Localized defective zone formation driven by selective Li+ extraction defines the high-voltage threshold of LiNi0.6Co0.2Mn0.2O2

Abstract

A structural investigation of an archetypal mid-Ni layered cathode, LiNi0.6Co0.2Mn0.2O2, charged to voltages between 4.3 V and 4.7 V, is performed to assess its structural stability at deep charge levels and determine its suitability for high-voltage cycling. Besides interparticle cracks, cracks within primary particles start to form above 4.5 V. The microstructure of primary particles is characterized by alternating bands of defect-free regions and areas containing numerous structural faults, likely caused by uneven Li ion extraction. Intraparticle cracks often originate at the boundary of these banded regions. Additionally, an unreported intermediate phase appears within the defective band. Electrochemical data confirm that 4.5 V (210 mAh g-1 at 0.1 C) is probably the limit at which LiNi0.6Co0.2Mn0.2O2 can be cycled without major capacity loss. This study reveals that the structural degradation of LiNi0.6Co0.2Mn0.2O2 during deep charging is highly localized due to the selective extraction of Li ions. Therefore, reducing the Li concentration difference at the cathode surface would prevent the formation of localized defective zones and enhance the cycling stability of LiNi0.6Co0.2Mn0.2O2 above 4.5 V.