Degradation mechanism of high-voltage single-crystal LiNi0.5Co0.2Mn0.3O2 cathode material

Abstract

Layered cathode materials have been successfully commercialized and applied to electric vehicles. To further improve improve the energy density of these marterials is still the main efforts in the market. Therefore, developing high-voltage LiNi x Co y Mn z O2 (x + y + z = 1, NCM) to achieve high energy density is particularly important. However, under high voltage cycling, NCM often exhibits rapid capacity degradation, which can be attributed to oxygen release, structural phase transition and particle cracking. In this work, the representative single-crystal LiNi0.5Co0.2Mn0.3O2 (NCM523) was studied under various high charge cut-off voltages. Analysis by x-ray diffraction (XRD), transmission electron microscope (TEM) and electron back scatter diffraction (EBSD) measurements indicated that the rock-salt phase is formed on the surface of the particles after high voltage cycling, which is responsible for the increase of impedance and the rapid decay of capacity. Therefore, inhibiting the formation of rock-salt phase is believed an effective strategy to address the failure of NCM under high voltages. These findings provide effective guidance for the development of high-voltage NCM.