The growing demand for sustainable energy solutions and the increasing environmental challenges necessitate significant advancements in energy storage technologies. Recent studies highlight carbon-nitride-based materials as promising candidates due to their high capacitance, low cost, and minimal volumetric expansion upon alkali-metal doping, making them viable for high-energy-density applications[1-3]. We explored alkali-metal carbon-nitride compounds (AMCNC) as potential battery materials, combining the advantages of both alkali metals and carbon-nitrides to enhance battery performance.

Previous studies provide compelling evidence that the properties of materials can undergo significant alterations when subjected to high pressure. [4-5] It is worth noting that most materials from AMCNC family haven’t been explored under high pressure. Motivated by these findings, we aim to investigate whether compressing AMCNC, specifically LiC4N3 and NaC4N3, under moderate pressure of a few tenths of gigapascal (GPa), can yield materials that are applicable as new battery materials with better efficiency. We note that both the LiC4N3 and NaC4N3 compounds, that are the subject of this study, are molecular solids and distinct from the conventional Li- and Na-intercalated g-C4N3. 

LiC4N3, as the starting material, was synthesized in-house and characterized at ambient conditions, confirming the successful synthesis. We performed a high-pressure Raman spectroscopy study up to 20 GPa, and an in-situ synchrotron Wide-Angle X-ray Scattering (WAXS) measurements up to 30 GPa using a diamond anvil cell (DAC). The results documented a pressure-induced polymerization above 15 GPa, concomitant with a crystalline-to-crystalline phase transformation and a possible metallization, as observed under the microscope.  In details,  Raman measurements documented that the C≡N bonds disappeared above 15 GPa, while a new peak at around 1500 cm-1, which resembled the internal E2g graphite peak (G-band), emerged above same pressure. It is plausible to assume that the molecular C≡N bonds were transformed into C=N extended bonds, and carbon bonds were connected to form carbon chains. The quenched material remained metastable and metallic after fully releasing the pressure and survived for a couple of hours after exposing to atmospheric conditions, while remaining stable for prolonged time when stored under inert atmosphere conditions. 
NaC4N3 has also been studied under high pressure using Raman spectroscopy and WAXS at synchrotron sources up to 32 GPa. At around 6 GPa and 20 GPa, two crystalline-to-crystalline phase transformations were observed, and the sample seemed metalized under the microscope. Similar to the case of LiC4N3  Raman spectra, we also conclude that the  C≡N triple bonds were transformed into extended double bonds, The quenched NaC4N3 could survive in the air longer than quenched- LiC4N3, but is still better to be stored in a glovebox.

 [1] Veith, G. M., Baggetto, L., Adamczyk, L. A., Guo, B., Brown, S. S., Sun, X. G., ... & Dudney, N. J. (2013). Electrochemical and solid-state lithiation of graphitic C3N4. Chemistry of Materials, 25(3), 503-508.
[2] Wu, M., Wang, Q., Sun, Q., & Jena, P. (2013). Functionalized graphitic carbon nitride for efficient energy storage. The Journal of Physical Chemistry C, 117(12), 6055-6059.
[3] Kraytsberg, A., & Ein-Eli, Y. (2017). A critical review-promises and barriers of conversion electrodes for Li-ion batteries. Journal of Solid State Electrochemistry, 21, 1907-1923.
[4] E. Stavrou, Y. Yao, A. F. Goncharov, S. Lobanov, J. M. Zaug, H. Liu, E. Greenberg, and Vitali B. Prakapenka. Synthesis of Xenon and Iron-Nickel Intermetallic Compounds at Earth’s Core Thermodynamic Conditions. Phys. Rev. Lett. 120, 096001(2018). 
 [5] E. Stavrou, S. Lobanov, H. Dong, A. R. Oganov, V. B. Prakapenka, Z. Konopkova and A. F. Goncharov. “Synthesis of ultra-incompressible sp3-hybridized carbon nitride, Chem. Mater. 28, 6925 (2016)
 

high pressure,phase transformations,alkali metals,carbon nitrides