Matjaž Spreitzer

Epitaxial LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) thin films are prepared via pulsed laser deposition to model fundamental electrochemical behavior and lithium-ion transport kinetics based on different crystallographic orientations and defect types. The observed growth direction and surface termination of NMC thin films are linked to surface energy minimization, primarily via the (104) and (003) planes. The shortest diffusion path for lithium-ion transport is achieved for a film thickness of ≈15 nm via optimal (100)-oriented growth of NMC, indicating selective growth direction of NMC domains. Analysis of interfaces and local crystal structure revealed two predominant types of defects: antiphase boundaries (APBs) and twinned domains, which are strictly related to the symmetry of the layered structure and columnar epitaxial growth of NMC domains. Electrochemical testing vs Li/Li⁺ at charge/discharge rates from C/10 up to 6 C showed that performance is influenced by both the crystallographic orientation of lithium transport pathways and the presence of structural defects. Specifically, (104)- and (1̅08)-oriented NMC thin films with twinned microstructure exhibited stable cycling, delivering specific discharge capacities of 66.2 μA cm^–2 μm^–1 (141.2 mAh g^–1) and 70.2 μA cm^–2 μm^–1 (149.4 mAh g^–1) at C/10, along with apparent lithium diffusion coefficients of 7.45 × 10^–15 cm² s^–1 and 7.95 × 10^–15 cm² s^–1, respectively. In contrast, (003)- and (1 0 16)-oriented thin films exhibited lower apparent lithium diffusion coefficients and limited functionality due to less favorable orientations of lithium slabs, higher density of APBs, and unit cell distortion. These factors contribute to a noticeable decline in average discharge voltage at higher discharge rates across all orientations except (104). This approach reveals an intrinsic correlation between the structural properties and electrochemical response of epitaxial NMC thin films and serves as a future guideline toward high-performance NMC cathodes.

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This study examines the structural and electrochemical behavior of epitaxial (104) oriented LiNi_1/3Mn_1/3Co_1/3O₂ (NMC 111) thin film cathodes prepared by pulsed laser deposition, aiming to elucidate the underlying mechanisms of reversible lithium-ion cycling. The effect of growth parameters on film morphology and crystal structure is thoroughly studied. The surface analysis confirms the oxidation states of transition metal ions to be Ni²⁺, Mn⁴⁺ and Co³⁺. Microstructural analysis reveals twinned domains in the NMC 111 layered structure, which conforms with its 4-domain crystallographic orientation. After NMC 111 thin film is charged to 4.2 V, a change in the local electronic structure of nickel and oxygen ions is observed by electron energy loss spectroscopy as a consequence of nickel oxidation. By utilizing ex situ reciprocal space mapping after charging to 4.2 V, a negative unit cell volume change was observed, compensated by an increased mosaic spread of NMC 111 lattice planes. This structural adjustment is reversibly maintained upon discharging to 3.0 V. Based on defined epitaxial structures, the reversible mechanism of lithiation and delithiation in NMC 111 thin films is determined on a structural level, providing detailed insight into its functionality. To address structural instability in the charged state, the electrochemical performance was enhanced by cooling the NMC 111 thin films under high oxygen pressure.

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