The Effects of Trace Yb Doping on the Electrochemical Performance of Li-Rich Layered Oxides.

Layered lithium-rich cathode materials are one of the most promising cathode materials owing to their higher mass energy density than the commercial counterparts. A series of trace Yb-doped lithium-rich cathode materials Li1.2 Mn0.54 Ni0.13 Co0.13-x Ybx O2 (0≤x≤0.050) were synthesized and the effects were investigated by XRD, X-ray photoelectron spectroscopy, and high-resolution TEM. The participation of Yb ions in electrochemical reactions and the larger binding energy of Yb-O than M-O (M=Mn, Ni, Co), which expands the lithium layer spacing and stabilizes the oxygen stacking, resulted in excellent performance of materials doped with a limited Yb content (x≤0.005). However, higher doping amounts (x>0.005) significantly increased the charge-transfer impedance and led to a sharp deterioration in electrochemical performance. The reason lies in the large difference in ionic radius between the transition metals (Mn, Co, and Ni) and Yb. There is an upper limit to the amount of Yb ions in the lattice. If the amount of Yb is higher than the limit, excess Yb ions enter the Li layers instead of staying in the transition-metal layers or even segregate on the surface and form electrochemically inert oxides.