NiCo₂O₄/CNT nanocomposites as bi-functional electrodes for Li ion batteries and supercapacitors

We report the in-situ synthesis of NiCo₂O₄ nanoparticles chemically bonded to multi-walled carbon nanotubes (NiCo₂O₄/CNT) based on a low-temperature, one-pot hydrothermal approach. The NiCo₂O₄/CNT 'composite' electrodes present excellent bi-functionality for applications in both Li-ion batteries (LIBs) and supercapacitors. They present a high Li-storage capacity of ∼1020 mAh g^-1 at 300 mA g^-1 after 200 cycles while a high pseudocapacitance of 680 F g^-1 is delivered when discharged at 1 A g^-1. Electrodes are also prepared using the 'physical mixture' of the same constituents at the same concentrations. They present much poorer Li-ion storage of ∼370 mAh g^-1 at 100 mA g^-1 after 100 cycles while their pseudocapacitances are at least 130-300 F g^-1 lower than those of the 'composite' at different current densities. Several morphological and functional characteristics are responsible for the excellent energy storage behavior of the 'composite' in comparison with the 'physical mixture' electrodes including (i) the strong attachment of uniformly dispersed NiCo₂O₄ nanoparticles on the functionalized CNTs with (ii) large surface areas and sites for efficient electrochemical reactions. The CNT substrates also function both (iii) as a conductive network for fast ion/electron transfer and (iv) as a cushion to accommodate the volume expansion during the charge/discharge cycles.