LiMO2 (M = Ni, Co) thin film cathode materials: a correlation between the valence state of transition metals and the electrochemical properties

Abstract

The electronic properties of the LiMO₂ (M = Ni, Co) thin film cathode materials grown by RF sputtering/co-sputtering are in situ studied by X-ray photoelectron spectroscopy (XPS). Stoichiometric Li_1.0Co_1.0O₂ thin films deposited on a heated substrate at T = 500–550 °C reveal the Co³⁺ (t_2g⁶e_g⁰) ground state configuration in the low spin (LS) state. Stoichiometry of the Li_x(Ni,Co)O₂ films and the valence and spin states of the Ni ions depend strongly on the growth conditions. The electronic configuration of stoichiometric Li_1.0Ni_0.5Co_0.5O₂ is described as the Ni³⁺ (t_2g⁶e_g¹) LS and Co³⁺ (t_2g⁶e_g⁰) LS states. The Li-deficient Li_x<1.0(Ni,Co)O₂ exhibits Ni²⁺ (t_2g⁶e_g²) in the high spin (HS) and Co³⁺ (t_2g⁶e_g⁰) in LS states. The reduction of the trivalent Ni ions to Ni²⁺ (t_2g⁶e_g²) with a HS state electronic configuration is related to the evaporation of Li₂O at elevated substrate temperatures coupled to a loss of O₂ due to an internal oxidation reaction of O²⁻ lattice ions induced by the strongly oxidizing Ni³⁺ ions. Owing to the stable Co³⁺ (t_2g⁶e_g⁰) with a LS state electronic configuration, Li_1.0Co_1.0O₂ thin films cycled to 4.2 V exhibit a very good electrochemical reversibility. Li_1.0Ni_0.5Co_0.5O₂ films annealed at the same temperature as for Li_1.0Co_1.0O₂ manifest a broadening of the oxidation/reduction peaks of the cyclic voltammogram (CV) curves with a strong current drop after the first step of the electrochemical Li-deintercalation. The observed irreversibility of the Li-intercalation/deintercalation process is attributed to instability of the Ni³⁺ (t_2g⁶e_g¹) ions. Temperatures of the deposition/annealing above 750 °C lead to the phase separation of the Li_x(Ni,Co)O₂ films, a strong Li deficiency, the occurrence of Co²⁺ (t_2g⁵e_g²) with HS ions and consequently a complete degeneration of the electrochemical cyclability.