Ionic Liquid Electrolytes for Safer Lithium Batteries: II. Performance in NMC and Graphite Electrodes

Abstract

Actually, the research on lithium battery materials is pushed towards large capacity anodes and high voltage cathodes. While graphite still represents the best choice as the anode material, the need for well-performing cathode materials is more and more stringent. Lithium-containing, mixed nickel-manganese-cobalt oxide (NMC) is one of the most investigated high operative voltage (> 4 V) cathode material also due to its large capacity value [1]. However, safety is still an open issue. Ionic liquids (ILs), molten salts at room temperature, were successfully proposed as non-flammable and non-volatile solvents for replacing the conventional, hazardous, toxic, organic compounds in lithium battery systems, aiming to enhance the safety of the device [2]. ILs can be favorably combined to obtain mixtures with improved properties, often not exhibited by single ionic liquid materials. Therefore, we have properly developed ternary IL mixtures, based on a blend of N-methyl-N-propyl pyrrolidinium bis(trifluoromethane sulfonyl)imide, PYR13TFSI, and methyl-N-propylpyrrolidinium bis(fluoro sulfonyl)imide, PYR13FSI, ionic liquids [3], to be addressed as electrolytes for lithium batteries. In the present work, we report the electrochemical performance of NMC cathodes and graphite anodes, manufactured using fluorine-free, water-soluble, natural binders as carboxymethylcellulose sodium salt (CMC), in LiTFSI-PYR13TFSI-PYR13FSI electrolyte. Results and Discussions The voltage vs. capacity profile of selected charge-discharge cycles, performed on NMC cathode tapes at 23 °C and 0.1C in LiTFSI-PYR13TFSI-PYR13FSI and (1M)LiPF6-EC/DMC (1:1 in weight) electrolytes, is reported in Figure 1. It is worth to note that both the charge (with the exception of the first half-cycle) and discharge features are practically overlapped, indicating high reversibility of the Li+ intercalation process and high columbic efficiency. The results clearly evidence how NMC cathodes are able, in the selected ionic liquid electrolyte, to deliver 90 % of the capacity discharged in organic solutions with a somewhat similar operative voltage. Cycling tests have revealed very good capacity retention with efficiency close to 100 % in the 0 -50 °C temperature range. Interesting capacity values (≥ 100 mA h g-1) were exhibited even at low temperatures (0 °C), but at low current rates (≤ 0.05), due to diffusive phenomena within the IL electrolytes. Figure 2 compares the voltage vs. capacity profile of selected discharge-charge cycles, performed on graphite anode tapes in LiTFSI-PYR13TFSI-PYR13FSI and (1M)LiPF6-EC/DMC (1:1 in weight) electrolytes. A small EC amount, e.g., 5 wt.%, was incorporated to the IL electrolyte in order to promote SEI growth onto graphite anode. The results have evidenced good reproducibility of the cycle features, indicating high reversibility of the lithium intercalation process, very good capacity retention and high coulombic efficiency. More than 90 % of the capacity value, delivered in conventional organic solutions, was exhibited in LiTFSI-PYR13TFSI-PYR13FSI + 5 wt.% electrolyte. Acknowledgements The authors wish to thank the financial support of the European Commission for the GREENLION project within the 7th Framework Program (Grant agreement n°: 28526).