To significantly increase the energy density of lithium-based batteries, the use of lithium metal as an anode is an option despite all of the associated challenges. Due to its high reactivity, lithium is covered with a passivation layer that may affect cell performance and reproducibility of electrochemical characterization. In most studies, this is ignored and lithium metal is used without considering the passivation layer and carrying out a proper characterization of the surface. Against this background, we systematically characterized various lithium samples with X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and complementary energy-dispersive X-ray spectroscopy (EDX), resulting in a complete three-dimensional chemical picture of the surface passivation layer. On all analyzed lithium samples, our measurements indicate a nanometer-thick inorganic passivation layer consisting of an outer lithium hydroxide and carbonate layer and an inner lithium oxide-rich region. The specific thickness and composition of the passivation layer depend on the treatment before use and the storage and transport conditions. Besides, we offer guidelines for experimental design and data interpretation to ensure reliable and comparable experimental conditions and results. Lithium plating through electron beam exposure on electrically contacted samples, the reactivity of freshly formed lithium metal even under ultrahigh-vacuum (UHV) conditions, and the decomposition of lithium compounds by argon sputtering are identified as serious pitfalls for reliable lithium surface characterization.

To overcome current challenges of lithium metal anodes (LMAs), which hinder their wide industrial application, the chemical composition of the lithium metal surface is an important factor. Due to its high reactivity and depending on the pre-treatment during processing, lithium is covered with a passivation layer composed of mainly Li 2 CO 3 , LiOH, and Li 2 O, what is mostly neglected in later electrochemical studies. Here, we investigate the effect of storage time and conditions on the surface passivation layer of commercial lithium foils, based on lithium surface characterization with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, finding that only sealed pouch bags can prevent lithium surface changes effectively. Otherwise, the passivation layer thickness increases steadily, even in gloveboxes with a low degree of contaminations. Testing the stored lithium foils in solid-state batteries with LLZO as model solid electrolyte, it is demonstrated that the solid electrolyte roughness and the applied pressure have a huge impact on the obtained impedance. While the passivation layer has no major effect on the interface resistance with a rough LLZO pellet and at high pressure, it clearly affects the interface resistance with smoother LLZO surfaces and at lower pressure. Consequently, the lithium passivation layer may hinder the application of the LMA in a solid-state battery what we discuss in depth. By reactivity experiments with model lithium surfaces, we show that water residuals are the main reason for the aging of lithium foil in gloveboxes. Additionally, nitrogen reacts with fresh lithium surfaces and lithium foils with an incomplete or damaged passivation layer. The results demonstrate that storage conditions are important factors for the surface state of lithium metal and consequently for the application as an anode material.

The physicochemical properties of thin metal oxide layers strongly depend on the layer thickness and thus differ significantly from their bulk counterpart. In this work, we present the growth of defined thin layers of CeO 2 within mesostructured ZrO 2 thin films using atomic layer deposition (ALD). The prepared films consist of a cubic ordered arrangement of 15 nm spherical mesopores induced by the used diblock copolymer poly(isobutylene)-block-poly(ethylene oxide) (PIB 50 -b-PEO 45 ), which allows studying the growth process and the successful coating of the interior pore surfaces via the combination of scanning electron microscopy (SEM), time-of-flight mass spectrometry (ToF-SIMS), and laser ellipsometry. These methods prove the CeO 2 layer growth and impregnation of the pores up to 100 ALD cycles, at which the interconnecting channels between the mesopore layers are filled completely impeding further transport of the gaseous CeO 2 precursors. X-ray photoelectron spectroscopy (XPS) and diffractometry (XRD) measurements point out the increased amount of Ce 3+ after a low number of ALD cycles and show the presence of cubic CeO 2 with increasing amount of ALD cycles, respectively. Impedance spectroscopic investigation further proves the formation of a continuous CeO 2 path through the entire porous network of the insulating ZrO 2 film and shows a strong influence of the layer thickness on the conductivity. All in all, our work presents the preparation of novel hybrid CeO 2 /ZrO 2 model systems, which enable us to tailor their physicochemical properties by changing the thickness of the active oxide layer, and promises improvements for their use as catalysts in oxidation reactions such as the HCl oxidation reaction or as a threeway catalytic converter in automotives.

Organic/inorganic interfaces greatly affect Li + transport in composite solid electrolytes (SEs), while SE/ electrode interfacial stability plays a critical role in the cycling performance of solid-state batteries (SSBs). However, incomplete understanding of interfacial (in)stability hinders the practical application of composite SEs in SSBs. Herein, chemical degradation between Li 6 PS 5 Cl (LPSCl) and poly(ethylene glycol) (PEG) is revealed. The high polarity of PEG changes the electronic state and structural bonding of the PS 4 3À tetrahedra, thus triggering a series of side reactions. A substituted terminal group of PEG not only stabilizes the inner interfaces but also extends the electrochemical window of the composite SE. Moreover, a LiF-rich layer can effectively prevent side reactions at the Li/SE interface. The results provide insights into the chemical stability of polymer/sulfide composites and demonstrate an interface design to achieve dendrite-free lithium metal batteries.

Capacity loss mechanisms of lithium-ion batteries (LIBs) are intensively studied. In this study, planar LiCoO2 (LCO) thin film electrodes are successfully prepared by pulsed laser deposition and used to explore a diffusion-controlled reversible capacity loss mechanism of layered oxide cathodes. It is found that the reversible capacity loss of LCO originates from asymmetric lithium chemical diffusion kinetics during (de)­lithiation, which is eventually rooted in the intrinsic lithium vacancy dependent diffusivity in the layered structure of LCO. The understanding of the reversible capacity loss may be highly beneficial for the development of LIBs and other systems based on intercalation compounds.

Cathode active materials (CAMs) in state-of-the-art lithium-ion batteries are mostly lithium-transition-metal oxides such as Li(Ni x Co y Mn z )O 2 (x + y + z = 1). To achieve optimum cycling stability and performance of the cathode, the extent of degradation processes and side reactions between CAMs and liquid or solid electrolytes has to be minimized. For this purpose, various coating strategies for CAMs have been developed in recent years. The underlying mechanism of the protective function of nanoscale coatings and their role for the enhanced cycling performance are mostly unclear, which is often based on incomplete characterization of the coating. Only a few analytical methods, such as X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray analysis, or X-ray photoelectron spectroscopy, have frequently been used in recent years, which often cannot provide enough information for a reliable and consistent picture of the very thin coating. For this reason, we demonstrate a systematic study on the analytical characterization of coated CAM using additional analytical methods. NCM622 coated with TiO 2 by atomic layer deposition is used as a model system and analyzed with SEM/EDX, focused ion beam scanning electron microscopy, scanning transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, low-energy ion scattering, and time-of-flight secondary ion mass spectrometry. The results highlight the advantages and disadvantages of each analytical method for the analysis of typical CAM coatings. The results demonstrate that a combination of the different methods is essential to understand CAM coatings and their properties in full detail.

Organic/inorganic interfaces greatly affect Li + transport in composite solid electrolytes (SEs), while SE/ electrode interfacial stability plays a critical role in the cycling performance of solid-state batteries (SSBs). However, incomplete understanding of interfacial (in)stability hinders the practical application of composite SEs in SSBs. Herein, chemical degradation between Li 6 PS 5 Cl (LPSCl) and poly(ethylene glycol) (PEG) is revealed. The high polarity of PEG changes the electronic state and structural bonding of the PS 4 3À tetrahedra, thus triggering a series of side reactions. A substituted terminal group of PEG not only stabilizes the inner interfaces but also extends the electrochemical window of the composite SE. Moreover, a LiF-rich layer can effectively prevent side reactions at the Li/SE interface. The results provide insights into the chemical stability of polymer/sulfide composites and demonstrate an interface design to achieve dendrite-free lithium metal batteries.

BackgroundSurface functionalization of orthopedic implants with pharmaceutically active agents is a modern approach to enhance osseointegration in systemically altered bone. A local release of strontium, a verified bone building therapeutic agent, at the fracture site would diminish side effects, which could occur otherwise by oral administration. Strontium surface functionalization of specially designed titanium-niobium (Ti-40Nb) implant alloy would provide an advanced implant system that is mechanically adapted to altered bone with the ability to stimulate bone formation.MethodsStrontium-containing coatings were prepared by reactive sputtering of strontium chloride (SrCl2) in a self-constructed capacitively coupled radio frequency (RF) plasma reactor. Film morphology, structure and composition were investigated by scanning electron microscopy (SEM), time of flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscopy (HR-TEM) was used for the investigation of thickness and growth direction of the product layer. TEM lamellae were prepared using the focused ion beam (FIB) technique. Bioactivity of the surface coatings was tested by cultivation of primary human osteoblasts and subsequent analysis of cell morphology, viability, proliferation and differentiation. The results are correlated with the amount of strontium that is released from the coating in biomedical buffer solution, quantified by inductively coupled plasma mass spectrometry (ICP-MS).ResultsDense coatings, consisting of SrOxCly, of more than 100 nm thickness and columnar structure, were prepared. TEM images of cross sections clearly show an incoherent but well-structured interface between coating and substrate without any cracks. Sr2+ is released from the SrOxCly coating into physiological solution as proven by ICP-MS analysis. Cell culture studies showed excellent biocompatibility of the functionalized alloy.ConclusionsTi-40Nb alloy, a potential orthopedic implant material for osteoporosis patients, could be successfully plasma coated with a dense SrOxCly film. The material performed well in in vitro tests. Nevertheless, the Sr2+ release must be optimized in future work to meet the requirements of an effective drug delivery system.Electronic supplementary materialThe online version of this article (10.1186/s40824-017-0104-8) contains supplementary material, which is available to authorized users.