In order to develop a model transferable to these a prior unknown structures, we have parameterized a Density Functional Tight-Binding (DFTB) model for Li x Si which is the main goal of the present paper. Although DFTB implies a stronger level of theory than classical force fields like ReaxFF, it is faster than conventional Density Functional Theory (DFT) calculations, enabling to study the dynamics of thousands of atoms. We also present a new parameterization approach that allows us to optimize the model by reproducing other observables beyond absolute energies. We use this idea to focus the model on reproducing the relative formation energies of the different Li x Si structures, a key feature to achieve a robust and transferable model for the entire composition range. The present new model shows excellent performances in the prediction of formation energies against DFT for several crystalline and amorphous structures.