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.

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We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB2. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E2g symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B σ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to quantitatively explaining the observed high Tc and boron isotope effect in MgB2 PACS numbers: 63.20. Ry, 63.20.Kr, 74.25.Jb, 74.25.Kc The recent discovery of superconductivity at 40 K in the MgB 2 binary alloy system [1] has triggered enormous interest in the structural and electronic properties of this class of materials. The system has a very simple crystal structure [2], where the boron atoms form graphitelike sheets separated by hexagonal layers of Mg atoms (see inset to Fig. 1). Our pseudopotential plane wave band structure calculations show that the bands near the Fermi level arise mainly from the p x,y σ bonding orbitals of boron, while the Mg does not contribute appreciably to the conductivity, in good agreement with initial reports from other groups [3][4][5]. In the case of graphite these σ bands are full, but for MgB 2 they are partially unoccupied, creating a hole-type [6] conduction band like the high-T c cuprates. In contrast to the cuprates, however, the normal-state conductivity is three-dimensional in nature instead of being highly anisotropic, thus eliminating the "weak-link" problem that has plagued widespread commercialization of the cuprates. The normal-state conductivity [6][7][8] is also one to two orders-of-magnitude higher than either the Nbbased alloys or Bi-based cuprates used in present day wires, and this feature combined with low cost and easy fabrication [8,9] could make this class of materials quite attractive for applications.From a fundamental point of view the central question is whether the high T c in this new system can be understood within the framework of a conventional electronphonon mechanism, or a more exotic mechanism is responsible for the superconducting pairing. The observed boron isotope effect [10] argues for an electron-phonon mechanism, while the positive Hall coefficient [6] suggests similarities with the cuprates [11]. To answer this question, we have carried out inelastic neutron scattering measurements of the phonon density of states, and compare these results with detailed first-principles calculations of the lattice dynamics (and electronic) calculations for MgB 2 . Excellent agreement is found between theory and experiment. More importantly, the numerical results demonstrate that the in-plane boron phonons near the zone-center (with E 2g symmetry at Γ) are very anharmonic and strongly coupled to the partially occupied c...

Li 2 B 12 H 12 , Na 2 B 12 H 12 , and their closo-borate relatives exhibit unusually high ionic conductivity, making them attractive as a new class of candidate electrolytes in solid-state Li-and Na-ion batteries. However, further optimization of these materials requires a deeper understanding of the fundamental mechanisms underlying ultrafast ion conduction. To this end, we use ab initio molecular dynamics simulations and density-functional calculations to explore the motivations for cation diffusion. We find that superionic behavior in Li 2 B 12 H 12 and Na 2 B 12 H 12 results from a combination of key structural, chemical, and dynamical factors that introduce intrinsic frustration and disorder. A statistical metric is used to show that the structures exhibit a high density of accessible interstitial sites and site types, which correlates with the flatness of the energy landscape and the observed cation mobility. Furthermore, cations are found to dock to specific anion sites, leading to a competition between the geometric symmetry of the anion and the symmetry of the lattice itself, which can facilitate cation hopping. Finally, facile anion reorientations and other low-frequency thermal vibrations lead to fluctuations in the local potential that enhance cation mobility by creating a local driving force for hopping. We discuss the relevance of each factor for developing new ionic conductivity descriptors that can be used for discovery and optimization of closo-borate solid electrolytes, as well as superionic conductors more generally.

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Solid lithium and sodium closo-polyborate-based salts are capable of superionic conductivities surpassing even liquid electrolytes, but often only at above-ambient temperatures where their entropically driven disordered phases become stabilized. Here we show by X-ray diffraction, quasielastic neutron scattering, differential scanning calorimetry, NMR, and AC impedance measurements that by introducing “geometric frustration” via the mixing of two different closo-polyborate anions, namely, 1-CB9H10 – and CB11H12 –, to form solid-solution anion-alloy salts of lithium or sodium, we can successfully suppress the formation of possible ordered phases in favor of disordered, fast-ion-conducting alloy phases over a broad temperature range from subambient to high temperatures. This result exemplifies an important advancement for further improving on the remarkable conductive properties generally displayed by this class of materials and represents a practical strategy for creating tailored, ambient-temperature, solid, superionic conductors for a variety of upcoming all-solid-state energy devices of the future.