Optimization of a potential manufacturing process for thin-film LiCoO2 cathodes

A radio frequency (RF) magnetron sputter deposition process designed for low-cost, high-rate fabrication of LiCoO2 (LCO) thin films was developed and used to fabricate approximately phase pure cathodes with the R (3) over barm crystal structure, strong (104) preferred orientation and high discharge capacities. Starting from a typical set of processing conditions for LCO films, the values of key deposition and post-deposition parameters were systematically changed to increase the deposition rate, simplify the sputter deposition process and minimize process gas use without compromising the structural properties and electrochemical performance. Cathodes were deposited onto Waspaloy (58Ni-19Cr-14Co-Mo-Ti-Al-Fe [wt%]) foil current collectors to provide enhanced thermal stability during post-deposition annealing. The best properties were attained using a process pressure of 0.5 Pa, an RF power density of 5.5 W cm(-2) and a post-deposition anneal in air at 600 degrees C for 2 h. A sintered sputter target, oxygen gas flow and substrate heating were not required. This set of conditions resulted in a high deposition rate of similar to 700-790 nm h(-1), allowing the formation of a relatively thick (similar to 2.8-3.2 mu m) film in 4 h. Furthermore, high maximum specific and areal discharge capacities of 132 mAh g(-1) / 62 mu Ah cm(-2) mu m(-1) and 172 mu Ah cm(-2) were attained during galvanostatic cycling at a rate of 0.1 C with an organic liquid electrolyte, which are among the highest values reported. Cycling rate tests showed that the thin-film LCO cathodes could deliver capacity up to a cycling rate of at least 2 C without causing any significant cell damage.