Graphene@hierarchical meso-/microporous carbon for ultrahigh energy density lithium-ion capacitors

Graphene-Based Cathode Materials for Lithium-Ion Capacitors: A Review

Lithium-ion capacitors (LICs) are attracting increasing attention because of their potential to bridge the electrochemical performance gap between batteries and supercapacitors. However, the commercial application of current LICs is still impeded by their inferior energy density, which is mainly due to the low capacity of the cathode. Therefore, tremendous efforts have been made in developing novel cathode materials with high capacity and excellent rate capability. Graphene-based nanomaterials have been recognized as one of the most promising cathodes for LICs due to their unique properties, and exciting progress has been achieved. Herein, in this review, the recent advances of graphene-based cathode materials for LICs are systematically summarized. Especially, the synthesis method, structure characterization and electrochemical performance of various graphene-based cathodes are comprehensively discussed and compared. Furthermore, their merits and limitations are also emphasized. Finally, a summary and outlook are presented to highlight some challenges of graphene-based cathode materials in the future applications of LICs.

Facile Synthesis of Graphene with Fast Ion/Electron Channels for High-Performance Symmetric Lithium-Ion Capacitors

With the battery-type anode and capacitor-type cathode, lithium-ion capacitors (LICs) are expected to exhibit both high energy and high power density but suffer from the mismatch of the electrode reaction kinetics and capacity. Herein, to alleviate the mismatch between the two electrodes and synergistically enhance the energy/power density, we design a method of microwave irradiation reduction to prepare graphene-based electrode material (MRPG/CNT) with fast ion/electron pathway. The three-dimensional structure of CNT intercalation to graphene inhibits the restacking of graphene sheets and improves the conductivity of the electrode material, resulting a rapid ion and electron diffusion channel. Due to its specific properties, MRPG/CNT materials can be used as both anode and cathode electrodes of LICs at the same time. As anode, MRPG/CNT shows a high capacity of 1200 mAh g^-1 as well as high rate performance. As cathode, MRPG/CNT displays a high capacity of 108 mAh g^-1 and the capacity retention of 100% after 8000 cycles. Coupling the prelithiated MRPG/CNT anode with MRPG/CNT cathode gives a full-graphene-based symmetric LIC, which achieves a high energy density of 232.6 Wh kg^-1 at 226.0 W kg^-1, 111.2 Wh kg^-1 at the ultrahigh power density of 45.2 kW kg^-1, and superior capacity retention of 86% after 5000 cycles. The structure design of this electrode provides a new strategy for alleviating the mismatch of LIC electrodes and constructing high-performance symmetrical LICs.

Oxidized-Polydopamine-Coated Graphene Anodes and N,P Codoped Porous Foam Structure Activated Carbon Cathodes for High-Energy-Density Lithium-Ion Capacitors

As a tradeoff between supercapacitors and batteries, lithium-ion capacitors (LICs) are designed to deliver high energy density, high power density, and long cycling stability. Owing to the different energy storage mechanisms of capacitor-type cathodes and battery-type anodes, engineering and fabricating LICs with excellent energy density and power density remains a challenge. Herein, to alleviate the mismatch between the anode and cathode, we ingeniously designed a graphene with oxidized-polydopamine coating (LG@DA1) and N,P codoped porous foam structure activated carbon (CPC750) as the battery-type anode and capacitor-type cathode, respectively. Using oxidized-polydopamine to stabilize the structure of graphene, increase layer spacing, and modify the surface chemical property, the LG@DA1 anode delivers a maximum capacity of 1100 mAh g^-1 as well as good cycling stability. With N,P codoping and a porous foam structure, the CPC750 cathode exhibits a large effective specific surface area and a high specific capacity of 87.5 mAh g^-1. In specific, the present LG@DA1//CPC750 LIC showcases a high energy density of 170.6 Wh kg^-1 and superior capacity retention of 93.5% after 2000 cycles. The success of the present LIC can be attributed to the structural stability design, surface chemistry regulation, and enhanced utilization of effective active sites of the anode and cathode; thus, this strategy can be applied to improve the performance of LICs.

Hierarchically Nanoporous Pyropolymers Derived from Waste Pinecone as a Pseudocapacitive Electrode for Lithium Ion Hybrid Capacitors

The non-aqueous asymmetric lithium ion hybrid capacitor (LIHC) is a tactical energy storage device composed of a faradic and non-faradic electrode pair, which aims to achieve both high energy and great power densities. On the other hand, the different types of electrode combinations cause severe imbalances in energy and power capabilities, leading to poor electrochemical performance. Herein, waste pinecone-derived hierarchically porous pyropolymers (WP-HPPs) were fabricated as a surface-driven pseudocapacitive electrode, which has the advantages of both faradic and non-faradic electrodes. The unique materials properties of WP-HPPs possessing high effective surface areas and hierarchically open nanopores led to high specific capacities of ~412 mA h g⁻¹ and considerable rate/cycling performance as a cathode for LIHCs. In particular, nanometer-scale pores, approximately 3 nm in size, plays a key role in the pseudocapacitive charge storage behaviors because open nanopores can transport solvated Li-ions easily into the inside of complex carbon structures and a large specific surface area can be provided by the effective active surface for charge storage. In addition, WP-HPP-based asymmetric LIHCs assembled with a pseudocapacitive counterpart demonstrated feasible electrochemical performance, such as maximum specific energy and specific power of ~340 Wh kg⁻¹ and ~11,000 W kg⁻¹, respectively, with significant cycling stability.

Boost Anion Storage Capacity Using Conductive Polymer as a Pseudocapacitive Cathode for High-Energy and Flexible Lithium Ion Capacitors

Current lithium ion capacitors (LICs) have been severely plagued by the insufficient anion storage capacity of porous carbon. This work reports the exploration of conductive polyaniline (PANi) as an anion intercalation cathode to enhance the PF₆^- storage via fast doping/undoping reactions. The PANi is electrodeposited on an electrospun carbon nanofiber (CNF) textile (denoted as PANi@CNF), which not only provides a robust support for PANi to increase its pseudocapacity but also renders a free-standing architecture for flexible devices. The PANi@CNF composite with a dominant capacitive storage characteristic reveals high specific capacities of 158.5 mAh g_PANi^-1 at 1 A g^-1 and 118.5 mAh g_PANi^-1 even at 20 A g^-1, which significantly surpasses state-of-the-art porous carbons. First-principle calculations revealed the coordination of PF₆^- anions with -NH groups of PANi⁺ via F atoms through ion-dipole electrostatic interaction, which are accompanied by electron transfer. By pairing with CNF as an anode, a thin and flexible LIC was assembled, which achieves maximum energies of 106.5 Wh kg^-1 under 769.0 W kg^-1 and 64.5 Wh kg^-1 under a super high power of 15087.1 W kg^-1, together with an impressive cycling stability of 70.3% after 9000 cycles at 10 A g^-1. These findings provide a facile strategy for high-energy and flexible LICs via anion storage pseudocapacitive materials.

Phenylphenol-Derived Carbon and Antimony-Coated Carbon Nanotubes as the Electroactive Materials of Lithium-Ion Hybrid Capacitors

Energy storage of the lithium-ion hybrid capacitor can be upgraded through adjusting the mismatched rate qualities between the positive and negative electrodes because the positive electrode of the electrical double layer (EDL) stores and releases electricity in a smaller quantity, yet much faster than the negative battery electrode. To increase the EDL capacity, nitrogen-doped carbon (KPN900) with a hollow-onion structure is prepared with phenylphenol, achieving a surface area above 3000 m² g^-1. The capacitance of KPN900 displays a diffusive component of 57 F g^-1, exceeding its capacitive counterpart at 10 mV s^-1. Moreover, its total capacitance reaches 168 F g^-1 at 1 mV s^-1 with a diffusive component of 112 F g^-1. On the other hand, the power of the negative electrode is improved through electrodeposition of metallic antimony on carbon nanotubes, Sb/CNTs, evidenced by the capacity of ∼250 mA h g^-1 at 1.0 A g^-1. Hence, the capacitor, with a 2:1 mass ratio of KPN900 to Sb/CNT, exhibits an effective trade-off between energy and power, distinct from the one-sided dependence on the carbon electrode of most hybrid capacitors. This capacitor stores 97 W h kg^-1 at a power level 0.12 kW kg^-1 and 17.4 W h kg^-1 at power 7.90 kW kg^-1.

Intensification of Pseudocapacitance by Nanopore Engineering on Waste-Bamboo-Derived Carbon as a Positive Electrode for Lithium-Ion Batteries

Nanoporous carbon, including redox-active functional groups, can be a promising active electrode material (AEM) as a positive electrode for lithium-ion batteries owing to its high electrochemical performance originating from the host-free surface-driven charge storage process. This study examined the effects of the nanopore size on the pseudocapacitance of the nanoporous carbon materials using nanopore-engineered carbon-based AEMs (NE-C-AEMs). The pseudocapacitance of NE-C-AEMs was intensified, when the pore diameter was ≥2 nm in a voltage range of 1.0~4.8 V vs Li⁺/Li under the conventional carbonate-based electrolyte system, showing a high specific capacity of ~485 mA·h·g⁻¹. In addition, the NE-C-AEMs exhibited high rate capabilities at current ranges from 0.2 to 4.0 A·g⁻¹ as well as stable cycling behavior for more than 300 cycles. The high electrochemical performance of NE-C-AEMs was demonstrated by full-cell tests with a graphite nanosheet anode, where a high specific energy and power of ~345 Wh·kg⁻¹ and ~6100 W·Kg⁻¹, respectively, were achieved.