|SEM images of nanorod devices in 1 M LiPF6 in the EC/DEC electrolyte: LiMn2O4 (a) 0, (b) 3, and (c) 9 h; LiAl0.1Mn1.9O4 (d) 0, (e) 3, and (f) 9 h; λ-MnO2 (g) 0, (h) 3, and (i) 9 h. Credit: ACS, Yang et al. Click to enlarge.|
An international team of researchers from Stanford University, Università degli Studi di Milano-Bicocca, and Korea Advanced Institute of Science and Technology (KAIST) is proposing the use of single nanostructure devices as a powerful new diagnostic tool for Li-ion batteries. A paper on their work was published online 6 October in the ACS journal Nano Letters.
The electrodes in lithium-ion batteries consist of particles with various sizes and shapes, conductive carbon, and polymer binders. During battery charge/discharge, the insertion or extraction of electrons and ions is accompanied by a series of other complex processes such as structure and phase transformation, volume change, materials dissolution, and side chemical reaction with electrolyte, the authors note. While a number of different technologies have been used for battery diagnostics, “the heterogeneous nature of ensemble electrodes averages all information and can not provide a direct correlation of electrochemical properties with the local morphology, structure, and chemical composition.”
The new approach, however:
...allows for the direct correlation of the electrochemical property with the structure on the same nanoscale particle...This work represents the first example of single nanostructure device battery diagnostics. We use single LiMn2O4 nanorod devices as an example in this study although this new methodology can generally be applied to a broad range of battery electrode materials. In the future, the capability of combining single nanostructure device diagnostics with in situ electron microscopy techniques can lead to a much deeper understanding of battery operational processes.
—Yang et al. 2009
The researchers, led by Dr. Yui Cui at Stanford, synthesized LiMn2O4 and Al-doped LiMn2O4 nanorods by a two-step method that combines hydrothermal synthesis of β-MnO2 nanorods and a solid state reaction to convert them to LiMn2O4 nanorods. λ-MnO2 nanorods were also prepared by acid treatment of LiMn2O4 nanorods.
They then studied the effect of electrolyte etching on these LiMn2O4-related nanorods by both SEM and single-nanorod transport measurement—the first time that the transport properties of this material have been studied at the level of an individual single crystalline particle.
Experiments showed that Al dopants reduce the dissolution of Mn3+ ions significantly and make the LiAl0.1Mn1.9O4 nanorods much more stable than LiMn2O4 against electrolyte etching, which is reflected by the magnification of both size shrinkage and conductance decrease.
These results correlated well with the better cycling performance of the Al-doped LiMn2O4 in Li-ion battery testing: LiAl0.1Mn1.9O4 nanorods achieved 96% capacity retention after 100 cycles at 1C rate at room temperature, and 80% at 60 °C, whereas LiMn2O4 shows worse retention of 91% at room temperature, and 69% at 60 °C. Temperature-dependent I-V measurements indicated that the sharp electronic resistance increase due to charge ordering transition at 290 K does not appear in the LiMn2O4 nanorod samples, suggesting good battery performance at low temperature.
In addition to the specific materials discussed in this study, we suggest that our method of single-nanorod measurement could be used as a new way to investigate the interaction between electrolyte and electrode materials, revealing relationships that current methods have not ascertained.
—Yang et al. 2009
The research was partially supported by the Global Climate and Energy Project at Stanford and King Abdullah University of Science and Technology (KAUST).
Yuan Yang, Chong Xie, Riccardo Ruffo, Hailin Peng, Do Kyung Kim and Yi Cui (2009) Single Nanorod Devices for Battery Diagnostics: A Case Study on LiMn2O4. Nano Lett., Article ASAP doi: 10.1021/nl902315u