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

Abstract

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.