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

Abstract

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.

Enhanced interface stability of ammine magnesium borohydride by in situ decoration of MgBr2·2NH3 nanoparticles

“…Meanwhile, the interaction effect between conductors and insulating additives, such as oxides and polymers, which enhances the ionic migration due to the formation of active interfaces and the creation of fresh ionic environment, has been often reported for solid-state electrolytes. 12,13,24,25 In the present study, the strong interaction between Mg(BH 4 ) 2 Á1.9NH 3 and MgBr 2 Á2NH 3 evidenced by expansion in the lattice spacing of MgBr 2 Á2NH 3 (Fig. 1c and Fig.…”

Low‐Cost Preparation of High‐Performance Na‐B‐H‐S Electrolyte for All‐Solid‐State Sodium‐Ion Batteries

“…These second phases were incorporated through the ball-milling method or the melt infiltration process; in other words, two phases were mixed using a physical approach. Ngene et al [13] revealed that this second-phase strategy could be attributed to the surface interaction between the oxides (Al 2 O 3 and SiO 2 ) and NaBH 4 . To strengthen the two-phase interface, the Na 2 B 12 H 12 phase was generated in situ through a gas-solid chemical reaction between NaBH 4 and B 2 H 6 .…”

“…As an inexpensive industrial reducing agent, the significant cost advantage of NaBH 4 has inspired interest in improving its Na + conductivity. The most straightforward way to improve conductivity is using a second‐phase incorporation strategy, such as NaBH 4 ‐C 60 , [ 12 ] NaBH 4 ‐Al 2 O 3 , [ 13 ] or NaBH 4 ‐SiO 2 . [ 13 ] The NaBH 4 ‐C 60 nanocomposite has shown an increase of four orders of magnitude in ionic conductivity compared with pure NaBH 4 .…”

“…The most straightforward way to improve conductivity is using a second‐phase incorporation strategy, such as NaBH 4 ‐C 60 , [ 12 ] NaBH 4 ‐Al 2 O 3 , [ 13 ] or NaBH 4 ‐SiO 2 . [ 13 ] The NaBH 4 ‐C 60 nanocomposite has shown an increase of four orders of magnitude in ionic conductivity compared with pure NaBH 4 . More recently, the NaBH 4 ‐Al 2 O 3 nanocomposite exhibited an ionic conductivity of 7.2 × 10 −6 S cm −1 at 80 °C.…”

“…Ngene et al. [ 13 ] revealed that this second‐phase strategy could be attributed to the surface interaction between the oxides (Al 2 O 3 and SiO 2 ) and NaBH 4 . To strengthen the two‐phase interface, the Na 2 B 12 H 12 phase was generated in situ through a gas‐solid chemical reaction between NaBH 4 and B 2 H 6 .…”