Going Beyond Sweep Voltammetry: Alternative Approaches in Search of the Elusive Electrochemical Stability of Polymer Electrolytes

Abstract

Solid polymer electrolytes (SPEs) are promising candidates for solid-state lithium-ion batteries. Potentially, they can be used with lithium metal anodes and high-voltage cathodes, provided that their electrochemical stability is sufficient. Thus far, the oxidative stability has largely been asserted based on results obtained with sweep voltammetry, which are often determined and reliant on arbitrary assessments that are highly dependent on the experimental conditions and do not take the interaction between the electrolyte and the electrode material into account. In this study, alternative techniques are introduced to address the pitfalls of sweep voltammetry for determining the oxidative stability of SPEs. Staircase voltammetry involves static conditions and eliminates the kinetic aspects of sweep voltammetry, and coupled with impedance spectroscopy provides information of changes in resistance and interphase layer formation. Synthetic charge-discharge profile voltammetry applies the real voltage profile of the active material of interest. The added effect of the electrode active material is investigated with a cutoff increase cell cycling method where the upper cutoff voltage during galvanostatic cycling is gradually increased. The feasibility of these techniques has been tested with both poly(ethylene oxide) and poly(trimethylene carbonate) combined with LiTFSI, thereby showing the applicability for several categories of SPEs. Supplementary material for this article is available online Next-generation high-energy-density batteries with lithium metal anodes and high-voltage cathode materials will require solid electrolytes that are more stable and safer than their liquid counterparts. In particular, solid polymer electrolytes (SPEs) constitute promising alternatives because of their wettability and adhesion to the electrodes, reduced flammability, scalability in production and being potentially low-cost. 1,2 Compatibility with lithium metal anodes and an inherent ability to resist dendrite growth are often cited as motivational factors and have been widely studied for SPE-based batteries. 3-6 Improved electrochemical stability compared to liquid electrolytes is also commonly cited as a motivation for solid-state electrolytes, but it is not clear whether this criterion is actually met. This ultimately comes down to a combination of clearly defining the electrochemical stability and finding suitable techniques to estimate this property correctly. It could be argued that there exists quite some confusion in the literature regarding the electrochemical stability limits of electro-lytes. A common misconception is, for example, the perceived equivalence between the anodic (oxidation) and cathodic (reduction) stability limits, and the HOMO and LUMO levels of the electrolyte. 7 While the HOMO and LUMO are molecular properties that may be obtained from the electronic structures of isolated molecules, redox potentials are thermodynamic properties that are dependent not only on the reacting molecules, but also their surroundings, concentration and the products formed in the reaction. This may lead to impossibly large electrochemical stability windows (ESWs) being suggested from, e.g., HOMO and LUMO levels obtained from density functional theory (DFT) calculations. 2,8 Moreover, the solvent and salt in conjunction must be taken into consideration as the electrolyte solution has its own distinct redox properties that are different from either of the individual components. 9