Nanomaterials for Lithium Intercalation

Nanotubes and Nanowires

Recently we have synthesised the first TiO₂ nanowires and nanotubes, but with the TiO₂–B crystal structure, the 5^th polymorph of TiO₂. The amount of lithium that may be inserted into TiO₂–B nanowires (up to Li_0.91TiO₂-B) is almost twice that of Li₄Ti₅O₁₂ and 30% higher than bulk TiO₂–B. The synthesis is cheap and easy and the materials are safe. The intercalation can be >99% reversible. All this means that the TiO₂–B nanowires are of considerable potential interest as a replacement for graphite negative electrodes used in the current generation of rechargeable lithium batteries.

From: Armstrong AR, Armstrong G, Canales J, Bruce PG. "TiO₂-B Nanowires". Angew. Chem. Int. Ed. 43 (17): 2286-2288 2004

Although there is some irreversibility on the first cycle, thereafter cyclability is excellent. The ability to cycle Li is superior to nanoparticulate anatase and TiO₂-B with an average particle size comparable to the diameter of the wires, demonstrating the value of the nanowire morphology. Such a morphology combines excellent transport between the wires with fast Li intercalation.

From: Armstrong AR, Armstrong G, Canales J, Garcia R, Bruce PG, "Lithium-ion intercalation into TiO₂-B nanowires". Adv. Mater. 17 (7): 862-865 2005

We have combined the TiO₂-B anode with nano LiFePO₄ and nano LiNi_0.5Mn_1.5O₄ cathodes to form a nanobattery operating in a different electrochemical potential window. Compared with conventional Li⁺-ion cells these deliver superior safety to graphite anodes (overcharge protection, thermodynamically stable) and with excellent rate performance.

Mesoporous Solids

There is much current interest in mesoporous solids, especially transition metal oxides because of their magnetic, optical and electrical properties and as catalysts. For many properties it is important to synthesize mesoporous materials with an ordered pore structure and crystalline walls. We have successfully synthesized the first example of a mesoporous iron oxide (specifically α-Fe₂O₃) with crystalline walls and present evidence for its unique magnetic behaviour, distinct from bulk α-Fe₂O₃, nanoparticulate α-Fe₂O₃, or mesoporous Fe₂O₃ with disordered walls.

From: Jiao F, Harrison A, Jumas J-C, Chadwick AV, Kockelmann W, Bruce PG "Ordered Mesoporous Fe₂O₃ with Crystalline Walls". J. Am. Chem. Soc. 128 (16): 5468-5474 2006

There are severe limitations to the range of transitional metal mesopores that may be synthesized directly. In particular, mixed valence materials or those composed of transition metals in oxidation states unstable in solution are difficult or impossible to prepare. We have synthesized for the first time ordered mesoporous Fe₃O₄ and γ-Fe₂O₃ with crystalline walls, in fact this is the first synthesis of any reduced mesoporous iron oxide (Fe₃O₄). Synthesis was achieved by reducing ordered mesoporous α-Fe₂O₃ to ordered mesoporous Fe₃O₄ and then oxidizing it to ordered mesoporous γ-Fe₂O₃. Preservation of the ordered mesoporosity despite conversion of the walls from hcp α-Fe₂O₃ to ccp Fe₃O₄ spinel demonstrates the stability of such mesoporous materials to significant structure phase transitions due to their ability to accomodate strain. Such post-template redox reactions to form mixed valence mesopores or other similar materials inaccessable by direct template syntheses is generally applicable.

From: Jiao F, Jumas J-C, Womes M, Chadwick AV, Harrison A, Bruce PG "Synthesis of Ordered Mesoporous Fe₃O₄ and γ-Fe₂O₃ with Crystalline Walls Using Post-Template Reduction/Oxidation". J. Am. Chem. Soc. 128 (39): 12905-12909 2006

By synthesizing mesoporous lithium intercalation electrodes significantly higher rates of intercalation/deintercalation can be obtained, important for their use as cathodes in high power lithium batteries such as for hybrid electric vehicles. We have synthesized the mesoporous Li intercalation compound LiMn₂O₄ spinel by a post-templating reaction demonstrating high rate performance at ambient temperature combined with far superior stability when operating at elevated temperatures, thus addressing an important challenge in the field.