Effects of Fluorine and Chromium Doping on the Performance of Lithium-Rich Li1+XMO2 (M = Ni, Mn, Co) Positive Electrodes

Lithium-rich metal oxides Li1+zMO2 (M = Ni, Co Mn, etc.) are promising positive electrode materials for high-energy lithium-ion batteries, with capacities of 250-300 mAh.g(-1) that closely approach theoretical intercalation limits. Unfortunately, these materials suffer severe capacity fade on cycling, among other performance issues. While ion substitution can improve the performance of many of these materials, the underlying mechanisms of property modification are not completely understood. In this work we show enhanced performance of the Li1+zMO2 electrode, consisting of Li2MnO3 (with C2/m space group) and LiMO2 (with R (3) over barm space group) phases, and establish the effects of cationic and anionic substitution on the phase and structure evolution underpinning performance changes. While the undoped material has a high capacity of similar to 270 mAh.g(-1), only 79% of this remains after 200 cycles. Including similar to 2% Cr in the material, likely at the R (3) over barm metal (3a) site, improved cycle performance by similar to 13%, and including similar to 5% F in the material, likely at the R (3) over barm oxygen (6c) site, enhanced capacity by similar to 4-5% at the expense of a similar to 12% decline in cycle performance. Moreover, Cr doping enhances energy density retention by similar to 13%, and F doping suppresses this by 17%. We find that these changes arise by different mechanisms. Both anionic and cationic substitution promote faster Li diffusion, by 48% and 20%, respectively, as determined using cyclic voltammetry and leading to better rate performance. Unlike anionic substitution, cationic substitution enhances structural stability at the expense of some capacity, by suppressing lattice distortion during Li insertion and extraction. This work implicates strategic cationic-anionic codoping for enhanced electrochemical performance in lithium-rich layered metal-oxide phases.