Ampere-hour-scale soft-package potassium-ion hybrid capacitors enabling 6-minute fast-charging

The study addresses the challenge of achieving extreme fast charging (≤10 min) in ampere-hour-scale energy storage devices, which is difficult for conventional lithium-ion batteries due to their rocking-chair intercalation mechanism, safety concerns at high current densities, and resource constraints. Potassium-ion systems offer advantages including abundant resources, comparable redox potential to lithium, lower propensity for metal deposition in carbonate electrolytes, and high ionic conductivity arising from small Stokes radius. However, K-ion batteries face issues such as sluggish intercalation kinetics, electrode volume changes, and cathode material limitations. The authors propose potassium-ion hybrid supercapacitors (PIHCs) that pair a capacitor-type cathode with a battery-type anode to combine high power and high energy while mitigating rocking-chair limitations. The research question is whether a holistic design—simultaneously tailoring electrode kinetics toward pseudocapacitive behavior, eliminating binders/current collectors to reduce parasitic reactions, and optimizing electrolyte parameters—can deliver Ah-scale PIHCs capable of 6-minute full charging with high energy density and stability.

Prior work has demonstrated coin-cell PIHCs with promising performance and recent attention to pouch cells with improved formation protocols. Nonetheless, commercialization is limited by capacity and kinetics mismatch between cathodes (insufficient capacitance though fast) and anodes (graphite with narrow 0.34 nm interlayer spacing leading to slow K+ intercalation), as well as electrolyte-induced corrosion of current collectors and binder side reactions at >4 V vs K/K+. Literature also reports strategies to improve K-ion storage via heteroatom-doped carbons, expanded interlayer spacing, and pseudocapacitive mechanisms, but practical Ah-scale devices with extreme fast charging remain scarce. These gaps motivate the present holistic optimization of materials, cell configuration, and electrolyte to bridge lab-scale advances and practical devices.

Materials and electrode design:

• Positive electrode (cathode): An ordered nitrogen-doped carbon nanotube array on mesoporous carbon (N-CNTs@MC) was derived from corncob biomass. Fe2O3-(N-CNTs)@MC was synthesized via Fe-catalyzed growth using melamine and FeSO4, followed by acid etching (1 M HCl, 60 °C, 4 h) and ultra-high-temperature annealing at 3000 °C in N2 to yield N-CNTs@MC with high graphitization. The composite exhibits a high BET surface area (1806.3 m2 g−1) and mesoporous structure (~5 nm and ~30 nm pores) with graphitic-N (~4.3 at.%). A free-standing film was fabricated by adding 10 wt% GO prior to annealing to obtain N-CNTs@MC-rGO and rolling to controlled thickness.
• Negative electrode (anode): MnO quantum dots inlaid carbon nanotubes (MnO@CNTs) were synthesized by a molecular beam template method: starch dispersed in water to form molecular beams, oxidized by KMnO4 (0.5 g), mixed with melamine (5 g), and annealed at 900 °C (N2, 2 h). MnO QDs embed between graphitic layers, expanding interlayer spacing to ~0.413 nm. A free-standing MnO@CNTs-rGO film (10 wt% rGO) with high conductivity and tunable thickness (5–20 µm) was prepared similarly.
• Binder/current-collector-free design: Both electrodes are free-standing conductive carbon papers (no metal foils, no polymer binder) to avoid electrolyte corrosion and side reactions and reduce inactive mass. Electrodes are connected to Al (positive) and Ni (negative) tabs in wound soft-pack architecture.

Electrochemical testing:

• Half-cells: 2032 coin cells assembled in Ar glovebox (<0.1 ppm O2/H2O). For N-CNTs@MC//K: electrolyte 1 M KPF6 in DMC/EMC (3:4 v/v), glass fiber separator, areal loading ~2.0 mg cm−2, ~30 µL electrolyte; voltage 3.0–4.8 V vs K/K+. For MnO@CNTs//K: 1 M KPF6 in DMC/DEC (1:1 v/v), areal loading ~1 mg cm−2, ~15 µL electrolyte. Pre-potassiation of MnO@CNTs anode performed via CV (0–3.0 V vs K/K+, 0.1 mV s−1, 3–5 cycles) until stabilization (<1% capacity loss).
• Full coin cells (PIHCs): N-CNTs@MC positive and pre-potassiated MnO@CNTs negative. Mass ratio (positive:negative active) ~3:1; areal loadings ≈3.0 mg cm−2 (positive) and ≈1.0 mg cm−2 (negative). Tested 0–4.8 V by galvanostatic cycling and CV; EIS performed at OCV (10 mV amplitude, 10 kHz–10 mHz).
• Pouch cells (Ah scale): Wound soft-pack using free-standing electrodes and separator layers, tab-welded to Al/Ni leads, injected with optimized electrolyte amount (E/C and density optimized). The 1 Ah cell total mass 18.76 ± 0.5 g (battery core 11.0 ± 0.2 g; electrolyte 4.5 ± 0.3 g). Testing at 25 ± 1 °C; rate tests up to 10 C; fast-charging protocol CC at 10 C for 5 min + CV hold at 4.8 V for 1 min; discharge at 1 C.

Characterization and computations:

• Morphology and structure: SEM, TEM, Cs-corrected STEM-HAADF, XRD, Raman, FTIR, XPS, BET. Conductivity measured via Hall effect at 300 K; N-CNTs@MC-rGO film conductivity (3.17 ± 0.01) × 10^5 S m−1.
• DFT (VASP): PBE-GGA, PAW, 500 eV cutoff, 5×5×1 k-mesh, structures relaxed to 10−5 eV convergence, ≥10 Å vacuum. Adsorption energies evaluated for PF6− on CNTs and N-doped CNTs, and K+ on graphene vs MnO@graphite; diffusion barriers via CI-NEB.

Energy/power calculations: E = ∫ I·V(t) dt / m; P = E / t. Mass basis: half-cells by working electrode; full coin cells by positive electrode mass; pouch cells by whole device.

• Materials and kinetics:
  • N-CNTs@MC positive exhibits high surface area (1806.3 m2 g−1), mesoporosity, and pseudocapacitive behavior (≈42% contribution at 2 mV s−1). Charge-transfer resistance reduced vs MC (~21 Ω vs ~32 Ω). PF6− adsorption stronger on N-doped CNTs (adsorption energy −10.21 eV) than on pure CNTs (−2.19 eV), supporting fast surface redox.
  • MnO@CNTs negative displays expanded interlayer spacing (~0.413 nm) and strong pseudocapacitive dominance (b ≈ 0.92; capacitive contributions rising from 64.8% at 0.1 mV s−1 to 96.6% at 20 mV s−1). Rate capacities (mAh g−1): 383.3, 338.2, 306.9, 264.6, 233.2, 226.5, 198.6 at 0.2, 0.5, 2, 5, 10, 20, 40 A g−1, far exceeding MWCNTs.
  • DFT: K+ adsorption stronger on MnO@graphite (≈−4.76 eV) vs graphene (≈−1.26 eV); K+ diffusion barrier much lower on MnO@graphite (≈0.92 eV) vs graphene (≈6.83 eV), explaining rapid intercalation.
• Half-cell performances:
  • N-CNTs@MC//K capacities (mAh g−1) at 1–20 A g−1: ~122.1, 104.8, 96.2, 90.3, 82.5, 75.4, 67.9 with excellent rate; ~92.8 mAh g−1 retained after 500 cycles at 5 A g−1.
• Full coin-cell PIHCs (N-CNTs@MC//MnO@CNTs):
  • Stable operation 0–4.8 V with deformed isosceles triangular charge–discharge profiles indicative of hybrid capacitive storage.
  • Rate capacities (mAh g−1, based on positive electrode) at 1, 2, 4, 6, 8, 10 A g−1: 120.6, 101.7, 92.3, 78.4, 69.1, 52.8.
  • Fast-charge test: full charge in ~16 s at 10 A g−1; discharge ~800 s at 0.2 A g−1; Coulombic efficiency ≈100%; specific power ≈26 kW kg−1 (positive-electrode basis).
  • Long-term cycling: ~90% capacity retention after 10,000 cycles at 10 A g−1; low self-discharge and strong voltage retention superior to MC//SC K-ion full cells.
• 1 Ah soft-pack pouch cells:
  • High voltage up to 4.8 V; device-level specific energy ≈140 Wh kg−1 (whole device mass basis) under 10 C; total cell mass 18.76 ± 0.5 g.
  • Fast charging: 10 C CC for 5 min + 1 min CV reaches full charge; can power a mobile phone.
  • Rate and cycling: after ~100 cycles at 10 C, capacity at 5 C returns to ~1 Ah; average CE ≈99.96% after activation; 88% capacity retention after 200 cycles at 10 C (0.88 Ah). Voltage retention ~99% at 25 ± 1 °C.
• Conductive, free-standing electrodes show electronic conductivity ~3.17 × 10^5 S m−1, comparable to Cu foil, enabling binder-/collector-free architecture and minimized inactive mass.

The work demonstrates that matching both electrodes toward pseudocapacitive kinetics—via N-doping and defect engineering in the cathode to enhance anion adsorption/redox and via MnO quantum-dot inlay to expand interlayer spacing and provide surface redox in the anode—overcomes the conventional capacity and kinetic mismatch of PIHCs. This design enables rapid, surface-dominated charge storage with minimal diffusion limitations, supporting extreme fast charging. Removing metal current collectors and polymer binders eliminates high-voltage corrosion and parasitic reactions, reduces inactive weight, and retains high electronic conductivity using free-standing carbon papers. Optimizing the electrolyte composition and amount (E/C ratio, density) further stabilizes operation up to 4.8 V, lowers self-discharge, and sustains high CE. Collectively, these strategies directly address the research goal: delivering Ah-scale PIHCs capable of 6-minute full charging while achieving device-level specific energy comparable to K-ion batteries and approaching Li-ion battery values, with strong cycling stability. The findings substantiate PIHCs as a viable pathway for fast-charging, high-energy storage in practical pouch-cell formats.

The study introduces a holistic strategy for potassium-ion hybrid capacitors that yields 1 Ah soft-pack devices with a 4.8 V window, 140 Wh kg−1 device-level specific energy, 6-minute full-charge capability (10 C), and robust cycling (88% capacity retention after 200 cycles at 10 C; low self-discharge and ~99% voltage retention at 25 °C). Key contributions include: (i) cathode/anode co-design toward pseudocapacitive behavior (N-doped, defect-rich N-CNTs@MC and spacing-expanded MnO@CNTs); (ii) binder- and current-collector-free, highly conductive free-standing electrodes; (iii) electrolyte optimization for high-voltage stability and minimal parasitic reactions. These approaches provide a pathway to practical, fast-charging K-based hybrid capacitors and offer transferable design principles for Li/Na batteries and hybrid systems. Future research could adapt the strategy to lower-cost carbon/expanded graphite materials, scale manufacturing, assess broader safety and environmental conditions, and optimize electrolytes for even higher voltage stability and longevity.

• Pouch-cell cycling data are reported to 200 cycles at 10 C; longer-term durability and calendar aging are not provided.
• Operation was assessed at 25 ± 1 °C; performance and safety under wider temperature ranges and abuse conditions were not detailed.
• The negative electrode requires pre-potassiation; practical integration of this step at scale may add process complexity.
• The N-CNTs@MC synthesis includes ultra-high-temperature (3000 °C) treatment, which may pose cost/energy challenges for large-scale manufacturing.
• High-voltage operation (to 4.8 V) in carbonate electrolytes, while stable here, may require further validation for extended lifetimes and varying duty cycles in real-world applications.
• Comprehensive safety testing (e.g., nail penetration, overcharge, thermal runaway) and environmental/aging effects on self-discharge were not reported.