MXenes are a family of two-dimensional materials, composed of early transition metal carbides, nitrides, and carbonitrides, with great potential in energy storage systems, in particular in electrodes for Li, Na, K-ion batteries. However, so far the capacities are not competitive. In this context, we investigate P and Si functionalized MXenes for metal-ion battery applications, using first-principles calculations, since P and Si provide reaction products with high ion content. Replacement of the F and OH ligands of Ti2C and V2C with P and Si is demonstrated to be feasible (energy barriers of less than 0.128 eV) and the ion diffusion barriers turn out to be less than 0.32 eV. Importantly, the Li, Na, and K capacities are predicted to be 1767 mAh g−1, 711 mAh g−1, and 711 mAh g−1, respectively, thus being much higher than in the case of F and OH functionalization.