Effects of volume strain due to Li-Sn compound formation on electrode potential in lithium-ion batteries

Sn and Sn-based compounds have attracted great interest as candidates for anode materials in lithium-ion batteries. Despite the great deal of attention focused on the effects of the volume change of the Sn anode during the lithiation/delithiation process on the cyclic property of the batteries, its influence on the electrode potential is still not well understood. In this study, by constructing a simple Sn-Li battery system, we have investigated the effects of the volume change associated with the formation of Li-Sn compounds on the electrode potential from the viewpoint of the Gibbs free energy and associated elastic-strain energy. Our experimental results show that (i) α-Sn, which is a low-temperature phase and in thermodynamic non-equilibrium at around 298 K (our experimental temperature), is also formed together with usual β-Sn after several cycles of the lithiation and delithiation processes and (ii) when a Sn plate-shape electrode is lithiated, the experimental electrode potential underruns the value expected thermodynamically. These experimental results can be consistently explained by considering the contribution of the elastic-strain energy to the chemical free energy of formation.