Fig. 1

(a) Schematic illustration of the cell configuration and operational mechanism of typical full PIBs[11]; (b) Comparison of the physical and electrochemical properties of Li, Na and K; (c) Electrochemical performance comparison of inorganic cathode materials for PIBs[11]"

Fig. 2

(a) Schematic illustrations of the crystal structures of O3-A1-x MeO2, P3-A1-x MeO2, and P2-A1-x MeO2(A=K)[11]; (b) Band structures between the M and ligand orbitals[24]; (c) O 2p nonbonding states and three cases of cationic and anionic redox[24]"

Fig. 3

(a) Enlarged HRTEM image of K0.77MnO2 ?H2O0.23[26]; (b) The first galvanostatic charge/discharge curves of the K0.77MnO2 ?H2O0.23 electrode at a current density of 20 mA/g and corresponding ex situ XRD patterns in the 2θ region of 10.5°～15.5°[26]; (c) Cycle performances and corresponding Coulombic efficiencies of the electrode at a current density of 100 mA/g for 100 cycles[26]; (d) In situ XRD characterization of K0.27(Mn0.98O2)?(H2O)0.53 electrode upon charge/discharge[43]; (e) Operando-XRD result of P2-K0.75[Ni1/3Mn2/3]O2 electrode[44]; (f) the corresponding calculated lattice parameters for o-XRD pattern[44]; (g)from experimentally calculated data and data predicted by first-principles calculations[13]; (h) Contour maps in selected 2θ ranges obtained from In operando XRD patterns of P2-K0.75MNFO2 during the charge-discharge process in the range of 1.5～3.9 V (K/K+)[13]"

Fig. 4

(a) Typical charge/discharge profiles of P3-type K0.5MnO2 at a current rate of 2 mA/g; (b)～(d) In situ XRD pattern taken for 2 h scanning rate per pattern; (e) XRD peak comparison of as-prepared (scan #1), scan #7, and scan #14 P3-K0.5MnO2 at 18°～20°; (f) comparison with simulated XRD patterns of the O3 and P3structures[48]; (g) P3-K0.75MnO2 and P3-K0.75[Co0.5Mn0.5]O2; (h) Predicted structural change in P3-K x [Co0.5Mn0.5]O2 (0.25≤ x ≤0.75); (i) operando XRD patterns of P3-K x [Co0.5Mn0.5]O2; (j) comparison of c-lattice parameters of P3-K x [Co0.5Mn0.5]O2 obtained by first-principles calculations and operando XRD data[57]; (k) Schematic illustration of the synthesis of peanut-like P3-type K0.45Mn0.5Co0.5O2 microparticles[59]; (l), (m) SEM images of P3-type K0.45Mn0.5Co0.5O2; (n) long-term cycling performance of K0.45Mn0.5Co0.5O2 at 300 mA/g; (o) Cyclic voltammograms at a scan rate of 0.03 mV/s"

Fig. 5

(a) In situ XRD characterization of K0.5Mn0.6Co0.2Fe0.1Mg0.1O2[61]; (b) Visualization of Jahn-Teller distortion[61]; (c) Long-term cycling performance of K0.5Mn0.6Co0.2Fe0.1Mg0.1O2 at 1 A/g[61]; (e) In situ XRD patterns of K0.35Mn0.8Fe0.1Cu0.1O2[66]; (f) HRTEM images of K0.35Mn0.8Fe0.1Cu0.1O2[66]; (g) Structural changes in K x Mn0.8Fe0.1Cu0.1O2 during K+ insertion[66]; (h) Cycling performance of K0.35Mn0.8Fe0.1Cu0.1O2 and KMO at 500 mA/g[66]"

Fig. 6

(a) Schematic diagram of the effect of Rb and Mg substitution on the crystal structure; (b) The stacking of P3-type and O3-type layered structures along the c-axis direction of the TM slabs; (c) Representative HRTEM images of the K0.45Rb0.05Mn0.85Mg0.15O2 powder and K0.45Rb0.05Mn0.85Mg0.15O2 test cathode placed in the battery environment without cycling; (d) The voltage curves and representative corresponding in situ XRD patterns of K0.45MnO2 and K0.45Rb0.05Mn0.85Mg0.15O2 at 70 mA/g in the range of 1.5—3.9 V.; (e) The c-lattice parameter of K0.45MnO2 and K0.45Rb0.05Mn0.85Mg0.15O2 calculated from in situ XRD patterns; (f) Charge GITT result of K0.45Rb0.05Mn0.85Mg0.15O2 at 20 mA/g, and the corresponding DK+; (g) Cycling performance of K0.45Rb0.05Mn0.85Mg0.15O2 at 200 mA/g[69]"

Fig. 7

Forecast of the main research directions and hot spots of PIBs in the future"