Ultrathin VO2 nanosheets self-assembled into 3D micro/nano-structured hierarchical porous sponge-like micro-bundles for long-life and high-rate Li-ion batteries

Nanomaterials, especially graphene-like 2D ultrathin ones that possess a wealth of unprecedented functionalities, can increase the discharge capacity of Li-ion batteries (LIBs), but they still suffer from poor cycling and rate performances due to their serious self-aggregation and pulverization. Constructing micro/nano-structures is a quite promising method to address the above issues. However, it remains a huge challenge to build 3D hierarchical porous micro/nano-structures self-assembled from ultrathin nanostructured building blocks, which has prompted extensively great interest. Herein, we report a facile "hydrolysis-controllable crystallization" strategy to controllably synthesize different dimensional (i.e. 1D, 2D, and 3D) VO2 (B) nanostructures by a simple one-step high-temperature mixing method under hydrothermal conditions. In particular, unique 3D micro/nano-structured hierarchical porous sponge-like micro-bundles (SLMBs) self-assembled from 2D crumpled ultrathin VO2 (B)@C nanosheets with a thickness of only similar to 3.1 nm (denoted as VO2 (B)@C-SLMBs) are synthesized. This is the first report on the synthesis of 2D ultrathin VO2 (B) nanomaterials. Importantly, the intrinsic VO2 (B) crystallization behavior and controllable synthesis mechanism of VO2 (B) micro/nano-structures are revealed for the first time by the density functional theory calculation. The as-synthesized VO2 (B)@C-SLMBs possess distinct structural advantages, i.e., large surface area, abundant meso/micropores, robust structure and conductive carbon frameworks, which make them exhibit excellent electrochemical performance in terms of long life, high rate, and large capacity as cathode materials of LIBs. The discharge capacity was 206 mA h g(-1) after 160 cycles at 100 mA g(-1), corresponding to 105% of the initial capacity. Even at a large current density of 1000 mA g(-1), they still exhibit a high retention of 104% after long period of 1000 cycles. These results indicate that the as-synthesized VO2 (B)@C-SLMBs have a great potential for long-life and high-rate cathode materials of next-generation LIBs.