Borohydride‐Scaffolded Li/Na/Mg Fast Ionic Conductors for Promising Solid‐State Electrolytes

Abstract

devices and for various types of consumer electronics. All-solid-state rechargeable batteries (ASSBs) utilizing solid-electrolyte separators rather than combustible liquid electrolytes possess the inherent advantages of enhanced safety and stability for state-of-the-art battery technologies. [1] Recently, ASSBs have attracted a resurgence of interest as ideal candidates for the next generation of electrochemicalenergy-storage devices. The superiority of ASSBs could be ascribed to the distinctive attributes of solid-state electrolytes (SSEs), including high Li-ion transference number and safety, and comparable ionic conductivity to their liquid counterparts. [2] The adoption of SSEs could offer new opportunities for the high-temperature electrical-energy-storage market and pave the way for the utilization of high-capacity electrodes, such as Li, Na, and sulfur. [2c,3] In contrast, in conventional batteries employing liquid electrolytes, the highcapacity electrodes may meet with detrimental side reactions, causing problems such as dendrite growth, reactivity, and/ or dissolution in the solvent. [4,5] With the construction of rigid solid electrolyte separators and stable interfaces in ASSBs, dendrite growth can be effectively mitigated, thus improving the safety of the batteries. [4,6] Owing to these benefits, in battery research, the number of studies on fabricating superionic SSEs has grown rapidly. Despite great progress gained these years, issues Borohydride solid-state electrolytes with room-temperature ionic conductivity up to ≈70 mS cm −1 have achieved impressive progress and quickly taken their place among the superionic conductive solid-state electrolytes. Here, the focus is on state-of-the-art developments in borohydride solid-state electrolytes, including their competitive ionic-conductive performance, current limitations for practical applications in solid-state batteries, and the strategies to address their problems. To open, fast Li/Na/Mg ionic conductivity in electrolytes with BH 4 − groups, approaches to engineering borohydrides with enhanced ionic conductivity, and later on the superionic conductivity of polyhedral borohydrides, their correlated conductive kinetics/thermodynamics, and the theoretically predicted high conductive derivatives are discussed. Furthermore, the validity of borohydride pairing with coated oxides, sulfur, organic electrodes, MgH 2 , TiS 2 , Li 4 Ti 5 O 12 , electrode materials, etc., is surveyed in solid-state batteries. From the viewpoint of compatible cathodes, the stable electrochemical windows of borohydride solid-state electrolytes, the electrode/electrolyte interface behavior and battery device design, and the performance optimization of borohydride-based solid-state batteries are also discussed in detail. A comprehensive coverage of emerging trends in borohydride solid-state electrolytes is provided and future maps to promote better performance of borohydride SSEs are sketched out, which will pave the way for their further development in the field of energy stora...

Complex Hydrides for Energy Storage, Conversion, and Utilization

“…Particularly, higher boranes and their derivatives with superionic conductivity of Li + and Na + have shown promising future. More detailed information can be found in the recent reviews 1b,12. It should be noted that the high conductivity is usually accompanied with low melting point/low phase transformation point, large‐size anion or layered structures, and sufficient defects in the materials, indicating a general research direction for optimization of ionic conduction.…”

“…Solid electrolytes of high ion conductivity could solve those problems and further promote the development of advanced batteries for clean energy utilization. A number of material systems including NASICON‐type, LISICON‐type, garnet‐type, perovskite‐type, sulfides, nitrides, hydrides, and halides have been actively explored for ion conduction, among which complex hydrides offer an attractive option because of their low grain boundary resistance, reduction stability, mechanical flexibility, and high ion conductivity 1b,12,119 . Table 3 lists some solid‐state Li and Na electrolytes based on complex hydrides that were developed recently.…”

“…Coincidentally, complex hydrides are composed of those cations and a relatively large sized [XH n ] anion. The coordination flexibility of the large anion with alkali or alkaline earth cation would create some favorable structural features, such as defects and/or low‐barrier diffusion channels, to facilitate cation migration within the lattice of complex hydrides . Starting from the finding of high Li‐ion conduction in the high‐temperature‐phase LiBH 4 , a variety of complex hydrides has been developed, some of which show superior ion conductivity to their nonhydride peers …”

Boron: Its Role in Energy‐Related Processes and Applications

“…In addition to applications in liquid electrolytes, anionic boron clusters have been discussed as building blocks for solid‐state metal‐ion batteries, especially because of the superionic conduction observed, for example, for Na 2 [ closo ‐B 12 H 12 ] and M 2 [ closo ‐1‐CB 9 H 10 ][ closo ‐1‐CB 11 H 12 ] (M=Li, Na) . The mixed salt Na 2 [ closo ‐B 12 H 12 ] 0.5 [ closo ‐B 10 H 10 ] 0.5 was used for the construction of a stable 3 V all‐solid‐state sodium‐ion battery, which exemplifies the potential of these solid electrolytes.…”

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

“…The role of MgB 12 H 12 in facilitating the improvement in ionic conductivity observed for oxidized Mg(BH 4 ) 2 should be considered. However, the ionic conductivity of MgB 12 H 12 has been predicted to be poor 36,45…”