Herein we present a solid electrolyte that adheres to the lithium surface and resists dendrite growth, both of which are needed for the development of high specific energy rechargeable batteries with lithium metal anodes. Nanostructured lamellar block copolymer electrolytes exhibit solid-like properties in the bulk, due to the presence of a randomly oriented granular structure, and liquid-like surface properties due to the formation of perpendicularly oriented lamellae at the lithium-electrolyte interface. The amount of charge that can be passed before short circuit in a symmetric lithium-polymer-lithium cell with nanostructured polystyrene-blockpoly(ethylene oxide) electrolytes is larger than that obtained with homopolymer poly(ethylene oxide) electrolytes by a factor ranging from 11 to 48. Grazing incident small angle X-ray scattering confirms that the microstructure of the block copolymer near the lithium-polymer interface has a perpendicular orientation. This orientation leads to a liquid-like behavior of the polymer at the interface due to the liquid crystalline symmetry of block copolymers. This combination of bulk and surface properties enhances the resistance to dendrites while maintaining electrode-electrolyte contact.

Energy-filtered transmission electron microscopy (EFTEM) was used to determine the distribution of lithium ions in solid polymer electrolytes for lithium batteries. The electrolytes of interest are mixtures of bis(trifluoromethane)sulfonimide lithium salt and symmetric poly(styrene-block-ethylene oxide) copolymers (SEO). In contrast to current solid and liquid electrolytes, the conductivity of SEO/salt mixtures increases with increasing molecular weight of the copolymers. EFTEM results show that the salt is increasingly localized in the middle of the poly(ethylene oxide) (PEO) lamellae as the molecular weight of the copolymers is increased. Calculations of the inhomogeneous local stress field in block copolymer microdomains, modeled using self-consistent field theory, provide a quantitative explanation for this observation. These stresses, which increase with increasing molecular weight, interfere with the ability of PEO chains to coordinate with lithium cations near the walls of the PEO channels where ion mobility is expected to be low.

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.