The Synthesis of Three‐Dimensional Hexagonal Boron Nitride as the Reinforcing Phase of Polymer‐Based Electrolyte for All‐Solid‐State Li Metal Batteries

Abstract 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 NH3. Glucose facilitates the homogeneous coating of boric acid onto the NaCl surface via functionalizing the NaCl surface. During the nitridation reaction, boron oxides (BO4 and BO3) form from a dehydration reaction of boric acid, which is then reduced to O2‐B‐N and O‐B‐N2 intermediates before finally being reduced to BN3 by NH3. 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.