Design of active sites in carbon materials for electrochemical potassium storage
摘要: 炭材料因低成本、无毒性和微观结构可调等优点被认为是最具应用潜力的钾离子电池负极材料，其电化学储钾行为与活性位点的类型密切相关。近年来，为了满足不同电化学储钾器件的应用需求，针对炭结构设计及其活性位点调控已取得大量研究进展。本文讨论了层间插层和离子吸附两种储钾机制的差异，以指导炭结构的合理设计。在此基础上，从库仑效率、容量、电位、倍率和稳定性等方面，综述了不同炭材料的活性位点演变规律及其对储钾性能的影响。同时，总结了炭材料用作钾离子全电池和钾离子电容器负极材料时的结构设计原则和储钾机制差异。并指出了炭材料储钾在活性位点设计方面需要解决的问题及今后研究和改进方向。Abstract: Carbon materials have attracted considerable attention as anodes for potassium ion batteries owing to their low-cost, nontoxicity, and controllable structures. The potassium storage behavior of carbon materials is highly associated with their active sites. In recent years, significant advances have been made in designing the active sites of carbon materials to meet the requirements of different potassium-based storage devices. Here, potassium storage mechanisms (intercalation and adsorption) for guiding the rational design of carbon materials are discussed. Based on these mechanisms, the review provides fundamental insight into the relationship between the structures and potassium storage performance of different carbon materials, including graphite, soft carbon, hard carbon, porous carbon, heteroatom-doped carbon, hybridized carbon and composited carbon. The structural design principles of carbon anode materials for potassium-ion full cell and potassium-ion capacitors are summarized based on the initial coulombic efficiency, capacity, potential plateau, rate performance, and cyclic stability. Finally, the problems and future research directions for the design of active sites in carbon materials for electrochemical potassium storage are considered.
Figure 4. (a) The initial discharge/charge profile of a graphite electrode at C/10 (1 C = 279 mA g-1) and (b) XRD patterns of electrodes corresponding to the marked stage compounds of charge in panel (a). (c) Waterfall representation XRD patterns at the first cycle and (d) the selected XRD patterns during the initial discharge/charge with different stages. (e) The dQ/dV profilesand (f) galvanostatic charge/discharge voltage profiles at 50 mA g-1 of CG and EG. (g) The structure of high-density K adsorbed around pores in graphene. (h) Electron charge density distribution differences and (i) the galvanostatic charge/discharge profiles of the NGF electrode(Reproduced with permission).
Figure 5. (a) Correlation between the microstructure of soft carbon and HTT. (b) The potassium storage mechanism of soft carbon at different stages during discharge. (c) Discharge/charge curves of soft carbon at different HTTs for cycles 1, 2 and 5. (d) The in-situ XRD spectra of SC-1200. (e) The discharge/charge profiles in the 1st cycle and cyclability test at 0.1 C of SC-1200, graphite, and HC-1200. (f) Schematic diagram of the major determinants affecting potential profiles of potassium storage. (g) Typical structure fragment and the calculated WAXS pattern based on RMC modeling. (h) Effect of the defect concentration on potential revealed by DFT calculation(Reproduced with permission).
Figure 6. (a) Correlation between the microstructure of CNFs and the annealing temperature. (b) The dQ/dV curves of CNF films after oxidation, the inset illustrates the main K+ storage mechanism in three different regions. (c) Selective in-situ Raman spectra of CNF-1250 with corresponding dQ/dV plot. The in-situ XRD pattern recorded on the first discharge/charge for (d) CNF-1250 and (e) CNF-2800. (f) Discharge/charge curves of CNFs at different working temperatures(Reproduced with permission).
Figure 7. TEM images and SAED patterns of hard carbon samples: (a) HC700 and (b) HC2000. (c) Schematic illustration of microstructures of the hard carbon samples prepared at 700 and 2000 °C. (d) Powder XRD patterns of carbon samples prepared at different HTTs. (e) Interlayer distances of carbon layers. (f) The charge-discharge curves of SP-HCs. (g) The dQ/dV curves of SP-HCs. (h) The K+ storage mechanism in hard carbon. (i) the capacities of sloping-voltage and plateau-voltage regions in relation to HTT. (j) Schematic diagram of K+ storage mechanism in CQDHC(Reproduced with permission).
Figure 8. (a) Schematic diagram of the synthesis procedure for the porous carbons by spray pyrolysis. TEM images of (b) HC-A, (c) HC-B and (d) HC-C. (e) Pore size distribution curves, (f) relationship between mesopore volume and ICE and (g) first charge/discharge curves of HC-A, HC-B and HC-C electrode at 0.1 A g−1. (h) In-situ Raman spectra of the HC-B electrode, and the corresponding variation of ID/IG ratio. (i) Capacitive contribution of different HCs during cycling process. (j) Scheme illustration of K+ storage mechanisms in HCs(Reproduced with permission).
Figure 9. (a) Schematic illustration of the synthesis process of NSCN. (b) SEM image of NSCN . (c) Schematic illustration for the synthesis of NPPC. (d) (i) SEM and (ii) high-magnification SEM images of NPPC. (e) Preparation principle of NHCS. (f) (i), (ii) SEM images, and (iii) TEM image of NHCS. (g) Synthesis protocol for hollow carbon nanospheres. (h) TEM image of the hollow carbon spheres by reaction for (i) 20 min and (ii) 40 min(Reproduced with permission).
Figure 10. (a) Schematic diagram of the synthesis of the P/C composite. (b) The cycling performance and (c) the rate performance of the P/C and pure graphite electrodes. (d) Schematic illustration of the structure evolution of P-CNT and P-CGCNT electrodes during cycling. (e) The capacity retention in different voltage ranges of P-CGCNT and P-CNT electrodes at the 5th and 60th cycles. (f) The cycling performance and (g) the rate performance of P-CGCNT, P-RCGCNT, and P-CNT electrodes(Reproduced with permission).
Figure 12. (a) Schematic illustration of the construction of CNSs. (b) Schematic illustration of the alignment of carbon atoms on the nanobubble surface to create the order-in-disorder structure. Schematic illustration of the K+ storage mechanism in CNSs obtained by (c) low and (d) high pyrolysis temperatures. (e) TEM images of CNS-1000. (f) Rate performance of CNS electrodes. (g) The variation of specific surface area, ID/IG and electric conductivity of CNSs with the pyrolysis temperature(Reproduced with permission).
Figure 13. (a) Schematic illustration of the preparation of the hard-soft carbon composites. (b) Unfavorable intercalation with accumulated K+ at one edge of graphite and local bending of the graphite layer locking K+. (c) Ex-situ XRD patterns and (d) Raman spectra of PI-700-P28 electrode at typical charge/discharge stages and their pristine states. (e) ID/IG ratios of PI-700-P28 electrode at different charge/discharge states. (f) GCD curves at 0.1 C of G (left), G-SC 3∶1 (middle) and SC (right). (g) Ex-situ XRD patterns of G, G-SC 3∶1 and SC. (h) Ex-situ Raman spectra of G, G-SC 3∶1 and SC (Reproduced with permission).
Figure 14. Potassium-ion full cell and potassium-ion hybrid capacitor based on the carbon electrode. (a) Schematic illustration of the graphite/PTCDA full cell. (b) Cycle stability at 30 mA g−1. (c) Charge/discharge curves at different current densities. (d) Schematic illustration of the hard carbon/KPB full-cell. (e) The long cycle performance of the full cell at 0.1 A g−1. (f) Schematic illustration for the working mechanism of PIHC. (g) Charge/discharge curves at 2 A g−1 and (h) prolonged cycling of the PIHC(Reproduced with permission).
Table 1. A comparison of the K+ storage performance for different carbon materials.
Capacity retention Mass loading
Ref. Graphite Graphite 273 at 0.028 A g−1 ~0.2 57.4 80 at 0.28 A g−1 50.8% after 50 cycles ~2  Expanded graphite 267 at 0.05 A g−1 ~0.3 81.6 175 at 0.2 A g−1 99% after 500 cycles ~2  N-doped graphitic carbon 266 at 0.05 A g−1 ~0.3 48.7 228.9 at 2 A g−1 188.9 after 2200 cycles 1-1.1  Polynanocr-ystalline graphite 224 at 0.02 A g−1 ~0.2 54.1 99 at 0.2 A g−1 50% after 240 cycles ~2  Graphite 260 at 0.028 A g−1 ~0.3 81.5 250 at 1.4 A g−1 99.9% after 300 cycles ~0.7  Soft carbon Soft carbon 264 at 0.028 A g−1 ~0.5 56.4 140 at 1.4 A g−1 81.4% after 50 cycles ~2 
N-doped carbon nanofibers
248 at 0.025 A g−1 ~0.65 49 101 at 20 A g−1 \ ~1.5  N-doped soft carbon 303 at 0.05 A g−1 ~0.5 30.9 141 at 5 A g−1 85.5% after 500 cycles ~1  N/S-doped soft carbon 359 at 0.1 A g−1 ~0.5 50 115 at 5 A g−1 79.5% after 200 cycles 1-1.2  Pitch-derived soft carbon (1200 °C) 296 at 0.028 A g−1 ~0.2 65 115.2 at 1.4 A g−1 93.2% after 50 cycles \  Pitch-derived soft carbon (1400 °C) 194 at 0.025 A g−1 ~0.27 70 110 at 0.5 A g−1 75% after 100 cycles ~2  Pitch-derived soft carbon (1500 °C) 240 at 0.028 A g−1 ~0.24 \ 80 at 1.4 A g−1 67.9% after 500 cycles ~2  Hard carbon Rhodanine-derived hard carbon 425 at 0.05 A g−1 \ \ 237.4 at 1 A g−1 90.4% after 10th to 400th cycles ~0.7  Biomass-derived hard carbon 442.4 at 0.03 A g−1 ~0.6 \ 175 at 2 A g−1 \ ~0.17  quantum dots in hard carbon 325 at 0.1 A g−1 \ 39 110 at 1 A g−1 75% after 150 cycles ~0.3  N/S-doped hard carbon 250 at 0.1 A g−1 ~0.5 35.2 174 at 3 A g−1 66% after 1200 cycles \  N/O-doped hard carbon 230 at 0.05 A g−1 \ 45.4 118 at 3 A g−1 72% after 1100 cycles ~0.9  N/P-doped hard carbon 312 at 0.05 A g−1 \ 44 179 at 5 A g−1 80% after 500 cycles \  Porous carbon Porous hard carbon 237.6 at 0.1 A g−1 ~0.8 44.4 81.6 at 2 A g−1 \ 1.2-1.6  Porous hard carbon 259 at 0.05 A g−1 ~0.5 44.6 214 at 2 A g−1 84% after 5000 cycles 1.1-1.3  Mesoporous carbon 460 at 0.05 A g−1 \ \ 110 at 4 A g−1 71.3% after 2000 cycles 0.8-1.2  Nanocapsules carbon 293 at 0.05 A g−1 ~0.5 30.9 151 at 5 A g−1 85.5% after 500 cycles ~1.  N/S co-doped nanocapsules carbon 408 at 0.05 A g−1 \ 33 149 at 5 A g−1 81% after 10th to 2000th cycles ~1.2 
N/P co-doped porous carbon
301 at 0.025 A g−1 ~0.5 60.04 126 at 10 A g−1 81.8% after 400 cycles 1-1.4  Hierarchical porous carbon 211.5 at 0.05 A g−1 \ 24.1 76.7 at 10 A g−1 53% after 100 cycles 0.8-1  Heteroatom-doping carbon N-doped carbon 248 at 0.025 A g−1 ~0.65 49 101 at 20 A g−1 \ ~1.5  S-doped carbon 310 at 0.05 A g−1 ~0.75 42 182.7 at 2 A g−1 68% after 2000 cycles ~2.4  P-doped carbon 323.5 at 0.05 A g−1 ~0.5 50.3 90 at 0.5 A g−1 53.7% after 500 cycles \  P-doped carbon 402.6 at 0.1 A g−1 ~0.75 \ 258 at 1 A g−1 \ \  N/O co-doped carbon 365 at 0.025 A g−1 \ 38 118 at 3 A g−1 69.5% after 1100 cycles ~0.9  N/S co-doped carbon 359 at 0.1 A g−1 \ 50 115 at 5 A g−1 92% after 1000 cycles 1-1.2  Hybrid and composite carbon sp2-sp3 carbon 425 at 0.05 A g−1 \ \ 237.4 at 1 A g−1 90.4% after 10th to 400th cycles ~0.7  sp-sp2 carbon 505 at 0.05 A g−1 \ \ 150 at 5 A g−1 90% after 2000 cycles ~0.8  Hard-soft carbon 376.8 at 0.025 A g−1 ~0.68 71.0 101.2 at 4 A g−1 \ ~1.2  Graphite-soft carbon 280 at 0.028 A g−1 ~0.8 67.3 \ \ 2-2.5  Graphite-hard carbon 253 at 0.05 A g−1 ~0.18 61.8 215.7 at 0.2 A g−1 97.5% after 1000 cycles \  Graphene-hard carbon 297.89 at 0.1 A g−1 ~0.25 \ 220 at 1 A g−1 99.4% after 3200 cycles ~1.2 
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