High voltage stability of LiCoO₂ particles with a nano-scale Lipon coating

For high-voltage cycling of rechargeable Li batteries, a nano-scale amorphous Li-ion conductor, lithium phosphorus oxynitride (Lipon), has been coated on surfaces of LiCoO₂ particles by combining a RF-magnetron sputtering technique and mechanical agitation of LiCoO₂ powders. LiCoO₂ particles coated with 0.36 wt% (∼1 nm thick) of the amorphous Lipon, retain 90% of their original capacity compared to non-coated cathode materials that retain only 65% of their original capacity after more than 40 cycles in the 3.0-4.4 V range with a standard carbonate electrolyte. The reason for the better high-voltage cycling behavior is attributed to reduction in the side reactions that cause increase of the cell resistance during cycling. Further, Lipon coated particles are not damaged, whereas uncoated particles are badly cracked after cycling. Extending the charge of Lipon-coated LiCoO ₂ to higher voltage enhances the specific capacity, but more importantly the Lipon-coated material is also more stable and tolerant of high voltage excursions. A drawback of Lipon coating, particularly as thicker films are applied to cathode powders, is the increased electronic resistance that reduces the power performance.