LiMn 2O 4 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 LiMn 2O 4 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-MnO 2 thin film was studied. Samples were prepared by pulsing LiO tBu and H 2O for different cycle numbers onto about 100 nm thick MnO 2 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/H 2O, the Li + ions penetrate only to the surface region of the β-MnO 2 film, and the samples form a mixture of β-MnO 2 and a lithium-deficient nonstoichiometric spinel phase Li xMn 2O 4 (0 < x < 0.5). When the lithium concentration exceeds x ≈ 0.5 in Li xMn 2O 4 (corresponding to 100 cycles of LiO tBu/H 2O), 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 N 2 at 600 °C after the lithium incorporation seemed to convert the films completely to the pure cubic spinel LiMn 2O 4.