<i>Anti</i>-Perovskite Li-Battery Cathode Materials

Through single-step solid-state reactions, a series of novel bichalcogenides with the general composition (Li₂Fe)<i>Ch</i>O (<i>Ch</i> = S, Se, Te) are successfully synthesized. (Li₂Fe)<i>Ch</i>O (<i>Ch</i> = S, Se) possess cubic <i>anti</i>-perovskite crystal structures, where Fe and Li are completely disordered on a common crystallographic site (3<i>c</i>). According to Goldschmidt calculations, Li⁺ and Fe²⁺ are too small for their common atomic position and exhibit large thermal displacements in the crystal structure models, implying high cation mobility. Both compounds (Li₂Fe)<i>Ch</i>O (<i>Ch</i> = S, Se) were tested as cathode materials against graphite anodes (single cells); They perform outstandingly at very high charge rates (270 mA g^–1, 80 cycles) and, at a charge rate of 30 mA g^–1, exhibit charge capacities of about 120 mA h g^–1. Compared to highly optimized Li_1–<i>x</i>CoO₂ cathode materials, these novel <i>anti</i>-perovskites are easily produced at cost reductions by up to 95% and, yet, possess a relative specific charge capacity of 75%. Moreover, these iron-based <i>anti</i>-perovskites are comparatively friendly to the environment and (Li₂Fe)<i>Ch</i>O (<i>Ch</i> = S, Se) melt congruently; the latter is advantageous for manufacturing pure materials in large amounts

New Monoclinic Phase at the Composition Cu₂SnSe₃ and Its Thermoelectric Properties

A new monoclinic phase (<i>m2</i>) of ternary diamond-like compound Cu₂SnSe₃ was synthesized by reaction of the elements at 850 K. The crystal structure of <i>m2</i>-Cu₂SnSe₃ was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group <i>Cc</i>, <i>a</i> = 6.9714(2) Å, <i>b</i> = 12.0787(5) Å, <i>c</i> = 13.3935(5) Å, β = 99.865(5)°, <i>Z</i> = 8). Thermal analysis and annealing experiments suggest that <i>m2</i>-Cu₂SnSe₃ is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, <i>m2</i>-Cu₂SnSe₃ is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu–Se and Sn–Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit <i>ZT</i> reaches a maximum value of 0.33 at 650 K

Crystal Structure and Physical Properties of Ternary Phases around the Composition Cu₅Sn₂Se₇ with Tetrahedral Coordination of Atoms

A new monoclinic selenide Cu₅Sn₂Se₇ was synthesized, and its crystal and electronic structure as well as thermoelectric properties were studied. The crystal structure of Cu₅Sn₂Se₇ was determined by electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data: space group <i>C</i>2, <i>a</i> = 12.6509(3) Å, <i>b</i> = 5.6642(2) Å, <i>c</i> = 8.9319(4) Å, β = 98125(4)°, <i>Z</i> = 2; <i>T</i> = 295 K. Thermal analysis and high-temperature synchrotron X-ray diffraction indicated the decomposition of Cu₅Sn₂Se₇ at 800 K with formation of the tetragonal high-temperature phase Cu_4.90(4)Sn_2.10(4)Se₇: space group <i>I</i>4̅2<i>m</i>, <i>a</i> = 5.74738(1) Å, <i>c</i> = 11.45583(3) Å; <i>T</i> = 873 K. Both crystal structures are superstructures to the sphalerite type with tetrahedral coordination of the atoms. In agreement with chemical bonding analysis and band structure calculations, Cu₅Sn₂Se₇ exhibits metal-like electronic transport behavior