Abstract
Glass-ceramic processes to prepare NASICON-type Na x Ti 2 (PO 4 ) 3 , which is an anode candidate for sodium-ion batteries, from 30Na 2 O–40TiO 2 –30P 2 O 5 glass have been investigated with varied atmospheric conditions. By annealing in the inert (N 2 ) atmosphere, the glass started crystalizing into NASICON-type NaTi 2 (PO 4 ) 3 with rhombohedral symmetry at ∼600 °C followed by crystallization of other phases to be multiphase mixtures at higher temperatures. In contrast, in the reducing (5% H 2 /Ar) atmosphere, NASICON-type Na 3 Ti 2 (PO 4 ) 3 with triclinic symmetry is crystallized at >∼800 °C. The formation of Na 3 Ti 2 (PO 4 ) 3 is associated with a loss of excess oxygens in the initial glass composition and a reduction of Ti from Ti 4+ to Ti 3+ . The reduction process upon the glass-to-ceramic conversion was traced by in-situ observation during the thermogravimetric analysis. It is also revealed that the electrochemical Na + -storage capabilities of the glass-ceramic electrodes are correlated with the Na-ion occupancy between these two phases, and their phase fractions affect the charge-discharge properties of Na-ion cells. Finer glass-ceramic powders could improve the electrochemical properties and achieve almost 80% of its theoretical capacity.
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