LAUSR

Hollow urchin-like NiCo2O4 microspheres (∼3 μm) with a large specific surface area (158.57 m2 g−1) have been synthesized by a facile template-free hydrothermal method and a morphology evolution mechanism of “bundles-solid spheres-hollow urchin-like microspheres” was proposed. The hollow urchin-like structure appears when the hydrothermal time is increased to 8 h, which can be accelerated by the addition of excess urea. Benefiting from the unique three-dimensional (3D) hollow structure and the desired composition, the NiCo2O4 microspheres exhibit an excellent reversible specific capacity for lithium ion batteries (991 mA h g−1 after 50 cycles) and sodium ion batteries (322.3 mA h g−1 after 50 cycles). The unique 3D hollow structure offers enough space to alleviate volume expansion caused by the Li+/Na+ insertion/extraction, and the perfect electrical conductivity of spinel binary metal oxides facilitates the transport of ions and electrons. A high capacitance contribution of 90% was achieved for LIBs at 0.3 mV s−1, while the capacitance contributions for SIBs were only 36% at 0.3 mV s−1 and 73% even at 5 mV s−1, which indicates that a capacitive-controlled charge storage mechanism plays a dominant role in the Li+ storage of NiCo2O4 microspheres. This work has guiding significance in the preparation of electrode materials with high electrochemical performance.