Fundamental investigations on the sodium-ion transport properties of mixed polyanion solid-state battery electrolytes

Abstract

Lithium and sodium (Na) mixed polyanion solid electrolytes for all-solid-state batteries display some of the highest ionic conductivities reported to date. However, the effect of polyanion mixing on the ion-transport properties is still not fully understood. Here, we focus on Na1+xZr2SixP3−xO12 (0 ≤ x ≤ 3) NASICON electrolyte to elucidate the role of polyanion mixing on the Na-ion transport properties. Although NASICON is a widely investigated system, transport properties derived from experiments or theory vary by orders of magnitude. We use more than 2000 distinct ab initio-based kinetic Monte Carlo simulations to map the compositional space of NASICON over various time ranges, spatial resolutions and temperatures. Via electrochemical impedance spectroscopy measurements on samples with different sodium content, we find that the highest ionic conductivity (i.e., about 0.165 S cm–1 at 473 K) is experimentally achieved in Na3.4Zr2Si2.4P0.6O12, in line with simulations (i.e., about 0.170 S cm–1 at 473 K). The theoretical studies indicate that doped NASICON compounds (especially those with a silicon content x ≥ 2.4) can improve the Na-ion mobility compared to undoped NASICON compositions.

Pushing Forward Simulation Techniques of Ion Transport in Ion Conductors for Energy Materials

“…The Natrium Super-Ionic CONdcutor (NaSICON) with the composition Na 1+ x Zr 2 Si x P 3– x O 12 is a well-known ceramic solid-electrolyte for all-solid-state Na-ion batteries, ,− with impressive ion-conductivities of ∼4 mS cm –1 at 298 K (for composition Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 ) . NaSICON provides a versatile framework, incorporating both cations (Na + and Zr 4+ ) and anion ( normalS normali O 4 4 − and normalP O 4 3 − ) moieties, which enable the number of Na + ions to vary between 1 and 4 per f.u.…”

“…Na + migration in Na 1+ x Zr 2 Si x P 3– x O 12 . (a) Minimal migration unit describing Na-ion migration in Na 1+ x Zr 2 Si x P 3– x O 12 , where Na follows a Na(2) ↔ Na(1) ↔ Na(2) hop.…”

“…As demonstrated by Deng et al, the diffusivities of Figure can be utilized to predict (via the Nernst–Einstein equation) the ionic conductivities as a function of Na composition and temperature in Na 1+ x Zr 2 Si x P 3– x O 12 . The predicted composition-dependent conductivities were found to be in good agreement with experimental values of bulk conductivities of Na 1+ x Zr 2 Si x P 3– x O 12 .…”

“…As demonstrated by Deng et al, the diffusivities of Figure can be utilized to predict (via the Nernst–Einstein equation) the ionic conductivities as a function of Na composition and temperature in Na 1+ x Zr 2 Si x P 3– x O 12 . The predicted composition-dependent conductivities were found to be in good agreement with experimental values of bulk conductivities of Na 1+ x Zr 2 Si x P 3– x O 12 . Note that to capture adequately the statistical disorder of the PO 4 /SiO 4 polyanion in NaSICON, each point of diffusivity in Figure represents the arithmetic average of 50 distinct spatial arrangements of the silicate and phosphate units at a given Na composition.…”

“…Average time for each Na + diffusion event (in ps) and total simulation time at temperatures of 373, 473, and 573 K and different sodium compositions, respectively. The sum of the time of each kMC event (left y -axis) provides the total simulation time (right y -axis) . Adapted with permission under a Creative Commons Attribution 4.0 International License from ref .…”