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 Na_3-xY_1-xZr_xCl₆ (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.

In the pursuit of urgently needed, energy dense solid-state batteries for electric vehicle and portable electronics applications, halide solid electrolytes offer a promising path forward with exceptional compatibility against high-voltage oxide electrodes, tunable ionic conductivities, and facile processing. For this family of compounds, synthesis protocols strongly affect cation site disorder and modulate Li⁺ mobility. In this work, we reveal the presence of a high concentration of stacking faults in the superionic conductor Li₃YCl₆ and demonstrate a method of controlling its Li⁺ conductivity by tuning the defect concentration with synthesis and heat treatments at select temperatures. Leveraging complementary insights from variable temperature synchrotron X-ray diffraction, neutron diffraction, cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, density functional theory, and electrochemical impedance spectroscopy, we identify the nature of planar defects and the role of nonstoichiometry in lowering Li⁺ migration barriers and increasing Li site connectivity in mechanochemically synthesized Li₃YCl₆. We harness paramagnetic relaxation enhancement to enable ⁸⁹Y solid-state NMR and directly contrast the Y cation site disorder resulting from different preparation methods, demonstrating a potent tool for other researchers studying Y-containing compositions. With heat treatments at temperatures as low as 333 K (60 °C), we decrease the concentration of planar defects, demonstrating a simple method for tuning the Li⁺ conductivity. Findings from this work are expected to be generalizable to other halide solid electrolyte candidates and provide an improved understanding of defect-enabled Li⁺ conduction in this class of Li-ion conductors.

Benchtop NMR spectrometers operating with low magnetic fields of 1–2 T at sub-ppm resolution show great promise as analytical platforms that can be used outside the traditional laboratory environment for industrial process monitoring. One current limitation that reduces the uptake of benchtop NMR is associated with the detection fields’ reduced sensitivity. Here we demonstrate how para-hydrogen (p-H₂) based signal amplification by reversible exchange (SABRE), a simple to achieve hyperpolarization technique, enhances agent detectability within the environment of a benchtop (1 T) NMR spectrometer so that informative ¹H and ¹³C NMR spectra can be readily recorded for low-concentration analytes. SABRE-derived ¹H NMR signal enhancements of up to 17 000-fold, corresponding to ¹H polarization levels of P = 5.9%, were achieved for 26 mM pyridine in d₄-methanol in a matter of seconds. Comparable enhancement levels can be achieved in both deuterated and protio solvents but now the SABRE-enhanced analyte signals dominate due to the comparatively weak thermally-polarized solvent response. The SABRE approach also enables the acquisition of ¹³C NMR spectra of analytes at natural isotopic abundance in a single scan as evidenced by hyperpolarized ¹³C NMR spectra of tens of millimolar concentrations of 4-methylpyridine. Now the associated signal enhancement factors are up to 45 500 fold (P = 4.0%) and achieved in just 15 s. Integration of an automated SABRE polarization system with the benchtop NMR spectrometer framework produces renewable and reproducible NMR signal enhancements that can be exploited for the collection of multi-dimensional NMR spectra, exemplified here by a SABRE-enhanced 2D COSY NMR spectrum.

In this study, the relationship between structure, cation disorder and Na-ion conduction properties is elucidated in a series of Na _{3− x} Y _{1− x} Zr _x Cl ₆ solid electrolytes.

The conversion of [IrCl(COD)(IMes)] (COD = cis,cis-1,5-cyclooctadiene, IMes = 1,3-bis(2,4,6-trimethyl-phenyl)imidazole-2-ylidene) in the presence of an excess of p-H₂ and a substrate (4-aminopyridine (4-AP) or 4-methylpyridine (4-MP)) into [Ir(H)₂(IMes)(substrate)₃]Cl is monitored by ¹H NMR spectroscopy using a benchtop (1 T) spectrometer in conjunction with the parahydrogen (p-H₂) based hyperpolarization technique signal amplification by reversible exchange (SABRE). A series of single-shot ¹H NMR measurements are also used in parallel to monitor these chemical changes that take place in solution through the life-time of the hyperpolarized response. Non-hyperpolarized high-field ¹H NMR control measurements were also undertaken to confirm that the observed time dependent changes relate directly to the underlying chemical evolution. The formation of [Ir(H)₂(IMes)(substrate)₃]Cl is further linked to the hydrogen isotope exchange reaction (HIE) which leads to the incorporation of deuterium into the ortho positions of 4-AP, where the source of deuterium is the solvent, methanol-d₄. Comparable reaction monitoring results are achieved at both high-field (9.4 T) and low-field (1 T) to further rationalise behavior. It is notable, that the low sensitivity of benchtop NMR enables the use of protio solvents, which is harnessed here to separate the effects of catalyst formation and substrate deuteration. Collectively, these methods illustrate how low-cost low-field NMR measurements provide unique insight into a complex catalytic process through a combination of hyperpolarization and relaxation data.