Fig 1

Comparison of relevant parameters between sodium and lithium."

Fig 2

Common cathode materials and related requirements for SIBs."

Table 1

Summary of basic structure information of polyanionic cathode materials for SIBs."

Fig 3

Schematic of mechanism illustration for sodium-ion battery."

Fig 4

Operating voltages and capacity characteristics of cathode materials for SIBs."

Fig 5

Energy diagram of iron-based mixed polyanion cathode materials for SIBs 19."

Fig 6

Structure and properties of NaFePO4. Schematic diagram of the crystal structure of (a) Sodium phosphorite type and (b) lithium phosphate-type NaFePO4 21; (c) charge-discharge curves of olivine-type NaFePO4 25; (d) transmission electron microscopy image of the phase transition from sodium phosphate phase NaFePO4 to amorphous FePO4 8; (e) schematic diagram of electrospinning preparation NaFePO4@C 26."

Fig 7

Structure and characterization of Na3Fe2(PO4)3. (a) Crystal structure of Na3Fe2(PO4)3 35; (b) XANES of Na3Fe2(PO4)3 35; (c) SEM images of NFP and K0.24NFP 37."

Fig 8

Crystal structure and performance of Na2FePO4F. (a) Schematic diagram of the crystal structure of Na2FePO4F 43; (b) charge discharge curve of Na2FePO4F/C electrode in the voltage range of 2.5–4.5 Ⅴ, with 5 cycles of 0.1 mV∙s−1, illustrated as CV curve 48."

Fig 9

Preparation and morphology of Na2FePO4F. (a) Synthesis mechanism of bilayer hollow spherical Na2FePO4F 55; (b, c) Scanning electron microscope (SEM) image of a porous spongy Na2FePO4F/C material 56."

Fig 10

Structure and related characterization of Na2FeP2O7. (a) Crystal structure of Na2FeP2O7 63; (b) charge-discharge curves and capacity differential curves of Na2FeP2O7 64; (c) XRD and Mössbauer spectra of Na2−xFeP2O7 (x = 0–1) samples 65."

Fig 11

Characterization, performance and preparation of Na4−αFe2+α/2(P2O7)2. (a) Voltage distribution of Na3.32Fe2.34(P2O7)2 74; (b) (011) and (−101) XRD spectra of the in-situ synchrotron 75; (c) schematic diagram of magnesium-doped CEI film with Na3.12Fe2.44(P2O7)2 and its mechanism of action 70; (d) synthesis process of Na3.12Fe2.44(P2O7)2/r-GO aerogel 76."

Fig 12

Crystal structure of common polyanionic iron-based sulfate compounds: (a) Na2Fe(SO4)2·2H2O 84, (b) Na2Fe(SO4)2·4H2O 85, (c) Na2Fe2(SO4)3 16, (d) NaFeSO4F·2H2O 85, (e) Na2FeSO4F3 85, (f) NaFeSO4F 94."

Fig 13

Properties and preparation of Na2+2xFe2−x(SO4)3 materials: (a) schematic diagram of possible composition, structure, and electrochemical curve changes of Na2+2xFe2−x(SO4)3 93; (b) comparison of the rate performance of NFS@C and NFS@C@GO electrodes 99; (c) schematic diagram of spherical shell synthesis of carbon nanotube network structure 93; (d) charge-discharge curves of NFS@C@2% CNTs material 99; (e) NFS@C@GO Schematic diagram of the synthesis of the sample 99."

Fig 14

The crystal structure and equivalent circuits of Na2FeSiO4: (a) crystal structure of Na2FeSiO4 109; (b) the equivalent circuits of models Ⅰ and Ⅱ used to simulate the first and second desodiation process, respectively 111."

Fig 15

Structure of Na2FeSiO4/CNTs: (a, b) TEM image of Na2FeSiO4/CNTs microspheres 118."

Fig 16

Structure and correlation characterization of Na4M3(PO4)2(P2O7): (a) crystal structure of Na4M3(PO4)2(P2O7) (M = Fe, Mn, Co, Ni) 4; (b) in situ XRD spectra of Na4Fe3(PO4)2(P2O7) 4; (c) charge-discharge curves of Na4Fe3(PO4)2(P2O7) 125; (d) schematic diagram of the synthesis of NFPP@rGO 126."

Fig 17

Structure and characterization of Na3Fe2(PO4)(P2O7): (a) crystal structure of NFFPP140; (b) rate performance of NFPP/rGO and NFPP/C at different rates; (c) corresponding charge/discharge curves of NFPP/rGO at different rates 141."

Fig 18

Structure and properties of NaFe2(PO4)(SO4)2: (a) projection plot of the crystal structure of NaFe2(PO4)(SO4)2 along the c-axis; (b) coordination of Na, Fe, and S atoms; (c) charge-discharge curves of NaFe2(PO4)(SO4)2 11."

Table 2

Comparison of iron-based polyanionic cathode materials for SIBs."