From Li ₃ BN ₂ to hBN: an <i>in situ</i> study

Abstract Lithium boron nitride (Li 3 BN 2 ) is a lithium-ion conducting material, interesting for energy storage, and belonging to the alkali/alkali-earth metal-BN compounds (e.g. Ba 3 B 2 N 4 and Mg–B–N). These compounds play a critical role as solvents for boron nitride crystal growth, especially for the high-pressure high-temperature process, which provides the most common source of hexagonal boron nitride (hBN), a cornerstone material in the two-dimensional community. Despite their strategic importance, the understanding of their physico-chemical thermal behavior remains poorly understood. Here we investigate the thermal stability of Li 3 BN 2 using in situ characterization techniques at multiple scales. Using complementary global and local-scale analyses, its structural, morphological, and energetic transformations are tracked. thermo-gravimetric analyses/differential scanning calorimetry analyses, combined with direct in situ observation of microstructural changes recorded by HT environmental electron microscopy and HT x-ray diffraction, provide essential insights on reactions occurring from moderate to high temperatures. They reveal the formation of a lithium intercalated hBN phase above 700 °C, followed by Li 3 BN 2 melting at 863 °C and its progressive vaporization. Additionally, Li 3 BN 2 ’s thermal expansion coefficients (in both a and c lattice directions) are reported for the first time. Its volumetric thermal expansion is estimated at 82 ± 1 10 −6 · K −1 . Above 1200 °C crystallization of hBN occurs, concomitant with a release of N 2 and highly reactive lithium-based gaseous species. These findings provide fundamental insights into BN-based solvents and highlight their critical role in optimizing hBN synthesis routes for 2D material applications.