First Principles Study of the Interfacial Structure Between Spinel LiMn2O4 and Protective Thin Films

Abstract

Spinel LiMn2O4 (LMO) is of considerable interest as a high voltage, low cost alternative to LiCoO2 cathodes for Li-ion batteries. A Mn3+ disproportionation reaction at the electrode surface, however, leads to Mn2+ dissolution into the electrolyte. Experimental work has determined that this process may lead to Mn2+ ion deposition on the counterelectrode, affecting full cell capacity retention.1 Deposition of thin oxide films on the electrode surface has demonstrated enhanced cyclability of LMO materials2, through mitigation of the Mn disproportionation and dissolution processes. Despite the efficacy of coated LMO cathodes, there is still much to be understood concerning the registry of thin films to the substrate. Electronic structure calculations may be used to further elucidate the mechanisms of thin film deposition on the surface, in order to gain atomistic insights regarding the performance of coated LMO cathodes. Density Functional Theory (DFT) calculations have been performed to evaluate the thermodynamic stability of LMO surface structures. The surface energies of low index off-stoichiometric LMO surface terminations are calculated within a constrained grand canonical ensemble, allowing for direct comparison of the thermodynamic stability relative to previously reported3–5 stoichiometric surface structures. This formalism provides a thermodynamic basis for the stable surface structures that are likely to be present for a given surface facet on LMO nanoparticles. We have further extended this approach to select high index LMO surface facets to evaluate the stability of potential model systems for steps, edges, or defects that may be present on LMO particles. Results are further compared with experimental X-ray reflectivity measurements on highly oriented LMO films. In order to better understand the nature of the interface between LMO and protective films, DFT calculations are used in direct comparison with atomic layer deposition (ALD) experiments to describe the mechanism by which ultrathin Al2O3 may register to the LMO substrate through alternating exposures of trimethylaluminum (TMA) and water. The stable surface terminations we have identified6 are used as model substrates to investigate the reaction of TMA with the LMO surface. DFT-calculated demethylation thermodynamics and kinetics suggest that TMA is likely to lose all of its methyl groups, which are coadsorbed on the surface through oxygen, in the first ALD half-cycle on both the (001) and (111) terraces of LMO. Moreover, TMA decomposition is calculated to be more exothermic on model stepped surfaces, suggesting defect passivation may preferentially occur under low TMA dosages. Mechanistic analyses of the foundational layers of protective film can provide important structural details toward an understanding of coated LMO performance. Such insights may facilitate the search for optimized film chemistries to suppress Mn dissolution from the LMO surface, while maintaining suitable Li ionic conductivity for enhanced performance.