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

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 Li1.0Co1.0O2 thin films deposited on a heated substrate at T = 500-550 degrees C reveal the Co3+ (t(2g)(6)e(g)(0)) ground state configuration in the low spin (LS) state. Stoichiometry of the Li-x(Ni,Co)O-2 films and the valence and spin states of the Ni ions depend strongly on the growth conditions. The electronic configuration of stoichiometric Li1.0Ni0.5Co0.5O2 is described as the Ni3+ (t(2g)(6)e(g)(1)) LS and Co3+ (t(2g)(6)e(g)(0)) LS states. The Li-deficient Li-x<1.0(Ni,Co)O-2 exhibits Ni2+ (t(2g)(6)e(g)(2)) in the high spin (HS) and Co3+ (t(2g)(6)e(g)(0)) in LS states. The reduction of the trivalent Ni ions to Ni2+ (t(2g)(6)e(g2)) with a HS state electronic configuration is related to the evaporation of Li2O at elevated substrate temperatures coupled to a loss of O-2 due to an internal oxidation reaction of O2- lattice ions induced by the strongly oxidizing Ni3+ ions. Owing to the stable Co3+ (t(2g)(6)e(g)(0)) with a LS state electronic configuration, Li1.0Co1.0O2 thin films cycled to 4.2 V exhibit a very good electrochemical reversibility. Li1.0Ni0.5Co0.5O2 films annealed at the same temperature as for Li1.0Co1.0O2 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(2g)(6)e(g)(1)) ions. Temperatures of the deposition/annealing above 750 degrees C lead to the phase separation of the Li-x(Ni,Co)O-2 films, a strong Li deficiency, the occurrence of Co2+ (t(2g)(5)e(g)(2)) with HS ions and consequently a complete degeneration of the electrochemical cyclability.