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Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal−sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li−S and the emerging Na−S and K−S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li−S are relatively mature, less progress has been made with Na−S and K−S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal−selenium and the metal−selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal−sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li-and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li−S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure−battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Metal oxide nanoparticles (MONPs) have widespread usage across many disciplines, but monitoring molecular processes at their surfaces in situ has not been possible. Here we demonstrate that MONPs give highly enhanced (×10 4 ) Raman scattering signals from molecules at the interface permitting direct monitoring of their reactions, when placed on top of flat metallic surfaces. Experiments with different metal oxide materials and molecules indicate that the enhancement is generic and operates at the single nanoparticle level. Simulations confirm that the amplification is principally electromagnetic and is a result of optical modulation of the underlying plasmonic metallic surface by MONPs, which act as scattering antennae and couple light into the confined region sandwiched by the underlying surface. Because of additional functionalities of metal oxides as magnetic, photoelectrochemical and catalytic materials, enhanced Raman scattering mediated by MONPs opens up significant opportunities in fundamental science, allowing direct tracking and understanding of application-specific transformations at such interfaces. We show a first example by monitoring the MONPassisted photocatalytic decomposition reaction of an organic dye by individual nanoparticles. KEYWORDS: Metal oxide, plasmons, surface-enhanced Raman scattering, photocatalysis, interface T ransition-metal oxides, due to the strong correlations of their d electrons, give rise to a wide variety of phenomena such as magnetism, ionic conduction, metal−insulator transitions, multiferroicity, and superconductivity. 1 As a result, they have an extensive range of applications that include fuel cells, batteries, catalysts, sensors, and microelectronics. 1 Despite the resulting importance of molecular binding and surface reactivity, their utilization in plasmonic applications has been prevented by the tuning of their localized surface plasmon resonance (LSPR) into the infrared. 2−6 Surface-enhanced Raman scattering (SERS) is a popular plasmonic application utilizing ultraviolet (UV), visible (VIS), or near-infrared (NIR) excitation, which overcomes the extremely small scattering cross section (∼10 −30 cm 2 per molecule) in conventional Raman scattering 7 to yield a technique that offers noninvasive and nondestructive fingerprint characterization 8 with extensive applications in chemical and biological sensing. The amplification in SERS stems primarily from the electromagnetic (EM) enhancement (up to 10 14 ) 9 obtained by excitation of SPR. 10 This is accompanied by typically smaller and system-dependent chemical enhancement as a result of formation of chargetransfer complexes between adsorbate and the surface. 11 Therefore, for efficient and sensitive SERS detection of molecules, nanoscale structures fabricated entirely with coinage metals (especially Ag and Au) have been the materials of choice since their SPR is easily excited in the vis or NIR regions. On the other hand, use of metal oxide nanoscale materials for enhanced Raman scattering has remained confi...

We simulate and discuss the local electric-field enhancement in a system of a dielectric nanoparticle placed very near to a metallic substrate. We use finite-element numerical simulations in order to understand the field-enhancement mechanism in this dielectric NP-on-mirror system. Under appropriate excitation conditions, the gap between the particle and the substrate becomes a "hot spot", i.e., a region of intense electromagnetic field. We also show how the optical properties of the dielectric NP placed on a metallic substrate affect the plasmonic field enhancement in the nanogap and characterize the confinement in the gap. Our study helps to understand and design systems with dielectric NPs on metallic substrates which can be equally as effective for SERS, fluorescence, and nonlinear phenomena as conventional all-metal plasmonic structures.

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