Fig. 4. Theoretical volumetric changes upon cell charge of lithium metal (top) and silicon-based (bottom) cells.

Volume change is visualized as a change in one dimension, namely thickness. In general, materials can expand in all three dimensions. The top panel shows that the deposition of 4 mAh/cm² of lithium metal would lead to an increase in cell thickness of about 19 µm per negative electrode layer, based on a specific capacity of 3860 mAh/g and a density of 0.53 g/cm³, i.e., a volumetric capacity of 2045 mAh/cm³. The bottom panel shows that at the end of charge, the same amount of lithium (i.e., lithium equivalents) in an alloying reaction with silicon to form Li₁₅Si₄ would lead to an increase in cell thickness per negative electrode layer of 12 µm, and a comparable overall negative electrode thickness of 18 µm. A density of 2.33 g/cm³ was used for pure silicon and a volumetric capacity of 2194 Ah/cm³ for Li₁₅Si₄. Positive electrode and electrolyte layer are assumed to have a constant thickness. Volumetric capacity and density determine cell energy density, affecting how much space a cell would occupy, e.g., in a battery pack. Increasing cell energy density can allow, for example, more electrode layers or cells to be integrated into the same space.