A stable cathode-solid electrolyte composite for high-voltage, long-cycle-life solid-state sodium-ion batteries

Rechargeable solid-state sodium-ion batteries (SSSBs) hold great promise for safer and more energy-dense energy storage. However, the poor electrochemical stability between current sulfide-based solid electrolytes and high-voltage oxide cathodes has limited their long-term cycling performance and practicality. Here, we report the discovery of the ion conductor Na3-xY1-xZrxCl6 (NYZC) that is both electrochemically stable (up to 3.8 V vs. Na/Na+) and chemically compatible with oxide cathodes. Its high ionic conductivity of 6.6 × 10−5 S cm−1 at ambient temperature, several orders of magnitude higher than oxide coatings, is attributed to abundant Na vacancies and cooperative MCl6 rotation, resulting in an extremely low interfacial impedance. A SSSB comprising a NaCrO2 + NYZC composite cathode, Na3PS4 electrolyte, and Na-Sn anode exhibits an exceptional first-cycle Coulombic efficiency of 97.1% at room temperature and can cycle over 1000 cycles with 89.3% capacity retention at 40 °C. These findings highlight the immense potential of halides for SSSB applications.


Supplementary
| Comparison among Na 3 YBr 6, Na 3 YCl 6, and Zr-substituted Na 3 YCl 6 (Na 2.5 Y 0.5 Zr 0.5 Cl 6 ). The crystal structure, thermodynamic stability (E hull ), Na + diffusion channel size, electronic band gap, and electrochemical stability window (EC window) are tabulated. The mean squared displacement of Na + for a 50 ps time scale at 800 K (MSD 50ps, 800K ) is negligible for Na 3 YCl 6 (NYC) and Na 3 YBr 6 (NYB), implying that NYC and NYB have a rigid structure with little to no Na + diffusivity.

System
Reaction energy w/ NaCrO 2 (eV/atom) Reaction energy w/Na (eV/atom) EC window (V) Na 3 YCl 6 -0.11 -0.13 0.6-3.8 Na 2.25 Y 0.25 Zr 0.75 Cl 6 -0.14 -0.34 1. and the results are shown in Supplementary Figure 3b. At room temperature the ionic conductivity of NYC was determined to be 9.5 x 10 -8 S/cm, several orders of magnitude lower than that of the Li counterpart, Li3YCl6 (1 x 10 -4 S/cm). 2 Supplementary Figure 3 | Characterization of Na 3 YCl 6 . a, Rietveld refinement result of the capillary XRD pattern of the as-synthesized Na 3 YCl 6 . The cell parameters and fitting parameters are in the insets. b, Room temperature Nyquist plot of Na 3 YCl 6 and the equivalent circuit used for fitting; the conductivity was determined to be 9.5 x 10 -8 S/cm.

Supplementary Note 2
Structural relaxations were carried out using known structures with the formula A2MX6 from the Materials Project (MP) as well as ICSD. The results are shown in Supplementary Figure 4a-b; it was determined that Na2ZrCl6 was isostructural to Na2TiF6 and has the space group P -3 m 1. 3 This result is consistent with a recent report that described the structure on Na2ZrCl6. 4 Supplementary Figure 4 | Determination of the structure of Na 2 ZrCl 6 . a, Computed XRD pattern of the determined Na 2 ZrCl 6 structure overlaid with experimental XRD data for the post heat-treated Na 2 ZrCl 6 . b, (top) the structure of Na 2 ZrCl 6 , space group P -3 m 1, is isostructural to (bottom) Na 2 TiF 6 . 3 c, Rietveld refinement result of the synchrotron XRD data of the post heat-treated Na 2 ZrCl 6 , in good agreement with the structural relaxation result. The cell parameters and fitting parameters are in the inset. d, Room temperature Nyquist plot and equivalent circuit fit for Na 2 ZrCl 6 .

Supplementary Note 3
The Na 3-x Y 1-x Zr x Cl 6 compounds were all subjected to a synthesis procedure of mixing, heating at 500°C, quenching, and subsequent ball milling per the Methods section. This is because the conductivity of heattreated Na 3-x Y 1-x Zr x Cl 6 is very low; in the case of heat-treated NYC before ball milling, the room temperature conductivity for NYC could not be measured as it is a poor Na + conductor. Similar to LYC, reducing the degree of crystallinity drastically increased the conductivity, so the ball milling procedure was used for every composition, and the ball-milled NYZC0.75 was the material incorporated into the ASSB.

Supplementary Figure 8 | XRD and Activation Energies before and after ball milling. The XRD and
Arrhenius plots, respectively, are shown before and after ball milling for a, b, Na 3 YCl 6 and c, d, Na 2.25 Y 0.25 Zr 0.75 Cl 6 . In both cases, ball milling was found to lessen the degree of crystallinity (by decreasing peak intensities) and raise the conductivity. High impedance or steady impedance growth is shown for Na 2.25 Y 0.25 Zr 0.75 Cl 6 with Na-Sn anodes as opposed to Na 3 PS 4 ; Na 3 PS 4 is more stable when paired with Na-Sn, consistent with the EC window calculations.

Supplementary Note 4
To explore a reduction in overall cell impedance, a rate capability test was conducted on a NYZC0.75 cell at room temperature and one that used half the amount of NPS (37.5mg). Such a cell was fabricated on top of a stainless-steel current collector (600mg), that served as the cell support, as opposed to the NPS-supported configuration. The results, along with the corresponding Nyquist plots of the cells, are shown in Supplementary Figure 13. The reduction in the amount of NPS has a direct effect on the rate performance of the cell; at room temperature, the cell could even run at 1C. The Nyquist plots showed that the electrolyte contributions to cell impedance (designated as Rb and Rint) were reduced by half (in total) for the stainless-steel supported cell, while the Rct component from the cathode remained about the same. This suggests that optimization of the ASSB form factor, notably with a thin electrolyte layer prepared by solution casting, would be a very promising avenue to explore for future work.  3d and Y 3d binding energies, respectively, for a-