Octahedral 16d Sites Research Articles

Enhancing Lithium Conductivity Using High-Valence Cations in Cubic Spinel Halide Solid Electrolytes.

Halide solid electrolytes for all-solid-state batteries have recently emerged as competitors to oxide and sulfide solid electrolytes due to their excellent electrochemical properties. This ab initio study unveils the dynamic nature of Li2Sc2/3Cl4, a rare superionic conductor among cubic spinel halide materials. Li ions in Li2Sc2/3Cl4 prefer to occupy some of the tetrahedral 8a, octahedral 16c, and octahedral 16d sites, leading to disordered Li distribution. Li ions in Li2Sc2/3Cl4 diffuse through the single-ion diffusion mechanism rather than the concerted diffusion mechanism, providing a high conductivity of 1.36 mS cm-1. Li ions at the 16d site diffuse as actively as those at the 8a/16c site, an unexpected result that runs counter to the conventional view. In Li2MgCl4, the same cubic spinel as Li2Sc2/3Cl4, Li ions at the 8a/16c site diffuse actively, but those at the 16d site are almost immobile, resulting in a very low conductivity of 5.3 × 10-4 mS cm-1. The extremely higher conductivity in Li2Sc2/3Cl4 than in Li2MgCl4 is because the concentration of Sc3+/Mg2+ cations blocking the movement of Li ions at the 16d site is lower in Li2Sc2/3Cl4 than in Li2MgCl4. Designing cubic spinel materials containing high-valence cations is proposed as a way to increase conductivity by reducing the concentration of multivalent cations that impede Li diffusion. This study sheds new light on how to control conductivity using site-dependent Li mobility in solid electrolytes.

Surface reconstructed layer with bulk high-valence Mo doping to achieve long-life LiMn2O4 cathode material

Mo doping plays a striking role in the stability enhancement of LiMn2O4 cathode materials. However, the underlying microscopic mechanism is still unclear. Here, we elucidate that the relationship between the atomic position of Mo doping in LiMn2O4 and the stability enhancement of Mo-doped LiMn2O4 by utilizing the spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM). It exhibits that the part of Mo6+ ions occupy empty Mn 16c site to form a surface reconstructed layer of rock-salt like phase on the outermost surface and other Mo6+ ions dope into the Mn octahedral 16d sites to form LiMoxMn2-xO4 in bulk, which is beneficial for inhibiting the parasitic side reactions. Concomitantly, this dual modification of bulk structure and surface can significantly enhance electrode reversibility and Li+ diffusion. As a result, excellent long cycling stability of the as-designed optimal LiMo0.01Mn1.99O4 after 1500 cycles with 61.61 % capacity retention at 10 C (1 C = 148 mAh g−1) is presented, with an initial discharge capacity of 88.30 mAh g−1. Our research provides a clever approach for regulating the surface structure/bulk architecture in LiMn2O4 cathode materials.

Cobalt doped spinel LiMn2O4 cathode toward high-rate performance lithium-ion batteries

Great efforts have been devoted to improving the cycling stability and the high-rate performance of spinel LiMn2O4 cathode. Herein, we demonstrate that Co doping can improve the electrochemical performance of spinel LiMn2O4. We elucidate the atomic position of Co in spinel LiMn2O4 configuration and the stability improvement mechanism of the Co-doped LiMn2O4 cathode by using the state-of-art spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM). The results show that the Co3+ ions occupy the Mn octahedral 16d sites to reconstruct a more robust LiCoxMn2−xO4 framework, which is beneficial for stabilizing the LiMn2O4 crystal structure by ameliorating Mn dissolution and inhibiting Jahn-Teller distortion. Concomitantly, the doped Co atoms can offer short path lengths for Li+ ions intercalation and deintercalation that leads to accelerated Li+ diffusion kinetics. The as-designed optimal LiCo0.05Mn1.95O4 presents a respectable capacity retention of 83.81 % after 1000 cycles at 10C (1C = 148 mAh g−1), with an initial discharge capacity of 86.31 mAh g−1. Especially, excellent capacity retention of 78.52 % is obtained after 1000 cycles even at a high current rate of 15C. Our research shed light on the microscopic mechanism of Co doping on the cycling stability enhancement of spinel LiMn2O4 toward high-rate performance LIBs.

Effect of the crystal structure and chemical bonding on the electronic and thermal transport in Cu2MeHf3S8 (Me – Mn, Fe, Co, Ni) thiospinels

Establishing the relationship between crystal structure and transport properties is an important issue that is directly connected with the applicability of functional materials. In this work, we present the analysis of the crystal structure, chemical bonding, and electronic and thermal transport properties of Cu2МеHf3S8 (Ме – Mn, Fe, Co, Ni) compounds. The increase of weighted mobility in the Mn → Fe → Со → Ni series as well as the change of the dominant scattering mechanism of charge carriers from scattering on point defects to the scattering on acoustic phonons explains the best electronic transport in Cu2NiHf3S8. Moreover, bonding inhomogeneity between the covalent δ(Co – S) and δ(Hf – S) from one side, and more ionic δ(Cu – S) interactions from the other side leads to low lattice thermal conductivity in Cu2MeHf3S8 (Me – Mn, Fe, Co, Ni) materials. The work also suggests the link between the occupation of the octahedral 16d site and the thermoelectric performance of the investigated thiospinels. Particularly, the best thermoelectric performance is observed in the case of the presence of two valence electrons in the d-level of atoms in octahedral voids, which can be essential for further enhancement of the thermoelectric performance in thiospinels.

Preparation of long cycle lifespan spinel LiMn2O4 cathode material by a dual-modified strategy

Spinel LiMn2O4 has been identified as a cathode material with extensive application potential for Li-ion batteries. However, during cycling, the Jahn−Teller effect and Mn dissolution have a substantial impact on its specific capacity and cycle span. Herein, combining Fe and Cr elements co-doping with single crystal truncated octahedron morphology is proposed to solve these problems. The Fe−Cr co-doping markedly restrains the Jahn−Teller effect, and promotes the development of crystal and selective growth of crystal surfaces in spinel LiMn2O4. The high exposure (111) surfaces of the truncated octahedron reduce the dissolution of Mn, and a tiny proportion of (110) and (100) surfaces increase the diffusion channels of Li+. HAADF-STEM characterization indicates that the Fe and Cr elements largely replace the Mn atoms at the octahedral 16d site. Because of these advantages, the optimal LiFe0.05Cr0.08Mn1.95-xO4 (LFCMO-8) delivers excellent electrochemical performance with 119.6 mAh·g−1 discharge capacity and 78.1% capacity retention after 500 cycles at 1 C. At comparatively higher current densities of 20 C and 30 C, the superior capacity retention ratios of 79.8% and 87.3% are still achieved after 2000 cycles. Even at 40 C, the LFCMO-8 sample can still reach 98.8% capacity retention rate after 2000 cycles. After 1000 cycles at 5 C, the LFCMO-8 sample still maintained a capacity retention of 45.4% at high temperature (55 °C). ICP-OES test combined with a V-shaped tube device indicates that the Mn dissolution of LCFMO-8 only accounted for 11.8% of LiFe0.05Mn1.95O4 (LFMO). This study serves as a meaningful reference for the preparation of high-performance LiMn2O4 cathode materials.

Cation distribution and bond energy of intermediate spinel Mg(Zn0.5Ti0.5)2xGa(2−2x)O4 microwave dielectric ceramics

Substitution of Ga3+ by [Zn0.5Ti0.5]3+ in MgGa2O4 improved the comprehensive properties of the resultant spinel. A continuous solid solution with a spinel structure was formed in Mg(Zn0.5Ti0.5)2xGa(2−2x)O4 (0 ≤ x ≤ 1); here, Zn2+ occupied the tetrahedral 8a site while Ti4+ occupied the octahedral 16d site, resulting in a decrease in the degree of inversion λ from 0.86 to 0. The optimal sintering temperature decreased from 1460 °C to 1220 °C, and the dielectric constant (εr) increased from 8.8 to 14.9; these changes could be ascribed to the greater polarizability of [Zn0.5Ti0.5]3+ relative to that of Ga3+. The temperature coefficient of resonant frequency (τf) of the material remarkably increased from −64.1 ppm/°C to −16.8 ppm/°C, which may be attributed to the increase in dielectric polarizability/molecular volume, bond energy, and bond valence in B-sites. The high-quality factor (Q × f) slightly decreased from 89,253 GHz to 73,432 GHz with decreasing λ and packing fraction. Optimal microwave dielectric properties were obtained from the sample with x = 1 and sample sintered at 1220 °C; in this case, εr = 14.9, Q × f = 73,432 GHz, and τf = −16.8 ppm/°C.

Improved electrochemical performance of LiMn1.5M0.5O4 (M=Ni, Co, Cu) based cathodes for lithium-ion batteries

LiMn2O4 and LiMn1.5M0.5O4 (M: Ni, Cu, Co) doped particles have been synthetized by sol-gel. Particles between 50 and 200 nm were obtained with the cubic spinel structure of the LiMn2O4. Ni doping shows a more efficient substitution in the octahedral 16d site, replacing the Mn3+ ion, improving the important drawback of poor cycling behavior of LiMn2O4. The average pore size decrease with the addition of the doped elements in the LiMn2O4 structure from 2.9 to 2.6 nm. Thermal analysis shows that the doped particles present higher thermal stability that the undoped ones. Electrochemical behavior of the cathodes prepared with each of the active materials show that the doping influenced the electrochemical performance of the active material. Thus, a specific capacity of 33, 74, 44 and 53 mAh g−1 (at C) and 74, 89, 59 and 69 mAh g−1 (at C/10) were obtained for LiMn2O4, LiMn1.5Ni0.5O4, LiMn1.5Cu0.5O4 and LiMn1.5Co0.5O4 cathodes, respectively. All cathodes present good electrochemical stability with low capacity fade of 0.5 and 3.1% for LiMn1.5Ni0.5O4 and LiMn1.5Cu0.5O4, respectively, after 50 cycles. These results show an improvement of electrochemical performance for LiMn2O4 doped with Ni, Cu and Co, demonstrating their suitability for lithium-ion battery systems.

Structural Peculiarities and Thermoelectric Study of Iron Indium Thiospinel.

The homogeneity range of ternary iron indium thiospinel at 873 K was investigated. A detailed study was focused on two distinct series (y=z): 1) a previously reported charge‐balanced (In0.67+0.33y□0.33−0.33y)tetr[In2−zFez]octS4 (A1‐series; □ stands for vacancy; the abbreviations “tetr” and “oct” indicate atoms occupying tetrahedral 8a and octahedral 16d sites, respectively) and 2) a new charge‐unbalanced (In0.67+y□0.33−y)tetr[In2−zFez]octS4 (A2‐series). Fe atoms were confirmed to exclusively occupy an octahedral position in both series. An unusual reduction of the unit cell parameter with increasing Fe content is explained by differences in the ionic radii between Fe and In, as well as by an additional electrostatic attraction originating from charge imbalance (latter only in A2‐series). The studied compound is an n‐type semiconductor, and its charge carrier concentration increases or decreases for larger Fe content within the A1‐ and A2‐series, respectively. The thermal conductivity κ tot is significantly reduced upon increasing vacancy concentration, whereas the change of power factor is insufficient to drastically improve the thermoelectric figure of merit.

Structural and Electrochemical Properties of Spherical LiMgxMn2–xO4 (x ≤ 0.10) for Secondary Lithium Ion Batteries

A series of spherical LiMgxMn2–xO4 (x ≤ 0.10) cathode materials were successfully synthesized by a co-precipitation technique and solid state route. The morphologies and crystal structures of LiMgxMn2–xO4 were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. All LiMgxMn2–xO4 samples exhibited a single phase LiMn2O4 spinel structure with good crystallinity. Mg2+ ions could completely occupy the octahedral (16d) site to substitute Mn3+ and Mn4+ ions. The effect of Mg2+ ions on the electrochemical performance of LiMgxMn2–xO4 was investigated by galvanostatic charge–discharge test and electrochemical impedance spectroscopy (EIS). The results showed that the Mg-doped LiMn2O4 possessed better cycling stability. For the LiMg0.06Mn194O4, the initial discharge capacity was 115 mAh/g and remained 111 mAh/g after 50 cycles at a constant current density of 148 mA/g (1 C-rate) during the voltage range 3.0–4.3 V. The capacity retention could reach 96.5%, which was higher than that of the LiMn2O4.

Neutron as a probe to study magnetization reversal in a spinel compound, CoCr1.9Fe0.1O4

Neutron depolarization and neutron diffraction techniques have been employed to investigate the origin of magnetization reversal in CoCr1.9Fe0.1O4 spinel compound. The spinel compound crystallizes in a face centered cubic (space group: Fd-3m) structure where Co2+ and (Cr/Fe)3+ ions occupy the tetrahedral 8a and octahedral 16d sites, respectively. The temperature dependent magnetization curves for magnetic fields ≤ 1 kOe show a magnetization reversal below the compensation temperature (Tcomp ∼ 40 K). The neutron depolarization measurements infer that the average magnetization of each domain becomes negligibly small at the TComp. Temperature dependent neutron diffraction study, a microscopic tool to understand sublattice magnetization, infers an antiparallel coupling between the tetrahedral (Co) and octahedral (Cr/Fe) site moments with ordered magnetic moments of −3.1(1) and 1.0(1) μB, respectively, at 30 K. Our neutron diffraction study also reveals that the unequal growth of the antiferromagnetically coupled tetrahedral and octahedral site moments across the TComp plays an important role in realizing the magnetization reversal in this spinel compound.

An Experimental and First Principles Study of the Energetics of Lithium Insertion into Fe3O4 and γ-Fe2O3

Inverse spinel magnetite, Fe3O4, has been studied as an electrode material for lithium-ion batteries due to its low cost, high abundance, and low environmental impact.1 Experimental discharge profiles of Fe3O4 initially show a concentration dependent voltage profile. The lithium concentration range of that profile varies as a function of nanoparticle size, with smaller particles exhibiting a larger intercalation regime.2,3 Thackeray et al. proposed initial lithium intercalation into magnetite on 16c sites, where the lattice accommodates a critical concentration of lithium without significant structural rearrangements. Above a critical concentration of lithium, Thackeray et al. proposed that with further discharge, lithium insertion to 16c sites leads to Fe8a displacement to vacant 16c sites in the material.4 It is also known that magnetite is iron deficient, Fe3-δO4 in the bulk and in the nanoparticulate forms with vacancies on the octahedral 16d sites.5–7 The most drastic manifestation of this oxidation process is maghemite, with δ=1/3, the topotactic oxidative product written as (Fe1 3+)8a[Fe1.66 3+♦0.33]16dO4 with ♦ as a 16d iron defect, or γ-Fe2O3. Menard et al. showed that the defects in nanoparticulate magnetite increase with decreasing nanoparticle size, but a thorough investigation of how defects affect the electrochemistry is yet to be presented.2 Using DFT+U calculations, we have investigated lithium intercalation in Fe3O4 magnetite. Our results show that stoichiometric Fe3O4 is not an intercalation material, with no stable phases for the LixFe3O4 0<x<1 convex hull. Resultantly, a voltage plateau at approximately 2.0 V is predicted, corresponding to a continuous phase transformation from inverse spinel Fe3O4 to rock-salt LiFe3O4with Li and Fe occupying 16c sites. This rules out the original hypothesis for a critical concentration of lithium hosted on the 16c sites resulting in a concentration dependent voltage at low concentrations of lithium in magnetite. We have performed DFT+U calculations on lithiated Fe3-δO4 with δ=0,1/8, and 1/3 to understand how 16d iron defects affect the open circuit potential of magnetite and therefore the discharge behaviour. In the presence of 16d iron defects, we have found that even at low concentrations of lithium, insertion on vacant 16c sites causes significant Fe8a rearrangements. When lithium is inserted on 16c positions, its proximity to Fe8a induces Coulombic repulsion between the Li16c and the Fe8a, and the resultant material is thermodynamically unstable against phase segregation. On the other hand, when lithium is inserted on vacant 16d sites available in the δ=1/8 and 1/3 cases, a structurally stable intercalated material results and DFT+U predicted voltages align with experimental observation. We further hypothesize that the concentration of defects is a function of nanoparticle size. Then we are able to explain the experimentally observed size-dependent open circuit potential and the variation of discharge capacity with particle size in a natural way. Finally, with DFT+U calculations we are able to provide a mechanism for intercalation and phase-segregation that is consistent with the nanoparticle size dependence observed in the electrochemical experiments. This work was supported as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under award #DE-SC0012673. This research used resources of the Center for Functional Nanomaterials, a U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory, Contract No. DE-SC0012704. The Extreme Science and Engineering Discovery Environment (XSEDE) was used, supported by National Science Foundation grant number ACI-1548562. The Yeti Shared HPC Cluster at Columbia University was used, which includes support from Empire State Development's Division of Science, Technology, and Innovation, contract number C090171. C. N. L. acknowledges the support of the National Science Foundation Graduate Research Fellowship under Grant No. DGE-11-44155. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. (1) Ito, S.; Nakaoka, K.; Kawamura, M.; Ui, K.; Fujimoto, K.; Koura, N. J. Power Sources 2005, 146, 319–322. (2) Menard, M. C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. Electrochim. Acta 2013, 94, 320–326. (3) Menard, M. C.; Takeuchi, K. J.; Marschilok, C.; Takeuchi, E. S. 2013, 15, 18539–18548. (4) Thackeray, M. M.; David, W. I. F.; Goodenough, J. B. Mater. Res. Bull. 1982, 17, 785–793. (5) Daniels, J. M.; Rosencwaig, a. J. Phys. Chem. Solids 1969, 30, 1561–1571. (6) Dieckmann, R. Berichte der Bunsen-Gesellschaft fur Phys. Chemie 1982, 86, 112–118. (7) Johnson, C. E.; Johnson, J. A.; Hah, H. Y.; Cole, M.; Gray, S.; Kolesnichenko, V.; Kucheryavy, P.; Goloverda, G. Hyperfine Interact. 2016, 237, 1–10.

Influence of iron doping on structure and electrochemical properties of Li4Ti5O12

Iron doped lithium titanates Li4+z-3yTi5-zFez+yO12 (0≤z+y≤0.3) have been synthesized and characterized by X-ray diffraction, scanning and transmission electron microscopy, 57Fe Mössbauer spectroscopy, and cyclic voltammetry. Their performance as anode materials for lithium-ion batteries was studied in a wide potential range (0.01 − 3V). Rietveld refinements and 57Fe Mössbauer spectroscopy data show that iron cations occupy both tetrahedral 8a and octahedral 16d sites in the spinel structure. At that, iron occupancy of the octahedral sites is approximately twice as much as the occupancy of the tetrahedral ones. Occupancy of octahedral sites by Fe3+ ions increases after cycling. Li4+z-3yTi5-zFez+yO12 (z+y=0.2) exhibits higher electrochemical capacity at high current densities than the pristine Li4Ti5O12. Moreover, capacity of doped material increases after cycling at high current densities.

Is Li-doped MgAl2O4 a potential solid electrolyte for an all-spinel Li-ion battery?

The ionic conductivity of Li-doped MgAl2O4 (LMAO), which is considered as a potential solid electrolyte for an all-spinel Li-ion battery, has not been investigated so far. Only NMR studies, which give information about the mobility or the Li-ion diffusion on local (microscopic) level, are performed on this system, but they do not give information about the long-range Li-ion diffusion, which is required for solid electrolyte application. Therefore, ionic conductivity study i.e., impedance spectroscopy, which provides insight into Li-ion diffusion on microscopic scale, is of great importance. The results reported here reveal for the first time ionic conductivity of LMAO ceramics and give insight into whether this material system has a potential for application as a solid electrolyte. The LMAO powders, with an average grain size of 55nm, are synthesized by nebulized spray pyrolysis (NSP) and processed into dense ceramics (95–98% TD) using spark plasma sintering (SPS). The decrease of the lattice parameter with Li-doping indicates the incorporation of lithium into the spinel structure. X-ray diffraction structural analysis shows a systematic decrease of the occupancy of Al-octahedral (16d) sites with Li-doping, indicating that a fraction of the Li+ ions are located on the octahedral (16d) sites, and at the same time in order to retain charge neutrality, some of the Al3+ ions are moved to the tetrahedral (8a) sites. The conductivity study reveals that even at a high temperature (~ 300°C) the value of the Li-ion conductivity of LMAO, in the order of 10−7S·cm−1 is poor and at room temperature it is substantially lower (~ 10−14S·cm−1). While the origin of this low conductivity remains unclear, it could be related either to the nano- and microstructure of the sintered ceramic or to the distribution of Li+ ions inside the lattice. Nevertheless, the results presented here suggest that more conductivity studies have to be performed in order to draw a final conclusion about potential of LMAO ceramics for application as a solid electrolyte in Li-ion batteries.

Effects of equimolar Mg (II) and Si (IV) co-doping on the electrochemical properties of spinel LiMn2−2xMgxSixO4 prepared by citric acid assisted sol–gel method

The spinel LiMn2−2xMgxSixO4 (x=0, 0.01, 0.03, 0.05 and 0.07) cathode materials were successfully synthesized by a citric acid assisted sol-gel process with LiOH·H2O, Mn(CH3COO)2·4H2O, Mg(NO3)2·6H2O and C8H20O4Si as raw materials. The crystal structures and morphologies of LiMn2−2xMgxSixO4 were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. All the obtained samples with good crystallinity were identified as the cubic spinel structure of LiMn2O4 and no impurity peaks were observed. Equimolar Mg2+ and Si4+ ions could completely occupy the octahedral (16d) sites to substitute Mn3+ and Mn4+ ions, respectively, which significantly improved the structural stabilization and suppressed the Jahn–Teller distortion. The effects of equimolar Mg2+ and Si4+ ions co-doping on the electrochemical properties of LiMn2−2xMgxSixO4 were investigated by galvanostatic charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that the equimolar Mg2+ and Si4+ ions co-doped LiMn2O4 presented better cycling retention and rate performance. For the optimal LiMn1.90Mg0.05Si0.05O4, the initial discharge capacity was 126.9mAhg−1 and remained 123.5mAhg−1 after 100 cycles at 0.5C in the voltage range of 3.20∼4.35V. The capacity retention could reach 97.3%, which was far higher than that of the undoped LiMn2O4.

Synthesis and Crystal Structure of Lithium Iron Titanium Oxides Li1/2-+1/2xFe5/2-3/2xTixO4 with 0 ≤ x ≤ 1.666

Li[Li1/3Ti5/3]O4 (LTO) is an ideal negative electrode material for long-cycle life lithium-ion battery (LIB), because of its “zero-strain” character [1]. In the LTO crystal lattice with Fd-3m space group, Li+ ions occupy both tetrahedral 8a and octahedral 16d sites, while Ti4+ ions sit the 16d site. We, thus, expect a solid solution compound Li1/2+1/2x Fe5/2–3/2x Ti x O4 (LFTO) between LTO and Fe[Li1/2Fe3/2]O4 via an intermediate spinel compound Li1/2Fe1/2[Li1/2Fe1/2Ti]O4. Fe[Li1/2Fe3/2]O4 is known to be one of the ferrites, which exhibit a soft ferromagnetic behavior. Here we report the results of powder X-ray diffraction (XRD) and electron diffraction (ED) measurements, and Raman spectroscopy analyses on the LFTO samples with 0 ≤ x ≤ 1.666. According to XRD measurements, all the LFTO samples were assigned as a spinel-framework structure. For the x = 0 sample, the extra diffraction lines were clearly observed in addition to the normal diffraction lines in the Fd-3m. The extra diffraction lines are attributed to the superlattice diffraction lines due to the ordering between Li+ and Fe3+ ions at the octahedral site. As x increases from 0.25, such superlattice diffraction lines disappear, but, surprisingly, superlattice diffraction lines appear again at x > 1.2. The crystal structure at x £ 0.25 and 1.2 < x < 1.6 are assigned as the P4332 space group. Combining with the result of x dependence of cubic lattice parameter (a c), the crystal structure of LFTO is classified into the four regions; (I) regular spinel structure of Fd3-m with x > 1.55, (II) superlattice spinel structure of P4332 with 1.2 ≤ x ≤ 1.55, (III) regular spinel structure of Fd3-m with 0.25 < x < 1.2, and (IV) superlattice spinel structure of P4332 with x ≤ 0.25.AcknowledgementThis work was supported by Grant-in-Aid for Scientific Research (C), 25410207, MEXT, Japan.

Li2-X FeTiO4 As a High Capacity Cathode Material for Lithium Ion Battery

Introduction Research focusing on iron-based cathode materials for lithium ion batteries tends to favor materials that operate on the (de) intercalation of one Li per Fe atom (based on Fe2+/Fe3+redox couple); thereby imposing an intrinsic limitation on the energy density of the cathode material. To drastically enhance the energy density, intercalation materials with more than one electron per Fe atom are needed. In commonly used cathode materials, however, more than one electron reaction has not been utilized for charge-discharge reaction. For the development of cathode materials which achieve more than one electron reaction, it is essential to investigate the reaction mechanism.This study investigates reaction mechanism of more than one electron reaction in cathode materials. Li2-x FeTiO4 is selected as an appropriate model compound, as both the Fe2+/Fe3+ as well as Fe3+/Fe4+ redox couples are expected to be utilized to achieve high capacity. The possibility of de (intercalation) of two lithiums from the spinel-type Li2-x FeTiO4 is examined and the mechanism underlying the Li+ (de) insertion mechanism in Li2-x FeTiO4is discussed. Experimental Spinel-type LiFeTiO4 was synthesized via the conventional solid state ceramics route. Electrodes prepared from this material were examined in two-electrode coin-type cells, using lithium metal as a counter electrode. Electrodes were prepared from LiFeTiO4 to which carbon black was added and ball-milled at 400 rpm for 6 hours. Polytetrafluoroethylene (PTFE) binder was thereafter added. The weight ratio of LiFeTiO4, carbon black and PTFE was 6:3:1. The electrolyte used was a 1 M solution of LiClO4 in ethylene carbonate / diethyl carbonate. Initial lithium extraction from LiFeTiO4 was carried out based on the capacity of one lithium per Fe atom. Cells were then cycled between 1.6V and 4.2V at various current densities. The electrochemical measurements were conducted at 55.0oC. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements were conducted at SPring-8 and at SR Center, Ritsumeikan University. Results and Discussion The pristine LiFeTiO4 was characterized by Rietveld analysis. As shown in Fig. 1, all the reflections of the XRD patterns of the pristine LiFeTiO4 are fully indexed to a cubic structure (space group Fd3m). The rietveld analysis shows that all cations are randomly distributed on the tetrahedral (8a) and octahedral (16d) sites. The refined lattice parameters are consistent with the previously reported values [1-3].The spinel-type LiFeTiO4 can be electrochemically delithiated below the voltage of 4.2 V, to obtain FeTiO4. The subsequent discharge and charge measurements show that a reversible capacity of about 250 mAhg-1can be obtained under C/20 rate at an average voltage of 2.5 V with good capacity retention.For the discharged and charged Li2-x FeTiO4, XRD and XAS measurements were conducted to elucidate the crystal and electronic structural changes underlying the high capacity in spinel-type LiFeTiO4. Fig. 2 shows XRD pattern of charged and discharged Li2-x FeTiO4. The spinel structure of LiFeTiO4 is maintained during both charge and discharge reaction. Additionally, changes in the lattice constants obey Vegard’s law, indicating a solid-solution behavior during the charge and discharge reaction in Li2-x FeTiO4. References M. A. Arillo, M. L. Lopez, E. Perez-Cappe, C. Pico, M. L. Veiga, Solid State Ion., 107, 307-312 (1998).M. A. Arillo, M. L. Lopez, C. Pico, M. L. Veiga, A. Jimenez-Lopez, E. Rodriguez-Castellon, J. Alloys Comp., 160-163, 317-318 (2001).M. A. Arillo, M. L. Lopez, C. Pico, M. L. Veiga, A. Jimenez-Lopez, M. L. Vega, Chem. Mater., 17, 4162-4167 (2005).

A new solid-state process for synthesis of LiMn 1.5Ni 0.5O 4− δ spinel

A new two-step solid-state process was developed for synthesis of a pure phase 4.7 V LiMn 1.5Ni 0.5O 4− δ (LMNO) spinel of good electrochemical properties. This process which was based on formation of stable Ni 1− x Mn 2O 4− δ ( x ≤ 0.33) spinel followed by subsequent lithiation, demonstrated the emerging of LiMn 1.5Ni 0.5O 4− δ frame work at temperatures of as low as 350 °C. During this lithiation process, migration of metal ions (Ni/Mn) from partially occupied tetrahedral 8a sites (in Ni 0.67Mn 2O 4− δ ) toward octahedral 16d site occurred. The resultant spinel material displayed low irreversible loss, a 97% columbic efficiency and 6% loss in discharge capacity after 100 cycles at 60 °C.

Defect processes in MgAl 2O 4 spinel

In perfect normal MgAl 2O 4 spinel the Mg 2+ ions occupy tetrahedral 8a sites and Al 3+ ions occupy octahedral 16d sites. In reality some cations are exchanged between the cation sublattices forming pairs of antisite defects and thus a degree of “inversion”. Here atomic simulation is used to investigate the influence that antisite defects have on the populations of other intrinsic defects, those associated with Schottky and Frenkel reactions. One consequence is that the total magnesium interstitial concentration is increased substantially over the aluminium interstitial concentration and the magnesium vacancy concentration is increased over the aluminium vacancy concentration but to a much smaller extent. The split structures of isolated interstitial defects and the stability of various defect clusters are also discussed.

Phase transition of lithiated-spinel Li 2Mn 2O 4 at high temperature

The phase transition of Li 2Mn 2O 4 spinel at high temperature was investigated by XRD, TG/DTA, average oxidation state of Mn and cyclic voltammeric techniques. The results reveal that the Li 2Mn 2O 4 spinel is unstable. At high temperature, it is easy to transform into [Li 2−2 x ] tet[Mn 2− x Li x ] octO 4, which accompanies the formation of Li 2MnO 3 impurities. The phase transition is associated with the transfer of Li + from tetrahedral 8a sites to octahedral 16d sites. With the increasing sintering temperature from 450 °C to 850 °C, the phase structure varies from lithiated-spinel Li 2Mn 2O 4 to Li 4Mn 5O 12-like to LiMn 2O 4-like and finally to rock-salt LiMnO 2-like. In addition, a way of determining x with average oxidation state of Mn and the content of Li 2MnO 3 was also demonstrated.

Crystallographic and magnetic structure in spinel system FeGa xCr 2−xS 4 by neutron diffraction

The samples of FeGa x Cr 2− x S 4 ( x=0.1, 0.3) were prepared by solid reaction method. The crystallographic structure and the magnetic properties of the obtained compounds were investigated by X-ray and neutron powder diffraction, vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The Cr ions occupy 16(d) site exclusively, while mixed spinel behaviors are developed with increase of Ga ions. Finally, it induces a large quadrupole splitting in octahedral (16d) sites. Neutron diffraction on FeGa x Cr 2− x S 4 above 10 K shows that there is no crystallographic distortion and reveals antiferromagnetic ordering. The magnetic moment of Fe 2+ (3.45 μ B) are found to be aligned antiparallel to Cr 3+ (−2.89 μ B), in the sample x=0.1. Below the Néel temperature, magnetic peaks exist on the crystal diffraction peaks. It proves that spin structure of the same species are aligned parallel.