Integrating Chemical Pre-Potassiation with Pre-Modulated KF-Rich Electrolyte Interfaces for Dual-Carbon Potassium Ion Hybrid Capacitor

Ultrahigh Pyridinic/Pyrrolic N Enabling N/S Co-Doped Holey Graphene with Accelerated Kinetics for Alkali-Ion Batteries

Carbonaceous materials hold great promise for K-ion batteries due to their low cost, adjustable interlayer spacing, and high electronic conductivity. Nevertheless, the narrow interlayer spacing significantly restricts their potassium storage ability. Herein, hierarchical N, S co-doped exfoliated holey graphene (NSEHG) with ultrahigh pyridinic/pyrrolic N (90.6 at.%) and large interlayer spacing (0.423 nm) is prepared through micro-explosion assisted thermal exfoliation of graphene oxide (GO). The underlying mechanism of the micro-explosive exfoliation of GO is revealed. The NSEHG electrode delivers a remarkable reversible capacity (621 mAh g-1 at 0.05 A g-1), outstanding rate capability (155 mAh g-1 at 10 A g-1), and robust cyclic stability (0.005% decay per cycle after 4400 cycles at 5 A g-1), exceeding most of the previously reported graphene anodes in K-ion batteries. In addition, the NSEHG electrode exhibits encouraging performances as anodes for Li-/Na-ion batteries. Furthermore, the assembled activated carbon||NSEHG potassium-ion hybrid capacitor can deliver an impressive energy density of 141 Wh kg-1 and stable cycling performance with 96.1% capacitance retention after 4000 cycles at 1 A g-1. This work can offer helpful fundamental insights into design and scalable fabrication of high-performance graphene anodes for alkali metal ion batteries.

Revealing the Effect of the Microstructure on Potassium Storage Behavior in a Two-Dimensional Mesoporous Carbon Anode

Hard carbon is considered as the most promising anode material for potassium-ion energy storage devices. Substantial progress has been made in exploring advanced hard carbons to solve the issues of sluggish kinetics and large volume changes caused by the large radius of K+. However, the relationship between their complicated microstructures and the K+ charge storage behavior is still not fully explored. Herein, a series of two-dimensional mesoporous carbon microcoins (2D-MCMs) with tunable microstructures in heteroatom content and graphitization degree are synthesized by a facile hard-template method and follow a temperature-controllable annealing process. It is found that high heteroatom content makes for surface-driven K+ storage behavior, which increases the capacity-contribution ratio from a high potential region, while a high graphitization degree makes for K+ intercalation behavior, which increases the capacity-contribution ratio from a low potential region. Electrochemical results from a three-electrode Swagelok cell demonstrate that a 2D-MCM anode with more capacity contribution from a low working region allows the porous carbon cathode to be operated in a much wider electrochemical window, thus storing more charge. As a result, potassium-ion capacitors based on the optimized 2D-MCM anode deliver a high energy density of 113 Wh kg-1 and an exhilarating power density of 51,000 W kg-1.

Halide-mediated endogenous ZnO domain-confined etching strategy: Realizing superior potassium storage in carbon anode

Coupling sites of nitrogen-dopants and intrinsic carbon defects (N/DC) are highly attractive to improve potassium-storage capacity and cycling stability, yet it is hard to effectively construct them. Herein, a novel strategy is proposed to establish abundant N/DC sites in N-doped carbon (ZIF8/NaBr-1-900) by pyrolyzing the mixture of metal-organic framework (ZIF8)/sodium bromide (NaBr). Systematic investigations disclose that the introduced NaBr can promote the full conversion of Zn-N4 moieties into zinc oxide (ZnO) via a "bait and switch" mechanism. Such formed endogenous ZnO can etch the carbon matrix of the confined domains around the N dopants during pyrolysis process, and meanwhile the released N-atoms from Zn-N4 moieties can largely form edge-N. As such, these N/DC coupling sites enable the resultant carbon to have a more significant capacitive behavior related to fast K-ion migration and high structural stability, leading to 255.3 mAh/g at 2 A/g with a prolonged cycle lifespan over 2000 cycles. Moreover, the assembled K-full battery presents a high energy density of 171.2 Wh kg-1 and excellent cyclability over 5000 cycles. This NaBr-mediated endogenous ZnO domain-confined etching strategy provides a new insight into the exploration of advanced carbon anode.

Ultrastable Graphite-Potassium Anode through Binder Chemistry

Graphite with abundant reserves has attracted enormous research interest as an anode of potassium-ion batteries (PIBs) owing to its high plateau capacity of 279 mAh g-1 at ≈0.2 V in conventional carbonate electrolytes. Unfortunately, it suffers from fast capacity decay during K+ storage. Herein, an ultrastable graphite-potassium anode is developed through binder chemistry. Polyvinyl alcohol (PVA) is utilized as a water-soluble binder to generate a uniform and robust KF-rich SEI film on the graphite surface, which can not only inhibit the electrolyte decomposition, but also withstand large volume expansion during K+ -insertion. Compared to the PVDF as binder, PVA-based graphite anode can operate for over 2000 cycles (running time of 406 days at C/3) with 97% capacity retention in KPF6 -based electrolytes. The initial Coulombic efficiency (ICE) of graphite anode is as high as 81.6% using PVA as the binder, higher than that of PVDF (40.1%). Benefiting from the strong adhesion ability of PVA, a graphite||fluorophosphate K-ion full battery is further built through 3D printing, which achieves a record-high areal energy of 8.9 mWh cm-2 at a total mass loading of 38 mg cm-2 . These results demonstrate the important role of binder in developing high-performance PIBs.

Co-activation for enhanced K-ion storage in battery anodes

The relative natural abundance of potassium and potentially high energy density has established potassium-ion batteries as a promising technology for future large-scale global energy storage. However, the anodes' low capacity and high discharge platform lead to low energy density, which impedes their rapid development. Herein, we present a possible co-activation mechanism between bismuth (Bi) and tin (Sn) that enhances K-ion storage in battery anodes. The co-activated Bi-Sn anode delivered a high capacity of 634 mAh g-1, with a discharge plateau as low as 0.35 V, and operated continuously for 500 cycles at a current density of 50 mA g-1, with a high Coulombic efficiency of 99.2%. This possible co-activation strategy for high potassium storage may be extended to other Na/Zn/Ca/Mg/Al ion battery technologies, thus providing insights into how to improve their energy storage ability.