Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) have been considered as promising candidates for solid-state Li-ion battery (LIB) electrolyte that can replace current liquid electrolyte system.[1] However, PEO-based SPEs have critical problems for practical use in that the ionic conductivity is relatively low at an ambient temperature, and the high interfacial resistance between the SPE and electrode is relatively high. For better performance of solid-state batteries, SPEs should improve the ionic conductivity, strengthen the mechanical properties, and reduce the interfacial resistance between SPE and electrode as well.[2]

In this study, we report highly promising SPEs for all-solid-state LIBs based on a novel cross-linker having poly(ethylene oxide) (PEO) branches with very high ionic conductivity, improved mechanical stability, and good electrochemical stability. In order to reduce the interfacial resistance between SPE and electrode, we introduced a semi-interpenetrating network system through an in-situ polymerization of SPE precursor solution, and a composite electrode that contained an ionic conducting component. In addition, we also introduced Li-ion conducting powders into the SPE to increase ionic conductivity and mechanical strength of the SPE. As prepared SPEs had relatively high ionic conductivities of ca. 5~7 x 10^-4 S/cm at an ambient temperature, good mechanical strengths, and electrochemically stable up to 5.2 V vs. Li/Li⁺. A cell composed of LiFePO₄ composite cathode, Li metal anode, and SPE showed good specific capacity of 140 mAh/g at 0.5 C, cycle stability, and rate capability (95 mAh/g at 2 C). Specially, when the ionic conducting powder introduced into the SPE, Li⁺ transference number rose by 50 %, and thus the cell exhibited improved rate capability and mechanical strength. Finally, all-solid-state pouch-type cells were assembled with the SPEs exhibited stable electrochemical performance, even under a severely bent and folded state. 

[1] A. Ghosh, C.S. Wang, P. Kofinasb, J. Electrochem. Soc. 2010, 157, A846.

[2] K. Xu, Chem. Rev. 2004, 104, 4303.