Felix Mattelaer

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.

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.

Manganese oxide films with controlled oxidation state for water splitting devices through a combination of atomic layer deposition and post-deposition annealing

Solar hydrogen devices combine the power of photovoltaics and water electrolysis to produce hydrogen in a hybrid form of energy production. To engineer these into integrated devices (i.e. a water splitting catalyst on top of a PV element), the need exists for thin film catalysts that are both transparent for solar light and efficient in water splitting. Manganese oxides have already been shown to exhibit good water splitting performance, which can be further enhanced by conformal coating on high surface-area structures. The latter can be achieved by atomic layer deposition (ALD). However, to optimize the catalytic and transparency properties of the water splitting layer, an excellent control over the oxidation state of the manganese in the film is required. So far MnO, Mn3O4 and MnO2 ALD have been shown, while Mn2O3 is the most promising catalyst. Therefore, we investigated the post-deposition oxidation and reduction of MnO and MnO2 ALD films, and derived strategies to achieve every phase in the MnO–MnO2 range by tuning the ALD process and post-ALD annealing conditions. Thin film Mn2O3 is obtained by thermal reduction of ALD MnO2, without the need for oxidative high temperature treatments. The obtained Mn2O3 is examined for solar water splitting devices, and compared to the as-deposited MnO2. Both thin films show oxygen evolution activity and good solar light transmission.

Effect of annealing atmosphere on LiMn2O4 for thin film Li-ion batteries from aqueous chemical solution deposition

In this study we demonstrate and explain the direct relationship between precursor chemistry and phase formation of LiMn2O4 powders and thin films from aqueous chemical solution deposition (CSD). The processing conditions applied to transform the precursor into the LiMn2O4 phase are investigated with a focus on the heating atmosphere and temperature. We found that the Mn2+ ions, used as a starting product, already partially oxidize into Mn3+/Mn4+ in the precursor solution. The Mn3+ ions present in the gel or the dried film are extremely sensitive to O2, leading to fast oxidation towards Mn4+. Here, we suggest that the oxygen, introduced in the precursor solution by the citrate complexing agent, suffices to oxidize the Mn2+ into Mn3+/Mn4+ which is crucial in the formation of phase pure spinel and stoichiometric LiMn2O4. Any additional oxygen, available as O2 during the final processing, should be avoided as it leads to further oxidation of the remaining Mn3+ into Mn4+ and to the formation of the γ-Mn2O3 and λ-MnO2 secondary phases. Based on these insights, the preparation of phase pure, spinel and stoichiometric LiMn2O4 in a N2 ambient was achieved both in powders and films. Moreover, the study of the precursor chemistry and final annealing leads to the possibility of reducing the final temperature to 450 °C, enabling the use of temperature and oxidation sensitive current collectors such as TiN. This inert ambient and low temperature processing of LiMn2O4 provides the opportunity to have large flexibility and compatibility with process conditions for other materials in the thin film battery stack, without undesired oxidations.

Plasma-enhanced atomic layer deposition of vanadium phosphate as a lithium-ion battery electrode material

Vanadium phosphate films were deposited by a new process consisting of sequential exposures to trimethyl phosphate (TMP) plasma, O2 plasma, and either vanadium oxytriisopropoxide [VTIP, OV(O-i-Pr)3] or tetrakisethylmethylamido vanadium [TEMAV, V(NEtMe)4] as the vanadium precursor. At a substrate temperature of 300 °C, the decomposition behavior of these precursors could not be neglected; while VTIP decomposed and thus yielded a plasma-enhanced chemical vapor deposition process, the author found that the decomposition of the TEMAV precursor was inhibited by the preceding TMP plasma/O2 plasma exposures. The TEMAV process showed linear growth, saturating behavior, and yielded uniform and smooth films; as such, it was regarded as a plasma-enhanced atomic layer deposition process. The resulting films had an elastic recoil detection-measured stoichiometry of V1.1PO4.3 with 3% hydrogen and no detectable carbon contamination. They could be electrochemically lithiated and showed desirable properties as lithium-ion...

Atomic layer deposition of vanadium oxides for thin-film lithium-ion battery applications

Amorphous VO2 thin films are deposited by atomic layer deposition (ALD) using tetrakis[ethylmethylamino]vanadium (TEMAV) as vanadium precursor and water or ozone as the oxygen source. The crystallisation and oxidation behaviour is investigated for different oxygen partial pressures between ambient air and 3.7 Pa, resulting in phase formation diagrams on SiO2, TiN and Pt substrates, demonstrating a series of stable vanadium oxide phases in the VO2–V2O5 series. Most of the obtained phases exhibit lithium intercalation behaviour in the 1.5–4.5 V vs. Li+/Li potential range, and demonstrate high volumetric capacities in the order of V2O5 < VO2 (B) < V6O13 < V3O7 < V4O9, with the latter at more than twice the capacity of the best commercial cathode materials.

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.

Electro-precipitation via oxygen reduction: a new technique for thin film manganese oxide deposition

Manganese oxide was deposited from a non-aqueous solution, dimethyl sulfoxide (DMSO), via the reduction of dissolved oxygen. The formed superoxide radical ion (O2−˙) reacts rapidly with the manganese ions forming a smooth and thin film (80 nm) of manganese oxide. From an in situ EQCM study, it could be concluded that MnO2 was the most probable oxide which was deposited at an average growth rate of 0.049 μg s−1 or 0.077 nm s−1. Since the direct deposition of a phase pure MnO2 layer was not confirmed by XRD, it is more likely that a variety of manganese oxides has been deposited during the electro-precipitation reaction and thus further optimization or post-treatments are required to obtain an active manganese oxide layer for thin film deposits. The key property of this new deposition technique is the self-limiting behavior, proven by rotating ring-disk electrode experiments. This is crucial to electrodeposit thin films conformally on high aspect ratio structures for 3D all-solid-state lithium-ion batteries or supercapacitors.