title: Substitution of Lithium for Magnesium, Zinc, and Aluminum in Li15 Si4 : Crystal Structures, Thermodynamic Properties, as well as (6) Li and (7) Li NMR Spectroscopy of Li15 Si4 and Li15-x Mx Si4 (M=Mg, Zn, and Al).
abstract: An investigation into the substitution effects in Li15 Si4 , which is discussed as metastable phase that forms during electrochemical charging and discharging cycles in silicon anode materials, is presented. The novel partial substitution of lithium by magnesium and zinc is reported and the results are compared to those obtained for aluminum substitution. The new lithium silicides Li14 MgSi4 (1) and Li14.05 Zn0.95 Si4 (2) were synthesized by high-temperature reactions and their crystal structures were determined from single-crystal data. The magnetic properties and thermodynamic stabilities were investigated and compared with those of Li14.25 Al0.75 Si4 (3). The substitution of a small amount of Li in metastable Li15 Si4 for more electron-rich metals, such as Mg, Zn, or Al, leads to a vast increase in the thermodynamic stability of the resulting ternary compounds. The (6,7) Li NMR chemical shift and spin relaxation time T1 -NMR spectroscopy behavior at low temperatures indicate an increasing contribution of the conduction electrons to these NMR spectroscopy parameters in the series for 1-3. However, the increasing thermal stability of the new ternary phases is accompanied by a decrease in Li diffusivity, with 2 exhibiting the lowest activation energy for Li mobility with values of 56, 60, and 62 kJ mol(-1) for 2, Li14.25 Al0.75 Si14 , and 1, respectively. The influence of the metastable property of Li15 Si4 on NMR spectroscopy experiments is highlighted.
authors.affiliation: Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747, Garching, Germany.
authors.forename: Volodymyr
authors.initials: V
authors.lastname: Baran
authors.affiliation: Department of Physics, University of Augsburg, Universitätsstrasse 1, 86159, Augsburg, Germany.
authors.forename: Leo
authors.initials: L
authors.lastname: van Wüllen
authors.affiliation: Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747, Garching, Germany. thomas.faessler@lrz.tum.de.
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Fässler
keywords: NMR spectroscopy; lithium; silicon; structure elucidation; thermodynamics