Phase Transition Mechanisms in Li_<i>x</i>CoO₂ (0.25 ≤ <i>x</i> ≤ 1) Based on Group–Subgroup Transformations

The basic structural chemistry of O3–Li_<i>x</i>CoO₂ (0.25 ≤ <i>x</i> ≤ 1) oxides is reviewed. Crystal chemical details of selected compositions and group–subgroup schemes are discussed with respect to phase transitions upon electrochemical or chemical deintercalation of the lithium atoms. Furthermore, the theoretical crystal structures of Li_<i>x</i>CoO₂ supercells (<i>x</i> = 0.75, 0.5, 0.33, and 0.25) are reported for the first time based on the combination of transmission electron microscopy (TEM) and X-ray (XRD) or neutron diffraction (ND) experiments. Li_0.75CoO₂ and Li_0.25CoO₂ supercells crystallize with the space group <i>R</i>3̅<i>m</i>, <i>a</i>₄ = 5.6234 Å and 5.624 Å, and <i>c</i>₄ = 14.2863 Å and 14.26 Å, respectively, whereas the Li_0.5CoO₂ supercell crystallizes with the space group <i>P</i>2₁/<i>m</i>, <i>a</i>₇ = 4.865 Å, <i>b</i>₇ = 2.809 Å, <i>c</i>₇ = 9.728 Å, and β₇ = 99.59°. The Li_0.33CoO₂ supercell may crystallize in different unit cells (hexagonal or orthorhombic or monoclinic). For Li_0.75CoO₂, the TEM superstructure reflections are due to only one type of lithium and vacancy ordering within the lithium layers; however, for <i>x</i> = 0.5, the superstructure reflections are due to an intergrowth of two Li_0.5CoO₂ monoclinic structures (<i>P</i>2/<i>m</i>, <i>a</i>₅ = 4.865(3) Å, <i>b</i>₅ = 2.809(3) Å, <i>c</i>₅ = 5.063(3) Å, β₅ = 108.68(5)°) with the lithium and vacancies alternating the 1<i>g</i> and 1<i>f</i> atomic positions, in two successive layers, along the <i>c</i> direction. For Li_0.33CoO₂, in most cases, the Li and vacancy ordering are similar to Li and Mn ordering in the Li₂MnO₃ structure. The phase transition mechanisms from O3–LiCoO₂ to O3–Li_0.25CoO₂ and from O3–LiCoO₂ to spinel–Li_0.5CoO₂ have been determined, and the structural relationship between O3–LiCoO₂ and Li₂MnO₃ has been discussed in detail

Structural Changes in Li₂CoPO₄F during Lithium-Ion Battery Reactions

The cobalt-based fluorophosphate Li₂CoPO₄F positive electrode has the potential to obtain high energy density in a lithium ion battery since its theoretical capacity is 287 mAh·g^–1 when two electrons can react reversibly. This material promises to charge/discharge with an extremely high redox-couple voltage of over 4.8 V vs Li/Li⁺. Bulk structural analyses including X-ray diffraction, Co K-edge X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) reveal that an orthorhombic Li_βCoPO₄F phase is produced from pristine Li₂CoPO₄F by a combination of solid-solution and two-phase reaction manners during the first charging process, and these phases reversibly transform during charge–discharge cycling. The results of ⁷Li MAS NMR and classical molecular dynamics simulations suggest that Li ions located at Li(1) sites intercalate/deintercalate through a 1D diffusion path along the <i>b</i> axis, whereas those located at Li(2) and Li(3) sites are fixed. The aforementioned analyses were successfully performed with the enhancement of electrochemical properties by use of a fluoroethylene carbonate-based electrolyte instead of an ethylene carbonate-based one and reducing its volume. Further enhancement was achieved by adding SiO₂ nanoparticles into the electrode slurry. The electrochemical results encourage the possibility of the intercalation/deintercalation of more than one Li ion from/into Li₂CoPO₄F during electrochemical cycling

Synthesis, Crystal Structure, and Properties of the Alluaudite-Type Vanadates Ag_2–<i>x</i>Na_<i>x</i>Mn₂Fe(VO₄)₃

The new members of the Ag_2–<i>x</i>Na_<i>x</i>Mn₂Fe­(VO₄)₃ (0 ≤ <i>x</i> ≤ 2) solid solution were synthesized by a solid-state reaction route, and their crystal structures were determined from single-crystal X-ray diffraction data. The physical properties were characterized by Mössbauer and electrochemical impedance spectroscopies, galvanostatic cycling, and cyclic voltammetry. These materials crystallize with a monoclinic symmetry (space group <i>C</i>2/<i>c</i>), and the structure is considered to be a new member of the <i>AA</i>′<i>MM</i>′₂(<i>X</i>O₄)₃ alluaudite family. The <i>A</i>, <i>A</i>′, <i>M</i>, and <i>X</i> sites are fully occupied by Ag⁺/Na⁺, Ag⁺/Na⁺, Mn²⁺, and V⁵⁺, respectively, whereas a Mn²⁺/Fe³⁺ mixture is observed in the <i>M</i>′ site. The Mössbauer spectra confirm that iron is trivalent. The impedance measurements indicate that the silver phase is a better conductor than the sodium phase. Furthermore, these phases exhibit ionic conductivities 2 orders of magnitude higher than those of the homologous phosphates. The electrochemical tests prove that Na₂Mn₂Fe­(VO₄)₃ is active as positive and negative electrodes in sodium-ion batteries

Synthesis and Characterization of the Crystal and Magnetic Structures and Properties of the Hydroxyfluorides Fe(OH)F and Co(OH)F

The title compounds were synthesized by a hydrothermal route from a 1:1 molar ratio of lithium fluoride and transition-metal acetate in an excess of water. The crystal structures were determined using a combination of powder and/or single-crystal X-ray and neutron powder diffraction (NPD) measurements. The magnetic structure and properties of Co­(OH)F were characterized by magnetic susceptibility and low-temperature NPD measurements. M­(OH)F (M = Fe and Co) crystallizes with structures related to diaspore-type α-AlOOH, with the <i>Pnma</i> space group, <i>Z</i> = 4, <i>a</i> = 10.471(3) Å, <i>b</i> = 3.2059(10) Å, and <i>c</i> = 4.6977(14) Å and <i>a</i> = 10.2753(3) Å, <i>b</i> = 3.11813(7) Å, and <i>c</i> = 4.68437(14) Å for the iron and cobalt phases, respectively. The structures consist of double chains of edge-sharing M­(F,O)₆ octahedra running along the <i>b</i> axis. These infinite chains share corners and give rise to channels. The protons are located in the channels and form O–H···F <i>bent</i> hydrogen bonds. The magnetic susceptibility indicates an antiferromagnetic ordering at ∼40 K, and the NPD measurements at 3 K show that the ferromagnetic rutile-type chains with spins parallel to the short <i>b</i> axis are antiferromagnetically coupled to each other, similarly to the magnetic structure of goethite α-FeOOH

Synthesis and Characterization of the Crystal and Magnetic Structures and Properties of the Hydroxyfluorides Fe(OH)F and Co(OH)F

The title compounds were synthesized by a hydrothermal route from a 1:1 molar ratio of lithium fluoride and transition-metal acetate in an excess of water. The crystal structures were determined using a combination of powder and/or single-crystal X-ray and neutron powder diffraction (NPD) measurements. The magnetic structure and properties of Co­(OH)F were characterized by magnetic susceptibility and low-temperature NPD measurements. M­(OH)F (M = Fe and Co) crystallizes with structures related to diaspore-type α-AlOOH, with the <i>Pnma</i> space group, <i>Z</i> = 4, <i>a</i> = 10.471(3) Å, <i>b</i> = 3.2059(10) Å, and <i>c</i> = 4.6977(14) Å and <i>a</i> = 10.2753(3) Å, <i>b</i> = 3.11813(7) Å, and <i>c</i> = 4.68437(14) Å for the iron and cobalt phases, respectively. The structures consist of double chains of edge-sharing M­(F,O)₆ octahedra running along the <i>b</i> axis. These infinite chains share corners and give rise to channels. The protons are located in the channels and form O–H···F <i>bent</i> hydrogen bonds. The magnetic susceptibility indicates an antiferromagnetic ordering at ∼40 K, and the NPD measurements at 3 K show that the ferromagnetic rutile-type chains with spins parallel to the short <i>b</i> axis are antiferromagnetically coupled to each other, similarly to the magnetic structure of goethite α-FeOOH

Synthesis and Characterization of the Crystal Structure and Magnetic Properties of the New Fluorophosphate LiNaCo[PO₄]F

The new compound LiNaCo­[PO₄]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo­[PO₄]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo­[PO₄]F crystallizes with orthorhombic symmetry, space group <i>Pnma</i>, with <i>a</i> = 10.9334(6), <i>b</i> = 6.2934(11), <i>c</i> = 11.3556(10) Å, and <i>Z</i> = 8. The structure consists of edge-sharing CoO₄F₂ octahedra forming CoFO₃ chains running along the <i>b</i> axis. These chains are interlinked by PO₄ tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie–Weiss behavior above 60 K with θ = −21 K. The specific heat and magnetization measurements show that LiNaCo­[PO₄]F undergoes a three-dimensional magnetic ordering at <i>T</i>_<i>mag</i> = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO₃ chain along the <i>b</i>-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the <i>a</i>-direction but have a noncollinear arrangement along the <i>c</i>-direction. The noncollinear spin arrangement implies the presence of spin conflict along the <i>c</i>-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations

Amorphous Metal Polysulfides: Electrode Materials with Unique Insertion/Extraction Reactions

A unique charge/discharge mechanism of amorphous TiS₄ is reported. Amorphous transition metal polysulfide electrodes exhibit anomalous charge/discharge performance and should have a unique charge/discharge mechanism: neither the typical intercalation/deintercalation mechanism nor the conversion-type one, but a mixture of the two. Analyzing the mechanism of such electrodes has been a challenge because fewer tools are available to examine the “amorphous” structure. It is revealed that the electrode undergoes two distinct structural changes: (i) the deformation and formation of S–S disulfide bonds and (ii) changes in the coordination number of titanium. These structural changes proceed continuously and concertedly for Li insertion/extraction. The results of this study provide a novel and unique model of amorphous electrode materials with significantly larger capacities