Lithium Ion Insertion Research Articles

Density Functional Theory Calculations of Lithium Adsorption and Insertion to Defect-Free and Defective Graphene

Density functional theory calculations with periodic boundary conditions were performed to clarify the interaction between lithium atoms and a graphene sheet. Three types of graphene sheets—defect-free, containing carbon vacancies VCn (n = 1, 2, 3, 4, 6, 10, 13, 16, and 24), and their hydrogen-terminated carbon vacancies—were examined in this study. We found that a lithium atom inserted into bare carbon vacancy VCn (n ≥ 3) is more stable than that in bulk lithium metal and that it is trapped by the vacancy. On the other hand, a lithium atom inserted into the hydrogen-terminated carbon vacancies is less stable than that in bulk metal. These results suggest that the electrochemical in-plane insertion of lithium ions into the bare carbon vacancies is possible, whereas the insertion into the hydrogen-terminated vacancies is unlikely because the precipitation of lithium metal is energetically more favorable.

The enhancement of polysulfide absorbsion in Li[sbnd]S batteries by hierarchically porous CoS2/carbon paper interlayer

The high-energy LiS battery suffers from poor cycling performance due to the shuttle effect of the polysulfides. Strategies must be adopted to suppress the diffusion of polysulfides into the electrolyte in LiS battery. In this work, for the first time, we adopt hydrophilic carbon paper anchored by hierarchically porous cobalt disulfides as the interlayer for capturing polysulfides through physical absorption and chemical bonding. Hierarchical pores can physically adsorb polysulfides, and moreover cobalt disulfide can trap the polysulfides by forming strong chemical interaction. The sulfur-graphene composite with a sulfur content of 70.5% delivers a high initial capacity of 1239.5 mAh g−1 at 0.2 C and retains a reversible capacity of 818 mAh g−1 after 200 cycles. In spite of a little capacity contribution by the insertion of lithium ions into cobalt disulfide for the initial cycles, it disappears in the subsequent cycling. Therefore, the as-developed porous transition metal disulfides on carbon paper as the interlayer could significantly enhance the cycling performance of LiS batteries.

Porous Si/C Composite Anode Materials for Lithium Ion Batteries

Among all the anode materials, silicon is one of the most attractive anode materials for lithium-ion batteries because of its highest capacity of 4200 mAh g-1, satisfactory voltage window for lithium insertion and extraction (<0.5 V vs. Li/Li+), high safety and low cost. However, the practical utilization of Si as anode material is hindered by its huge volume changes (300%) induced by the lithium-ion insertion and extraction processes, culminating in the pulverization of electrode structure as well as the poor cycling performance. In addition, the low electrical conductivity of pristine Si and the difficulty in the formation of stable solid-electrolyte interface (SEI) films have also restrained the commercialization of Si anode material. Here we report a series of silicon based composites [1-10], such as self-supporting grapheme/Si film, three dimensional porous carbon/silicon composites, three dimensional silicon/TiOx/carbon framework, etc, aiming at solving the problems of current silicon anode materials. Those porous silicon based anode materials have demonstrated to be effective in maintaining good electronic contact among particles and accommodating the mechanical stresses experienced by the huge volume change of active silicon associated with Li insertion/extraction processes.

How the Conditions of Preparation Process Affect the Electrochemical Properties of C/Sn Anode Materials?

How the conditions of preparation process affect the electrochemical properties of C/Sn anode materials? The application of tin in commercially available Li-ion batteries is limited by the poor cyclic performance due to the pulverization caused by cyclic volume changes of elementary cell, during the insertion and extraction processes of lithium ions [1]. Consequently, it results in loss of electrical contact between active material and current collector. One of the ways to overcome this issue is development of tin-carbon nanocomposite, in which nanometric tin grains are encapsulated in a flexible and conductive carbon layers. Such nanocomposites are able to provide appropriate electrochemical properties, only if the formed carbon layer is flexible, has appropriate porosity which featuring local free space to compensate volume changes. The goal of the presented studies is to examine the effects of carbonization temperature as well as origin and content of carbon on the electrochemical performance of carbon-tin nanocomposites. Electrode materials consisted of tin-based nanograins encapsulated in carbon buffer matrix of different origins (starch or water soluble polymer) were obtained in a simple and inexpensive process. The tin precursor (SnO2) was prepared using a modified reverse nanoemulsion method (w/o) and then was coated by a various source of carbon (potato starch or modified poly(N-vinylformamide)) [2,3]. The carbon-tin precursors were pyrolyzed, providing formation of tin-based nanograins encapsulated in buffer matrix. The optimal condition for preparation process was studied using thermal analysis method (EGA-TGA/DTG/SDTA). The structure and the morphology of the samples were characterized by X-Ray diffraction analysis (XRD), low-temperature nitrogen adsorption method (N2-BET) and transmission electron microscopy (TEM) as well. The quality of carbon matrix in resulting composites was determined by Raman spectroscopy (RS) measurements. The working electrodes were prepared from a carbon-tin active material and polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) solvent on copper foil. R-2032 coin type cells were assembled using obtained electrodes combined with Li metal discs as a counter electrode. The electrolyte consisted of a solution of 1M LiPF6in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with EC/DEC molar ratio of 1:1. The lithium cells were characterized by cyclic voltammetry (CV), charge-discharge tests (CELL TEST) as well as electrochemical impedance spectroscopy (EIS). The thermal analysis results enabled determination of the optimal carbonization temperature for obtaining carbon-tin anode materials. N2-BET data as well as TEM images indicated that samples, in which the potato starch was used as a carbon source, have high specific surface area and pore volume as well as reveal presence of the opened pores through the composites. The electrochemical tests showed that better encapsutation of tin nanograins in carbon buffer matrix resulted in higher gravimetric capacity and better cell performance. The C/Sn material in which carbon matrix was fabricated from modified polymer exhibited higher gravimetric capacity and better cell performance (589 mAh g-1 after 70 cycles). The authors acknowledge a financial support from the National Science Centre of Poland under research grant No. 2012/07/N/ST8/03725. [1] S. Ding, X.W. Lou, Nanoscale, 3 (2011) 3586[2] M. Molenda, A. Chojnacka, M. Bakierska, R. Dziembaj, Mater. Technol., 29 (2014) A88[3] A. Chojnacka, M. Molenda, M. Bakierska, R. Dziembaj, Procedia Eng., 98 (2014) 2

A Computational Study of Lithium-Ion Insertion and Diffusion in the NiTi2O2(PO4)2 Anode Material for Lithium-Ion Batteries

Polyanionic compounds currently attract considerable interest as electrode materials for Li and Na-ion batteries, owing to their apparent gain in electrochemical performance. Titanium oxyphosphate MTi2O2(PO4)2 (M = Ni, Fe, Co) compounds have been targeted as potential anode materials since NiTi2O2(PO4)2 was identified by Belharouak and Amine as a polyanionic host material for Li-ion battery electrodes [1]. Following this pioneering work, other groups have reported a number of structurally related compounds. Key issues regarding the electrochemical mechanisms and structural changes upon Li-ions insertion have been investigated through a combination of several experimental techniques. The main concern is that titanium oxyphosphate materials lose crystallinity and become amorphous during lithiation [2-7] with a typical mixed insertion-conversion mechanism during the first discharge. In this context, high-energy spectroscopic techniques have enabled a more careful analysis of the changes in composition and the oxidation states of the materials components at different intercalation steps [8]. However, despite the many efforts, the results found on MTi2O2(PO4)2 in literature are still incomplete and contradictory, and a complete picture of the complexity of the electrode reactions is missing. For instance, lithium ions insertion sites, lithium ions diffusion, and migration pathways are not yet known. Obtaining such insights for complex polyhedral structures from experiments remains a challenge. To date, most of the research on this subject has been devoted to the experimental analysis, and to our knowledge no theoretical studies have been reported. The present study uses state of the art computational tools based on density functional theory (DFT) in order to gain valuable insights into crystal structure, Li-ions insertion sites, phase stability upon Li-ions insertion and diffusion pathways and energetics in NiTi2O2(PO4)2, which are crucial for a complete understanding of the electrochemical behavior of this anode material. In order to probe the validity of our computational approach, structural optimization of NiTi2O2(PO4)2 was performed based on the bulk crystal structure observed experimentally [9]. The crystal structure provides three-dimensionally connected empty sites for intercalation of guest ions, and NiTi2O2(PO4)2 can thus be described as [Ni]2a [□]2d [□]2b [□]2c [Ti2 ]4e[O10 ]4e[P2 ]4e . Several lithiation scenarios are investigated in our calculations based on the model of pristine NiTi2O2(PO4)2. In these calculations, the evolution of pressure, volume, lattice parameters and local structure changes were identified upon the insertion of 1 to 6 lithium ions, using all possible combinations of occupation. Upon Li insertion, the pressure in the cell increases and can vary by an order of magnitude depending on the number of Li ions and the site occupation combination. Allowing a volume change of the cell relieves the pressure. We report on these pressure/volume changes for different Li concentrations and site occupations and put them into context of the electrochemical performance during battery cycling. We also report on diffusion barriers and pathways between high symmetry sites for different Li concentrations.

Sandwich-structured graphene sheets@LiNi0.5Mn1.5O4@graphene sheets composites as cathode materials for lithium ion batteries with high rate performance

Sandwich-structured graphene sheets@LiNi0.5Mn1.5O4@graphene sheets (GNRs@LNMO@GNRs) composites have been synthesized by growing graphene sheets(GNRs) on the surface of LiNi0.5Mn1.5O4 (LNMO) via an in-situ method of graphitizing organic compounds. The microstructure, surface morphology and electrochemical performances of GNRs@LNMO@GNRs composites have been examined by X-ray diffraction(XRD), transmission electron microscope(TEM) and different electrochemical tests. The TEM results show that the LNMO nanoparticles are homogeneously dispersed and attached on the GNRs surface. Electrochemical tests demonstrate that the GNRs@LNMO@GNRs electrode exhibits higher discharge capacity, higher coulombic efficiency, better cycling performance and rate capability than the pristine LNMO electrode. These findings reveal that distributing LNMO nanoparticles uniformly on the GNRs might not only provide a plenty of accessible active sites for the fast diffusion of lithium ions, decrease the electrochemical reaction resistance and improve the electronic conductivity, but also suppress the volume changes associated with lithium ion insertion and extraction processes, which result in the improved electrochemical performance of the GNRs@LNMO@GNRs. This study offers an effective way to obtain LNMO cathode materials of lithium ion batteries with high rate performance.

Direct Mapping of Charge Distribution during Lithiation of Ge Nanowires Using Off-Axis Electron Holography.

The successful operation of rechargeable batteries relies on reliable insertion/extraction of ions into/from the electrodes. The battery performance and the response of the electrodes to such ion insertion and extraction are directly related to the spatial distribution of the charge and its dynamic evolution. However, it remains unclear how charge is distributed in the electrodes during normal battery operation. In this work, we have used off-axis electron holography to measure charge distribution during lithium ion insertion into a Ge nanowire (NW) under dynamic operating conditions. We discovered that the surface region of the Ge core is negatively charged during the core-shell lithiation of the Ge NW, which is counterbalanced by positive charge on the inner surface of the lithiated LixGe shell. The remainder of the lithiated LixGe shell is free from net charge, consistent with its metallic characteristics. The present work provides a vivid picture of charge distribution and dynamic evolution during Ge NW lithiation and should form the basis for tackling the response of these and related materials under real electrochemical conditions.

Macroporous Nanostructured Nb2O5 with Surface Nb4+ for Enhanced Lithium Ion Storage Properties

AbstractMacroporous Nb2O5 (MP‐Nb2O5) has been synthesized using dispersed polystyrene microspheres (PS) as template followed by annealing in air. The structural characterization showed that the diameters of the macropores are around 200 nm and the average particle size of the composition is 20–50 nm. XPS revealed the presence of low valence Nb4+ and oxygen vacancies on the surface of the resulting product introduced during the pyrolysis of PS. Such a unique combination of macroporous nanostructure and tetravalent niobium ions enables the electrode with superior lithium ion insertion properties, such as high specific capacity (≈190 mA h g−1 at 0.5C) and rate capability. Even at a current density of 1.6 A g−1, an average capacity of 129.2 mA h g−1 can still be obtained. These findings demonstrate MP‐Nb2O5 is a promising candidate for high‐rate lithium ion storage applications.

Effect of lithium intercalation on the structural and electronic properties of layered LiFeSO4OH and layered FeSO4OH using first-principle calculations

The effect of lithium intercalation on the structural and electronic properties of layered LiFeSO4OH and layered FeSO4OH cathode material is investigated using first-principle calculations. Results of Mulliken analysis, band gap, density of state, and charge density before and after intercalation are discussed. The importance of the strong covalent character of S–O and Fe–O bonds in high charge/discharge cycles and thermodynamic stabilities is confirmed. Analysis of the partial density of state reveals that the main contributor to the strong S–O covalent bond is the hybridization between the S 3s, S 3p electron state and the O 2p electron state. The Fe 3d orbital also plays an important role in cathode performance because it dominates the conduction and valence bands and radically changes upon the removal and insertion of lithium ion inside the cathode materials. The charge density elucidates the ionic and covalent characters of the bond inside the cathode.

Highly reversible insertion of lithium into MoO2 as an anode material for lithium ion battery

MoO2 has gained renewed attention as a safe oxide anode host material for lithium ion insertion because of its high gravimetric/volumetric capacity and highly stable cycling behavior. However, these recent results are completely contrary to previous reports. To confirm that MoO2 is an appropriate anode material as well as further understand lithium ion reactions when inserted into MoO2, we combine electrochemical characterization of MoO2 electrodes and ex situ X-ray diffraction analysis with first principle calculations. Theoretical capacity of the MoO2 electrode (∼209 mAh g−1) and stable capacity retention up to 100 cycles are simultaneously attained using a proper particle size and type of binder. Ex situ XRD analysis with first principle calculations of the phase transformation of MoO2 electrodes shows that MoO2 undergoes reversible structural changes upon lithiation and subsequent delithiation, clearly demonstrating that nanostructured MoO2 can be used as an anode material for highly reliable lithium ion batteries.

Effects of amorphous V2O5 coating on the electrochemical properties of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 as cathode material for Li-ion batteries

Lithium-rich layered oxide Li[Li0.2Mn0.54Ni0.13Co0.13]O2 coated with V2O5 layers (labeled as LMNCO@V2O5) has been synthesized and its electrochemical properties as cathode material for lithium ion batteries have been measured and compared with pristine Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (labeled as LMNCO) and LMNCO–V2O5 composite. As a lithium ions insertion host material, both the V2O5 in the LMNCO@V2O5 and the LMNCO–V2O5 can reduce the irreversible capacity losses and improve the Coulombic efficiencies of the cathode in the first charge-discharge cycle. However, for improving the cycling stabilities and the high-rate capabilities of the LMNCO, the effects of the V2O5 coating layers in the LMNCO@V2O5 are far beyond the effects of the V2O5 nanoparticles in the LMNCO–V2O5. When charge-discharged galvanostatically at 25 mA g−1 between 2.0 and 4.8 V (vs. Li+/Li), the LMNCO@V2O5 with 3 wt.% V2O5 exhibits a discharge capacity of 279.5 mAh g−1 in the first cycle and maintains a discharge capacity of 269.1 mAh g−1 after 50 cycles, with capacity retention of 96.3%. In contrast, the discharge capacity of the pristine LMNCO changes from 251.2 mAh g−1 in the initial cycle to 202.2 mAh g−1 in the 50th cycle, with capacity retention of 80.2%; and the LMNCO–V2O5 shows the same capacity fading trend as the pristine LMNCO, with no obvious improvement in the capacity retention. At high rate of 1250 mA g−1, the discharge capacity of the LMNCO@V2O5 can reach 113.6 mAh g−1, which is much higher than the capacities that the pristine LMNCO and the LMNCO–V2O5 can reach. Different effects of V2O5 are due to their different roles in the cathode materials. While the V2O5 coating layer in the LMNCO@V2O5 can reduce the charge transfer resistance at the electrode-electrolyte interfaces and improve the transportation of lithium ions among the LMNCO particles, the V2O5 nanoparticles in the LMNCO–V2O5 can only work as a Li+ ions insertion host material.

Furfuryl Alcohol-Derived Vitreous Carbon Coating on Si/ Silicide Nanocomposite Anode of Li-Ion Batteries:

Silicon has been widely studied as an anode material for lithium ion batteries because of high gravimetric and volumetric capacity. However, lithium ion insertion and extraction results in a volumetric expansion of about 400%, which leads to pulverization of the silicon anode during electrochemical cycling. One of the methods to mitigate this problem is to coat a thin carbon layer on silicon particles. It can effectively increase the electrical conductivity, improve the chemical and/or physical properties of the surface layer. In addition, the anode particles can be protected from direct contact with electrolyte, leading to enhanced cycle life. Most of the carbon coating layer tried so far is graphitic starting from different types of aromatic hydrocarbon sources.Here, we have adopted a new carbon coating with vitreous carbon derived from furfuryl alcohol. The vitreous carbon structure, formed after carbonization heat treatment at relatively lower temperatures, with inherently higher strength, hardness, resistance to chemical reaction, as well as lower porosity and permeability may result in better carbon coating layer. For the anode material, we combined chemically inert phase such as silicides with silicon into nanostructured composite particles which should have a much lower volumetric expansion after lithiation than pure silicon particles.The specific surface area which would affect the irreversible capacity and charge retention ratio of the cell could be controlled by change of the heat treatment condition. The specific surface area was changed between 7m2/g and 70m2/g after coating of the alloy powder with the specific surface area of 3m2/g. By optimizing heat treatment conditions, we could obtained much improved initial capacity as well as significantly better charge retention ratio after 20 cycles.

Silicon Non-Metal Compounds As New Promising Materials for Lithium Ion Batteries

Due to an increasing energy demand caused by an expanding global population and the rapid development of new technologies, a high priority to a further development of new battery systems is given. In fact, there is a great interest in Lithium Ion Batteries as they offer a high energy density and a high rate capability [1]. Owing to a high theoretical capacity silicon is a promising anode material to replace commercially used graphite [2]. The electrochemical energy storage originates from an alloying-dealloying mechanism where different lithium-silicon phases like Li12Si7, Li15Si4 or Li22Si5 were formed [2]. This mechanism conducts capacity fading caused by volume expansion during the insertion and extraction of lithium ions. Therefore, nanostructured silicon is expected to meet future criteria defined for new electrochemical energy storage systems. A new promising way to produce silicon nanoparticles is the application of silicon non-metal compounds and their decomposition through an electrochemical in situ conversion mechanism. In addition, the non-metal serves as a Li-containing matrix that buffers the volume expansion during charging and discharging. Moreover, the synthesis of silicon non-metal compounds via planetary ball mill is environmentally benign and easy scalable. Initial studies show that phase-pure produced silicon diphosphide (SiP2) is electrochemical active over a wide potential range. The first discharge cycle reaches a capacity closely to theory (about 3100 mAh/g). Continuous cycling is stable about 100 cycles. Cyclic voltammetry experiments give an insight into the behavior of the redox properties of materials which is in this case very similar compared to pure silicon. From this observation a similar reaction of silicon diphosphide as known from silicon is indicated. Ex situ XRD results confirm the proposed reaction mechanism and refers to the formation of lithium phosphide which serves as buffering matrix. The production of smaller particles is challenging and the large capacity fading after the first cycle needs to be explained and to be overcome. Both points are taken as prospective tasks. [1] C.-M. Park, J.-H. Kim, H. Kim, H.-J. Sohn: Li-alloy based anode materials for secondary batteries. In: Chem. Soc. Rev. VI. 39, 2010, S. 3115–3141. [2] S. Goriparti, E. Miele, F. De Angelis, E. De Fabrizio, R. P. Zaccaria, C. Capiglia: Review on recent progress of nanostructured anode materials for Li-ion batteries. In: J. Power Sources VI. 257, 2014, S. 421–443.

Effects of Ni/Co Doping on the Properties of LiFeaNibCocPO4 High-Performance Olivine Cathodes for Lithium Batteries

Nowadays, the rapid development in portable electronics, load leveling/peak shaving for the power grid and electric automotive, requires significant progress in high voltage and high capacity storage systems [1,2]. Lithium batteries are, to date, the most promising systems that can sustain this demand [3]; they have high specific energy, high efficiency and a long lifespan [4]. Lithium cobalt oxide (LiCoO2 ) based cathode materials currently dominate the market [5], but, due to a low working potential (3.0 – 4.0 V vs. Li) and to a high cost and toxicity, there is a broad scope for the development of new cathodic materials [6]. Lithium-transition metal-phosphates (LiMPO4 , M=Co, Fe, Mn or Ni) show very good performance: their olivine structure with a 2D framework of crossed tunnels allows the insertion and de-insertion of lithium ions during the discharge/charge of the battery [7]. The highest specific capacity is reached by lithium iron phosphate (LiFePO4 ), but at low potential, while the highest working potential can be obtained using lithium cobalt phosphate (LiCoPO4 ) or lithium nickel phosphate (LiNiPO4 ), however, the lifespan and the specific capacity become very low [8-10]. In this work we describe the synthesis and the characterization of a new family of high voltage cathodic materials based on lithium-transition metal mixture-phosphates of iron, nickel and cobalt, in order to best take advantage of all the positive characteristics of each element presents in the structure (high voltage and high capacity) [11]. Five materials have been produced, varying the Ni/Co molar ratio; the effect of different degrees of Co and Ni doping on structure, morphology and electrochemical properties have been thoroughly studied. The stoichiometry is evaluated using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), the thermal stability is investigated by High Resolution – Thermo Gravimetric Analyses (HR-TGA), morphology and size distribution are characterized by Field Emission - Scanning Electron Microscopy (FE-SEM) and High-Resolution Transmission Electron Microscopy (HR-TEM) (see Figure 1); the structure is examined by powder X-Ray Diffraction (XRD) as well as variety of IR spectroscopy techniques. Electrochemical characterization is achieved by Cyclic Voltammetry (CV) and charge/discharge tests (see Figure 2). Indeed, the proposed materials are good cathodic candidates for the development of high voltage lithium batteries: the best of our materials LFNCP0.61 showed a specific capacity and a specific energy of 125 mAh∙g-1 and 560 mWh∙g-1, respectively. Acknowledgements The authors thank, a) the strategic project “From Materials for membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this study; b) the “Centro studi di economia e tecnica dell’energia Giorgio Levi Cases” for grants to G.P. and E.N. References 1 M. Armand and J. M. Tarascon Nature 451, 652-657, (2008). 2 B. Dunn, H. Kamath and J. M. Tarascon Science 334, 928-935, (2011). 3 V. Di Noto, T. A. Zawodzinski, A. M. Herring, G. A. Giffin, E. Negro and S. Lavina Int. J. Hydrogen Energy 37, 6120-6131, (2012). 4 B. Scrosati and J. Garche J. Power Sources 195, 2419-2430, (2010). 5 K. Zaghib, A. Mauger, H. Groult, J. B. Goodenough and C. M. Julien Mater. 6, 1028-1049, (2013). 6 K. Zaghib, J. Dubé, A. Dallaire, K. Galoustov, A. Guerfi, M. Ramanathan, A. Benmayza, J. Prakash, A. Mauger and C. M. Julien J. Power Sources 219, 36-44, (2012). 7 V. A. Streltsov, E. L. Belokoneva, V. G. Tsirelson and N. K. Hansen Acta Crystallogr., Sect. B: Struct. Sci. B49, 147-153, (1993). 8 N. N. Bramnik, K. G. Bramnik, T. Buhrmester, C. Baehtz, H. Ehrenberg and H. Fuess J. Solid State Electrochem. 8, 558-564, (2004). 9 A. K. Padhi, K. S. Nanjundaswamy and J. B. Goodenough J. Electrochem. Soc. 144, 1188-1194, (1997). 10 J. Wolfenstine and J. Allen J. Power Sources 142, 389-390, (2005). 11 G. Pagot, F. Bertasi, G. Nawn, E. Negro, G. Carraro, D. Barreca, C. Maccato, S. Polizzi and V. Di Noto Adv. Funct. Mater. 25, 4032-4037, (2015). Figure 1

Vanadium Oxyfluoride/Few-Layer Graphene Composite as a High-Performance Cathode Material for Lithium Batteries.

Metal oxyfluoride compounds are gathering significant interest as cathode materials for lithium ion batteries at the moment because of their high theoretical capacity and resulting high energy density. In this regard, a new and direct approach is presented to synthesize phase-pure vanadium oxyfluoride (VO2F). The structure of VO2F was identified by Rietveld refinement of the powder X-ray diffraction (XRD) pattern. It crystallizes in a perovskite-type structure with disorder of the oxide and fluoride ions. The as-synthesized VO2F was tested as a cathode material for lithium ion batteries after being surface-coated with few-layer graphene. The VO2F delivered a first discharge capacity of 254 mA h g(-1) and a reversible capacity of 208 mA h g(-1) at a rate of C/20 for the first 20 cycles with an average discharge voltage of 2.84 V, yielding an energy density of 591 W h kg(-1). Improved rate capability that outperforms the previous report has been achieved, showing a discharge capacity of 150 mA h g(-1) for 1 C. The structural changes during lithium insertion and extraction were monitored by ex-situ XRD analysis of the electrodes discharged and charged to various stages. Lithium insertion results in an irreversible structural change of the anion lattice from (3)/4 cubic close packing to hexagonal close packing to accommodate the inserted lithium ions while keeping the overall space-group symmetry. For the first time we have revealed a structural change for the ReO3-type structure of as-prepared VO2F to the RhF3 structure after lithiation/delithiation, with structural changes that have not been observed in previous reports. Furthermore, the new synthetic approach described here would be a platform for the synthesis of new oxyfluoride compounds.

Construction of an Artificial Ferrimagnetic Lattice by Lithium Ion Insertion into a Neutral Donor/Acceptor Metal-Organic Framework.

Construction of a molecular system in which the magnetic lattice exhibits long-range order is one of the fundamental goals in materials science. In this study, we demonstrate the artificial construction of a ferrimagnetic lattice by doping electrons into acceptor sites of a neutral donor/acceptor metal-organic framework (D/A-MOF). This doping was achieved by the insertion of Li-ions into the D/A-MOF, which was used as the cathode of a Li-ion battery cell. The neutral D/A-MOF is a layered system composed of a carboxylate-bridged paddlewheel-type diruthenium(II,II) complex as the donor and a TCNQ derivative as the acceptor. The ground state of the neutral form was a magnetically disordered paramagnetic state. Upon discharge of the cell, spontaneous magnetization was induced; the transition temperature was variable. The stability of the magnetically ordered lattice depended on the equilibrium electric potential of the D/A-MOF cathode, which reflected the electron-filling level.

Construction of an Artificial Ferrimagnetic Lattice by Lithium Ion Insertion into a Neutral Donor/Acceptor Metal–Organic Framework

AbstractConstruction of a molecular system in which the magnetic lattice exhibits long‐range order is one of the fundamental goals in materials science. In this study, we demonstrate the artificial construction of a ferrimagnetic lattice by doping electrons into acceptor sites of a neutral donor/acceptor metal–organic framework (D/A‐MOF). This doping was achieved by the insertion of Li‐ions into the D/A‐MOF, which was used as the cathode of a Li‐ion battery cell. The neutral D/A‐MOF is a layered system composed of a carboxylate‐bridged paddlewheel‐type diruthenium(II,II) complex as the donor and a TCNQ derivative as the acceptor. The ground state of the neutral form was a magnetically disordered paramagnetic state. Upon discharge of the cell, spontaneous magnetization was induced; the transition temperature was variable. The stability of the magnetically ordered lattice depended on the equilibrium electric potential of the D/A‐MOF cathode, which reflected the electron‐filling level.

One-Pot Fabrication of Hierarchical Nanosheet-Based TiO2 -Carbon Hollow Microspheres for Anode Materials of High-Rate Lithium-Ion Batteries.

Hierarchical and hollow nanostructures have recently attracted considerable attention because of their fantastic architectures and tunable property for facile lithium ion insertion and good cycling stability. In this study, a one-pot and unusual carving protocol is demonstrated for engineering hollow structures with a porous shell. Hierarchical TiO2 hollow spheres with nanosheet-assembled shells (TiO2 NHS) were synthesized by the sequestration between the titanium source and 2,2'-bipyridine-5,5'-dicarboxylic acid, and kinetically controlled etching in trifluoroacetic acid medium. In addition, annealing such porous nanostructures presents the advantage of imparting carbon-doped functional performance to its counterpart under different atmospheres. Such highly porous structures endow very large specifics surface area of 404 m(2) g(-1) and 336 m(2) g(-1) for the as-prepared and calcination under nitrogen gas. C/TiO2 NHS has high capacity of 204 mA h g(-1) at 1 C and a reversible capacity of 105 mA h g(-1) at a high rate of 20 C, and exhibits good cycling stability and superior rate capability as an anode material for lithium-ion batteries.

Mesocrystal MnO cubes as anode for Li-ion capacitors

Mesocrystal MnO cubes consisted of nanocrystals (~16nm) with percolated nanopores (~10.5nm) and a porosity of 45% were synthesized through polyvinyl alcohol (PVA) assisted hydrothermal growth and homoepitaxial aggregation. The resulting MnO crystal makes up of appreciable amount of Mn3+, ~11.8%, ascribed to incomplete reduction during the hydrothermal growth, and the presence of such an appreciable amount of Mn3+ is thought to lead to the enhancement of Li ion diffusion coefficient from 6.96×10−14cm2/s to 3.33×10−13cm2/s. Carbon coating derived from polyvinyl alcohol and homoepitaxial connection of nanocrystals provides excellent charge and mass transfer pathways. Such mesocrystal MnO cubes enhanced the contact of electrolyte and electrode materials, at the same time the active nanocrystals with trivalent manganese ions and cationic vacancies would promote the conversion reaction for lithium-ion insertion and extraction, leading to a high capacity of 637mAh/g at 100mA/g in a relatively smaller voltage range of 0.05–2.50V, as compared with a voltage window of 0.01–3V used by other research groups. The high voltage (4V) Li-ion capacitor, a full cell with mesocrystal MnO cubes as anode and activated carbon as cathode, demonstrated excellent cycling performance with the degradation rate of 0.002% per cycle, and the achieved maximum energy in full capacitor reached 227Whkg−1 that calculated on the total weight of the active materials in both electrodes.

Graphene-nanosheet-wrapped LiV3O8 nanocomposites as high performance cathode materials for rechargeable lithium-ion batteries

A novel graphene-nanosheet-wrapped LiV3O8 nanoflakes (GNS/LiV3O8) nanocomposite has been generated by sheet-to-sheet self-assembly of ultrathin LiV3O8 nanoflakes and graphene nanosheets. When used as a cathode material for lithium-ion batteries, the GNS/LiV3O8 nanocomposites show superior rate capability and excellent cycling stability. Discharge capacities of approximately 328.7, 305.3, 276.9, 251.4, and 209.3 mAh g−1 are achieved at current densities of 2, 5, 10, 20, and 50C, respectively. A reversible capacity of approximately 287.2 mAh g−1 is retained even after 100 cycles at 1.0 A g−1 (about 3C), approximately 88.3% of the initial discharge capacity. It is believed that the unique nanoflake morphology of LiV3O8 and the surface modification by graphene nanosheets contribute to the improved kinetics of lithium-ion diffusion, excellent structural stability and superior electrochemical performance. The structural evolution of LiV3O8 species upon charging and discharging is investigated by in situ X-ray diffraction technique. Anisotropic lattice expansion is found occurring along a, b and c axes upon the insertion of lithium ions into the crystal structure of LiV3O8.