The Interplay of Oxygen Functional Groups and Folded Texture in Densified Graphene Electrodes for Compact Sodium‐Ion Capacitors

Sodium‐ion capacitors (SICs) can effectively combine high energy density with high power density, and are especially appropriate for high‐power demanding applications of large‐scale stationary energy storage. Surface‐induced pseudocapacitive charge storage based on porous or nano carbon materials is regarded as the most promising candidate for SICs. Unfortunately, their ultralow packing densities severely restrict their practical applications. A novel approach toward ultrafast high‐volumetric SICs based on folded‐graphene electrodes has already been demonstrated and showed quite competitive performance. In this work, it is further proved that oxygen functional groups and folded texture are two key elements for high‐volumetric sodium storage of folded‐graphene electrodes. Through a simple and controllable method, of thermal treatment in inert atmosphere, both the oxygen functional groups and folded texture can be quantitatively manipulated to better investigate the individual contribution and mutual interplay. It is illustrated that oxygen functional groups are crucial to superior capacitive sodium storage while folded texture is not only the origin for high‐volumetric sodium storage but also beneficial for both capacitive and additional diffusion‐controlled sodium storage. Inspired by above‐mentioned conclusion, more rational designs and effective preparation of advanced structure and novel materials can be realized to better promote the development of high‐volumetric SICs.     

Cascade‐Type Prelithiation Approach for Li‐Ion Capacitors

An industry‐relevant method for pre‐lithiation of lithium‐ion capacitors to balance the first charge irreversibility is demonstrated, which addresses the prime bottleneck for their market integration. Based on a composite positive electrode that integrates pyrene monomers and an insoluble lithiated base, Li₃PO₄, a “cascade‐type” process involving two consecutive irreversible reactions is proposed: i) oxidative electropolymerization of the pyrene moieties releases electrons and protons; ii) protons are captured by Li₃PO₄ and exchanged for a stoichiometric amount of Li⁺ into the electrolyte. (¹H, ¹⁹F, and ³¹P) NMR spectroscopy, operando X‐ray diffraction, and Raman spectroscopy support this mechanism. By decoupling the irreversible source of lithium ions from electrons, the cascade‐type pre‐lithiation allows the simultaneous enhancement of the capacity of the positive electrode, thanks to p‐doping of the resulting polymer. Remarkably, the proton scavenging properties of Li₃PO₄ also boost the polymerization process, which enables a 16% increase in capacity without detrimental effect on power properties and cyclability. Full cells integrating a cheap carbon black based negative electrode, show much‐improved capacity of 17 mAh g^‐1_electrodes (44 F g^‐1_electrodes, 3–4.4 V) and excellent stability over 2200 cycles at 1 A g^‐1. Thanks to its versatile chemistry and flexibility this approach in principle can be applied to any kind of ion‐battery.     

Rational Construction of Nitrogen‐Doped Hierarchical Dual‐Carbon for Advanced Potassium‐Ion Hybrid Capacitors

Potassium‐ion hybrid capacitors (PIHCs), elaborately integrate the advantages of high output power as well as long lifespan of supercapacitors and the high energy density of batteries, and exhibit great possibilities for the future generations of energy storage devices. The critical next step for future implementation lies in exploring a high‐rate battery‐type anode with an ultra‐stable structure to match the capacitor‐type cathode. Herein, a “dual‐carbon” is constructed, in which a three‐dimensional nitrogen‐doped microporous carbon polyhedron (NMCP) derived from metal‐organic frameworks is tightly wrapped by two‐dimensional reduced graphene oxide (NMCP@rGO). Benefiting from the synergistic effect of the inner NMCP and outer rGO, the NMCP@rGO exhibits a superior K‐ion storage capability with a high reversible capacity of 386 mAh g^?1 at 0.05 A g^?1 and ultra‐long cycle stability with a capacity of 151.4 mAh g^?1 after 6000 cycles at 5.0 A g^?1. As expected, the as‐assembled PIHCs with a working voltage as high as 4.2 V present a high energy/power density (63.6 Wh kg^?1 at 19 091 W kg^?1) and excellent capacity retention of 84.7% after 12 000 cycles. This rational construction of advanced PIHCs with excellent performance opens a new avenue for further application and development.     

Phosphorene as Cathode Material for High‐Voltage,Anti‐Self‐Discharge Zinc Ion Hybrid Capacitors

Output voltage and self‐discharge rate are two important performance indices for supercapacitors, which have long been overlooked, though these play a very significant role in their practical application. Here, a zinc anode is used to construct a zinc ion hybrid capacitor. Expanded operating voltage of the hybrid capacitor is obtained with novel electrolytes. In addition, significantly improved anti‐self‐discharge ability is achieved. The phosphorene‐based zinc ion capacitor exploiting a “water in salt” electrolyte with a working potential can reach 2.2 V, delivering 214.3 F g^?1 after 5000 cycles. The operating voltage is further extended to 2.5 V through the use of an organic solvent as the electrolyte; the solvent is prepared by adding 0.2 m ZnCl₂ into the tetraethylammonium tetrafluoroborate in propylene carbonate (Et₄NBF₄/PC) solvent, and it exhibits 105.9 F g^?1 even after 9500 cycles. More importantly, the phosphorene‐based capacitors possess excellent anti‐self‐discharge performance. The capacitors retain 76.16% of capacitance after resting for 300 h. The practical application of the zinc ion capacitor is demonstrated through a flexible paper‐based printed microcapacitor. It is believed that the developed zinc ion capacitor can effectively resolve the severe self‐discharge problem of supercapacitors. Moreover, high‐voltage zinc ion capacitors provide more opportunities for the application of supercapacitors.     

Surface Pseudocapacitive Mechanism of Molybdenum Phosphide for High‐Energy and High‐Power Sodium‐Ion Capacitors

Sodium‐based energy storage technologies are potential candidates for large‐scale grid applications owing to the earth abundance and low cost of sodium resources. Transition metal phosphides, e.g. MoP, are promising anode materials for sodium‐ion storage, while their detailed reaction mechanisms remain largely unexplored. Herein, the sodium‐ion storage mechanism of hexagonal MoP is systematically investigated through experimental characterizations, density functional theory calculations, and kinetics analysis. Briefly, it is found that the naturally covered surface amorphous molybdenum oxides layers on the MoP grains undergo a faradaic redox reaction during sodiation and desodiation, while the inner crystalline MoP remains unchanged. Remarkably, the MoP anode exhibits a pseudocapacitive‐dominated behavior, enabling the high‐rate sodium storage performance. By coupling the pseudocapacitive anode with a high‐rate‐battery‐type Na₃V₂O₂(PO₄)₂F@rGO cathode, a novel sodium‐ion full cell delivers a high energy density of 157 Wh kg^?1 at 97 W kg^?1 and even 52 Wh kg^?1 at 9316 W kg^?1. These findings present the deep understanding of the sodium‐ion storage mechanism in hexagonal MoP and offer a potential route for the design of high‐rate sodium‐ion storage materials and devices.     

Fast Sodium Storage in TiO2@CNT@C Nanorods for High‐Performance Na‐Ion Capacitors

Na‐ion capacitors have attracted extensive interest due to the combination of the merits of high energy density of batteries and high power density as well as long cycle life of capacitors. Here, a novel Na‐ion capacitor, utilizing TiO₂@CNT@C nanorods as an intercalation‐type anode and biomass‐derived carbon with high surface area as an ion adsorption cathode in an organic electrolyte, is reported. The advanced architecture of TiO₂@CNT@C nanorods, prepared by electrospinning method, demonstrates excellent cyclic stability and outstanding rate capability in half cells. The contribution of extrinsic pseudocapacitance affects the rate capability to a large extent, which is identified by kinetics analysis. A key finding is that ion/electron transfer dynamics of TiO₂@CNT@C could be effectively enhanced due to the addition of multiwalled carbon nanotubes. Also, the biomass‐derived carbon with high surface area displays high specific capacity and excellent rate capability. Owing to the merits of structures and excellent performances of both anode and cathode materials, the assembled Na‐ion capacitors provide an exceptionally high energy density (81.2 W h kg^?1) and high power density (12 400 W kg^?1) within 1.0–4.0 V. Meanwhile, the Na‐ion capacitors achieve 85.3% capacity retention after 5000 cycles tested at 1 A g^?1.     

Exploiting High‐Performance Anode through Tuning the Character of Chemical Bonds for Li‐Ion Batteries and Capacitors

A high‐performance anode material, MnNCN, is synthesized through a facile and low‐cost method. The relationship between electrochemical properties and chemical composition is explored on the scientific considerations that can provide an insight on designing expected materials. MnNCN with the long bonding length of 2.262 Å in Mn? N and weak electronegativity of 3.04 Pauling units in N leads to a lower charge/discharge potential than that of MnO owing to the character of chemical bonds transformed to covalent dominating from ionic dominating in MnO. Covalent character increases the ratio of sharing electrons that decreases the migration energy of electrons in electrochemical reaction, which enhances the reactive reversibility and stability of electrode material. MnNCN delivered a reversibly specific capacity of 385 mA h g^?1 at 5 A g^?1 in a Li‐ion half cell. Besides, a Li‐ion hybrid capacitor with a high voltage of 4 V presents energy and power densities of respective 103 Wh kg^?1 and 8533 W kg^?1 and cycles at 5 A g^?1 without detectable degradation after 5000 cycles.     

Thermally Durable Lithium‐Ion Capacitors with High Energy Density from All Hydroxyapatite Nanowire‐Enabled Fire‐Resistant Electrodes and Separators

The reliability and durability of lithium‐ion capacitors (LICs) are severely hindered by the kinetic imbalance between capacitive and Faradaic electrodes. Efficient charge storage in LICs is still a huge challenge, particularly for thick electrodes with high mass loading, fast charge delivery, and harsh working conditions. Here, a unique thermally durable, stable LIC with high energy density from all‐inorganic hydroxyapatite nanowire (HAP NW)‐enabled electrodes and separators is reported. Namely, the LIC device is designed and constructed with the electron/ion dual highly conductive and fire‐resistant composite Li₄Ti₅O₁₂‐based anode and activated carbon‐based cathode, together with a thermal‐tolerant HAP NW separator. Despite the thick‐electrode configuration, the as‐fabricated all HAP NW‐enabled LIC exhibits much enhanced electrochemical kinetics and performance, especially at high current rates and temperatures. Long cycling lifetime and state‐of‐the‐art areal energy density (1.58 mWh cm^?2) at a high mass loading of 30 mg cm^?2 are achieved. Benefiting from the excellent fire resistance of HAP NWs, such an unusual LIC exhibits high thermal durability and can work over a wide range of temperatures from room temperature to 150 °C. Taking full advantage of synergistic configuration design, this work sets the stage for designing advanced LICs beyond the research of active materials.     

Fast Redox Kinetics in Bi‐Heteroatom Doped 3D Porous Carbon Nanosheets for High‐Performance Hybrid Potassium‐Ion Battery Capacitors

Potassium‐ion hybrid capacitors (PIHCs) hold the advantages of high‐energy density of batteries and high‐power output of supercapacitors and thus present great promise for the next generation of electrochemical energy storage devices. One of the most crucial tasks for developing a high‐performance PIHCs is to explore a favorable anode material with capability to balance the kinetics mismatch between battery‐type anodes and capacitor‐type cathode. Herein, a reliable route for fabricating sulfur and nitrogen codoped 3D porous carbon nanosheets (S‐N‐PCNs) is reported. Systematic characterizations coupled with kinetics analysis indicate that the doped heteroatoms of sulfur and nitrogen and the amplified graphite interlayer can provide ample structural defects and redox active sites that are beneficial for improving pseudocapacitive activity, enabling fast kinetics toward efficient potassium‐ion storage. The S‐N‐PCNs are demonstrated to exhibit superior potassium storage capability with a high capacity of 107 mAh g^?1 at 20 A g^?1 and long cycle stability. The as‐developed PIHCs present impressive electrochemical performance with an operating voltage as high as 4.0 V, an energy density of 187 Wh kg^?1, a power density of 5136 W kg^?1, and a capacity retention of 86.4% after 3000 cycles.     

Lithium Ion Capacitors in Organic Electrolyte System: Scientific Problems,Material Development,and Key Technologies

Lithium ion capacitors (LICs), which are hybrid electrochemical energy storage devices combining the intercalation/deintercalation mechanism of a lithium‐ion battery (LIB) electrode with the adsorption/desorption mechanism of an electric double‐layer capacitor (EDLC) electrode, have been extensively investigated during the past few years by virtue of their high energy density, rapid power output, and excellent cycleability. In this review, the LICs are defined as the devices with an electrochemical intercalation electrode and a capacitive electrode in organic electrolytes. Both electrodes can serve as anode or cathode. Throughout the history of LICs, tremendous efforts have been devoted to design suitable electrode materials or develop novel type LIC systems. However, one of the key challenges encountered by LICs is how to balance the sluggish kinetics of intercalation electrodes with high specific capacity against the high power characteristics of capacitive electrode with low specific capacitance. Herein, the developments and the latest advances of LIC in material design strategies and key techniques according to the basic scientific problems are summarized. Perspectives for further development of LICs toward practical applications are also proposed.     

High Energy and High Power Lithium‐Ion Capacitors Based on Boron and Nitrogen Dual‐Doped 3D Carbon Nanofibers as Both Cathode and Anode

High energy density at high power density is still a challenge for the current Li‐ion capacitors (LICs) due to the mismatch of charge‐storage capacity and electrode kinetics between capacitor‐type cathode and battery‐type anode. In this work, B and N dual‐doped 3D porous carbon nanofibers are prepared through a facile method as both capacitor‐type cathode and battery‐type anode for LICs. The B and N dual doping has profound effect in tuning the porosity, functional groups, and electrical conductivity for the porous carbon nanofibers. With rational design, the developed B and N dual‐doped carbon nanofibers (BNC) exhibit greatly improved electrochemical performance as both cathode and anode for LICs, which greatly alleviates the mismatch between the two electrodes. For the first time, a 4.5 V “dual carbon” BNC//BNC LIC device is constructed and demonstrated, exhibiting outstanding energy density and power capability compared to previously reported LICs with other configurations. In specific, the present BNC//BNC LIC device can deliver a large energy density of 220 W h kg^?1 and a high power density of 22.5 kW kg^?1 (at 104 W h kg^?1) with reasonably good cycling stability (≈81% retention after 5000 cycles).     

Folding protein alpha-carbon chains into compact forms by Monte Carlo methods.

A method is presented for generating folded chains of specific amino acid sequences on a simple cubic lattice. Monte Carlo simulations are used to transform extended geometries of simplified alpha-carbon chains for eight small monomeric globular proteins into folded states. Permitted chain transitions are limited to a few types of moves, all restricted to occur on the lattice. Crude residue-residue potentials derived from statistical structure data are used to describe the energies for each conformer. The low resolution structures obtained by this procedure contain many of the correct gross features of the native folded architectures with respect to average residue energy per nonbonded contact, segment density, and location of surface loops and disulfide pairs. Rms deviations between these and the native X-ray structures and percentage of native long-range contacts found in these final folded structures are 7.6 +/- 0.7 A and 48 +/- 3%, respectively. This procedure can be useful for predicting approximate tertiary interactions from amino acid sequence.     

Flexible Quasi‐Solid‐State Sodium‐Ion Capacitors Developed Using 2D Metal–Organic‐Framework Array as Reactor

Achieving high‐performance Na‐ion capacitors (NICs) has the particular challenge of matching both capacity and kinetics between the anode and cathode. Here a high‐power NIC full device constructed from 2D metal–organic framework (MOFs) array is reported as the reactive template. The MOF array is converted to N‐doped mesoporous carbon nanosheets (mp‐CNSs), which are then uniformly encapsulated with VO₂ and Na₃V₂(PO₄)₃ (NVP) nanoparticles as the electroactive materials. By this method, the high‐power performance of the battery materials is enabled to be enhanced significantly. It is discovered that such hybrid NVP@mp‐CNSs array can render ultrahigh rate capability (up to 200 C, equivalent to discharge within 18 s) and superior cycle performance, which outperforms all NVP‐based Na‐ion battery cathodes reported so far. A quasi‐solid‐state flexible NIC based on the NVP@mp‐CNSs cathode and the VO₂@mp‐CNSs anode is further assembled. This hybrid NIC device delivers both high energy density and power density as well as a good cycle stability (78% retention after 2000 cycles at 1 A g^?1). The results demonstrate the powerfulness of MOF arrays as the reactor for fabricating electrode materials.