Experimental determination of enthalpies of formation of Li17Si4, Li16.42Si4 and Li13Si4

Abstract

The hydrogen absorption equilibria have been investigated to determine the enthalpy of formation of the lithium rich silicides Li17Si4, Li16.42Si4 and Li13Si4. For the corresponding experiments, two different sample types of Li17Si4: pure silicide and a stoichiometric mixture of lithium hydride and silicon (ratio 17:4), as well as Li16.42Si4 were used. The measurements were performed at three different temperatures (450 °C, 475 °C, 500 °C). The calculation of the enthalpies of formation by using the slopes of the classical van´t Hoff plot resulted in uncertainties of at least 10%. Due to this unsatisfying large error an alternative evaluation method was applied linking up the hydrogen equilibrium pressure peq(H2) from the sorption measurements and our recently published precise heat capacity and entropy data of the lithium silicides. The values obtained in this manner for the standard enthalpies of formation (Li17Si4 - (mol atom)−1; Li13Si4 = -24.4 ± 0.3 kJ (mol atom)−1 and Li16.42Si4 = -22.2 ± 0.5 kJ (mol atom)−1 show strongly reduced uncertainties that are acceptable for CALPHAD calculations to obtain a largely improved phase diagram description completely based on an experimental thermodynamic dataset.