Thermodynamische Untersuchungen in den Systemen Lithium-Silicium und Lithium-Zinn

Lithium-Ion-Batteries have an excellent potential for energy storage. Currently, the dominating anode material is graphite with an energy storage capacity of 339 mAh g-1. An attractive alternative to the commercial used graphite are lithium silicides, as well as lithium stannides. These materials are characterized by much greater storage capacities and low self-discharge potentials. For the application of these two different materials in Lithium-Ion-Batteries, fundamental knowledge of the physicochemical properties, especially reliable thermodynamic data are essential. Until now, the existence of seven lithium silicides are certainly proven. These include the six stable phases Li17Si4, Li16.42Si4, Li13Si4, Li7Si3, Li12Si7, the high pressure phase LiSi and the metastable one Li15Si4. For the first five mentioned phases reliable heat capacities and standard entropies were already determined in the first funding period of the priority program 1473. In case of the lithium stannides seven phases are secured. However, no reliable experimental thermodynamic basic data exists. For this reason, the aim of this study is the experimental determination of precise thermodynamic values of the two last-named lithium silicides (LiSi and Li15Si4), as well as for the lithium stannides Li17Sn4, Li7Sn2, Li13Sn5 and Li7Sn3. The synthesized phase pure samples were characterized by means of X-ray diffraction, thermal and chemical analysis. The focus of this work was directed to the experimental determination of the heat capacity in a temperature range from 2 K to the decomposition temperature of each investigated phase. Therefore, two different calorimeters were used: a Physical Property Measurement System (Quantum Design) from 2 K to 300 K and a DSC 111 (Setaram) starting from 300 K. The experimental data were given with an accuracy of 1 % to 2 % above 20 K and rises up to 20 % below 20 K. The measurement at low temperatures allowed the calculation of the standard entropy, plus the determination of electronic and lattice contributions to the heat capacity. Furthermore, the second main part of this study was the provision of the enthalpy of formation of the investigated lithium silicides and lithium stannides based on hydrogen sorption measurements in a Sieverts-Apparatus. For the first time pressure-composition-isotherms of lithium silicides starting with Li17Si4, LiH:Si (Li:Si =17:4), Li16.42Si4 and LiSi were recorded. As initial material for the lithium stannides Li17Sn4, LiH:Sn (Li:Sn = 17:4), Li7Sn2 and LiH:Sn (Li:Sn = 7:2) were used. The commercial used van ́t-Hoff-Plot analysis resulted in uncertainties of at least 10 %. For this reason, an alternative evaluation method was applied linking the experimental heat capacities and standard entropies with the equilibrium pressure from the sorption measurements. This method resulted in errors of less than even 1 %. The results of this work provide a complete thermodynamic data set for the Li-Si-System. The calculation of the Li-Si-phase diagram is in good agreement to the experimental known literature data. A validation of the basic data determined in this work was performed for the lithium stannides. The achievements of this work present a significant contribution to the data basis for thermodynamic calculations and for the understanding of phase and electrochemical equilibria, which is of great importance for battery development as well as for the basic understanding of