Synthesis & characterization of mesoporous LiCoO2 and LiMn2O4 thin films

Abstract

This work focuses on the adaptation of molten salt assisted self-assembly (MASA) method for the synthesis and characterization of mesoporous LiCoO2 and LiMn2O4 materials without a polymerizing agent in the media. The MASA process is a new method to synthesize mesoporousthin films and monoliths. Fresh gel and calcined solid samples were characterized by using XRD, Raman, N2 sorption, FTIR POM, SEM, TEM and UV-Vis measurements. The first step of the process involves the preparation of clear solutions that contain two hydrated salts, LiNO3.xH2O and Co(NO3)2.6H2O (or Mn(NO3)2.6H2O), CTAB (cethyltrimethylammonium bromide, ionic surfactant) (or CTAN), 10- lauryl ether (C12EO10, ionic surfactant) and ethanol (or H2O) as volatile solvents. Additionally, HNO3 is needed in the case of LiMn2O4 to prevent formation of Mn(OH)2(H2O)4 complex during and/or before the assembly process. The clear solutions are spin coated over various substrates to produce the salt-surfactant LLC mesophase that resists to high temperature treatments (300- 550 oC) in air to form first examples of mesoporous metal lithiates. Thesis is designed to give the insights on how to change several parameters that control the features like purity, pore size, surface area and crystallinity of the LiCoO2 and LiMn2O4 mesostructures by determining solvent type, thickness of the LLC film, acid amount, salt uptake, calcination temperature, calcination time and surfactant used for the synthesis. Ethanol was determined to be a good solvent to prepare the clear solutions. The samples, prepared by spin coating gave better results than the ones prepared by drop casting to obtain uniform pores with less side products. Addition of HNO3 has no effect on side product formation in the synthesis of LiCoO2 but it is vital to obtain a stable, homogenous solution in the case of LiMn2O4 preparation. LiCoO2 samples, prepared using CTAB are semicrystalline at lower temperatures and stable up to 550 oC but it undergoes decomposition to Co3O4 and Li2O at higher temperatures. Presence of CTAB in the reaction media has a positive effect on the uniformity pores. However; due to the role of Brion on formation of side products, CTAN has been used for further experimentation. Thin films of the calcined samples showed small angle diffraction corresponds to preserved order of the mesophase during calcination. Mesoporous pure HT- LiCoO2 synthesized with MASA is the first example in the literature, having a 65 m2 /g specific surface area and 15 nm pore size. Moreover; mesoporous pure LiMn2O4 could be obtained both using CTAB and CTAN with 82 m2 /g specific surface area and 11 nm pore size.