Comparative Study of Fluorinated and Non-Fluorinated Electrolytes in Li[NixMnyCoz]O2/Graphite Pouch Cells

Abstract

The fluorinated electrolyte system, fluoroethylene carbonate (FEC)/bis (2,2,2-trifluoroethyl) carbonate (TFEC) (1/1 w/w), has shown promising performance when Li[Ni0.42Mn0.42Co0.16]O2(NMC442)/graphite pouch cells were operated at 4.5 V.1 Therefore Li[Ni1/3Mn1/3Co1/3]O2(NMC111), NMC442, Li[Ni0.5Mn0.3Co0.2]O2(NMC532), Li[Ni0.6Mn0.2Co0.2O2](NMC622), Al2O3 coated NMC532 and Al2O3 coated NMC622/graphite pouch cells were examined using the FEC/TFEC electrolyte system and compared to cells made with the traditional electrolyte solvent system: ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3/7 w/w). Both electrolyte systems contained 1M LiPF6 and 2% prop-1-ene-1,3-sultone (PES) plus 0.5% methylene methane disulfonate (MMDS) as electrolyte additives.2,3 This systematic study was performed using the ultra high precision charger (UHPC) at Dalhousie University, electrochemical impedance spectroscopy (EIS), gas evolution measurements and long term cycling tests. A wide upper cut-off voltage window was explored during extensive UHPC testing (from 4.1 V to 4.6 V). The charge end-point capacity slippage, capacity fade, columbic efficiency (CE), impedance change during cycling and gas evolution were compared to measure the effects of different electrolyte solvents on different NMC/graphite pouch cells. Figures 1(a-d) show that the coulombic efficiency (CE) of the FEC/TFEC system is higher than that of the EC/EMC system for all uncoated NMC grades except for NMC622 when the upper cut-off voltage is 4.5 V. This suggests better long term cycling performance at high voltage when FEC/TFEC is used, except for NMC622. The reactions between charged NMC grades (4.5 V) and these two electrolyte systems at elevated temperatures were also investigated by accelerating rate calorimetry (ARC). Figures 1(g-h) show that FEC/TFEC does not affect the thermal performance of NMC532 and NMC622 compared to EC/EMC. However, the self-heating rate of charged NMC111 or charged NMC442 reaction with FEC/TFEC is higher than that of EC/EMC after a temperature greater than 150°C. This means that cell designers will need to may close attention to the impact of fluorinated solvents on cell safety.