Designing Highly Conductive Sodium‐Based Metal Hydride Nanocomposites: Interplay between Hydride and Oxide Properties

• Published in: Advanced functional materials Vol. 33; no. 13
• Main Authors: Kort, Laura M., Brandt Corstius, Oscar E., Gulino, Valerio, Gurinov, Andrei, Baldus, Marc, Ngene, Peter
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01-03-2023
• Subjects: Aluminum oxide; complex hydride electrolytes; Electrolytes; Interface reactions; interfacial ion conduction; Ion currents; Materials science; Metal hydrides; Metal oxides; Molten salt electrolytes; nanocomposite solid electrolytes; Nanocomposites; nanoconfined electrolytes; Room temperature; Silicon dioxide; Sodium; sodium‐ion conductors; Solid electrolytes; Space charge; Structural analysis; Weight reduction
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.202209122

Sodium‐based complex hydrides have recently gained interest as electrolytes for all‐solid‐state batteries due to their light weight and high electrochemical stability. Although their room temperature conductivities are not sufficiently high for battery application, nanocomposite formation with metal oxides has emerged as a promising approach to enhance the ionic conductivity of complex hydrides. This enhancement is generally attributed to the formation of a space charge layer at the hydride‐oxide interface. However, in this study it is found that the conductivity enhancement results from interface reactions between the metal hydride and the oxide. Highly conductive NaBH4 and NaNH2/oxide nanocomposites are obtained by optimizing the interface reaction, which strongly depends on the interplay between the surface chemistry of the oxides and the reactivity of the metal hydrides. Notably, for NaBH4, the best performance is obtained with Al2O3, while NaNH2/SiO2 is the most conductive NaNH2/oxide nanocomposite with conductivities of, respectively, 4.7 × 10−5 and 2.1 × 10−5 S cm−1 at 80 °C. Detailed structural characterization reveals that this disparity originates from the formation of different tertiary interfacial compounds, and is not only a space charge effect. These results provide useful insights for the preparation of highly conductive nanocomposite electrolytes by optimizing interface interactions.
Through formation of nanocomposites with mesoporous Al2O3 and SiO2, the Na‐ion conductivity of the complex hydrides NaBH4 and NaNH2 is increased by three orders of magnitude at room temperature. This interface‐induced conductivity enhancement is related to interaction/reaction at the hydride‐oxide interface which is mostly dictated by the surface chemistry of the metal oxide.