Transition metal dissolution mechanisms and impacts on electronic conductivity in composite LiNi0.5Mn1.5O4 cathode films

The high-voltage LiNi0.5Mn1.5O4 (LNMO) spinel cathode material offers high energy density storage capabilities without the use of costly Co that is prevalent in other Li ion battery chemistries (e.g., LiNixMnyCozO2 (NMC)). Unfortunately, LNMO-containing batteries suffer from poor cycling performance due the intrinsically coupled processes of electrolyte oxidation and transition metal dissolution that occurs at high voltage. In this work, we use operando electron paramagnetic resonance (EPR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy to demonstrate that transition metal dissolution in LNMO is tightly coupled to HF formation (and thus, electrolyte oxidation reactions as detected with operando and in situ solution NMR), indicative of an acid-driven disproportionation reaction that occurs during delithiation (i.e., battery charging). X-ray photoelectron emission microscopy (XPEEM) provided surface sensitive and localized X-ray absorption spectroscopy (XAS) measurements that indicate that disproportionation is enabled by surface reconstruction that occurs upon charging that leads to surface Mn3+ sites on the LNMO particle surface that can disproportionate into Mn2+(dissolved) and Mn4+(s). During discharge of the battery, we observe high quantities of metal fluorides (particularly rich in MnF2) deposit in the CEI layer on the LNMO particles as well as the conductive carbon additives on the composite surface. Electronic conductivity measurements indicate that the MnF2 decreases film conductivity by 3-fold compared to LiF, suggesting that this CEI may impede both the ionic and electronic properties of the cathode. Our data indicate that in order to prevent transition metal dissolution and the associated side reactions in spinel-type cathodes (particularly those that operate at high voltage like LNMO), the use of electrolytes that offer improved anodic stability and prevent acid byproducts will be necessary.