LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) and related materials have been focused as alternative cathode materials of Li-ion rechargeable batteries (LIBs) to LiCoO₂due to its higher specific capacity and lower Co usage. For the view point of practical use, the increase of working voltage from 3.6 V to 4.6 V will be responsible to further enhancement of the energy density of the LIBs. However, the oxidative reaction at the interface between liquid electrolytes and electrodes at higher voltage of 4.3 V causes drastic capacity fade through the impedance growth and degradation of the NCM electrodes during the charge-discharge operation.

The surface functionalization of cathode with thin oxide layer of electrochemically-inactive materials has been proposed to suppress these unfavorable reactions. Indirect contact between the cathode and electrolytes plays a key role for the electrochemical stabilization, however these oxide layer possibly works as an insulating layer, leading to impedance growth. On the basis of these backgrounds, we propose herein the surface modification of NCM cathode with self-assembled monolayer of NbO_x nanosheet as well as fluorocarbon-containing organosilane molecule for stabilizing the electrode at high voltage. Through systematic studies, we found the coatings suppress effectively the oxidative decomposition of the electrolyte and disorder in the layered structures on the NCM surface, which result in impedance growth. After 100 cycles at voltage ranges 4.6 V–2.8 V more than 70% of the initial discharge capacity was retained in the coated electrodes. In comparison, the discharge capacity completely faded in the bare NCM cathode. Furthermore, according to Raman spectroscopy, the nanosheets stabilize the delithiated NCM lattice with rock-salt structure and inhibit transformation to the spinel structure.