pubs.acs.org/JPCL Letter Yttrium−Sodium Halides as Promising Solid-State Electrolytes with High Ionic Conductivity and Stability for Na-Ion Batteries Yu Qie, Shuo Wang, Sijie Fu, Huanhuan Xie, Qiang Sun,* and Puru Jena Cite This: J. Phys. Chem. Lett. 2020, 11, 3376−3383 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: All-solid-state sodium-ion batteries (ASIBs) are promising candidates for large-scale energy storage applications. To build such a battery system, efficient solid-state electrolytes (SSEs) with high sodium ionic conductivity at room temperature and good electrochemical stability as well as interface compatibility are required. In this work, using density functional theory combined with molecular dynamics simulation and a phase diagram, we have studied the potential of yttrium halide-based materials (Na3YX6, where X = Cl or Br) with inherent cation vacancies as diffusion carriers for solid electrolytes in ASIBs. A great balance between electrochemical stability and ionic conductivity found in these two systems overcomes the shortcomings of sulfide- and oxide- based SSEs. In particular, these two materials show Na+ conductivities of 0.77 and 0.44 mS cm−1 at 300 K and wide electrochemical windows of 0.51−3.75 and 0.57−3.36 V, and good interfacial stability with Na metal anode and high-potential polyanion (fluoro)phosphate cathode materials, respectively. These features make halide-based materials promising efficient solid-state electrolytes for Na-ion batteries. fi Sodium-ion batteries (SIBs) are considered as a promising sul de-based materials (Na3PS4 and other derivatives) are alternative to lithium-ion batteries (LIBs) for large-scale energy known for their higher ionic conductivities and easier synthetic storage systems because sodium is highly abundant, environ- method of simple cold pressing but suffer from an intrinsic 1−4 mentally friendly, and inexpensive. Remarkable progress has narrow electrochemical stability window and serious interface − been made in developing room-temperature SIBs based on side reaction.22 24 To facilitate pragmatic large-scale ASIB 5−7 ff organic liquid electrolytes. However, these SIBs su er from applications, developing sodium SSEs with high RT ionic safety issues arising from the volatile and flammable organic 8,9 conductivity and a wide electrochemical stability window at the Downloaded via PEKING UNIV on June 15, 2020 at 04:47:26 (UTC). liquid electrolytes like their lithium counterparts. In this same time is imperative. regard, using inorganic nonflammable solid-state electrolytes (SSEs) to prepare all-solid-state sodium-ion batteries (ASIBs) Recently, a new class of lithium halide materials have been is a good strategy for eliminating the safety concerns caused by reported as attractive SSEs for LIBs with high RT ionic liquid electrolytes. In addition, the wide electrochemical conductivities, excellent interface compatibility, and wide See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 25 window and excellent mechanical properties of SSEs enable electrochemical windows. Wang et al. further confirmed the use of high-energy density electrodes such as Na metal that the smooth energy landscape is intrinsic in a halogen anodes and high-voltage cathodes, including Na2FePO4F and anion sublattice, and the high anodic stability originates from 26 Na3V2(PO4)3, which further improves the battery performance. the antioxidant halogen anions. Moreover, Sun et al. This has led to considerable interest in the research on sodium successfully synthesized a Li3InCl6 SSE and experimentally 10−14 SSEs. proved its superior Li-ion conductivity, excellent electro- For solid-state SIBs, currently most used SSEs are oxides, chemical stability, and high stability in ambient air.27,28 fi sul des, and their derivatives, which face considerable Considering the similarity between LIBs and SIBs, the challenges because of an undesirable trade-off between ionic − question of whether halide-based materials can be used as conductivity and stability.15 17 For example, the oxide solid electrolytes (β-alumina, NASICON, etc.) generally possess good chemical stability and wide electrochemical windows, but Received: January 2, 2020 most of them exhibit only reasonable ionic conductivities at Accepted: April 13, 2020 − high temperatures (∼10−4 Scm−1 at room temperature)18 20 Published: April 13, 2020 and require harsh synthesis processes for preparing the electrode−oxide SSE composites. This might induce grain- boundary resistance and poor interface contact.21 In contrast, © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.jpclett.0c00010 3376 J. Phys. Chem. Lett. 2020, 11, 3376−3383 The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letter Figure 1. Crystal structures of (a) NYC and (b) NYB. (c) Face-to-face and (d) edge-to-edge connected NaCl6 octahedra in NYC. (e) Edge-to- edge connected NaBr6 octahedra in NYB. The octahedra and tetrahedra are shown as yellow and pink polyhedra, respectively. Na, Y, Cl, and Br atoms are represented as yellow, teal, green, and brown spheres, respectively. SSEs for SIBs with both high ionic conductivity and good NYC or NYC) in equilibrium with the chemical potential of stability naturally arises. element Na is constructed.39,40 The decomposition reaction To address this question, in this study, we investigate the energy is defined as ff ionic di usivity, electrochemical window, and interfacial Δ=[]−−Δopen μμ μ − EEEnD (phase,Na )eq C eq (C,Na ) (phase) Na Na stability of sodium yttrium halides Na3YX6 (X = Cl or Br) fi μ Δ using rst-principles theory. Our results show that Na3YCl6 where Na is the chemical potential of Na metal and nNa is and Na3YBr6 possess high RT Na ionic conductivities of 0.77 the number difference of element Na from the original −1 and 0.44 mS cm , respectively, wide electrochemical windows composition C. The given phase is stable within a certain range − μ Δ (around 0.6 3.5 V), and high chemical stability toward of Na where nNa is zero. traditional NIBs cathodes, especially for the high-potential Chemical Stability of Interfaces. The interface pseudobinary polyanion cathode materials [for example, Na2MnPO4F, reaction energy is calculated as Na3V2(PO4)3, etc.]. Moreover, the small reaction energy (<50 meV/atom) between Na metal and NYC (NYB) ΔExD(SE, electrode, ) makes it possible to use the Na metal anode, leading to =[ExeqC(C,C,) interface SSE electrode ] high-energy density ASIBs. In addition, Na3YCl6 and Na3BrCl6 are both electronic insulators and possess good phase, thermal, − Exinterface(SSE, electrode, ) and dynamical stability. All of these features make halide materials promising candidates for sodium SSEs. where CSSE and Celectrode are the compositions of SSE and electrode materials, respectively, normalized to one atom per 41,42 ■ COMPUTATIONAL METHODS formula. x is the molar fraction of the SSE. The crystal structure and energy of most materials are obtained from the Density Functional Theory. In this work, all calculations are MP database.34 based on the density functional theory (DFT) and the Vienna Geometries and Stabilities. As the crystalline structures of 29 ab initio Simulation Package (VASP) within the projector Na YX (X = Cl or Br) have Na partial occupancies, we first 30 3 6 augmented-wave (PAW) approach. Generalized gradient generated all symmetrically distinctive structures (see Figure approximation (GGA) with the Perdew−Burke−Ernzerhof S1) and then conducted DFT calculations to identify the (PBE) function31 is used to account for exchange and geometries with the lowest energy for each halide. The ground- correlation potential. The Heyd−Scuseria−Ernzerhof state structures are displayed in Figure 1. The optimized 32,33 2 (HSE06) hybrid functional is used for an accurate band Na3YCl6 (NYC) structure has a D3 space symmetry group gap. The parameters in the calculations, including the energy (No. 150, P321) consisting of two parallel sets of YCl6 cutoff and k-point density, are consistent with those of octahedra arranged along different orientations, which is very Materials Project34 (MP). different from the structure proposed by Stenzel and Meyer43 ̅ Diffusion and Conductivity. The mean square displacement reported in 1993 for Na3YCl6 with an R3 space group (No. (MSD) method with elevated temperatures is employed to 148). Here, the YCl octahedra align in the same direction. ff 6 calculate the di usivity and conductivity based on ab initio Na3YBr6 (NYB) possesses an fcc-like anion arrangement with a 3 molecular dynamics (AIMD) simulations. The data analysis is space symmetry group of C2 (No. 5, C2). In both cases, the carried out with the aimd module developed under the yttrium and sodium atoms are 6-fold coordinated with 35−38 pymatgen python framework. More details are provided in halogens, forming NaX6 and YX6 (X = Cl or Br) octahedra the Supporting Information. with high stability.43 In addition, there are two empty cation Electrochemical Stability. To evaluate the electrochemical sites near the Y site, acting as built-in cation vacancy sites, stability of NYC and NYB, the grand potential phase diagram which play an important role in ion conductivity as known in μ 44 45 46 of a given phase [Ceq(C, Na)] with the composition C (C = many SSEs such as LixScCl3+x, LATP, LGPS, and 3377 https://dx.doi.org/10.1021/acs.jpclett.0c00010 J. Phys. Chem. Lett. 2020, 11, 3376−3383 The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letter Figure 2. Arrhenius plots of the Na+ diffusion coefficient in (a) NYC and (b) NYB obtained from AIMD simulations. (c) Isosurface of the Na+ probability density distribution (yellow) in (c) NYC and (d) NYB at 600 K, with an isosurface value of 0.00025 e/Bohr.3 LLZO.47,48 Such partial occupation sites lead to a smoother Na+ migration in the yttrium halide-based SSEs, the Arrhenius energy landscape and act as available sites for ion migration. As diagrams of Na+ diffusivity in NYC and NYB at different presented in Figure 1c, the NaCl6 octahedra are connected in a temperatures from 600 to 1000 K are plotted in panels a and b face-to-face manner along the c direction in NYC, where Na of Figure 2, respectively.