Substitution of Lithium for Magnesium, Zinc, and Aluminum in Li15Si4: Crystal Structures, Thermodynamic Properties, as well as 6Li and 7Li NMR Spectroscopy of Li15Si4 and Li15−xMxSi4 (M=Mg, Zn, and Al)

Abstract An investigation into the substitution effects in Li 15 Si 4 , 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 Li 14 MgSi 4 ( 1 ) and Li 14.05 Zn 0.95 Si 4 ( 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 Li 14.25 Al 0.75 Si 4 ( 3 ). The substitution of a small amount of Li in metastable Li 15 Si 4 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 T 1 ‐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 , Li 14.25 Al 0.75 Si 14 , and 1 , respectively. The influence of the metastable property of Li 15 Si 4 on NMR spectroscopy experiments is highlighted.