Magnesium doping improved characteristics of high voltage cycle of layered cathode of sodium ion battery

Driven by global demand for new energy, Li-ion batteries (LIBs) have developed rapidly due to their competitive performance. Although LIBs show the advantages of high capacity and good cycling stability, their disadvantages such as uneven distribution of lithium resources are gradually exposed. Therefore, with abundant reserves, Na-ion batteries (NIB) have become one of the most promising solutions to make up for the deficiency of Li-ion battery. The NIBs layered oxide cathodes have the most potential applications of cathode material due to their high specific capacity (167 mAh·g^–1 in 2.4–4.3 V) and simple synthesis method. However, improving the cycling stability of layered cathode materials is one of the keys to their large-scale industrialization. To develop high capacity and cycling stability cathode materials, the Mg²⁺ is substituted for Ni²⁺ in NaNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂ (NCMT), thereby obtaining a NaNi_0.35Mg_0.05Cu_0.1Mn_0.4Ti_0.1O₂ (NCMT-Mg) cathode material. The NCMT-Mg has a high reversible specific capacity of 165 mAh·g^–1 in a voltage window of 2.4–4.3 V. The reversible specific capacity of about 110 mAh·g^–1 at 0.1 C after 350 cycles with a capacity retention of 67.3% is about 13% higher than the counterpart of NCMT. The irreversible reaction is suppressed from P'3 phase to X phase for NCMT. The ex-XRD spectrometers further prove that the NCMT-Mg shows a P3 and X mixed phase after being initially charged to 4.3 V, but the NCMT shows an X phase. The irreversible phase transition is suppressed to increase the cycling stability. The inactive Mg²⁺ replaces Ni²⁺, reducing the charge compensation and stabilizing the structure, the inactive Mg²⁺ can activate the charge compensation of Ni²⁺/Cu²⁺. The electrochemical activity increases from 77% to 86%. The high capacity and excellent cycling stability prove that the NCMT-Mg structure remains intact after various current rates have been tested. The long cycling stability mechanism is further systematically studied by using various technologies. The present work will provide an important reference for developing high-performance Na-ion cathode materials.