Properties of Li-graphite and LiFePO4 electrodes in LiPF6-sulfolane electrolyte

Highlights: → Paper describes properties of a new electrolyte for lithium-ion batteries, based on sulfolane (TMS). The system shows increased non-flammability in comparison to classical electrolytes in cyclic carbonates. → It has been found that a co-solvent (vinylene carbonate) is necessary to form solid electrolyte interface (SEI). The protective SEI was studied with electrochemical impedance spectroscopy (EIS) and with scanning electron microscopy (SEM). → It has shown that specific capacity of graphite anode and LiFePO4 cathode are comparable to those characteristic for classical systems, but with increased safety. - Abstract: Sulfolane (also referred to as tetramethylene sulfone, TMS) containing LiPF6 and vinylene carbonate (VC) was tested as a non-flammable electrolyte for a graphite |LiFePO4 lithium-ion battery. Charging/discharging capacity of the LiFePO4 electrode was ca. 150 mAh g-1 (VC content 5 wt%). The capacity of the graphite electrode after 10 cycles establishes at the level of ca. 350 mAh g-1 (C/10 rate). In the case of the full graphite |1 M LiPF6 + TMS + VC 10 wt% |LiFePO4 cell, both charging and discharging capacity (referred to cathode mass) stabilized at a value of ca. 120 mAh g-1. Exchange current density for Li+ reduction on metallic lithium, estimated from electrochemical impedance spectroscopy (EIS) experiments, was jo(Li/Li+) = 8.15 x 10-4 A cm-2. Moreover, EIS suggests formation of the solid electrolyte interface (SEI) on lithium, lithiated graphite and LiFePO4 electrodes, protecting them from further corrosion in contact with the liquid electrolyte. Scanning electron microscopy (SEM) images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. No graphite exfoliation was observed. The main decomposition peak of the LiPF6 + TMS + VC electrolyte (TG/DTA experiment) was present at ca. 275 deg. C. The LiFePO4(solid) + 1 M LiPF6 + TMS + 10 wt% VC system shows a flash point of ca. 150 deg. C. This was much higher in comparison to that characteristic of a classical LiFePO4 (solid) + 1 M LiPF6 + 50 wt% EC + 50 wt% DMC system (Tf ∼ 37 deg. C).