On the Discrepancy between Local and Average Structure in the Fast Na+ Ionic Conductor Na2.9Sb0.9W0.1S4.

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS43- by WS42- in Na2.9Sb0.9W0.1S4 leads to a very high ionic conductivity of 41 mS cm-1 at room temperature. While pristine Na3SbS4 crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4 crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4 remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+ diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.

Ionic Conductivity of the NASICON-Related Thiophosphate Na1+x Ti2-x Gax (PS4 )3.

Inspired by the recent interest in fast ionic conducting solids for electrolytes, the ionic conductivity of a novel ionic conductor Na1+x Ti2-x Gax (PS4 )3 has been investigated. Using X-ray diffraction and impedance spectroscopy the sodium ionic conductivity in this compound was demonstrated, in which bond valence sum analysis suggests a tunnel diffusion for Na+ . Substitution with Ga3+ leads to an increasing Na+ content, an expansion of the lattice and an increasing conductivity with increasing x in Na1+x Ti2-x Gax (PS4 )3 . Given the relation to the NASICON family, upon replacement of the phosphate by a thiophosphate group, a rich structural chemistry can be expected in this class of materials. This work demonstrates the potential for making NaTi2 (PS4 )3 an ideal system to study structure-property relationships in ionic conductors.

Halide and Sulfide Electrolytes in Cathode Composites for Sodium All-Solid-State Batteries and their Stability.

Sodium all-solid-state batteries may become a novel storage technology overcoming the safety and energy density issues of (liquid-based) sodium ion batteries at low cost and good resource availability. However, compared to liquid electrolyte cells, contact issues and capacity losses due to interface reactions leading to high cell resistance are still a problem in solid-state batteries. In particular, sulfide-based electrolytes, which show very high ionic conductivity and good malleability, exhibit degradation reactions at the interface with electrode materials and carbon additives. A new group of solid electrolytes, i.e., sodium halides, shows wider potential windows and better stability at typical cathode potentials. A detailed investigation of the interface reactions of Na3SbS4 and Na2.4Er0.4Zr0.6Cl6 as catholytes in cathodes and their cycling performance in full cells is performed. X-ray spectroscopy, time-of-flight spectrometry, and impedance spectroscopy are used to study the interface of each catholyte with a transition metal oxide cathode active material. In addition, impedance measurements were used to study the separator electrolyte Na3SbS4 with the catholyte Na2.4Er0.4Zr0.6Cl6. In conclusion, cathodes with Na2.4Er0.4Zr0.6Cl6 show a higher stability at low C-rates, resulting in lower interfacial resistance and improved cycling performance.

Protective NaSICON Interlayer between a Sodium-Tin Alloy Anode and Sulfide-Based Solid Electrolytes for All-Solid-State Sodium Batteries.

This paper presents a suitable combination of different sodium solid electrolytes to surpass the challenge of highly reactive cell components in sodium batteries. The focus is laid on the introduction of ceramic Na3.4Zr2Si2.4P0.6O12 serving as a protective layer for sulfide-based separator electrolytes to avoid the high reactivity with the sodium metal anode. The chemical instability of the anode|sulfide solid electrolyte interface is demonstrated by impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Na3.4Zr2Si2.4P0.6O12 disk shows chemical stability with the sodium metal anode as well as the sulfide solid electrolyte. Impedance analysis suggests an electrochemically stable interface. Electron microscopy points to a reaction at the Na3.4Zr2Si2.4P0.6O12 surface toward the sulfide solid electrolyte, which does not seem to affect the performance negatively. The results presented prove the chemical stabilization of the anode-separator interface using a Na3.4Zr2Si2.4P0.6O12 interlayer, which is an important step toward a sodium all-solid-state battery. Due to the applied pressure that is mandatory for battery cells with sulfide-based cathode composite, the use of a brittle ceramic in such cells remains challenging.

Anharmonic Lattice Dynamics in Sodium Ion Conductors.

We employ terahertz-range temperature-dependent Raman spectroscopy and first-principles lattice dynamical calculations to show that the undoped sodium ion conductors Na3PS4 and isostructural Na3PSe4 both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice-Na+ ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. Na3PSe4 shows an almost purely displacive character with the soft modes disappearing in the cubic phase as the change in symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. Na3PS4 instead shows an order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining Raman active in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice-mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity of these Na+ ion conductors.