(Invited) Novel Method for Conformal LiMn2O4 thin Films Fabrication on Planar and 3D Microstuctured Substrates
Authors: Labyedh Nouha , Timmermans Marina , Mattelaer Felix , Mees Maarten J. , Detavernier Christophe , Vereecken Philippe M. 
Published: February 2020

In this work we report for the first time on a novel LiMn2O4 (LMO) thin film fabrication method. Using this fabrication method crystalline LMO thin films were prepared on both planar and 3D microstructured Si pillar substrates. The process is based on a solid state reaction between an electrolytic manganese dioxide (EMD) and Li2CO3 stacked-layers upon thermal annealing. The main advantage of using the electrochemical deposition technique to prepare the MnO2 layer is its capability of providing a conformal film deposition on high aspect ratio substrates. Moreover the electrolytic manganese dioxide film shows a high porosity (~50%) which will help later to release the mechanical strain during the Li-ion insertion and extraction process. The Li2CO3 deposition was done either by chemical solution deposition (CSD) or atomic layer deposition (ALD). The temperature dependence of the EMD conversion was investigated. The annealing time was adjusted based on the film thicknesses. Further, different current collector materials such as the noble Pt, Ni and MnO2seed layer on TiN were evaluated for the EMD conversion.

The phase and morphology of the prepared LiMn2O4 films were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The XRD patterns show the characteristic LMO diffraction peaks confirming the formation of the crystalline phase. SEM images display a continuous and conformal film on both planar and 3D microstructured substrates. The stoichiometry of the fabricated LMO films was evaluated by X-ray photoelectron spectroscopy (surface spectra and depth profile analyses) and elastic recoil detection (ERD) analyses. The results indicate that the fabricated film has a stoichiometry close to the theoretical stoichiometry of LiMn2O4with a homogeneous elements distribution through the layer. In addition, in-situ XRD measurements were performed in order to investigate the conversion mechanism evolution upon annealing and its kinetics.

The electrochemical performance of the fabricated films was examined using cyclic voltammetry and charge and discharge measurements. The cyclic voltammogram reveals sharp redox peaks centered around 3 and 4V (vs. Li+/Li) corresponding to Li+ ion insertion/extraction in/from the spinel LiMn2O4. Figure 1.b shows that the converted EMD film shows a similar electrochemical performance to a LiMn2O4 film prepared by RF-sputtering. Also, the lithiation and delithiation curves show the typical LMO potential plateaus. About 100% of the maximum theoretical capacity was reached at 0.1 C. Furthermore, a study of the electrochemical performance of the prepared film based on its annealing time and temperature was done. The LiMn2O4 coated Si Pillar array showed 19 times higher capacity than the film prepared with the same conditions on planar substrate. The relevance of this technique for the fabrication of high rate and high capacity 3D thin-film Li-ion batteries will be discussed.

Figure 1