Next-generation batteries will require higher energy densities, cyclability, safety, and affordability to adequately meet energy storage needs. Li-metal is a promising, high-capacity anode that can be incorporated into Li-ion, Li-S, and Li-air systems. However, the loss of lithium into the solid electrolyte interphase (SEI) and into lithium deposits that become electrically disconnected (‘dead’ Li) are major, interrelated contributors to Coulombic efficiency losses in the Li-metal system. In this work, we use a custom-designed electrochemical cell to measure the XRD throughout plating and stripping of lithium onto a copper substrate to monitor the presence of lithium and crystalline SEI components during cycling. We obtain quantitative results on dead lithium development, which we compare to Coulombic efficiency measurements. Our results show variations in dead lithium formation in across different electrolytes and cycling conditions. Further understanding of capacity loss in lithium metal anodes is likely to lead to better-designed protection layers and electrolytes which improve anode performance.