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

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.

Automated summary

It has been reported that the oxidative stability of a prototypical polymer electrolyte can reach up to 4.6 V versus Li/Li+, but the onset of PEO oxidation actually occurs at a much lower voltage of around 3.2 V. At 3.6 V, the chain of the PEO is oxidized, resulting in the formation of the strong acid HTFSI which can chemically attack the PEO to form methanol and 2-methoxyethanol. Cycling a solid-state lithium-metal battery with a high-energy positive electrode showed enhanced capacity fading when the upper cut-off voltage was increased beyond 3.6 V versus Li/Li+, highlighting the need for protective layers at the electrode-electrolyte interface.