Scandium-Substituted Na₃Zr₂(SiO₄)₂(PO₄) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors

As possible electrolyte materials for all-solid-state Na-ion batteries (NIBs), scandium-substituted Na₃Zr₂­(SiO₄)₂(PO₄) in the structure of NASICONs (Na superionic conductors) has received hardly any attention so far, although among all the trivalent cations, Sc³⁺ might be the most suitable substitution ion for Na₃Zr₂(SiO₄)₂(PO₄) because the ionic radius of Sc³⁺ (74.5 pm) is the closest to that of Zr⁴⁺ (72.0 pm). In this study, a solution-assisted solid-state reaction (SASSR) method is described, and a series of scandium-substituted Na₃Zr₂(SiO₄)₂(PO₄) with the formula of Na_3+<i>x</i>Sc_<i>x</i>Zr_2‑<i>x</i>(SiO₄)₂(PO₄) (NSZSP<i>x</i>, 0 ≤ <i>x</i> ≤ 0.6) have been prepared. This synthesis route can be applied for powder preparation on a large scale and at low cost. With increasing degrees of scandium substitution, the total conductivity of the samples also increases. An optimum total Na-ion conductivity of 4.0 × 10^–3 S cm^–1 at 25 °C is achieved by Na_3.4Sc_0.4Zr_1.6­(SiO₄)₂(PO₄) (NSZSP0.4), which is the best value of all reported polycrystalline Na-ion conductors. The possible reasons for such high conductivity are discussed

Fast Na⁺ Ion Conduction in NASICON-Type Na_3.4Sc₂(SiO₄)_0.4(PO₄)_2.6 Observed by ²³Na NMR Relaxometry

The bulk diffusion of Na in Na_3.4Sc₂(SiO₄)_0.4(PO₄)_2.6 was investigated by ²³Na NMR relaxometry in the temperature range from 250 to 670 K. These measurements reveal fast Na diffusion with hopping rates of 3 × 10⁸ s^–1 for the Na⁺ ions at 350 K and activation barriers for single Na⁺ ion jumps of (0.20 ± 0.01) eV. From these values a diffusion coefficient of <i>D</i> = 6.4 × 10^–12 m²/s and a Na ion conductivity of σ_Na = 4 mS/cm (both at 350 K) can be estimated. Measurements on two samples, one stored in air and one stored in Ar, do not show significant differences, which reveals that these NMR measurements are probing the bulk diffusion while conductivity measurements usually are also influenced by grain boundaries that can be affected by the moisture level during storage

Characterization and Optimization of La_0.97Ni_0.5Co_0.5O_3−δ-Based Air-Electrodes for Solid Oxide Cells

On the basis of previous studies of perovskites in the quasi-ternary system LaFeO₃–LaCoO₃–LaNiO₃, LaNi_0.5Co_0.5O₃ (LNC) is chosen as the most promising air-electrode material in the series for solid oxide cells (SOCs). In the present study, A-site deficiency of LNC is discussed and La_0.97Ni_0.5Co_0.5O₃ (LNC97) is selected as the optimal composition. Compatibility of LNC97 with 8 mol % Y₂O₃ stabilized ZrO₂ (8YSZ) is analyzed and compared with that of the state-of-the-art air-electrode La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and 8YSZ. Targeting to the requirements of high-performance SOC air-electrodes (high electronic and ionic conductivity and high catalytic activity for the oxygen reduction reaction), LNC97-based air-electrodes are tailored, characterized and optimized by symmetric-cell tests. Principles of air-electrode design for SOCs are revealed accordingly. Long-term measurement of the symmetric cells over a period of 1000 h is performed and possible degradation mechanisms are discussed. Full cells based on optimized LNC97 air-electrodes are also tested. Lower reactivity with 8YSZ in comparison to LSCF and a similar performance render LNC97 a very competitive candidate to substitute LSCF as air-electrode material of choice for SOCs