Double Paddle‐Wheel Enhanced Sodium Ion Conduction in an Antiperovskite Solid Electrolyte

Abstract

Antiperovskite (AP) structure compounds (X3AB, where X is an alkali cation and A and B are anions) have the potential for highly correlated motion between the cation and a cluster anion on the A or B site. This so-called "paddle-wheel" mechanism may be the basis for enhanced cation mobility in solid electrolytes. Here we show, through combined experiments and modeling, the first instance of a double paddle-wheel mechanism, leading to fast sodium ion conduction in the antiperovskite Na3-xO1-x(NH2)x(BH4). As the concentration of amide (NH2 -) cluster anions is increased, large positive deviations in ionic conductivity above that predicted from a vacancy diffusion model are observed. Using EIS, PXRD, synchrotron XRD, neutron diffraction, AIMD, and NMR, we characterize the cluster anion rotational dynamics, and find that cation mobility is influenced by the rotation of both NH2and BH4species, resulting in sodium ion conductivity a factor of 10 2 higher at x=1 than expected for the vacancy mechanism alone. Generalization of this phenomenon to other compounds could accelerate fast ion conductor design and exploration.

Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries

“…By simultaneously substituting the amide (NH 2 ) À cluster at the B site and replacing the A site with (BH 4 ) À in the Na 3 BA antiperovskite, a ''double paddle-wheel'' effect was proposed in Na 3 (NH 2 )(BH 4 ) with both rotational cluster groups promoting Na + conduction, in distinct ways. 724 In this dual-cluster-based antiperovskite, as the slowly rotating amide cluster obeyed the ''paddle-wheel mechanism'', the indirect correlation between the fast-rotating borohydride cluster and Na + diffusion was observed for the first time, which may explain the failed attempt at the replication of fast-conducting Na 3 O(BH 4 ). Meanwhile, by comparing the calculated jump attempt frequencies of carrier migrations in Na 3 OBr and Na 3 O(BH 4 ), the (BH 4 ) À cluster was proved to mitigate the activation energy and facilitate the ion hopping by a large margin.…”

Unveiling the structure and ion dynamics of amorphous Na3−xOHxCl antiperovskite electrolytes by first-principles molecular dynamics

“…The rotational disorder due to the hydroxyl OH − anions, along with other anions, is a feature common to other solid electrolytes, such as crystalline Na 3 OBH 4 13 and Na 3−x O 1−x (NH 2 ) x (BH 4 ). 95 Ion migration occurs via a mechanism that combines the concerted motion of ions with large quasi-permanent reorientations of the surrounding hydroxyl groups. This latter effect, known as the paddlewheel mechanism, 30,96,97 is typically observed in hightemperature crystalline polymorphs.…”

Dynamic Lone Pairs and Fluoride-Ion Disorder in Cubic-BaSnF4

“…37−40 Orientational disorder can be static, where each polyatomic subunit has a fixed average orientation over experimentally relevant time scales, 41,42 or dynamic, where the polyatomic subunits rotate and reorient. 40,43,44 In some solid electrolytes, this reorientational dynamics of the host framework is thought to couple to the diffusive dynamics of the mobile-ion species, giving rise to a so-called "paddlewheel" effect. 21,38,43,45 While orientational disorder in solid electrolytes is usually discussed in the context of molecular or polyanion orientational degrees of freedom, materials that contain posttransition metals with "stereoactive" lone pairs, such as Sn or Bi, may exhibit electronic orientational disorder.…”

“…40,43,44 In some solid electrolytes, this reorientational dynamics of the host framework is thought to couple to the diffusive dynamics of the mobile-ion species, giving rise to a so-called "paddlewheel" effect. 21,38,43,45 While orientational disorder in solid electrolytes is usually discussed in the context of molecular or polyanion orientational degrees of freedom, materials that contain posttransition metals with "stereoactive" lone pairs, such as Sn or Bi, may exhibit electronic orientational disorder. 46 These cations, when in an oxidation state two fewer than their formal maximum (e.g., Sn II or Ba II ), have a formal electron configuration with a filled s-orbital as their last valence shell.…”

“…One such design principle arises from the observation that solid electrolytes with some form of host-framework disorder often have significantly higher ionic conductivities than related compounds with well-ordered host frameworks. ,,− Framework-disordered solid electrolytes typically exhibit one of two classes of disorder: occupational disorder, where two or more distinct species occupy the same crystallographic positions, ,,,,,− or orientational disorder, where molecular or polyatomic subunits within the host framework have different disordered orientations. − Orientational disorder can be static, where each polyatomic subunit has a fixed average orientation over experimentally relevant time scales, , or dynamic, where the polyatomic subunits rotate and reorient. ,, In some solid electrolytes, this reorientational dynamics of the host framework is thought to couple to the diffusive dynamics of the mobile-ion species, giving rise to a so-called “paddlewheel” effect. ,,, …”