Monolithic cathodes of optimized porosity prepared by sintering LiCoO2 powders provide high volume utilization and surprising stability under electrochemical cycling. Combined with a novel packaging approach, ultrahigh energy densities in small volumes are enabled. The microbatteries have volumes <6 mm(3) and provide sustained similar to 2.5 h discharges with energy densities of 400-650 W h L-1.