Integrating Ethereal Molecular Backbones into the Ester Solvent with High Solubility of Nitrate for High-Voltage Li Metal Batteries.

Abstract

The high-energy-density Li metal batteries require high-voltage cathode, low negative/positive capacity (N/P) ratio and lean electrolyte. Despite the all-fluorinated electrolytes with severe corrosion, the development of ester electrolytes is stagnant due to the incompatibility of ester solvent with Li metal anode. Hence, various electrolyte additives have been developed. Among them, LiNO₃ is considered as the most effective electrolyte additive for improving the reversibility of Li deposition. Unfortunately, their solubility into the ester solvent is extremely low. This investigation suggests that the strong ionic bonds in LiNO₃ and the low solvation energy of ester solvent are the main triggers for the insolubility of LiNO₃ in the ester electrolyte. Hence, a new organic nitrate salt (N-propyl-N-methylpyrrolidinium nitrate (Py₁₃NO₃)) with large organic cations and a new liner ester solvent (dipropyleneglycol methyl ether acetate (DPGMEA)) is designed, which integrates the ethereal molecular backbones into the ester solvent. Consequently, the electrolyte containing 1.2 m lithium bis(fluorosulfonyl)imide (LiFSI), 0.3 m Py₁₃NO₃ and 0.1 m lithium disfluorophosphate (LiPO₂F₂) in fluoroethylene carbonate (FEC):DPGMEA (2:8) showcases excellent electrochemical performance in high-voltage Li metal batteries. Eventually, the "1 Ah level" Li||LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) pouch cell (N/P ratio ≈1.2; electrolyte/capacity (E/C) ratio ≈2.5 g Ah^-1) exhibits excellent cycle life over 150 times in the designed electrolyte.