We have studied the cycle life of two polyhedral oligomeric silsesquioxane-b-poly(ethylene oxide)-b-polyhedral oligomeric silsesquioxane (POSS-PEO-POSS) block copolymer electrolytes differing primarily in molecular weights and composition using lithium/polymer/lithium symmetric cells. The higher molecular weight electrolyte, labeled H, has a higher storage modulus, G _el . However, the volume fraction of the conducting phase in the low molecular weight electrolyte, labeled L, is higher and this leads to a four-fold increase in limiting current density, i _L. Measurement of ionic conductivity provides insight into the reason for the observed differences in limiting current density. The average lifetime of symmetric cells with electrolyte L was slightly higher than that of cells with electrolyte H. The combined effect of mechanical and electrochemical properties of electrolytes on the stability of lithium electrodeposition was quantified by examining two dimensionless parameters, i/i _L and G _el /G _Li , introduced in the theory developed by Barai and Srinivasan [Phys. Chem. Chem. Phys., 19, 20493–20505 (2017)]. This theory predicts the regime of stable lithium electrodeposition as a function of these two parameters. Despite large differences in G _el and i _L between the two electrolytes, we show that similar cell lifetimes are consistent with the theoretical predictions of unstable lithium electrodeposition without resorting to any adjustable parameters.