Nucleation-density-regulated dimensional evolution of growth unit from 2D nanosheets to 1D nanoneedles in self-assembled hierarchical NiCo2O4 for enhanced lithium storage

Abstract

Bimetallic oxide anodes with multi-electron conversion capabilities require hierarchical multi-dimensional structures to provide both a stable framework and abundant lithium-storage active sites for balance of cycling stability and reversible capacity. However, achieving precise dimension (D) regulation in these structures, particularly through surfactant-free synthesis, remains a significant challenge. Herein, a surfactant-free supersaturation-driven nucleation-control approach was developed to construct self-assembled hierarchical flower-like (SAHF) NiCo₂O₄ structures (2D/3D or 1D/3D) using a facile urea-induced hydrothermal co-precipitation process. By precisely adjusting urea concentration, controlled supersaturation and nucleation density further affect the driving force for isotropic growth, which governed the dimensional evolution from 2D nanosheets to 1D nanoneedles within the SAHF framework, a breakthrough in morphology regulation without external additives. Our systematic investigations revealed that the unique 1D/3D SAHF NiCo₂O₄ significantly outperformed 2D/3D architectures, showcasing superior Li-storage capabilities, including remarkable rate performance and a striking 116% capacity retention after 200 cycles compared to only 58% for 2D/3D counterparts. The real differential capacity/voltage (dQ/dV) analysis suggests that the increased cycling capacity is possibly linked to the dynamic growth of Li-containing gel-like film (corresponding to the raised 1.1 V couple) and the gradually increased reversible evolution of Co²⁺/Co³⁺ (the newly emerged 2.2 V couple) during cycling. Galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) analyses confirmed fast ion diffusion and enhanced charge transfer kinetics in the 1D/3D SAHF structure. Importantly, in situ X-ray diffraction (XRD) uncovered previously unrecognized dynamic structural changes with a gradual loss of long-range order in NiCo₂O₄ over the initial three cycles, challenging the established view of immediate disordering after the first discharge. Moreover, it was uniquely experimentally observed that CoO formation occurred prior to NiO during the first discharge, and partially unreoxidized Ni and Co persisted throughout cycling, both of which were previously unreported phenomena. This study not only provides a precise surfactant-free dimension-control strategy for tailoring hierarchical bimetallic oxides but also unveils fundamental insights into their growth dynamics and unprecedented electrochemical behavior, offering new pathways for enhancing lithium-ion storage performance.