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Figure 3. a) Potential profiles of the 24 and 50‘ cycle at 0.1 C in the voltage range 3.5-1.5 and 4.0-1.5 V. Reproduced with permission.!45] Copyright 2016, Electrochemical Society. b) Cycling performance of P2-Kg;MnO, and P3-Kg 4s;MnO, at 20 mA g™!. Reproduced with permission.!46] Copyright 2019, Elsevier. c) Structural changes of P3-Ky ,MnO, during charge and discharge. Reproduced with permission.[*7] Copyright 2017, Wiley.

Figure 3 a) Potential profiles of the 24 and 50‘ cycle at 0.1 C in the voltage range 3.5-1.5 and 4.0-1.5 V. Reproduced with permission.!45] Copyright 2016, Electrochemical Society. b) Cycling performance of P2-Kg;MnO, and P3-Kg 4s;MnO, at 20 mA g™!. Reproduced with permission.!46] Copyright 2019, Elsevier. c) Structural changes of P3-Ky ,MnO, during charge and discharge. Reproduced with permission.[*7] Copyright 2017, Wiley.

Related Figures (19)

Figure 1. a) Schematic illustration of working mechanism of KIB, b) standard redox potential of various metal anodes, and c) comparison of Shannon’s ionic radius and Stokes radius in propylene carbonate of Lit, Nat and Kt.
Figure 1. a) Schematic illustration of working mechanism of KIB, b) standard redox potential of various metal anodes, and c) comparison of Shannon’s ionic radius and Stokes radius in propylene carbonate of Lit, Nat and Kt.
Figure 2. Crystal structures of O3-, P3-, and P2-type layered metal oxides.
Figure 2. Crystal structures of O3-, P3-, and P2-type layered metal oxides.
Figure 4. a) aren mechanism of the dual interphase layers composed of a SEI layer and spinel interlayer on P2-KMO during cycles. Reproducec with permission.'>3] Copyright 2019, Elsevier. b) Schematic illustration of the synthesis process of AIF; @S-KMO. c) Cycling performance at 50 mA g- before and after refreshing the potassium metal anodes and electrolyte. Reproduced with permission.|*4] Copyright 2019, Wiley.
Figure 4. a) aren mechanism of the dual interphase layers composed of a SEI layer and spinel interlayer on P2-KMO during cycles. Reproducec with permission.'>3] Copyright 2019, Elsevier. b) Schematic illustration of the synthesis process of AIF; @S-KMO. c) Cycling performance at 50 mA g- before and after refreshing the potassium metal anodes and electrolyte. Reproduced with permission.|*4] Copyright 2019, Wiley.
Figure 5. a) Voltage curves of A//AxCoO, (A = Li, Na, and K). Reproduced with permission.!*3] Copyright 2017, Royal Society of Chemistry. b) Typical charge/discharge profile at a current rate of 2 mA g~! and in situ XRD patterns of P2-Ky ¢CoO,. Reproduced with permission.[>4 Copyright 2017, Wiley. c) Rate capability of s-KCO and i-KCO at different current rates; Typical charge-discharge curves of s-KCO//K and hard carbon//K in a half-cell and s-KCO//hard carbon full-cell configurations. Reproduced with permission.[°7] Copyright 2018, American Chemical Society.
Figure 5. a) Voltage curves of A//AxCoO, (A = Li, Na, and K). Reproduced with permission.!*3] Copyright 2017, Royal Society of Chemistry. b) Typical charge/discharge profile at a current rate of 2 mA g~! and in situ XRD patterns of P2-Ky ¢CoO,. Reproduced with permission.[>4 Copyright 2017, Wiley. c) Rate capability of s-KCO and i-KCO at different current rates; Typical charge-discharge curves of s-KCO//K and hard carbon//K in a half-cell and s-KCO//hard carbon full-cell configurations. Reproduced with permission.[°7] Copyright 2018, American Chemical Society.
Figure 6. a) Thermodynamic stability of layered KMO, compounds. b) Typical voltage-capacity curves at a current rate of 5 mA g~! and c) in situ XRD patterns of O3-KCrO,. Reproduced with permission.!#°] Copyright 2018, American Chemical Society.
Figure 6. a) Thermodynamic stability of layered KMO, compounds. b) Typical voltage-capacity curves at a current rate of 5 mA g~! and c) in situ XRD patterns of O3-KCrO,. Reproduced with permission.!#°] Copyright 2018, American Chemical Society.
Figure 7. a) Crystal structure of Kg5V,Os b) galvanostatic charge/discharge voltage profiles, c) rate capability of Kg5V2O5. Reproduced with permission.|7"] Copyright 2018, Wiley. d) Schematic illustration of chemical pre-intercalation synthesis approach. Reproduced with permission.!74] Copy- right 2018, American Chemical Society. e) Ex situ XRD patterns of the K,V3;Og electrode at various charge/discharge states at 10 mA g~!. Reproduced with permission.!73] Copyright 2019, Royal Society of Chemistry.
Figure 7. a) Crystal structure of Kg5V,Os b) galvanostatic charge/discharge voltage profiles, c) rate capability of Kg5V2O5. Reproduced with permission.|7"] Copyright 2018, Wiley. d) Schematic illustration of chemical pre-intercalation synthesis approach. Reproduced with permission.!74] Copy- right 2018, American Chemical Society. e) Ex situ XRD patterns of the K,V3;Og electrode at various charge/discharge states at 10 mA g~!. Reproduced with permission.!73] Copyright 2019, Royal Society of Chemistry.
Figure 8. a) CV curves of s-KFMO electrode at a scanning rate of 0.1 mV s~! and charge/discharge curves of s-KFMO cathode at 20 mA g~', b) cycling performance of s-KFMO//hard carbon full cell. Reproduced with permission.!*4] Copyright 2018, Wiley.
Figure 8. a) CV curves of s-KFMO electrode at a scanning rate of 0.1 mV s~! and charge/discharge curves of s-KFMO cathode at 20 mA g~', b) cycling performance of s-KFMO//hard carbon full cell. Reproduced with permission.!*4] Copyright 2018, Wiley.
Figure 9. a) Charge—discharge curves of P3-Ko 4s5Mng5Cog5O>, b) cyclic stability of P3-Ko 45 Mg 5Cog 502. Reproduced with permission.|°" Copyright 2019, Elsevier. c) Formation energy of P3-K,[Cog;Mng5]O, (0 < x < 1), d) comparison of experimentally measured GITT charge/discharge curve and predicted voltage profile, and comparison of Mn—-O bonding distances between e) P3-Kg75;MnO, and f) P3-Kg 75[Cogs;Mng5]O2. Reproduced with permission.|®4] Copyright 2019, Elsevier.
Figure 9. a) Charge—discharge curves of P3-Ko 4s5Mng5Cog5O>, b) cyclic stability of P3-Ko 45 Mg 5Cog 502. Reproduced with permission.|°" Copyright 2019, Elsevier. c) Formation energy of P3-K,[Cog;Mng5]O, (0 < x < 1), d) comparison of experimentally measured GITT charge/discharge curve and predicted voltage profile, and comparison of Mn—-O bonding distances between e) P3-Kg75;MnO, and f) P3-Kg 75[Cogs;Mng5]O2. Reproduced with permission.|®4] Copyright 2019, Elsevier.
Figure 10. a) Charge—discharge profiles of NNMO during Kt exchange process, b) comparison of dQ/dV curves of NNMO in Nat and K* electrolytes c) Comparison of rate capability of KNMO. Reproduced with permission.[41] Copyright 2019, Elsevier. d) Formation energies of P2/02-K,[Nij/3Mn3/3]O with various K contents. e) Comparison between experimentally measured charge/discharge curves and redox potentials predicted from first-principle calculations, f) predicted structural change of P2/O2-K,[Ni,,3Mn3/3]O2 as a function of K content. Reproduced with permission. [84] Conprignt 2020 Wiley. g) In situ XRD patterns of P2-Kg 44Nig. aot 7302 alecirade ‘collected during the first and second charge/discharge at 10 mA g™! in the voltag range of 1.54.0 V. Reproduced with permission.!®! Copyright 2019, Wiley. Operando synchrotron XRD patterns and calculated lattice parameters fo h,i) Kg 5 MnO, and j,k) Ko.s[Nio ; Ming 9]O. Reproduced with permission.!’] Copyright 2019, American Chemical Society.
Figure 10. a) Charge—discharge profiles of NNMO during Kt exchange process, b) comparison of dQ/dV curves of NNMO in Nat and K* electrolytes c) Comparison of rate capability of KNMO. Reproduced with permission.[41] Copyright 2019, Elsevier. d) Formation energies of P2/02-K,[Nij/3Mn3/3]O with various K contents. e) Comparison between experimentally measured charge/discharge curves and redox potentials predicted from first-principle calculations, f) predicted structural change of P2/O2-K,[Ni,,3Mn3/3]O2 as a function of K content. Reproduced with permission. [84] Conprignt 2020 Wiley. g) In situ XRD patterns of P2-Kg 44Nig. aot 7302 alecirade ‘collected during the first and second charge/discharge at 10 mA g™! in the voltag range of 1.54.0 V. Reproduced with permission.!®! Copyright 2019, Wiley. Operando synchrotron XRD patterns and calculated lattice parameters fo h,i) Kg 5 MnO, and j,k) Ko.s[Nio ; Ming 9]O. Reproduced with permission.!’] Copyright 2019, American Chemical Society.
Figure 11. High-resolution XPS spectra of a) Mn 2p and b) Mg 2p in H-Kg 7Mng 7Mng 302. Reproduced with permission.!°2] Copyright 2019, Wiley Galvanostatic charge—discharge profiles of c) H-Kg 7MnO, and d) H-Kg 7Mno 7Mgy 302. Reproduced with permission.[%3] Copyright 2020, Elsevier. In sitt XRD patterns and corresponding voltage—capacity curve of e) P2- Ks;9 MnO, and f) P2- Ks;gMn7/9Tiz;9O7 at 10 mA g—!. Reproduced with permission.|2° Copyright 2020, Elsevier. Dimensional changes at various states-of-charge in g) P2-Kg 7[Crg.g5SD9 75]O2 and h) P2-Ko ¢yNag.og[Cro.g5 Sbo,15]O2. The solic and hollow circles denote changes in the c-axis and a-axis lengths, respectively. The XRD patterns for (002) and (010) peaks are shown in the left anc right panels, respectively. The (010) peaks are magnified by 10. Reproduced with permission.!"2] Copyright 2020, Electrochemical Society.
Figure 11. High-resolution XPS spectra of a) Mn 2p and b) Mg 2p in H-Kg 7Mng 7Mng 302. Reproduced with permission.!°2] Copyright 2019, Wiley Galvanostatic charge—discharge profiles of c) H-Kg 7MnO, and d) H-Kg 7Mno 7Mgy 302. Reproduced with permission.[%3] Copyright 2020, Elsevier. In sitt XRD patterns and corresponding voltage—capacity curve of e) P2- Ks;9 MnO, and f) P2- Ks;gMn7/9Tiz;9O7 at 10 mA g—!. Reproduced with permission.|2° Copyright 2020, Elsevier. Dimensional changes at various states-of-charge in g) P2-Kg 7[Crg.g5SD9 75]O2 and h) P2-Ko ¢yNag.og[Cro.g5 Sbo,15]O2. The solic and hollow circles denote changes in the c-axis and a-axis lengths, respectively. The XRD patterns for (002) and (010) peaks are shown in the left anc right panels, respectively. The (010) peaks are magnified by 10. Reproduced with permission.!"2] Copyright 2020, Electrochemical Society.
Figure 12. The electrochemical performances for the pristine P3-Ko 45Nig ;Cog;MnggO,, Mg-doped, and Al-doped cathodes. a) Galvanostatic charge/discharge curves and b) cycling performance at 20 mA g~!. Reproduced with permission.!°9! Copyright 2020, Elsevier. c) Operando synchrotron XRD patterns for the P3-Kg 5[MnggFeg.jNigJOz and d) the lattice parameters calculated from operando SXRD patterns during the charge—discharge processes. Reproduced with permission.!'°"] Copyright 2020, Elsevier. e,f) Schematic from the DFT calculation for Mn—O bonding distances of Kg ¢ MnO, and Kg gMnogNig;Tig1Oz, respectively. Predicted K* diffusion paths and activation barrier energy in the Kg gMnggNig Tig. ;Oz by first-principles calcu- lation, g) Kr to Kg, h) Kr to K,, i) K, to K, and j) corresponding energy barriers for the migration pathways displayed in (g—i) by using the NEB method. Reproduced with permission. 1103) Copyright 2021, Elsevier.
Figure 12. The electrochemical performances for the pristine P3-Ko 45Nig ;Cog;MnggO,, Mg-doped, and Al-doped cathodes. a) Galvanostatic charge/discharge curves and b) cycling performance at 20 mA g~!. Reproduced with permission.!°9! Copyright 2020, Elsevier. c) Operando synchrotron XRD patterns for the P3-Kg 5[MnggFeg.jNigJOz and d) the lattice parameters calculated from operando SXRD patterns during the charge—discharge processes. Reproduced with permission.!'°"] Copyright 2020, Elsevier. e,f) Schematic from the DFT calculation for Mn—O bonding distances of Kg ¢ MnO, and Kg gMnogNig;Tig1Oz, respectively. Predicted K* diffusion paths and activation barrier energy in the Kg gMnggNig Tig. ;Oz by first-principles calcu- lation, g) Kr to Kg, h) Kr to K,, i) K, to K, and j) corresponding energy barriers for the migration pathways displayed in (g—i) by using the NEB method. Reproduced with permission. 1103) Copyright 2021, Elsevier.
Table 1. (Continued). Table 2. Electrochemical properties of layered oxide cathodes in full-cell KIBs.
Table 1. (Continued). Table 2. Electrochemical properties of layered oxide cathodes in full-cell KIBs.
over 30 000 cycles at 10 A g—1, demonstrating excellent cyclic sta- bility (Figure 14c). The mass ratio between anode and cathode in the full cell KIC must be balanced according to their working po- tentials to obtain high energy/power density. Wei et al.!!! fabri- cated an aqueous KIC with a layered Kp 59,Mny 92,0, cathode and AC anode. The Ko 99,M1p 9960,//AC displayed high energy and power densities of 70 W kg“! and 6000 W kg“, respectively, and a capacitance retention of 89.3% after 10 000 cycles. The unique layered structure facilities the rapid diffusion of K* ions and pro- vides space for charge storage. from cathode surface diffuse back to the electrolyte.!174) Most cur- rent research efforts are focused on developing battery-type an- ode materials for KICs. Such materials include carbon-based ma- terials, metal oxides, metal chalcogenides, MXenes and organic materials that follow different storage mechanisms such as inter- calation, conversion, and alloying (Figure 14a).!1?773] Conversely, carbon-based capacitor-type cathodes are extensively studied, but literature reports on the battery-type cathodes for KIC are rare, perhaps because potassium-ion storage systems are in an early development stage and suitable battery-type cathodes have not yet been explored.!!*4! Although the anode materials exhibit superior capacities, they must be combined with suitable cathode materi- als to achieve high energy and power density devices. Ramasamy et al.8] reported a non-aqueous KIC consisting of a layered P3- Ko.45 Mp 5 Coy 5O, battery-type cathode combined with a commer- cial activated carbon (AC) anode (Figure 14b). The constructed KIC delivered a high energy and power density of 43 Wh kg"! and 30 kW kg“, respectively, and retained 88% of its energy density
over 30 000 cycles at 10 A g—1, demonstrating excellent cyclic sta- bility (Figure 14c). The mass ratio between anode and cathode in the full cell KIC must be balanced according to their working po- tentials to obtain high energy/power density. Wei et al.!!! fabri- cated an aqueous KIC with a layered Kp 59,Mny 92,0, cathode and AC anode. The Ko 99,M1p 9960,//AC displayed high energy and power densities of 70 W kg“! and 6000 W kg“, respectively, and a capacitance retention of 89.3% after 10 000 cycles. The unique layered structure facilities the rapid diffusion of K* ions and pro- vides space for charge storage. from cathode surface diffuse back to the electrolyte.!174) Most cur- rent research efforts are focused on developing battery-type an- ode materials for KICs. Such materials include carbon-based ma- terials, metal oxides, metal chalcogenides, MXenes and organic materials that follow different storage mechanisms such as inter- calation, conversion, and alloying (Figure 14a).!1?773] Conversely, carbon-based capacitor-type cathodes are extensively studied, but literature reports on the battery-type cathodes for KIC are rare, perhaps because potassium-ion storage systems are in an early development stage and suitable battery-type cathodes have not yet been explored.!!*4! Although the anode materials exhibit superior capacities, they must be combined with suitable cathode materi- als to achieve high energy and power density devices. Ramasamy et al.8] reported a non-aqueous KIC consisting of a layered P3- Ko.45 Mp 5 Coy 5O, battery-type cathode combined with a commer- cial activated carbon (AC) anode (Figure 14b). The constructed KIC delivered a high energy and power density of 43 Wh kg"! and 30 kW kg“, respectively, and retained 88% of its energy density
Figure 13. a) Charge—discharge profiles of P2-No g4CoO, in Na-ion cell and K-ion cell. b) Ex situ XRD measurements of the pristine, charged, and dis: charged electrodes depicting the successful desodiation and potassiation of the pristine compound to form Ky ;CoO,. Reproduced with permission.!18] Copyright 2017, Royal Society of Chemistry. c) The diffusivity is calculated from GITT analysis for Nag 9Crg.9Rug 10. Reproduced with permission.!"" Copyright 2019, Royal Society of Chemistry. d) Schematic diagram depicting two possible routes for reversible Kt-insertion/deinsertion in P’3. Nag52CrO7. Small spheres (blue) represent Na* ions. Arrows indicate the direction of contraction/expansion along the c-axis with Kt-insertion. Re- produced with permission.!'!] Copyright 2018, American Chemical Society.
Figure 13. a) Charge—discharge profiles of P2-No g4CoO, in Na-ion cell and K-ion cell. b) Ex situ XRD measurements of the pristine, charged, and dis: charged electrodes depicting the successful desodiation and potassiation of the pristine compound to form Ky ;CoO,. Reproduced with permission.!18] Copyright 2017, Royal Society of Chemistry. c) The diffusivity is calculated from GITT analysis for Nag 9Crg.9Rug 10. Reproduced with permission.!"" Copyright 2019, Royal Society of Chemistry. d) Schematic diagram depicting two possible routes for reversible Kt-insertion/deinsertion in P’3. Nag52CrO7. Small spheres (blue) represent Na* ions. Arrows indicate the direction of contraction/expansion along the c-axis with Kt-insertion. Re- produced with permission.!'!] Copyright 2018, American Chemical Society.
Figure 14. a) Various battery-type anode materials tested in KICs. Reproduced with permission.!!?2] Copyright 2021, American Chemical Society. b) Schematic illustration of a KIC fabricated with a battery-type Ko 4; Mng 5 Cog 502 cathode and AC anode. c) Ragone plot of Kg 4s Ming 5Cog 5O7//AC device. Inset: First few charge/discharge cycles at 10 A g~!. Reproduced with permission.[2"] Copyright 2019, Elsevier.
Figure 14. a) Various battery-type anode materials tested in KICs. Reproduced with permission.!!?2] Copyright 2021, American Chemical Society. b) Schematic illustration of a KIC fabricated with a battery-type Ko 4; Mng 5 Cog 502 cathode and AC anode. c) Ragone plot of Kg 4s Ming 5Cog 5O7//AC device. Inset: First few charge/discharge cycles at 10 A g~!. Reproduced with permission.[2"] Copyright 2019, Elsevier.
Figure 15. Crystal structures of a) Kg g¢CoO, (KCO), b) Kg ¢CoO;.3Ng porous nanoframe (KCO PNF) and c) Kt migration energy barriers of KCO and KCO PNF. Reproduced with permission.!15°] Copyright 2020, Elsevier. de) Finite element model of strain variation during volume expansion and f,g) SEM images of conventional KMNC and YS-KMNC after cycling. Reproduced with permission.!'®] Copyright 2022, Wiley.
Figure 15. Crystal structures of a) Kg g¢CoO, (KCO), b) Kg ¢CoO;.3Ng porous nanoframe (KCO PNF) and c) Kt migration energy barriers of KCO and KCO PNF. Reproduced with permission.!15°] Copyright 2020, Elsevier. de) Finite element model of strain variation during volume expansion and f,g) SEM images of conventional KMNC and YS-KMNC after cycling. Reproduced with permission.!'®] Copyright 2022, Wiley.
Figure 16. a) First charge/discharge profiles of full cells at 52 mA g~!. Reproduced with permission.!®8] Copyright 2020, Elsevier. b) Charge—discharge profiles of a graphite electrode in a Li cell with 1 m LiPF,/EC:DMC (black line), Na cell with NaPF,/EC:DEC (blue line) and in K cell with 1 m KFSI/EC:DEC (red line). Reproduced with permission.|2°] Copyright 2018, Wiley. c) Discharge capacity and average charge/discharge voltage of selected anode mate- rials employed in potassium-ion batteries. Reproduced with permission.!!”*] Copyright 2021, Wiley.
Figure 16. a) First charge/discharge profiles of full cells at 52 mA g~!. Reproduced with permission.!®8] Copyright 2020, Elsevier. b) Charge—discharge profiles of a graphite electrode in a Li cell with 1 m LiPF,/EC:DMC (black line), Na cell with NaPF,/EC:DEC (blue line) and in K cell with 1 m KFSI/EC:DEC (red line). Reproduced with permission.|2°] Copyright 2018, Wiley. c) Discharge capacity and average charge/discharge voltage of selected anode mate- rials employed in potassium-ion batteries. Reproduced with permission.!!”*] Copyright 2021, Wiley.
Table 3. Summary of metal elements characteristics and functions in K*-layered oxide cathodes. are low-cost, abundant, and environmentally benign. The Ti** substation increased the valence state of Mn** that suppresses the J-T distortions. Further, the electrochemical inactive Ti** pre- vents the layer gliding and thus improving structural stability dur- ing K+ deintercalation/intercalation.!!°! Generally, the doping of electrochemically inactive metal reduces the specific capacity and thus the doping amount must be adjusted to obtain enhanced ca- pacity and stability. The doping metals with high electronegativity (Sb>*, TeS+) could provide high operating voltage due to the in- ductive effect.!!21°5] In a word, developing K*-layered oxide ma- terials with suitable elements and compositions could possibly improve structural stability, specific capacity, average operating voltage, and cycling stability. K*-ion layered oxides destabilize the layered structure and form K*-ion-deficient compounds or other 3D structures. In half-cell configurations, the initial potassium loss related to the solid elec- trolyte interface (SEI) formation is compensated by the potas- sium metal anode. However, a full-cell KIB constructed using a carbon-based anode and a K*-deficient layered cathode demon- strates low capacity because the cathode is the K*-ion reser- voir. To overcome these issues, K*-rich layered compounds can be synthesized by introducing suitable metal cations. Further- more, prepotassiation, as used in LIBs, can also be employed in KIBs. Kt-ion layered oxide compounds demonstrate steeper volt- age curves and more variation in stepwise voltage than their Lit and Na* counterparts mainly because strong K*t-K* repulsion causes Kt /vacancy ordering as a function of Kt-ion content and transition-metal charge ordering, thereby leading to numerous phase transitions and voltage steps. Further studies are required to design multimetal compounds with various redox-active and -inactive metals to suppress K*/ordering and obtain smoother voltage profiles for K*-ion layered oxide compounds.
Table 3. Summary of metal elements characteristics and functions in K*-layered oxide cathodes. are low-cost, abundant, and environmentally benign. The Ti** substation increased the valence state of Mn** that suppresses the J-T distortions. Further, the electrochemical inactive Ti** pre- vents the layer gliding and thus improving structural stability dur- ing K+ deintercalation/intercalation.!!°! Generally, the doping of electrochemically inactive metal reduces the specific capacity and thus the doping amount must be adjusted to obtain enhanced ca- pacity and stability. The doping metals with high electronegativity (Sb>*, TeS+) could provide high operating voltage due to the in- ductive effect.!!21°5] In a word, developing K*-layered oxide ma- terials with suitable elements and compositions could possibly improve structural stability, specific capacity, average operating voltage, and cycling stability. K*-ion layered oxides destabilize the layered structure and form K*-ion-deficient compounds or other 3D structures. In half-cell configurations, the initial potassium loss related to the solid elec- trolyte interface (SEI) formation is compensated by the potas- sium metal anode. However, a full-cell KIB constructed using a carbon-based anode and a K*-deficient layered cathode demon- strates low capacity because the cathode is the K*-ion reser- voir. To overcome these issues, K*-rich layered compounds can be synthesized by introducing suitable metal cations. Further- more, prepotassiation, as used in LIBs, can also be employed in KIBs. Kt-ion layered oxide compounds demonstrate steeper volt- age curves and more variation in stepwise voltage than their Lit and Na* counterparts mainly because strong K*t-K* repulsion causes Kt /vacancy ordering as a function of Kt-ion content and transition-metal charge ordering, thereby leading to numerous phase transitions and voltage steps. Further studies are required to design multimetal compounds with various redox-active and -inactive metals to suppress K*/ordering and obtain smoother voltage profiles for K*-ion layered oxide compounds.
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