An Explorative Approach for Formulating Highly Performant, Scalable, Lithium Battery Separators

Introduction The lithium-ion technology has revolutionized the energy storage market and the demand for highly performant devices is rapidly expanding, also capable of satisfying hard/challenging operative conditions required in many technological sectors. For instance, large-scale applications (particularly, deep-water drilling devices, gas/oil industry, but also stationary power sources and automotive) require batteries able of safely operating even at high temperatures (around or above 100 °C), while maintaining acceptable performance and cycle life without significant degradation [1,2]. However, commercial Li-ion batteries (LIBs) are temperature limited as they can only occasionally overcome 50-60 °C. The presence of volatile and flammable organic electrolyte solvents can lead to a dangerous chain of events such as overpressure, cell venting, burning and explosion, with rapid cell dismantling [3]. In addition, the LiPF 6 salt (generally used in standard LIB electrolytes) is thermally unstable and, in the presence of even moisture and/or oxygen traces, is able of generating HF acid, thereby irreversibility ageing the electrochemical device and leading to cell performance decay [4-5]. In this scenario, an appealing approach for overcoming this drawback is the design of non-volatile, non-flammable, more thermally robust electrolyte formulations able of withstanding high temperatures [2]. Ionic liquids, molten salts below 100 °C (often at room temperature or below), were proposed as advanced electrolyte solvents for improving the safety and reliability of LIB devices [6] due to their appealing peculiarities (i.e., no measurable vapor pressure, marked flame retardant properties, fast ion transport properties, high chemical/electrochemical/thermal stability, good power solvent) [7]. Phosphonium-based ionic liquids are expected to exhibit higher thermal and electrochemical stability compared to those containing ammonium cations [8-12]. Experimental In the present work, the attention has been focused on the tetrabutylphosphonium (P 4444 ) + cation, which has been selected because the steric hindrance and the symmetry of its structure are expected of allowing high thermal and electrochemical stability (with respect to the reduction process) [8-12], although these factors do not favor the ion transport properties and the low melting temperature. The (P 4444 ) + cation, commercially available as bromine salt (easily handled and purified), was coupled with selected anions of the per(fluoroalkylsulfonyl)imide family for their appealing thermal/electrochemical stability and good transport properties [9,13]. The (P 4444 ) + -based ionic liquids (PILs) were synthesized and purified according to an eco-friendly procedure, reported in detail elsewhere [7], which requires water as the only processing solvent. The quality control of the PIL materials was checked in terms of NMR, FT-IR, EDX and UV-Vis analysis where the physicochemical properties were studied through DSC and TGA techniques. The electrochemical characteristics were also examined in the presence of the LiTFSI salt (PIL:LiTFSI mole ratio = 4:1) in terms of ionic conductivity and electrochemical stability. Results The PIL materials were successfully synthesized with purity level overcoming 99.9 wt.%, i.e., in particular, the halide, moisture and lithium content was found below 5 ppm. The PIL electrolytes have exhibited very good thermal robustness (well above 250 °C) in conjunction with wide electrochemical stability window (close to 4.8 V vs the Li + /Li° redox couple). Fast ion transport properties (largely exceeding 10 -3 S cm -1 ) were recorded at temperatures ranging from 80-120 °C. These results make the (P 4444 ) + -based electrolytes rather appealing for high temperature lithium battery systems. The results are here presented and discussed. References [1] G.-T. Kim, et al. , Ionic Liquid-Based Electrolyte Membranes for Medium-High Temperature Lithium Polymer Batteries, Membranes 2018, 8, 41 [2] D. R. Wright, et al. , Review on high temperature secondary Li-ion batteries, Science Direct, Energy Procedia 151 (2018) 174–181. [3] S. Shahid, et al. , Energy Conversion and Management: X 16 (2022). [4] S. Li, et al. , Electrochim. Acta 129 (2014). [5] P. Murmann, et al. , Electrochim Acta 114 (2013). [6] S. Passerini, et al. , Lithium Polymer Batteries Based on Ionic Liquids in Polymers for Energy Storage and Conversion , Vikas Mittal editor, John Wiley and Scriverner Publishing, USA, 2013 [7] M. Montanino, et al. ,Electrochim. Acta 96 (2013) 124-133. [8] K.J. Fraser, et al. , Aust. J. Chem. 62 (2009) 309-321. [9] K. Tsunashima, et al. , Electrochem. Commun. 9 (2007) 2353-2358. [10] P.J. Griffin, et al. , J. Chem. Phys. 142 (2015) 084501. [11] P.J. Carvalho, et al., J. Chem. Phys. 140 (2014) 064505. [12] F. Chen, et al. , J. Chem. Phys. 148 (2018) 193813-193819. [13] G.B. Appetecchi, et al., Electrochim. Acta 56 (2011) 1300-1307. Acknowledgements The authors would like to acknowledge the financial support from the European Battery Innovation (EuBatIn) – IPCEI Project. E.D.S thanks the Electrical, Materials and Nanotechnology Engineering Doctoral Course of La Sapienza University of Rome for the financial support.