Effect of Mixtures of Lithium Hexafluorophosphate (LiPF6) and Lithium Bis(fluorosulfonyl)imide (LiFSI) as Salts in Li[Ni1/3Mn1/3Co1/3]O2/Graphite Pouch Cells

Abstract

The effect of mixtures of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) with different molar ratios in the electrolyte of Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells was studied using the ultra-high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and gas evolution measurements. Ethylene carbonate: ethyl methyl carbonate (EC:EMC, 3:7 wt.% ratio) solvent was used as the base solvent in these studies. Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells containing both LiPF6 and LiFSI (with a total salt content of 1 M) with or without 2% VC showed smaller or similar self-discharge, lower charge transfer resistance (Rct), and smaller amounts of gas evolution during formation and during storage at high temperature, compared to cells containing only 1 M LiPF6 with and without 2% VC, respectively. For cells without 2% VC, cells with 0.3 M LiPF6 + 0.7 M LiFSI showed the smallest self-discharge and the lowest Rct after 40°C storage. For cells with 2% VC, cells with 0.5 M LiPF6 + 0.5 M LiFSI + 2% VC showed the lowest voltage drop and the lowest Rct after 40°C storage. The UPHC cycling data showed that cells containing LiPF6:LiFSI mixtures with 2% VC showed similar coulombic efficiency (CE), and similar charge end-point capacity slippage compared to cells with 1 M LiPF6 + 2% VC. The combination of LiFSI and LiPF6 in electrolytes that contain 2% VC can bring benefits of improved storage properties and reduced gas evolution at high temperature while maintaining all other properties of the cells in experiments limited to 4.2 V. However, preliminary experiments at voltages up to 4.45 V suggest that LiFSI may lead to increased transition metal dissolution (Ni, Mn and Co) compared to lithium bistrifluoromethane sulfonyl imide (LiTSFI), another salt additive used to improve storage properties and reduce gassing.