Researchers from the University of Western Ontario (UWO), the University of Toronto and Glabat Solid-State Battery Inc. have proposed a novel strategy to extend the electrochemical stability window (ESW) of PEO-based solid polymer electrolytes (SPEs) based on their new understanding of the ESW limiting factor. A paper on their work is published in the RSC journal Energy & Environmental Sciences.

Schematic of PEGDME solid polymer electrolyte with improved Li stability and extended electrochemical stability windows in Li-LiNi_0.5Mn_0.3Co_0.2O₂ pouch cells. Yang et al.

Poly (ethylene oxide) (PEO)-based SPEs has demonstrated its promising electrochemical performance in Li-LiFePO₄ batteries due to its high flexibility and relatively high ionic conductivity at operating temperatures. However, the narrow ESW limits their combination with high-voltage cathodes and the limiting factor remains unknown.

Here, poly (ethylene glycol) (PEG) and poly (ethylene glycol) dimethyl ether ‎(PEGDME) with different terminal groups are selected to answer this question. We found that the reactive terminal –OH group is the limiting factor towards high-voltage and Li anode. Replacing the –OH with more stable –OCH₃ extended the ESW from 4.05 to 4.3 V as well as improved the Li-anode compatibility (Li-Li symmetric cells stably run for 2500 h at 0.2 mA cm^-2).

Its practical application is further proved by the PEGDME-based pouch cells. The 0.53 mA cm^-2 Li-LiFePO⁴ and 0.47 mAh cm^-2 Li-LiNi_0.5Mn_0.3Co_0.2O₂ ASSLB pouch cells demonstrated high capacity retention of 97% and 90% after 210 cycles and 110 cycles, respectively.

—Yang et al.

To achieve higher energy density, high-voltage all-solid-state lithium batteries (ASSLBs) have attracted increasing attention, which requires the solid-state electrolytes (SSEs) with wide electrochemical stability windows (ESW, typically >4.2 V) and high-stability against Li anode.

Nevertheless, poly (ethylene oxide) (PEO), the most widely used solid polymer electrolyte (SPE), can’t tolerate a high-voltage over 4 V. Either the main chain (-C-O-C-) or the terminal hydroxide group (-OH) is the limiting factor for the narrow ESW remaining unknown.

Prof. Xueliang (Andy) Sun’s team from Western University and Prof. Chandra Veer Singh from University of Toronto clarified that the terminal -OH group is the limiting factor via ration experimental design.

Replacing the unstable terminal -OH with more stable groups (e g. -OCH₃) has been demonstrated to be effective to extend the ESW of PEO-based SPEs.

The SPE with -OCH₃ shows improved Li stability and high oxidization resistance, enabling the assembled high-voltage ASSLBs with excellent cycling stability.

Resources

• Xiaofei Yang, Chandra Veer Singh, Xueliang Sun, et al. (2020) “Determining the limiting factor of the electrochemical stability window for PEO-based solid polymer electrolytes: main chain or terminal –OH group?” Energy & Environmental Sciences doi: 10.1039/D0EE00342E