Enhancing Electrochemical Properties Of Layered Fe/Mn-Based Cathode Materials By Dual-Cationic Doping For Sodium-Ion Batteries

The over depletion of non-renewable energy and growing concerns about climate warming have triggered intermittent renewable clean energy to be an inevitable choice to promote energy transformation.The rapid development of lithium-ion batteries（LIBs）has totally changed our daily life.However,the limited lithium resource and its climbing price restrict the large-scale energy storage of LIBs for clean energy.Sodium-ion batteries（SIBs）have been considered as one of the most promising technologies for large-scale energy storage owing to the advantage of sodium resources which are rich,wide-range and low-cost.It is well-known that cathode materials dominate the overall performance and cost of sodium-ion batteries.Layered Fe/Mn-based cathode materials have attracted much attention because of its high theoretical specific capacity and non-toxic abundant resource,but they have been still facing the challenges,such as undesirable detrimental phase transition caused by large radius of Na⁺,fast electrochemical performance degradation,material instability and sluggish sodium ion diffusion.To alleviate the above problems,the bulk structure of P2-Na_0.7Fe_0.4Mn_0.6O₂ has been rationally designed and improved by dual-cationic doping,two new cathode materials(P2/O3-Na_0.7Li_0.1Ni_0.1Fe_0.2Mn_0.6O₂ and P2-Na_0.7Cu_0.2Fe_0.2Mn_0.5Ti_0.1O₂)were obtained,with the greatly boosted electrochemical performances for SIBs.The two main contents of the thesis are summarized below.（1）Li⁺/Ni²⁺co-doping induced P2/O3-Na_0.7Li_0.1Ni_0.1Fe_0.2Mn_0.6O₂ with excellent electrochemical performanceTo solve the problems of P2-Na_0.7Fe_0.4Mn_0.6O₂（FM）such as unfavorable high-voltage phase transition,sluggish sodium-ion transport kinetics and poor air stability,the dual strategy of Li⁺/Ni²⁺co-doping and intergrown phase construction was performed.An air-stable P2/O3-Na_0.7Li_0.1Ni_0.1Fe_0.2Mn_0.6O₂（LN）was prepared by conventional solid-state method,in which Li⁺stabilizes the structure,Ni²⁺with high ionic conductivity improves the rate performance,and the intergrown O3 phase induced by Li⁺and Ni²⁺further limits slab gliding during the charging and discharging process.Results of XRD Rietveld refinement and HRTEM visualization confirm the existence of P2/O3intergrown structure.The LN cathode maintains a high capacity retention of89.2%in the voltage range of 2.5-4.3 V after 100 cycles at 1C,which is significantly higher than that of FM（34.6%）.At the high current density of 10C,LN delivers 57.7%of the initial capacity,and a capacity retention of up to 74.6%after 500 cycles.Characterization by in-situ XRD,GITT and DSCV reveals the reversible phase transition of P2/O3-P2/P3-P2/O3 and rapid sodium ion transfer kinetics during the charge-discharge process of LN,which guarantee the long-term cycle life and excellent rate performance.In addition,the crystal structure of LN has no change upon 7 days of air exposure,demonstrating its great air stability.（2）Cu²⁺/Ti⁴⁺co-doped,low-cost P2-Na_0.7Cu_0.2Fe_0.2Mn_0.5Ti_0.1O₂ with rapid and stable sodium-ion storageGenerally,the reported cationic doping strategies generally greatly increase the cost of layered Fe/Mn-based cathodes.Therefore,it is of great interest to improve electrochemical performance via a cost-effective strategy.Herein,a low-cost P2-Na_0.7Cu_0.2Fe_0.2Mn_0.5Ti_0.1O₂（CT）cathode material was designed and prepared via Cu²⁺/Ti⁴⁺co-doping.In the CT cathode,Cu²⁺boosts the rate performance and greatly suppresses the phase transition at high voltage,while Ti⁴⁺inhibits the Jahn-Teller effect of Mn³⁺at low voltage to strengthen structural stability and reduces transition metal dissolution.The CT cathode delivered a high reversible capacity of 130 m Ah g^-1 at 0.1C（2-4.2 V）,favorable cyclability（71.1%capacity retention after 300 cycles at 5C）and decent rate capability（37%capacity retention at 10C compared to 0.1C）.GITT and DSCV results show that the diffusion coefficients of Na⁺of CT at high and low voltage region are significantly improved.In-situ XRD monitoring reveals that the Cu²⁺/Ti⁴⁺co-doped CT inhibits the harmful phase transition,and converts to reversible P2-P2’-P2 phase transition process.The results of ICP of cathode and XPS of Na anode indicate that the co-doping enables CT significantly reduced the dissolution of transition metal ions.These results strongly demonstrate the reason for the greatly boosted electrochemical performance of CT.Moreover,after 30 days of exposure to air and even 3 days of immersion in water,the structure of the CT material remained in good condition,clearly reflecting an excellent air/water stability resulting from the synergistic contribution of Cu²⁺and Ti⁴⁺to improving the reserve stability.