Nanoscale Ionic Materials for Nafion Based Nanocomposites Membranes As Single Lithium-Ion Conducting Polymer Electrolytes for Lithium Sulfur Batteries

Abstract

Currently, rechargeable batteries with the lithium–sulfur (Li–S) chemistry has attracted great interest as one of the most promising candidates for next generation electrochemical energy storage systems. Research into these high energy density devices is critical to the development of thinner, lighter, and lower cost battery systems. One of the biggest obstacles for practical applications of Li-S batteries is caused by the soluble nature of the highly ordered lithium polysulfides (Li2Sn) in the organic electrolytes and induce a so-called “shuttle effect”. A solid-state electrolyte (SPEs) could be a valid alternative in terms of reducing the polysulfides dissolution and shuttle, as well as to protect the lithium metal anode and to minimize dendrite formation, which is beneficial for improving the safety and cycle life of Li−S batteries. SPEs are typically dual-ion conductor systems both cations and anions are mobile and cause a concentration polarization leading to poor performances of batteries. Recently, single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) have been proposed for polymer electrolytes, where anions are covalently bonded to the polymer, inorganic backbone, or immobilized by anion acceptors and only the Li+ cation will contribute to a permanent flow of charge. They have advantages over conventional dual-ion conducting SPEs such as unity transference number, absence of harmful effect of anion polarization, extremely low rate of Li dendrite growth and immobilization of the lithium polysulfides in the lithium-sulfur (Li-S) batteries. Polymer electrolytes based on ionomers (e.g., Nafion) with easily ionizable groups (e.g., sulfonic groups covalently bonded to the polymer side-chains, −CF2SO3 −) are promising thanks to the high concentration of weakly coordinating anions (counterions). In this work, lithiated Nafion and Nafion-nanocomposites membranes based on Nanoscale Ionic Materials (NIMs) were synthesized, and their ionic conductivity and lithium transference number were investigated in common nonaqueous organic solvents (EC/PC and Glymes).A thorough and systematic study of the lithium-ion transport was conducted by p 1H and 7Li pulsed field gradient (PFG) NMR spectroscopy and electrochemical impedance spectroscopy (EIS), while the mechanical properties of the film electrolytes have been tested by dynamic mechanical analysis (DMA) in a wide temperature range. The electrochemical studies have been conducted both in Li/Li symmetric cell and in secondary Li-S cells. The preliminary results are very interesting, showing ionic conductivities of the order of 5 × 10-4 S/ cm at 25°C satisfactory properties in terms of stability window and stability of the lithium stripping. The lithium transport number is very close to unity thus confirming the complete immobilization of the negative charge carriers.