Lithium dendrite formation was studied in polymer electrolytes consisting of lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) dissolved in polystyrene-b-poly(ethylene oxide) block copolymers. Both Li-polymer-FePO4 batteries and Li-polymer-Li symmetric cells were cycled at constant current until voltage responses characteristic of dendritic failure were observed. The amount of charge passed before this observation was defined as the lifetime of the cell. Battery lifetimes were found to be significantly longer than symmetric cell lifetimes. Both battery and symmetric cell lifetimes increased with increasing electrolyte thickness, as predicted by the model of Monroe and Newman. Post-mortem analysis of the lithium surfaces were conducted after failure. The lithium electrodes from failed batteries were much smoother than those from failed symmetric cells. Dendritic structures with characteristic length scales in the 10-30 mu m range were seen in batteries. In contrast, the length scales of the dendritic structures seen in symmetric cells were in the 100-300 mu m range. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.030303jes] All rights reserved.