Nanomaterials for Lithium Intercalation

Lithium intercalation into solid hosts is the fundamental mechanism underpinning the operation of electrodes in rechargeable lithium batteries. We seek to synthesise new lithium intercalation compounds with unusual properties or combinations of properties. We are especially interested in nanomaterials since the nanoscale can enhance intercalation properties.

Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox

 

Pictu1

Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. R. A. House, L. Jin, U. Maitra, K. Tsuruta, J. W. Somerville, D. P. Forstermann, F. Massel, L. Duda, M. R. Roberts & P. G. Bruce. Energy & Environmental Science, 11, 926-932 (2018).

Arguably the greatest barrier to improving the specific energy of the Li-ion battery is the cathode. While Si offers a route to increase the capacity at the anode this needs to be balanced by higher energy storage cathodes. The capacity of the cathode is limited by storing electrons on the transition metal ions alone, such as in LiMn2O4, where electrons are stored on the Mn3+/4+ redox couple. There is a great deal of interest in increasing charge storage in transition metal oxide cathodes beyond the limit of transition metal redox activity. While redox reactions on sulfur in transition metal sulfides are well known only recently has O redox activity in transition metal oxides been recognised. Here we report a new intercalation cathode material, a lithium manganese oxyfluoride based on Li2MnO2F, with a high capacity to store charge by invoking redox activity on the Mn cations and O anions. It has a disordered rocksalt structure, which avoids the structural changes and consequent severe changes in voltage observed for O-redox layered transition metal oxide cathodes during the 1st cycle. This material is produced using a facile ball milling synthesis.

 

Nanotube and Nanowire

We have synthesised the first TiO2 nanowires and nanotubes, but with the TiO2–B crystal structure, the 5th polymorph of TiO2. The amount of lithium that may be inserted into TiO2–B nanowires (up to Li0.91TiO2-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 LiNi0.5Mn1.5O4 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.