Atomic Layer Deposition of Nitrogen-Doped Al Phosphate Coatings for Li-Ion Battery Applications.

In situ nitrogen doping of aluminum phosphate has been investigated in two different plasma-enhanced atomic layer deposition (PE-ALD) processes. The first method consisted of the combination of trimethyl phosphate plasma (TMP*) with a nitrogen plasma and trimethyl aluminum (TMA), that is, TMP*-N2*-TMA. The second method replaces TMP* with a diethylphosphoramidate plasma (i.e., DEPA*-N2*-TMA) of which the amine group could further aid nitrogen doping and/or eliminate the need for a nitrogen plasma step. At a substrate temperature of 320 °C, the TMP*-based process showed saturated growth (0.8 nm/cycle) of a nitrogen-doped (approximately 8 atom %) Al phosphate, while the process using DEPA* showed a similar amount of nitrogen but a significantly higher growth rate (1.4 nm/cycle). In the latter case, nitrogen doping could also be achieved without the nitrogen plasma, but this leads to a high level of carbon contamination. Both films were amorphous as-deposited, while X-ray diffraction peaks related to AlPO4 appeared after annealing in a He atmosphere. For high coating thickness (>2 nm), a significant increase in the Li-ion transmittance was found after nitrogen doping, although the coating has to be electrochemically activated. At lower thickness scales, such activation was not needed and nitrogen doping was found to double the effective transversal electronic conductivity. For the effective transversal ionic conductivity, no conclusive difference was found. When a lithium nickel manganese cobalt oxide (NMC) powder is coated with one ALD cycle of N-doped Al phosphate, the rate capability and the energy efficiency of the electrode improves.

Thermal and Plasma-Enhanced Atomic Layer Deposition of Yttrium Oxide Films and the Properties of Water Wettability.

The atomic layer deposition (ALD) of yttrium oxide (Y2O3) is investigated using the liquid precursor Y(EtCp)2(iPr-amd) as the yttrium source with thermal (H2O) and plasma-enhanced (H2O plasma and O2 plasma) processes, respectively. Saturation is confirmed for the growth of the Y2O3 films with each investigated reactant with a similar ALD window from 150 to 300 °C, albeit with a different growth rate. All of the as-deposited Y2O3 films are pure and smooth and have a polycrystalline cubic structure. The as-deposited Y2O3 films are hydrophobic with water contact angles >90°. The water contact angle gradually increased and the surface free energy gradually decreased as the film thickness increased, reaching a saturated value at a Y2O3 film thickness of ∼20 nm. The hydrophobicity was retained during post-ALD annealed at 500 °C in static air for 2 h. Exposure to polar and nonpolar solvents influences the Y2O3 water contact angle. The reported ALD process for Y2O3 films may find potential applications in the field of hydrophobic coatings.

Wet-Chemical Synthesis of 3D Stacked Thin Film Metal-Oxides for All-Solid-State Li-Ion Batteries.

By ultrasonic spray deposition of precursors, conformal deposition on 3D surfaces of tungsten oxide (WO3) negative electrode and amorphous lithium lanthanum titanium oxide (LLT) solid-electrolyte has been achieved as well as an all-solid-state half-cell. Electrochemical activity was achieved of the WO3 layers, annealed at temperatures of 500 °C. Galvanostatic measurements show a volumetric capacity (415 mAh·cm-3) of the deposited electrode material. In addition, electrochemical activity was shown for half-cells, created by coating WO3 with LLT as the solid-state electrolyte. The electron blocking properties of the LLT solid-electrolyte was shown by ferrocene reduction. 3D depositions were done on various micro-sized Si template structures, showing fully covering coatings of both WO3 and LLT. Finally, the thermal budget required for WO3 layer deposition was minimized, which enabled attaining active WO3 on 3D TiN/Si micro-cylinders. A 2.6-fold capacity increase for the 3D-structured WO3 was shown, with the same current density per coated area.

Amorphous and Crystalline Vanadium Oxides as High-Energy and High-Power Cathodes for Three-Dimensional Thin-Film Lithium Ion Batteries.

Flexible wearable electronics and on-chip energy storage for wireless sensors drive rechargeable batteries toward thin-film lithium ion batteries. To enable more charge storage on a given surface, higher energy density materials are required, while faster energy storage and release can be obtained by going to thinner films. Vanadium oxides have been examined as cathodes in classical and thin-film lithium ion batteries for decades, but amorphous vanadium oxide thin films have been mostly discarded. Here, we investigate the use of atomic layer deposition, which enables electrode deposition on complex three-dimensional (3D) battery architectures, to obtain both amorphous and crystalline VO2 and V2O5, and we evaluate their thin-film cathode performance. Very high volumetric capacities are found, alongside excellent kinetics and good cycling stability. Better kinetics and higher volumetric capacities were observed for the amorphous vanadium oxides compared to their crystalline counterparts. The conformal deposition of these vanadium oxides on silicon micropillar structures is demonstrated. This study shows the promising potential of these atomic layer deposited vanadium oxides as cathodes for 3D all-solid-state thin-film lithium ion batteries.

Molecular layer deposition of "vanadicone", a vanadium-based hybrid material, as an electrode for lithium-ion batteries.

Molecular layer deposition (MLD) of hybrid organic-inorganic thin films called "vanadicones" was investigated using tetrakisethylmethylaminovanadium (TEMAV) as the metal precursor and glycerol (GL) or ethylene glycol (EG) as the organic reactant. Linear and continued growth could only be achieved with GL as the organic reactant. The TEMAV/GL process displayed self-limiting reactions for both precursor and reactant pulses in the temperature range from 80 °C to 180 °C, with growth rates of 1.2 to 0.5 Å per cycle, respectively. Infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed the hybrid nature of the films. From X-ray reflectivity, the density was estimated at 2.6 g cm-3. A series of 21 nm vanadicone films were subjected to annealing under oxidizing (air) or inert (He) atmospheres at 500 °C. During annealing in air, the film crystallized to the V2O5 phase and all carbon was removed from the film. The films annealed in helium remained amorphous and retained most of their carbon content. Electrochemical measurements revealed lithium-ion activity during cyclic voltammetry in all treated films, while the as deposited film was inactive. In the 2.9 to 3.5 V vs. Li+/Li potential region, no improvement over the V2O5 reference was observed. However, the helium annealed samples outperformed V2O5 in terms of capacity, rate performance and cyclability when charged and discharged in the 1.0 to 3.5 V vs. Li+/Li region. This result enables the application of VxOy-based hybrid electrodes in a wider potential range without sacrificing the stability and performance.

Heterogeneous TiO2/V2O5/Carbon Nanotube Electrodes for Lithium-Ion Batteries.

Vanadium pentoxide (V2O5) is proposed and investigated as a cathode material for lithium-ion (Li-ion) batteries. However, the dissolution of V2O5 during the charge/discharge remains as an issue at the V2O5-electrolyte interface. In this work, we present a heterogeneous nanostructure with carbon nanotubes supported V2O5/titanium dioxide (TiO2) multilayers as electrodes for thin-film Li-ion batteries. Atomic layer deposition of V2O5 on carbon nanotubes provides enhanced Li storage capacity and high rate performance. An additional TiO2 layer leads to increased morphological stability and in return higher electrochemical cycling performance of V2O5/carbon nanotubes. The physical and chemical properties of TiO2/V2O5/carbon nanotubes are characterized by cyclic voltammetry and charge/discharge measurements as well as electron microscopy. The detailed mechanism of the protective TiO2 layer to improve the electrochemical cycling stability of the V2O5 is unveiled.

Molecular layer deposition of "titanicone", a titanium-based hybrid material, as an electrode for lithium-ion batteries.

Molecular layer deposition (MLD) of hybrid organic-inorganic thin films called "titanicones" was achieved using tetrakisdimethylaminotitanium (TDMAT) and glycerol (GL) or ethylene glycol (EG) as precursors. For EG, in situ ellipsometry revealed that the film growth initiates, but terminates after only 5 to 10 cycles, probably because both hydroxyls react with the surface. GL has a third hydroxyl group, and in that case steady state growth could be achieved. The GL process displayed self-limiting reactions for both reactants in the temperature range from 80°C to 160°C, with growth rates of 0.9 to 0.2 Å per cycle, respectively. Infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the hybrid nature of the films, with a carbon atomic concentration of about 20%. From X-ray reflectivity, the density was estimated at 2.2 g cm(-3). A series of films was subjected to water etching and annealing under air or He atmosphere at 500°C. The carbon content of the films was monitored with FTIR and XPS. Almost all carbon was removed from the air annealed and water treated films. The He annealed samples however retained their carbon content. Ellipsometric porosimetry (EP) showed 20% porosity in the water etched samples, but no porosity in the annealed samples. Electrochemical measurements revealed lithium ion activity during cyclic voltammetry in all treated films, while the as-deposited film was inactive. With increasing charge current, the He annealed samples outperformed amorphous and anatase TiO2 references in terms of capacity retention.