Effect of Counter Ion Placement on Conductivity in Single-Ion Conducting Block Copolymer Electrolytes

Single-ion conducting block copolymer electrolytes were prepared in which counter ions were tethered to the polymer backbone to achieve a lithium transference number of unity. Through tailored anionic synthesis, the influence of counter ion placement on conductivity was investigated. Incorporating the anions outside the ion-conducting [poly(ethylene oxide)-based] block, such as in poly(lauryl methacrylate)-block-poly(lithium methacrylate)-block-poly[(oxyethylene)9 methacrylate], known as PLMA-bPLiMA-b-POEM, and P(LMA-r-LiMA)-b-POEM, caused lithium ions to dissociate from the carboxylate counter ions upon microphase separation of the POEM and PLMA blocks, yielding conductivities of 10-5 S/cm at 70DGC. In contrast, incorporating anions into the conducting block, as in PLMA-b-P(LiMA-r-OEM), rendered the majority of lithium ions immobile, resulting in conductivities one to two orders of magnitude lower over the range of temperatures studied for equivalent stoichiometries. Converting the carboxylate anion to one that effectively delocalized charge through complexation with the Lewis acid BF3 raised the conductivity of the latter system to values comparable to those of the other electrolyte architectures. Ion dissociation could thus be equivalently achieved by using a low charge density counter ion (COOBF-3) or by spatially isolating the counter ion from the ion-conducting domains by microphase separation.