LiCoO2 (LCO) is the dominant cathode for 3C-type lithium-ion batteries. However, it suffers from severe structural and interfacial deterioration above 4.55 V. Herein, a gradient entropy (GE) surface architecture approach is proposed to stabilize LCO to an ultrahigh cutoff voltage of 4.7 V. This customized architecture is mediated by a homogeneous self-encapsulation layer via the exceptional chelating capability of phytic acid with multiple metal ions (Mg/Al/Ni) on the LCO surface. Upon calcination, a distinctive GE-LCO features a gradient surface architecture with entropy gradually decreasing from exterior to interior, which facilitates the high retention of bulk electrochemical activity while leveraging high-entropy effects to stabilize the surface. Specifically, the higher-entropy surface lowers the Gibbs free energy, producing a thermodynamically stable outer surface. Kinetically, P-Mg-Ni dopants expand the Li channels that enhance Li+ mobility. The cocktail effect substantially stabilizes the surface oxygen by modulating Co 3d-O 2p hybridization. Moreover, the gradient-entropy layer raises the kinetic barrier, thus significantly inhibiting interfacial Co migration. Comprehensive analysis reveals that these coupled entropy-driven mechanisms improve the electrochemical-mechanical stability of the outmost surface, meanwhile preserving its highly electrochemically active bulk. Consequently, GE-LCO exhibits a high capacity of 230.9 mAh/g (0.1C) and superior capacity retention of 80.6% at 4.7 V after 100 cycles.
Zhang et al. (Tue,) studied this question.