The escalating need for efficient electrical energy storage has intensified the pursuit of more advanced rechargeable batteries. Modern lithium-ion batteries (LIBs) have surpassed previous secondary batteries in both energy and current density. However, the performance of modern LIBs is constrained by their graphite anodes, whose theoretical maximum capacity has already been reached. To surpass this limit, a new anode material is needed, and transition metal sulfides are an excellent class of candidate materials, given their higher capacity, abundance, and good stability. This study investigates CoSx-CNT anodes in LIBs, where the material achieved an impressive 560 mAh g−1 initial charge capacity. However, as the cycling continued, the charge capacity of the cell increased, reaching 659 mAh g−1 after 90 cycles. The increase was attributed to increasing reversible reduction of the organic liquid on the growing solid electrolyte interface (SEI), which continued to reversibly store charge even as the conventional Li2S conversion reaction declined. This research demonstrated the potential beneficial effect of the growth of the SEI on the electrode’s reversible charge capacity, as opposed to previous studies in which SEI growth worsened cell performance. Insights from this system offer an improved means of studying the SEI’s properties and formation, as well as valuable knowledge for enhancing the cycle life and charge capacity of even non-TMS LIB anodes.