Li-Ion Capacitor Integrated with Nano-network-Structured Ni/NiO/C Anode and Nitrogen-Doped Carbonized Metal-Organic Framework Cathode with High Power and Long Cyclability

Metal-organic frameworks and their derivatives for metal-ion (Li, Na, K and Zn) hybrid capacitors

Metal-ion hybrid capacitors (MIHCs) hold particular promise for next-generation energy storage technologies, which bridge the gap between the high energy density of conventional batteries and the high power density and long lifespan of supercapacitors (SCs). However, the achieved electrochemical performance of available MIHCs is still far from practical requirements. This is primarily attributed to the mismatch in capacity and reaction kinetics between the cathode and anode. In this regard, metal-organic frameworks (MOFs) and their derivatives offer great opportunities for high-performance MIHCs due to their high specific surface area, high porosity, topological diversity, and designable functional sites. In this review, instead of simply enumerating, we critically summarize the recent progress of MOFs and their derivatives in MIHCs (Li, Na, K, and Zn), while emphasizing the relationship between the structure/composition and electrochemical performance. In addition, existing issues and some representative design strategies are highlighted to inspire breaking through existing limitations. Finally, a brief conclusion and outlook are presented, along with current challenges and future opportunities for MOFs and their derivatives in MIHCs.

Hierarchical Stratiform of a Fluorine-Doped NiO Prism as an Enhanced Anode for Lithium-Ion Storage

Doping is regarded as a prominent strategy to optimize the crystal structure and composition of battery materials to withstand the anisotropic expansion induced by the repeated insertion and extraction of guest ions. The well-known knowledge and experience obtained from doping engineering predominate in cathode materials but have not been fully explored for anodes yet. Here, we propose the practical doping of fluorine ions into the host lattice of nickel oxide to unveil the correlation between the crystal structure and electrochemical properties. Multiple ion transmission pathways are created by the orderly two-dimensional nanosheets, and thus the stress/strain can be significantly relieved with trace fluorine doping, ensuring the mechanical integrity of the active particle and superior electrochemical properties. Density functional theory calculations manifest that the F doping in NiO could improve crystal structural stability, modulate the charge distribution, and enhance the conductivity, which promotes the performance of lithium-ion storage.

Boosting lithium-ion storage performance by ultrafine bimetal carbides nanoparticles coupled with Hollow-like carbon composites

Metallic carbides demonstrated tremendous application potential in energy conversion field deriving from their distinctive electrochemical activity and chemical stability. Herein, a molybdenum-based hybrid self-template strategy was adopted to confine ultrafine molybdenum carbides and tungsten carbides nanoparticles in N, P-codoped carbon nanotubes (MoC/WC@N, P-CNTs) for enhanced lithium-ion storage. Specifically, hierarchical MoW-polydopamine nanotubes were prepared via a self-template strategy, which employed Mo₃O₁₀(C₆H₈N)₂·2H₂O nanowires as the template. Ultrafine MoC and WC nanoparticles embedded in ultrathin carbon nanosheets could be obtained rationally after carbonization treatment, which could not only prevent carbides nanoparticles from agglomeration and oxidation, but also endow the rapid electron transfer rate. Thus, MoC/WC@N, P-CNTs displayed outstanding lithium storage abilities with great rate property and long-term cycling durability. The stable specific capacity of 475.0 mAh g^-1 could be preserved at high current intensity of 5.0 A g^-1 after 1000 cycles, which was one of the best performances for metal carbides anodes. Furthermore, the successful fabrication of lithium-ion hybrid capacitors (LIHCs) delivered the maximum energy density of 117 Wh kg^-1 and power density of 6571 W kg^-1. Moreover, the superior capacity retention of 89.7 % after 2000 cycles also indicated the excellent cycling stability. The present work highlights a self-template strategy for designing nanostructures toward efficient energy storage and conversion fields.

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.

High performance Li-ion capacitor fabricated with dual graphene-based materials

Lithium-ion capacitors (LICs) are now drawing increasing attention because of their potential to overcome the current energy limitations of supercapacitors and power limitations of lithium-ion batteries. In this work, we designed LICs by combining an electric double-layer capacitor cathode and a lithium-ion battery anode. Both the cathode and anode are derived from graphene-modified phenolic resin with tunable porosity and microstructure. They exhibit high specific capacity, superior rate capability and good cycling stability. Benefiting from the graphene-enhanced electrode materials, the all graphene-based LICs demonstrate a high working voltage (4.2 V), high energy density of 142.9 Wh kg^-1, maximum power density of 12.1 kW kg^-1 with energy density of 50 Wh kg^-1, and long stable cycling performance (with ∼88% capacity retention after 5000 cycles). Considering the high performance of the device, the cost-effective and facile preparation process of the active materials, this all graphene-based lithium-ion capacitor could have many promising applications in energy storage systems.

CuCo2O4 nanoparticles wrapped in a rGO aerogel composite as an anode for a fast and stable Li-ion capacitor with ultra-high specific energy

To meet practical application requirements, high specific energy and specific power and excellent cyclability are highly desired.

Na0.76V6O15/Activated Carbon Hybrid Cathode for High-Performance Lithium-Ion Capacitors

Lithium-ion hybrid capacitors (LICs) are regarded as one of the most promising next generation energy storage devices. Commercial activated carbon materials with low cost and excellent cycling stability are widely used as cathode materials for LICs, however, their low energy density remains a significant challenge for the practical applications of LICs. Herein, Na_0.76V₆O₁₅ nanobelts (NaVO) were prepared and combined with commercial activated carbon YP50D to form hybrid cathode materials. Credit to the synergism of its capacitive effect and diffusion-controlled faradaic effect, NaVO/C hybrid cathode displays both superior cyclability and enhanced capacity. LICs were assembled with the as-prepared NaVO/C hybrid cathode and artificial graphite anode which was pre-lithiated. Furthermore, 10-NaVO/C//AG LIC delivers a high energy density of 118.9 Wh kg⁻¹ at a power density of 220.6 W kg⁻¹ and retains 43.7 Wh kg⁻¹ even at a high power density of 21,793.0 W kg⁻¹. The LIC can also maintain long-term cycling stability with capacitance retention of approximately 70% after 5000 cycles at 1 A g⁻¹. Accordingly, hybrid cathodes composed of commercial activated carbon and a small amount of high energy battery-type materials are expected to be a candidate for low-cost advanced LICs with both high energy density and power density.

Three-Dimensional Topotactic Host Structure-Secured Ultrastable VP-CNO Composite Anodes for Long Lifespan Lithium- and Sodium-Ion Capacitors

Performance degradation of lithium/sodium-ion capacitors (LICs/SICs) mainly originates from anode pulverization, particularly the alloying and conversion types, and has spurred research for alternatives with an insertion mechanism. Three-dimensional (3D) topotactic host materials remain much unexplored compared to two-dimensional (2D) ones (graphite, etc.). Herein, vanadium monophosphide (VP) is designed as a 3D topotactic host anode. Ex situ electrochemical characterizations reveal that there are no phase changes during (de)intercalation, which follows the topotactic intercalation mechanism. Computational simulations also confirm the metallic feature and topotactic structure of VP with a spacious interstitial position for the accommodation of guest species. To boost the electrochemical performance, carbon nano-onions (CNOs) are coupled with 3D VP. Superior specific capacity and rate capability of VP-CNOs vs lithium/sodium can be delivered due to the fast ion diffusion nature. An exceptional capacity retention of above 86% is maintained after 20 000 cycles, benefitting from the topotactic intercalation process. The optimized LICs/SICs exhibit high energy/power densities and an ultrastable lifespan of 20 000 cycles, which outperform most of the state-of-the-art LICs and SICs, demonstrating the potential of VP-CNOs as insertion anodes. This exploration would draw attention with regard to insertion anodes with 3D topotactic host topology and provide new insights into anode selection for LICs/SICs.