State-of-the-art anodes of potassium-ion batteries: synthesis, chemistry, and applications

Abstract

The growing demand for green energy has fueled the exploration of sustainable and eco-friendly energy storage systems. To date, the primary focus has been solely on the enhancement of lithium-ion battery (LIB) technologies. Recently, the increasing demand and uneven distribution of lithium resources have prompted extensive attention toward the development of other advanced battery systems. As a promising alternative to LIBs, potassium-ion batteries (KIBs) have attracted considerable interest over the past years owing to their resource abundance, low cost, and high working voltage. Capitalizing on the significant research and technological advancements of LIBs, KIBs have undergone rapid development, especially the anode component, and diverse synthesis techniques, potassiation chemistry, and energy storage applications have been systematically investigated and proposed. In this review, the necessity of exploring superior anode materials is highlighted, and representative KIB anodes as well as various structural construction approaches are summarized. Furthermore, critical issues, challenges, and perspectives of KIB anodes are meticulously organized and presented. With a strengthened understanding of the associated potassiation chemistry, the composition and microstructural modification of KIB anodes could be significantly improved.

Fig. 1. (a) Various features of potassium in batteries. (b) Literature distribution of KIB based research from 2008 to 2020 in the website of Web of Science, and (c) corresponding number of citations.

Fig. 2. Overview of the preparation and application of various KIB anodes.

Fig. 3. Preparation of KIB anodes through hydrothermal treatment. (a) Morphology of SnO₂ NSs grown on stainless steel mesh. Reproduced with permission. Copyright 2018, Elsevier. (b) Hydrothermal synthesis of oriented mesoporous graphitic carbon nanospring and its TEM image. Reproduced with permission. Copyright 2019, Wiley. (c) Schematic structure of hydrothermally synthesized MoSe₂/MXene@C. Reproduced with permission. Copyright 2019, American Chemical Society. (d) Synthesis process of the MoO₂/rGO composite as a PIB anode. Reproduced with permission. Copyright 2019, Wiley. (e) Geometric structure of (NH₄)₂Mo₃S₁₃; the yellow, cyan, blue, and white spheres represent S, Mo, N, and H atoms, respectively. Reproduced with permission. Copyright 2020, American Chemical Society. (f) Schematic illustration of the hydrothermally synthesized ZnSe CS/C. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (g) Morphology and elemental mapping of the hydrothermal carbon hollow bowls. Reproduced with permission. Copyright 2019, American Chemical Society. (h) Elemental distribution of molybdenum and sulfur in synthesized MoS₂ NWs. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (i) Schematic structure of hydrothermal MoS₂@rGO and (j) the flower-like MoS₂ without graphene. Reproduced with permission. Copyright 2019, Elsevier.

Fig. 4. Synthesis of KIB anodes via mechanical ball milling method. (a) Bright-field image and an overlay of ball mill-synthesized Co₃O₄–Fe₂O₃/C anode (color scheme: cobalt-green; iron-red; carbon-yellow). Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (b) Schematic diagram of the ball milling of the P/C composite. Reproduced with permission. Copyright 2018, Elsevier. (c) TEM image of the milled Sn₄P₃/C NPs. Reproduced with permission. Copyright 2017, American Chemical Society. (d) Overlay of the maps of phosphorus (red) and carbon (green). Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (e) Schematic ball milling of the antimony-graphite composites. Reproduced with permission. Copyright 2019, Wiley. (f) Schematic illustration of structural configuration of activated RP/C-based PIB electrode. Reproduced with permission. Copyright 2019, Wiley. (g) TEM and SEM images of the ball-milled RP/MoS₂ hybrid. Reproduced with permission. Copyright 2019, Wiley. (h) Ball milling process of SnP₃/C nanocomposite. Reproduced with permission. Copyright 2019, American Chemical Society.

Fig. 5. Preparation of KIB anodes through chemical etching process. (a) Schematic of the synthesis of a-Ti₃C₂ MNRs via etching method. Reproduced with permission. Copyright 2017, Elsevier. (b) Schematic illustrations of three etched nanoporous samples: np-Ge₃₀, np-Ge₂₀ and np-AlGe. Reproduced with permission. Copyright 2019, Elsevier. (c) Morphology images of Ti₃C₂ MXene, M-NTO, and M-KTO. Reproduced with permission. Copyright 2017, American Chemical Society. (d) Schematic illustrations of the fabrication process of BC–NCS by etching SiOC ceramic spheres using molten KOH. Reproduced with permission. Copyright 2019, Elsevier. (e) Schematic illustration of the acid etching process of graphitic nanocarbons. Reproduced with permission. Copyright 2019, Wiley. (f) TEM image of the silicon carbide-derived carbon anode synthesized using a concise etching approach. Reproduced with permission. Copyright 2020, Wiley.

Fig. 6. Synthesis of KIB anodes through facile annealing. (a) Schematic illustration of synthesis process of Bi@C samples through facile annealing. Reproduced with permission. Copyright 2020, Elsevier. (b) Structure of NTO/rGO films and its cross-sectional TEM image. Reproduced with permission. Copyright 2018, Wiley. (c) Nitrogen mapping (orange) of facile annealed N-doped carbon nanofibers, and (d) TEM image of their hollow structure. Reproduced with permission. Copyright 2018, Nature. (e) AFM image of pure chitin before annealing process. Reproduced with permission. Copyright 2017, Elsevier. (f) TEM image of the annealed Zn NPs confined in carbon network and their elemental mapping. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry. (g) Crystal structure of the facile annealed KVPO₄F anode. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (h) Schematic diagram of the annealing mechanism of the PMA–MA supermolecule. Reproduced with permission. Copyright 2020, Wiley.

Fig. 7. Co-precipitation process for the preparation of KIB anodes. (a) Schematic depicting the precipitation of the SnP_0.94@GO composite. Reproduced with permission. Copyright 2019, Elsevier. (b) SEM image of precipitated Na₂C₆H₂O₆. Reproduced with permission. Copyright 2018, Elsevier. (c) Morphology and schematic diagram of the Fe–Mo selenide@N-doped carbon anode. Reproduced with permission. Copyright 2018, Elsevier. (d) Morphology and Sb distribution in the Sb-NP@PC anode. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (e) Schematic illustration and (f) SEM image of Sb–Co precipitate. Reproduced with permission. Copyright 2019, Elsevier. (g) Schematic illustration and (h) microstructure of the nanoflake-interlaced carbon microspheres. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry.

Fig. 8. Electrospinning, spray drying, electrodeposition, and CVD. (a) Electrospinning process and the corresponding microstructure of Sb@CN nanofibers. Reproduced with permission. Copyright 2020, Elsevier. (b) As-spun preasphaltene/PAN (b1) and PAN (b2) fibers. Reproduced with permission. Copyright 2020, American Chemical Society. (c) SEM images of CuO/Cu-NCNFs. Reproduced with permission. Copyright 2020, Elsevier. (d) Schematic illustration of the spraying process and (e) corresponding elemental mapping of the CoTe₂–C composite. Reproduced with permission. Copyright 2020, Elsevier. (f) SEM of the obtained N-doped interconnected carbon spheres and the NaCl/C₆H₁₇N₃O₇ precursor for spraying. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (g) Morphology of the activated crumpled graphene after the spray drying process. Reproduced with permission. Copyright 2020, Wiley. (h) Schematic of the electrodeposition process of SnO₂@CF and (i) XRD spectrum after deposition. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (j) CVD for the fabrication of red P@AC. Reproduced with permission. Copyright 2020, Elsevier. (k) Microstructure of multiwalled hierarchical carbon nanotube after CVD. Reproduced with permission. Copyright 2018, Wiley. (l) CVD for the synthesis of graphitic carbon foam. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

Fig. 9. Potassium storage chemistry and applications of Sb-based alloying-type anodes. (a) High-magnification SEM image of MXene@Sb anode, and (b) cycling performances of MXene@Sb and bulk Sb. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (c) Schematic illustration of the solvothermal reaction and structural morphology of the Sb/CNS composite. Reproduced with permission. Copyright 2018, Elsevier. (d) Morphology of the Sb@PC anode. Reproduced with permission. Copyright 2018, Elsevier. (e) Electrochemical behavior of the Sb–C–rGO anode, and (f) its rate performance. Reproduced with permission. Copyright 2019, American Chemical Society. (g) Schematic illustration of the potassium storage behavior of 3D SbNPs@C. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry. (h) TEM image and Sb mapping of an individual Sb@CSN sphere, and (i) typical second charge/discharge profile at 50 mA g⁻¹. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

Fig. 10. Potassium storage chemistry and applications of Sn-, Bi-based alloying-type anodes. (a) Schematic synthesis and TEM image of the 3D-HPCS anode. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry. (b) Microstructure and (c) potassium storage performance of the Sn@RGO composite. Reproduced with permission. Copyright 2019, Elsevier. (d) Distribution of Sn in the obtained Sn–C composite. Reproduced with permission. Copyright 2016, The Royal Society of Chemistry. (e) Alloying and dealloying processes in Bi electrode. Reproduced with permission. Copyright 2018, Wiley. (f) Application of the Bi-based anode in a KIB full cell. Reproduced with permission. Copyright 2018, Wiley. (g) Schematic illustration and galvanostatic charge–discharge profiles of the Bi@3DGFs anode. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (h) TEM and (i) cycling performance of the Bi@C electrode. Reproduced with permission. Copyright 2019, Wiley. (j) SEM image and elemental mapping of carbon-coated double-shell bismuth hollow boxes. Reproduced with permission. Copyright 2019, Elsevier. (k) Schematic illustration of Bi@N-CT and its corresponding electrochemical performance. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (l) TEM image of a Bi-based composite structure comprising Bi nanorod networks confined in a N, S co-doped carbon matrix. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry.

Fig. 11. Potassium storage chemistry and applications of phosphorus-based alloying-type anodes. (a) TEM image of P@HC (75 wt% P content) and (b) its charge/discharge curves at various current densities. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (c) Schematic illustration of the synthesis process for RP@CN composite, and (d) its calculated formation energy. Reproduced with permission. Copyright 2018, Wiley. (e) Schematic illustration of the potassiation/depotassiation process of hollow carbon fibers coated with RP, and nanostructured RP confined in the N-PHCNF matrix, and their potassium storage performance. Reproduced with permission. Copyright 2019, American Chemical Society. (f) SEM and line scanning of P@TBMC and (g) its electrochemical performance. Reproduced with permission. Copyright 2018, Elsevier. (h) Rate capability for the black phosphorus/carbon electrodes. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry.

Fig. 12. Potassium storage chemistry and applications of conversion-type anodes. (a) Diffusion pathway of the K⁺ ions in the FeP structure, and its corresponding potassium storage performance. Reproduced with permission. Copyright 2019, American Chemical Society. (b) SEM image of CoS@G. Reproduced with permission. Copyright 2017, Wiley. (c) Surface morphology of the conversion-type Co₃O₄–Fe₂O₃ electrodes. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (d) TEM of the synthesized Cu_xS and Cu₂S@NC, and (e) the long cycling performance of Cu₂S@NC. Reproduced with permission. Copyright 2020, American Chemical Society. (f) SEM mapping characterization and (g) discharge and charge performances of the Mn–Fe–Se/CNT composite. Reproduced with permission. Copyright 2019, Elsevier. (h) SEM of the flower-like VPO₄ anode material. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

Fig. 13. Potassium storage chemistry and applications of intercalation-type anodes. (a) TEM of the ultrathin nanoribbons of potassium titanate and (b) its rate performance. Reproduced with permission. Copyright 2017, American Chemical Society. (c) Crystal structure of the M-KTO anode. Reproduced with permission. Copyright 2016, The Electrochemical Society. (d) Electron localization functions of Ti₂CO₂ with two K layers. Reproduced with permission. Copyright 2014, American Chemical Society. (e) Microstructure and projection of the K₂Ti₈O₁₇ structure along the [100] direction. Reproduced with permission. Copyright 2016, The Royal Society of Chemistry. (f) SEM of nanocubic KTi₂(PO₄)₃ and HRTEM image of KTi₂(PO₄)₃/C. Reproduced with permission. Copyright 2016, The Royal Society of Chemistry. (g) Comparison of K⁺-(de)intercalation in three different intercalation-type anodes. Reproduced with permission. Copyright 2019, Wiley. (h) Microstructure of hierarchical tubular TiO₂-carbon MTs. Reproduced with permission. Copyright 2019, Elsevier. (i) XPS spectra of the K₂Ti₆O₁₃ electrode at full potassiation (0.01 V) and depotassiation (3.0 V) states, and its corresponding potassium storage performance at various current densities. Reproduced with permission. Copyright 2019, Elsevier. (j) Illustration of PDDA-modified N-rich porous CNS/Ti₃C₂ hybrids. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry.

Fig. 14. Potassium storage chemistry and applications of anodes with a conversion–alloy mechanism. (a) SEM images and elemental mapping of SnO₂@CF with the photograph of the freestanding SnO₂@CF inserted. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (b) Schematic illustration of the 3D SnO₂@C and (c) its corresponding rate performance. Reproduced with permission. Copyright 2019, Elsevier. (d) SEM of the SnO electrode before and after cycling without cracks recognized. Reproduced with permission. Copyright 2018, American Chemical Society. (e) Morphology of SnO₂–G–C nanofibers and (f) its electrical conductivity measurement. Reproduced with permission. Copyright 2018, Elsevier. (g) Morphology and (h) rate capabilities of the SnS₂/graphene nanocomposite. Reproduced with permission. Copyright 2019, Elsevier. (i) Illustration of the synthesized SnS₂@C@rGO as the anode in a KIB. Reproduced with permission. Copyright 2019, Wiley. (j) SEM and (k) schematic diagram of the crystal structure of the BiOCl anode material. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

Fig. 15. Potassium storage chemistry and applications of anodes with conversion–alloy mechanism. (a) Microstructure of obtained SnS₂@rGO composite. Reproduced with permission. Copyright 2019, Wiley. (b) Electrochemical performance of SnS₂–rGO composite with an energy-filtered TEM image (color scheme: red-sulfur, green-carbon) inserted. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (c) Schematic crystal structure of SnS₂ in N, S–C/SnS₂ NSs, and its corresponding elemental mappings. Reproduced with permission. Copyright 2020, Elsevier. (d) Photographs of the Sb₂S₃–GO precursor solution and Sb₂S₃–SNG composite and (e) CV curves of Sb₂S₃–SNG/KVPO₄F–C full cells. Reproduced with permission. Copyright 2017, Springer. (f) Schematic representation of the nanocrystalline Sb₂S₃ coated by rGO NSs, and (g) charge/discharge curves at various current densities. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (h) Illustration of the potassium storage behavior of the Sn₄P₃@C active material. Reproduced with permission. Copyright 2019, Elsevier.

Fig. 16. Potassium storage chemistry and applications of conversion–decomposition-type anodes. (a) Morphology of crystal-pillar-like CuSe. (b) In situ XRD patterns of the CuSe electrode obtained during the first electrochemical cycle. (c) Charge–discharge curves and (d) long cycling performance of CuSe electrodes in KIB. Reproduced with permission. Copyright 2019, Wiley.

Fig. 17. Potassium storage chemistry and applications of intercalation–conversion-type anodes. (a) Microstructure of MXene and (b) subsequent MXene/MoS₂ composite. Reproduced with permission. Copyright 2019, Elsevier. (c) Illustration of VS₄@C precursor and final V₅S₈@C product, and (d) side view of K atom diffusion pathways in V₅S₈. Reproduced with permission. Copyright 2019, American Chemical Society. (e) Potassium storage reaction mechanism of FeS₂ anode, disclosing its reversible intercalation/deintercalation and conversion reactions during cycles. Reproduced with permission. Copyright 2018, Wiley. (f) Synthesis of the rGO@p-FeS₂@C composite and (g) discharge/charge profiles of its initial three cycles. Reproduced with permission. Copyright 2019, Elsevier. (h) Element mapping and (i) cycling performance of MoSe₂/N–C anode material. Reproduced with permission. Copyright 2018, Wiley.

Fig. 18. Potassium storage chemistry and applications of organic anodes. (a) EELS spectra of ADAPTS organic compound at low (left) and high (right) energy loss regimes. Reproduced with permission. Copyright 2018, Wiley. (b) Charge–discharge curves of K₄PTC@CNT anode during the three initial cycles. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (c) Long cycle profiles of K₂TP and K₂PC organic anodes for 100 cycles. Reproduced with permission. Copyright 2017, Elsevier. (d) Cycling performance of CoTP and CoTP/super P in K-ion cells (60 mA g⁻¹). Reproduced with permission. Copyright 2017, Elsevier. (e) Possible six-electron storage mechanism for organic PTCDI anode. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

Fig. 19. Potassium storage chemistry and applications of graphite anodes. (a) Focused ion beam-based GB sample preparation (left) and its cross-sectional view (right). (b) XRD patterns derived from DFT calculations for KC_x models. Reproduced with permission. Copyright 2019, Wiley. (c) Cycling performance of graphite with two different electrolytes in KIB cells. Reproduced with permission. Copyright 2019, Wiley. (d) HRTEM image, (e) response in different electrolytes and (f) cycling performance of graphite anode material. Reproduced with permission. Copyright 2018, Elsevier.

Fig. 20. Potassium storage chemistry and applications of non-graphite carbonaceous anodes. (a) Microstructure of phosphorus and oxygen dual-doped graphene anode. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (b) Schematic diagram of the hybrid potassium storage route and the protection barrier formation on crystal carbon@graphene microsphere during the cycling process. Reproduced with permission. Copyright 2019, American Chemical Society. (c) SEM images of sulfur-grafted carbon material with (d) hollow sphere morphology. Reproduced with permission. Copyright 2019, Wiley. (e) Long cycling performance of the hollow carbon nanospheres and activated hollow carbon nanospheres at 2 A g⁻¹. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry. (f) Two-step wrinkle formation mechanism of carbon tube-based anode, (g) illustration of assembled K-ion cell and (h) its corresponding rate performance. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry.

Fig. 21. Potassium storage chemistry and applications of non-graphite carbonaceous anodes. (a) Morphology of 3D interconnected nitrogen-doped hierarchical porous carbon and (b) its cycling performance. Reproduced with permission. Copyright 2019, American Chemical Society. (c) Microstructure of nitrogen-doped porous carbon and (d) density of states in different carbon structures. Reproduced with permission. Copyright 2018, Wiley. (e) SEM and photograph images of highly graphitized carbon nanofibers. Reproduced with permission. Copyright 2020, Elsevier. (f) Illustration of tri-doped carbon for KIB from AHF@COF and (g) its Raman spectrum. Reproduced with permission. Copyright 2020, Wiley. (h) Potassium storage capability of nitrogen-rich porous CNSs. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry.

Fig. 22. Full cell assembly with counterpart cathodes (layered cathodes and hexacyanometallate cathodes). (a) SEM images of P2-K_0.41CoO₂ and (b) charge and discharge curves of K//P2-K_0.41CoO₂ cell. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (c) TEM images and the corresponding EDX mapping of P3-K_xCrO₂. Reproduced with permission. Copyright 2018, The Royal Society of Chemistry. (d) Schematic illustration of a K-ion full cell using a NO-WCT anode and P3-K_0.69CrO₂ cathode, and (e) the corresponding deep cycling performance. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (f) SEM image of potassium iron hexacyanoferrate dihydrate particles. Reproduced with permission. Copyright 2016, Wiley. (g) Face-centered cubic structure of NiHCF with Prussian blue crystal structure. Reproduced with permission. Copyright 2011, American Chemical Society. (h) Structural illustration and (i) morphology of KMHCF powder. Reproduced with permission. Copyright 2017, American Chemical Society. (j) Voltage profiles of KMFCN cathode and KTP@C anode, and (k) the corresponding charge/discharge curve of potassium full cell. Reproduced with permission. Copyright 2019, Wiley.

Fig. 23. Full cell assemble with counterpart cathodes (polyanionic cathodes and organic cathodes). (a) Morphology of bare KFSF (up) and KFSF@G (down). Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (b) The elemental distribution in K₃V₂(PO₄)₃/C. Reproduced with permission. Copyright 2017, The Royal Society of Chemistry. (c) Morphology of robust KVP₂O₇ cathode and the corresponding phosphorus distribution, and (d) its activation energy barrier. Reproduced with permission. Copyright 2018, Wiley. (e) Schematic working mechanism and (f) its galvanostatic charge and discharge curve of KIB full cell. Reproduced with permission. Copyright 2020, The Royal Society of Chemistry. (g) TEM images of PTCDA rod, and (h) charge/discharge profiles of the PTCDA electrode for the 2nd to 5th cycle. Reproduced with permission. Copyright 2015, Elsevier. (i) Unit molecular structure and (j) morphology of PAQS. Reproduced with permission. Copyright 2016, Elsevier. (k) Schematic diagram of the full cell with graphite anode and PTCDA cathode, and (l) its cycle stability at current density of 30 mA g⁻¹. Reproduced with permission. Copyright 2019, Wiley.

Fig. 24. Current main challenges in KIB application. (a) SEM images of the unstable Sb–C electrode. Reproduced with permission. Copyright 2019, Elsevier. (b) Unstable cycling performance of the ball-milled graphite electrode. Reproduced with permission. Copyright 2019, The Royal Society of Chemistry. (c) Performance of PFSA-K membrane and pristine separator at −20 °C and 150 °C. Reproduced with permission. Copyright 2020, Elsevier. (d) Cycling performance of SnSb/C with CMC + PAA binder and various electrolytes. Reproduced with permission. Copyright 2020, Elsevier. (e) Battery combustion image. Reproduced with permission. Copyright 2020, Elsevier. (f) Flammability test for TMP and EC-DEC based electrolyte. Reproduced with permission. Copyright 2020, Wiley. (g) Schematic illustration of SEI phase change during the charge/discharge process of BPCS anode. Reproduced with permission. Copyright 2020, American Chemical Society. (h) TEM images of the interface layer between graphite and DEGDME after 20 cycles. Reproduced with permission. Copyright 2019, Elsevier. (i) Optical images of the pristine graphite and potassiated graphite with LCE, HCE, and LHCE, and (j) schematic illustrations of solution structures of LCE, HCE, and LHCE. Reproduced with permission. Copyright 2019, Wiley.