Two-dimensional holey Co₃O₄ nanosheets for high-rate alkali-ion batteries: From rational synthesis to in situ probing

A general template-directed strategy is developed for the controlled synthesis of two-dimensional (2D) assembly of Co₃O₄ nanoparticles (ACN) with unique holey architecture and tunable hole sizes that enable greatly improved alkali-ion storage properties (demonstrated for both Li and Na ion storage). The as-synthesized holey ACN with 10 nm holes exhibit excellent reversible capacities of 1324 mAh/g at 0.4 A/g and 566 mAh/g at 0.1 A/g for Li and Na ion storage, respectively. The improved alkali-ion storage properties are attributed to the unique interconnected holey framework that enables efficient charge/mass transport as well as accommodates volume expansion. In situ TEM characterization is employed to depict the structural evolution and further understand the structural stability of 2D holey ACN during the sodiation process. The results show that 2D holey ACN maintained the holey morphology at different sodiation stages because Co₃O₄ are converted to extremely small interconnected Co nanoparticles and these Co nanoparticles could be well dispersed in a Na₂O matrix. These extremely small Co nanoparticles are interconnected to provide good electron pathway. In addition, 2D holey Co₃O₄ exhibits small volume expansion (6%) compared to the conventional Co₃O₄ particles. The 2D holey nanoarchitecture represents a promising structural platform to address the restacking and accommodate the volume expansion of 2D nanosheets for superior alkali-ion storage.