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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-21488-7 A solid-state architecture for rechargeable sodium-ion batteries has garnered substantial research interest in 1–5 recent years . By replacing flammable organic liquid electrolytes with solid electrolytes (SEs), solid-state sodium-ion batteries (SSSB) promise not only higher safety, but also poten- tially enable higher voltage cathodes, metal anodes, and stacking architectures to achieve higher energy densities. In addition, the higher abundance of sodium relative to lithium makes sodium- ion batteries a more cost-effective alternative, especially for large- scale grid storage applications where low operating costs are more strongly emphasized than a high energy density6. However, an ideal SE has to meet a stringent set of requirements, namely high ionic conductivity, low electronic conductivity, and electro- chemical, chemical, and mechanical compatibility with electrodes. While major breakthroughs have been made in achieving liquid- like ionic conductivity values in sulfide SEs, their poor electro- chemical and chemical interfacial stability against common electrodes remains a critical bottleneck for practical applications. Recently, two lithium halide superionic conductors, Li3YCl6 and Li3YBr6, have been reported as promising SEs for solid-state lithium-ion batteries7. Exhibiting reasonable Li+ conductivities in the range of 0.5-0.7 mS cm−1, the most interesting feature of these halide SEs is their electrochemical and chemical stability, demon- strated via compatibility with the 4 V LiCoO2 cathode7. As a result, more studies have since emerged on halide SEs (Li3InCl6 and LixScCl3+x) that also exhibit high Li+ diffusivity, compatibility with LiCoO2, and facile processability8–10. In addition to these, there has also been a report on aliovalent substitution with Zr, yielding the halide Li3−xM1−xZrxCl6 (M = Er, Y), where the introduction of vacancies led to an increase in the ionic conductivity up to the order of 10−3 S/cm at room temperature11. Interestingly, unlike the fast Li-ion conducting sulfides or oxides, fast Li-ion conduction in these halide frameworks do not require a bcc anion sublattice, allowing a much wider selection of compositions when designing halide SE chemistries10. It is important to note that to a first approximation, the oxidative electrochemical stability of SEs are determined by anion chemistry10,12,13, and for halides, it generally follows the trend F > Cl>Br>I10,13. In contrast to the Li halides mentioned, the Na analogs Na3YCl6 and Na3YBr6 have been relatively less studied; previous studies have reported experimental ionic conductivities on the order of 10−4–10−6 S/cm at 500 K. These materials are therefore expected to have much lower room-temperature ionic con- ductivities than their lithium counterparts and thus impractical for SE applications14. Here, we report the data-driven develop- ment of Na3-xY1-xZrxCl6 (NYZCx) as a new class of sodium SEs exhibiting high ionic conductivities as well as excellent electro- chemical and chemical stability up to 3.8 V vs Na/Na+. Using density functional theory (DFT) calculations, it was predicted that aliovalent doping of Y3+ with Zr4+ would improve the Na+ conductivity of Na3YCl6 by three orders of magnitude, while retaining a wide electrochemical window and good chemical stability. A SSSB comprising a NaCrO2:NYZC0.75:vapor grown carbon fibers (VGCF) composite cathode with Na3PS4 (NPS) as the SE and a Na-Sn (2:1) anode exhibited an extremely high first cycle Coulombic efficiency (CE) of 97.6% at room temperature. Even when cycled at 40 °C and a rate of 1 C, the SSSB displayed stable electrochemical performance over 1000 cycles with 89.3% capacity retention. Results and discussion Electrochemically stable and conductive Na3-xY1-xZrxCl6. Unlike its lithium counterpart, Na3YCl6 (NYC) (Fig. 1a, space group: P21/n) does not exhibit partial occupancy in the 2d and 4e Na sites, which may explain its lower ionic conductivity. A series of ions (Ti4+, Zr4+, Hf4+, and Ta5+) were evaluated as potential aliovalent dopants for Y3+ to increase the concentration of defects and thus the ionic conductivity of Na3-(z-3)xY3+1-xMz+xCl62,4,15–17. The effect of ionic substitu- tion on the phase stability of NYC is shown in Supplementary Figure 1. Zr4+ is predicted to exhibit a low dopant formation energy and is also low-cost, due to the abundance of Zr. Furthermore, the enthalpies of mixing of the NYC-Na2ZrCl6 (NZC) pseudo-binary system are low, as shown in Fig. 1b. The electrochemical window of Na3-xY1-xZrxCl6 (NYZCx) was investigated using the grand potential phase diagram approach18,19. Consistent with previous studies on the Li analogs, NYZCx SEs exhibit wide electrochemical windows, with a particularly high oxidation limit of ~3.8 V vs Na/Na+ (Fig. 1c). This high oxidation limit is maintained regardless of Zr content. However, the reduction limit narrows (from 0.6 V for NYC to 1.5 V for NYZCx), due to the higher thermodynamic reduction potential of Zr4+ compared to Y3+. The oxidation limit of 3.8 V for NYZCx indicates that it could be compatible with the NaCrO2 cathode, which has an operating voltage window of 2–3.6 V vs Na/Na+20. In contrast, sulfide SEs, such as Na3PS4, have oxidation limits of only ~2.5 V vs Na/Na+21. In addition, the reaction energies of NYZC0.75 with NaCrO2 and with metallic Na were found to be less negative compared to NPS (Supplementary Table 2). The crystalline form of the end members NYC and NZC exhibit a closed-pack arrangement of [YCl6]3− and [ZrCl6]2− polyanions, respectively. With increasing x in NYZCx, the unit cell volume increases, which results in a widening of the Na+ diffusion channels, as shown in Supplementary Figure 2. NVT ab initio molecular dynamics (AIMD) simulations were carried out at 600−1000 K for NYZCx for x = 0, 0.375, 0.5, and 0.75. For NYC, AIMD simulations indicate no diffusion of Na+ ions even at elevated temperatures (Supplementary Table 1), consistent with its poor ionic conductivity. With Zr4+ doping, Na+ diffusivity increases substantially (Fig. 1d). Due to the high cost of ab initio methods, NVT AIMD simulations (constrained to the pre-equilibrated volume) were limited to small supercells and temperatures above 500K to ensure sufficient diffusion statistics. To probe the diffusivity at lower temperatures, a highly accurate ML-IAP based on the moment tensor potential formalism was developed using snap- shots extracted from the AIMD trajectories as well as ground state and strained structures of NYC, NZC and the highest conductivity NYZC0.75 (see Methods section for details)22–25. To our knowledge, this is the first work that demonstrates the use of AIMD simulations to fit a ML-IAP. As shown in Fig. 1d, NVT MD simulations of NYZC0.75 carried out using this ML-IAP reproduces the AIMD diffusivities to good accuracy at 600–1000 K. The validated ML-IAP was then applied for NpT MD simulations of NYZC0.75 using a much larger cell (592 atoms) over much longer time scales (up to 10 ns) and under constant atmospheric pressure at 350 to 650K. Interestingly, a non- Arrhenius behavior is observed; there is a transition between two linear regimes at around 500–550K. Similar step changes in diffusion characteristics have been previously observed experi- mentally in NYC and other superionic conductors14,26. The activation barrier for diffusion in the low temperature regime (<500 K) is predicted to be 594 meV, and the room temperature Na+ conductivity is predicted to be 1.4 × 10−5 S cm−1, which is two orders of magnitude higher than that of NYC. NYZCx compounds were synthesized using stoichiometric amounts of NaCl, YCl3, and ZrCl4 (see Methods). The parent compound NYC was first synthesized (detailed in Supplementary Note 1) and its XRD pattern and corresponding Rietveld Refinement results are shown in Supplementary Fig. 3a and Supplementary Table 3. These results are consistent with previous 2 NATURE COMMUNICATIONS | (2021)12:1256 | https://doi.org/10.1038/s41467-021-21488-7 | www.nature.com/naturecommunicationsPDF Image | cathode-solid electrolyte composite sodium-ion
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