William I. F. David

Lithium insertion into manganese spinels

Lithium has been inserted chemically and electrochemically into Mn3O4 and Li[Mn2]O4 at room temperature. From X-ray diffraction, it is shown that the [Mn2]O4 subarray of the A[B2]X4 spinels remains unperturbed and that the electrons compensating for the Li+-ion charge reduce Mn3+ to Mn2+ in Mn3O4 and Mn4+ to Mn3+ in Li[Mn2]O4. In LixMn3O4, the tetragonal distortion due to a cooperative Jahn-Teller distortion by octahedral-site Mn3+ ions decreases with x from ca = 1.157 for x = 0 to ca = 1.054 for x = 1. The system Li1+x[Mn2]O4, is cubic at x = 0 and tetragonal (ca = 1.161) at x = 1.2. Electrochemical data reveal a two-phase region in the Li1+xMn2O4 system and a maximum xm = 1.25. X-ray diffraction confirms the coexistence of a cubic and a tetragonal phase in the compositional range 0.1 ≤ x ≤ 0.8. The X-ray data also show that the inserted Li+ ions occupy the interstitial octahedral positions of the spinel structure. However, in LixMn3O4 the tetrahedral-site Mn2+ions are displaced from the A positions to the interstitial octahedral positions, as in LixFe3O4, whereas the tetrahedral-site Li+ ions in Li[Mn2]O4 remain on the A sites.

Structural Phase Transitions in the Fullerene C60

High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerence C60 in the temperature range 5 K to 320 K. Solid C60 adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the high-temperature face-centred-cubic phase (T > 260 K), the C60 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single 111 direction. In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists.

DASH : a program for crystal structure determination from powder diffraction data

DASH is a user-friendly graphical-user-interface-driven computer program for solving crystal structures from X-ray powder diffraction data, optimized for molecular structures. Algorithms for multiple peak fitting, unit-cell indexing and space-group determination are included as part of the program. Molecular models can be read in a number of formats and automatically converted to Z-matrices in which flexible torsion angles are automatically identified. Simulated annealing is used to search for the global minimum in the space that describes the agreement between observed and calculated structure factors. The simulated annealing process is very fast, which in part is due to the use of correlated integrated intensities rather than the full powder pattern. Automatic minimization of the structures obtained by simulated annealing and automatic overlay of solutions assist in assessing the reproducibility of the best solution, and therefore in determining the likelihood that the global minimum has been obtained.

Structural characterization of the lithiated iron oxides LixFe3O4 and LixFe2O3 (0<x<2)

Lithium has been inserted into Fe3O4 and α-Fe2O3 at room temperature both chemically and electrochemically; a compositional range 0<x<2 has been established for both LixFe3O4 and LixFe2O3. Powder X-ray-diffraction data of Li1.5Fe3O4 indicate that the [Fe2]O4 subarray of the spinel structure remains intact; the A-site Fe3+ ions are displaced to empty octahedral positions and the Li+ ions in excess of x = 1 are located in tetrahedral sites. Lithiation of α-Fe2O3 causes the anion array to transform from hexagonal to cubic close packing; in this case the Li+ ions are distributed over both 16c and 16d octahedral sites of the cubic space group Fd3m.

Structure and electrochemistry of lithium cobalt oxide synthesised at 400°C

A novel LiCoO2 compound has been prepared by the reaction of Li2CO3 and CoCO3 at 400°C. Unlike the well-known LiCoO2 structure that is synthesised at higher temperature (900°C) and contains Li+ and Co3+ ions in discrete layers between planes of close-packed oxygen ions, the structure of LiCoO2 (400°C) has approximately 6% cobalt within the lithium layers. The electrochemical properties of LiCoO2 (400°C) differ significantly from LiCoO2 (900°C). Whereas electrochemical extraction from LixCoO2 (900°C) in room-temperature lithium cells takes place as a single-phase reaction above 3.9V for x≤0.9, electrochemical extraction from LixCoO2 (400°C) occurs as a two-phase reaction at an open-circuit voltage of 3.61V for 0.1<x<0.95. Because LixCoO2 (400°C) is a less oxidizing material than its high-temperature analogue, it is expected to be more stable in many of the organic-based electrolytes that are currently employed in lithium cells.

Structure refinement of the spinel-related phases Li2Mn2O4 and Li0.2Mn2O4

Abstract The crystal structures of the lithium-rich and lithium-deficient spinel phases Li2[Mn2]O4 and Li0.2[Mn2]O4 have been determined by neutron-diffraction techniques. Structure refinements confirm earlier reports that the [Mn2]O4 framework of the Li[Mn2]O4 spinel remains intact during both lithium insertion and extraction, but demonstrate unequivocally that in Li2[Mn2]O4 the Li+ ions reside in face-shared tetrahedra and octahedra of the cubic-close-packed oxygen-anion array; in Li0.2[Mn2]O4 the Li+ ions are located randomly on only the tetrahedral sites of the spinel structure.

Tuning the Decomposition Temperature in Complex Hydrides: Synthesis of a Mixed Alkali Metal Borohydride

Metal borohydrides continue to attract considerable interest as potential hydrogen storage materials owing to their very large gravimetric hydrogen densities. In terms of the equally important performance parameter, the thermal decomposition temperature Tdec, it has emerged [1–3] that the degree of charge transfer between the metal cation and the BH4 anion is a key component for any ultimate materials design. Although manipulation of Tdec in single-cation borohydrides is clearly limited by the intrinsic properties of the individual metal (its characteristic electronegativity, for example), double and multiple cation substitution allows more extensive and precise control of Tdec. Although this approach is supported by bulk thermochemical studies, there are few X-ray structural investigations of double cation substitutions in these important materials. Such structural information is key to assessing whether genuine new multinary compounds are formed, or whether microscopic segregation of constituent phases takes place. Herein we report the first synthesis and crystal structure determination of a mixed alkali metal borohydride, LiK(BH4)2. Importantly in this new material, the observed decomposition temperature lies between that of the constituent phases. This finding of a genuine, dual-cation single-phase material offers the real prospect of chemical control of Tdec by the manipulation of multication combinations. X-ray diffraction data of thirteen samples with varying initial ratio LiBH4:KBH4 were collected. LiK(BH4)2 (see Figure 1) was identified from the data as having the space group Pnma and approximate lattice parameters a= 7.9134 4, b= 4.4907 4, and c= 13.8440 4. The b-axis lattice parameter is very similar to that of orthorhombic LiBH4 (4.43686(2) 4), suggesting a degree of structural similarity between the phases. The BH4 units in LiK(BH4)2 form an approximately tetrahedral coordination around the lithium ion, which is similar to that found in orthorhombic LiBH4. The Li B bonds are greater in LiK(BH4)2 than in LiBH4 but with a narrower range of angles (see Supporting Information). The larger Li···B separations observed in the new phase may originate from the presence of potassium cations in the structure, which are considerably larger than their lithium counterparts (Li ionic radius 0.59 4, K ionic radius 1.38 4 in tetrahedral coordination). The arrangement of the BH4 units in LiK(BH4)2 and KBH4 differs considerably. KBH4 has an octahedral arrangement of BH4 units, whereas those in LiK(BH4)2 might be best described as monocapped trigonal prisms (see Supporting Information). The K···B distances in KBH4 are 3.364 4, [8] whereas in LiK(BH4)2 the (seven) distances are 3.404(3)(18) 4 (twice), 3.409(3) 4 (twice), 3.431(3) 4 (twice), and 3.475(3) 4 (once). It is thought that these larger separations arise because of the greater number of BH4 units present around the potassium cation. The BH4 units in LiK(BH4)2 appear to be distorted in a similar manner to those reported in the orthorhombic structure of LiBH4 by Soulie et al. [6] (see Supporting Information). Specifically, while KBH4 has all equivalent B H bonds, those in orthorhombic LiBH4 [6] and LiK(BH4)2 are separated into two equivalent pairs. LiK(BH4)2 was found to have a narrower range of B H bond lengths than LiBH4 Figure 1. A schematic diagram of a) the proposed LiK(BH4)2 structure and b) that of orthorhombic LiBH4. K crimson, Li yellow, B green, H gray.

Structural aspects of lithium-manganese-oxide electrodes for rechargeable lithium batteries

The structural characteristics of “LixMnOy” electrodes, prepared by reacting manganese oxides with lithium salts at 400°C–900°C, for ambient temperature, rechargeable lithium cells have been determined by powder X-ray and neutron diffraction studies. The identification of structure types and cyclic voltammetry data show that the rechargeability of the “LixMnOy” electrodes in Li“LixMnOy” cells can be attributed predominantly to a spinel component of the system LixMn2−zO4 (0≤x≤1.33 and 0≤z≤0.33) which includes the spinel phases LiMn2O4, Li2Mn4O9 and Li4Mn5O12.

Alpha manganese dioxide for lithium batteries: A structural and electrochemical study

Abstract A highly crystalline α-MnO 2 phase has been synthesised by acid treatment of Li 2 MnO 3 . A neutron-diffraction study has shown that the stoichiometry of this phase is A 0.36 Mn 0.91 O 2 (or MnO 2 ·0.2A 2 O) where A refers predominantly to H + ions and a very minor concentration of Li + ions. Heat-treatment at 300°C leaves a virtually anhydrous α-MnO 2 product. The absence of any foreign cation such as K + , Na + or Rb + within the channels of the structure has raised the possibility of utilizing the α-MnO 2 framework as a high performance electrode for secondary lithium cells. Preliminary electrochemical data indicate that capacities in excess of 200 mAh/g are achievable from these α-MnO 2 electrodes in room-temperature lithium cells. Cyclic voltammograms show that lithium is inserted into α-MnO 2 in a two-step process and that this process is reversible.

The heat capacity of solid C60

Abstract High resolution heat capacity measurements of the prototypic fullerene, C60, are presented between 13 and 300 K. The well-documented first-order phase transition is clearly observed at 257.6 K and is associated with enthalpy and entropy changes of 7.54 and 30.0 J K−1 mol−1, respectively. A more subtle transition is observed at 86.8 K. This transition is attributable to the onset of orientational glass behaviour related to the kinetics of molecular reorientation. The rate of the enthalpy relaxation at the glass transition is accurately reproduced by a simple exponential function. The Arrhenius parameters describing the temperature dependence of the relaxation time are Ea = 22.2 ± 1.0 kJ mol−1 and τ0 = 4×10−11±1s. At low temperatures, solid C60 is a glassy crystal in which molecular orientational disorder is frozen in as a consequence of the long relaxation time of the molecular reorientation.