A Strategy towards Light-Absorbing Coatings Based on Optically Black Nanoporous Alumina with Tailored Disorder

This work provides a conceptually new way of thinking about the light-absorbing mechanism in additive-free black porous anodic alumina (black PAA, or b-PAA) layers obtained via “burning” anodizing regime. The new insight into the controllable photonic effects in PAA allows the implementation of the optical blackening method based on the deliberate randomization of the initially well-ordered nanopore arrangement. The proposed black coloration mechanism rests solely on the destructive interference of light after its multiple scattering. Similar effects have been earlier considered for some natural or artificially created biomimetic structures (e.g., the so-called “moth eye effect”, or the coloration mechanism in the Neurothemis tullia dragonfly wings). Comprehensive analysis confirmed that the chemical composition of b-PAA has only a minor influence on the color changes and the optical density increase, and that the light-absorbing properties most likely result from the structural effects. The new functional 2D materials exhibit strong adhesion to aluminum surface, are cost-effective and suitable for application under harsh thermal or UV-light conditions. They are potentially useful for manufacturing of optical devices or heat-resistant coatings in aerospace technologies, as well as solid supports for biological filtration and fluorescence imaging

High-temperature structure of Co3O4: Understanding spinel inversion using in situ and ex situ measurements

International audienc

Synthetic Tailoring of Ionic Conductivity in Multicationic Substituted, High‐Entropy Lithium Argyrodite Solid Electrolytes

Superionic conductors are key components of solid‐state batteries (SSBs). Multicomponent or high‐entropy materials, offering a vast compositional space for tailoring properties, have recently attracted attention as novel solid electrolytes (SEs). However, the influence of synthetic parameters on ionic conductivity in compositionally complex SEs has not yet been investigated. Herein, the effect of cooling rate after high‐temperature annealing on charge transport in the multicationic substituted lithium argyrodite Li₆.₅[P₀.₂₅Si₀.₂₅Ge₀.₂₅Sb₀.₂₅]S₅I is reported. It is demonstrated that a room‐temperature ionic conductivity of ∼12 mS cm⁻¹ can be achieved upon cooling at a moderate rate, superior to that of fast‐ and slow‐cooled samples. To rationalize the findings, the material is probed using powder diffraction, nuclear magnetic resonance and X‐ray photoelectron spectroscopy combined with electrochemical methods. In the case of moderate cooling rate, favorable structural (bulk) and compositional (surface) characteristics for lithium diffusion evolve. Li₆.₅[P₀.₂₅Si₀.₂₅Ge₀.₂₅Sb₀.₂₅]S₅I is also electrochemically tested in pellet‐type SSBs with a layered Ni‐rich oxide cathode. Although delivering larger specific capacities than Li₆PS₅Cl‐based cells at high current rates, the lower (electro)chemical stability of the high‐entropy Li‐ion conductor led to pronounced capacity fading. The research data indicate that subtle changes in bulk structure and surface composition strongly affect the electrical conductivity of high‐entropy lithium argyrodites

LiMO2 (M = Ni, Co) thin film cathode materials: A correlation between the valence state of transition metals and the electrochemical properties

The electronic properties of the LiMO2 (M = Ni, Co) thin film cathode materials grown by RF sputtering/co-sputtering are in situ studied by X-ray photoelectron spectroscopy (XPS). Stoichiometric Li1.0Co1.0O2 thin films deposited on a heated substrate at T = 500–550 °C reveal the Co3+ (t2g6eg0) ground state configuration in the low spin (LS) state. Stoichiometry of the Lix(Ni,Co)O2 films and the valence and spin states of the Ni ions depend strongly on the growth conditions. The electronic configuration of stoichiometric Li1.0Ni0.5Co0.5O2 is described as the Ni3+ (t2g6eg1) LS and Co3+ (t2g6eg0) LS states. The Li-deficient Lix<1.0(Ni,Co)O2 exhibits Ni2+ (t2g6eg2) in the high spin (HS) and Co3+ (t2g6eg0) in LS states. The reduction of the trivalent Ni ions to Ni2+ (t2g6eg2) with a HS state electronic configuration is related to the evaporation of Li2O at elevated substrate temperatures coupled to a loss of O2 due to an internal oxidation reaction of O2− lattice ions induced by the strongly oxidizing Ni3+ ions. Owing to the stable Co3+ (t2g6eg0) with a LS state electronic configuration, Li1.0Co1.0O2 thin films cycled to 4.2 V exhibit a very good electrochemical reversibility. Li1.0Ni0.5Co0.5O2 films annealed at the same temperature as for Li1.0Co1.0O2 manifest a broadening of the oxidation/reduction peaks of the cyclic voltammogram (CV) curves with a strong current drop after the first step of the electrochemical Li-deintercalation. The observed irreversibility of the Li-intercalation/deintercalation process is attributed to instability of the Ni3+ (t2g6eg1) ions. Temperatures of the deposition/annealing above 750 °C lead to the phase separation of the Lix(Ni,Co)O2 films, a strong Li deficiency, the occurrence of Co2+ (t2g5eg2) with HS ions and consequently a complete degeneration of the electrochemical cyclability

Preparation and characterization of carbon foams–LiCoPO4 composites

The preparation and characterization of composites consisting of carbon foams coated with olivine structured lithium cobalt phosphate is reported. The composites are prepared by a Pechini assisted sol–gel process and treated under different conditions in flowing nitrogen and in flowing air. The structural, morphological and electrochemical properties were found to be dependent upon the annealing conditions as time and atmosphere. After annealing in nitrogen, the formation of the LiCoPO 4 phase on the foams is observed at T = 730 ◦ C. The photoelectron emission spectra reveal divalent cobalt ions in the composites. The LiCoPO 4 /foam composites deliver a discharge specific capacity of 100 mAh g – 1 , at a discharge rate of C/25 and room temperature. After a pre-annealing in air then the annealing in nitrogen, the voltammetric curves show values of the reduction/oxidation processes at 4.6 V and 5.2 V respectively. The electrochemical measurements revealed a decrease of the capacity fade ( ∼ 26% at C/10, RT). A plateau in the specific capacity as a function of the potential has been observed

Single-Source Magnetic Nanorattles By Using Convenient Emulsion Polymerization Protocols

A novel strategy to achieve easily scalable magneto-responsive nanoceramics with core/shell and nanorattle-type or yolk/shell architectures based on a ferrocene-containing polymer precursor is described. Monodisperse nanorattle-type magnetic particles are obtained by using convenient semicontinuous emulsion polymerization and Stöber process protocols followed by thermal treatment. The particles are characterized by TGA, TEM, WAXS, DLS, XPS, and Raman spectroscopy. Herein, established synthetic protocols widen opportunities for the convenient bottom-up strategies of various ferrocene-precursor-based spherical architectures for advanced ceramics with potential applications within fields of sensing and stimuli-responsive nanophotonics

Temperature induced reduction of the trivalent Ni ions in LiMO2 (M=Ni, Co) thin films

We report on an in-situ photoelectron spectroscopy study of the LiMO2 (M = Ni,Co) thin film layered materials. In the stoichiometric Lix(NiCo)O2 films grown by RF-magnetron sputtering at 20–600 °C, Ni3+ ions exist in the low-spin state. The growth temperatures T > 650 °C lead to better crystallinity of the films, Li-deficiency, and reduction of Ni3+ to Ni2+ in the high-spin state. The Co3+ ions in the low spin state are stable at 400–670 °C. The missing electron at the Ni-site is compensated by a less negative charge at the oxygen site or by the oxygen deficiency in Lix(NiCo)O2

High Voltage Electrodes for Li-Ion Batteries and Efficient Water Electrolysis: An Oxymoron?

We demonstrate that key parameters for efficient electrocatalytic oxidation of water are the energetics of the redox complexes associated with their ionization and electrochemical potentials coupled to the change of metal–oxygen band hybridization. We investigate the catalytic activity of the LiCoPO4–LiCo2P3O10 tailored compound, which is a 5 V cathode material for Li-ion batteries. The reason for the weak catalytic activity of the lithiated compound toward the oxygen evolution reaction is a large energy difference between the electronic states involved in the electrochemical reaction. A highly active catalyst is obtained by tuning the relative energetic position of the electronic levels involved in the charge transfer reaction, which in turn are governed by the lithium content. A significant lowering of the overpotential from >550 mV to ∼370 mV at 10 mA cm–2 is achieved via a decrease of the ionization potential and shifting the electrochemical potential near the electronic states of the molecule, thereby facilitating water oxidation

The Effect of Interfacial Charge Distribution on Chemical Compatibility and Stability of the High Voltage Electrodes (LiCoPO 4 , LiNiPO 4 )/Solid Electrolyte (LiPON) Interface

Solid electrolytes hold the promise of improved safety and superior electrochemical stability in energy storage systems. Among those, electrolytes with phosphate anions are expected to be more stable at high operating voltages, thereby providing even higher energy density. The key challenge is to control the boundary conditions at the cathode/electrolyte interface, which impact drastically the functionality of the energy storage devices. Here, the evolution of the chemical composition and electronic properties of the interface forms upon consequent deposition of solid electrolyte (lithium phosphorous oxynitride [LiPON]) onto the 5 V LiCoPO4 and LiNiPO4 carbon‐free thin film cathode materials is in situ studied by comprehensive electron spectroscopy experiments combined with the energy band diagram approach. It is demonstrated that the driving forces for interfacial reactivity are the band bending direction and the double layer potential drop at the electrode–electrolyte interface coupled to an unfavorable electrochemical potential shift of involved electronic states upon contact formation. The probability for interfacial chemical reactions is essentially increased at small energy differences in the ionization potentials of the cathode material and electrolyte, whereas a large energy difference ensures their chemical compatibility

The stability of the SEI layer, surface composition and the oxidation state of transition metals at the electrolyte-cathode interface impacted by the electrochemical cycling: X-ray photoelectron spectroscopy investigation

The stability of the valence state of the 3d transition metal ions and the stoichiometry of LiMO2 (M=Co, Ni, Mn) layered oxides at the surface-electrolyte interface plays a crucial role for energy storage applications. The surface oxidation/reduction of the cations caused by the contact of the solids to air or to the electrolyte results in the blocking of the Li-transport through the interface that leads to the fast batteries deterioration. The influence of the end-of-charge voltage on the chemical composition and the oxidation state of 3d transition metal ions, as well as the stability of the solid-electrolyte interface (SEI), which is formed during the electrochemical Li-deintercalation/intercalation of the LiCoO2 (LCO) and Li(Ni,Mn,Co)O2 (LNMCO), have been investigated by X-ray photoelectron spectroscopy (XPS). While the chemical composition of SEI is similar for both layered oxide surfaces, the electrochemical cycling to some critical voltage values lead to the disappearance of the SEI layer. The breaking and the weakening of the chemical bonds in the chemical compounds of the SEI is assigned with the lattice stress induced by the lattice expansion which is stronger at the higher end-of charge voltages. By the analysis of the shape of the 2p and 3s photoelectron emissions we show that the formation of the SEI layer correlates with the partial reduction of the trivalent Co ions at the electrolyte-LCO interface and amount of the Co2+ ions is increased as the SEI vanishes. In opposite, the Mn4+, Co3+ and Ni2+ ions at the SEI-LNMCO interface are stable under the electrochemical cycling to higher end-of-charge voltage. A correlation between deterioration of the LCO and LNMCO batteries and the change of electronic structure at the surface/interface after the electrochemical cycling has been found. The dissolution of the SEI layer might be the reason for the fast deterioration of the Li-ion batteries