Carbon inverse opal macroporous monolithic structures as electrodes for Na-ion and K-ion batteries

Highly ordered three-dimensionally structured carbon inverse opals (IOs) produced from sucrose are stable electrodes in sodiumion and potassium-ion batteries. The walls of the ordered porous carbon structure contain short-range graphitic areas. The interconnected open-worked structure defines a conductive macroporous monolithic electrode that is easily wetted by electrolytes for Na-ion and K-ion systems. Electrochemical characterization in half-cells against Na metal electrodes reveals stable discharge capacities of 25 mAh g−1 at 35 mA g−1 and 40 mAh g−1 at 75 mA g−1 and 185 mA g−1 . In K-ion half cells, the carbon IO delivers capacities of 32 mAh g−1 at 35 mA g−1 and ∼25 mAh g−1 at 75 mA g−1 and 185 mA g−1 . The IOs demonstrate storage mechanisms involving both capacitive and diffusion-controlled processes. Comparison with non-templated carbon thin films highlights the superior capacity retention (72% for IO vs 58% for thin film) and cycling stability of the IO structure in Na-ion cells. Robust structural integrity against volume changes with larger ionic radius of potassium ions is maintained after 250 cycles in kion cells. The carbon IOs exhibit stable coulombic efficiency (>99%) in sodium-ion batteries and better coulombic efficiency during cycling compared to typical graphitic carbons.