Facile Self‐Forming Superionic Conductors Based on Complex Borohydride Surface Oxidation

Abstract

Complex hydrides have attracted significant attention as better inorganic solid‐state electrolytes owing to their lightweight and good compatibility with metal anodes (Li, Na, and/or Mg) for all solid‐state batteries. However, high ionic conductivity is usually observed at high temperatures upon the stabilization of adequate crystalline phases enabling fast ionic mobility. Here, an extremely simple strategy to significantly increase the ionic conductivity of complex borohydrides is reported. By exposing complex borohydrides to oxygen, the rearrangement of surface atoms upon the oxidation of borohydride particles and the resulting defects lead to extremely high ionic conductivity. NaBH4 and LiBH4 exposed to 5% O2 show an ionic conductivity of ≈10−3 S cm−1 at 35 °C. Similarly, oxidized Mg(BH4)2 displays a conductivity of ≈10−6 S cm−1 at 25 °C instead of 9.63 × 10−13 S cm−1. To the best of the authors' knowledge, this is to date, the simplest approach to tune the properties of borohydrides toward high ionic conductivity at room temperature as it does not rely on the difficult synthesis of large cage boron based anions to substitute BH4− and allow better ionic conduction paths. Owing its simplicity, the finding has the potential to enable new avenues toward the realization of viable complex borohydride based solid‐state electrolytes.

The Nature of Interface Interactions Leading to High Ionic Conductivity in LiBH4/SiO2 Nanocomposites

“…No clear correlation was discovered between the relative abundance of an oxidized species and the fraction of highly dynamic ions of LiBH 4 . Although some oxidation can be beneficial for the ionic conductivity of (nonconfined) complex metal hydrides, we expect this effect to be much less significant in nanoconfined LiBH 4 due to the abundant silicon oxide and the absence of oxygen gas during synthesis. It is possible that boron and the oxygen of the Q′ 3 site interact, yet our REDOR experiments show that the coupling between boron and Q′ 3 is much weaker than the interaction of boron with the mii site.…”

Hydrides compounds for electrochemical applications

“…It results from the reorientational dynamics of the anions and their ability to create a flat energy landscape for the cation diffusion [53]. Intensive efforts have been devoted to suppressing or reducing the temperature of the phase transition below room temperature through nano-sizing and defects [54], partial substitution of boron by carbon to form closo-carboborates such as NaCB9H10 [52], and anion mixing such as Na4(B12H12)(B10H10) [55]. By combining the two latter approaches, ionic conductivity above 1 mScm −1 at room temperature and oxidative electrochemical stability up to 4.5 V vs. Na + /Na was obtained for Na4(CB11H12)(B12H12)) [55] 6 .…”

A Wide Temperature 10 V Solid-state Electrolyte with a Critical Current Density of over 20 mA cm-2

“…25 However, the critical aws of this strategy are the synchronous reduction in the Li-ion conductivities in bulk SEs, especially at RT, and the deterioration of mechanical behaviors due to the high hardness and brightness characteristics of the inert llers. 20 Other scholars have tried to introduce covalently bonded molecules 26,27 and adopt elemental substitution by modifying the cations and anions to form ion/covalent-bonded inorganic compounds [28][29][30][31][32] either into the grain boundaries or electrode interfaces for SEs, which decrease the electronic conductivities for SEs to a certain extent. Nevertheless, the thermal, chemical and electrochemical stabilities of the composite SEs decline signi cantly, 22,33 which causes interfacial incompatibilities to Li anodes, leading to spontaneous reactions and structural deteriorations.…”