Boost For Battery Performance

Lithium-ion batteries may get a shot in the arm, thanks to a modified version of a not-so-new material. Substituting one transition metal for another in the lithium compound commonly used in electrodes may boost the ability of future batteries to store and release energy quickly, according to a new study.

Because they pack more energy on a size and weight basis than other types of batteries, rechargeable lithium batteries corner the market in the portable electronics arena (C&EN, Feb. 13, page 79). Other types of products could benefit from lithium batteries' high storage capacity (energy density). But manufacturers have been slow to implement the batteries because for some applications-for example, heavy-duty portable power tools and hybrid electric vehicles-lithium batteries haven't generally been able to discharge or recharge quickly enough.

Now, a combined theoretical and experimental study finds that the charge and discharge rates of batteries featuring well-ordered lithium nickel manganese oxide [Li(Ni_0.5Mn_0.5)O₂] electrodes are significantly faster than those employing the conventional electrode material, lithium cobalt oxide (Science 2006, 311, 977). The findings counter earlier studies in which the nickel-manganese compound had been judged to be a poor candidate for improving the performance of lithium batteries.

The new study was conducted by Gerbrand Ceder, a materials science professor at Massachusetts Institute of Technology; MIT graduate student Kisuk Kang and postdoc Ying (Shirley) Meng; and their collaborators at the State University of New York, Stony Brook.

Lithium cobalt oxide (LiCoO₂) and a variety of related materials under investigation for electrode applications exhibit a layered structure in which sheets of transition-metal cations are separated from layers of lithium by oxygen, Ceder explains. The layered structure is central to the charging and discharging mechanism, which is mediated by diffusion of lithium ions through the material.

"We knew from the theoretical investigation that in order to increase the rate of lithium diffusion, we needed a layered structure that was well-ordered," Ceder says. The group also determined that the size difference between the alkali (Li) ions and transition-metal ions governs the ordering of the layers. In the case of Li(Ni_0.5Mn_0.5)O₂, because lithium and nickel are so similar in size, standard methods for preparing the compound led to materials with roughly 10% disorder due to Li/Ni site swapping. According to the researchers, that disorder is the source of the poor charge and discharge rates measured previously.

So the group prepared the sodium analog of the material to exploit the relatively large size difference between Na⁺ and Li⁺. When they exchanged the ions in the product, they obtained a well-ordered version of the lithium compound.

Describing the work as "excellent," Linda F. Nazar, a chemistry professor at Ontario's University of Waterloo, notes that "if the high degree of cation ordering, and hence excellent kinetics, is sustained on long-term cycling, this material could well prove important to the next generation of lithium-ion batteries, especially if direct synthetic routes are found."