Co-MnO2 Nanorods for High-Performance Sodium/Potassium-Ion Batteries and Highly Conductive Gel-Type Supercapacitors

Boosting the Energy Density of Bowl-Like MnO2@Carbon Through Lithium-Intercalation in a High-Voltage Asymmetric Supercapacitor with "Water-In-Salt" Electrolyte

Highly concentrated "'water-in-salt"' (WIS) electrolytes are promising for high-performance energy storage devices due to their wide electrochemical stability window. However, the energy storage mechanism of MnO2 in WIS electrolytes-based supercapacitors remains unclear. Herein, MnO2 nanoflowers are successfully grown on mesoporous bowl-like carbon (MBC) particles to generate MnO2/MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO2 particles during the fast charging/discharging processes, but also facilitate the electron transfer and ion diffusion within the whole electrode, resulting in significant enhancement of the electrochemical performance. An asymmetric supercapacitor, assembled with MnO2/MBC and activated carbon (AC) and using 21 m LiTFSI solution as the WIS electrolyte, delivers an ultrahigh energy density of 70.2 Wh kg-1 at 700 W kg-1, and still retains 24.8 Wh kg-1 when the power density is increased to 28 kW kg-1. The ex situ XRD, Raman, and XPS measurements reveal that a reversible reaction of MnO2 + xLi+ + xe-↔LixMnO2 takes place during charging and discharging. Therefore, the asymmetric MnO2/MBC//AC supercapacitor with LiTFSI electrolyte is actually a lithium-ion hybrid supercapacitor, which can greatly boost the energy density of the assembled device and expand the voltage window.

Intercalation chemistry engineering strategy enabled high mass loading and ultrastable electrodes for High-Performance aqueous electrochemical energy storage devices

Aqueous electrochemical energy storage devices (AEESDs) are considered one of the most promising candidates for large-scale energy storage infrastructure due to their high affordability and safety. Developing electrodes with the merits of high energy density and long lifespan remains a challenging issue toward the practical application of AEESDs. Research attempts at electrode materials, nanostructure configuration, and electronic engineering show the limitations due to the inherent contradictions associated with thicker electrodes and ion-accessible kinetics. Herein, we propose an intercalation chemistry engineering strategy to enhance the electrolyte ion (de)intercalation behaviors during the electrochemical charge-discharge. To validate this strategy, the prototypical model of a high-mass-loading MnO2-based electrode is used with controlled intercalation of Na+ and H2O. Theoretical and experimental results reveal that an optimal content of Na+ and H2O on the MnO2-based electrode exhibits superior electrochemical performance. Typically, the resultant electrode exhibits an impressive areal capacitance of 1551 mF/cm2 with a mass loading of 9.7 mg/cm2 (at 1 mA/cm2). Furthermore, the assembled full-cell with obtained MnO2-based electrode delivers a high energy density of 0.12 mWh/cm2 (at 20.02 mW/cm2) and ultra-high cycling stability with a capacitance retention percentage of 89.63 % (345 mF/cm2) even after 100,000 cycles (tested over 72 days).

Synergetic Sn Incorporation-Zn Substitution in Copper-Based Sulfides Enabling Superior Na-Ion Storage

Transition-metal sulfides have been regarded as perspective anode candidates for high-energy Na-ion batteries. Their application, however, is precluded severely by either low charge storage or huge volumetric change along with sluggish reaction kinetics. Herein, an effective synergetic Sn incorporation-Zn substitution strategy is proposed based on copper-based sulfides. First, Na-ion storage capability of copper sulfide is significantly improved via incorporating an alloy-based Sn element. However, this process is accompanied by sacrifice of structural stability due to the high Na-ion uptake. Subsequently, to maintain the high Na-ion storage capacity, and concurrently improve cycling and rate capabilities, a Zn substitution strategy (taking partial Sn sites) is carried out, which could significantly promote Na-ion diffusion/reaction kinetics and relieve mechanical strain-stress within the crystal framework. The synergetic Sn incorporation and Zn substitution endow copper-based sulfides with high specific capacity (≈560 mAh g-1 at 0.5 A g-1 ), ultrastable cyclability (80 k cycles with ≈100% capacity retention), superior rate capability up to 200 A g-1 , and ultrafast charging feature (≈4 s per charging with ≈190 mAh g-1 input). This work provides in-depth insights for developing superior anode materials via synergetic multi-cation incorporation/substitution, aiming at solving their intrinsic issues of either low specific capacity or poor cyclability.

Scalable Quasi-Solid-State Supercapacitor for Wide-Temperature Wearable Devices

Quasi-solid-state supercapacitors have wide application prospects in flexible and scalable electronics, which require high capacity, simple form factor, and excellent mechanical robustness. However, it is a challenge to have all these benefits in one material. Addressing this, we report a composite hydrogel with excellent mechanical durability and freezing resistance. The designed composite hydrogel acts both as a load-bearing layer to maintain its structure during deformation and as a permeable binder to stimulate the interfacing between the conductive electrode and the electrolyte to reduce the interface resistance. Flexible supercapacitors are assembled with composite hydrogels and high-performance MnO₂/carbon cloth, which has excellent performance and can store energy at different temperatures or bending states. These results show that the tough hydrogel facilitates the improvement of electrical and mechanical stability, showing great potential in wide-temperature wearable devices.

Compression-tolerant supercapacitor based on NiCo2O4/Ti3C2Tx MXene/reduced graphene oxide composite aerogel with insights from density functional theory simulations

Compression-tolerant electrodes are critical for developing next-generation wearable energy storage devices. However, most of previous studies on compressible electrodes focus on carbon-based materials, whereas metal-based materials such as spinel metal oxide with faradaic nature have been rarely studied due to their lack of compressibility. Herein, NiCo₂O₄ (NCO) microtubes assembled by ultrathin and mesoporous nanosheets, are deposited on/into Ti₃C₂T_x MXene/reduced graphene oxide aerogel (MGA), an intrinsically compressible host template with high conductivity and specific surface areas. The optimized NCO/MGA-300 sample shows a reversible compressive strain of 60% and a superior durability. Density functional theory (DFT) calculations reveal that the NCO/MGA-300 heterojunction has high electronic conductivity, fast electron transfer ability, and low adsorption energy for OH^- ions. As a result, the NCO/MGA-300 electrode exhibits superb electrochemical performance in terms of its high gravimetric capacitance (1633F g^-1 at 1 A g^-1), rate performance (1492F g^-1 at 10 A g^-1), and remarkable cycling stability of 86.6% after 10,000 charge-discharging cycles. Moreover, an assembled asymmetric supercapacitor based on compressible NCO/MGA-300 shows stable electrochemical performances under different compressive strains (20%. 40% and 60%), or after 100 compression-release cycles. This research finding demonstrates the possibility of metal-based electrode for wearable devices with high energy storage capability and good compressibility.

Phase-engineered cathode for super-stable potassium storage

The crystal phase structure of cathode material plays an important role in the cell performance. During cycling, the cathode material experiences immense stress due to phase transformation, resulting in capacity degradation. Here, we show phase-engineered VO₂ as an improved potassium-ion battery cathode; specifically, the amorphous VO₂ exhibits superior K storage ability, while the crystalline M phase VO₂ cannot even store K⁺ ions stably. In contrast to other crystal phases, amorphous VO₂ exhibits alleviated volume variation and improved electrochemical performance, leading to a maximum capacity of 111 mAh g^-1 delivered at 20 mA g^-1 and over 8 months of operation with good coulombic efficiency at 100 mA g^-1. The capacity retention reaches 80% after 8500 cycles at 500 mA g^-1. This work illustrates the effectiveness and superiority of phase engineering and provides meaningful insights into material optimization for rechargeable batteries.

Versatile iodine-doped BiOCl with abundant oxygen vacancies and (110) crystal planes for enhanced pollutant photodegradation

Crystal plane regulation, defect engineering, and element doping can effectively solve the problems of large band gaps, poor light absorption, and fast recombination of BiOCl. In this work, iodine-doped BiOCl (I/BiOCl) nanowafers with abundant (110) crystal planes and oxygen vacancies (OV) were prepared by a simple hydrothermal method and assessed for pollutant photodegradation. I/BiOCl with a molar ratio of I to Cl of 0.6 (I_0.6/BiOCl) degraded under visible light 95.8% of the toxic dye rhodamine B and 85.1% of the persistent antibiotic tetracycline in 5 and 10 min, respectively. In comparison, unmodified BiOCl photodegraded only between 42.0% and 48.2% of these critical water pollutants. Furthermore, I_0.6/BiOCl was highly stable with most of its photocatalytic activity remaining after 4 cycles. Three reasons explain the excellent photodegradation properties of I_0.6/BiOCl. First, the doped photocatalyst grew abundant (110) crystal planes, which inhibits the recombination of photogenerated electron-hole pairs. Second, the large quantity of OV present in I_0.6/BiOCl increased active sites for reactive oxygen species generation, improved photogenerated charge separation, and pollutants adsorption. Lastly, I_0.6/BiOCl had a modified electronic band structure enhancing light absorption. Overall, these results describe a promising photocatalyst capable of degrading efficiently major pollutants with different structures.

Spectroscopic Monitoring of the Electrode Process of MnO2@rGO Nanospheres and Its Application in High-Performance Flexible Micro-Supercapacitors

Structural instability is a major obstacle to realizing the high performance of a MnO₂-based pseudocapacitor material. Understanding its structure transformation in the process of electrochemical reaction, therefore, plays an important role in the efficient enhancement of rate capacity and stability. Herein, a stable MnO₂@rGO core-shell nanosphere is first synthesized by a liquid-liquid interface deposition further combined with the electrostatic self-assembly method. The structural transformation process of the MnO₂@rGO electrode is monitored by ex situ Raman and X-ray diffraction spectroscopy during the charging-discharging process. It is found in the first discharging process that layered-MnO₂ transforms into the spinel-Mn₃O₄ phase with K⁺ ion intercalation. From the second charging, the spinel-Mn₃O₄ phase is gradually adjusted to a more stable λ-MnO₂ with a three-dimensional tunnel structure, finally realizing the reversible intercalation/deintercalation of K⁺ ions in the λ-MnO₂ tunnel structure during subsequent cycling, which can be attributed to the presence of oxygen vacancies formed by the lengthening of the Mn-O bond and losing oxygen in the MnO₆ octahedral unit with K⁺ ion intercalation/deintercalation. Meanwhile, the MnO₂@rGO electrode demonstrates a high specific capacitance of 378 F g^-1 at 1 A g^-1 and excellent cycling stability with a capacitance retention of up to 89.5% after 10 000 cycles at 10 A g^-1. Furthermore, the assembled symmetric micro-supercapacitor delivers a high areal energy density of 1.01 μWh cm^-2, superior cycling stability with no significant capacity decay after 8700 cycles, and a capacity retention rate of almost 100% after 2000 bending cycles, showing great mechanical flexibility and practicability.

A Versatile Strategy for Achieving Fast-Charging Batteries via Interfacial Engineering: Pseudocapacitive Potassium Storage without Nanostructuring

The rapid transport of alkali ions in electrodes is a long-time dream for fast-charging batteries. Though electrode nanostructuring has increased the rate-capability, its practical use is limited because of the low tap density and severe irreversible reactions. Therefore, development of a strategy to design fast-charging micron-sized electrodes without nanostructuring is of significant importance. Herein, a simple and versatile strategy to accelerate the alkali ion diffusion behavior in micron-sized electrode is reported. It is demonstrated that the diffusion rate of K⁺ ions is significantly improved at the hetero-interface between orthorhombic Nb₂ O₅ (001) and monoclinic MoO₂ (110) planes. Lattice distortion at the hetero-interface generates an inner space large enough for the facile transport of K⁺ ions, and electron localization near oxygen-vacant sites further enhances the ion diffusion behavior. As a result, the interfacial-engineered micron-sized anode material achieves an outstanding rate capability in potassium-ion batteries (KIBs), even higher than nanostructured orthorhombic Nb₂ O₅  which is famous for fast-charging electrodes. This is the first study to develop an intercalation pseudocapacitive micron-sized anode without nanostructuring for fast-charging and high volumetric energy density KIBs. More interestingly, this strategy is not limited to K⁺ ion, but also applicable to Li⁺ ion, implying the versatility of interfacial engineering for alkali ion batteries.

Graphene Quantum Dots Pinned on Nanosheets-Assembled NiCo-LDH Hollow Micro-Tunnels: Toward High-Performance Pouch-Type Supercapacitor via the Regulated Electron Localization

A combined delicate micro-/nano-architecture and corresponding surface modification at the nanometer level can co-tailor the physicochemical properties to realize an advanced supercapacitor electrode material. Herein, nanosheets-assembled nickel-cobalt-layered double hydroxide (NiCo-LDH) hollow micro-tunnels strongly coupled with higher-Fermi-level graphene quantum dots (GQDs) are reported. The unique hollow structure endows the electrolyte accessible to more electroactive sites, while 2D nanosheets have excellent surface chemistry, which favors rapid ion/electron transfer, synergistically resulting in more super-capacitive activities. The experimental and density functional theory calculations recognize that such a precise decoration generally tunes the charge density distribution at the near-surface due to the Fermi-level difference of two components, thus regulating the electron localization, while decorating with conductive GQDs co-improves the charge mobility, affording superior capacitive response and electrode integrity. The as-acquired GQDs@LDH-2 electrode yields excellent capacitance reaching ≈1628 F g^-1 at 1 A g^-1 and durable cycling longevity (86.2% capacitive retention after 8000 cycles). When coupled with reduced graphene oxide-based negative electrode, the hybrid device unveils an impressive energy/power density (46 Wh kg^-1 / 7440 W kg^-1 ); moreover, a flexible pouch-type supercapacitor can be constructed based on this hybrid system, which holds high mechanical properties and stable energy and power output at various situations, showcasing superb application prospects.

Electrode-Electrolyte Interfacial Chemistry Modulation for Ultra-High Rate Sodium-Ion Batteries

Sodium-ion batteries capable of operating at rate and temperature extremes are highly desirable, but elusive due to the dynamics and thermodynamics limitations. Herein, a strategy of electrode-electrolyte interfacial chemistry modulation is proposed. The commercial hard carbon demonstrates superior rate performance with 212 mAh g^-1 at an ultra-high current density of 5 A g^-1 in the electrolyte with weak ion solvation/desolvation, which is much higher than those in common electrolytes (nearly no capacity in carbonate-based electrolytes). Even at -20 °C, a high capacity of 175 mAh g^-1 (74 % of its room-temperature capacity) can be maintained at 2 A g^-1 . Such an electrode retains 90 % of its initial capacity after 1000 cycles. As proven, weak ion solvation/desolvation of tetrahydrofuran greatly facilitates fast-ion diffusion at the SEI/electrolyte interface and homogeneous SEI with well-distributed NaF and organic components ensures fast Na⁺ diffusion through the SEI layer and a stable interface.