(83f) Atomic Layer Deposition of Protective Coatings on LiMn2O4 Cathodes

Spinel LiMn₂O₄ (LMO) is a low cost, non-toxic, and thermally stable lithium ion battery electrode with reasonable energy and power densities. Despite these advantages, however, its cycling performance is limited, due in large part to Mn dissolution from the electrode surface.¹ Mn dissolution is proposed to proceed following charge disproportionation between two Mn³⁺ cations, resulting in Mn⁴⁺ (insoluble) and Mn²⁺ (soluble) ion formation.² In turn, protective surface coatings have emerged as a promising strategy to mitigate Mn dissolution. Coatings have been proposed to (1) increase the concentration of surface Mn⁴⁺ ions to inhibit Mn³⁺ disproportionation, and (2) provide a physical barrier between Mn ions and the electrolyte.³ Despite the enhanced performance of coated electrodes, there remains a lack of fundamental understanding regarding the registry of protective films to the electrode surface, as well as the role of the electrode/film interfacial structure towards electrochemical performance.

In this work, first principles density functional theory (DFT) calculations are used to evaluate the formation of Al₂O₃ coatings by atomic layer deposition (ALD), a self-limiting vacuum technique for thin film growth. Al₂O₃ film growth is considered through the prototypical reaction sequence of alternating trimethylaluminum (TMA) and H₂O half-reactions. DFT calculations, along with in situ experiments (FTIR, QMS, QCM), demonstrate that the TMA precursor undergoes significant decomposition in early ALD cycles, leading to site blocking sub-monolayer coverages of Al₂O₃. Through electrochemical cycling, LMO electrodes with sub-monolayer coatings show enhanced capacity in comparison to fully conformal films formed upon further ALD. Through DFT studies of reaction pathways on thermodynamically stable⁴ (001), (111) terraces, in addition to (511) step models, we outline the structure sensitivity of film growth during the early stages of ALD. The findings show that film nucleation is most facile at the more reducible surface facets, that given their near-surface electronic structure, are also most likely to undergo disproportionation and dissolution. These trends help provide an explanation for enhanced cycling performance with only 1-2 ALD cycles, wherein defects on the LMO surface may be selectively passivated. We will address how the trends outlined for the TMA/H₂O system can be extended to control the onset of film growth based on precursor selection.

This research was 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

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