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.

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.

Hybrid battery cells combining liquid electrolytes (LEs) with inorganic solid electrolyte (SE) separators or different SEs and polymer electrolytes (PEs), respectively, are developed to solve the issues of single-electrolyte cells. Among the issues that can be solved are detrimental shuttle effects, decomposition reactions between the electrolyte and the electrodes, and dendrite propagation. However, the introduction of new interfaces by contacting different ionic conductors leads to other problems, which cannot be neglected before commercialization is possible. The interfaces between the different types of ionic conductors (LE/SE and PE/SE) often result in significant charge-transfer resistances, which increase the internal resistance considerably. This review highlights studies evaluating the interfacial resistances and activation barriers in such systems to present an overview of the issues still hampering hybrid battery systems. The interfaces between different SEs in hybrid all-solid-state batteries (SSBs) are considered as well. In addition, a short summary of physicochemical models describing heteroionic interfaces-interfaces between two different ion conductors-is given in an attempt to explain high interface resistances. In doing so, we hope to inspire future work on the crucial topic of interface optimization toward better SSBs.

Solid-state batteries have gained increasing attention with the discovery of new inorganic solid electrolytes, some of which rival the ionic conductivity of liquid electrolytes. With the additional benefit of being...

Electrochemical impedance spectroscopy (EIS) is a powerful technique to study interface kinetics in lithium ion batteries. In order to separate the contributions of the working and counter electrode, a reference electrode (RE) is crucial. However, size and shape of the RE strongly influence the quality of the measurement data. In this study we define two criteria that enable the theoretical quality assessment of a wire-shaped RE as a function of its diameter. With help of a simplified Newman-type battery model we show that small wire diameters are essential in order to obtain meaningful EIS data, in particular when using polymer electrolytes, which have comparably low ionic conductivity. Therefore, a gold plated tungsten wire with 10 μm diameter is chosen as preferred RE. It is placed between two layers of electrolyte and lithiated before use to ensure a long-term stable reference potential. Using this setup, artifact-free EIS spectra are obtained for lithium/lithium and lithium/LFP cells with polymer electrolyte. Finally, two applications are shown for which a RE is indispensable: (I) the symmetry of anodic and cathodic lithium dissolution and deposition kinetics is characterized; (II) the growth of the anode and cathode impedance of a lithium/LFP cell during cycling is monitored.

Lithium-ion batteries (LIBs) play a crucial role in today’s consumer goods and transportation market due to their superior ability to store energy. While the energy and power density has been strongly increased since its commercialization in 1991, LIBs will soon reach a limit, which cannot be overcome with conventional liquid electrolyte based systems [1]. However, solid-state batteries (SSB) based on solid electrolytes (SE) offer the possibility to further enhance the energy and power density [2]. Sulfide-based SEs show superior conductivities of several mS/cm at room temperature. Their ductile nature allows for cold pressing and results in good electrode contacting and hence lower interfacial resistance compared to oxide-based SEs [3]. However, sulfide-based SEs are not stable against lithium and the formation of an interphase has been shown in theory and experiment. The degradation of argyrodite-type Li6PS5X (X = Cl, Br, I) in contact with lithium has been investigated by Wenzel et al. using in-situ XPS. They reported Li3P, Li2S and LiX as decomposition products, which is in accordance to a thermodynamic analysis based on first-principle calculations carried out by Zhu et al. [4,5]. A promising approach to prevent the lithium/Li6PS5X interface from decomposition is the introduction of an interlayer. While many examples about sputtering thin interlayers onto the SE or lithium have been reported, only few reports can be found about thin polymer interlayers. However, the latter could be more attractive for commercial applications as they can be applied by less expensive techniques than sputtering. By the introduction of a polymer interlayer between lithium and Li6PS5X, an extra electrolyte/electrolyte interface in addition to the lithium/electrolyte interface is formed. While the lithium/electrolyte interface has already been intensively studied for several electrolytes, only little is known about the electrolyte/electrolyte interface especially in case of the polymer/solid electrolyte interfaces. In previous studies, the specific resistances for the La0.55Li0.35TiO3\PEO20:LiCF3SO3 (2200 Ωcm2, 80°C) [6], the Ohara\PEO16:LiCF3SO3 (32 Ωcm2, 80°C) [7], the Ohara\PEO10:LiTFSI (47 Ωcm2, 40°C) [7], and the Li7La3Zr2O12/PEO20:LiClO4 (9000 Ωcm2, 70°C) [8] interfaces have been reported. However, these studies use oxide-based SEs while to the best of our knowledge no comprehensive study on the interface between polymer electrolyte and sulfide-based SE exists. In the present study, we electrochemically and analytically investigate the interface between argyrodite-type Li6PS5Cl and poly(ethylene oxide): lithium bis(trifluoromethanesulfonyl)imide (PEO:LiTFSI). The PEO:LiTFSI electrolyte is prepared solvent-free in order to exclude any solvent-related decomposition reactions at the Li6PS5Cl/PEO:LiTFSI interface. In the first part, the Li6PS5Cl/PEO:LiTFSI interface is studied via electrochemical impedance spectroscopy (EIS). Two separated processes, which occur at different characteristic frequencies, are identified using an in-house developed four-point measurement setup (Figure 1a). The development of the cell impedance is monitored over 10 days at 80°C and reveals the rise of a medium-frequency process over time. In order to study the interface analytically, the polymer is removed and X-ray Photoelectron Spectroscopy (XPS) measurements are carried out after different ageing times. A clear evolution of decomposition products at the Li6PS5Cl/PEO:LiTFSI interface is observed (Figure 1b). These observations are confirmed via Time-of-Flight secondary ion mass spectrometry (ToF-SIMS). Possible decomposition pathways are discussed. References [1] J. Janek, P. Adelhelm, in: Lithium-Ion Batteries: Basics and Applications (ed. R. Korthauer), Springer, pp. 187–207 (2018). [2] A. L. Robinson, J. Janek, MRS Bull., 39(12), 1046 (2014). [3] J. Janek, W.G. Zeier, Nat. Energy, 1(9), 1167 (2016). [4] Y. Zhu, X. He, Y. Mo, ACS Appl. Mater. Interfaces, 7(42), 23685 (2015). [5] S. Wenzel, S.J. Sedlmaier, C. Dietrich, W.G. Zeier, J. Janek, Solid State Ionics, 318, 102 (2018). [6] T. Abe, M. Ohtsuka, F. Sagane, Y. Iriyama, Z. Ogumi, J. Electrochem. Soc., 151(11), A1950 (2004). [7] W.E. Tenhaeff, K.A. Perry, N.J. Dudney, J. Electrochem. Soc., 159(12), A2118 (2012). [8] F. Langer, M.S. Palagonia, I. Bardenhagen, J. Glenneberg, F. La Mantia, R. Kun, J. Electrochem. Soc., 164(12), A2298 (2017). Figure 1

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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 ScV O (0 ≤ x ≤ 2.00) oxide defect model system. For all ScV O 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 V preference toward the 8b site. In situ X-ray diffraction experiments were used to explore the oxidation of the ScV O phases. In all cases ScVO was found as a final product, accompanied by ScO for x < 1.0 and VO 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.