Li-ion batteries dominate portable energy storage due to their exceptional power and energy characteristics. Yet, various consumer devices and electric vehicles demand higher specific energy and power with longer cycle life. Here we report a full-cell battery that contains a lithiated Si/graphene anode paired with a selenium disulfide (SeS₂) cathode with high capacity and long-term stability. Selenium, which dissolves from the SeS₂ cathode, was found to become a component of the anode solid electrolyte interphase (SEI), leading to a significant increase of the SEI conductivity and stability. Moreover, the replacement of lithium metal anode impedes unwanted side reactions between the dissolved intermediate products from the SeS₂ cathode and lithium metal and eliminates lithium dendrite formation. As a result, the capacity retention of the lithiated silicon/graphene - SeS₂ full cell is 81% after 1,500 cycles at 268 mAg_SeS2 ^-1. The achieved cathode capacity is 403 mAhg_SeS2 ^-1 (1,209 mAhcm_SeS2 ^-3).