Rare Earth Oxide-Assisted Sintered NASICON Electrolyte Composed of a Phosphate Grain Boundary Phase with Low Electronic Conductivity

A NASICON-type electrolyte was considered to be one of the most promising electrolytes for solid-state Na metal batteries. However, its lower ion conductivity compared to a liquid electrolyte and the formation of Na dendrites hinder its practical application. Herein, NASICON-type (Na3Zr2Si2PO12) solid-state electrolytes with developed ionic conductivity and declined electronic conductivity were synthesized through the rare earth oxide-assisted sintering method, such as Sm2O3 and Ho2O3. With the presence of Sm2O3 and Ho2O3 during sintering, the formed phosphate grain boundary phase adjusts the Si/P ratio in the NASICON structure with higher Na+ occupancy and then enhances the ionic conductivity of electrolytes. On the other hand, the formed phosphate grain boundary phase with low electronic conductivity prevents the movement of electrons at the grain boundary, reducing the probability that electrons combine with Na+ at the grain boundary to form Na0, thereby restricting the formation of dendrites along grain boundaries. In addition, the added Sm2O3 and Ho2O3 play the role of fluxing agents to increase the densification of ceramics, further enabling the enhancement of ionic conductivity and restriction of dendrites in the voids. As a result, the obtained NZSP-0.2Sm and NZSP-0.3Ho electrolytes deliver critical current density (CCD) values of 0.85 and 0.65 mA cm–2, respectively, at room temperature. Application of the obtained electrolytes in Na metal batteries is evaluated by assembling Na3V2(PO4)3|NZSP-0.2Sm/0.3Ho|Na cells, which deliver high discharge capacity values of 102.6 and 101.8 mAh g–1 at 0.5 C after 100 cycles with capacity retention ratios of 98.3 and 98.6%, respectively. The presented results indicated that rare earth oxide-assisted sintering is an effective route to improve the ionic conductivity and restrict dendrite formation for oxide ceramic solid-state electrolytes.