Synthesis,non-isothermal kinetic and thermodynamic studies of the formation of LiMnPO4 from NH4MnPO4·H2O precursor

NH₄MnPO₄·H₂O was successfully synthesized by precipitating method. The LiMnPO₄ was successfully generated through solid state reaction between synthesized NH₄MnPO₄·H₂O precursor and Li₂CO₃. The morphologies were observed to depend on the reaction temperatures. The thermal decomposition of NH₄MnPO₄·H₂O and the formation process of LiMnPO₄ were confirmed by TG/DTG/DTA, FTIR, AAS/AES, XRD and SEM methods. The average crystallite size of NH₄MnPO₄·H₂O, Mn₂P₂O₇ and LiMnPO₄ were found to be around 51.2, 44.9 and 48.1 nm, respectively. The non–isothermal kinetic parameters (kinetic triplet: E_α, A, g(α)) of the formation process of LiMnPO₄ were evaluated from TG data by using Ozawa–Flynn–Wall and Kissinger–Akahira–Sunose methods. The iterative methods of both equations were carried out to determine the exact values of E_α. The Coats–Redfern equation and kinetic compensation effects were successfully applied to confirm the activation energy and the most probable mechanism functions of the formation of LiMnPO₄. The thermodynamic functions (ΔH^≠, ΔS^≠, ΔG^≠) of the transition state complexes of the formation of LiMnPO₄ were calculated from the kinetic parameters for the first time.     

Molecular wiring of LiMnPO4 (olivine) by ruthenium(II)-bipyridine complexes

LiMnPO₄ (olivine) was surface-modified by two different complexes: Ru-bis(4,4′-diethoxycarbonyl-2,2′-bipyridine)(4,4′-dicarboxylate-2,2′-bipyridine) and Ru-bis(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-dinonyl-2,2′bipyridine). These complexes have redox potentials of 4.45 and 4.25 V vs. Li/Li⁺, respectively, and are both active for molecular wiring of LiMnPO₄. The surface-confined Ru(II)/Ru(III) redox reaction propagates across the monolayer via hole-hopping, allowing a subsequent chemical delithiation of the underneath olivine towards MnPO₄. The activity of LiMnPO₄ is about half of that of LiFePO₄ (olivine) at similar experimental conditions.     

Crystal chemistry of the olivine-type LixFePO4 system () between 25 and 370 °C

The crystal chemistry of two initial mixtures of LiFePO₄ and heterosite FePO₄ (0.5LiFePO₄ + 0.5FePO₄; 0.75LiFePO₄ + 0.25FePO₄) was investigated through Neutron diffraction at 350 and 370 °C, respectively, and at room temperature after cooling. At 350 and 370 °C, Li_0.5FePO₄ and Li_0.75FePO₄ are refined as olivine-type single phases, in which Li⁺ ions are disordered. Significant anisotropic microstrains, within the (001)_Pmnb planes, occur, which may be accounted for by heterogeneous distance distributions within and between the [100]_Pmnb Li⁺ channels. On cooling back to room temperature, Li_0.5FePO₄ and Li_0.75FePO₄ single phases separate into mixtures of FePO₄ + Li_0.64FePO₄ and LiFePO₄ + Li_0.64FePO₄, respectively. The Li content of this metastable intermediate phase may correspond to the occupancy of 2 out of 3 Li sites within [100]_Pmnb Li⁺ channels. In Li_0,64FePO₄, average LiO bonds are longer than in LiFePO₄, whereas FeO bond lengths are shortened, due to a fraction of Fe(III). This may be at the origin of the metastability of such intermediate phase, and thus of the two-phase mechanism between LiFePO₄ and FePO₄.     

Spray-drying as a tool to disperse conductive carbon inside Na<Subscript>2</Subscript>FePO<Subscript>4</Subscript>F particles by addition of carbon black or carbon nanotubes to the precursor solution

In this work, Na₂FePO₄F-carbon composite powders were prepared by spray-drying a solution of inorganic precursors with 10 and 20 wt% added carbon black (CB) or carbon nanotubes (CNTs). In order to compare the effect of CB and CNT when added to the precursor solutions, the structural, electrochemical, and morphological properties of the synthesized Na₂FePO₄F-xCB and Na₂FePO₄F-xCNT samples were systematically investigated. In both cases, X-ray diffraction shows that calcination at 600 °C in argon leads to the formation of Na₂FePO₄F as the major inorganic phase. ⁵⁷Fe Mössbauer spectroscopy was used as complementary technique to probe the oxidation states, local environment, and identify the composition of the iron-containing phases. The electrochemical performance is markedly better in the case of Na₂FePO₄F-CNT (20 wt%), with specific capacities of about 100 mAh/g (Na₂FePO₄F-CNT) at C/4 rate vs. 50 mAh/g for Na₂FePO₄F-CB (20 wt%). SEM characterization of Na₂FePO₄F-CB particles revealed different particle morphologies for the Na₂FePO₄F-CNT and Na₂FePO₄F-CB powders. The carbon-poor surface observed for Na₂FePO₄F-CB could be due to a slow diffusion of carbon in the droplets during drying. On the contrary, Na₂FePO₄F-CNT shows a better CNT dispersion inside and at the surface of the NFPF particles that improves the electrochemical performance.     

The possibility of manganese disorder in LiMnPO4 and its effect on the electrochemical activity

Orthorhombic structured LiMnPO₄ was synthesized by a hydrothermal method. The possibility of manganese disorder in LiMnPO₄ was studied using powder X-ray diffraction and X-ray absorption fine structure analysis. A manganese-rich model was proposed for the hydrothermally synthesized LiMnPO₄. It is found that the extent of Mn²⁺ disorder on the Li⁺ sites was suppressed by increasing the reaction temperature, which led to an enhanced electrochemical activity. These observations are explained on the basis of the manganese-rich model, in which the disordered Mn²⁺ on the Li⁺ sites may act as a blockage in one-dimensional lithium ion transport pathway, thus reducing the electrochemical activity of the LiMnPO₄ prepared at low temperatures.     

A Theoretical study on the charge and discharge states of Na-ion battery cathode material,Na1+xFePO4F

Na₂FePO₄F is a promising cathode material for a Na-ion battery because of its high electronic capacity and good cycle performance. In this work, first principle calculations combined with cluster expansion and the Monte Carlo method have been applied to analyze the charge and discharge processes of Na₂FePO₄F by examining the voltage curve and the phase diagram. As a result of the density functional theory calculation and experimental verification with structural analysis, we found that the most stable structure of Na_1.5FePO₄F has the P2₁/b11 space group, which has not been reported to date. The estimated voltage curve has two clear plateaus caused by the two-phase structure composed of P2₁/b11 Na_1.5FePO₄F and Pbcn Na₂FePO₄F or Na₁FePO₄F and separated along the c-axis direction. The phase diagram shows the stability of the phase-separated structure. Considering that Na₂FePO₄F has diffusion paths in the a- and c-axis directions, Na₂FePO₄F has both innerphase and interphase diffusion paths. We suggest that the stable two-phase structure and the diffusion paths to both the innerphase and interphases are a key for the very clear plateau. We challenge to simulate a nonequilibrium state at high rate discharge with high temperature by introducing a coordinate-dependent chemical potential. The simulation shows agreement with the experimental discharge curve on the disappearance of the two plateaus. © 2018 Wiley Periodicals, Inc.     

Na2FePO4F/Biocarbon Nanocomposite Hollow Microspheres Derived from Biological Cell Template as High-Performance Cathode Material for Sodium-Ion Batteries

We have successfully synthesized Na₂FePO₄F/biocarbon nanocomposite hollow microspheres from Fe^III precursor as cathodes for sodium-ion batteries through self-assembly of yeast cell biotemplate and sol-gel technology. The carbon coating on the nanoparticle surface with a mesoporous structure enhances electron diffusion into Na₂FePO₄F crystal particles. The improved electrochemical performance of Na₂FePO₄F/biocarbon nanocomposites is attributed to the larger electrode−electrolyte contact area and more active sites for Na⁺ on the surface of hollow microspheres compared with those of Na₂FePO₄F/C. The Na₂FePO₄F/biocarbon nanocomposite exhibits a high initial discharge capacity of 114.3 mAh g⁻¹ at 0.1 C, long-cycle stability with a capacity retention of 74.3 % after 500 cycles at 5 C, and excellent rate capability (70.2 mAh g⁻¹ at 5 C) compared with Na₂FePO₄F/C. This novel nanocomposite hollow microsphere structure is suitable for improving the property of other cathode materials for high-power batteries.     

不同碳源合成Na2MnPO4F/C及其作为锂离子电池正极材料的性能

采用湿法球磨和原位热解碳包覆相结合的方法, 分别以硬脂酸、柠檬酸、聚乙二醇-6000 (PEG-6000)、β-环糊精为碳源, 制备了不同结构的Na₂MnPO₄F/C 复合材料, 并研究了它们作为锂离子电池正极材料的电化学行为. 通过X射线衍射(XRD)、扫描电镜(SEM)、BET比表面积测试、恒流充放电等表征手段, 比较和分析了产物的结构、形貌及电化学性能. 研究结果表明, 由不同碳源制备的材料在形貌和颗粒尺寸上有明显差异, 进而对它们的电化学性能造成很大影响. 影响电化学性能的关键因素在于材料一次颗粒的大小. 其中, 以柠檬酸为碳源制备的样品呈现出典型的微纳结构和最小的一次颗粒(10-40 nm). 并给出最佳的电化学性能: 在1.5-4.8 V电压范围内, 以5 mA·g^-1充放电电流获得的首次放电比容量约为80 mAh·g^-1, 且循环稳定性良好.     

碳和石墨烯对磷酸锰锂材料性能的影响

在采用溶剂热法制备磷酸锰锂的基础上,以蔗糖和石墨烯为碳源,制备了裂解碳和石墨烯含量不同的磷酸锰锂/碳/石墨烯复合材料,研究了裂解碳和石墨烯对材料性能的影响。采用扫描电镜（SEM）和透射电镜（TEM）对材料的形貌进行了表征。裂解碳包覆可以提高LiMnPO₄纳米片表面的电子导电性,对于材料性能的改善起到主要的作用;石墨烯可以提高纳米片之间的电子和离子导电性,改善材料的电化学性能。电化学测试表明,当裂解碳含量为4%、石墨烯含量为2%时,LiMnPO₄电极具有较好的电化学性能,在0.5C下的放电比容量为139.1 mAh·g^-1,循环100次后,容量保持率为93.6%。与添加单一碳和单一石墨烯的LiMnPO₄电极相比,该电极在0.5C下的放电比容量分别提高了35.0%和48.6%。     

Identifying the Structural Evolution of the Sodium Ion Battery Na2FePO4F Cathode

Na₂FePO₄F is a promising cathode material for Na‐ion batteries owing to its relatively high discharge voltage and excellent cycling performance. Now, the long‐ and short‐range structural evolution of Na₂FePO₄F during cycling is studied by in situ high‐energy X‐ray diffraction (XRD), ex situ solid‐state nuclear magnetic resonance (NMR), and first‐principles DFT calculations. DFT calculations suggest that the intermediate phase, Na_1.5FePO₄F, adopts the space group of P2₁/c, which is a subgroup (P2₁/b11, No. 14) of Pbcn (No. 60), the space group of the starting phase, Na₂FePO₄F, and this space group provides a good fit to the experimental XRD and NMR results. The two crystallographically unique Na sites in the structure of Na₂FePO₄F behave differently during cycling, where the Na ions on the Na2 site are electrochemically active while those on the Na1 site are inert. This study determines the structural evolution and the electrochemical reaction mechanisms of Na₂FePO₄F in a Na‐ion battery.     

Synthesis and electrode performance of carbon coated Na2FePO4F for rechargeable Na batteries

Carbon-coated Na₂FePO₄F is synthesized by a simple solid-state method with ascorbic acid as carbon source. Structural characterization of Na₂FePO₄F by synchrotron X-ray diffraction, scanning/transmission electron microscopy, and Raman spectroscopy reveals that ascorbic acid effectively suppresses the particle growth of Na₂FePO₄F, forming the nano-sized carbon coated materials. Electrode performance of Na₂FePO₄F for rechargeable sodium batteries is also examined. The carbon-coated Na₂FePO₄F sample (1.3 wt% carbon) delivers initial discharge capacity of 110 mAh g^-1 at a rate of 1/20 C (6.2 mA g^-1) with well-defined voltage plateaus at 3.06 and 2.91 V vs. Na metal. The sample also shows acceptable capacity retention and rate capability as the positive electrode materials for rechargeable Na batteries, which is operable at room temperature.     

THERMODYNAMIC STUDY OF GASEOUS MANGANESE PHOSPHATES MnPO3 and MnPO2

In the present work, the stability of gaseous manganese-containing salts was proved by the high-temperature mass spectrometric method. New previously unreported species were found, MnPO₃ and MnPO₂. On the basis of equilibrium constants measured for gas-phase reactions the standard formation enthalpies for MnPO₃ and MnPO₂ were determined as ?602.0 ± 10.0 kJ/mole and ?299.0 ± 11.5 kJ/mole, respectively, and the standard atomization enthalpies as 1950 ± 14 kJ/mole and 1397 ± 14 kJ/mole, respectively.     

碳和石墨烯对磷酸锰锂材料性能的影响

在采用溶剂热法制备磷酸锰锂的基础上,以蔗糖和石墨烯为碳源,制备了裂解碳和石墨烯含量不同的磷酸锰锂/碳/石墨烯复合材料,研究了裂解碳和石墨烯对材料性能的影响。采用扫描电镜(SEM)和透射电镜(TEM)对材料的形貌进行了表征。裂解碳包覆可以提高LiMnPO_4纳米片表面的电子导电性,对于材料性能的改善起到主要的作用;石墨烯可以提高纳米片之间的电子和离子导电性,改善材料的电化学性能。电化学测试表明,当裂解碳含量为4%、石墨烯含量为2%时,LiMnPO_4电极具有较好的电化学性能,在0.5C下的放电比容量为139.1 m Ah·g-1,循环100次后,容量保持率为93.6%。与添加单一碳和单一石墨烯的LiMnPO_4电极相比,该电极在0.5C下的放电比容量分别提高了35.0%和48.6%。     

磷酸铁锂废料中FePO4·2H2O提取及其杂质形成机理

采用化学共沉淀方法从磷酸铁锂废料中提取FePO₄·2H₂O,并研究了回收过程中杂质形成的机理。在热力学计算基础上绘制了298和363 K时Fe-P-Li-H₂O体系的电势(φ)-pH图,结果表明当pH≤5.0时,Fe(OH)₃相可以自发地转成FePO₄·2H₂O相,从而得到高纯的FePO₄·2H₂O。但实验结果发现当溶液中铁、磷的物质的量之比(n_Fe∶n_P)为1∶1,合成pH为1.5～2.2时得到的FePO₄·2H₂O中存在Fe(OH)₃杂质,这是因为在共沉淀过程中少量Fe³⁺以Fe(OH)₃快速沉淀,而陈化时Fe(OH)₃相转化速率慢,因此FePO₄·2H₂O中含有Fe(OH)₃     

Amorphous iron phosphate/carbonized polyaniline nanorods composite as cathode material in sodium-ion batteries

As-prepared polyaniline (PANI) nanorods have been used to synthesize an iron phosphate/polyaniline (FePO₄/PANI) composition through the microemulsion technique. After sintering at 460 °C under a nitrogen protective atmosphere, the PANI carbonized, yielding the amorphous iron phosphate/carbonized polyaniline nanorods (FePO₄/CPNRs) composite, which acts as the cathode material in sodium-ion batteries (SIBs). The electrochemical performance of FePO₄/CPNRs composite shows an initial discharge specific capacity of 140.2 mAh g^?1, with the discharge specific capacity being maintained at 134.4 mAh g^?1 after the 120th cycle, up to 87.9 % of the theoretical capacity (154.1 mAh g^?1 for NaFePO₄), as well as an excellent rate capability in sodium-ion batteries. Compared with pure FePO₄, the electrochemical performance has been greatly improved. On the one hand, using the CPNRs as conductive medium significantly improves electronic transport. On the other hand, the FePO₄ sphere of nanoscale particles, which has a large specific surface area, can promote an active material/electrolyte interface reaction and improve the speed of sodiation and desodiation during the charge and discharge process. The amorphous FePO₄/CPNRs composite shows outstanding electrochemical performance as competitive cathode material in SIBs.     

Synthesis of Two‐Dimensional Transition‐Metal Phosphates with Highly Ordered Mesoporous Structures for Lithium‐Ion Battery Applications

Materials with ordered mesoporous structures have shown great potential in a wide range of applications. In particular, the combination of mesoporosity, low dimensionality, and well‐defined morphology in nanostructures may exhibit even more attractive features. However, the synthesis of such structures is still challenging in polar solvents. Herein, we report the preparation of ultrathin two‐dimensional (2D) nanoflakes of transition‐metal phosphates, including FePO₄, Mn₃(PO₄)₂, and Co₃(PO₄)₂, with highly ordered mesoporous structures in a nonpolar solvent. The as‐obtained nanoflakes with thicknesses of about 3.7 nm are constructed from a single layer of parallel‐packed pore channels. These uniquely ordered mesoporous 2D nanostructures may originate from the 2D assembly of cylindrical micelles formed by the amphiphilic precursors in the nonpolar solvent. The 2D mesoporous FePO₄ nanoflakes were used as the cathode for a lithium‐ion battery, which exhibits excellent stability and high rate capabilities.     

A novel route for FePO4 olivine synthesis from sarcopside oxidation

Heterosite FePO₄ is synthesized for the first time by direct thermal oxidation of sarcopside Fe₃(PO₄)₂. Both FePO₄ and Fe₃(PO₄)₂ have a pseudo olivine structure. Complete isostructural conversion of sarcopside into FePO₄ is achieved at a temperature of 450 °C within 3 days according to the reaction Fe₃(PO₄)₂ + ¾ O₂ → 2 FePO₄ + ½ Fe₂O₃ which leads to the extraction of iron from the sarcopside structure. Appropriate heating ramp must be applied in order to avoid the crystallization of Fe₇(PO₄)₆. Electrochemical performances of the oxidation product are consistent with those of olivine FePO₄.     

Effect of Proton Transfer on Electrocatalytic Water Oxidation by Manganese Phosphates

The effect of proton transfer on water oxidation has hardly been measurably established in heterogeneous electrocatalysts. Herein, two isomorphous manganese phosphates (NH₄MnPO₄ ⋅ H₂O and KMnPO₄ ⋅ H₂O) were designed to form an ideal platform to study the effect of proton transfer on water oxidation. The hydrogen-bonding network in NH₄MnPO₄ ⋅ H₂O has been proven to be solely responsible for its better activity. The differences of the proton transfer kinetics in the two materials indicate a fast proton hopping transfer process with a low activation energy in NH₄MnPO₄ ⋅ H₂O. In addition, the hydrogen-bonding network can effectively promote the proton transfer between adjacent Mn sites and further stabilize the Mn^III−OH intermediates. The faster proton transfer results in a higher proportion of zeroth-order in [H⁺] for OER. Thus, proton transfer-affected electrocatalytic water oxidation has been measurably observed to bring detailed insights into the mechanism of water oxidation.     

Synthesis of MnPO4·H2O by refluxing process at atmospheric pressure

Serrabrancaite (MnPO₄·H₂O) was synthesized by oxidizing Mn(H₂PO₄)₂ with NaClO solution using a refluxing process at atmospheric pressure, and a mixed solution of MnCl₂ and H₃PO₄ could substitute for Mn(H₂PO₄)₂ in the process. The products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Hureaulite was formed when single solution of Mn(H₂PO₄)₂ was refluxed for 12 h at 60 °C. Rodlike hureaulite was fabricated by refluxing reaction of 30 mmol Mn(H₂PO₄)₂ and 60 mmol NaClO solution with adding hydrochloric acid within 40 mmol. Granular hureaulite was formed by refluxing of 30 mmol MnCl₂ and 60 mmol NaClO solution with adding phosphoric acid within 30 mmol. For the two above-mentioned reaction systems, MnPO₄·H₂O was prepared by adding hydrochloric acid no less than 80 mmol and phosphoric acid no less than 60 mmol respectively. MnPO₄·H₂O yield increased with elevating reflux temperature, and increased firstly and then decreased with increasing additional amount of acid. The highest recovery yield of MnPO₄·H₂O reached 84.1% when Mn(H₂PO₄)₂ was performed as bivalent manganese source, and approached 74.0% when MnCl₂ and H₃PO₄ were used instead.     

Surface modification of FePO4 particles with conductive layer of polypyrrole

Polypyrrole–FePO₄ powder was synthesized by an oxidative polymerization of pyrrole monomer on the surface of FePO₄ powder. The polymerization reaction was initiated using hydrogen peroxide in an acidified solution and catalysed with Fe³⁺. The samples were investigated by light microscopy (LM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). These methods confirmed the presence of polypyrrole on FePO₄ particles and its homogeneous distribution in the composite material. To determine the PPy content in the PPy–FePO₄ composites a thermogravimetric analysis was used. Cyclic voltammetry curves (CV) were measured and compared in a non-aqueous lithium salt solution for electrodes consisting of pellets made from pure FePO4 and FePO₄/PPy. Electrochemical impedance spectroscopy (EIS) showed that coating of PPy significantly decreases the charge transfer resistance of PPy–FePO₄ electrodes.