Degradation mechanisms of lithium-rich nickel manganese cobalt oxide cathode thin films

This paper reports a method to prepare Li-rich NMC (Li_1.2Mn _0.55Ni_0.15Co_0.1O₂) thin film cathodes for Li-ion batteries using RF magnetron sputtering and post-annealing in O₂. Thin film cathodes with high reversible capacities (260 mA h g^-1) and potential profiles similar to those of the powder material have been obtained. Structural and electrochemical studies show that the grown materials consist of a layered structure with trigonal symmetry in which Li/TM ordering is partially achieved. Using XPS we determine that the surface is comprised of Mn⁴⁺, Co³⁺ and Ni²⁺ cations inside an O^2- framework. The loss mechanisms of these electrodes have been studied after 184 cycles. The data after cycling shows the absence of Li/TM ordering, confirming that Li₂MnO₃ activation is irreversible, while electron diffraction data indicates extensive structural modifications upon cycling. In addition, we identified that the surface chemistry is dominated by inorganic species (LiF, Li_x′PO _y′F_z′, Li_xPF_y), along with small amounts of organic species with C-O and O-CO groups such as PEO, LiOR and RCO₂Li. Moreover, XPS results indicate that Ni and Co migrate into the bulk while the reduction of Mn⁴⁺ into Mn³⁺ is clearly evidenced, as expected from the activation of Li₂MnO ₃ domains and discharging to 2.5 V.