Size effects of LiFePO₄

Changbao Zhu, Jelena Popovic

Recently, LiFePO₄ has received much attention as a potential of high rate cathode material for electric vehicle and hybrid electric vehicle application. However, size effects in LiFePO₄need to be clarified in order to arrive at deeper conceptual understanding of its electrochemical behaviour.

In this project, metastable LiFePO₄ (nanocrystalline and amorphous phase) is investigated using thermodynamic considerations combined with obtained experimental results. The effect of size as well as the impact of amorphization are strongly dependent on the storage mechanism (single phase vs. two phase model), size distribution and surface energy.

The thermodynamic effects in the LiFePO₄ are very complex and even staging phenomena are observed, which challenges the traditional two phase models. Various staging phenomena were observed for LiFePO₄of different sizes. The corresponding thermodynamics and kinetics for phase transition process are discussed.

Figure 1. (a), (b) Different Gibbs free energy curves in the LiFePO4/FePO4 systems as function of Li content with the existence of low energy staging phase. Note that is the Gibbs energy per number FePO4-units andis the ratio of number of Li and number of FePO4. (c) The chemical potential of Li, owing to the definition of and , as derivative of one above curves. — Figure 1. (a), (b) Different Gibbs free energy curves in the LiFePO₄/FePO₄ systems as function of Li content with the existence of low energy staging phase. Note that is the Gibbs energy per number FePO₄-units andis the ratio of number of Li and number of FePO₄. (c) The chemical potential of Li, owing to the definition of and , as derivative of one above curves.

Figure 1. (a), (b) Different Gibbs free energy curves in the LiFePO₄/FePO₄ systems as function of Li content with the existence of low energy staging phase. Note that is the Gibbs energy per number FePO₄-units andis the ratio of number of Li and number of FePO₄. (c) The chemical potential of Li, owing to the definition of and , as derivative of one above curves.

Selected publications:

C. Zhu, L. Gu, L. Suo, J. Popovic, H. Li, Y. Ikuhara, and J. Maier Size-dependent staging and phase transition in LiFePO₄/FePO₄ Advanced Functional Materials, published online, 2013. DOI: 10.1002/adfm.201301792

L. Gu, C. Zhu, H. Li, Y. Yu, C. L. Li, S. Tsukimoto, J. Maier, and Y. Ikuhara Direct Observation of Lithium Staging in Partially Delithiated LiFePO₄ at Atomic Resolution Journal of the American Chemical Society 133 (13), 4661–4663 (2011). DOI: 10.1021/ja109412x

Size effects of LiFePO4

Size effects of LiFePO₄