High Energy Density Rechargeable Batteries Based on Li Metal Anodes. The Role of Unique Surface Chemistry Developed in Solutions Containing Fluorinated Organic Co-solvents

To date, lithium ion batteries are considered as a leading energy storage and conversion technology, ensuring a combination of high energy and power densities and prolonged cycle life. A critical point for elaboration of high energy density secondary Li batteries is the use of high specific capacity positive and negative electrodes. Among anode materials, Li metal anodes are considerably superior due to having the highest theoretical specific capacity (3860 mAh g^–1) and lowest negative redox potential (−3.040 V vs a standard hydrogen electrode). Combination of Li metal anodes with Li[NiCoM]O₂-layered cathodes with a high stable specific capacity of about 200 up to 250 mAh g^–1 is particularly attractive. The development of advanced electrolyte solutions which ensure effective passivation of the electrodes’ surfaces is of critical importance. Considerable efforts have been focused on fluorinated organic co-solvents and specifically fluoroethylene carbonate (FEC) due to the formation of thin, flexible Li-ions-conducting surface films with excellent protective properties. However, in the FEC-based solutions, detrimental “cross talk” between the Li anodes and the Li[NiCoM]O₂ cathodes leads to worsening of the passivation of Li metal anodes, consumption of the electrolyte solutions, and limited cycle life of full Li|Li[NiCoM]O₂ cells cycled with a low amount of the electrolyte solution and practical cycling parameters. The addition of difluoroethylene carbonate (DFEC) co-solvent with lower LUMO energy leads to a significant improvement in the cycling behavior of full cells. Using fluorinated co-solvents possessing synergistic effects is very promising and paves the way for developing rechargeable batteries with the highest energy density.