Electrochemical and electron microscopic characterization of thin-film LiCoO2 cathodes under high-voltage cycling conditions

Thin-film LiCoO2 batteries using the Lipon electrolyte exhibit excellent cycleability up to 4.4 V. With the enhanced voltage limit for repeatable cycling, the thin-film cathode delivers a specific capacity of 170 mAh/g, which is 22% higher than that for previous LiCoO2 cathodes cycled to 4.2 V. No evidence of grain fracture was detected by transmission electron microscopy (TEM) in the cathode layer after cycling to 4.4 V. When cycled to >4.4 V, capacity fades and cell resistance increases more rapidly, attributed to the onset of a phase transition at >4.4 V. Fracture of grains was observed by TEM in the cathode layer after cycling to 5 V. Nanocrystalline films show better cycleability to 5 V than films with a larger grain size.