NATURE MATERIALS | VOL 10 | SEPTEMBER 2011 | www.nature.com/naturematerials 649

news & views

Electrochemical energy storage systems, which can deliver electricity on demand, lie at the heart of today’s

mobile electronic devices and electrically powered vehicles. On a larger scale,

such systems can secure the supply of renewable energy through the buffering of intermittent resources such as wind and the Sun. The growing need for energy storage systems has prompted intense academic

research activities and huge industrial investments in the field of rechargeable batteries1,2. The lithium-ion battery, which was introduced by Sony Corporation in 1991, now plays a major role in consumers’

SOLID ELECTROLYTES

Lithium ions on the fast trackThe solvent-based electrolytes used at present in lithium-ion batteries can be unsafe for large-scale applications. A crystalline electrolyte with high ionic conductivity could soon enable all-solid energy storage systems.

Christian Masquelier

MATERIA

L WITN

ESS

When fullerene chemistry took off in the 1990s, one enticing use of the carbon-cage molecules was as a protective wrapping for trapped molecules, isolating them from their environment. This, it was suggested, might offer a kind of matrix-isolation technique for studying sensitive chemical species under ambient conditions — a notion explored previously using other cage complexes1. Trapped metal atoms — the first class of endohedral fullerene compounds reported — might be used as contrast agents for magnetic resonance imaging, or perhaps even as qubits for quantum computing2,3. If encapsulation could be made reversible, maybe fullerenes could be used to chaperone drug molecules to their target cells.

Some of these possibilities continue to show promise; no doubt others will turn out to have been over-optimistic. The enterprise of trapping molecules to study in isolation has now been furthered significantly by Kurotobi and Murata, who have used inventive chemistry to open up a hole in C60, insert a single water molecule, and then seal up the cage again4. They report a crystal structure of H2O@C60 (the @ sign denoting encapsulation) complexed to a nickel porphyrin, with the lone H2O localized in the cage centre.

Kurotobi and Murata suggest that H2O@C60 might offer a rare opportunity to study the water molecule in a situation where it is not coordinated to

other species via either hydrogen bonds or lone-pair dative bonding. Well, perhaps. The nominally hydrophobic interior of the fullerene cage might certainly lead one to imagine that the water molecule behaves almost as if in vacuo. And the ultraviolet-visible spectrum of the complex is almost identical to that of empty C60, suggesting almost no interaction between the H2O and the carbon cage, while the 13C NMR spectrum shows all the carbons to be identical, indicating that the water molecule rotates rapidly on the NMR timescale. Yet it would be surprising if indeed there was no interaction between the hydrogen atoms and the aromatic rings of the cage, as hydrogen bonding to the π electrons of benzene rings has been reported for other encapsulated water molecules, both in synthetic supramolecular complexes5 and in a hydrophobic protein cavity6.

It’s in the latter regard that H2O@C60 might prove to be a useful model compound. Several proteins have hydrophobic interior spaces in which a few water molecules might lodge. In the cytokine interleukin-1β, for example, such a cavity hosts between one and four water molecules on average6. There is predominantly a water dimer in the cavity itself, but it is not totally isolated: lone water molecules in two channels opening onto the cavity sometimes connect the dimer to the exterior7.

Such trapped water molecules seem to have significantly slowed dynamics relative to the bulk8, although it’s

not clear whether this depends on there being several water molecules present so that the dynamics become cooperative. If that’s so, the trapped water might be expected to have a different dielectric constant from the bulk, altering its solvent properties9. If the same chemistry will work for larger fullerenes, Kurotobi and Murata might have devised a valuable method for exploring the characteristics of such small, hydrophobically encapsulated water clusters. ❐

References1. Cram, D. J., Tanner, M. E. & Thomas, R.

Angew. Chem. Int. Ed. 30, 1024–1027 (1991).2. Harneit, W. Phys. Rev. A 65, 032322 (2002).3. Ju, C., Suter, D. & Du, J. Phys. Rev. A 75, 012318 (2007).4. Kurotobi, K. & Murata, Y. Science 333, 613–616 (2011).5. Atwood, J. L., Hamada, F., Robinson, K. D., Orr, G. W. &

Vincent, R. L. Nature 349, 683–684 (1991).6. Somani, S., Chng, C. P. & Verma, C. S. Proteins

67, 868–885 (2007).7. Yu, B., Blaber, M., Gronenborn, A. M., Clore, G. M.

& Caspar, D. L. D. Proc. Natl Acad. Sci. USA 96, 103–108 (1999).

8. Modig, K., Rademacher, M., Lücke, C. & Halle, B.  J. Mol. Biol. 332, 965–977 (2003).

9. Despa, F., Fernández, A. & Berry, R. S. Phys. Rev. Lett. 93, 228104 (2004).

CAGED WATER

PHILIP BALL

© 2011 Macmillan Publishers Limited. All rights reserved

650 NATURE MATERIALS | VOL 10 | SEPTEMBER 2011 | www.nature.com/naturematerials

news & views

everyday life, and it is regarded as the key technology for the further development of hybrid or all-electric cars2,3. It followed the demonstration of facile and reversible lithium-ion insertion and extraction from LiCoO2 (the positive electrode) and graphite (the negative electrode), respectively. Besides the positive and negative electrodes, the electrolyte through which the ions move during charge and discharge cycles has a critical role in the performance of lithium-ion batteries. It is the subject of extensive scrutiny, because it impacts the safety, toxicity and long-term cyclability of such high-energy-density electrochemical storage systems4. Writing in Nature Materials, Kamaya et al.5 now report the discovery of a crystallized solid inorganic electrolyte whose lithium-ion conductivity even exceeds that of solvent-based systems, and which can be applied in all-inorganic energy storage systems.

The high operating voltage of a lithium-ion electrochemical cell (~4 V) prohibits the use of aqueous electrolytes, and most commercial cells use solutions of dissociated salts such as LiPF6, LiBF4 or LiBC4O8 in organic alkyl carbonate solvents. These solvents are usually complex mixtures of ethylene-, dimethyl-, diethyl- and ethylmethyl carbonates with additives. Despite the great advances in volumetric and gravimetric energy and power density that have been made, such electrolyte compositions pose a number of challenges, in particular for large-scale applications in electric vehicles or stationary energy storage. At high operating voltages, these electrolytes can undergo unwanted reactions with electrodes made from structurally unstable layered

transition-metal oxides that are detrimental to the long-term cyclability of such energy storage systems. Furthermore, they may decompose and generate dangerous thermal runaways4. Many alternatives to purely organic solvent-based liquid electrolytes have hence been proposed in the literature, and some have been implemented in real batteries. These materials include solid polymers, ionic liquids, inorganic (ceramic) electrolytes and their combinations in so-called hybrid electrolyte systems, which, however, still suffer from slow ionic transport (particularly at low temperature) and high cost6.

The new inorganic crystallized composition that Kamaya et al. report — Li10GeP2S12 — shows the highest lithium-ion conductivity ever measured for a solid, about 12 mS cm–1 at 300K. The material possesses a three-dimensional structure that consists of (Ge0.5P0.5)S4 tetrahedra, PS4 tetrahedra, LiS4 tetrahedra, and LiS6 octahedra. Based on careful structural determination from powder neutron diffraction data, the researchers at Tokyo Institute of Technology and Toyota Motor Corporation found highly anisotropic conduction of Li+ ions along one crystal direction, namely through partially occupied LiS4 tetrahedra and interstitial positions that are connected by a common edge (see Fig. 1). They derive a low activation energy for ionic conduction (24 kJ mol–1) in this new material, which enables its use at low temperature and leads to a conductivity of 10–3 S Cm–1 at 245 K. Moreover, a wide electrochemical stability versus Li0 is demonstrated, which is particularly important for cycling stability and safety issues, which have been major

drawbacks in existing systems such as Li3N, Li1/3–xLi3xNbO3, LiTi1.7Al0.3(PO4)3 and LiBH4–LiI superionic conductors. Finally, the researchers realize all-solid-state batteries using LiCoO2 and indium as positive and negative electrodes, respectively, which operate at 3.3 V with excellent reversibility.

The results by Kamaya et al. demonstrate how continuous investment into energy storage systems — such as the 30-year-long effort that Japanese authorities and companies in particular have made in lithium-ion technology — can lead to significant breakthroughs arising from previously disregarded chemistries. Other examples, which were inspired by the pioneering work of Takahashi7, include sulphide glassy and crystallized compositions in the Li2S–SiS2–LiI, Li2S–SiS2–Li3PO4 and Li2S–GeS2–P2S5 systems8,9.

Most certainly, the work of Kamaya et al. will be followed by further studies from other groups, as the implementation of the new superionic conductor Li10GeP2S12 in real lithium-ion energy storage systems will be contingent on, for instance, the large-scale ceramic processing of air-sensitive sulphides, the cost-effective use of Ge and the mechanical stability of such a battery over prolonged cycling. Overall, the work of Kamaya et al. will probably stir significant and renewed interest in ceramic solid electrolytes, and it may lead to breakthroughs similar to the discoveries of beta alumina and NASICON-type structures, which are nowadays used in Na–S batteries. Recent demonstrations of all-solid-state batteries9,10 illustrate the wide variety of chemistries and ceramic processing techniques that could be envisaged to further improve the performance of energy storage systems based on this new solid electrolyte, for example by reducing the interfacial resistance between active components. ❐

Christian Masquelier is at the Université de Picardie Jules Verne, Amiens, France. e-mail: christian.masquelier@u-picardie.fr

References1. Yang, Z. et al. Chem. Rev. 111, 3577–3613 (2011).2. Tarascon, J. M. & Armand, M. Nature 414, 359–367 (2001).3. Armand, M. & Tarascon, J. M. Nature 451, 652–657 (2008).4. Hammami, A., Raymond, N. & Armand, M. Nature

424, 635–636 (2003).5. Kamaya, et al. Nature Mater. 10, 682–686 (2011).6. Knauth, P. Solid State Ionics 180, 911–916 (2009).7. Takahashi, T. & Yamamoto, O. Electrochim. Acta

11, 779–789 (1966).8. Kanno, R. & Murayama, M. J. Electrochem. Soc.

148, A742–A746 (2001).9. Tatsumisago, M. & Hayashi, A. Funct. Mater. Lett. 1, 31–36 (2008).10. Aboulaich, A. et al. Adv. Energ. Mater. 1, 179–183 (2011).

+ -In

e–

>

LiCoO2 Li10GeP2S12

Li+ Li+ Li+ Li+ Li+ Li+ Li+ Li+

Figure 1 | Schematic view of the all-solid-state lithium-ion battery reported by Kamaya and colleagues. During charging, lithium ions (grey) travel with high mobility from the positive LiCoO2 electrode to the negative indium electrode via partially occupied LiS4 tetrahedra and interstitial positions in the new superionic conductor Li10GeP2S12 (ref. 5).

© 2011 Macmillan Publishers Limited. All rights reserved