Ionic Liquids, Synthesized and Utilized by Greener Methods, for the Application in High-Voltage Lithium-Ion Batteries
Authors: Palluzzi Matteo , Tsurumaki Akiko , Matic Aleksandar , D'Angelo Paola , Navarra Maria Assunta 
Published: February 2024

Due to their high energy density and reliability, lithium-ion batteries (LIBs) are a crucial energy storage system for portable electronic devices, which has dominated the market for more than 30 years. To increase their energy density even more, it is imperative to replace the commonly employed cathode materials (such LiCoO2) with cathode materials that can operate at higher potentials, like LiNi0.5Mn1.5O4 (LNMO) working at 4.7V vs Li+/Li. However, standard electrolytes, typically containing carbonate-based solvents (e.g., LP30), are unstable at such high potentials and suffer from oxidation decomposition. To mitigate this problem, ionic liquids (ILs) have recently been proposed as an additive to them. Despite this, their application in industrial-scale is hindered by the high-cost and environmentally unfriendliness of their synthesis [1]. In this study, ILs containing oxalatoborate anions—known to form the so-called cathode-electrolyte interphase (CEI), a protective layer on the cathode surface [2]—have been obtained by using mostly water as solvent, to minimize the impact of the synthesis on the environment. Four different ILs have been synthesized: N-ethoxyethyl-N-methylpiperidinium bis(oxalato)borate (PIP1,2O2BOB), N-ethoxyethyl-N-methylpiperidinium difluoro(oxalate)borate (PIP1,2O2DFOB), N-propyl-N-methylpiperidinium bis(oxalato)borate (PIP1,3BOB) and N-propyl-N-methylpiperidinium difluoro(oxalate)borate (PIP1,3DFOB). All of them have been characterized by IR spectroscopy and thermal analysis (DSC and TGA). The thermal analyses highlighted that all the ILs are thermally stable up to 280°C. Furthermore, PIP1,2O2DFOB and PIP1,2O2BOB did not show any crystallization behavior even at sub-zero temperatures, but only a glass-transition (at -70 °C and -30 °C, respectively). The improved liquid property is related to the presence of an ether oxygen in the cation structure.
The four ILs have been then used as the additive for LP30 at a concentration of 0.3M and the resulting mixtures have been tested in Li|LP30+IL|LNMO coin-cells at 1C (147 mA g-1) for over 150 charge/discharge cycles. The results obtained showed that the addition of the ILs to the electrolyte has ensured more stable charge/discharge cycles, with higher coulombic efficiency and improved discharge capacity retention especially in the case of BOB-based ILs, while Li|LP30|LNMO cells prepared as a reference failed after about 100 cycles. On the promise that the ILs play an important role at the surface of cathode, another approach has been tested: 3 wt% of IL has been added directly to the slurry during the preparation of LNMO electrodes. These IL-added electrodes have been tested in Li|LP30|LNMO+IL coin-cells. Remarkable improvements in the stabilities of the cells have been observed also in this case. In IL-added electrodes, the total amount of IL in the system is almost 40 times smaller than that used in LP30+IL mixtures as the electrolytes. These results show that presence of ILs closer to the electrode is crucial to obtain an improvement of the stabilities of high-voltage cathodes. The combination of greener synthesis and utilization is expected to enhance the sustainability of LIBs.
[1] E. Simonetti, M. De Francesco, M. Bellusci, G. Kim, F. Wu, S. Passerini, G. B. Appetecchi, ChemSusChem, 2019, 12, 4946– 4952
[2] A. Tsurumaki, M. Branchi, A. Rigano, R. Poiana, S. Panero, M.A. Navarra, Electrochimica Acta, 2019, 315, 17-23

Figure 1