Lithium-ion conducting solid polymer electrolytes (SPEs) are promising materials to replace flammable liquid electrolytes to solve the safety and performance issues in conventional rechargeable lithium batteries [1]. Poly(ethylene oxide) (PEO) has been studied extensively as baseline materials for SPEs. It is well known that pure PEO crystallizes at room temperature, hampering lithium ion migration and resulting in low ionic conductivity. This drawback hinders PEO based SPEs applied in real battery configuration that usually requires ionic conductivity greater than 10^-4 S cm^-1 at 25 ℃. In general, suppression of polymer crystallinity is favorable for SPEs to enhance the ionic conductivity [2]. Here, we synthesized a semi-interpenetrating polymer network (s-IPN) from the mixture of poly(propylene carbonate) (PPC), poly(ethylene glycol) methylether acrylate (PEGMEA), poly(ethylene glycol) diacrylate (PEGDA), and lithium bis(trifluoromethane) sulfonimidate (LiTFSI) salt using one-pot thermal curing method. The amorphous PPC performs as film forming agent, whereas the crosslinked network with PEG pendants provides lithium conducting channel. The s-IPN architecture inhibits the phase segregation of two different polymers. Curing temperature, crosslinker PEGDA content and salt concentration were optimized carefully to attain a flexible film with ionic conductivity reaching 5×10^-6 S cm^-1 at ambient temperature. The ionic conductivity and working potential window were correlated with the thermal and physical properties of the s-IPN electrolytes. Differential scanning calorimetry measurements have shown a decrease in the crystallinity of the s-IPN electrolytes. The electrolyte with higher salt concentration has lower ionic conductivity, consistent with higher glass transition temperature of PEG chain. The ionic conductivity was further enhanced by incorporating lithium ion conductive ceramic powder, lithium lanthanum zirconate (LLZO) as a filler into the s-IPN matrix, even though LLZO aggregation occurred during film forming. The optimized s-IPN has potential as a baseline material for future solid-state electrolyte designs.

Acknowledgements

We gratefully acknowledge the financial support from the NASA MIRO Program #NNX15AP44A and the National Science Foundation under Award #162-6449.

References

[1] Kang Xu. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503–11618
[2] Yu Wang, Wei-Hong Zhong. Development of Electrolytes towards Achieving Safe and High-Performance Energy-Storage Devices: A Review. CHEMELECTROCHEM 2015, 2, 22–36