Cyclic Phosphate-Based Nonflammable Electrolytes for High Energy and Safe Lithium-Ion Batteries
Authors: Zheng Qifeng , Yamada Yuki , Shang Rui , Nakumura Eiichi , Yamada Atsuo 
Published: December 2020

With the rapid development of renewable energy technologies, electric vehicles, and portable and wearable electronics, high-performance energy-storage devices are in ever-increasing demand. Among the various commercial energy-storage technologies, lithium-ion batteries have dominated the market in portable consumer electronics due to their high operating voltages and energy densities that can be attributed to their use of organic electrolytes.However, for the commercialization of large-scale applications, such as electric vehicles, further improvements in the energy and power densities must be achieved without sacrificing safety. The majority of current safety issues associated with Li-ion batteries arise from the utilization of highly volatile and flammable organic carbonates as solvents and the chemically unstable lithium hexafluorophosphate (LiPF6) as the salt in commercial electrolytes. Safety is becoming the hottest topic in the area of Li-ion batteries (LIBs). Although various safety measures have been applied, accidental fires still happen frequently due to the use of highly flammable organic electrolytes. As such, a solution to this issue is the use of a non-flammable organic solvent; however, no such solvent has been reported that is also suitable for use in LIBs.
Herein, we, a joint team of battery researchers and organic chemists, designed and synthesized a new non-flammable fluorinated cyclic phosphate solvent (Figure 1) [1], namely, 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEP), for use in LIBs. As a rationale, we fused the chemical structural features of a conventional battery solvent (i.e., the ability to form an electrode-protective layer via ring-opening polymerization) and a fire retardant (i.e., the ability to trap H· radicals, which support combustion). An ethylene carbonate-free, LiPF6-free non-flammable electrolyte was prepared by dissolving 0.95 M LiN(SO2F)2 (LiFSA) in TFEP/2,2,2-trifluoroethyl methyl carbonate (FEMC, 1:3 v/v). Due to its ability to form a stable SEI, highly reversible and stable cycling was achieved for the graphite anode in such a non-flammable electrolyte without the use of high salt concentrations or SEI-forming additives. Moreover, the TFEP solvent enabled the stable cycling of high-voltage LiNi0.5Mn1.5O4 (LNMO) and high-capacity LiNi1/3Mn1/3Co1/3O2 (NMC) cathodes even in a dilute lithium bis(fluorosulfonyl)imide (LiFSI) electrolyte. This was attributed to its unique functions of forming an F-rich passivation layer that can prevent corrosion of the Al current collector and forming a polymeric cathode electrolyte interphase layer to prevent electrolyte oxidation and transition metal dissolution.
Furthermore, silicon-based materials are promising anodes for next-generation Lithium-ion batteries because of their specific capacities, but they practical applications have been hindered by huge volume change, limited cycle life, and safety concerns. We found that cyclic phosphate-based electrolytes could also significantly improve the long-term cycling stability of SiO anode owing to the formation of a highly elastic inorganic-organic bilayer interphase. A LiNi0.6Mn0.2Co0.2|SiO cell with a high cutoff voltage of 4.4V shows 70.0 % capacity retention after 300 cycles with an average Coulombic efficiency of 99.9%.
Further optimization of the unique chemical structure of this electrolyte would be expected to lead to the development of high-energy-density, sustainable, and safe Li-ion batteries. This work addresses the trade-off between battery voltage and safety, enhancing both factors simultaneously, which opens new frontiers in electrolyte developments toward safe LIBs with higher energy densities.

Reference:

[1]. Zheng Q, Yamada Y, Shang R, Ko S, Lee Y-Y, Kim K, Nakamura E, Yamada A. A cyclic phosphate-based battery electrolyte for high voltage and safe operation. Nat. Energy 2020, 5(4): 291-298.

Figure 1