The Structural Design And Redox Mechanisim Of Fe-based Polyanionic Cathode Materials For Na-ion Storage

Sodium ion batteries（SIBs）exhibit promising prospects for large-scale energy storage due to abundant resources and low cost.It is greatly important for the design of sodium ion battery cathodes with high energy density,cost-effectiveness,and high safety for the development of SIBs.The Fe-based polyanionic cathode materials possess the advantages of abundant reserves,environmental benign,and high safety.This thesis first summarizes the latest research progress of the main SIBs cathode materials in terms of structure,performance,phase transition,and charge-discharge mechanism,and the focus is the high-safety Na_3.32Fe_2.34（P₂O₇）₂ and high-energy-density Na_2+2xFe_2-x（SO₄）₃（0<x<0.4）materials.It is clear that the key to improving performance is to solve the bottlenecks,such as low electronic conductivity and poor structural stability.Therefore,the corresponding electrochemical performance is effectively improved via structural modulation and morphology construction.And the reaction mechanism is also revealed.The main research contents and conclusions of this thesis are as follows:（1）The poor electronic conductivity of pyrophosphates with the strong bond of P-O is caused by the structural characteristics of polyanions,which can be improved by designing single-dimensional carbon materials.However,it is still a challenge to improve the cycle stability of Na_3.32Fe_2.34（P₂O₇）₂ by using only single-dimensional carbon.To effectively improve the electronic conductivity and cycle life of the Na_3.32Fe_2.34（P₂O₇）₂ material,a solid-state method was used to prepare an N-doped multiple-dimensional carbon matrix,which in situ coated the Na_3.32Fe_2.34（P₂O₇）₂particles and reduced their sizes.The versatile N-doped carbon structures include carbon rods,carbon nanosheets,and carbon skeletons,which are resulted from the chemical interaction between glucose and diammonium hydrogen phosphate during high-temperature calcination.Carbon-coated Na_3.32Fe_2.34（P₂O₇）₂@C exhibits a discharge specific capacity of 100 m Ah g^-1 at 0.1 C and achieves a high capacity retention of～90%after 1000 cycles at 1 C,which are higher than those in the pristine material(90 m Ah g^-1 and 67%).Even cycling 1000 times at 1 C under 0℃,the capacity retention of the modified electrode is as high as 94.5%.Such an improved electrochemical activity origins from:（a）N-doped carbon generates more defects and active sites,and（b）the hybrid carbon matrix can provide high electronic conductivity and work as a buffer to minimize volume changes of the composite.The improved kinetic properties of Na ion diffusion in Na_3.32Fe_2.34（P₂O₇）₂@C is demonstrated by the GITT technique,and the ex situ XRD indicates that a stable crystal structure is retained after 2000cycles at 5 C.This work provides a practical strategy for constructing high-performance Na_3.32Fe_2.34（P₂O₇）₂cathode materials.（2）As a safe and sustainable cathode material for reversible sodium ion storage,the electrochemical performance of Na_3.32Fe_2.34（P₂O₇）₂ can be further promoted via ion-doping.However,the rationale behind is still unclear.In this study,Na⁺and Fe²⁺cations were partially replaced by K⁺and Mg²⁺,respectively,in which the dopants worked as pillars to support the crystal framework and thus reduce the potential hysteresis.After 1000 cycles at 1 C,the capacity retention of Mg-doped electrode（～98%）is higher than that of the K-doped electrode（～83%）.Comprehensive study indicates that the structure can be stabilized by inhibiting Fe migration via Mg-doping,which may be the key to improving performance.The reason is that although K⁺and Mg²⁺are stable pillars to provide enhanced stability,good reversibility,and low voltage polarization,while the occupation of Mg at Fe sites can also inhibit Fe migration by adjusting the Coulomb repulsion between Fe³⁺and Mg²⁺and thus renders long cycling life.Overall,it is fundamentally important to study the decoupled substitutions in this Na_3.32Fe_2.34（P₂O₇）₂ cathode material and investigate their positive role in promoting the storage of Na ions.Finally,this work demonstrates that proper cationic pillaring is greatly useful for achieving high-performance cathode materials.（3）Furthermore,Na_3.32Fe_2.11Ca_0.23（P₂O₇）₂ material was prepared by calcium selective doping,and the Na-ion storage mechanism and thermal stability were investigated in depth.Ca-doping can significantly contribute to the electrochemical stability and operational safety of the electrode materials.The structural superiority provided by Na_3.32Fe_2.11Ca_0.23（P₂O₇）₂ helps to achieve a high capacity retentions of 81.7%after1000 cycles at 1 C,much better than the un-doped electrode（15.5%）.According to the high temperature in situ XRD test（In situ HR-XRD）,it is found that the charged Na_3.32-xFe_2.34（P₂O₇）₂（0<x≤1）electrodes has undergone a thermal phase transition from triclinic（P1）to monoclinic（P2 1/c）phase,but this transition in the charged Na_3.32-xFe_2.11Ca_0.23（P₂O₇）₂（0<x≤1）has been increased from 450℃to 500℃,indicating that Ca-doping can provide the better thermal stability.Based on the results of in situ X-ray synchrotron radiation（SXRD）and in situ X-ray near edge absorption spectroscopy（XANES）,it’s found that the redox mechanism of Na_3.32Fe_2.11Ca_0.23（P₂O₇）₂ is an ideal solid-solution process with high structural reversibility,which can greatly reduce the structural distortion caused by de-intercalation/intercalation of sodium ions,and thus obtains a long cycle life.Meanwhile,it is first revealed that the voltage curve is highly correlated with the variable lattice parameter b value,as reflected by the different potential plateauses corresponding to different b-value regions.For exapmle,the voltage range corresponding to the b-value plateau segment（～9.428?）is about 2.9 V～3.12 V.More importantly,the lattice parameter b can also be used as a sensitive indicator of the Na content or specific capacity in the electrode materials during the cycles.This underlying regulation of b value and voltage plateau as well as Na content may provide an opportunity to understand the changes in voltage plateaus and specific capacities associated with other systems and the rationale behind.（4）In addition to high safety,high energy density is also the key to the development of Na-ion cathode materials.According to the―induction effect‖,replacing（P₂O₇）₂^4-with the more electronegative SO₄^2-can obtain a higher operational voltage.Therefore,the alluaudite Na_2+2xFe_2-x（SO₄）₃（0<x<0.4）cathode material possesses a high theoretical specific energy density(～540 Wh Kg^-1,equial to Li Fe PO₄).However,the current performance is unsatisfying and is greatly limited by the two major bottlenecks,i.e.low electronic conductivity and high moisture sensitivity.Traditional in situ surface coating strategies through wet chemistry and thermolysis are impracticable due to the moisture sensitivity and poor thermal stability.In this study,the high-voltage Na_2+2xFe_2-x（SO₄）₃ cathode material was successfully synthesized through the solid-state route.The precursor obtained by ball milling was calcined at 350℃to obtain the target material,and graphene with high electronic conductivity was added to improve its electrochemical performance.The initial reversible capacity of the pristine Na_2+2xFe_2-x（SO₄）₃is only 65 m Ah g^-1 at 0.1 C,wheras it can be lifted to 106 m Ah g^-1 in Na_2+2xFe_2-x（SO₄）₃@graphene.The rate capability is impressive with a reversible discharge capacity of 51 m Ah g^-1 at 20 C,which is about 3 times higher than that in the pristine sample(16m Ah g^-1).Moreover,a capacity retention of>98%is retained over 700 cycles at low temperature（0°C）.This strategy of graphene-assisted synthesis is expected to provide deep insight for the optimization and industrial production of Na_2+2xFe_2-x（SO₄）₃ cathodes materials.In summary,this thesis mainly focuses on the sustainable iron-based polyanionic cathode materials,aiming to effectively solve the low electronic conductivity and poor structural stability of Na_3.32Fe_2.34（P₂O₇）₂ and Na_2+2xFe_2-x（SO₄）₃ materials.The results demonstrate that in-situ coating of carbon with differently-dimensional structures for high-safety Na_3.32Fe_2.34（P₂O₇）₂,and the addition of graphene in high-voltage Na_2+2xFe_2-x（SO₄）₃,are effective strategies to enhance the electronic conductivity as well as electrochemical performance.Moreover,through the electrochemically inactive K⁺,Mg²⁺,and Ca²⁺doping,a deep insight into the voltage drop phenomenon and the mechanism of structural stability of Na_3.32Fe_2.34（P₂O₇）₂ were obtained.Especially,Ca²⁺selective doping greatly improves the electrochemical performance and operational safety of Na_3.32Fe_2.34（P₂O₇）₂.Combining with the in-situ characterizations,the mechanism of charge-discharge and the sodium-ion diffusion was revealed.The above work pioneers an effective strategy for designing high-safety,cost-effective and long-cycle-life cathode materials for Na-ion storage.The corresponding strategy can be extended to other energy materials to prolong the battery life and promote operational safety.