Effect of Yield Stress on Stability of Block Copolymer Electrolytes against Lithium Metal Electrodes

We have studied the electrochemical properties of a poly(ethylene oxide)-b-poly(acryloisobutyl polyhedral oligomeric silsesquioxane) (PEO-POSS(10-4)) diblock copolymer and a POSS-PEO-POSS(2-10-2) triblock copolymer mixed with lithium bis(trifluoromethanesulfonyl)imide salt. Both the copolymers have similar molecular weights and conducting phase volume fractions. While the ionic conductivity of the diblock copolymer was a factor of 5 higher than that of the triblock copolymer, the storage shear moduli of the two polymers were within experimental error. The PEO chains in the triblock copolymer form bridges between adjacent POSS lamellae, and this impedes plastic deformation; there are no bridging PEO chains in the diblock copolymer. As a result, the yield stress of the triblock copolymer is a factor of 3 higher than that of the diblock copolymer. This relatively small difference in yield stress has a large effect on the morphology of protrusions that form on the anode. Dendrites are formed in the diblock copolymer while globular protrusions are formed in the triblock copolymer, resulting in an increase in the cycle life of Li/polymer/Li symmetric cells containing the triblock copolymer by a factor of 50.