Voltage-dependent formation of cathode–electrolyte interphase with independent metallic layer in LiNi0.8Mn0.1Co0.1O2 cathode for high-energy density lithium-ion batteries

Abstract

Layered LiNi0.8Mn0.1Co0.1O2 (NMC811) has emerged as the promising cathode material for next generation lithium-ion batteries owing to its high capacity. The double redox reaction between Ni2+/Ni3+ and Ni3+/Ni4+ is the major source of the large capacity, but it often leads to the formation of an unstable cathode–electrolyte interphase (CEI) layer. Herein, the correct nature of the CEI layer for the NMC811 battery has been studied with a particular attention to the voltage dependency (4.3 V vs. 4.7 V). A CEI metallic layer consisting of transition metal (TM) composites (i.e. 54.9MnOx, 54.9MnF3, 58.7NiO, 58.7NiF3, 58.9CoOx, and 58.9CoF3) has been long considered as a part of inorganic layer. However, the present study found that the metallic layer can be present as an independent CEI layer depending on the cut-off voltages. To further understand the nature of the CEI formation with the cathode material, a density-functional theory was conducted. The Fermi level with respect to the redox couples of the TMs/oxygen orbitals and their covalency not only affect the cathode stability but also modulate the CEI property. It is also proposed that the covalency of TM(3d)-O(2p) is the key factor that determines the localization of the TM metallic layer.