Restraining the polarization increase of Ni-rich and low-Co cathodes upon cycling by Al-doping
The Ni-rich and low-Co cathodes (LiNixCoyMn1-x-yO2, x > 0.9, y ≤ 0.03) have the advantages of high capacity and low cost. However, the sharp increase in polarization upon cycling is unfavorable to the cycling performance. Herein, the mechanism of polarization increase is explored in detail, and Al-doping is proposed to restrain the polarization increase upon cycling. Firstly, electrochemistry test, In-situ XRD, GITT and In-situ EIS are combined to provide new insights for understanding the polarization increase process. In addition, the mechanism of that Al-doping suppress the polarization increase is also investigted. During cycling, the main cause for the increased polarization of the Al-free LiNi0.94Co0.03Mn0.03O2 (NCM 94) cathodes is attribute to the kinetic reasons. It is worth noting that the change of Li dynamics and charge-transfer impedance is more obvious at high SOC, which is consistent with the evolution of polarization. In contrast, because of the stabilizing effect of Al-doping on the structure, the Al-doped LiNi0.92Co0.03Mn0.03Al0.02O2 (NCMA 92) cathodes can alleviate polarization increase during prolonged cycling. In detail, Al-doping transforms the two-phase reaction into a quasi-single-phase process at high SOC, which is conducive to improving the structural stability, maintaining the connection between primary particles, reducing the inner-surface degradation, and thus resulting in faster electrochemical kinetics than NCM 94. Benefiting from the alleviative polarization increase, the NCMA 92 cathodes demonstrate a capacity retention of 92% after 100 cycles, while the NCM 94 only 69%. In this article, the insights on the polarization increase process of Ni-rich and low-Co cathodes will further the understanding of its cycling behavior and contribute to the efforts of improving its electrochemical performance.