Polynanocrystalline Graphite: A New Carbon Anode with Superior Cycling Performance for K‑Ion Batteries

We synthesized a new type of carbonpolynanocrystalline graphiteby chemical vapor deposition on a nanoporous graphenic carbon as an epitaxial template. This carbon is composed of nanodomains being highly graphitic along <i>c</i>-axis and very graphenic along <i>ab</i> plane directions, where the nanodomains are randomly packed to form micron-sized particles, thus forming a polynanocrystalline structure. The polynanocrystalline graphite is very unique, structurally different from low-dimensional nanocrystalline carbon materials, e.g., fullerenes, carbon nanotubes, and graphene, nanoporous carbon, amorphous carbon and graphite, where it has a relatively low specific surface area of 91 m²/g as well as a low Archimedes density of 0.92 g/cm³. The structure is essentially hollow to a certain extent with randomly arranged nanosized graphite building blocks. This novel structure with disorder at nanometric scales but strict order at atomic scales enables substantially superior long-term cycling life for K-ion storage as an anode, where it exhibits 50% capacity retention over 240 cycles, whereas for graphite, it is only 6% retention over 140 cycles

A General Phase-Transfer Protocol for Mineral Acids and Its Application in the Large-Scale Synthesis of Highly Nanoporous Iron Phosphate in Nonaqueous Solvent

As a general protocol for transferring mineral acids from an aqueous solution to an organic phase, mineral acids are extracted with secondary carbon primary amine (C_9–11)₂CHNH₂ (commercial code: N1923) into an organic phase (e.g., heptane or benzene) because of the complexation reaction and the formation of typical reversed micelles. Based on this principle, a novel approach for a large-scale synthesis of highly nanoporous iron phosphate particles is developed via the formed RNH₃⁺/H₂PO₄^– (H₂O)/oil reversed micelle system and ethanol–Fe³⁺ solutions. Synthetic conditions, such as H₃PO₄ concentration in reversed micelles and Fe³⁺ concentration in ethanol–Fe³⁺ solution are investigated and optimized. The product is characterized using transmission electron microscopy, Brunauer–Emett–Teller, thermogravimetric analysis, X-ray diffraction, and Fourier transform infrared spectroscopy. The as-obtained iron phosphate is flocculent and highly porous, exhibiting a high reported surface area of 144 m²/g. The synthetic procedure is relatively simple and is suitable for large-scale fabrication, and the used organic amines can be recycled. The power of this approach is demonstrated using other kinds of organic amines, such as tri-n-octylamine (TOA) and tri-C_8–10-alkylmethyl ammonium chloride (N263), as phase-transfer reagents exhibiting promising application in the synthesis of highly nanoporous materials

Control of Gradient Concentration Prussian White Cathodes for High-Performance Potassium-Ion Batteries

Owing to their abundant resources and low cost, potassium-ion batteries (PIBs) have become a promising alternative to lithium-ion batteries (LIBs). However, the larger ionic radius and higher mass of K+ propose a challenging issue for finding suitable cathode materials. Prussian whites (PWs) have a rigid open framework and affordable synthesis method, but they suffer quick capacity fade due to lattice volume change and structural instability during K+ insertion/extraction. Here, we prepared controllable gradient concentration KxFeaNibMn1–a–b[Fe(CN)6]y·zH2O particles via a facile coprecipitation process, demonstrating high-performance potassium-ion storage. The high-Mn content in the interior can minimize capacity loss caused by electrochemically inert Ni and achieve a high reversible capacity; meanwhile, the high-FeNi content in the exterior can alleviate the volume change of the core material upon cycling, thus enhancing structural stability. Taking the above synergistic effect, the controllable gradient concentration PWs deliver a high reversible capacity of 109.8 mAh g–1 at 100 mA g–1 and good capacity retention of 77.8% after 200 cycles. The gradient concentration PWs can retain structural integrity and stability during long-term cycling. This work provides a prospective strategy to fabricate PWs with stable structure and excellent electrochemical performance for developing high-performance PIBs

Electrochemically Expandable Soft Carbon as Anodes for Na-Ion Batteries

Na-ion batteries (NIBs) have attracted great attention for scalable electrical energy storage considering the abundance and wide availability of Na resources. However, it remains elusive whether carbon anodes can achieve the similar scale of successes in Na-ion batteries as in Li-ion batteries. Currently, much attention is focused on hard carbon while soft carbon is generally considered a poor choice. In this study, we discover that soft carbon can be a high-rate anode in NIBs if the preparation conditions are carefully chosen. Furthermore, we discover that the turbostratic lattice of soft carbon is electrochemically expandable, where <i>d</i>-spacing rises from 3.6 to 4.2 Å. Such a scale of lattice expansion only due to the Na-ion insertion was not known for carbon materials. It is further learned that portions of such lattice expansion are highly reversible, resulting in excellent cycling performance. Moreover, soft carbon delivers a good capacity at potentials above 0.2 V, which enables an intrinsically dendrite-free anode for NIBs

Insights on the Mechanism of Na-Ion Storage in Soft Carbon Anode

Graphite is the commercial anode for lithium-ion batteries; however, it fails to extend its success to sodium-ion batteries. Recently, we demonstrated that a low-cost amorphous carbonsoft carbon exhibits remarkable rate performance and stable cycling life of Na-ion storage. However, its Na-ion storage mechanism has remained elusive, which has plagued further development of such carbon anodes. Here, we remedy this shortfall by presenting the results from an integrated set of experimental and computational studies that, for the first time, reveal the storage mechanism for soft carbon. We find that sodium ions intercalate into graphenic layers, leading to an irreversible quasi-plateau at ∼0.5 V versus Na⁺/Na as well as an irreversible expansion seen by in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD). Such a high-potential plateau is correlated to the defective local structure inside the turbostratic stacking of soft carbon and the associated high-binding energies with Na ions, suggesting a trapping mechanism. On the other hand, soft carbon exhibits long sloping regions above and below the quasi-plateau during the first sodiation, where the sloping regions present highly reversible behavior. It is attributed to the more defects contained by soft carbon revealed by neutron total scattering and the associated pair distribution function studies

Insights on the Mechanism of Na-Ion Storage in Soft Carbon Anode

Graphite is the commercial anode for lithium-ion batteries; however, it fails to extend its success to sodium-ion batteries. Recently, we demonstrated that a low-cost amorphous carbonsoft carbon exhibits remarkable rate performance and stable cycling life of Na-ion storage. However, its Na-ion storage mechanism has remained elusive, which has plagued further development of such carbon anodes. Here, we remedy this shortfall by presenting the results from an integrated set of experimental and computational studies that, for the first time, reveal the storage mechanism for soft carbon. We find that sodium ions intercalate into graphenic layers, leading to an irreversible quasi-plateau at ∼0.5 V versus Na⁺/Na as well as an irreversible expansion seen by in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD). Such a high-potential plateau is correlated to the defective local structure inside the turbostratic stacking of soft carbon and the associated high-binding energies with Na ions, suggesting a trapping mechanism. On the other hand, soft carbon exhibits long sloping regions above and below the quasi-plateau during the first sodiation, where the sloping regions present highly reversible behavior. It is attributed to the more defects contained by soft carbon revealed by neutron total scattering and the associated pair distribution function studies

Defective Hard Carbon Anode for Na-Ion Batteries

Hard carbon as an anode is critical for the near-future commercialization of Na-ion batteries. However, where Na ions are located at different states of charge with respect to the local structures of hard carbon remains a topic that is under debate. Recently, some groups, including ours, have suggested a structure–property correlation that assigns the slope capacity in galvanostatic charge/discharge curves to the binding of Na ions to structural defects of hard carbon. To test this correlation, herein, we prepared a highly defective hard carbon by microwaving a carbon that was obtained by pyrolysis of cellulose at 650 °C. After this microwave treatment for just 6 s, the reversible capacity of the hard carbon increased from 204 to 308 mAh/g, which is significantly higher than that of hard carbon annealed at 1100 °C for 7 h (274 mAh/g). The microwave treatment not only is energy-efficient but also retains a high extent of the structural vacancies in hard carbon, as demonstrated by neutron total scattering and the associated pair distribution function results. Indeed, such a defective structure exhibits a slope capacity much higher than that of the conventional hard carbon. This work serves as one of the first examples of rationally designed hard carbon guided by the new Na-ion storage mechanism. Furthermore, microwave heating represents a promising strategy for fine-tuning the structures of hard carbon for Na-ion batteries

Low-Surface-Area Hard Carbon Anode for Na-Ion Batteries via Graphene Oxide as a Dehydration Agent

Na-ion batteries are emerging as one of the most promising energy storage technologies, particularly for grid-level applications. Among anode candidate materials, hard carbon is very attractive due to its high capacity and low cost. However, hard carbon anodes often suffer a low first-cycle Coulombic efficiency and fast capacity fading. In this study, we discover that doping graphene oxide into sucrose, the precursor for hard carbon, can effectively reduce the specific surface area of hard carbon to as low as 5.4 m²/g. We further reveal that such doping can effectively prevent foaming during caramelization of sucrose and extend the pyrolysis burnoff of sucrose caramel over a wider temperature range. The obtained low-surface-area hard carbon greatly improves the first-cycle Coulombic efficiency from 74% to 83% and delivers a very stable cyclic life with 95% of capacity retention after 200 cycles

High Energy Density Aqueous Electrochemical Capacitors with a KI-KOH Electrolyte

We report a new electrochemical capacitor with an aqueous KI-KOH electrolyte that exhibits a higher specific energy and power than the state-of-the-art nonaqueous electrochemical capacitors. In addition to electrical double layer capacitance, redox reactions in this device contribute to charge storage at both positive and negative electrodes via a catholyte of IO_<i>x</i>^–/I^– couple and a redox couple of H₂O/H_ad, respectively. Here, we, for the first time, report utilizing IO_<i>x</i>^–/I^– redox couple for the positive electrode, which pins the positive electrode potential to be 0.4–0.5 V vs Ag/AgCl. With the positive electrode potential pinned, we can polarize the cell to 1.6 V without breaking down the aqueous electrolyte so that the negative electrode potential could reach −1.1 V vs Ag/AgCl in the basic electrolyte, greatly enhancing energy storage. Both mass spectroscopy and Raman spectrometry confirm the formation of IO₃^– ions (+5) from I^– (−1) after charging. Based on the total mass of electrodes and electrolyte in a practically relevant cell configuration, the device exhibits a maximum specific energy of 7.1 Wh/kg, operates between −20 and 50 °C, provides a maximum specific power of 6222 W/kg, and has a stable cycling life with 93% retention of the peak specific energy after 14 000 cycles

A Hydrocarbon Cathode for Dual-Ion Batteries

We have demonstrated, for the first time, a polycyclic aromatic hydrocarbon (PAH), crystalline and readily available coronene, exhibits highly reversible anion-storage properties. Conventional graphite anion-insertion electrodes operate at potentials >4.5 V vs Li⁺/Li, requiring electrolyte additives or the use of ionic liquids as electrolytes. The coronene electrode shows flat plateaus at 4.2 V (charge) and 4.0 V (discharge) in a standard alkyl carbonate electrolyte and delivers a reversible discharge capacity of ∼40 mA h g^–1. Ex situ characterization reveals that coronene retains its crystalline structure and chemical bonding upon initial PF₆^– incorporation. Coronene–PF₆ electrodes show impressive cycling stability: 92% capacity retention after 960 cycles. The discovery of the reversible anion-storage properties of coronene may open new avenues toward dual-ion batteries based on PAHs as electrodes