Experimental determination of the enthalpies of formation of the lithium silicides Li7Si3 and Li12Si7 based on hydrogen sorption measurements

This work focuses on the determination of the standard molar enthalpies of formation ΔfH°(298) of the lithium silicides Li7Si3 and Li12Si7. For this purpose the hydrogenation/dehydrogenation equilibria in the system Li-Si-H were investigated at three different temperatures (450 °C, 475 °C, 500 °C) using a Sieverts type apparatus. The measurements are based on two different sample types: (i) pure silicide Li7Si3 (Li2.33Si) and (ii) a stoichiometric mixture of lithium hydride and silicon in the ratio of Li:Si = 2.33:1. The calculation of the standard enthalpies of formation by using the slopes of the classical Van't Hoff plot results in uncertainties of at least 10%. Due to this relatively large error, an alternative evaluation method based on the linking of the hydrogen equilibrium pressures peq(H2) from the sorption measurements with recently published precise heat capacity and entropy data of the lithium silicides was applied. The obtained values for the standard enthalpy of formation amount to ΔfH°(298) = (−25.3 ± 0.3) kJ/(mol atom) for Li7Si3 and ΔfH°(298) = (−23.5 ± 0.3) kJ/(mol atom) for Li12Si7 exhibiting strongly reduced uncertainties.