A Universal Converse Voltage Process for Triggering Transition Metal Hybrids In Situ Phase Restruction toward Ultrahigh‐Rate Supercapacitors

Defect engineering holds great promise for precise configuration of electrode materials for dramatically enhanced performance in the field of energy storage, but the high energy/large time cost and lack of control involved in this process represent a serious limit to its use. In response, a low‐energy‐cost and ultrafast universal converse voltage process is developed to effectively activate the capacitive performance of transition metal compounds integrated on carbon fiber paper, including Co‐, Ni‐, Mn‐, Fe‐, and Cr‐based hybrids. As a representative example, this process triggers a phase conversion from cobalt hydroxide to electric‐field‐activated CoOOH (EA‐CoOOH), leading to the formation of molecular structure with abundant defects, lattice disorders, and connecting holes, responsible for an enhanced performance within 10 min at room temperature. Moreover, the retained Co²⁺ in EA‐CoOOH results in increased activity, confirmed by density functional theory calculations. Consequently, these EA‐CoOOH hybrids deliver a capacitance value of 832 F g^?1 at a current density of 1 A g^?1 and exhibit a retention rate up to 78% (649 F g^?1) at a super‐large current density of 200 A g^?1. This technology paves a way for ultrafast configuration/modulation of defects on advanced materials toward application in the fields of energy and catalysis.     

High‐Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

The voltage limit for aqueous asymmetric supercapacitors is usually 2 V, which impedes further improvement in energy density. Here, high Na content Birnessite Na_0.5MnO₂ nanosheet assembled nanowall arrays are in situ formed on carbon cloth via electrochemical oxidation. It is interesting to find that the electrode potential window for Na_0.5MnO₂ nanowall arrays can be extended to 0–1.3 V (vs Ag/AgCl) with significantly increased specific capacitance up to 366 F g^?1. The extended potential window for the Na_0.5MnO₂ electrode provides the opportunity to further increase the cell voltage of aqueous asymmetric supercapacitors beyond 2 V. To construct the asymmetric supercapacitor, carbon‐coated Fe₃O₄ nanorod arrays are synthesized as the anode and can stably work in a negative potential window of ?1.3 to 0 V (vs Ag/AgCl). For the first time, a 2.6 V aqueous asymmetric supercapacitor is demonstrated by using Na_0.5MnO₂ nanowall arrays as the cathode and carbon‐coated Fe₃O₄ nanorod arrays as the anode. In particular, the 2.6 V Na_0.5MnO₂//Fe₃O₄@C asymmetric supercapacitor exhibits a large energy density of up to 81 Wh kg^?1 as well as excellent rate capability and cycle performance, outperforming previously reported MnO₂‐based supercapacitors. This work provides new opportunities for developing high‐voltage aqueous asymmetric supercapacitors with further increased energy density.     

Compressing Carbon Nanocages by Capillarity for Optimizing Porous Structures toward Ultrahigh‐Volumetric‐Performance Supercapacitors

High volumetric energy density combined with high power density is highly desired for electrical double‐layer capacitors. Usually the volumetric performance is improved by compressing carbon material to increase density but at the much expense of power density due to the deviation of the compressed porous structure from the ideal one. Herein the authors report an efficient approach to increase the density and optimize the porous structure by collapsing the carbon nanocages via capillarity. Three samples with decreasing sizes of meso‐ and macropores provide us an ideal model system to demonstrate the correlation of volumetric performance with porous structure. The results indicate that reducing the surplus macropores and, more importantly, the surplus mesopores is an efficient strategy to enhance the volumetric energy density while keeping the high power density. The optimized sample achieves a record‐high stack volumetric energy density of 73 Wh L^?1 in ionic liquid with superb power density and cycling stability.     

Conversion Synthesis of Self‐Standing Potassium Zinc Hexacyanoferrate Arrays as Cathodes for High‐Voltage Flexible Aqueous Rechargeable Sodium‐Ion Batteries

Prussian blue analogs exhibit great promise for applications in aqueous rechargeable sodium‐ion batteries (ARSIBs) due to their unique open framework and well‐defined discharge voltage plateau. However, traditional coprecipitation methods cannot prepare self‐standing electrodes to meet the needs of wearable energy storage devices. In this work, a water bath method is reported to grow microcube‐like K₂Zn₃(Fe(CN)₆)₂·9H₂O on carbon cloth (CC) using Zn nanosheet arrays as the zinc source and reducing agent, directly serving as a self‐standing cathode. Benefiting from fast ion diffusion and high conductivity, the cathode delivers a high areal capacity of 0.76 mAh cm^?2 at 0.5 mA cm^?2 and excellent capacity retention of 57.9% as the current density increases to 20 mA cm^?2. By coupling with NaTi₂(PO₄)₃ grown on CC as an anode, a quasi‐solid‐state flexible ARSIB with a high output voltage plateau of 1.6 V is successfully assembled, exhibiting a superior areal capacity of 0.56 mAh cm^?2 and energy density of 0.92 mWh cm^?2. In particular, the device shows admirable mechanical flexibility, maintaining 90.3% of initial capacity after 3000 bending cycles. This work is anticipated to open a new avenue for the rational design of self‐standing electrodes used in high‐voltage flexible ARSIBs.     

A Flexible Film toward High‐Performance Lithium Storage: Designing Nanosheet‐Assembled Hollow Single‐Hole Ni–Co–Mn–O Spheres with Oxygen Vacancy Embedded in 3D Carbon Nanotube/Graphene Network

Ternary transition metal oxides (TMOs) are highly potential electrode materials for lithium ion batteries (LIBs) due to abundant defects and synergistic effects with various metal elements in a single structure. However, low electronic/ionic conductivity and severe volume change hamper their practical application for lithium storage. Herein, nanosheet‐assembled hollow single‐hole Ni–Co–Mn oxide (NHSNCM) spheres with oxygen vacancies can be obtained through a facile hydrothermal reaction, which makes both ends of each nanosheet exposed to sufficient free space for volume variation, electrolyte for extra active surface area, and dual ion diffusion paths compared with airtight hollow structures. Furthermore, oxygen vacancies could improve ion/electronic transport and ion insertion/extraction process of NHSNCM spheres. Thus, oxygen‐vacancy‐rich NHSNCM spheres embedded into a 3D porous carbon nanotube/graphene network as the anode film ensure efficient electrolyte infiltration into both the exterior and interior of porous and open spheres for a high utilization of the active material, showing an excellent electrochemical performance for LIBs (1595 mAh g^?1 over 300 cycles at 2 A g^?1, 441.6 mAh g^?1 over 4000 cycles at 10 A g^?1). Besides, this straightforward synthetic method opens an efficacious avenue for the construction of various nanosheet‐assembled hollow single‐hole TMO spheres for potential applications.