Multiscale Ionic Diffusion in Novel Sodium Solid-State Amorphous Electrolytes

Currently, lithium-based ionic conductors and transition metal-based cathodes are the primary focus in the development of solid states batteries (SSBs)[1]. However, sustainable energy storage solutions require materials that are more abundant and less socially critical than lithium and transition metals. Sodium is one of the most promising candidates to replace lithium [2]. The less polarizing sodium ions could be in principle more mobile in a solid ionic conductor than lithium, which could lead to faster charging times; it could also offer the possibility of using thicker electrodes, which would, in turn, deliver higher capacities and energy density. However, the development of sodium-based solid-state batteries has been hindered mainly due to the chemical instability of sodium-ion conducting solid electrolytes. Halide-based ionic conductors attracted large interest as solid electrolyte candidates because of their suggested electrochemical oxidation stability and deformability. However, most of the discovered sodium metal halides exhibit relatively low ionic conductivities. Recently, a new class of mechanochemically stabilized sodium metal oxyhalides NaMOCl₄ (M = Nb, Ta) has been developed using ball milling synthesis method [3]. The obtained compounds NaNbOCl₄ and NaTaOCl₄ are low-crystalline or fully amorphous materials, exhibiting nevertheless a very high ionic conductivity of 1.2 and 1.5 mS cm⁻¹, respectively. The proposed structure of these materials consists of MCl₄O₂ (M = Nb, Ta) octahedra with Cl^- occupying the equatorial sites and the O^2- placed in the apical sites. The octahedra connected by the oxygen anions in (ab) plane but weekly coupled in (bc) plane forming one-dimensional chains. The distance between the chains is about 4 Å, leading to a well pronounced intermediate range order. Na⁺ cations are loosely placed in cavities between the octahedral chains and bonded to MCl₄O₂ (M = Nb, Ta) octahedra via interactions with Cl ions.
We have investigated Na nanoscale diffusion in NaTaOCl4 using quasielastic neutron spectroscopy (QENS) and obtained important information about nanoscale mechanism of Na ion diffusion and intermolecular interactions of Na ions with local environment. Thus, we observed two distinctive diffusive processes linked to short and intermediate range orders. The first process is realized through jumps with the characteristic length of about 2 Å and could originate from structural rearrangements of neighboring Na ions in cavities between TaCl₄O₂ octahedra. The second process indicates the existence of jumps with the length of about 4 Å, giving the reason to assume that this process is coupled to the Na+ diffusion from one metal cluster to another leading to the long-range charge transport. The data show the onset of ionic diffusion at temperature of about 200K, with diffusion coefficients ranging in (2-5)*10^-7 cm²/s depending on temperature.
References:
[1]. Janek, W.G. Zeier, W. G. Nat. Energy 1, 16141 (2016);
[2]. S. Ohno, W.G. Zeier, Nature Energy, 7, pages 686–687 (2022);
[3] T. Zhao, B. Samanta, X. Martinez de Irujo-Labalde, W. Zeier et al, ACS Materials Lett. 2024, 6, 3683−3689