Na-ion conducting Na 1+x [Sn x Ge 2Àx (PO 4 ) 3 ] (x = 0, 0.25, 0.5, and 0.75 mol%) glass samples with NASICON-type phase were synthesized by the melt quenching method and glass-ceramics were formed by heat treating the precursor glasses at their crystallization temperatures. XRD traces exhibit formation of most stable crystalline phase NaGe 2 (PO 4 ) 3 (ICSD-164019) with trigonal structure. Structural illustration of sodium germanium phosphate [NaGe 2 (PO 4 ) 3 ] displays that each germanium is surrounded by 6 oxygen atom showing octahedral symmetry (GeO 6 ) and phosphorous with 4 oxygen atoms showing tetrahedral symmetry (PO 4 ). The highest bulk Na + ion conductivities and lowest activation energy for conduction were achieved to be 8.39 9 10 À05 S/cm and 0.52 eV for the optimum substitution levels (x = 0.5 mol%, Na on Na-Ge-P network. CV studies of the best conducting Na 1.5 [Sn 0.5 Ge 1.5 (PO 4 ) 3 ]glass-ceramic electrolyte possesses a wide electrochemical window of 6 V. The structural and EIS studies of these glass-ceramic electrolyte samples were monitored in light of the substitution of Ge by its larger homologue Sn.conductivity, electrical properties, glass-ceramics, microstructure
| INTRODUCTIONInvestigations on Na-ion battery technology have attracted significantly in the last few years as emerging storage technology for stationary applications such as grid-scale, mini grid energy storage devices, and also for renewable energy integration such as solar and wind power where gravimetric energy can be compromised. 1,2 Solid-state electrolytes having high Na + ion conductivity are of interest for safest energy storage, lowest cost, long cycle life, and high rate capabilities which are critical to develop room-temperature rechargeable sodium batteries. 3 Systematic studies on electrolyte materials relatively limited in order to realize safer sodium ion batteries with improved performance that operate under ambient conditions in view of the fact that there is hardly accepted negative electrode. Hence, the current challenge is to investigate, characterize, and optimize suitable solid-state electrolyte materials with higher Na + ion conductivity, and engineer the desired properties to provide stable interfacial contact. Alkali oxide (Li/Na) glass and glass-ceramic electrolytes are being widely used in storage and sensing devices because of their better conducting and physicochemical properties than their crystalline counter parts and further modified as a function of composition and controlled crystallization. [4][5][6] Hayashi et al. have been reported on chemical, mechanical, and ionic conductivity studies to closely meet desired current densities of various glass and glass-ceramic electrolytes mixed with various modifiers.7-9 Among several oxide glass electrolytes for sodium ion secondary batteries, sodium-