Tomasz Pietrzak1,Marek Wasiucionek1,Jerzy Garbarczyk1
Warsaw Univ. of Technology1
Tomasz Pietrzak1,Marek Wasiucionek1,Jerzy Garbarczyk1
Warsaw Univ. of Technology1
<b>Introduction</b><br/>It has been well established that nanostructured materials for energy applications (cathodes, anodes, solid electrolytes) perform better than their coarse-grained polycrystalline counterparts. Most of these nanomaterials were obtained by solid state reaction techniques. Our group, however, has systematically explored thermal crystallization of glasses as a simpler alternative way to synthesize nanostructured materials for energy storage. We have carried a series of studies on thermal nanocrystallization [1] of glassy analogues of cathode materials for sodium and lithium batteries (e.g. V<sub>2</sub>O<sub>5</sub> [2], LiFePO<sub>4</sub> [3], Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub> [4]) and solid electrolytes (e.g. Bi<sub>2</sub>O<sub>3</sub> – oxygen ion conductor [5]). Thanks to this alternative technique, it was possible to synthesize nanostructured cathodic and electrolytic materials exhibiting interesting morphology and unique physical properties [6].<br/><br/><b>Nanocrystalline cathode materials</b><br/>In the case of the cathode materials, nanocrystallization was used to improve the poor conductivity of those materials without introducing carbon additives. A giant and irreversible increase of the conductivity (even by a factor as high as 10<sup>9</sup>) was obtained. The microstructure of the samples consisted of 5–50 nm nanocrystallites. The conductivity increase has been attributed to appearance of highly disordered shells around such small grains (cores), which facilitated the electron hopping, e.g. between aliovalent V<sup>5+</sup> / V<sup>4+</sup> or Fe<sup>3+</sup> / Fe<sup>2+</sup> centers.<br/><br/><b>Nanocrystalline electrolytes</b><br/>Crystalline δ–Bi<sub>2</sub>O<sub>3</sub> is the best O<sup>2–</sup> ion conductor, but it is stable in a relatively narrow temperature range 729–825 °C only. Its very high ionic conductivity (1 S/cm at 750 °C) has motivated many researchers to look for a method to stabilize this fluorite-type structure to lower temperature. So far the successful strategies to achieve the stabilization of the delta phase have included doping (e.g. by rare-earth elements) or synthesis in form of thin films.<br/>By thermal nanocrystallization of Bi<sub>2</sub>O<sub>3</sub>-based glasses, we have succeeded to stabilize nanosize grains of δ–like phase confined in a glassy matrix down to RT [7]. The phase remained stable for at least one year of storage at ambient conditions. This method may be used to stabilize other phases in the future.<br/><br/><b>Acknowledgments</b><br/>This research was funded by POB Energy and POB TechMat of Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) program.<br/><br/><b>References</b><br/>1. T.K. Pietrzak et al., <i>Materials Science and Engineering B</i> <b>213</b> (2016) 140–147.<br/>2. T.K. Pietrzak et al., <i>Journal of Power Sources</i> <b>194</b> (2009) 73–80.<br/>3. J.E. Garbarczyk et al., <i>Solid State Ionics</i> <b>272</b> (2015) 53–59.<br/>4. T.K. Pietrzak et al., <i>International Journal of Applied Glass Science</i> <b>11</b> (2020) 87–96.<br/>5. T.K. Pietrzak et al., <i>Solid State Ionics</i> <b>323</b> (2018) 78–84.<br/>6. T.K. Pietrzak et al., <i>Nanomaterials</i> <b>11</b> (2021) 1321.<br/>7. T.K. Pietrzak et al., <i>Scientific Reports</i> <b>11</b> (2021) 19145.