Highly Efficient Functionalization of hBN with Lithium Oxalate: A Multifunctional Platform for Composites, Ion Transport, and Spin Labeling

TL;DR

This work develops a scalable, solvent-free mechanochemical route to covalently functionalize hBN with lithium oxalate, producing Li$_2$(BN)$_6$C$_2$O$_4$ (LBNCO), a lamellar solid with combined ionic conduction, chemical robustness, and thermal resilience. Comprehensive characterization (XRD, XPS, FTIR, Raman, NMR, ESR, TGA-DSC-MS) and DFT simulations reveal dual-edge (zigzag and armchair) functionalization with oxalate ligands, partial Li intercalation, and stable radical centers that enable spin-labeling. LBNCO exhibits predominantly ionic transport with a wide band gap ($E_g \approx 5.82$–$6.06$ eV) and enhanced conductivity upon Li-doping by >$10^{2}$, suggesting utility as a separator and interfacial coating in Li-metal batteries and as a functional filler in polymer composites. The study introduces a general, solvent-free platform for creating lithium-rich, boron nitride-based solids with tunable ion conduction, thermal stability, and radical functionality, extendable to other alkali ions and applications in energy storage and materials science.

Abstract

The development of multifunctional solid-state materials is key to advancing lithium-ion batteries with enhanced safety and simplified architectures. Here, we report a scalable, highly efficient (near $100\%$), solvent-free mechanochemical synthesis of hexagonal boron nitride (hBN) functionalized with lithium oxalate (Li$_2$C$_2$O$_4$), yielding a novel lamellar composite that functions both as a lithium-ion conductor and separator. The high-energy milling process promotes exfoliation of hBN and covalent attachment of oxalate groups at edge and defect sites, forming a brown, nanocrystalline material with uniform lithium distribution. The composite exhibits room-temperature ionic and negligible electronic conductivity, thermal stability at least up to $350~^{\circ}$C, and hosts stable free radicals enabling its use as a spin label. The synthesis produces no byproducts and can be extended towards lithium doping via secondary mechanochemical steps, creating highly doped, chemically stable phases that host additional Li for ionic conduction. These results introduce a new class of lithium-rich, boron nitride-based solids for solid-state batteries, combining ion conduction, mechanical robustness, and thermal resilience in a single material platform.

Figures (14)

• Figure 1: a) The synthesis of Li$_{2}$(BN)$_{6}$C$_{2}$O$_{4}$ (LBNCO) from its constituents using a high energy ball mill (Fe balls) or a planetary ball mill (Zr balls), and a HRTEM image of the resulting material showing the nanocrystalline nature of it and the lattice periodicity in the $c$-axis direction. b) Proposed reaction steps for both types of edges (zigzag and armchair) supported by DFT-based modeling and spectroscopic evidence. c) Calcination of the LBNCO material results in a color change of the material: while up to $400~^{\circ}$C it darkens, above $600~^{\circ}$C it immensely lightens reaching a pastel white color above $800~^{\circ}$C. d) Powder X-ray diffraction patterns of LBNCO and its calcined derivatives. The vertical lines refer to the reflections of crystalline hBN; milling results in a turbostratic material.
• Figure 2: a) Thermal decomposition of LBNCO as monitored by TGA (upper panel) and MS (lower panel) with Ar as carrier gas. The mass is normalized to the initial sample mass, $m/m_0$. Right axis of the upper panel shows a numerical derivative, $\mathrm{d}m/\mathrm{d}T$, where negative peaks indicate mass-loss events, emphasized by vertical dashed lines for direct comparison with the MS ion current signals. The MS traces are identified as H$_{2}$O ($m/z = 18)$, N$_{2}$ ($m/z = 28$), HCN ($m/z = 27$) and CO$_{2}$ ($m/z = 44$). b) FTIR absorbance spectra of LBNCO and its calcined derivatives, together with hBN, cBN and Li$_{2}$C$_{2}$O$_{4}$. The vertical lines refer to the peaks of the main bands of Li$_{2}$C$_{2}$O$_{4}$. c) Isotope effects in the FTIR spectra of LBNCO (upper panel) and Li$_{2}$C$_{2}$O$_{4}$ (lower panel). The vertical lines refer to the peaks of the main bands of Li$_{2}$C$_{2}$O$_{4}$ with natural abundance isotopes.
• Figure 3: The B, N, and Li 1s XPS spectra and their deconvolution of LBNCO and its $400~^{\circ}$C and $700~^{\circ}$C calcined derivatives.
• Figure 4: NMR (a-f) and ESR (g-i) spectra of LBNCO and its calcined derivatives. The intensities are scaled for a better visualization of relative changes a) $^{11}$B together with hBN and cBN as reference materials. b) Ratio of sp$^3$ boron compared to the total number of B atoms. c) $^{7}$Li shown together with Li$_{2}$C$_{2}$O$_{4}$ and LiBOB for reference. d) $^{7}$Li spin-lattice, $T_1$, relaxation times obtained using the inversion-recovery pulse sequence. Please note the $4$-fold shortening of the relaxation time in the functionalized materials compared to lithium oxalate. Calcination further decreases the relaxation time. LiBOB is also given as a reference. e) $^{13}$C with $^{13}$C-enriched LBNCO and Li$_{2}$C$_{2}$O$_{4}$ for reference (natural abundance; Savitzky--Golay filtered). The structure, as well as the chemical shifts obtained from DFT calculations are also depicted. f) $^{14}$N, a $95$ w% hBN (sp$^2$) mixed with $5$ w% NH$_{4}$Cl sp$^3$ used for reference. Inset shows a full rotational spectra noting that no further nitrogen configurations are present. g) Measured derivative X-band cw ESR signal and its deconvolution to a Lorentzian and a Gaussian component of the as-prepared LBNCO. Graphics show the corresponding structures and electron densities. h) Mass spin-susceptibility and spin-counts. i) $g$-factors (relative to the free electron $g$-factor, $g_{\text{e}}$) associated with the Lorentzian and Gaussian ESR signals.
• Figure 5: Arrhenius plot of the electrical transport measurement showing activated temperature dependence consistent with fixed-range hopping. The lithium-doping of the material results in the increase of conductivity by over two orders of magnitude. The inset presents the same data as resistivity versus temperature.