Design Principles for Rotational Cluster Anion [BH₄]⁻ Promote Superionic Conductivity in Sodium-Rich Antiperovskite Na₃OBH₄

Abstract

The ionic conductivity is determined by the activation energies and the jump attempt frequencies of carrier migrations between adjacent sites. However, few works in the past have considered the influence of the attempt frequency. In this work, the attempt frequencies of carrier migrations in Na3OBH4 and Na3OBr are calculated using density functional theory and written as the contribution of each element using frequency analysis, which can be used to calculate the conductivity of a carrier in combination with the defect concentration, migration barrier of carrier migration in equilibrium. The results show that the conductivity of sodium cations can be greatly improved by substituting Br− with the [BH4]− tetrahedral anion in an antiperovskite structure due to two reasons: (1) the paddle-wheel mechanism (PWM) of the [BH4]− anionic groups can effectively reduce the activation energy and (2) the [BH4]− anionic groups with a high vibration frequency undoubtedly improve the carrier jump attempt frequency by the buffering effect. The quasi-harmonic approximation (QHA) is used to describe the isobaric behavior of Na3OBH4 and Na3OBr, and the results show that carriers have a higher jump attempt frequency in the zero-pressure compared with the constant volume. The ionic conductivity (σ) in Na3OBr and Na3OBH4 are obtained from the Arrhenius equation. The results show that the calculated sodium ionic conductivity of Na3OBH4 is 3–4 orders of magnitude higher than that of Na3OBr at room temperature. The calculated sodium ionic conductivity is lower than the experimental measurement, and the discrepancy is attributed to the overestimation of the calculated migration barrier and the underestimation of the defect concentration.

Insights into the electrochemical properties of Li2FeS2 after FeS2 discharging

“…The thermodynamics of these three point defects as charge-neutral systems were systematically explored, including the low-energy defect configurations and defect formation energy. Using the Kröger–Vink notation, 40 the reactions involving these defects can be as expressed by:…”

“…With the Li + migration energy barrier as the foundation, we estimated the diffusion coefficient ( D ) of Li + in Li 2 FeS 2 for comparison with the experimental data. Eqn (11) can be used to calculate the diffusion coefficient ( D ): 40 where d is the hopping distance (the linear distance from the initial state to the final state), v 0 is the vibration frequency and can be approximated as 10 13 Hz, E a is the Li + migration energy barrier, and K b and T are the Boltzmann constant and absolute temperature.…”

“…The structural stability and phase transition of the electrode materials during the charging/discharging process have an important influence on their electrochemical performance, especially the cycle stability. 40 In particular, the thermodynamically most stable phase was LiFeS 2 (P % 1) in the process of Li + intercalation into FeS 2 (PA %…”

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

“…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. 725 Even though the calculated ionic conductivity of cluster-based antiperovskites still cannot match the high value from the experimental result, 276 such a difference can be interpreted by the overestimated migration barrier and underestimated defect concentration. In this manner, a variety of polyanion clusters were predicted to accomplish the high ionic conductivity and low activation energy of cluster-based antiperovskite SEs, such as (AlF 4 ) À , (ClO 4 ) À , (Icl 4 ) À , and (IO 4 ) À , among which Na 3 SAlF 4 manifested the highest ionic conductivity of 65.5 mS cm À1 and the activation energy of 0.19 eV based on first-principles DFT calculations, 726 due to the multifold optimizations including active lattice dynamics, widened diffusion channels and stabilized geometry structure.…”

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

“…The synthesis consisted of a 3-step process starting with ball milling and liquid phase processes, followed by heat treatment at 380-400 °C and water removal under high vacuum to promote the creation of the Li 6 OS(BH 4 ) 2 product, Li 2 O + Li 2 S + 2Li(BH 4 ) / Li 6 OS(BH 4 ) 2 . 91,98 In the study, 91 the band gap of the electrolyte was determined to be 4.03 eV. Furthermore, a symmetric cell was fabricated as Li/Li 6 OS(BH 4 ) 2 /Li; this cell is reported to have a long cycling stability of about 3000 h at 60 mA cm −2 current density, even though the increment of the current density seems to reduce the cycling stability.…”