Unveiling the Roles of Trace Fe and F Co-doped into High-Ni Li-Rich Layered Oxides in Performance Improvement

Abstract

High-Ni Li-rich layered oxides (HNLOs) derived from Li-rich Mn-based layered oxides (LRMLOs) can effectively mitigate the voltage decay of LRMLOs but normally suffered from decreased capacity and cycling stability. Herein, an effective, simple, and up-scalable co-doping strategy of trace Fe and F ions via a facile expanded graphite template-sacrificed approach was proposed for improving the performance of HNLOs. The trace Fe and F co-doping can far more effectively improve both its rate capability and cycling stability in a synergistic manner compared to the introduction of individual Fe cations and F anions. The co-doping of Fe and F increased the Li-O bonds by a magnitude far larger than the summation of the increments by their individual doping, quite favorable for the performance. The trace Fe doping can escalate the capacity and enhance the rate capability significantly by increasing the components of lower valence transition metals to activate their redox reactions more effectively and improving both the electronic and ionic conduction. In contrast, trace F can improve the cycling stability remarkably by lowering the O 2p band top to suppress the lattice oxygen escape effectively which were revealed by density functional theory calculations. The co-doped cathode exhibited excellent cycling stability with a superior capacity retention of 90% after 200 cycles at 1 C, much higher than 64% for the pristine sample. This study offers an idea for synergistically improving the performance of Li-rich layered oxides by co-doping trace Fe cations and F anions simultaneously, which play a complementary role in performance improvement.