Rapid thermal annealing effect on surface of LiNi_1-xCo_xO₂ cathode film for thin-film microbattery

The effect of rapid thermal annealing (RTA) on the surface of a LiNi_1-xCo_xO₂ cathode film is examined by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and auger electron spectroscopy (AES). It is found that the as-deposited LiNi_1-xCo_xO₂ film undergoes a surface reaction with oxygen in the air, due to the high activity of lithium in the film. AES spectra indicate that the surface layer consists of lithium and oxygen atoms. The RTA process at 500 °C eliminates the surface layer to some extent. An increase in annealing temperature to 700 °C results in complete elimination of the surface layer. The surface evolution of the LiNi_1-xCo_xO₂ film with increasing annealing time at 700 °C is examined by means of AFM examination. It is found that the surface layer, which is initially present in the form of an amorphous like-film, becomes agglomerated and then vaporizes after 5 min of annealing. A thin-film microbattery (TFB), fabricated by using the LiNi_1-xCo_xO₂ film without a surface layer, exhibits more stable cycliability and a higher specific discharge capacity of 60.2 μAh cm^-2 μm than a TFB with an unannealed LiNi_1-xCo_xO₂ film. Therefore, it is important to completely eliminate the surface layer in order to achieve high performance from all solid-state thin-film microbatteries.