Pristine Surface Investigation of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ towards Improving Capacity and Rate-capability for Lithium-ion Batteries

Understanding the nature of pristine surface associated with lithium- and manganese-rich cathodes is the key to optimizing the electrochemical performance for Li-ion batteries. In this work, Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathodes with phase and element homogeneity were synthesized by a facile hydroxide co-precipitation strategy. Two routes of adjusting lithium content and post-treatment were applied to tune the surface structure of pristine samples. It is found that the sample with 5 wt%-excess lithium and after post-treatment exhibits excellent capacity and rate-capability: delivering initial discharge capacities of 254 mAh/g at current densities of 100 mA/g, 200 mAh/g at 1000 mA/g, and 142 mAh/g at 4000 mA/g. The capacity of this cathode at 4 A/g thereby remains 55 percentage of the discharge capacity at 100 mA/g. The reason for the better performance of this cathode was attributed to the removal of surficial amorphous Li₂CO₃ layer, as revealed by the detailed analyses of transition electron micrograph and X-ray photoelectron spectrum. The amorphous layer was demonstrated by electrochemical impedance to show a block effect on increasing the interfacial resistance. This work may shed light on the importance of pristine surface and provide a hint for searching cathodes of high-energy Li-ions batteries with excellent capacity and rate properties.