Fire-Preventing LiPF₆ and Ethylene Carbonate-Based Organic Liquid Electrolyte System for Safer and Outperforming Lithium-Ion Batteries

Abstract

Battery safety is an ever-increasing significance to guarantee consumer's safety. Reducing or preventing the risk of battery fire and explosion is a must for battery manufacturers. Major reason for the occurrence of fire in commercial lithium-ion batteries is the flammability of conventional organic liquid electrolyte, which is typically composed of 1 M LiPF 6 salt and ethylene carbonate (EC)-based organic solvents. Herein, we report the designed 1 M LiPF 6 and EC-based nonflammable electrolyte including methyl(2,2,2-trifluoroethyl)carbonate, which breaks the conventional perception that EC-based liquid electrolyte is always flammable. The designed electrolyte also provides high anodic stability beyond the conventional charge cut-off voltage of 4.2 V. A graphite∥Li-Ni 0.6 Co 0.2 Mn 0.2 O 2 lithium-ion full cell with our designed EC-based nonflammable electrolyte with a small fraction of vinylene carbonate additive under an aggressive condition of 4.5 V charge cutoff voltage, 0.5C rate, and 45 °C exhibits increased capacity, reduced interfacial resistance, and improved performance and rate capability. A basic understanding of how a high-voltage cathode−electrolyte interface and anode−electrolyte interface are stabilized and how failure modes are mitigated by fire-preventing electrolyte is discussed.

Design of Fire‐Resistant Liquid Electrolyte Formulation for Safe and Long‐Cycled Lithium‐Ion Batteries

“…Surface Ni 2+ -O and Mn 2+ -O are notoriously soluble in the electrolyte and are the reasons for the surface structural degradation and disorder from the hexagonal-layered to the cubic phase and the structural disorder propagation from the surface to the bulk along with oxygen gas release for maintaining the charge neutrality. [2,9,11,20,23,24] In that aspect, the prevention or reduction in the fraction of Ni 2+ -O and Mn 2+ -O when using the TFA-based EL is a mitigation signature of the metal-dissolution event that has been a fatal failure mode of Ni-rich NCM cathodes in Li-ion batteries with traditional EL system particularly under high-voltage and/or high-temperature operating conditions (Figure S8, Supporting Information).…”

“…After cycling with the TFA-based EL with FEC, new strong peaks of ester compounds were observed, overlying the peaks of pristine PVDF binder (Figure 3e-i). [8,11,25] Because the same amount of FEC additive was commonly used in both electrolytes, the stronger absorbance of ester compounds upon the use of TFA-based EL, i.e., an ester-rich SEI layer, is due to the decomposition of PC and TFA solvents (Figure S10, Supporting Information). A similar remark pertains to Li 2 CO 3 .…”

“…[2,[8][9][10][11] The unique characteristics of fluorinated organic solvents originate from the lowered highest occupied molecular orbital (HOMO) energy level from non-fluorinated counterparts, obtained by density functional theory (DFT) calculation results, because of the electron-withdrawing effect of the F atom and thus improving the thermodynamic and electrochemical stability. [2,8,[10][11][12]…”

Non-flammable liquid electrolytes for safe batteries

“…16 Chung et al modified a 1.0 M LiPF 6 and EC:EMC-based electrolyte system, by substituting a hydrogen atom with a fluorinated methyl group in the EMC molecule, resulting in 1.0 M LiPF 6 in EC:FEMC. 25 The electrolyte not only suppressed flammability, but was also shown to be highly effective in terms of improving the cycling performance of graphite// Li-Ni 0.6 Co 0.2 Mn 0.2 O 2 cells. With addition of the commonly used vinylene carbonate (VC) additive, the full cell even outperformed conventional electrolytes with VC (186 mA h g À1 , 11% increase) and remained non-flammable (Fig.…”

“…Fig.1Discharge capacity vs. cycle number for conventional electrolyte and non-flammable electrolyte. Reproduced from Chung et al,25 with permission from American Chemical Society.…”