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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-21488-7 Fig. 4 Effect of octahedra rotation on Na+ diffusivity. Plots of the probability density (isosurface value = 5 × 10−4) of a Na+ in Na3YCl6, b Na+ in Na2.25Y0.25Z0.75Cl6, c Cl− in Na3YCl6 and d Cl− in Na2.25Y0.25Z0.75Cl6, over 100 ps of AIMD simulations at 600 K. The motion of Na+ and Cl− in Na3YCl6 are relatively localized, while macroscopic Na+ diffusion with (Zr/Y)Cl6 octahedral rotation are observed in Na2.25Y0.25Z0.75Cl6. e Na+ diffusivity at 800 K (D800K, in cm2/s) for varying Zr content in Na3−xY1−xZrxCl6, compared with a selective dynamics simulation with Cl− ions frozen in space, which shows negligible Na+ diffusivity. These AIMD calculations therefore indicate that an increase in cell volume or in octahedral motion significantly improves Na+ conductivity - both effects are closely related and most likely responsible for enhanced Na+ diffusion kinetics in NYZC0.75, as compared to NYC. In addition, an analysis of Na+ motion in NYZC0.75 was carried out using the trajectories from the ML-IAP NpT MD simulations at 500 K and 550 K, i.e., below and above the transition point for the two linear regimes in Fig. 1d, respectively. It was found that the Na+ diffusion topology changes from being quasi-2D to being 3D at the transition temperature, accompanied by a sharp increase in the degree of YCl63-/ZrCl62- octahedra rotation (Supplementary Fig. 11). We may therefore surmise that the much lower barriers for Na+ diffusion in the high- temperature regime compared to the low-temperature regime is due to the activation of additional rotational modes and diffusion pathways above the transition temperature. These results high- light the cooperative interplay between increased lattice volume36 and octahedral rotations in enhancing the long-range Na+ conductivity in this framework. Cathode composite for a long cycle-life solid-state sodium battery. Given the high cathodic stability and conductivity of NYZC0.75, cells comprising NYZC0.75 in a composite with the NaCrO2 cathode and Na3PS4 as the SE were constructed; a schematic is shown in Fig. 5a. For comparison, a control cell using Na3PS4 alone, without NYZC0.75, was also constructed (Supplementary Fig. 12a). At 20 °C at a rate of C/10 (Fig. 5b, c for NYZC0.75 and Supplementary Fig. 12b, c for NPS), it is evident that the first cycle Coulombic efficiency (CE) drastically increased in the NYZC0.75 cell (from 71.9% to 97.6%). This observed first cycle CE for the NYZC0.75 cell is the highest among those reported for Na ASSBs that use NaCrO2 as the cathode5,37–39. We believe that the high cathodic stability of NYZC0.75 protects the Na3PS4 SE from oxidation by NaCrO2, and in turn the Na3PS4 SE forms a stable passivating interface with the Na-Sn anode7. This is consistent with results from symmetric cell experiments carried out with NYZCx and NPS with the Na-Sn alloys Na15Sn4 (0.1 V vs. Na/Na+) and Na-Sn 2:1 (0.3V vs. Na/Na+), as shown in Supplementary Fig. 1340. Based on the results, Na-Sn 2:1 was chosen for its stability with NPS. To study the rate capability of the NYZC0.75 cell configuration, additional cells were constructed and tested at C/2 (after the first 5 cycles at C/10) at both 20 °C and 40 °C (Fig. 5d–g, respectively). At 20 °C, there is a noticeable drop in specific capacity (from 101 to 53.7 mAh/g) after switching to a rate of C/2. This is due to several reasons: one, the NPS layer is relatively thick (~800 μm) and the conductivity of NYZC0.75 is in the order of 10−5 S/cm. It is important to note that the cyclical behavior in Fig. 5e is due to temperature variations in the glovebox, as the cell was not inside a temperature-controlled chamber. At 40 °C, where the conductivity of NYZC0.75 is in the range of 1–2 × 10−4 S/cm, thus increasing the kinetics, the drop in capacity is negligible (from 104 to 101 mAh/g) when switching to a rate of C/2. For this particular SSSB, the average CE is 99.96%, which yields a capacity retention of 88% after 500 cycles. Furthermore, another NYZC0.75-NPS SSSB was constructed and cycled at 40 °C and at a rate of 1 C (Fig. 5h, i). Although there is a slight drop in capacity (78 mAh/g) compared to the cell cycled at 40 °C and C/2 (101 mAh/g), the cell cycled at 1 C lasted over 1000 cycles with a capacity retention of 89.3%, highlighting the superior stability of NYZC0.75 when paired with NaCrO2. To date, this is the highest capacity retention obtained 6 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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