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

Abstract

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.

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“…[95,106] Nonetheless, a recent study reported an increase in the oxidative stability from 4.05 to 4.3 V versus Li/Li + , for PEG and PEGDME-based electrolytes, respectively. [107] Accordingly, a higher reactivity of the terminal hydroxyls would be in agreement with the early oxidation of hydroxyls in PEO reported by Seidl et al [105] Another recent study reported higher oxidative stability for a cross-linked polymer electrolyte containing free hydroxyls compared to PEG, which was caused by the restricted mobility and reactivity of the hydroxyls in the cross-linked electrolyte. [108] The diverging results reported in the recent literature or their conflicting interpretation indicate that the evaluation of the absolute stability limit of PEO-based electrolytes is still challenging.…”

“…[ 107 ] Accordingly, a higher reactivity of the terminal hydroxyls would be in agreement with the early oxidation of hydroxyls in PEO reported by Seidl et al. [ 105 ] Another recent study reported higher oxidative stability for a cross‐linked polymer electrolyte containing free hydroxyls compared to PEG, which was caused by the restricted mobility and reactivity of the hydroxyls in the cross‐linked electrolyte. [ 108 ]…”

“…This finding was further challenged by Seidl et al, [ 105 ] who coupled electrochemical and spectroscopic methods, and identified the onset for PEO/LiTFSI degradation at much lower voltages, that is, below 4 V versus Li/Li + . In particular, they detected that the deprotonation of PEO terminal OH groups initiates at 3.2 V, which, resulted in the formation of the strong acid HTFSI that further attacks the PEO ether main chain.…”

“…Consequently, according to the authors, polyether-based electrolytes could be used with high-voltage cathodes, up to at least 4.5 V, by hindering dendrites growth, for example, using mechanically robust crosslinked polymer electrolytes. [103,104] This finding was further challenged by Seidl et al, [105] who coupled electrochemical and spectroscopic methods, and identified the onset for PEO/LiTFSI degradation at much lower voltages, that is, below 4 V versus Li/Li + . In particular, they detected that the deprotonation of PEO terminal OH groups initiates at 3.2 V, which, resulted in the formation of the strong acid HTFSI that further attacks the PEO ether main chain.…”

“…Moreover, the LiCoO 2 electrode cycled in combination with the SPE yielded a surface catalytic effect, resulting in H 2 evolution at only 4.2 V. This was mitigated by employing a LATP surface coating of LiCoO 2 , which provided stability to the system up to 4.6 V. [111] Seidl et al demonstrated H 2 evolution from LiTFSI in PEO when in contact with Li metal, and the evolution of species such as methanol and 2-methoxyethanol during a voltage ramp to 5 V versus Li/Li + . [105] Such decomposition and gas evolution can be circumvented by a sandwich-structured HSE, as demonstrated by Li et al [217] The study employed a PAN-based electrolyte on the cathode side and a PEO-based electrolyte on the anode (Li metal) side. The LLTO nanofibers were used as an inorganic filler in each phase and as a polymer-in-ceramic center layer of the sandwich.…”