Dissecting the Solid Polymer Electrolyte–Electrode Interface in the Vicinity of Electrochemical Stability Limits

Abstract

Proper understanding of solid polymer electrolyte–electrode interfacial layer formation and its implications on cell performance is a vital step toward realizing practical solid-state lithium-ion batteries. At the same time, probing these solid–solid interfaces is extremely challenging as they are buried within the electrochemical system, thereby efficiently evading exposure to surface-sensitive spectroscopic methods. Still, the probing of interfacial degradation layers is essential to render an accurate picture of the behavior of these materials in the vicinity of their electrochemical stability limits and to complement the incomplete picture gained from electrochemical assessments. In this work, we address this issue in conjunction with presenting a thorough evaluation of the electrochemical stability window of the solid polymer electrolyte poly­(ε-caprolactone):lithium bis­(trifluoromethanesulfonyl)­imide (PCL:LiTFSI). According to staircase voltammetry, the electrochemical stability window of the polyester-based electrolyte was found to span from 1.5 to 4 V vs Li+/Li. Subsequent decomposition of PCL:LiTFSI outside of the stability window led to a buildup of carbonaceous, lithium oxide and salt-derived species at the electrode–electrolyte interface, identified using postmortem spectroscopic analysis. These species formed highly resistive interphase layers, acting as major bottlenecks in the SPE system. Resistance and thickness values of these layers at different potentials were then estimated based on the impedance response between a lithium iron phosphate reference electrode and carbon-coated working electrodes. Importantly, it is only through the combination of electrochemistry and photoelectron spectroscopy that the full extent of the electrochemical performance at the limits of electrochemical stability can be reliably and accurately determined.

Degradation of Styrene-Poly(ethylene oxide)-Based Block Copolymer Electrolytes at the Na and K Negative Electrode Studied by Microcalorimetry and Impedance Spectroscopy

“…This, however, is very challenging to show in SPE-based batteries, since removal of the electrolyte phase the way it is commonly done for liquid electrolytes typically leads to destruction of the electrode-electrolyte interface. 61 At the same time preparation of cross-sections for instance by sputter techniques can readily induce irreversible changes.…”

Polymer-in-Salt Electrolyte from Poly(caprolactone)-graft-polyrotaxane for Ni-Rich Lithium Metal Batteries at Room Temperature

“…In addition to the metallic lithium interfacial stability, the PGPE/NCM811 interfacial stability is also critical for the fluent Li + flux, ultimately affecting the solid-state battery’s lifespan. However, improving the compatibility between the SPE and the high-voltage cathode active materials remains a significant challenge, as several factors, including the (electro)­chemical stability of SPEs, the characteristics of the cathodic active materials, etc., all have a considerable impact on this. − To gain a better understanding of the high-voltage tolerance of PGPE, electron microscopy observations are performed on the NCM811-based cathode surface after cycling. Figure a depicts the scanning electron microscopy (SEM) image of the cathode electrode surface prior to cycling, while Figure b shows a transmission electron microscopy (TEM) image of the NCM811 particle prior to cycling.…”

Solid-state polymer electrolytes in lithium batteries: latest progress and perspective

“…8f ). 127 To study the interfacial contact, Chen et al developed a poly (ether-ester)-based poly( pyrrolidone chloride) (PDCL-SPE) with adjustable composition and structure through direct bulk of cyclic monomers CL and poly(1,5-dioxepan-2-one) (PDXO). The amorphous structure, lower glass transition temperature, as well as synergistic effects from the ether and carbonyl groups facilitated the dissociation of lithium salts and transport of lithium ions even after long-term cycling (600 h).…”