This proposal presents a new class of electrolytes based on fluorinated polymers that have high thermal- and electrochemical-stability for use in advanced high-voltage lithium ion batteries (LIBs). Fluorinated polymers, such as poly(tetrafluoroethylene) (PTFE), will be chosen as a matrix for electrolyte solutions with respect to their extreme chemical inertness and thermal stability. PTFE-based polymer electrolytes are desired but have not yet made successful in their preparation because the extreme inertness of PTFE restricts the absorption of electrolytes. Meanwhile, we have focused on ionic liquids (ILs), an inflammable molten salt, as electrolytes for fluorinated polymers. Because ILs are composed of organic ions having structural diversity, we can deem that ILs are imbued with the possibility for showing a high compatibility so as to avoid the repellence of ILs from PTFE. Some fluoro-functionalized ILs originally prepared in my research last year have successfully resulted in the formation of homogeneous composites with PTFE. Taking advantages of this achievement, this year I propose ternary composites based on PTFE, commercialized electrolyte solutions, and fluorinated-ILs, where fluorinated-ILs are expected to play a crucial role as a binder between PTFE and the electrolyte solution. At the same time, fluorinated-ILs are also expected to act as an excellent flame-retardant and an additive to broaden the electrochemical stability window. With respect to the aim of this research, the ratio of all components will be optimized in terms of thermal- and electrochemical-stability. The applicability of the composites to advanced-high voltage LIBs (>5V) will be discussed by evaluating real batteries with the composites. The composites are expected to enable the safe use of high voltage cathodes which cannot be attained by using conventional electrolytes. Throughout this research, I intend to establish peculiar PTFE as a unique and reliable matrix for LIBs.
The PTFE-based polymer electrolytes proposed here will provide benefits both in industrial- and scientific-concepts that will make this research more important and innovative; first of all the polymer electrolytes promise the development of high energy and safe LIBs; second, the combination of PTFE and ILs is quite unique and this will have a great impact, both on the field of ILs and fluorinated polymers.
The first benefit is of course related to the improvement in the thermal-stability of electrolytes. As introduced before, most accidents from LIBs consisting a conventional electrolyte solution are related to thermal runaway. The replacement of flammable electrolytes with inflammable polymer electrolytes, where vapor pressure is suppressed by the ILs, will surely improve the safety and credibility of existing LIBs. The successfully prepared composites will bring benefits to the LIB industry, especially for high temperature applications such as electro vehicles. This research will give us also a new knowledge about the fluoro-functionalized ILs in terms of their thermal stability. As of now, non-functionalized ILs have been used as a flame retardant for the electrolytes in LIBs. However, there is no report with respect to the effect of the fluorine content in ILs on their thermal stability, even though there is the evident that a lot of fluorinated additives are used as a flame retardant in industry. This is because that it is quite difficult to modify ILs with fluorine and increase the fluorine content in ILs. By optimizing the structure of fluoro-functionalized ILs considering the knowledge obtained, ILs will be established as an excellent flame retardant. In addition to this, the use of fluoro-functionalized ILs is beneficial in terms of electrochemical stability. The fluorinated-ILs were found to have an electrochemical potential window reaching to 6 V, which is wide enough for advanced-high voltage LIBs. This wide electrochemical potential window cannot be achieved with the conventional carbonate-based electrolyte solution. The use of the ILs is expected to expand the diversity of cathode materials in LIBs that cannot be combined with the conventional electrolyte solutions. This research can be extended to further developments of new high voltage cathodes. Considering these facts, we can deem that the polymer electrolytes proposed in this research will contribute to improve the safety and the energy density of LIBs.
In contrast to the industrial- and applicational-point of view, I would like to emphasize that this material is quite unique in terms of polymer chemistry because the use of PTFE as a matrix for liquid material is desirable but not general so far. Only a few materials have been prepared by modifying PTFE via gamma radiation or using modified tetrafluoroethylene during their polymerization. The materials proposed here will be a new class of polymer materials based on PTFE. This research is imbued with potentials that expand the applications of PTFE. In this research, I functionalized ILs with fluorine to use them as a binder between PTFE and conventional electrolyte solutions. The ILs will be the innovative material that can support the absorption of conventional electrolytes into PTFE. In this research, electrolyte solutions were selected as a partner of PTFE aiming at the development of a polymer electrolyte for LIBs. However, the target of soaking material can be varied considering various purposes. As it is described as the biggest advantage of ILs, ILs have structural diversity because they are composed organic ions. This means that ILs can be further designed with not only fluoro-alkyl chain but also other functions in order to bind PTFE and other specific materials. This research is applicable to other research field and able to expand the application of PTFE. By taking those two advantages, I hope the composites based on PTFE will lead to successful and safe electrolytes for advanced high-voltage LIBs, and in turn be followed by other research in a wide range of fields.