The role of yttrium content in improving electrochemical performance of layered lithium-rich cathode materials for Li-ion batteries

The promising layered lithium-rich cathode materials, Li _1.2Mn_0.6-xNi_0.2Y_xO₂ (0 ≤ x ≤ 0.05), have been synthesized by substituting Mn⁴⁺ in Li_1.2Mn_0.6Ni_0.2O₂ with unusually large Y³⁺ ions, in order to improve their cycling performance and rate capability. An oxalate co-precipitation method is adopted in the synthetic process. X-ray diffraction (XRD) patterns show that, other than as a dopant, the yttrium element is found to become Y₂O₃ or LiYO ₂ in excess Y³⁺-doped samples. The effects of yttrium content on the electrochemical properties of the lithium-rich materials are investigated by electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge tests as well. It demonstrates that the high capacity retention (240.7 mA h g^-1 after 40 cycles at 0.1 C rate) and superior rate capability (184.5 mA h g^-1 after 40 cycles at 1 C rate) have been achieved by the lithium-rich materials with a suitable amount of Y³⁺ doping. The "super-large" Y³⁺ can expand Li ⁺-diffusing channels in the layered structure and stabilize the material structure.