Improving Room Temperature Ionic Conductivity of Na_3–xK_xZr₂Si₂PO₁₂ Solid-Electrolytes: Effects of Potassium Substitution

Abstract

The battery safety and cost remain major challenges for developing nextgeneration rechargeable batteries. All-solid-state sodium (Na)-ion batteries are a promising option for low-cost as well as safe rechargeable batteries by using abundant resources and solid electrolytes. However, the operation of solid-state batteries is limited due to the low ionic conductivity of solid electrolytes. Therefore, it is essential to develop new compounds that feature a high ionic conductivity and chemical stability at room temperature. Herein, we report a potassium-substituted sodium superionic conductor solid electrolyte, Na 3−x K x Zr 2 Si 2 PO 12 (0 ≤ x ≤ 0.2), that exhibits an ionic conductivity of 7.734 × 10 −4 S/cm −1 at room temperature, which is more than 2 times higher than that of the undoped sample. The synchrotron powder diffraction patterns with Rietveld refinements revealed that the substitution of large K-ions resulted in an increased unit cell volume, widened the Na diffusion channel, and shortened the Na−Na distance. Our work demonstrates that substituting a larger cation on the Na site effectively widens the ion diffusion channel and consequently increases the bulk ionic conductivity. Our findings will contribute to improving the ionic conductivity of the solid electrolytes and further developing safe next-generation rechargeable batteries.

Crystal Chemistry of NaSICONs: Ideal Framework, Distortion, and Connection to Properties

“…40 In another, experimental, study, it was shown that 10% substitution of larger K + for Na + in monoclinic NaSICON-type Na 3−x K x Zr 2 Si 2 PO 12 increases conductivity and decreases activation energy. 41 This is consistent with the strong response of the framework to the size mismatch between the A and the A site, discussed in Section 2.4 and illustrated in Figure 11c. Based on that, we can formulate at least two practical recommendations for optimizing NaSICON-type frameworks to increase ionic conductivity, as follows.…”

“…The increase of the [LiO 6 ] polyhedra volume in turn was shown to correlate with the reduced migration activation energy calculated by the Nudged Elastic Band method . In another, experimental, study, it was shown that 10% substitution of larger K + for Na + in monoclinic NaSICON-type Na 3– x K x Zr 2 Si 2 PO 12 increases conductivity and decreases activation energy . This is consistent with the strong response of the framework to the size mismatch between the A and the A site, discussed in Section and illustrated in Figure c.…”

A perspective on the building blocks of a solid-state battery: from solid electrolytes to quantum power harvesting and storage

“…Considering the sodium ion version, Na 1+x Zr 2 Si x P 3−x O 12 , the highest ionic conductivity obtained is for x = 2, Na 3 Zr 2 PSi 2 O 12 with an ionic conductivity of approximately 1.13 mS cm −1 at RT. 22 However, several doping strategies have been employed to achieve higher ionic conductivities: (1) doping with potassium ions, which demonstrated for Na 2.303 -K 0.099 Zr 2 Si 1.880 P 1.106 O 12 an ionic conductivity of 0.8 mS cm −1 at RT; 23 (2) doping with Ge, Na 3.125 Zr 1.75 Sc 0.125 Ge 0.125 Si 2 PO 12 , resulted in total ionic conductivity 4.6 mS cm −1 ; 24 (3) doping with Sc, Na 3.4 Zr 1.6 Sc 0.4 Si 2 PO 12 provides an RT ionic conductivity of 4.0 mS cm −1 . 22 As for perovskite electrolytes, one notable example is Li 3x La (2/3)−x TiO 3 LLTO.…”

A hierarchical approach to designing a Na-rich phosphide solid-state electrolyte for Na-ion batteries

“…4,5 Therefore, designing solid-state electrolytes (SSEs) for NIBs has attracted tremendous research attention in recent years. 6–13 To date, SSEs for NIBs have been mostly based on oxides, sulfides and halides, 14,15 such as β-alumina, 16,17 NASICON, 18,19 Na 3 PS 4 (ref. 20) and Na 3 OCl.…”