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

Novel Potassium-Ion Hybrid Capacitor Based on an Anode of K₂Ti₆O₁₃ Microscaffolds

To fill the gap between batteries and supercapacitors requires integration of the following features in a single system: energy density well above that of supercapacitors, cycle life much longer than Li-ion batteries, and low cost. Along this line, we report a novel nonaqueous potassium-ion hybrid capacitor (KIC) that employs an anode of K₂Ti₆O₁₃ (KTO) microscaffolds constructed by nanorods and a cathode of N-doped nanoporous graphenic carbon (NGC). K₂Ti₆O₁₃ microscaffolds are studied for potential applications as the anode material in potassium-ion storage for the first time. This material exhibits an excellent capacity retention of 85% after 1000 cycles. In addition, the NGC//KTO KIC delivers a high energy density of 58.2 Wh kg^–1 based on the active mass in both electrodes, high power density of 7200 W kg^–1, and outstanding cycling stability over 5000 cycles. The usage of K ions as the anode charge carrier instead of Li ions and the amenable performance of this device suggest that hybrid capacitor devices may welcome a new era of beyond lithium

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

Identify the Removable Substructure in Carbon Activation

Activated carbon plays a pivotal role in achieving critical functions, such as separation, catalysis, and energy storage. A remaining question of carbon activation is which substructures in amorphous carbon are preferentially removed during activation. Herein, we report the first structure–activation correlation elucidated on the basis of unprecedented comprehensive characterization on carbon activation. We discover that activation under CO₂ preferentially removes graphenic layers that are more defective. Therefore, the resulting activated carbon contains thinned turbostratic nanodomains that are of a higher local graphenic order. The mechanistic insights explain why more defective soft carbon is “burned” under CO₂ at a much faster rate than hard carbon. The mechanism leads to an activation-based design principle of mesoporous carbon. Guided by this principle, a bimodal micromesoporous carbon is prepared simply by CO₂ activation. Our findings may cause a paradigm shift for the rational design of nanoporous carbon

High Capacity of Hard Carbon Anode in Na-Ion Batteries Unlocked by PO_<i>x</i> Doping

The capacity of hard carbon anodes in Na-ion batteries rarely reaches values beyond 300 mAh/g. We report that doping PO_<i>x</i> into local structures of hard carbon increases its reversible capacity from 283 to 359 mAh/g. We confirm that the doped PO_<i>x</i> is redox inactive by X-ray adsorption near edge structure measurements, thus not contributing to the higher capacity. We observe two significant changes of hard carbon’s local structures caused by doping. First, the (002) <i>d</i>-spacing inside the turbostratic nanodomains is increased, revealed by both laboratory and synchrotron X-ray diffraction. Second, doping turns turbostratic nanodomains more defective along <i>ab</i> planes, indicated by neutron total scattering and the associated pair distribution function studies. The local structural changes of hard carbon are correlated to the higher capacity, where both the plateau and slope regions in the potential profiles are enhanced. Our study demonstrates that Na-ion storage in hard carbon heavily depends on carbon local structures, where such structures, despite being disordered, can be tuned toward unusually high capacities