Piperidinium-based ionic liquid electrolyte with linear solvent and LiODFB for LiFePO4/Li cells at room and high temperature

Abstract

Lithium difluoro(oxalato)borate (LiODFB) combines the advantages of the salts LiBOB and LiBF₄ when used in electrolytes for lithium ion cells. However, compared with the common Li salts, such as LiPF₆ or LiTFSI, LiODFB shows poor solubility in ionic liquids. In order to increase the solubility of LiODFB in ionic liquid, dimethyl carbonate (DMC) has been introduced into N-allyl-N-methylpiperidinium bis(trifuoromethansulfonyl)imide (PP_13*TFSI)-based electrolytes as a co-solvent additive. The binary electrolyte 0.4 mol L⁻¹ LiODFB- PP_13*TFSI/DMC (8 : 2 v/v) has been prepared and investigated. For comparison, the electrolytes 0.1 mol L⁻¹ LiODFB-PP_13*TFSI and 0.4 mol L⁻¹ LiODFB-DMC have also been prepared. Li/LiFePO₄ half cells containing the novel binary electrolyte were found to exhibit excellent electrochemical performance, maintaining a discharge capacity and coulombic efficiency of 158 mAh g⁻¹ and 99.21%, respectively, after 100 cycles. The tremendous improvement is ascribed to the joint oxidative decomposition of ODFB⁻ and TFSI⁻ anions to form a perfect solid electrolyte interphase (SEI) on the LiFePO₄ electrode. Further, discharge capacities of 147 mAh g⁻¹ and 128 mAh g⁻¹ were obtained at the rates of 1C and 2C, respectively. Besides, Li/LiFePO₄ half cells with the novel binary electrolyte could still reach up to a first discharge capacity of 158 mAh g⁻¹ and first coulombic efficiency of 90% at 60 °C.