Morphological evolution of hollow NiCo2O4 microspheres and their high pseudocapacitance contribution for Li/Na-ion battery anodes

Abstract

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.