Grafting Nitrophenyl Groups on Carbon Surfaces by Diazonium Chemistry to Suppress Irreversible Reactions in High-Voltage LiNi0.5Mn1.5O4 Positive Electrodes

Abstract

Nitrophenyl groups are grafted onto the surface of a conductive carbon using diazonium chemistry. The as-modified carbon is loaded as a conducting agent in a spinel-structured LiNi0.5Mn1.5O4 (LNMO) composite positive electrode to examine the beneficial effects given by the surface-grafted nitrophenyl groups. The Li/LNMO cell fabricated with unmodified carbon exhibits poor Coulombic efficiency due to parasitic reactions, such as the oxidative decomposition of the electrolyte and oxidation of the conductive carbon, prevailing at the working voltage of LNMO (4.6–4.9 V vs. Li/Li+). These reactions appear to occur mainly on the carbon surface on account of its ∼20 times larger area than that of LNMO. However, use of the nitrophenyl-modified carbon greatly improves the Coulombic efficiency of the Li/LNMO cell, indicating suppression of such parasitic reactions. Differential electrochemical mass spectrometry reveals that nitrophenyl grafting effectively suppresses carbon oxidation at high voltage. Although suppression of electrolyte oxidation is only moderate with nitrophenyl-modified carbon, CO2 generation by carbon oxidation is greatly suppressed. The latter feature can be attributed to the electron withdrawing ability of the nitro groups, which imparts the conductive carbon with high stability against electrochemical oxidation.