Solid electrolytes with sufficiently high conductivities and stabilities are the elusive answer to the inherent shortcomings of organic liquid electrolytes prevalent in today’s rechargeable batteries. We recently revealed a novel fast-ion-conducting sodium salt, Na2B12H12, which contains large, icosahedral, divalent B12H122− anions that enable impressive superionic conductivity, albeit only above its 529 K phase transition. Its lithium congener, Li2B12H12, possesses an even more technologically prohibitive transition temperature above 600 K. Here we show that the chemically related LiCB11H12 and NaCB11H12 salts, which contain icosahedral, monovalent CB11H12− anions, both exhibit much lower transition temperatures near 400 K and 380 K, respectively, and truly stellar ionic conductivities (> 0.1 S cm−1) unmatched by any other known polycrystalline materials at these temperatures. With proper modifications, we are confident that room-temperature-stabilized superionic salts incorporating such large polyhedral anion building blocks are attainable, thus enhancing their future prospects as practical electrolyte materials in next-generation, all-solid-state batteries.

Na2 B10 H10 exhibits exceptional superionic conductivity above ca. 360 K (e.g., ca. 0.01 S cm(-1) at 383 K) concomitant with its transition from an ordered monoclinic structure to a face-centered-cubic arrangement of orientationally disordered B10 H10 (2-) anions harboring a vacancy-rich Na(+) cation sublattice. This discovery represents a major advancement for solid-state Na(+) fast-ion conduction at technologically relevant device temperatures.

Both LiCB9H10 and NaCB9H10 exhibit liquid‐like cationic conductivities (≥0.03 S cm−1) in their disordered hexagonal phases near or at room temperature. These unprecedented conductivities and favorable stabilities enabled by the large pseudoaromatic polyhedral anions render these materials in their pristine or further modified forms as promising solid electrolytes in next‐generation, power devices.

Novel ultrathin Li(2)MnSiO(4) nanosheets have been prepared in a rapid one pot supercritical fluid synthesis method. Nanosheets structured cathode material exhibits a discharge capacity of ~340 mAh/g at 45 ± 5 °C. This result shows two lithium extraction/insertion performances with good cycle ability without any structural instability up to 20 cycles. The two-dimensional nanosheets structure enables us to overcome structural instability problem in the lithium metal silicate based cathode materials and allows successful insertion/extraction of two complete lithium ions.

Complex hydrides have energy storage-related functions such as i) solid-state hydrogen storage, ii) electrochemical Li storage, and iii) fast Li-and Naionic conductions. Here, recent progress on the development of fast Li-ionic conductors based on the complex hydrides is reported. The validity of using them as electrolytes in all-solid-state lithium rechargeable batteries is also examined. Not only coated oxides but also bare sulfi des are found to be applicable as positive electrode active materials. Results related to fast Na-ionic conductivity in the complex hydrides are presented. In the last section, the future prospects for battery assemblies with high-energy densities, and Mg ion batteries with the liquid and the solid-state electrolytes are discussed. Figure 1. Crystal structures of a) LT phase and b) HT phase of LiBH 4 . [ 16 ]Adv. FEATURE ARTICLEto fast Li-ionic conduction, the advantages of the use of complex hydrides as rechargeable battery electrolytes are as follows, iii) Fast Li-and Na-ionic conductions: One of the important challenges in the fi eld of battery research is the development of fast ionic conductors because this class of materials enables the assembly from micro- [ 24 ] to bulk-type [25][26][27] all-solid-state rechargeable cells, which allows the broader applications. With this background, various solid-state electrolytes have been developed so far, however, the materials showing suffi cient ionic conductivity as well as good stability in the voltage ranges for battery operation are limited to a few cases. [ 28 ] Therefore, novel solid-state electrolytes are urgently required for the development of future generation batteries. Since the discovery of fast Li-ionic conduction in the HT phase of LiBH 4 , [ 29 ] we have developed numerous solid-state electrolytes based on the complex hydrides that exhibit fast Li-and Na-ionic conductions. [ 30 ] Herein, we briefl y report recent progress in the development of the complex hydrides that exhibit fast Li-ionic conduction. All-solid-state Li rechargeable batteries were subsequently assembled using coated-LiCoO 2 and TiS 2 positive electrodes, and LiBH 4 -based electrolytes. The validity of the use of the complex hydrides as electrolytes in a rechargeable battery was examined on the basis of charge-discharge measurements. In the following section, our recent development of fast Na-ionic conductors based on the complex hydrides is summarized. We discuss the future prospects for the development of high energy density rechargeable batteries, and Mg-ion batteries that use nonaqueous and solid-state electrolytes based on the complex hydrides. FEATURE ARTICLEhexagonal phase of LiBH 4 (fast Li-ion conduction phase) even at room temperature when x exceeds 0.25. And then, the solid-solution, Li 4 (BH 4 ) 3 I, exhibits fast Li-ionic conductivity of 2 × 10 −5 S cm −1 at 300 K. [ 34,[51][52][53] This phenomenon might be due to increased neighboring distance of [BH 4 ] − [ 54 ] and induced lattice anharmonicity by the partial substitution of I − instead of ...

By a variety of techniques including X-ray powder diffraction, quasielastic neutron scattering, and AC impedance, we have probed the effect of mechanical milling on the phase behaviors of the different lithium and sodium closo-borate salt compounds containing B 12 H 12 2-, B 10 H 10 2-, and CB 11 H 12anions. We have found that the crystallite-size reduction and disordering effects of such milling enables the room-T stabilization of their high-T-like superionic-conducting phases. This demonstrates a viable strategy for better exploiting the impressive cation mobilities that are typically restricted to somewhat higher temperatures for this class of compounds.

In this study, we assembled a bulk-type all-solid-state battery comprised of a TiS 2 positive electrode, LiBH 4 electrolyte, and Li negative electrode. Our battery retained high capacity over 300 discharge-charge cycles when operated at 393 K and 0.2 C. The 2 nd discharge capacity was as high as 205 mAh g -1 , corresponding to a TiS 2 utilization ratio of 85 %. The 300 th discharge capacity remained as high as 180 mAh g -1 with nearly 100 % coulombic efficiency from the 2 nd cycle. Negligible impact of the exposure of LiBH 4 to atmospheric-pressure oxygen on battery cycle life was also confirmed. To investigate the origin of the cycle durability for this bulk-type all-solid-state TiS 2 /Li battery, electrochemical measurements, thermogravimetry coupled with gas composition analysis, powder X-ray diffraction measurements and first-principles molecular dynamics simulations were carried out. Chemical and/or electrochemical oxidation of LiBH 4 occurred at the TiS 2 surface at the battery operating temperature of 393 K and/or during the initial charge. During this oxidation reaction of LiBH 4 with hydrogen (H 2 ) release just beneath the TiS 2 surface, a third phase, likely including Li 2 B 12 H 12 , precipitated at the interface between LiBH 4 and TiS 2 . Li 2 B 12 H 12 has a lithium ionic conductivity of log(σ / S cm -1 ) = -4.4, charge transfer reactivity with Li electrodes, and superior oxidative stability to LiBH 4 , and thereby can act as a stable interface that enables numerous discharge-charge cycles. Our results strongly suggest that the creation of such a stable interfacial layer is due to the propensity of forming highly stable, hydrogen-deficient polyhydro-closo-polyborates such as Li 2 B 12 H 12 , which are thermodynamically available in the ternary Li-B-H system.

Stable battery operation of a bulk-type all-solid-state lithium-sulfur battery was demonstrated by using a LiBH4 electrolyte. The electrochemical activity of insulating elemental sulfur as the positive electrode was enhanced by the mutual dispersion of elemental sulfur and carbon in the composite powders. Subsequently, a tight interface between the sulfur-carbon composite and the LiBH4 powders was manifested only by cold-pressing owing to the highly deformable nature of the LiBH4 electrolyte. The high reducing ability of LiBH4 allows using the use of a Li negative electrode that enhances the energy density. The results demonstrate the interface modification of insulating sulfur and the architecture of an all-solid-state Li-S battery configuration with high energy density.