Molecular gate effects observed in fluoroalkylsilane self-assembled monolayers grafted on LiNi0.5Mn1.5O4 cathodes: an application to efficient ion-exchange reactions

Abstract

We demonstrated the role of fluoroalkylsilane (FAS) self-assembled monolayers in improving the high-voltage durability and C-rate capabilities of spinel LiNi_0.5Mn_1.5O₄ (LNMO) cathodes. The influence of the cathode–electrolyte interface (CEI) layer formation was assessed and compared with various linear carbonates and fluoroethylenecarbon (FEC) additives using electrochemical impedance and X-ray photoemission spectroscopies. The densely grafted FAS monolayer provided a less resistive CEI with a nearly three times lower charge transfer resistance owing to the formation of lower amounts of Li₂CO₃ and LiPF₆ salt degradation byproducts and the significant reduction of Mn dissolution by almost a third in full cells after cycling. Notably, high voltage stabilities dependent on non-cyclic carbonates were proportional to the depth of their HOMO-level positions. However, the addition of FEC drastically degraded the capacity retention in the FAS-grafted LNMO cathode systems during the cycles with the a thick CEI layer formation. This suggests that it was because of the enhancement of affinity, driven by the strong intermolecular interaction among fluorine-containing compounds. Furthermore, we found that the Li⁺ diffusion coefficient was larger than that for bare LNMO electrodes, which suggested significant changes in the kinetics of the Li⁺ exchange reaction (Li⁺ adsorption and diffusion processes) on the electrode/electrolyte associated with polarization mitigation. We believe that the grafted FAS layer plays a role as a molecular gate, and while blocking the diffusion of uncoordinated free-carbonate solvents to the electrode surface, it tends to preferentially permeate both solvated Li⁺ and counter PF₆⁻, and partially permeate fluorine-substituted additives, which nearly eliminate the oxidation decomposition of free-carbonate solvents and promote an efficient Li⁺-exchange reaction.