The Role of Ultrathin Coatings on the Interfacial Reactions at LiMn2O4 Surfaces

The spinel lithium manganese oxide (LiMn₂O₄, or LMO) is an established cathode technology for lithium ion batteries that suffers from a well-known phenomenon whereby Mn dissolves from the surface through the disproportionation reaction of Mn(III), and the subsequent deposition of Mn(II) on the anode.  LMO can serve as a model system for exploring various mitigation strategies that will enrich our understanding of electrochemical corrosion at charged interfaces in general and may ultimately lead to better batteries.  Consequently, the Center for Electrical Energy Science: Tailored Interfaces  (CEES, a DOE-funded Energy Frontier Research Center), is exploring the influence of ultrathin coatings and surface treatments on the interfacial reactivity during lithiation reactions in LMO by combining advanced synthesis, in situ probes, and theory.  As part of this effort, we have applied  single-layer graphene coatings to LMO films, and discovered that these coatings improve capacity retention following electrochemical cycling. X-ray photoelectron spectroscopy (XPS) depth profiling, reveals that the graphene coating inhibits manganese depletion in the LMO film. Additionally, cross-sectional TEM demonstrates that a stable solid-electrolyte interphase layer is formed on the graphene coating, which screens the LMO from direct contact with the electrolyte, thereby inhibiting dissolution. Density functional theory (DFT) calculations have provided plausible mechanisms for this behavior.  In a different effort, we have examined inert metal layers to prevent Mn dissolution.  We found that 3-nm-thick Au shells prepared by electroless deposition could reduce Mn dissolution by up to 88%.  Similarly, LiAlO₂ thin films deposited on LiNi_0.5Mn_1.5O₄ surfaces by atomic layer deposition can improve the electrochemical stability.  The LiAlO₂ films are 100x more ionically conducting compared to Al₂O₃, and, consequently, provide a higher discharge capacity compared to Al₂O₃ and allow for higher rates.  Finally, we have explored the role of coherent coatings of a few nm-thick surface LiMn_2-xTi_xO₄, deposited on LMO. This approach protects the bulk LMO from acid corrosion but is also electrochemically active and, therefore, maintains the ion and charge transport channels on the surface. The surface-doped LMO has >2-fold improvement in electrochemical performance with respect to uncoated LMO in terms of cyclability and capacity at elevated temperature.  In this presentation I will survey these ongoing research efforts into understanding the role of ultrathin coatings on interfacial reactions at LMO surfaces.

This work was supported as part of the Center for Electrical Energy Storage: Tailored Interfaces, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.