Matteo PALLUZZI1, Akiko TSURUMAKI1, Paola D'ANGELO1, Maria Assunta NAVARRA1
1Sapienza University of Rome, Department of Chemistry, Rome, Italy
Lithium-ion batteries (LIBs) are a vital energy storage system for portable electronic devices, by virtue of their high energy density and reliability. The replacement of commonly used cathode materials (such as LiCoO2) with a cathode material capable of working at higher potentials, for example LiNi0.5Mn1.5O4 (LNMO) is a crucial request to further improve their energy density. However, traditional electrolytes generally contain carbonate-based solvents which are not stable at such high potentials due oxidation on the cathode surface. Recently, ionic liquids (ILs) have been used as additive to the electrolytes to improve their thermal and electrochemical stabilities. In this study, two ILs with borate anions, known to form a protective layer on the cathode surface, the so called cathode-electrolyte interphase (CEI), are synthesized, by using greener methods to reduce environmental impact during the synthesis.
N-ethoxyethyl-N-methylpiperidinium bromide (PIP1,2O2Br) has been synthesized by a conventional quaternization but using water as the solvent. The salt has been subsequentially purified by stirring it overnight with activated carbon in water and filtrated. Anion exchange reaction has been performed by mixing PIP1,2O2Br with either lithium bis(oxalato)borate (LiBOB) or lithium difluoro(oxalato)borate (LiDFOB) to obtain PIP1,2O2BOB and PIP1,2O2DFOB. The two ILs have been then extracted using dichloromethane.
The two ILs, PIP1,2O2BOB and PIP1,2O2DFOB, exhibited a glass transition at -30 °C and -70 °C, respectively, without crystallization behavior in differential scanning calorimetry. The absence of crystallization even at sub-zero temperature is related to the presence of an oxygen in the cation structure. Thermal gravimetric analysis demonstrated that the two ILs are thermally stable up to at least 280 °C. In the cyclic voltammetry the oxidation peak has been observed in the first cycle for both the ILs, but the oxidation current decreased during successive cycles, confirming the formation of CEI.
Then the ILs have been used as the additive for battery components, specifically electrolyte and LNMO-electrode. In the former case, 1M LiPF6 in EC/DMC (LP30) was chosen as the electrolyte, and the IL was added at concentrations of 0.1/0.3/0.5 M. In the latter case, the slurry of LNMO containing the 3wt% IL was deposited on an Al foil to form the electrodes. By using these prepared materials, lithium-metal cells have been assembled. A notable improvement of the coulombic efficiency with only a minimal decrease in the specific discharge capacity was observed when PIP1,2O2BOB was added to the electrode. Further tests are being carried out to identify the best concentration of IL to be added either to the electrolyte or to the cathode to ensure the best performances. Thereafter the two approaches will be combined, adding IL both to the electrolyte and to the cathode.