Unraveling the Voltage‐Dependent Oxidation Mechanisms of Poly(Ethylene Oxide)‐Based Solid Electrolytes for Solid‐State Batteries

• Published in: Advanced materials interfaces Vol. 9; no. 8; pp. 2100704 - n/a
• Main Authors: Seidl, Lukas, Grissa, Rabeb, Zhang, Leiting, Trabesinger, Sigita, Battaglia, Corsin
• Format: Journal Article
• Language: English
• Published: Weinheim John Wiley & Sons, Inc 01.03.2022
• Subjects: Cycles; Electric potential; Electrochemical oxidation; electrochemical stability; Electrolytes; Ethylene oxide; Ions; Lithium; Li‐metal batteries; Molten salt electrolytes; NMC electrodes; Oxidation; PEO oxidation mechanism; poly(ethylene oxide) (PEO); Polyethylene oxide; Solid electrolytes; solid‐state batteries; Spectrometry; Voltage
• ISSN: 2196-7350; 2196-7350
• DOI: 10.1002/admi.202100704

Using galvanostatic techniques, an oxidative stability up to 4.6 V versus Li/Li+ and beyond has been reported for the prototypical polymer electrolyte consisting of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in poly(ethylene oxide) (PEO). However, no long‐term cycling of a battery with this high cut‐off voltage has been demonstrated. Electrochemical and spectroscopic/spectrometric methods are employed to critically reinvestigate the electrochemical oxidation mechanisms of PEO electrolytes. It is found that the onset of PEO oxidation occurs at much lower voltage of around 3.2 V versus Li/Li+, at which the terminal OH group is deprotonated. At 3.6 V, the chain of the PEO is oxidized. Both processes result in the formation of the strong acid HTFSI, which in turn chemically attacks the PEO to form methanol and 2‐methoxyethanol. A stable cycling of a solid‐state lithium‐metal battery with a high‐energy LiNi0.8Mn0.1Co0.1O2 (NMC811) posititve electrode to an upper cut‐off voltage of 3.6 V versus Li/Li+ is demonstrated, however, resulting in enhanced capacity fading when increasing the upper cut‐off voltage to 3.8 V versus Li/Li+ or higher. Thus, operating PEO electrolytes beyond 3.6 V versus Li/Li+ requires protective layers at the positive electrode‐electrolyte interface to prevent PEO oxidation. The electrochemical oxidation of poly(ethylene oxide) (PEO)‐based solid electrolytes for Li‐metal batteries is triggered at around 3.2 V, where the terminal OH‐group is oxidized. Around 3.6 V, the polymeric ether‐chain is deprotonated, resulting in the formation of the strong bis(trifluoromethansulfonyl)imide acid (HTFSI). With the breakdown of the ether‐chain, the functionality of the PEO as electrolyte is lost.