Effect of Lithium-Ion Concentration on Morphology and Ion Transport in Single-Ion-Conducting Block Copolymer Electrolytes

Abstract

Single-ion-conducting polymers are ideal electrolytes for rechargeable lithium batteries as they eliminate salt concentration gradients and concomitant concentration overpotentials during battery cycling. Here we study the ionic conductivity and morphology of poly(ethylene oxide)-b-poly(styrenesulfonyllithium(trifluoromethylsulfonyl)imide) (PEO-b-PSLiTFSI) block copolymers with no added salt using ac impedance spectroscopy and small-angle X-ray scattering. The PEO molecular weight was held fixed at 5.0 kg mol–1, and that of PSLiTFSI was varied from 2.0 to 7.5 kg mol–1. The lithium ion concentration and block copolymer composition are intimately coupled in this system. At low temperatures, copolymers with PSLiTFSI block molecular weights ≤4.0 kg mol–1 exhibited microphase separation with crystalline PEO-rich microphases and lithium ions trapped in the form of ionic clusters in the glassy PSLiTFSI-rich microphases. At temperatures above the melting temperature of the PEO microphase, the lithium ions were released from the clusters, and a homogeneous disordered morphology was obtained. The ionic conductivity increased abruptly by several orders of magnitude at this transition. Block copolymers with PSLiTFSI block molecular weights ≥5.4 kg mol–1 were disordered at all temperatures, and the ionic conductivity was a smooth function of temperature. The transference numbers of these copolymers varied from 0.87 to 0.99. The relationship between ion transport and molecular structure in single-ion-conducting block copolymer electrolytes is qualitatively different from the well-studied case of block copolymers with added salt.