As a result, suggested Tien Duong, of the US Department of Energy (DOE), at the recent 1^st International Conference on Advanced Lithium Batteries for Automobile Applications at Argonne National Laboratory, manganese spinel systems can meet HEV and PHEV-10 (plug-in hybrid with a 10-mile electric range) requirements, but would be harder pressed to be used in a PHEV-40 application.

The use of the nanorods could address some of those issues. In comparing their nanorod materials to a cathode material fabricated from commercial powders (LiMn₂O₄ electrochemical grade, Sigma Aldrich), the researchers observed that:

...the nanorod morphology has a much higher charge capacity than the commercial powders at higher power rates. At the lowest current (from cycle 1 to 5) both the samples have a specific capacity around 110 mAh/g but increasing the rate to 0.2 C (from cycle 6 to 10) leads a large difference in performance: the specific capacity of the nanorod electrode remains almost constant between 110 and 105 mAh/g while that of commercial powder decreases to 70 mAh/g.

In most of the literature a larger amount of conductive agent (up to 20%) is added to the active LiMn₂O₄ material and the specific discharge capacity obtained at moderate rates such as 0.2 C is comparable to that of our nanorods. However, we point out that in our nanorod case, reducing the amount of carbon black (10 wt %) and increasing the active material ratio means that more charges can be stored in the electrode regardless of the obtained LiMn₂O₄ specific capacity. This advantage results from the one-dimensional electron transport and large surface area of the nanorods.

The difference of specific charge capacity between the nanorods and commercial powders becomes larger with the further rate increase. At the highest current (148 mA/g), the nanostructured electrode can deliver a specific charge capacity (100 mAh/g) twice of the commercial powders (50 mAh/g).

Capacity retention after 100 cycles was 85%. The team suggests that the large surface-to-volume ratio of the nanorods enhances greatly the kinetics of the LiMn₂O₄ electrodes.

The research is supported by the Global Climate and Energy Project (GCEP) at Stanford and King Abdullah University of Science and Technology (KAUST). In March, KAUST named Dr. Cui (one of the Stanford researchers who showed the potential of silicon nanowires for improving li-ion energy capacity and cycle life, earlier post), as one of its initial Global Research Partnership (GRP) Investigators. (Earlier post.)

Resources

• Do Kyung Kim, P. Muralidharan, Hyun-Wook Lee, Riccardo Ruffo, Yuan Yang, Candace K. Chan, Hailin Peng, Robert A. Huggins, and Yi Cui (2008) Spinel LiMn₂O₄ Nanorods as Lithium Ion Battery Cathodes. ASAP Nano Lett., doi: 10.1021/nl8024328