Fig. 1

The main classification of potassium polyanionic positive materials in recent years"

Fig. 2

(a) Schematic diagram of the synthesis mechanism of K3V2(PO4)3/C[36]; (b) N2 adsorption-desorption isotherm at K3V2(PO4)3/C (Illustration: Pore size distribution curve of K3V2(PO4)3/C)[36]; (c) Synthesis diagram of preparation process and mechanism of K3V2(PO4)3/C bundling nanowires[39]; (d) XRD diagram of K3V2(PO4)3/C bundled nanowires at 700—900℃[39]; (e) In-situ XRD plot and voltage overlay plot during 100 mAh/g charging and discharging[39]; (f) Long-life cycling performance of K3V2(PO4)3/C bundled nanowires at 1000 and 2000 mA/g[39]; (g) Rietveld refinement diagram of SXRD of K3V2(PO4)3 structure[43]; (h) The unit structure of K3V2(PO4)3 evolves with CV curve in the range of 1.0—4.0 V[43]"

Fig. 3

(a) TGA curve for K3V3(PO4)4·H2O[44]; (b) Charge-discharge curve of K3V3(PO4)4·H2O from the 1st to the 100th cycle[44]; (c) Perspective views of K3V3(PO4)4·H2O potential K+ diffusion channels parallel to the (a) x-z basal plane [001][44]; (d) Schematic diagram of the charge-discharge curve and crystal structure of KFePO4 when the current density is 7.1 mA/g[48]; (e) Schematic diagram of the charge-discharge curve and crystal structure of KMnPO4 when the current density is 7.1 mA/g[48]; (f) N2 adsorption-desorption isotherms for KFePO4/C (Illustration: Pore size distribution curve of KFePO4/C)[49]; (g) DOS of K3Ti2(PO4)3[53]; (h) Structural evolution of KTP/C: a) Two-dimensional images of the manipulation of XRD data at the beginning of the first charge and discharge and the second discharge[53]; (i) XANES spectra of Ti K-edge measured at OCV, end of discharge (1 V) and charge (4 V) for KTP/C electrode and references (TiO2 and TiN)[53]"

Fig. 4

(a) Color and CV curves before and after the reaction of various pyrophosphates[54]; (b) Charge-discharge curves of KVP, KVP-LC, KVP-HC at 0.2c[55]; (c) P2p ex situ XPS diagram[55]; (d) Ex-situ XRD patterns of the KVP-LC electrodes at different charged-discharged states[55]"

Fig. 5

(a) Charge/discharge profiles and cycle performance of KVPO4F[62]; (b) Charge/discharge profiles and cycle performance of KVOPO4[62]; (c) K-ion migration path 3 and K migration barrier in Cs-y-KVPF[63]; (d) Contour maps of in situ XRD patterns collected during the first charge/discharge of the Cs-5-KVPF electrode at a voltage ranging from 2.5 to 5.0 V[63]; (e) DSC curves of KVPF and Cs-5-KVPF electrodes inthe fully charged state[63]; (f) Comparison of the energy density of KVPF@C-PMS with other cathode materials[64]"

Fig. 6

(a) Voltage profiles of de-sodiation of Na3V2(PO4)2F3 and the following potassiation/de- potassiation[65]; (b) Compassion of the voltage and capacities of as-prepared K3V2(PO4)2F3 with different types of cathodes reported until now[65]; (c) A typical charge-discharge curve of Li x K0.15VPO4F at 0.2 C.(The inset demonstrates the capacity retention and Coulomb efficiency in the cycling ability test at 1 C rate)[67]; (d) SEM image of K1+δ VOPO4F[68];(e) Ex situ XPS of K1+δ VOPO4F charge to 4.5 V[68]; (f) Ex situ XPS of K1+δ VOPO4F discharge to 2.0 V[68]; (g) Schematic illustration for designing KVPF@3DC composite material[69]; (h) The cycling performance of KVPF@3DC at elevated temperature (55 ℃) with the current density of 50 mA/g[69]; (i) Schematic diagram of the interface change during repeated charge-discharge process at elevated temperature of KVPF@3DC[69]"

Fig. 7

(a) BVEL of o-KFeSO4F[71]; (b) BVEL of m-KFeSO4F[71]; (c) Normalized Fe K-edge XANES spectra of KFMg005SF,a charged KFMg005SF electrode, and a discharged KFMg005SF electrode[73]; (d) In-situ XRD patterns in the 2h range of 14.5°–17.5°, 27.2°–29.9°, and 31.0°–34.2° and the concerned charge/discharge profiles at 10 mA/g[74]; (e) GITT profiles and the relevant D(K+) values of KFSF@CNTs/DEG[74]; (f) Representative charge–discharge profiles of the full cell[74]"

Table 1

Electrochemical performance and spatial configuration of polyanionic cathode materials for potassium-ion batteries in recent years[46, 54, 62, 65, 71]"