Phase Structure Regulation And Performance Optimization Of P2-type Layered Mangan-based Cathode Material For Sodium-ion Battery

Lithium-ion batteries have been applied commercially on large scale because of their high energy density,excellent cycle performance and easy preparation.Especially with the emergence of portable electronic devices,the demand for lithium batteries is increasing,but the maldistribution of lithium with limited reserve in the earth’s crust greatly restricts the future application of LIBs as large-scale commercial energy storage devices.Therefore,it is urgent to develop a new energy storage system as a substitute for lithium-ion batteries.Sodium comes into people’s eyes because of its similar physical and chemical properties to lithium.Because of abundant sodium reserves,uniform distribution,low price,and the working principle of sodium-ion battery is the same as that of lithium-ion battery,sodium-ion battery has gradually become a research hotspot.The cathode material of sodium ion battery determines the energy density and reversible capacity of the battery,so developing the cathode material of sodium ion battery is the key to promote its application.The layered Mn-based cathode material has become a research hotspot with high specific capacity and simple preparation technology.The P2 phase Mn-based material（P2-Na_xMn O₂）has an open Na⁺channel and fascinating rate performance.However,P2-Na_xMn O₂ material has John-Teller effect,adverse phase transition P2-OP4/O2,and abnormally high initial coulomb efficiency（ICE）,which limits its practical application.Based on the above problems,P2-Na_xMn O₂ was modified by phase structure regulation and ion doping in this paper.1.The Ni/Cu co-doped P2/O3-Na_0.75Mn_1-yNi_y-zCu_zO₂ material was designed,John-Teller effect and P2-O2 adverse phase transition were inhibited while regulating the phase structure.Ni doping can adjust the ratio of P2:O3 to regulate the ICE.Cu doping can enlarge the c parameter of O3 phase,while the irreversible P2-O2 phase transition is inhibited,and the structural stability is improved.The optimal P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ with 92 wt%O3 phase shows a capacity of 154.8 m Ah g^-1 with ICE of 112.1%at 20 m A g^-1,superior rate performance(95.9 m Ah g^-1 at 1.2 A g^-1),and good cycling performance in the wide temperature range of-40～30℃.Furthermore,the phase evolution and electrochemical kinetics for P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ is investigated by in（ex）situ measurements,electrochemical tests,and density functional theory（DFT）calculations.This work is expected to provide meaningful guidance to develop novel P2/O3-Na_xMn_1-yM_yO₂ cathode for high-performance SIBs.Additionally,the Na-ion full battery based on P2/O3-Na_0.75Mn_0.6Ni_0.3Cu_0.1O₂ cathode and hard carbon anode can achieve a remarkable energy density of 306.9 Wh kg^-1 with a power density of 695.5 W kg^-1 at 200 m A g^-1 in the range of 1.4-4.1 V.2.P2-Na_0.67Li_0.24Mn_0.76O₂ was designed,and high capacity cathode material was obtained by introducing high voltage oxygen anion redox reaction.The phase structure and morphology of P2-Na_0.67Li_0.24Mn_0.76O₂ was characterized by XRD,SEM and TEM.The material can provide 240 m A g^-1 discharge capacity at the current density of 20 m A g^-1 in the range of 1.5-4.5 V,respectively.But the capacity decay is fast(at 200 m A g^-1 current density,the retention rate of 200 cycles is 40.8%),the electrolyte was further modified,Li PO₂F₂ film-forming additive was added,and the capacity retention rate was 100%after200 cycles at the current density of 200 m A g^-1.SEM test of the material after cycling proved that Li PO₂F₂ additive could prevent the formation and expansion of surface microcracks,EIS test of different cycles proved that Li PO₂F₂ additive could effectively reduce charge transfer resistance,and kinetic test proved that Li PO₂F₂ additive can not affect the kinetic properties of the material.When the full battery is assembled,it can provide 130 m Ah g^-1 discharge specific capacity at 1.4-4.4 V.