Development of Effective, Safer and Fast UV-Cured Solid Gel Polymer Electrolytes for Lithium-O2 Rechargeable Batteries

Abstract

More now than ever, the development of greener, safer, cheaper and more powerful rechargeable batteries is considered a goal of strategic importance for several sectors including transportation (i.e. electric vehicles). Nowadays, one of the most important types of rechargeable batteries are lithium-ion batteries as they provide relatively high energy density (~160 Wh/kg) and long cyclability. However, these still have many limitations, especially on energy efficiency or rate capability. Beyond Lithium-ion, many efforts are being put to develop polymers and materials that can be implemented in more powerful and advanced technologies such as lithium-metal or lithium-air/ lithium –O2 batteries.Lithium-O2 batteries represent one of the most appealing candidates for battery electric vehicles (BEV) due to its remarkable theoretical high energy density, similar to fossil fuels (~11,000 Wh/kg). Aprotic liquid electrolyte cells, also called “non-aqueous”, are the most popular due to their highest theoretical capacity. Amongst all common aprotic solvents, tetraethylene glycol dimethyl ether (TEGDME) is the most used liquid electrolyte due its low volatility, wide electrochemical window (beyond 4.5 V versus Li0/Li+) or good solubility of metal alkali salts. However, its liquid nature prompts some drawbacks related to safety issues such as the potential leaking of the toxic and flammable organic electrolyte in the cell.Solid gel polymer electrolytes (GPEs) represent a plausible and compromising solution to tackle some of the safety challenges associated to these conventional liquid-based Li-O2 batteries. In these systems, a solvent (often named plasticizer) is trapped within the polymer network, achieving intermediate ionic conductivities (~10−3 S·cm−1) much higher and closer to liquid systems (~10−2 S·cm−1) than the so-called solid polymer electrolytes (SPEs) (~10−5 S·cm−1).In this presentation, we will present a family of cross-linked robust GPEs based on TEGDME. The GPEs were prepared by fast UV-photopolymerisation and investigated as solid electrolytes for Li-O2 batteries. Two families of GPEs have been developed: single-ion GPEs (in which the counter anion is attached to the polymeric backbone) and dual-ion conductive GPEs (in which LiTFSI salt is dissolved in the plasticizer). Both types of GPEs, single ion and dual ion lithium conductors, have been compared for the first time on Li-O2 cells.Both types of GPEs presented high ionic conductivity at room temperature (1.6·10−4 S·cm−1 and 1.4·10−3 S·cm−1 for single ion or dual ion, respectively). First, their performance was investigated in symmetrical Li|Li cells. In this case, the dual-ion GPE showed an outstanding behavior where the overpotential was <0.2 V vs Li0/Li+ for more than 40 hours at a current density as highs as ±1 mA·cm−2. On the other hand, in full Li-O2 configuration, the single ion GPE cell showed superior discharge capacity, up to 2.38 mAh·cm−2. In addition, a dynamic discharge characterization technique is proposed here as a method for evaluating the polarization effect in electrolytes during discharge in an easy, quantifiable and reproducible manner.Overall, we will discuss the rapid and effective synthesis and properties of several types of GPEs using state-of-the-art materials and new and greener aprotic solvents for Li-O2 cells. Figure 1