High lithium conductivities in weakly-ionophilic low-dimensional block copolymer electrolytes

The structure of amphiphilic low-dimensional copolymer electrolytes I of similar overall composition but prepared by different synthetic procedures X and Y are described. I are copolymers of poly [2,5,8,11,14-pentaoxapentadecamethylene(5-alkyloxy-1,3-phenylene)] (CmO5) and poly [2,-oxatrimethylene(5-alkyloxy-1, 3-phenylene)] (CmO1) where the alkyl side chains having m carbons are hexadecyl or mixed dodecyl/octadecyl (50150). H-1 NMR shows that the copolymers have 50% (m = 16) or only 18 and 13% of CmO5 units and DSC indicates that the copolymers have 'block' sequencing of CmO1 and CmO5 segments. Molecular dynamics modelling indicates that in CmO5 Li+ and BF4- ions are separated by Li+ encapsulation in tetraethoxy segments but in ionophobic CmO1 units the salt is mostly present as neutral aggregates decoupled from the polymer. Conductivities of these microphase-separated mixtures with salt-bridge amphiphilic polyethers II and III of each system are similar. They have low temperature dependence over the range 20 degrees C to 110C at similar to 10(-3) S cm(-1). Li-7 NMR linewidth measurements confirm high lithium mobilities at -20 degrees C. A conduction mechanism is proposed whereby Li+ hopping takes place along rows of decoupled aggregates (dimers/quadrupoles) within an essentially block copolymer structure. Subambient measurements to -10 degrees C gave a conductivity of 4 x 10(-5) S cm(-1). (c) 2005 Elsevier Ltd. All rights reserved.