Role of Cobalt Content in Improving the Low-Temperature Performance of Layered Lithium-Rich Cathode Materials for Lithium-Ion Batteries

Layered lithium-rich cathode material, Li_1.2Ni_0.2-xCo_2xMn_0.6-xO₂ (x = 0-0.05) was successfully synthesized using a sol-gel method, followed by heat treatment. The effects of trace amount of cobalt doping on the structure, morphology, and low-temperature (-20 °C) electrochemical properties of these materials are investigated systematically. X-ray diffraction (XRD) results confirm that the Co has been doped into the Ni/Mn sites in the transition-metal layers without destroying the pristine layered structure. The morphological observations reveal that there are no changes of morphology or particle size after Co doping. The electrochemical performance results indicate that the discharge capacities and operation voltages are drastically lowered along with the decreasing temperature, but their fading rate becomes slower when increasing the Co contents. At -20 °C, the initial discharge capacity of sample with x = 0 could retain only 22.1% (57.3/259.2 mAh g^-1) of that at 30 °C, while sample with x = 0.05 could maintain 39.4% (111.3/282.2 mAh g^-1). Activation energy analysis and electrochemical impedance spectroscopy (EIS) results reveal that such an enhancement of low-temperature discharge capacity is originated from the easier interface reduction reaction of Ni⁴⁺ or Co⁴⁺ after doping trace amounts of Co, which decreases the activation energy of the charge transfer process above 3.5 V during discharging.