Growth of the solid electrolyte interphase (SEI) is the primary driver of capacity fade in lithium-ion batteries. Despite its importance to this device and intense research interest, the fundamental mechanisms underpinning SEI growth remain unclear. In this work, we measure the electrochemical growth of the SEI with precision coulometric measurements on different relevant carbon electrodes, including graphite and carbon black. For both materials, we report large, decaying lithiation capacities and smaller, constant delithiation capacities, implying the SEI grows primarily on lithiation. This observation is especially striking at low rates (C/10), rates at which most charging/discharging phenomena can be approximated as symmetric as they approach equilibrium. We determine the asymmetry in irreversible SEI growth is coupled to the asymmetries in reversibleintercalation and deintercalation of carbon. This work illustrates an interesting electrochemical coupling phenomenon and furthers our understanding of SEI growth in carbon anodes.