The combination of both high mechanical stability due to internal volume compensation and high capacities by incorporated silicon renders silicon–carbon (Si–C) void structures attractive anode materials for lithium-ion solid-state batteries (LISSBs). Evaluating the lithiation mechanism of Si–C composite (30 wt% Si, 1473 mAh g_Si–C⁻¹) electrodes in half-cells versus lithium using Li₆PS₅Cl as a solid electrolyte also reveals participation of the carbon matrix. Consequently, the storage of additional capacity in the carbon shell of the Si–C particles [= carbon (over)lithiation] is investigated in full cells versus nickel-rich NCM (n/p < 1) and compared with non-overlithiated Si–C|SE|NMC cells (n/p > 1). The absence of metallic lithium deposition is evidenced by ⁷Li Nuclear Magnetic Resonance spectroscopy, indicating solely (over)lithiation of the carbon matrix without lithium plating. Finally, the investigation of application-relevant pouch cells (n/p > 1) demonstrates minimal breathing behavior together with high reversibility even at stack pressures as low as 5 MPa.