Spinel LiMn₂O₄ (LMO) is an inexpensive, non-toxic cathode material for lithium ion batteries. Its high reduction potential (~4.1 V vs. Li/Li⁺) and excellent rate capability have made LMO competitive with the industry standard cathode material, LiCoO₂. Despite its advantages, however, reaction with HF in the electrolyte at temperatures greater than 55°C leads to disproportionation of Mn³⁺ cations at the electrode surface. This process may be followed by dissolution of Mn²⁺ into the electrolyte and subsequent deposition on the counterelectrode, which in turn affects capacity retention of the cell.¹ Ultrathin protective films have been shown to suppress Mn dissolution from LMO, thus exhibiting superior capacity retention with respect to the uncoated electrodes.² There is little known, however, regarding the mechanisms for the formation of these films or of the substrate/film interfacial structure at the atomic level. Electronic structure calculations can assist in elucidating important structural features of these interfaces, ultimately facilitating the screening of optimized film chemistries to achieve peak performance.

In this presentation, we describe a series of Density Functional Theory (DFT) calculations to rigorously describe thermodynamically the structure of various LMO surface terminations. Although certain low-index LMO surface structures has been investigated previously,^3–5 additional low energy surface structures have been identified through the application of a grand canonical thermodynamic formalism to describe the stability of off-stoichiometric surfaces. Moreover, several stable high-index surface structures have also been identified, which may be representative of edges or defects on LMO particles.

The stable surface structures identified for LMO are next used as model substrates to understand the formation of ultrathin films via atomic layer deposition (ALD).  We describe, in particular, the formation of Al₂O₃films by ALD through alternating trimethylalumina (TMA) and water half-cycle exposures. DFT calculations are used to determine the thermodynamics and kinetics of TMA decomposition on the (001) and (111) surfaces of LMO. Based on calculated demethylation thermodynamics and kinetics, we demonstrate that TMA is likely to lose all methyl groups in the first half-cycle, which are coadsorbed through oxygen on the LMO surface.  We further describe how model defects impact the initial ALD deposition mechanisms, and we thereby draw general conclusions concerning the growth of ultrathin alumina films on LMO nanoparticles. More generally, determining the structure of the first monolayer of these protective films is an important step towards gaining a detailed understanding of the enhanced performance of coated electrodes. Such atomistic insights can assist in providing predictions for optimal film chemistries to suppress transition metal dissolution from the cathode surface.

(1)       Zhan, C.; Lu, J.; Jeremy Kropf, A.; Wu, T.; Jansen, A. N.; Sun, Y.-K.; Qiu, X.; Amine, K. Nat. Commun. 2013, 4, 2437.

(2)       Park, J. S.; Meng, X.; Elam, J. W.; Hao, S.; Wolverton, C.; Kim, C.; Cabana, J. Chem. Mater. 2014, 26(10), 3128–3134.

(3)       Benedek, R.; Thackeray, M. M. Phys. Rev. B 2011, 83(19), 195439.

(4)       Karim, A.; Fosse, S.; Persson, K. A. Phys. Rev. B 2013, 87(7), 075322.

(5)       Kim, S.; Aykol, M.; Wolverton, C. Phys. Rev. B 2015, 92 (11), 115411.