Nonstoichiometry of Li-rich cathode material with improved cycling ability for lithium-ion batteries

Lithium-rich layered oxides are considered as promising cathode materials for lithium-ion batteries due to its high capacity, but the rapid decay of capacity and operating voltage are great challenges to achieve its commercial application. In this work, the nonstoichiometry of Li-rich layered oxide Li_1.2Mn_0.6Ni_0.2O₂ was designed by directly declining the Mn amounts in the form of Li_1.2Mn_xNi_0.2O₂ (x = 0.59, 0.57, 0.55). The nonstoichiometric sample Li_1.2Mn_0.55Ni_0.2O₂ exhibits a capacity of 170.73 mAh g^-1 at 0.5 C, a little lower than 187.29 mAh g^-1 of Li_1.2Mn_0.6Ni_0.2O₂, however, better cycling stability of operating voltage and capacity is attained with the reduction of Mn amounts, compared to that of Li_1.2Mn_0.6Ni_0.2O₂. The capacity retention of Li_1.2Mn_0.55Ni_0.2O₂ is enhanced to 88.7% via 74.7% of Li_1.2Mn_0.6Ni_0.2O₂ after 100 cycles at 0.5 C. The declining value of operating voltage for Li_1.2Mn_0.55Ni_0.2O₂ is 0.200 V as compared to 0.559 V for Li_1.2Mn_0.6Ni_0.2O₂. X-ray photoelectron spectra (XPS) was employed to confirm the existence of Ni³⁺ in the nonstoichiometric samples, and the amounts of Ni³⁺ increase along the Mn contents decrease. The improvement of electrochemical properties for nonstoichiometric samples is attributed to the presence of Ni³⁺ due to Ni³⁺ can defer the transition of layered-to-spinel structure through decreasing the Li/Ni mixing.