High-entropy alloy (HEA) electrocatalysts offer tunable multi-element synergy and intrinsic structural robustness for oxygen evolution reaction (OER), yet integrating high active-site exposure with desirable lattice and electronic structures remains a significant challenge. Here we present an ultrafast thermal explosion strategy for one-step synthesis of carbon-encapsulated nanoreactors uniformly embedding FeCoNiCuAl HEA nanoparticles. Ultrafast heating of metal-chloride-loaded carbon black drives rapid chloride decomposition and gas evolution, inflating the carbon into a hierarchical porous network and inducing the formation and spatial confinement of HEA nanoparticles. This nanoreactor architecture with confined microenvironments maximizes accessible active sites and accelerates mass/charge transport, yielding lower OER overpotentials than commercial RuO₂. Their applicabilities in overall water splitting (OWS) and rechargeable Zn-air batteries further confirm the potential for practical energy storage integration. Local structural investigations reveal that the incorporation of Al element could reduce the first-shell coordination number, enhancing the adsorption capacity of the active sites. Density functional theory (DFT) calculations further demonstrate that the synergistic interaction between Al and neighboring metal sites facilitates the efficient OER, and Al-induced modulation of electronic structure lowers the desorption barrier of the *OOH intermediate, thereby accelerating OER kinetics. This explosion-driven nanoreactor strategy provides a general, one-step route to engineer carbon nanoreactors embedding HEA electrocatalysts, opening new avenues for advanced clean energy catalysis and practical device integration.

Topochemical transformation has emerged as a promising method for fabricating two-dimensional (2D) materials with precise control over their composition and morphology. However, the large-scale synthesis of ultrathin 2D materials with controllable thickness remains a tremendous challenge. Herein, we adopt an efficient topochemical synthesis strategy, employing a confined reaction space to fabricate ultrathin 2D Sn₄P₃ nanosheets in large-scale. By carefully adjusting the rolling number during the processing of Sn/Al foils, we have successfully fabricated Sn₄P₃ nanosheets with varied layer thicknesses, achieving a remarkable minimum thickness of two layers (~ 2.2 nm). Remarkably, the bilayer Sn₄P₃ nanosheets display an exceptional initial capacity of 1088 mAh·g⁻¹, nearing the theoretical value of 1230 mAh·g⁻¹. Furthermore, we reveal their high-rate property as well as outstanding cyclic stability, maintaining capacity without fading more than 3000 cycles. By precisely controlling the layer thickness and ensuring nanoscale uniformity, we enhance the lithium cycling performance of Sn₄P₃, marking a significant advancement in developing high-performance energy storage systems.