UV cross-linked, lithium-conducting ternary polymer electrolytes containing ionic liquids

Abstract

In this manuscript is reported an attempt to prepare high ionic conductivity lithium polymer electrolytes by UV cross-linking the poly(ethyleneoxide) (briefly called PEO) polymer matrix in presence of the plasticizing lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and an ionic liquid of the pyrrolidinium family (N-alkyl-N-methylpyrrolidinium TFSI) having a common anion with the lithium salt. It is demonstrated that polymer electrolytes with room temperature ionic conductivities of nearly 10⁻³ S cm⁻¹ could be obtained as a result of the reduced crystallinity of the ternary electrolytes. The results clearly indicate that the cross-linked ternary electrolyte shows superior mechanical properties with respect to the non-cross-linked electrolytes and higher conductivities with respect to polymer electrolytes containing none or less ionic liquid.

Introduction

Rechargeable lithium batteries outpaced all other battery systems in the consumer portable electronic and telecommunications markets within the last few years. However, to power the hybrid and/or pure electric vehicles of tomorrow, lithium batteries have to provide even higher energy and/or higher power densities, better cyclability, reliability, and, overall, safety. Lithium metal batteries with their theoretically high gravimetric energy and power densities are a desired solution. However, the liquid organic electrolytes that have warranted the great success to present lithium-ion batteries, appears to be not useful because of their high reactivity with Li-metal that results in poor performance and uneven (dendritic) anode deposition. This latter phenomenon raises serious safety issues considering that the conventional electrolytes for Li-ion batteries are mostly composed of volatile flammable solvents, which could easily cause fire or explosion of the battery upon dendritic short-circuit. Actually, the safety issue caused by the electrolytes’ fire and explosion hazards is indeed present in large Li-ion batteries and, so far, it has prevented the wide deployment of Li-ion batteries in hybrid and electric vehicles. The thermal runaway of a Li-ion battery is a catastrophic, uncontrollable event that occasionally happens even in long established applications such as laptop PC and cell phones [1].

Lithium metal, polymer electrolyte batteries (LMPBs) have been proposed since early 1980s as the solution to the safety issues. So far, however, LMPBs are substantially limited to high-temperature operations since solvent-free polymer electrolytes, such as those with dissolved lithium salt in poly(ether/glycols), are characterized by a relative low ionic conductivity at room temperature.

Most of the work on polymer electrolytes, so far, has been focusing on poly(ethylene oxide) (PEO). PEO is an inert polymer and its application as electrolyte host has been intensively studied for almost four decades now [2]. The fact that PEO builds complexes with Li salts and displays both thermal and interfacial stabilities [3] makes it a promising candidate as polymer electrolyte host. However, at room temperature the ionic conductivity of Li salts dissolved in PEO is limited because the highly symmetrical repeating ethylene oxide units tend to crystallize. Amorphous areas are necessary for sufficient ionic conductivity [4], [5]. Li salts with large anions free the Li⁺ from the strong EO coordination. An ionic liquid as additional salt, providing the same anion seems to promote this process and the voluminous cations create “free-volume” [6], [7] for diffusion. Recently, the combination of a polymer and an ion conducting, electrochemically stable ionic liquid has been explored as a polymer-based electrolyte in lithium ion batteries [8]. A broad and stable amorphous region of PEO was created. Ionic liquids are perfectly suitable for battery applications, because they present high chemical and thermally stability, negligible vapour pressure, non-flammability and in some cases high electrochemical stability and hydrophobicity [9], [10], [11], [12].

The ionic liquid N-methyl-N-butylpyrrolidinium bis(trifluoromethansulfonyl)imide (PYR₁₄TFSI) [13] and lithium bis(trifluoromethansulfonyl)imide (LiTFSI) as conducting salt are used in this work [14], [15], [16]. The membranes are easily prepared by a solvent-free technique in which the components are mixed together and hot pressed, a considerably more attractive preparation procedure for industrial applications than the typically used solvent-casting. The resulting PEO–RTIL–LiTFSI materials are true dry solid polymer electrolytes (SPE) consisting solely of commercial high molecular weight PEO and two salts.

Conductivities of about 10⁻⁴ S cm⁻¹ at or near ambient temperature are needed to match the requirements for application in batteries for electronic devices and electric vehicles [17]. The ionic conductivity of the new SPE is almost two orders of magnitude higher than that of ionic-liquid-free PEO–LiTFSI electrolyte [8] and if the PEO/IL/LiTFSI ratio is increased from 10/1/1 (by mole) to 10/2/1, the ionic conductivity is further increased, from 10⁻⁴ S cm⁻¹ to 3 × 10⁻⁴ S cm⁻¹ at 20 °C. Unfortunately, the mechanical stability of the electrolyte membrane suffers at increasing IL content, where sticky materials are obtained. In the attempt to obtain room temperature highly conductive polymer electrolytes without depleting their mechanical properties, we have “in situ” UV photo-irradiated the PEO chains, in presence of LiTFSI and PYR₁₄TFSI, to obtain chemically cross-linked membranes. UV-induced cross-linking of the PEO chains was performed with benzophenone (Bp) as cross-linking agent thus allowing to obtain processable thin polymer films.

This work is a contribution to the development of safe lithium batteries, which can be operated without the use of volatile and combustible electrolyte liquids.