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Glass-ceramic processes to prepare NASICON-type Na_xTi_2(PO_4)_3, which is an anode candidate for sodium-ion batteries, from 30Na_2O–40TiO_2–30P_2O_5 glass have been investigated with varied atmospher...ic 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_3Ti_2(PO_4)_3 with triclinic symmetry is crystallized at >∼800 °C. The formation of Na_3Ti_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.続きを見る
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