Modification Of P2-Na_0.67Mn_0.62Ni_0.38O₂ Cathode Material For Sodium-Ion Batteries

With the increasing exhaust of lithium resources and uneven distribution over the world,the price of lithium-ion batteries（LIBs）is increasing recently.Sodium-ion batteries（SIBs）are intensively studied because of low cost and Na enrich resources.Among many cathode materials for sodium-ion batteries,P2-type layered manganese-based cathode materials possess the characteristics of short Na⁺transport path,low diffusion energy barrier,high energy density,low cost,and environmental protection.However,they suffer from an issue of irreversible phase transformation during the charge and discharge processes.Especially,at a high charge voltage,the primary phase transformation of volume change（P2-O2 phase transformation）leads to poor cycling reversibility.In this paper,Ti doping,Nb doping and adjusting the cut-off charge voltage were utilized to suppress the P2-O2 phase transition at 4.2 V（vs.Na⁺/Na）.Moreover,through co-doping of Li and Nb,the interlayer spacing of sodium layer was maintained,and the average valence state of manganese was enhanced.As a result,the Jahn-Teller effect of trivalent manganese was weakened,and the dissolution of divalent manganese in the electrolyte was inhibited.The research contents are listed as follows:（1）P2-type layered Na_0.67Mn_0.62Ni_0.38-xTi_xO₂materials were prepared by a solid-state method.The as-prepared samples were characterized by scanning electron microscope（SEM）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）technologies.Their electrochemical performance was evaluated by cyclic voltammetry（CV）,galvanostatic charge-discharge（GCD）and galvanostatic intermittent titration technique（GITT）.The effect of Ti-doping amount on the electrochemical properties of the cathode materials were investigated.XRD results indicate that Ti-doping enlarges the interlayer spacing of sodium layer,which would enhance the initial discharge capacity and sodium diffusion coefficient.The P2-O2phase transition is effectively suppressed under high potentials,and the crystalline structure is stabilized,making sodium intercalation process more reversible.When the molar amount of Ti doped is 0.10,the resultant sample displays the best electrochemical performance.It can deliver an initial specific capacity of 151 m Ah g^-1at 20 m A g^-1over the potential range of 1.5-4.5 V（vs.Na⁺/Na）,and retain 92 m Ah g^-1after 100 cycles,accompanying with a capacity retention of 67.6%.At the current density of 500 m A g^-1,the initial discharge capacity remains 58 m Ah g^-1.The discharge capacity can maintain 79.1%of the initial one when the current density recovers to 20 m A g^-1,exhibiting a good rate capability.The results suggest that suitable doping of Ti can improve the electrochemical performance of the Na_0.67Mn_0.62Ni_0.38O₂cathode material for SIBs.（2）The P2-type Na_0.67Mn_0.62Ni_0.38-xNb_xO₂cathode materials were prepared by a solid-state method.The as-prepared samples were characterized by SEM,TEM,XRD and XPS technologies.Their electrochemical performance was evaluated by CV,GCD and electrochemical impedance spectroscopy（EIS）.The results indicate that the Nb-doping increases the interlayer spacing of sodium layer,which increases the first charge-discharge capacity.At the same time,doping of Nb effectively inhibits the phase transition of P2-O2.Therefore,the sample shows an improved cycling reversibility even charged to a high voltage.The effect of doping amount of Nb on the electrochemical performance was investigated in detail.When the molar amount of doped Nb is 0.08,the resultant sample presents the best electrochemical performance.It can deliver an initial capacity of 163 m Ah g^-1at the current density of 20 m A g^-1over the voltage range of 1.5-4.5 V,and the capacity retention is 62.9%after 100cycles.At the current density of 500 m A g^-1,it can deliver a reversible capacity of 56m Ah g^-1.The results indicate that suitable doping of Nb can improve the electrochemical performance of the Na_0.67Mn_0.62Ni_0.38O₂cathode material for SIBs.（3）In order to further improve the cyclic reversibility of Nb-doped samples,by adjusting the charging cut-off voltage and avoiding the P2-O2 phase transition in the high potential region,the reversibility of the sodium ion deintercalation process is effectively improved.At this time,the influence of cyclic reversibility is mainly due to the poor redox reversibility of Mn³⁺/Mn⁴⁺.The lithium/niobium co-doped P2 type Na_0.67Mn_0.62Ni_0.18Li_0.15-xNb_xO₂cathode material is prepared by solid-state method.When the molar ratios of doped Li and Nb are 0.10:0.05,the resultant sample exhibits the best electrochemical performance.The sample can deliver a high initial discharge capacity.The Jahn-Teller effect of Mn³⁺and the dissolution of Mn²⁺in the electrolyte are effectively weakened,which improves the rate capability and cycling performance distinctly.The specific discharge capacity can reach 93 m Ah g^-1at the current density of 500 m A g^-1over the voltage range of 1.5-4.0 V.When the current returns to 20 m A g^-1,the capacity retention rate is 85.6%.At the current density of 100 m A g^-1,the capacity retention rate is 81.6%after 100 cycles.The results demonstrate that the electrochemical performance of Na_0.67Mn_0.62Ni_0.38O₂can be improved significantly by the synergistic effect of suitable co-doping of Li/Nb and regulating of charge terminate voltage when used as cathode material of SIBs.