Grafting Nitrophenyl Groups on Carbon Surfaces by Diazonium Chemistry to Suppress Irreversible Reactions in High-Voltage LiNi_0.5Mn_1.5O₄Positive 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 LiNi 0.5 Mn 1.5 O 4 (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, CO 2 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.

Understanding the Conductive Carbon Additive on Electrode/Electrolyte Interface Formation in Lithium-Ion Batteries via in situ Scanning Electrochemical Microscopy

“…In addition, the redox peaks at 4.56/4.10 V have obvious larger anodic peak than the cathodic peak, demonstrating the existence of an irreversible oxidation process. The anodic peak can be induced by anion intercalation, oxidative decomposition of electrolyte and conductive carbon oxidation (itself or its functional groups) (Qi et al, 2014(Qi et al, , 2015Soon et al, 2018). With these three contributions, the reversible part could be the (de)intercalation of PF − 6 anions to/from the electrodes (Qi et al, 2014).…”

“…In comparison, the electrode cycled in the high-potential region has fewer inorganic species, which can be told from the lower amount of Li1s, F1s, and much higher C1s element concentration (Table 1). In addition, the much higher ratio between C-C bonds and carbon oxygen bonds demonstrates that the electrode surface has less oxygen-containing species and less electrolyte decomposition (Soon et al, 2018;Tatara et al, 2019).…”