Hollow nanostructures offer promising potential for advanced energy storage and conversion applications. In the past decade, considerable research efforts have been devoted to the design and synthesis of hollow nanostructures with high complexity by manipulating their geometric morphology, chemical composition, and building block and interior architecture to boost their electrochemical performance, fulfilling the increasing global demand for renewable and sustainable energy sources. In this Review, we present a comprehensive overview of the synthesis and energy-related applications of complex hollow nanostructures. After a brief classification, the design and synthesis of complex hollow nanostructures are described in detail, which include hierarchical hollow spheres, hierarchical tubular structures, hollow polyhedra, and multi-shelled hollow structures, as well as their hybrids with nanocarbon materials. Thereafter, we discuss their niche applications as electrode materials for lithium-ion batteries and hybrid supercapacitors, sulfur hosts for lithium-sulfur batteries, and electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. The potential superiorities of complex hollow nanostructures for these applications are particularly highlighted. Finally, we conclude this Review with urgent challenges and further research directions of complex hollow nanostructures for energy-related applications.

Hierarchical hollow nanostructures with complex shell structures are attractive for efficient storage of electrochemical energy. However, reports on the designed formation of such delicate architectures are rare. Starting from a framework of Co-based zeolitic imidazolate (ZIF-67) nanocubes, we demonstrate a templateengaged formation of double-shelled hollow structures with CoS-nanoparticleassembled nanoboxes surrounded by outer CoS-nanosheet-constructed shells (denoted as CoS-NP/CoS-NS DSNBs). Delicate manipulation of the templateengaged reaction between ZIF-67 and water leads to the formation of ZIF-67/ Co(OH) 2 -nanosheet yolk-shelled structures, which are then transformed into CoS-NP/CoS-NS DSNBs through the reactions with Na 2 S. As a result of the unique assembly of subunits with different dimensionalities, the CoS-NP/CoS-NS DSNBs have a high specific surface area, suitable mesopores, and good structural robustness, which contribute to exceptional performance as a battery-type electrode. Moreover, a hybrid supercapacitor is fabricated by using CoS-NP/CoS-NS DSNBs as the positive electrode and activated carbon as the negative electrode. Impressively, these optimized devices can be operated stably in a wide potential range of 1.6 V with greatly enhanced energy density and power capability. The device is stable for 10,000 cycles with a capacitance retention of 88%.

A novel natural drying (ND) strategy for low-cost and simple fabrication of graphene aerogels (GAs) is highlighted. The as-formed NDGAs exhibit ultralarge reversible compressibility (99%) and tunable Poisson's ratio behaviors (-0.30 < ν < 0.46), which suggests promising applications in soft actuators, soft robots, sensors, deformable electronic devices, drug release, thermal insulator, and protective materials.

Hierarchical Fe3 O4 hollow spheres constructed by nanosheets are obtained from solvothermally synthesized Fe-glycerate hollow spheres. With the unique structural features, these hierarchical Fe3 O4 hollow spheres exhibit excellent electrochemical lithium-storage performance.