Alkyl chain length effects of hydroxyl-functionalized imidazolium ionic liquids in the ionothermal synthesis of LiFePO₄

Abstract

LiFePO₄ is one of the most promising cathode materials for lithium ion batteries. To investigate the role of the cation in the ionothermal synthesis of LiFePO₄, three imidazolium-based ionic liquids, containing hydroxyl-terminated alkyl chains of various lengths, were synthesized using microwave-assisted methods: 1-(3-hydroxypropyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, 1-(6-hydroxyhexyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide, and 1-(9-hydroxynonyl)-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide. An ionothermal synthesis was attempted with each ionic liquid, using various precursors. The resulting solid products were investigated using powder X-ray diffraction and solid state nuclear magnetic resonance spectroscopy. The yield of LiFePO₄ increased with an increase in the hydroxyl-terminated alkyl chain length on the imidazolium.

Graphical Abstract

Keywords:

• Li ion batteries
• LiFePO4
• ionic liquids
• ionothermal synthesis
• MAS NMR