Tuesday, 31 May 2016
Exhibit Hall H (San Diego Convention Center)
Here, we use the density functional theory (DFT) calculations to investigate the (001) and (111) surface structure of the LiMn2O4 (LMO) spinel cathode materials.1 We first test the effect of slab termination, relaxation, and reconstruction, as well as electronic and magnetic ordering for Mn to examine previous discrepancies on the calculated DFT surface energy for LMO in the literature. We reveal that the relative stability of (001) and (111) LMO surfaces significantly vary depending on the relaxation schemes and surface reconstruction. While our initial DFT calculations testing various numerical and physical factors represent a “closed” thermodynamic system at 0 K, in actuality, the system is open to exchange of materials to react with O2 molecule or Li+ ions. Therefore, we further investigate the surface stability of LMO as a function of the oxygen and lithium chemical potentials. Our calculations show that the (001) and (111) DFT surface energies are very close, suggesting the spectrum of cube-like to octahedron-like particles shapes can be observed under different experimental synthesis condition. We predict that higher temperature and high lithium chemical potentials should be avoided to mitigate the Mn dissolution in LMO, where the synthesized LMO particles are likely to contain more (001) surfaces that are more prone to Mn dissolution. In contrast, the (111) surfaces that are composed of Li-rich surface layers could be more resistant to the Mn dissolution.
Reference: [1] S. Kim, M. Aykol, and C. Wolverton, Phys. Rev. B, 2015, 92, 115411.
Acknowledgements: This work was supported by Northwestern-Argonne Institute of Science and Engineering (NAISE).
Figure Caption: Different (001) to (111) surface energy ratio and the corresponding representative LMO particles shapes (as a function of lithium and oxygen chemical potential).