Engineering a faster battery

Today's laptop computers, cell phones, hybrid vehicles, and other technologies rely on rechargeable batteries. As discussed in Physics Today, December 2008, page 43, batteries— in particular, the popular lithium-ion batteries— typically have a high energy density but a low power density: They can't deliver their stored energy particularly quickly. Often the limiting step in Li⁺ batteries is not getting the ions through the electrolyte and electrode structure but getting them into the active electrode material itself. Using nanoscale materials in the electrodes and doping the materials are among present techniques to improve battery rates. Now Byoungwoo Kang and Gerbrand Ceder of MIT have shown that using particles of a common electrode material, lithium iron phosphate (LiFePO₄), covered with a glassy coating of iron-doped lithium phosphate can significantly increase the charging and discharging rates. Moreover, the particles and coating can be formed together in a single step. In test experiments, the researchers obtained discharge rates 100 times as fast as today's commercial Li⁺ batteries. The researchers suggest that the amorphous coating may improve Li⁺ transport across the surface of the electrode particles; uncoated LiFePO₄, in contrast, conducts ions poorly except in a narrow range of directions. Additionally, they say that the coating may modify the surface potential and provide adsorption sites for a range of ion energies. (B. Kang, G. Ceder, Nature 458, 190, 2009.) — Richard J. Fitzgerald