mechanism. For the first time, we report the preparation of NdB6 nanotubes. The morphology and crystalline structure are characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Transmission electron microscopy images show that they have different growth directions: [111], [001], and [110], respectively. In addition, detailed growth mechanisms of the nanobelts, nanoawls, and nanotubes are presented. A droplet induced self-catalyzed mechanism, self-catalyzed with a vapor–solid mechanism, and diffusion limited self-catalyzed mechanism are proposed to explain the growth of nanobelts, nanoawls, and nanotubes, respectively.

Lanthanum–praseodymium hexaboride (LaxPr1−xB6) nanoawls have been fabricated via a simple flux-controlled method using lanthanum (La) powders, praseodymium (Pr) powders and boron trichloride (BCl3) gas as starting materials at 1050 °C. Scanning electron microscopy (SEM) shows that the tapered nanoawls have a length of 2–10 μm and a diameter ranging from 50 to 300 nm at the roots and 10–80 nm at the tips. Transmission electron microscopy (TEM) reveals that the nanoawls are single crystalline with the preferred growth direction along [110]. Raman spectra indicate that the T1u mode splitting effect occurs in this ternary rare-earth hexaboride. In addition, a concentration gradient is necessary to create these awl-like structures where the combination of self-catalyzed and vapor–solid growth is responsible for the growth of tapered LaxPr1−xB6 nanoawls.

A Co9S8/C nanocomposite is prepared using a solid-state reaction followed by a facile mechanical ball-milling treatment, with sucrose as the carbon source. The phases, morphologies, and detailed structures of the Co9S8/C nanocomposite are well-characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. When evaluated as an anode material for sodium-ion batteries, the Co9S8/C nanocomposite electrode displays a reversible capacity of ∼567 mA h g–1 in the initial cycle and maintains a reversible capacity of ∼320 mA h g–1 after 30 cycles, indicating a larger capacity and a stable cycling performance. For comparison, the electrochemical performances of Co9S8 and Co9S8/C samples synthesized using the solid-state reaction are also displayed. The ex situ XRD and transmission electron microscopy tests demonstrate that Co9S8 undergoes a conversion-type sodium storage mechanism.

Li2Mn1-xFex(PO3)4 (x = 0, 0.2, 0.4, 0.6, 0.7) solid solution phase has been successfully prepared via solid-state reaction. The Rietveld refinement results indicate that the Li2Mn1-xFex(PO3)4 (x = 0, 0.2, 0.4, 0.6, 0.7) solid solutions with orthorhombic structure can be obtained and the lattice parameters (including a, b, c, and V) decrease with the increasing of Fe concentration. Partial substitution of manganese with iron enhances the electrochemical performance; there, the discharge-specific capacity of the samples obviously increases from 21 mAh/g for x = 0 to 59 mAh/g for x = 0.7, which is 85 % capacity of that one lithium removal. The cyclic voltammetric (CV) curves present the Mn2+/Mn3+ redox couple situated at 4.6 and 1.8 V and Fe2+/Fe3+ redox couple located at 4.3 and 2.3 V, which can be observed in cathodic and anodic sweeps. Such a low discharge potential value for M2+/M3+ redox couple may be attributed to the zigzag [(PO3)1−]n chains in this structure.

A new Co-base sodium metaphosphate compound, NaCo(PO3)3, has been synthesized here by solid-state method. The crystal structure is refined by the Rietveld method, and the results reveal that NaCo(PO3)3 has an orthorhombic structure with the space group of P212121 and lattice parameters of a = 14.2453(2) Å, b = 14.2306(1) Å, and c = 14.2603(2) Å. Its typical morphology and chemical composition are confirmed by scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). The valence states of all elements and the internal/external vibrational modes of NaCoP3O9 compound are measured by X-ray photoelectron and vibrational spectrum, where a typical feature of the (PO3)− polyanion group is observed. Meanwhile, the electrochemical properties of NaCo(PO3)3 cathode for sodium-ion batteries are also elevated and an initial discharge capacity of 33.8 mAh/g can be obtained at 0.05 C within 1.5–4.2 V. After 20 cycles, a discharge capacity of 26.7 mAh/g can be obtained and a well-kept oxidation–reduction plateau is still observed for NaCo(PO3)3 cathode, indicating the good reversibility of this metaphosphate electrode.

We report the catalyst-free synthesis of uniform distributed single-crystalline LaxNd1−xB6 nanowires by simply heating mixed La and Nd powders to the required temperature in an inlet flux of mixed gases (H2, Ar and BCl3). FE-SEM, HRTEM, SAED, EDS, element mapping, XRD and Raman scattering results show that the LaxNd1−xB6 nanowires are structurally uniform and well-doped single crystals. Based on our experimental results, a dominant VLS-like mechanism with a self-catalytic growth mechanism was proposed and depicted conceptually. The nanowires display an excellent field emission performance with a low turn-on field value of ∼4.12 V μm−1 when evaluated as an electron emitter. Attempts were also made to understand the morphological influence of reaction time, reaction temperature and the proportion of the La and Nd powders.

•32 Ah prismatic cell overcharge test was firstly reported.•The cell temperature difference between inside and outside can reach 100 °C before fire.•The reaction between cathode and electrolyte is the key for fast heat generation.•Li-plating was observed after overcharge test.Safety behaviors of a 32 Ah prismatic lithium-ion battery are investigated under abusive charge conditions by monitoring the internal and external cell temperature variation. Results show that the cell internal temperature can reach 235 °C before firing, which is almost 140 °C higher than the cell external temperature. Although the cell resistance increases abruptly due to electrolyte oxidization when the cell is firstly overcharged to its maximum voltage (5.10 V), the cell internal temperature keeps a low temperature of 50 °C without notable temperature rise. However, the cathode/electrolyte interface becomes highly reactive as the cell is further overcharged. Cell internal temperature goes up to more than 200 °C accompanied with massively gas production when the cell is overcharged to 180% SoC. Post-overcharge analysis on both cathode and anode indicates that lithium plating during overcharge is the major cause responsible for thermal runaway because the observed cell temperature is well above the melting point of lithium metal.

•One dimensional Li2MoO4 nanostructures including nanorods and nanotubes have been successfully fabricated via a simple sol-gel method firstly.•Possible crystal formation mechanisms are proposed for these one dimensional Li2MoO4 nanostructures.•These one dimensional Li2MoO4 nanostructure electrode materials present outstanding rate abilities and cycle capabilities in electrochemical performance compared to the carbon-free powder sample when evaluated as anode materials for Lithium-ion batteries.•The carbon-coated Li2MoO4 nanotube electrode improves the charging/discharging capacities of graphite even after applying 60 cycles at very high current density.One dimensional Li2MoO4 nanostructures including nanorods and nanotubes have been successfully fabricated via a simple sol-gel method adding Li2CO3 and MoO3 powders into distilled water with citric acid as an assistant agent and carbon source. Our experimental results show that the formation of the one dimensional nanostructure morphology is evaporation and crystallization process with self-adjusting into a rod-like hexagonal cross-section structure, while the citric acid played an important role during the formation of Li2MoO4 nanotubes under the acidic environment by capping, stabilizing the {1010} facet of Li2MoO4 structure and controlling the concentration of H+ (pH value) of the aqueous solution. Finally, basic electrochemical performance of these one dimensional Li2MoO4 nanostructures including nanorods and nanotubes evaluated as anode materials for lithium-ion batteries (LIBs) are discussed, for comparison, the properties of carbon-free powder sample synthesized by solid-state reaction are also displayed. Experimental results show that different morphology and carbon-coating on the surface have an important influence on electrochemical performance.

•Na2MoO4 is firstly valuated as an anode material for lithium-ion batteries.•Carbon-coated nanoplate α-Na2MoO4 sample is synthesized firstly via a facile sol–gel method.•Residual carbon and reducing atmosphere would not change the valence of Mo (+6).•Carbon-coated nanoplate α-Na2MoO4 presents outstanding rate abilities and cycle capabilities compared to the carbon-free sample.•The Li storage mechanism of α-Na2MoO4 is conversion reaction confirmed by the ex-situ XRD and HRTEM results.The carbon-coated α-Na2MoO4 nanoplate sample was fabricated via a facile sol–gel method involving the subsequent annealing under a reducing atmosphere to decompose the organic carbon source. X-ray diffraction with Rietveld refinement, high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) results show that single-phase α-Na2MoO4 can be obtained even under the presence of carbon and reducing atmosphere. When evaluated as an anode material for lithium-ion batteries, the carbon-coated α-Na2MoO4 nanoplate electrode displays a discharge and recharge capacity of 806 mAh g−1 and 409 mAh g−1 respectively in the first cycle, while a reversible discharge–charge capacity of 350 mAh g−1 can be retained after 30 cycles at 30 mAh g−1. A capacity of ∼320 mAh g−1 at 30 mAh g−1 can still recover after 50 cycles even following the discharge/charge process with the high current density of 480 mAh g−1. Meanwhile, carbon-free and carbon-coated α-Na2MoO4 powders fabricated via a solid state reaction were also prepared for comparison. Furthermore, the structure change of α-Na2MoO4 and its Li storage mechanism upon lithiation and delithiation process are studied by ex-situ XRD and TEM in below.

•The in-situ carbon-coated Li3VO4 was synthesized by simple solid-state reaction method.•The electrochemical performance of Li3VO4/C at various temperatures was systematically studied.•The electrochemical analysis shows that Li3VO4/C has better performance at higher temperature.The carbon-coated Li3VO4 (Li3VO4/C) sample was synthesized by simple solid-state reaction method using glucose as carbon source. Rietveld refinement, XPS and element analysis results show that, though it is synthesized in the presence of carbon and reducing atmosphere, both the single-phase Li3VO4/C and the valence of vanadium of +5 can be retained. The SEM and TEM images reveal that Li3VO4/C composite has uniform particles with size less than 1 μm. Electrochemical testing results show that Li3VO4/C at high operation temperatures holds both higher specific capacity and cyclic performance than that of low temperatures. The initial discharge capacities for the Li3VO4/C electrodes at temperatures of −20, 0, 25 and 50 °C are 312, 600, 760 and 721 mAh g−1 with the coulombic efficiency of 40.45%, 72.09%, 74.34% and 73.41%, respectively. Even at a high discharge/charge rate of 15 C, the capacities of the Li3VO4/C electrodes at −20, 0, 25 and 50 °C still can retain about 20, 120, 370 and 450 mAh g−1, respectively. The CV results demonstrate that the higher operation temperature can decrease the voltage polarization of the electrode, thus benefit the electrochemical performance of the Li3VO4/C electrode. In addition, the EIS results indicate that larger charge-transfer resistance and smaller lithium diffusion coefficient can be obtained at low operation temperatures, which should be one of the major reasons for its poor low-temperature performance of the Li3VO4/C electrode.

Dual-rare earth hexaborides PrxNd1−xB6 nanowires have been successfully synthesized on silicon substrates by a one-step CVD method at a temperature of 1030 °C. The quasi-aligned nanowires are shown to be structurally uniform and well-doped single crystals based on comprehensive analysis. The present preparation technique is an effective and invaluable method to develop and optimize dual-REB6 nanostructured emitters.

Neodymium hexaboride (NdB6) submicroawls have been fabricated via a simple flux-controlled self-catalyzed method using neodymium (Nd) powders and boron trichloride (BCl3) as starting materials at 1000 °C. Scanning electron microscopy (SEM) reveals that the submicroawls are tapered, with a length of 2–5 μm and a diameter ranging from approximately 0.1–0.3 μm at the roots and 5–50 nm at the tips. Transmission electron microscopy (TEM) shows that the submicroawls are single crystalline with the preferred growth direction along [001]. For systematic research, we have discussed the morphological change of NdB6 submicron structures by varying reaction temperatures, catalysts and duration. Moreover, a multistage growth model of the NdB6 submicroawls is proposed.

•The phase relations of Li2O–MnO–P2O5 ternary system were reported for the first time.•A high-pressure Mn3(PO4)2 was prepared under normal the solid-state reaction.•The detail structure for high-pressure Mn3(PO4)2 was presented for the first time.•The solid solubility of Li1+xMn1−xPO4 (−0.05 ⩽ x ⩽ 0.03) were reported for the first time.•The electrochemical properties of Li1+xMn1−xPO4 were studied.The phase relations of Li2O–MnO–P2O5 ternary system under reducing atmosphere have been systematically investigated by means of X-ray diffraction. Inferior to what we expected, no other new lithium manganese phosphates exist within the Li2O–MnO–P2O5 ternary system under the reducing atmosphere. A high-pressure phase Mn3(PO4)2 with graftonite Fe3(PO4)2-type structure can be easily obtained in the MnO–P2O5 system under the ordinary solid-state reaction conditions in H2/Ar atmosphere and its detail structure is presented. In addition, the solid solubility of Li1+xMn1−xPO4 is determined as −0.05 ⩽ x ⩽ 0.03. The lattice parameters and electrochemical properties of Li1+xMn1−xPO4 with x content are investigated. The electrochemical test results show that excess Li-ion (x > 0) or the excess Mn-ion (x < 0) in LiMnPO4 has an unfavorable effect on the electrochemical properties caused by the deterioration of the lithium diffusion along the one-dimensional tunnels.Subsolidus phase relations of the LiO2–MnO–P2O5 system: (A) Li1+xMn1−xPO4 (−0.05 ⩽ x ⩽ 0.03); (B) Li2MnP2O7; (C) Li2Mn(PO3)4; and (D) LiMn(PO3)3; dots: single phase; square: two phases; triangle: three phases.The phase relations of Li2O–MnO–P2O5 ternary system under reducing atmosphere have been systematically investigated by means of X-ray diffraction for the first time. A high-pressure phase Mn3(PO4)2 can be easily obtained in the MnO–P2O5 system and its detail structure is presented. In addition, the solid solubility of Li1+xMn1−xPO4 is determined as −0.05 ⩽ x ⩽ 0.03, and electrochemical properties of Li1+xMn1−xPO4 are investigated.

•A metaphosphate NaFe(PO3)3 compound is reported here for the first time.•Its structure and morphology are confirmed by XRD, SEM and TEM.•The vibrational spectra of NaFe(PO3)3 compound is resolved and summarized.•A typical paramagnetic behavior is observed for NaFe(PO3)3 within 10–300 K.•Its sodium deinsertion/insertion properties are also presented and discussed.A metaphosphate NaFe(PO3)3 compound has been obtained by solid state method here. This title compound belongs to the orthorhombic structure with the space group of P212121 (No. 19) and lattice parameters of a = 14.3390(3) Å, b = 14.3374(2) Å, c = 14.3642(1) Å. Its structure can be described as made up of infinite (PO3)1− chains connected by FeO6 octahedra, while the monovalent Na cations are located in the tunnels determined by the connection of PO4 tetrahedrons and FeO6 octahedrons. Besides, the morphology, vibrational spectra, magnetic and sodium deinsertion/insertion properties of NaFe(PO3)3 compound are also presented and discussed here for the first time, which can provide more information about the physicochemical properties of this metaphosphate compound.

Carbon-coated Li2MoO4 hexagonal hollow nanotubes were fabricated via a facile sol–gel method involving the solution synthesis of Li2MoO4 with subsequent annealing under an inert atmosphere to decompose the organic carbon source. To the best of our knowledge, this is the first report on the synthesis of Li2MoO4 nanotubes. More significantly, we have found that Li2MoO4 can be used as an anode material for lithium-ion batteries (LIBs). When evaluated as an anode material, the carbon-coated Li2MoO4 hollow nanotubes show an excellent electrochemical performance with a high reversible capacity (∼550 mA h g−1) after 23 cycles, good rate capability and cycling stability. Meanwhile, carbon-free Li2MoO4 sample, fabricated via a solid state reaction, was also prepared for comparison. The Li storage mechanism has been investigated in-detail by advanced XPS, in situ XRD and HRTEM.

•Carbon-coated Li3VO4 composite is firstly synthesized by simple sol–gel method.•Residual carbon and reducing atmosphere would not change the valence of V (+5).•The carbon-coated Li3VO4 shows a reversible capacity of ∼480 mAh g−1 at 0.18C.•The carbon-coated Li3VO4 presents outstanding rate abilities and cycle capabilities.Carbon-coated Li3VO4 sample is synthesized by a simple sol–gel method. X-ray diffraction and X-ray photoelectron spectroscopy results show that single-phase Li3VO4 can be obtained in a reducing atmosphere with the valence of vanadium of +5. The final product sintered at 650 °C demonstrates a favorable electronic conductivity with 6.12% residual carbon. Electrochemical testing shows that the carbon-coated Li3VO4 sample display a discharge capacity of 662.6 mAh g−1 and 507.4 mAh g−1 at 0.18C in the first and the second cycle, respectively, and a reversible capacity (charge capacity) of ∼480.0 mAh g−1 can be obtained in the second cycle. Furthermore, the discharge–charge capacity of the sample can retain ∼180 mAh g−1 and ∼90 mAh g−1 at 12C and 40C, respectively.

The structure and electrochemical performances of Si/SiOx/C (SSC) and graphite-SSC composite (G-SSC) are evaluated in half cells and full cells as anode materials for lithium ion batteries. It is found that the SSC material shows a lithium storage capacity of 3 to 5 times higher than that of pure graphite while the G-SSC composite exhibits a desirable cycling stability (+80%@400cycles). Structural characterization reveals no particle cracks in the G-SSC composite. The Si particle is surrounded with lithium silicates, which provide protection against the electrolyte invasion and electrolyte decomposition but permits the Li+ ions to pass through. The graphite coating layer, on the other hand, acts as an electric conductive connector and volume buffer. These structural features make the SSC and G-SSC attractive as anode materials for high-performance commercial lithium ion batteries.

Designed Graphite-Si/SiOx/C composite electrodes for rechargeable lithium-ion batteries are prepared with different binder of carboxymethyl cellulose-styrene butadiene rubber (CMC-SBR) and polyimide (PI). Electrode performance of composites highly depends on the selection of binder. The Si-based/graphite composite electrode containing PI binder shows very stable cycle stability with the retention higher than 95 % after 30 cycles; however, the capacity of composite electrode with CMC-SBR binder fades to less than 80 % after 20 cycles. The improvement mechanism of PI binder is characterized by SEM, EDS mapping, adhesive strength test, and electric performance test. The surface of anode film does not show crack after several cycles, and the SEI on the surface of Si/SiOx/C particle is characterized. It is found that anode film peeing off strength matches well with the composite cycle stability. This result is further supported with cell disassembly result. We believe that improvement of anode film adhesion strength is an effective way to get stable long cycle life.

•Co-doped Li9V3(P2O7)3(PO4)2/C cathode materials have been synthesized by using sol-gel method for the first time.•The X-ray photoelectron spectroscopy analysis reveal that V4+ ion exists in the V3+ parent when Co2+ ion-doping is presented in Li9V3-xCox(P2O7)3(PO4)2/C compounds.•Proper amounts of Co ions can be incorporated into the crystal structure of Li9V3(P2O7)3(PO4)2/C and the resulting compounds exhibit smaller particle size, lower charge-transfer resistance and faster lithium-ion diffusion.•Co-doped Li9V2.96Co0.04(P2O7)3(PO4)2/C compound exhibits the best cyclic performance and rate ability among all the samples.A series of Co-doped Li9V3 − xCox(P2O7)3(PO4)2/C (x = 0.00–0.10) compounds have been prepared by sol–gel method and the Rietveld refinement results indicate that pure-phase Li9V3 − xCox(P2O7)3(PO4)2/C (x = 0.00–0.10) compounds with trigonal structure can be obtained. Their electrochemical performance has been investigated and the results show that, although the initial specific capacity decreased with Co doping at a lower current rate, both cycle performance and rate capability have excited improvement with proper Co-doping content. Li9V2.96Co0.04(P2O7)3(PO4)2/C compound presents the best cyclic ability and rate performance. The enhancement of cyclic ability and rate performance may be attributed to enhanced specific surface area and improved lithium-ion diffusion during the proper amount of Co-doping (x = 0.04) in V sites.

A series of Mn-doped Li9V3−xMnx(P2O7)3(PO4)2/C (x = 0.00, 0.02, 0.05, 0.08, and 0.10) compounds have been successfully prepared by using sol–gel method. ICP, XRD, and Rietveld refinement results analyses indicate that Mn ions were sufficiently doped in Li9V3(P2O7)3(PO4)2 without changing its trigonal structure, and the cell parameters (including a, c, and V) of Li9V3−xMnx(P2O7)3(PO4)2/C (x = 0.00, 0.05, and 0.10) increase with the increasing of x. The X-ray photoelectron spectroscopy (XPS) analysis reveals that the manganese oxidation state was +2 and both V4+ and V3+ ions were present in the Li9V2.9Mn0.1(P2O7)3(PO4)2 compound. The electronic conductivity increases monotonically with Mn-doped content x. The electrochemical insertion/extraction properties of Li9V3−xMnx(P2O7)3(PO4)2/C (x = 0.00, 0.02, 0.05, 0.08, and 0.10) phases are also presented. The Mn-doped phases exhibit smaller redox voltage polarization, higher cycle performance and better rate capability than that of the pristine one. Particularly, Li9V2.9Mn0.1(P2O7)3(PO4)2/C shows both good rate capability and best capacity recovered ability among all the samples. The enhancement of rate performance and cyclic capability may be attributed to the optimizing particle size, enhancement of electronic conductivity, lithium ion mobility and structural stability owing to the proper amount of Mn-doping in V sites.

Carbon-coated Li3V2(PO4)3 was firstly prepared at 850 °C via two-step reaction method combined sol–gel and conventional solid-state synthesis by using VPO4/carbon as an intermediate. Two different carbon sources, citric acid and glucose as carbon additives in sequence, ultimately deduced double carbon-coated Li3V2(PO4)3 as a high-rate cathode material. The Li3V2(PO4)3/carbon with 4.39% residual carbon has a splendid electronic conductivity of 4.76×10−2 S cm−1. Even in the voltage window of 2.5–4.8 V, the Li3V2(PO4)3/carbon cathode can retain outstanding rate ability (170.4 mAh g−1 at 1.2 C, 101.9 mAh g−1 at 17 C), and no degradation is found after 120 C current rate. These phenomena show that the two-step carbon-coated Li3V2(PO4)3 can act as a fast charge-discharge cathode material for high-power Li-ion batteries. Furthermore, it's believed that this synthesize method can be easily transplanted to prepare other lithiated vanadium-based phosphates.Highlights► A novel carbon-coating method for Li3V2(PO4)3 via two-step reaction is presented. ► The sol–gel and solid-state reaction are combined by using VPO4 as an intermediate. ► The end product has a high conductivity of ∼10−2 S cm-1 with 4.39% residual carbon. ► Even in 2.5–4.8 V, the Li3V2(PO4)3/C cathode can retain outstanding rate ability. ► No degradation is found after Li3V2(PO4)3/C testing at extreme high rate (120 C).

Layered Li-rich vanadium phosphate, Li9V3(P2O7)3(PO4)2, is a novel and potential cathode material for lithium-ion batteries. It possesses both facile ion mobility due to its two-dimensional pathways, and high theoretical capacity (173.5 mAh g−1) because of its ability to extract six lithium ions (per formula) from the trigonal framework accompanied with the double-electron reaction of vanadium. In this study, we first correlate the structural characters with the electrochemical process by using a combined experimental and computational method. The electrochemical recrystallization of Li9V3(P2O7)3(PO4)2 is accomplished along with a metastable superstructure phase in different but related space group. Nevertheless, the structure as well as oxidation state can be easily recovered on reduction-oxidation, and the volume change is minimal. Furthermore, the electrochemical voltage-composition profile is predicted and understood as emerging from site energetics and redox couples via first-principles calculations.

Nanostructured rare-earth hexaborides (REB6) are promising materials for photonic and electronic applications due to their unique characteristic. These include high melting point, hardness, chemical stability, low work function, low volatility at high temperatures, superconductivity, magnetic properties, efficiency, thermionic emission, and narrow band semiconductivity. This article focuses on recent developments regarding the synthesis, characterization, and applications of REB6 nanostructures. We first summarize information regarding the classification and crystal chemistry of REB6. Next, we examine the means by which researchers have successfully synthesized REB6. We consider the structural properties and morphology of REB6, and the growth mechanism involved in their fabrication. Finally, we offer suggestions for the use of REB6 nanostructures in photonic and electronic applications, and identifying four areas for further research.

Single phase Li9V3(P2O7)3(PO4)2 is synthesized at 750 °C via solid-state reaction method for the first time. The Rietveld refinement results show that the trigonal system (space group: P3¯c1) with the lattice parameters a = 0.9724 nm, c = 1.3596 nm are obtained. Its intrinsic electrical conductivity of 1.43 × 10−8 S cm−1 is higher than that of LiFePO4 and as the same order of Li3V2(PO3)4. The electrochemical measurement results show that there are two plateaus (3.77 V and 4.51 V) and three plateaus (3.77 V, 4.51 V and 4.75 V) in the potential ranges of 2.0–4.6 V and 2.0–4.8 V, respectively. In the range of 2.0–4.6 V, two discharge plateaus (4.46 V and 3.74 V) can be observed and 110 mAh g−1 of discharge capacity is achieved. The Rietveld refinement result of the X-ray diffraction (XRD) data at the end of discharge after the first cycle suggests that the structural reversibility can be retained during electrochemical reactions in Li9V3(P2O7)3(PO4)2. In the range of 2.0–4.8 V, almost six lithium ions are extracted and the trigonal structure is still recovered after 30 cycles. Therefore, this novel layered vanadium monodiphosphate offers a promising candidate as cathode material for lithium-ion batteries.

Cr-doped Li9V3−xCrx(P2O7)3(PO4)2 (x = 0.0–0.5) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li9V3−xCrx(P2O7)3(PO4)2 (x = 0.0–0.5) with trigonal structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content. Li9V2.8Cr0.2(P2O7)3(PO4)2 compound presents the good electrochemical rate and cyclic ability. The enhancement of rate and cyclic capability may be attributed to the optimizing particle size, morphologies, and structural stability during the proper amount of Cr-doping (x = 0.2) in V sites.Highlights► The Cr-doped Li9V3(P2O7)3(PO4)2 compounds were successfully prepared by the sol–gel method. ► Both cycle performance and rate capability of Li9V3−xCrx(P2O7)3(PO4)2 have excited improvement with moderate Cr-doping content. ► Li9V2.8Cr0.2(P2O7)3(PO4)2 compound presented more better electrochemical rate and cyclic ability.

Layered monodiphosphate Li9V3(P2O7)3(PO4)2 can be synthesized by direct solid-state reaction using either hydrogen or carbon as the reducing agent at the sintered temperature of 750 °C. When the temperature is higher than 800 °C, Li9V3(P2O7)3(PO4)2 begins to decompose into Li3V2(PO4)3 and Li4P2O7. The measurement results of electronic conductivity, magnetization, and electrochemical impedance spectroscopy are reported for the first time. After carbon coating, the electronic conductivity comes to 2.07 × 10−3 S cm−1, which is the same order of magnitude as that of carbon-coated LiFePO4 and Li3V2(PO3)4. Li-ion diffusion coefficient (4.19 × 10−10 cm2 s−1) for carbon-uncoated Li9V3(P2O7)3(PO4)2 is close to that of LiCoO2 and much higher than that of LiFePO4. Li9V3(P2O7)3(PO4)2 exhibits a paramagnetic behavior in the temperature range of 5−300 K, which is consistent with the result from our X-ray photoelectron spectroscopy analysis where the oxidation state of vanadium is +3 in the Li9V3(P2O7)3(PO4)2 compound. The favorable electronic and ionic properties suggest that Li9V3(P2O7)3(PO4)2 can be a potential cathode material for Li-ion batteries.

Co-doped Li3V2−xCox(PO4)3/C (x = 0.00, 0.03, 0.05, 0.10, 0.13 or 0.15) compounds were prepared via a solid-state reaction. The Rietveld refinement results indicated that single-phase Li3V2−xCox(PO4)3/C (0 ≤ x ≤ 0.15) with a monoclinic structure was obtained. The X-ray photoelectron spectroscopy (XPS) analysis revealed that the cobalt is present in the +2 oxidation state in Li3V2−xCox(PO4)3. XPS studies also revealed that V4+ and V3+ ions were present in the Co2+-doped system. The initial specific capacity decreased as the Co-doping content increased, increasing monotonically with Co content for x > 0.10. Differential capacity curves of Li3V2−xCox(PO4)3/C compounds showed that the voltage peaks associated with the extraction of three Li+ ions shifted to higher voltages with an increase in Co content, and when the Co2+-doping content reached 0.15, the peak positions returned to those of the unsubstituted Li3V2(PO4)3 phase. For the Li3V1.85Co0.15(PO4)3/C compound, the initial capacity was 163.3 mAh/g (109.4% of the initial capacity of the undoped Li3V2(PO4)3) and 73.4% capacity retention was observed after 50 cycles at a 0.1 C charge/discharge rate. The doping of Co2+into V sites should be favorable for the structural stability of Li3V2−xCox(PO4)3/C compounds and so moderate the volume changes (expansion/contraction) seen during the reversible Li+ extraction/insertion, thus resulting in the improvement of cell cycling ability.

Single-phase lithium manganese borate, LiMnBO3, was obtained at the temperature higher than 850 °C by one-step solid state reaction without using carbon black in the starting materials. The initial specific discharge capacity for the cathode active material was 75.5 mAh/g at the current density of 5 mA/g and the mean fade of capacity was 0.09% per cycle except for the first cycle. The LiMnBO3 compound maintained a specific discharge capacity of 42.3 mAh/g even at the current density of 50 mA/g and the capacity fade per cycle was only 0.2% during 40 cycles. The cyclic voltammograms (CV) curves show that the Mn3+/Mn2+ redox couple situated at 2.23 and 4.13 V can be clearly observed during anodic and cathodic sweeps. Combined the cyclic voltammograms results with the X-ray diffraction patterns of electrodes before and after cycling, where no significant change of the peak currents and the peak potentials during cycling, it was anticipated that the extraction and insertion of Li-ions are totally reversible in this compounds and the hexagonal structure for LiMnBO3 can be maintained after long cycles under high charge and discharge rate.

High tap density Li3V2(PO4)3 cathode materials were synthesized using mixed LiF and LiNO3 as lithium precursors, LiNO3 was used as the sintering agent. Rietveld refinement results show that no impurities phases are detected in products. Particle size distribution and tap density measurement results show that particle size and tap density of products can be increased by the addition of LiNO3. Electrochemical characterization results show that electrochemical performance of products is declined with the increase in contents of LiNO3 in the lithium precursors. Only a small amount of LiNO3 added in the lithium precursors (mole ratio of LiNO3 to LiF is 1:9) can increase the tap density and also retain the good performance of products. Scanning electron microscopy (SEM) images indicate that the samples prepared by mixed lithium precursors present particles agglomerate, and the particle size increased with increase in contents of LiNO3. Large amount of LiNO3 added in the lithium precursors induces the particles to become spheric and smooth, which worsens the performance. The particles obtained with the mole ratio of LiNO3 to LiF in 1:9 show a flake-like shape with a high specific surface area, which leads to good electrochemical performance.

LiFePO4 samples have been synthesized by mixing stoichiometric amounts of (NH4)2HPO4, FeC2O4·2H2O, and LiF. During synthesis, carbon gel was used as the carbon source. Single-phase LiFePO4 can be formed when the heating temperature ranges from 650 to 800 °C and it is decomposed into Li4P2O7, Li3PO4, Fe2P, and Li3P7 when the temperature comes to 850 °C. We find that the ratio of the lattice parameter (a/c) decreases with the increasing temperature, thereby increasing the Li+ diffusion channel length. Both the decrease of a/c and the abrupt crystal growth are expected to contribute to the monotonic decrease of the initial capacity of the samples. The sample heated at 650 °C with a smaller uniform particle size and relative higher specific surface area (8.2 m2/g) shows an excellent electrochemical performance. The initial specific capacity of 156.7(3) mAh/g is obtained at the rate of C/10.

Cr-doped Li3V2−xCrx(PO4)3/C (x = 0, 0.05, 0.1, 0.2, 0.5, 1) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li3V2−xCrx(PO4)3/C with monoclinic structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content in Li3V2−xCrx(PO4)3/C. Li3V1.9Cr0.1(PO4)3/C compound presents an initial capacity of 171.4 mAh g−1 and 78.6% capacity retention after 100 cycles at 0.2C rate. At 4C rate, the Li3V1.9Cr0.1(PO4)3/C can give an initial capacity of 130.2 mAh g−1 and 10.8% capacity loss after 100 cycles where the Li3V2(PO4)3/C presents the initial capacity of 127.4 mAh g−1 and capacity loss of 14.9%. Enhanced rate and cyclic capability may be attributed to the optimizing particle size, carbon coating quality, and structural stability during the proper amount of Cr-doping (x = 0.1) in V sites.

Lithium iron phosphate (LiFePO4) cathode material has been synthesized by a solid-state reaction. The XRD patterns of the samples show that the single-phase LiFePO4 compounds can be obtained in our experimental conditions. According to Popa theory, using the result from Rietveld refinement, the shape and the size of crystallite can be obtained. The result shows that the use of carbon gel in precursors do not change the structure of the crystal, but it can inhibit the particle growth and restrain the anisotropy growth of the grain at a lower temperature. At a higher temperature, carbon-coated LiFePO4 shows an anisotropy growth, i.e. growth rate along (1 0 0) crystal plane is more rapid than that of (1 1 1) crystal plane. In our experimental conditions, a spherical carbon-coated LiFePO4 can be synthesized successfully at 650 °C. The electrochemical testing indicated that the spherical carbon-coated LiFePO4 had the excellent performance. Its initial specific capacities were 156.7 mAh g−1 under the rate of C/10. At the 50th cycle, the reversible specific capacities were found to approach 151.2 mAh g−1 (the ratio of 96.5% of initial capacity).

LiFeBO3 cathode material has been synthesized successfully by solid-state reaction using Li2CO3, H3BO3 and FeC2O4·2H2O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO3, C-LiFeBO3, are also prepared. The amount of carbon coated on LiFeBO3 particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO3, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO3. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO3 may be an attractive cathode for lithium-ion batteries.

A liquid-based sol–gel method was developed to synthesize nanocarbon-coated Li3V2(PO4)3. The products were characterized by XRD, SEM and electrochemical measurements. The results of Rietveld refinement analysis indicate that single-phase Li3V2(PO4)3 with monoclinic structure can be obtained in our experimental process. The discharge capacity of carbon-coated Li3V2(PO4)3 was 152.6 mAh/g at the 50th cycle under 1C rate, with 95.4% retention rate of initial capacity. A high discharge capacity of 184.1 mAh/g can be obtained under 0.12C rate, and a capacity of 140.0 mAh/g can still be held at 3C rate. The cyclic voltammetric measurements indicate that the electrode reaction reversibility is enhanced due to the carbon-coating. SEM images show that the reduced particle size and well-dispersed carbon-coating can be responsible for the good electrochemical performance obtained in our experiments.

Large-scale calcium hexaboride (CaB6) nanostructures have been successfully fabricated with self-catalyst method using calcium (Ca) powders and boron trichloride (BCl3) gas mixed with hydrogen and argon. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) were used to characterize the compositions, morphologies, and structures of the samples. Our results show that the nanowires are highly single crystals elongated preferentially in the [1 1 0] direction. The growth mechanism based on the self-catalyst process is simply discussed.Large-scale calcium hexaboride (CaB6) nanostructures have been successfully fabricated with self-catalyst method using calcium (Ca) powders and boron trichloride (BCl3) gas mixed with hydrogen and argon. Our results show that the nanowires are highly single crystals elongated preferentially in the [1 1 0] direction.

Large scale NdB6 nanowires have been successfully fabricated for the first time using a self-catalyst method with Nd powders and boron trichloride (BCl3) gas mixed with hydrogen and argon. X-ray diffraction, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the samples. Transmission electron microscopy (TEM) reveals that the NdB6 nanowires are single crystals with cubic structure. Our investigation forms part of a series of studies for finding comparatively inexpensive methods to prepare RB6 nanomaterials.

Monoclinic Li3V2(PO4)3 can be synthesized by solid-state reaction using either hydrogen or carbon as the reducing agent when sintering temperatures are higher than 800 °C. The initial capacity of Li3V2(PO4)3 synthesized using hydrogen as the reducing agent increases with increasing sintering temperature T and then for T > 900 °C decreases monotonically, and the sample synthesized at 900 °C present the highest initial capacity of 146.3 mAh g−1, but exhibit poor cycle performance. The scanning electron microscope (SEM) images show that Li3V2(PO4)3 particles with small uniform particle size can be obtained at 900 °C. X-ray diffraction patterns of electrodes before and after cycling indicate that the capacity fading is not related to structure collapse. The carbon-coated Li3V2(PO4)3 (LVP/C) composites are synthesized by carbo-thermal reduction method at the optimized temperature of 900 °C. The LVP/C exhibit good cycle performance (137.5 mAh g−1 at 50th cycle under 1C rate, 94.6% of initial discharge capacity) and rate behavior (111.0 mAh g−1 under 5C rate for initial discharge) for the fully de-lithiated (3–4.8 V) samples. Our results suggest, based on the SEM images, that the good capacity retention and rate performance are owing to the nanometer size carbon webs coated the Li3V2(PO4)3 particles with both the greater specific surface area and the small uniform particle size.

Monoclinic lithium vanadium phosphate, Li3V2(PO4)3, has been successfully synthesized using LiF as lithium source. The one-step reaction with stoichiometric composition and relative lower sintering temperature (700 °C) has been used in our experimental processes. The solid-state reaction mechanism using LiF as lithium precursor has been studied by X-ray diffraction and Fourier transform infrared spectra. The Rietveld refinement results show that in our product sintered at 700 °C no impurity phases of VPO4, Li5V(PO4)2F2, or LiVPO4F can be detected. The solid-state reaction using Li2CO3 as Li-precursor has also been carried out for comparison. X-ray diffraction patterns indicate that impurities as Li3PO4 can be found in the product using Li2CO3 as Li-precursor unless the sintering temperatures are higher than 850 °C. An abrupt particle growth (about 2 μm) has also been observed by scanning electron microscope for the samples sintered at higher temperatures, which can result in a poor cycle performance. The product obtained using LiF as Li-precursor with the uniform flake-like particles and smaller particle size (about 300 nm) exhibits the better performance. At the 50th cycle, the reversible specific capacities for Li3V2(PO4)3 measured between 3 and 4.8 V at 1C rate are found to approach 147.1 mAh/g (93.8% of initial capacity). The specific capacity of 123.6 mAh/g can even be hold between 3 and 4.8 V at 5C rate.

Layered Li-rich vanadium phosphate, Li9V3(P2O7)3(PO4)2, is a novel and potential cathode material for lithium-ion batteries. It possesses both facile ion mobility due to its two-dimensional pathways, and high theoretical capacity (173.5 mAh g−1) because of its ability to extract six lithium ions (per formula) from the trigonal framework accompanied with the double-electron reaction of vanadium. In this study, we first correlate the structural characters with the electrochemical process by using a combined experimental and computational method. The electrochemical recrystallization of Li9V3(P2O7)3(PO4)2 is accomplished along with a metastable superstructure phase in different but related space group. Nevertheless, the structure as well as oxidation state can be easily recovered on reduction-oxidation, and the volume change is minimal. Furthermore, the electrochemical voltage-composition profile is predicted and understood as emerging from site energetics and redox couples via first-principles calculations.

We report the catalyst-free synthesis of uniform distributed single-crystalline LaxNd1−xB6 nanowires by simply heating mixed La and Nd powders to the required temperature in an inlet flux of mixed gases (H2, Ar and BCl3). FE-SEM, HRTEM, SAED, EDS, element mapping, XRD and Raman scattering results show that the LaxNd1−xB6 nanowires are structurally uniform and well-doped single crystals. Based on our experimental results, a dominant VLS-like mechanism with a self-catalytic growth mechanism was proposed and depicted conceptually. The nanowires display an excellent field emission performance with a low turn-on field value of ∼4.12 V μm−1 when evaluated as an electron emitter. Attempts were also made to understand the morphological influence of reaction time, reaction temperature and the proportion of the La and Nd powders.