[en] Powdery hexagonal boron nitride (h-BN), as an important material for electrochemical energy storage, has been typically synthesized in bulk and one/two-dimensional (1/2D) nanostructured morphologies. However, until now, no method has been developed to synthesize powdery three-dimensional (3D) h-BN. This work introduces a novel NaCl-glucose-assisted strategy to synthesize micron-sized 3D h-BN with a honeycomb-like structure and its proposed formation mechanism. We propose that NaCl acts as the template of 3D structure and promotes the nitridation reaction by adsorbing NH${}_3$. Glucose facilitates the homogeneous coating of boric acid onto the NaCl surface via functionalizing the NaCl surface. During the nitridation reaction, boron oxides (BO${}_4$ and BO${}_3$) form from a dehydration reaction of boric acid, which is then reduced to O${}_2$-B-N and O-B-N${}_2$ intermediates before finally being reduced to BN${}_3$ by NH${}_3$. When incorporated into polyethylene oxide-based electrolytes for Li metal batteries, 5 wt % of 3D h-BN significantly enhances ionic conductivity and mechanical strength. Consequently, this composite electrolyte demonstrates superior electrochemical stability. It delivers 300 h of stable cycles in the Li//Li cell at 0.1 mA cm${}^{-<!--\; ???\; -->2}$ and retains 89 % of discharge capacity (138.9 mAh g${}^{-<!--\; ???\; -->1}$) after 100 cycles at 1 C in the LFP//Li full cell. (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH)