Magnesium-doped improved cycling to high voltages of layered cathode of sodium ion battery

Driven by global new energy demand, Li-ion batteries (LIBs) have been developed rapidly owing to its competitive performance. Although LIBs show the advantages of high capacity and good cycling stability, their disadvantages such as uneven distribution of lithium resources is 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. NIBs layered oxide cathodes are the most potential for practical 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 replacement of Mg²⁺ for Ni²⁺ was used for NaNi_0.4Cu_0.1Mn_0.4Ti_0.1O₂(NCMT) and obtained the 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 the 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 NCMT. The irreversible reaction was suppression from P'3 phase to X phase for NCMT. The ex-XRD spectrometers future prove that the NCMT-Mg shows a P'3 and X mixed phase after initial charge to 4.3 V, but the NCMT shows a X phase. The irreversible phase transition is suppressed to increase the cycling stability. The inactive Mg²⁺ replaces Ni²⁺ reducing the charge compensation and stabling the structure, the inactive Mg²⁺can activate the charge compensation of Ni²⁺/Cu²⁺. The electrochemical active was activated from 77% to 86%. The high capacity and excellent cycling stability prove that the NCMT-Mg structure maintains intact after various current rates test. The long cycling stability mechanism was further systematically studied by various technologies. The work will provide an important reference for developing high-performance Na-ion cathode materials.