Electrochemical Performance of Highly Conductive Nanocrystallized Glassy Alluaudite-Type Cathode Materials for NIBs

Alluaudite-type materials are systematically attracting more attention as prospective cathode materials for sodium-ion batteries. It has been demonstrated that optimized thermal nanocrystallization of glassy analogs of various cathode materials may lead to a significant increase in their electrical conductivity. In this paper, three alluaudite-like glasses (Na2Fe3(PO4)(3)-FFF, Na2VFe2(PO4)(3)-VFF, and Na2VFeMn(PO4)(3)-VFM) were synthesized and subjected to an optimized thermal nanocrystallization. This procedure resulted in nanostructured samples with increased electrical conductivity at room temperature: 5 x 10(-7) S/cm (FFF), 7 x 10(-5) S/cm (VFM), and 6 x 10(-4) S/cm (VFF). The nanocrystalline microstructure was also evidenced by ultra-high-frequency impedance spectroscopy (up to 10 GHz) and proposed electrical equivalent circuits. Prototype electrochemical cells were assembled and characterized with voltage cutoffs of 1.5 and 4.5 V. The electrochemical performance was, however, modest. The gravimetric capacity varied between the studied materials, but did not exceed 35 mAh/g. Capacity retention after ca. 100 cycles was satisfactory. Further optimization of the residual-glass-to-nanocrystallite volume ratio would be desirable.