Thermodynamic Characterization of the Hydrogen Sorption Reaction of Lithium Silicides
Authors: Biedermann Franziska , Jürich Nadine , Thomas Daniel , Hüttl Regina , Seidel Jürgen , Bohmhammel Klaus , Mertens Florian 
Published: February 2020

In the modern society new energy storage systems have become a basic need not only as a mobile energy source but also in the context of the global energy transition. Currently, the dominating anode material in lithium ion batteries is Graphite. It´s main drawback is the limited storage capacity and the low energy density (372 mAh g-1). Therefore, Silicon seems to be an attractive alternative for the next generation anode material. It offers an extremely large theoretical specific energy density (Li17Si4: 4054 mAh g-1) and low discharge potentials. Furthermore, lithium silicides show high reversible hydrogen storage capacity and may be used as safe hydrogen storage material [1,2]. For the application of lithium silicides in lithium ion batteries fundamental knowledge of the physicochemical properties of the various phases is essential. Especially accurate thermodynamic data is crucial for the understanding of the Li-Si phase diagram, phase transitions and electrochemical equilibria as well as for any rational design of materials for battery applications.

Numerous investigations of the binary Li-Si system were reported in literature, however, only few of them provided reliable thermodynamic data. Therefore, we recently reported experimental heat capacities and entropies of the five lithium silicides Li17Si4, Li16.42Si4, Li13Si4, Li7Si3 and Li12Si7
[1,3]. For thermodynamic calculations the enthalpies of formation are also required but the uncertainty of available data in literature is about 10 %, which is unacceptable for reliable equilibrium calculations.

Motivated by this situation we recently focused our activity on the accurate determination of the standard enthalpies of formation ΔFH°(298K) of the lithium silicides Li17Si4, Li13Si4, Li7Si3 and Li12Si7. For this purpose, the hydrogen absorption and desorption equilibria at three different temperatures (450°C, 475°C and 500°C) were investigated using a Sieverts type apparatus. Two different sample types were used for the measurements: (i) a stoichiometric mixture of lithium hydride and silicon and (ii) pure silicide. The determination of the enthalpy of formation was performed by applying the van´t Hoff relation. This approach resulted in uncertainties of at least 10 %. This unsatisfying result directed our attention to an alternative evaluation method. Combining the hydrogen equilibrium pressure peq(H2) from the sorption measurements with our precise heat capacity and entropy data of the lithium silicides finally provided values for the standard enthalpies of formation with an error of less than 2 %. Utilizing the complete set of experimental thermodynamic data, the Li-Si phase diagram can now be calculated (CALPHAD) with excellent accuracy.

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