K-ion and Na-ion storage performances of Co 3 O 4 -Fe 2 O 3 nanoparticle-decorated super P carbon black prepared by a ball milling process.

The hybridisation of Co3 O4 and Fe2 O3 nanoparticles dispersed in a super P carbon matrix is proposed as a favourable approach to improve the electrochemical performance (reversible capacity, cycling stability and rate capability) of the metal oxide electrodes in metal-ion batteries. Hybrid Co3 O4 -Fe2 O3 /C is prepared by a simple, cheap and easily scalable molten salt method combined with ball-milling and used in sodium-ion and potassium-ion batteries for the first time. The electrode exhibits excellent cycling stability and superior rate capability in sodium-ion cells with a capacity recovery of 440 mA h g-1 (93% retention) after 180 long-term cycles at 50-1000 mA g-1 and back to 50 mA g-1 . In contrast, Co3 O4 -Fe2 O3 , Co3 O4 and Fe2 O3 electrodes display unsatisfactory electrochemical performance. The hybrid Co3 O4 -Fe2 O3 /C is also reactive with potassium and capable of delivering a reversible capacity of 220 mA h g-1 at 50 mA g-1 which is comparable with the most reported anode materials for potassium-ion batteries. The obtained results broaden the range of transition metal oxide-based hybrids as potential anodes for K-ion and Na-ion batteries, and suggest that further studies of these materials with potassium and sodium are worthwhile.