Block Copolymer Solid Battery Electrolyte with High Li-Ion Transference Number

Abstract

The electrochemical properties of a solid polymer electrolyte consisting of a diblock copolymer and lithium bis͑oxalato͒borate, LiBC 4 O 8 ͑LiBOB͒ salt, is reported. The spherical microphase-separated diblock copolymer is composed of a majority poly͑eth-ylene oxide͒ ͑PEO͒ block and a minority random copolymer block of methyl methacrylate ͑MMA͒ and lithium salt of methacrylic acid ͑MAALi͒, PEO-b-͑PMMA-ran-PMAALi͒. Salt-optimized electrolyte membranes of the diblock copolymer and LiBOB salt exhibited an average lithium-ion transference number ͑T Li +͒ value of 0.9 at room temperature ͑21-23°C͒. The solid-state flexible, translucent polymer electrolyte shows a wide electrochemical stability window and excellent interface properties with the lithium metal electrode. The combination of these properties makes these block copolymer membranes viable electrolyte candidates for flexible lithium-ion batteries.The widespread use of high energy density lithium-ion-based batteries has been constrained by the safety concerns of its reactive components. Thus, the need to design, synthesize, and optimize polymer structures for improved energy storage is of critical importance. Also, advances in lithium cathode chemistries have led to the development of batteries that are capable of delivering a high discharge potential ͑Ͼ4.5 V͒, 1-3 driving the need for electrolyte materials that are capable of withstanding such high voltage conditions. Unlike traditional liquid electrolytes, solid-state polymer electrolytes possess the high electrochemical stability window required for application in high voltage battery setups. Electrolyte membranes incorporating poly͑ethylene oxide͒ ͑PEO͒ with a lithium salt have long been proposed as a viable candidate. 4-6 However, such materials have poor room-temperature ionic conductivity due to the semicrystalline nature of PEO. In addition, PEO-based electrolytes have the intrinsic disadvantage of possessing a low lithium-ion transference number ͑T Li +͒, exhibiting a value between 0.2 and 0.5. 7-10 These polymer-lithium salt systems behave more like an aniontransporting medium, resulting in a charge gradient across the medium. This increases the polarization across the polymer film, leading to loss of power drawn from the battery. 11-13 Studies done by Doyle et al. showed that better performance is exhibited by polymer materials with T Li + = 1.0 than those possessing an order of magnitude greater conductivity but a poorer T Li + value. 11 Hence, the interest is to develop a conductive polymer with high ionic conductivity as well as high lithium-ion transference number.Numerous electrolytes have been reported that possess single-ion transport properties with lithium-ion transference numbers approaching unity. Single-ion polyelectrolyte systems have been successfully developed where the anion or the counterion is effectively tethered to the polymer matrix. 14-16 Further, the use of various anion trappers has been shown to effectively boost the cation transference number. 17-20 These approaches, however,...

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