Operando Observations and First-Principles Calculations of Reduced Lithium Insertion in Au-Coated LiMn2O4

The deposition of protective coatings on the spinel LiMn2O4 (LMO) lithium-ion battery cathode is effective in reducing Mn dissolution from the electrode surface. Although protective coatings positively affect LMO cycle life, much remains to be understood regarding the interface formed between these coatings and LMO. Using operando powder X-ray diffraction with Rietveld refinement, it is shown that, in comparison to bare LMO, the lattice parameter of a model Au-coated LMO cathode is significantly reduced upon relithiation. Less charge passes through Au-coated LMO in comparison to bare LMO, suggesting that the reduced lattice parameter is associated with decreased Li+ solubility in the Au-coated LMO. Density functional theory calculations show that a more Li+-deficient near-surface is thermodynamically favorable in the presence of the Au coating, which may further stabilize these cathodes through suppressing formation of the Jahn-Teller distorted Li2Mn2O4 phase at the surface. Electronic structure and chemical bonding analyses show enhanced hybridization between Au and LMO for delithiated surfaces leading to partial oxidation of Au upon delithiation. This study suggests that, in addition to transition metal dissolution from electrode surfaces, protective coating design must also balance potential energy effects induced by charge transfer at the electrode-coating interface.