ABSTRACT
Composite polymer electrolytes (CPEs), consisting of solid electrolyte particles embedded within a solid polymer electrolyte matrix, are promising materials for all-solid-state batteries because of their mechanical properties and scalable production processes. In this study, CPEs consisting of PEO20:LiTFSI blended with 1, 10, and 40 wt % (CPE40) of the Li6PS5Cl electrolyte filler are prepared by a slurry-based process. The incorporation of Li6PS5Cl improves the lithium-ion conductivity from 0.84 mS cm-1 (PEO20:LiTFSI) to 3.6 mS cm-1 (CPE40) at 80 °C. Surface-sensitive X-ray photoelectron spectroscopy (XPS) reveals LiF, polysulfides, and Li3PO4 on the CPE surface, originating from decomposition reactions between PEO20:LiTFSI and Li6PS5Cl. The decomposition products influence the formation of the solid electrolyte interphase (SEI) at the lithium metal | CPE interface, resulting in a reduced SEI resistance of 3.3 Ω cm2 (CPE40) compared to 5.8 Ω cm2 (PEO20:LiTFSI) at 80 °C. The SEI growth follows a parabolic rate law and the growth rate declines from 1.2 Ω cm2 h-0.5 (PEO20:LiTFSI) to 0.57 Ω cm2 h-0.5 (CPE40) during thermal aging at 80 °C. By substituting CPEs for PEO20:LiTFSI in lithium plating and stripping experiments, the increase in SEI resistance was reduced by more than 75%. In order to get a deeper understanding of the SEI formation process, in situ XPS measurements were carried out where the lithium metal is successively deposited on the CPE sample and XPS is measured after each deposition step. On the basis of these measurements, a multistep decomposition mechanism is postulated, including the formation of LiF and Li2S as key components of the SEI.
ABSTRACT
All-solid-state lithium metal batteries using thiophosphate solid electrolytes (SE) present a promising alternative to state-of-the-art lithium-ion batteries due to their potentially superior energy and power. However, reactions occurring at the lithium metal | SE interface result in an increasing internal resistance and limited cycle life. A stable solid polymer electrolyte (SPE) may be used as protective interlayer to prevent the SE from direct contact and reaction with lithium metal. This creates a new and rarely studied heteroionic interface between the inorganic SE and the SPE, which we investigate here. The interface resistance between argyrodite-type Li6PS5Cl and a poly(ethylene oxide)/LiTFSI-based SPE is quantified by four-point electrochemical impedance measurements using two wire-shaped reference electrodes (2.4 Ω cm2 at 80 °C). Two distinct processes are observed and attributed to lithium-ion conduction through a formed solid-polymer electrolyte interphase (SPEI) and an ionic charge-transfer (CT) process. The SPEI predominantly consists of polysulfides and lithium fluoride (LiF), as identified by X-ray photoelectron spectroscopy (XPS) analysis. A temperature-enhanced SPEI growth is observed using electrochemical impedance spectroscopy (EIS) and depth profiling combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results highlight the importance of four-point measurements to determine electrolyte-electrolyte interface properties. Overall, the low resistance and low activation energy of the SPEI makes the SPE interlayer an attractive candidate to protect Li6PS5Cl from decomposition at the lithium metal anode.
ABSTRACT
Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that directly convert chemical energy of fuels into electricity with high efficiency. Because of their fuel flexibility, low emissions, high conversion efficiency, no moving parts, and quiet operation, they are considered as a promising energy conversion technology for low carbon future needs. Solid-state oxide and proton conducting electrolytes play a crucial role in improving the performance and market acceptability of SOFCs. Defect fluorite phases are some of the most promising fast oxide ion conductors for use as electrolytes in SOFCs. We report the synthesis, structure, phase diagram, and high-temperature reactivity of the Sc(2- x)V xO3+δ (0 ≤ x ≤ 2.00) oxide defect model system. For all Sc(2- x)V xO3.0 phases with x ≤ 1.08 phase-pure bixbyite-type structures are found, whereas for x ≥ 1.68 phase-pure corundum structures are reported, with a miscibility gap found for 1.08 < x < 1.68. Structural details obtained from the simultaneous Rietveld refinements using powder neutron and X-ray diffraction data are reported for the bixbyite phases, demonstrating a slight V3+ preference toward the 8b site. In situ X-ray diffraction experiments were used to explore the oxidation of the Sc(2- x)V xO3.0 phases. In all cases ScVO4 was found as a final product, accompanied by Sc2O3 for x < 1.0 and V2O5 when x > 1.0; however, the oxidative pathway varied greatly throughout the series. Comments are made on different synthesis strategies, including the effect on crystallinity, reaction times, rate-limiting steps, and reaction pathways. This work provides insight into the mechanisms of solid-state reactions and strategic guidelines for targeted materials synthesis.
