Intercalation of Lithium Ions from Gaseous Precursors into β-MnO₂ Thin Films Deposited by Atomic Layer Deposition

LiMn₂O₄ is a promising candidate for a cathode material in lithium-ion batteries because of its ability to intercalate lithium ions reversibly through its three-dimensional manganese oxide network. One of the promising techniques for depositing LiMn₂O₄ thin-film cathodes is atomic layer deposition (ALD). Because of its unparalleled film thickness control and film conformality, ALD helps to fulfill the industry demands for smaller devices, nanostructured electrodes, and all-solid-state batteries. In this work, the intercalation mechanism of Li⁺ ions into an ALD-grown β-MnO₂ thin film was studied. Samples were prepared by pulsing LiO^tBu and H₂O for different cycle numbers onto about 100 nm thick MnO₂ films at 225 °C and characterized with X-ray absorption spectroscopy, X-ray diffraction, X-ray reflectivity, time-of-flight elastic recoil detection analysis, and residual stress measurements. It is proposed that for <100 cycles of LiO ^tBu/H₂O, the Li⁺ ions penetrate only to the surface region of the β-MnO₂ film, and the samples form a mixture of β-MnO₂ and a lithium-deficient nonstoichiometric spinel phase Li_xMn₂O₄ (0 < x < 0.5). When the lithium concentration exceeds x ≈ 0.5 in Li_xMn₂O₄ (corresponding to 100 cycles of LiO ^tBu/H₂O), the crystalline phase of manganese oxide changes from the tetragonal pyrolusite to the cubic spinel, which enables the Li⁺ ions to migrate throughout the whole film. Annealing in N₂ at 600 °C after the lithium incorporation seemed to convert the films completely to the pure cubic spinel LiMn₂O₄.