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

摘要

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 (PP13* TFSI)-based electrolytes as a co-solvent additive. The binary electrolyte 0.4 mol L(-1)1 LiODFB-PP13* TFSI/DMC (8 : 2 v/v) has been prepared and investigated. For comparison, the electrolytes 0.1 mol L-1 LiODFB-PP13* 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 degrees C.