The importance of combining disorder with order for Li-ion insertion into cryogenically prepared nanoscopic ruthenia

Cryogenically prepared RuO2 (cryo-RuO2), a material known for its ability to "self-wire" into continuous, nanoscopic electronic pathways, is proposed as an electrode for Li-ion microbatteries with three-dimensionally interpenetrated components. We determined processing guidelines that optimize Li-ion uptake in cryo-RuO2 powders by varying the solid-state structure of cryo-RuO2 with thermal processing at 50-250 deg C in flowing O2(g) or Ar(g). The highly disordered structure of as-prepared cryo-RuO2 is transformed to rutile RuO2 at 200 deg C in O2(g), resulting in a 60% loss of Li-ion capacity (as-prepared: 214 mA h g-1; rutile: 84 mA h g-1). In contrast, thermal processing in Ar(g) preserves structural disorder in the cryo-RuO2, even up to 250 deg C. The highest Li-ion capacity occurs for the treatment that mixes order (crystallinity) with disorder: > 250 mA h g-1 for cryo-RuO2 heated in oxygen to 50 deg C. This study provides processing guidelines to achieve fabrication of 3-D microbattery architectures containing a nanoscopic RuO2 electrode component.