Structural changes and mechanisms of reversible electrochemical lithium-ion cycling in (104) oriented LiNi _1/3 Mn _1/3 Co _1/3 O ₂ thin film cathodes prepared by pulsed laser deposition

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 <i>ex situ</i> 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.