Numerous studies have shown that protective surface coatings are effective in reducing the well-documented problem of Mn ion dissolution from the surface of the spinel LiMn₂O₄ (LMO) Li-ion battery cathode.^[1] Although recent computational work has identified many promising coating chemistries based on bulk thermochemical properties,^[2,3] there remains much to be understood regarding the interface formed between protective coatings and the LMO electrode and how such interfacial properties may influence electrochemical performance. In this work, first principles density functional theory calculations are performed on models for both oxide and metallic coatings on the LMO surface.

First, we elucidate the mechanism for the onset of Al₂O₃ film growth by atomic layer deposition (ALD) and show that precursor decomposition in early ALD pulses leads to adsorption site blocking and sub-monolayer film growth.^[4] During ALD, the trimethylaluminum [Al(CH₃)₃] precursor loses its -CH₃ groups to the LMO surface, where the Al(CH₃)_x* and CH₃* surface adsorbates each behave as Lewis bases (electron donors), where near-surface Mn ions are the Lewis acidic adduct (electron acceptor). These studies are extended to various thermodynamically stable low- and high-index LMO surface models,^[5] demonstrating that ALD on LMO is structure-sensitive toward stepped sites, which are more electron accepting as suggested by calculated oxygen vacancy formation energies. Moreover, we find that the thermodynamics of decomposed intermediates are linearly related to the corresponding oxygen vacancy formation energy, or the relative Lewis acidity of near-surface Mn ions, of each given surface facet and termination. We propose that selective ALD leads to partial coatings formed at defect sites which may be particularly vulnerable to Mn dissolution.

Next, we model the process of delithiation near the surface of an LMO coated with metallic Au, which effectively mitigates Mn dissolution. Moreover, Au coatings have been shown to improve kinetics and do not strip under the potentials where LMO electrochemistry occurs.^[6] DFT calculations show that a Li⁺-deficient near-surface is thermodynamically favored for Au-coated LMO. We demonstrate through local density of states and chemical bonding analyses that Au hybridizes with LMO upon delithiation, leading to partial oxidation on Au electronic states rather than full localization of holes on Mn ions. The Au coating has a lower work function than LMO, driving electron transfer from Au to LMO. This is also associated with downward bending of the LMO conduction band in response to Fermi level pinning at the LMO/Au interface. Operando X-ray diffraction experiments show that these interfacial effects may translate to a reduction in the amount of Li⁺ that can intercalate into the bulk of Au-coated LMO.

These studies help elucidate the structure-property relationships of protective coatings at the LMO surface. In particular, this work highlights the driving forces for nucleation and growth of protective coatings by ALD, as well as the influence of interfacial electronic structure on intercalation chemistry of coated electrode materials.

This research is supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

References

[1] G. Xu, Z. Liu, C. Zhang, G. Cui, L. Chen, J. Mater. Chem. A 2015, 3, 4092.

[2] M. Aykol, S. Kirklin, C. Wolverton, Adv. Energy Mater. 2014, 4, 1400690.

[3] M. Aykol, S. Kim, V. I. Hegde, D. Snydacker, Z. Lu, S. Hao, S. Kirklin, D. Morgan, C. Wolverton, Nat. Commun. 2016, 7, 13779.

[4] L. Chen, R. E. Warburton, K.-S. Chen, J. A. Libera, C. Johnson, Z. Yang, M. C. Hersam, J. P. Greeley, J. W. Elam, Chem 2018, 4, 2418.

[5] R. E. Warburton, H. Iddir, L. A. Curtiss, J. Greeley, ACS Appl. Mater. Interfaces 2016, 8, 11108.

[6] J. L. Esbenshade, M. D. Fox, A. A. Gewirth, J. Electrochem. Soc. 2015, 162, A26.