Electrophoretically Deposited p-Phenylene Diamine Reduced Graphene Oxide Ultrathin Film on LiNi0.5Mn1.5O4 Cathode to Improve the Cycle Performance.

Spinel LiNi0.5Mn1.5O4 (LNMO) has been considered as one of the most promising candidate cathode materials for power lithium-ion batteries. However, its cycle performance suffers from the increasing impedance of the LNMO-cathode/electrolyte interface (LNMO-CEI) layer caused by parasitic reactions on the electrode surface at high operating potentials. To address the capacity degradation upon cycling, we present a feasible way to realize electrode modification by electrophoretically deposited graphene ultrathin films on the exterior surface of the LNMO cathodes without decreasing the electrode density. A p-phenylene diamine reduced graphene oxide (pPD-rGO) film with an area density of 20 μg/cm2 not only increases the capacity retention rate of the 1000th cycle at 4.2-5.2 V from 71.7 to 81.7% but also boosts the specific capacity from 110.6 to 122.4 mAh/g. X-ray photoelectron spectroscopy (XPS) spectra reveal that the pPD-rGO film with Lewis-base nature increases the content of LiF and reduces the number of RCFx groups in the cycled electrode, indicating the consumption of high-potential-generated F radicals by the pPD-rGO film. Such consumption favors the formation of a robust interphase between the pPD-rGO film and the electrolyte, which could hinder the sustained production of F radicals, consequently stabilize the LNMO-CEI layer, and improve the cycle performance. An electrophoretically deposited Lewis-acid GO film of 20 μg/cm2 reduces the specific capacity and fails to work as the pPD-rGO film. The chemical process for the formation of interphase on the GO film is similar to that on the bare LNMO electrode.