Hydrofluoroether electrolytes for lithium-ion batteries: Reduced gas decomposition and nonflammable

摘要

The optimum combination of high energy density at the desired power sets lithium-ion battery technology apart from the other well known secondary battery chemistries. However, this is besieged by thermal instability of the electrolyte. This "Achilles heel" still remains a significant safety issue and unless this propensity is improved the promise of widespread adoption of Li-ion batteries for Transportation application may not be realized. With this in mind we launched a systematic study to evaluate fluoro solvents that are known to be nonflammable, for thermal and electrochemical performances. We investigated hydro-fluoro-ethers (HFE) (1) 2-trifluoromethyl-3-methoxyperfluoropentane {TMMP} and (2) 2-trifluoro-2-fluoro-3-difluoropropoxy-3-difluoro-4-fluoro-5-trifluoropentane {TPTP} in Sandia-built cells. Thermal properties under near abuse conditions that exist in thermal runaway environment and the electrochemical characteristics for these electrolytes were measured. In the thermal ramp (TR) measurement, EC:DEC:TPTP-1 M LiBETI (or TFSI or LiPF6) electrolytes exhibited no ignition/fire. Similar behavior was observed for the EC:DEC:TMMP-1 M LiBETI. Further, in ARC studies the HFE electrolytes generated less gas by 50% compared to the EC:EMC-1.2 M LiPFG {CAR-1} electrolyte. Although in all cases the HFEs generated less gas, the onset of gas generation appears to depend on the salt. For the LiBETI and TFSI containing HFEs the onset is pushed out by ~80 C and for the LiPF6 the onset is comparable to that of the CAR-1. The solution ionic conductivity of these HFE electrolytes was lower (4-5 times) than that of the CAR-1 electrolyte however, the electrochemical performance was comparable. For example, full cells in 2032 type coin cells containing LiMN033Ni0.33Co0.33O2 cathode and carbon anode showed around 5mAh capacity and the computed specific capacity was ~154mAh for all the electrolytes. In half-cells against lithium the cathode and anode gave specific capacity on the order of 170mAh and 340 mAh respectively. These electrolytes when tested in 18,650 cells containing the above cathode and anode also showed comparable capacity. Further, the voltage stability window was not compromised by the HFEs. ARC measurements on 18,650 full cells showed less gas generation for the HFE electrolytes compared to CAR-1 electrolyte.