Researchers from MIT, Argonne National Laboratory and UC Berkeley led by Dr. Gerbrand Ceder (now at UC Berkeley/Lawrence Berkeley Lab as of 1 July, formerly at MIT) have developed a new class of high capacity cation-disordered oxides—lithium-excess nickel titanium molybdenum oxides (Li-Ni-Ti-Mo, or LNTMO)—for Li-ion cathode materials which deliver capacities up to 250 mAh/g. A paper on their work is published in the RSC journal Energy & Environmental Science.
In the rechargeable Li-ion battery, cathodes reversibly release and insert (de-intercalation and intercalation) lithium ions during charge and discharge, respectively. Intercalation and de-intercalation must not cause permanent change to the crystal structure of the material over cycling, lest capacity fade. Conventionally, electrochemists have looked to well-ordered close-packed oxides for cathode materials, especially layered rocksalt-type lithium–transition metal oxides (Li-TM oxides) and ordered spinels. In a 2014 paper in Science, however, Dr. Ceder and his colleagues outlined the potential for disordered materials.
In these ordered compounds, Li sites and pathways (a 2D slab in the layered oxides and a 3D network of tetrahedral sites in the spinels) are separated from the TM sublattice, which provides stability and electron storage capacity. Having well-ordered structures where there is little or no intermixing between the Li and the TM sublattice is generally considered important for obtaining high-capacity cathode materials with good cycle life. In some cases, improvements in ordering have led to notable increases in power or energy density. Here, we show that this “ordering paradigm” may have led the community to overlook a large class of cathode materials in which Li and TM share the same sublattice in a random (disordered) fashion; some of these materials may offer higher capacity and better stability relative to the layered oxides.—Lee et al. (2014)
In the Science paper, Ceder and his colleagues chose Li1.211Mo0.467Cr0.3O2 (LMCO) to show that lithium diffusion can be facile in disordered materials. They demonstrated that this is due to percolation of a certain type of active diffusion channels in disordered materials with the introduction of sufficient Li excess.
Disordered Li-excess rocksalts have considerable advantages over layered materials. We find that the changes in lattice parameters and volume, as a function of Li concentration, are very small in disordered materials (<1% in LMCO), which will lead to less mechanical stress and capacity loss in an electrode. Furthermore, as they have more homogeneous cation distribution, they tend to experience less change in local environment of the lithium ions as a function of state of charge. This change in environment is particularly problematic in layered structures where the slab spacing decreases considerably when large amounts of Li are removed, leading to a substantial reduction of Li mobility.
However, in cation-disordered structures, homogeneously distributed cations should lead to a Li diffusivity that is more independent of the Li concentration, as is the case for electrode materials with the spinel- and olivine-type structures. … Our results may explain why disorder has not been pursued as a strategy before: Most materials synthesized are near stoichiometry (LiTMO2), which is well below the percolation threshold for 0-TM diffusion. Therefore, these materials quickly lose their capacity upon disorder as it renders typical 1-TM channels inactive, while 0-TM channels are not percolating. As a result, disorder may have appeared to be a counterintuitive strategy. In contrast, our analysis points to cation-disordered materials as a class of materials that can exhibit high capacity and high energy density, thereby offering hope for substantial improvements in the performance of rechargeable Li batteries.—Lee et al. (2014)
As described in the new paper, the researchers designed their LNTMO materials based on the percolation theory which predicts lithium diffusion to become facile in cation-disordered oxides as the lithium-excess level increases (x > 1.09 in LixTM2-xO2). As conceptualized and demonstrated in the earlier paper, the reversible capacity and rate capability in these compounds improve considerably with lithium excess.
The variant Li1.2Ni1/3Ti1/3Mo2/15O2 delivered up to 250 mAh/g and 750 Wh/kg (~3080 Wh/l) at 10 mA/g.
The team proposed strategies for further improvements, setting new guidelines for the design of high performance cation-disordered oxides for rechargeable lithium batteries.
Jinhyuk Lee, Dong-Hwa Seo, Mahalingam Balasubramanian, Nancy Twu, Xin Li and Gerbrand Ceder (2015) “A new class of high capacity cation-disordered oxides for rechargeable lithium batteries: Li-Ni-Ti-Mo oxides” Energy Environ. Sci. doi: 10.1039/C5EE02329G
Jinhyuk Lee, Alexander Urban, Xin Li, Dong Su, Geoffroy Hautier, and Gerbrand Ceder (2014) “Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries” Science 343 (6170), 519-522 doi: 10.1126/science.1246432