Liu, Y.; Ning, D.; Zheng, L.; Zhang, Q.; Gu, L.; Gao, R.; Zhang, J.; Franz, A.; Schumacher, G.; Liu, X.: Improving the electrochemical performances of Li-rich Li1.20Ni0.13Co0.13Mn0.54O2 through a cooperative doping of Na+ and PO43- with Na3PO4. Journal of Power Sources 375 (2018), p. 1-10
10.1016/j.ipowsour.2017.11.042

Abstract:
Li-rich layered oxide cathodes suffer from poor rate capability, voltage decay and inferior cycling stability. Herein, we propose a novel synergistic strategy to improve the electrochemical performances of Li-rich Li1.20Ni0.13Co0.13Mn0.54O2 by the co-doping of Na+ and PO4 3−. The co-doping of Na+ for Li and PO4 3− for Mn is simultaneously achieved using Na3PO4 as a dopant. The co-doping of Na+ and PO4 3− not only enhances the high-rate performance (106.4 mAhg−1@10C) and capacity retention (93.8%@1C@100 cycles) but also mitigates the voltage decay owing to the synergistic effect of Na+ and PO4 3− co-doping. The synergistic mechanism is unraveled based on neutron diffraction, aberration-corrected scanning transmission electron microscope, Xray photoelectron spectroscopy, ex-situ X-ray absorption spectra, ex-situ X-ray diffraction, electrochemical impedance spectroscopy and electrochemical measurements. The co-doping of Na+ and PO4 3− enlarges the interlayer spacing and suppresses Li/Ni mixing which increases Li+ diffusivity and enhances the rate capability. Meanwhile, the co-doping of Na+ and PO4 3− shrinks the thickness of the slabs, weakens the TM-O covalency and alleviates the volume change in the charge/discharge process which improves the layered structure stability and the cycling performances. This study presents some new insights into designing high performance cathode materials through a cooperative modulation of different crystal sites doping.