3D thin-film microbatteries are attracting a lot of interest due to their fast charging capability and higher capacity provided by the surface area enhancement. The thin-film deposition techniques (e.g. RF-sputtering and CVD) typically used in case of planar thin-film microbatteries cannot be used for the 3D microbatteries. Indeed, for the latter, the thin-film needs to be conformally coated over high aspect ratio microstructured substrates which cannot be done using line-of-sight sputtering techniques. In this paper, we report on a new conformal thin-film fabrication technique for the preparation of 3D LiMn₂O₄ (LMO) thin-film electrodes. LiMn₂O₄ is an interesting cathode material for Li-ion batteries due to its high capacity and low volume change during the Li-ion insertion/extraction process. Moreover, the cubic spinel structure of LMO helps reaching a fast charging which is one of the key requirements for 3D thin-film batteries. Our novel fabrication technique is based on the electrochemical deposition of MnO₂ thin-film and its conversion upon thermal treatment to LiMn₂O₄ using a Li₂CO₃ precursor.

Using this fabrication technique up to 350 nm conformal and continuous LMO films were prepared on different current collector materials both on planar substrates and on Si pillar arrays as 3D substrates providing an area enhanced of 21x (see SEM images in figure 1 (a, b, and b’)). Elastic recoil detection (ERD) analysis show that the fabricated films indeed have a stoichiometry close to the theoretical stoichiometry of LiMn₂O₄ and with a homogeneous element distribution throughout the layer. The prepared planar and 3D LMO thin-film are electrochemically active with comparable rate performance. The full theoretical capacity is reached at 0.1 C and 55% of the capacity is still retained at high C-rates of 10C (see figure 1 (c)). As expected from the area enhancement, the capacity for the 3D LMO electrode is 21 times that of planar LMO electrode.

Finally, a functional solid-state Li-ion battery was demonstrated using our 3D LMO thin-film cathode and a Li-foil anode using a nanocomposite solid electrolyte (SCE).