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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 O-2, 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 spinet and stoichiometric LiMn2O4. Any additional oxygen, available as O-2 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 gamma-Mn2O3 and lambda-MnO2 secondary phases. Based on these insights, the preparation of phase pure, spinet and stoichiometric LiMn2O4 in a N-2 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 degrees 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

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LiMnO (LMO) is interesting from the viewpoint of its energy storage applications as it is a cathode in lithium ion batteries (LIB), which contains no rare, toxic or expansive elements, while it provides a high theoretical capacity (148 mA h g) at a reasonable voltage (4 V region) and a higher thermal stability compared to cobalt based cathodes and has a good rechargeability and cycling stability due to its spinel structure. Low temperature synthesis routes for cathode materials are currently gaining attention, in order to decrease the ecological footprint of the final LIB. Here, the crystallization temperature of LMO by a citrate based solution-gel synthesis was significantly lowered, to as low as 250 °C by the addition of ethanol to the precursor. The role of ethanol in this synthesis process was explored. It was found to lead to a considerable increase in the oxidation rate of the redox couple Mn/Mn, a lowering of the precursor decomposition temperature by 200 °C, besides a drastic decrease in the crystallization temperature (reaching 250 °C). Moreover, the main cause was identified to be an esterification reaction of ethanol with the carboxylic acid in the precursor complexes, taking place before the oxide formation. The insights obtained strengthen the knowledge regarding citrato-Mn/Mn complexes present in aqueous solution-gel synthesis routes and are relevant for the preparation of various manganese containing oxides. Moreover, the precursor developed opens up a new possibility for the low temperature synthesis of LMO powders and thin films for application in LIB. In the case of thin film batteries, the low temperature processing provides compatibility with other materials in the thin film battery stack, avoiding undesired oxidations or interfacial reactions.

By ultrasonic spray deposition of precursors, conformal deposition on 3D surfaces of tungsten oxide (WO 3 ) 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 WO 3 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 WO 3 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 WO 3 and LLT. Finally, the thermal budget required for WO 3 layer deposition was minimized, which enabled attaining active WO 3 on 3D TiN/Si micro-cylinders. A 2.6-fold capacity increase for the 3D-structured WO 3 was shown, with the same current density per coated area.