Currently, Li metal anode development is restricted by low Coulombic efficiency, poor cycle life, and safety concerns. Each of these challenges stem from the extreme reactivity of Li metal and the resulting undesirable interactions at the electrode/electrolyte interface. Dendrites grow during electrodeposition, consuming electrolyte, leading to dead Li formation and capacity fade, and eventually causing short circuiting. The limited knowledge of the dynamically evolving Li metal surface during extended cycling has prevented rational design of methods to mitigate these undesirable behaviors. Our recent work elucidated the correlation of galvanostatic voltage traces with morphology evolution on Li metal anodes during early cycles, providing a “window” into cells which traditionally are difficult to monitor with operando techniques [1]. The present work builds on that understanding by studying the evolution of the voltage trace shape and the dead Li layer over extended cycling in both Li-Li symmetric and full cells containing Li metal anodes (Figure 1). In this work [2], we focus on understanding: 1) the evolution of the voltage trace from the peaking shape seen in Figure 1a to the arcing seen in Figure 1b during extended cycling in both half and full cells, 2) how the accumulation of a dead Li layer impacts mass transport, and 3) why dead Li causes capacity fade and failure of full cells containing Li metal anodes. Continuous accumulation of dead Li on the anode surface leads to the formation of a tortuous porous interphase. This layer creates larger interfacial concentration gradients which lead to large overpotentials. Based on experimental results and modeling, we provide a detailed understanding of the effects that mass transport through complex interphases has on capacity fade, voltage traces, and failure of Li metal anodes in half and full cell configurations. We demonstrate that these effects at the anode have significant impacts on full cell performance, in many cases driving failure. This result underscores the importance of understanding the dynamic morphology evolution on anode surfaces and shows that Li-Li symmetric cells are a powerful tool to study the underlying physical phenomena occurring during cycling. This study enables evaluation of methods of reducing dead Li formation and rational design of solutions for improved performance in Li-metal batteries, an important step towards making Li metal rechargeable batteries a commercial reality. References 1. K. N. Wood, E. Kazyak, A. F. Chadwick, K.-H. Chen, J.-G. Zhang, K. Thornton, N. P. Dasgupta, ACS Cent. Sci. 2, 790 (2016). 2. K.-H. Chen, K. N. Wood, E. Kazyak, W. S. LePage, A. L. Davis, A. J. Sanchez, N. P. Dasgupta, Submitted. Figure 1