Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon Nanotube Fibers for Ultrahigh-Energy-Density All-Solid-State Wearable Asymmetric Supercapacitors

Unraveling and regulating electrode-electrolyte integration towards high-performing MoS2 loaded fiber shaped supercapacitor

Molybdenum disulfide (MoS2) is touted as a highly promising material for fiber-shaped supercapacitors (FSCs) but limited by its low capacitance and unsatisfactory cycling stability. Here, we report a MoS2 deposited stainless steel wire (MoS2@SSW) that can be electrochemically intercalated with dual ions (Na+ and H+). A high capacitance of ∼1632.7 mF cm-2 at 0.4 mA cm-2 and high capacitance retention of 84.25 % after 10,000 cycles under sulfuric acid/sodium sulfate (H2SO4/Na2SO4) aqueous electrolyte is recorded for this system. This is mainly attributed to the pre-intercalation of H+ into MoS2 to form MoS-SH. This process redistributes the local charge regions on MoS2 surface and lowers the energy barrier for Na+ migration in MoS2 to facilitate the adsorption and intercalation of Na+. Notably, MoS2@SSW FSCs are integrated into three-dimensional space textiles as a proof-of-concept. This novel exploration of the nanointerface resulted by electrolyte engineering offers a feasible design paradigm for the development of high-performing FSCs.

Hierarchical Spatial Confinement Unlocking the Storage Limit of MoS2 for Flexible High-Energy Supercapacitors

Molybdenum sulfide (MoS2) is a promising electrode material for supercapacitors; however, its limited Mo/S edge sites and intrinsic inert basal plane give rise to sluggish active electronic states, thus constraining its electrochemical performance. Here we propose a hierarchical confinement strategy to develop ethylene molecule (EG)-intercalated Co-doped sulfur-deficient MoS2 (Co-EG/SV-MoS2) for efficient and durable K-ion storage. Theoretical analyses suggest that the intercalation-confined EG and lattice-confined Co can enhance the interfacial K-ion storage capacity while reducing the K-ion diffusion barrier. Experimentally, the intercalated EG molecules with mildly reducing properties induced the creation of sulfur vacancies, expanded the interlayer spacing, regulated the 2H-1T phase transition, and strengthened the structural grafting between layers, thereby facilitating ion diffusion and ensuring structural durability. Moreover, the Co dopants occupying the initial Mo sites initiated charge transfer, thus activating the basal plane. Consequently, the optimized Co-EG/SV-MoS2 electrode exhibited a substantially improved electrochemical performance. Flexible supercapacitors assembled with Co-EG/SV-MoS2 delivered a notable areal energy density of 0.51 mW h cm-2 at 0.84 mW cm-2 with good flexibility. Furthermore, supercapacitor devices were integrated with a strain sensor to create a self-powered system capable of real-time detection of human joint motion.

Different metal cation-doped MnO2/carbon cloth for wide voltage energy storage

Aqueous gel supercapacitors, as an important component of flexible energy storage devices, have received widespread attention for their fast charging/discharging rates, long cycle life and high electrochemical stability under mechanical deformation condition. However, the low energy density of aqueous gel supercapacitors has greatly hindered their further development due to the narrow electrochemical window and limited energy storage capacity. Therefore, different metal cation-doped MnO₂/carbon cloth-based flexible electrodes herein are prepared by constant voltage deposition and electrochemical oxidation in various saturated sulphate solutions. The influence of different metal cations as K⁺, Na⁺ and Li⁺ doping and deposition conditions on the apparent morphology, lattice structure and electrochemical properties are explored. Furthermore, the pseudo-capacitance ratio of the doped MnO₂ and the voltage expansion mechanism of the composite electrode are investigated. The specific capacitance and pseudo-capacitance ratio of the optimized δ-Na_0.31MnO₂/carbon cloth as MNC-2 electrode could be reached 327.55 F/g at 10 mV/s and 35.56% of the pseudo-capacitance, respectively. The flexible symmetric supercapacitors (NSCs) with desirable electrochemical performances in the operating range of 0-1.4 V are further assembled with MNC-2 as the electrodes. The energy density is 26.8 Wh/kg at the power density of 300 W/kg, while the energy density can still reach 19.1 Wh/kg when the power density is up to 1150 W/kg. The energy storage devices with high-performance developed in this work can provide new ideas and strategic support for the application in portable and wearable electronic devices.

Nanoporous Conducting Polymer Nanowire Network-Encapsulated MnO2-Based Flexible Supercapacitor with Enhanced Rate Capability and Cycling Stability

Transition-metal-oxide-based electrochemical electrodes usually suffer from poor electron and ion transport, leading to deteriorated rate performance and cycling stability. Herein, we address these issues by developing a facile "conducting encapsulation" strategy toward a nanoporous PEDOT nanowire/MnO₂ nanoparticle/PEDOT nanowire composite electrode. Through encapsulation of the PEDOT nanowire network, the overall electrochemical performance of the resultant composite electrode is substantially enhanced. Specifically, the rate capability and capacitance retention are improved by ∼48.2 and ∼33%, respectively, which are 89.8% at 0.8-40 mA/cm² and 93% after 3000 charge/discharge cycles at 2.0 mA/cm², respectively. Moreover, the specific capacitance is increased by ∼6 times of that of the MnO₂@PEDOT NW electrode at ∼200 mA/cm². We find that a nanoporous conducting nanowire network that encapsulates a MnO₂ nanoparticle layer can provide efficient electron and ion transport paths and stabilize the structure of MnO₂ from collapse during charge/discharge cycling and mechanical deformation. This strategy can be applied to other pseudocapacitive material-based electrochemical electrodes, such as transition-metal oxides and conducting polymers.

Layer-by-Layer Heterostructure of MnO2@Reduced Graphene Oxide Composites as High-Performance Electrodes for Supercapacitors

In this paper, δ-MnO₂ with layered structure was prepared by a facile liquid phase method, and exfoliated MnO₂ nanosheet (e-MnO₂) was obtained by ultrasonic exfoliation, whose surface was negatively charged. Then, positive charges were grafted on the surface of MnO₂ nanosheets with a polycation electrolyte of polydiallyl dimethylammonium chloride (PDDA) in different concentrations. A series of e-MnO₂@reduced graphene oxide (rGO) composites were obtained by electrostatic self-assembly combined with hydrothermal chemical reduction. When PDDA was adjusted to 0.75 g/L, the thickness of e-MnO₂ was ~1.2 nm, and the nanosheets were uniformly adsorbed on the surface of graphene, which shows layer-by-layer morphology with a specific surface area of ~154 m²/g. On account of the unique heterostructure, the composite exhibits good electrochemical performance as supercapacitor electrodes. The specific capacitance of e-MnO₂-0.75@rGO can reach 456 F/g at a current density of 1 A/g in KOH electrolyte, which still remains 201 F/g at 10 A/g. In addition, the capacitance retention is 98.7% after 10000 charge-discharge cycles at 20 A/g. Furthermore, an asymmetric supercapacitor (ASC) device of e-MnO₂-0.75@rGO//graphene hydrogel (GH) was assembled, of which the specific capacitance achieves 94 F/g (1 A/g) and the cycle stability is excellent, with a retention rate of 99.3% over 10000 cycles (20 A/g).

Arrayed Heterostructures of MoS2 Nanosheets Anchored TiN Nanowires as Efficient Pseudocapacitive Anodes for Fiber-Shaped Ammonium-Ion Asymmetric Supercapacitors

Nonmetallic ammonium ions that feature high safety, low molar mass, and small hydrated radius properties have shown great advantages in wearable aqueous supercapacitors. The construction of high-energy-density flexible ammonium-ion asymmetric supercapacitors (AASCs) is promising but still challenging due to the lack of high-capacitance pseudocapacitive anodes. Herein, freestanding core-shell heterostructures supported on carbon nanotube fibers were designed by anchoring MoS₂ nanosheets on nanowires (MoS₂@TiN/CNTF) as anodes for AASCs. With contributions of abundant active sites and conspicuous synergistic effects of multiple components for arrayed heterostructure engineering, the developed MoS₂@TiN/CNTF anodes exhibit a specific capacitance of 1102.5 mF cm^-2 at 2 mA cm^-2. Theoretical calculations confirm the dramatic enhancement of the binding strength of ammonium ions on the MoS₂ shell layer at the heterostructure, where a built-in electric field exists to accelerate the charge transfer. By utilizing a MnO₂/CNTF cathode and NH₄Cl/poly(vinyl alcohol) (PVA) as a gel electrolyte, quasi-solid-state fiber-shaped AASCs were successfully constructed, achieving a specific capacitance of 351.2 mF cm^-2 and an energy density of 195.1 μWh cm^-2, outperforming most recently reported fiber-shaped supercapacitors. This work provides a promising strategy to rationally design heterostructure engineering of MoS₂@TiN nanoarrays toward advanced anodes for application in next-generation AASCs.

Sodium Pre-Intercalation-Based Na3-δ-MnO2@CC for High-Performance Aqueous Asymmetric Supercapacitor: Joint Experimental and DFT Study

Electrochemical energy storage devices are ubiquitous for personal electronics, electric vehicles, smart grids, and future clean energy demand. SCs are EES devices with excellent power density and superior cycling ability. Herein, we focused on the fabrication and DFT calculations of Na₃-δ-MnO₂ nanocomposite, which has layered MnO₂ redox-active sites, supported on carbon cloth. MnO₂ has two-dimensional diffusion channels and is not labile to structural changes during intercalation; therefore, it is considered the best substrate for intercalation. Cation pre-intercalation has proven to be an effective way of increasing inter-layered spacing, optimizing the crystal structure, and improving the relevant electrochemical behavior of asymmetric aqueous supercapacitors. We successfully established Na⁺ pre-intercalated δ-MnO₂ nanosheets on carbon cloth via one-pot hydrothermal synthesis. As a cathode, our prepared material exhibited an extended potential window of 0-1.4 V with a remarkable specific capacitance of 546 F g^-1(300 F g^-1 at 50 A g^-1). Moreover, when this cathode was accompanied by an N-AC anode in an asymmetric aqueous supercapacitor, it illustrated exceptional performance (64 Wh kg^-1 at a power density of 1225 W kg^-1) and incomparable potential window of 2.4 V and 83% capacitance retention over 10,000 cycles with a great Columbic efficiency.

Current advances and challenges in nanosheet-based wearable power supply devices

Nowadays, wearable devices mainly exist in the form of portable accessories with various functions, connecting various kinds of terminals like mobile phones to form various wearable systems. In a wearable system, the wearable power supply device is the key component as energy dispenser for all devices. Nanosheets, a kind of two-dimensional material, which always displays a high surface-to-volume ratio and thus is lightweight and has remarkable conductive as well as electrochemical properties, have become the optimal choice for wearable power supply devices. The development and status of nanosheet-based wearable power supply devices including nanosheet-based wearable batteries, nanosheet-based wearable supercapacitors, nanosheet-based wearable self-powered energy suppliers are introduced in this article. Besides, the future opportunities and challenges of wearable devices are discussed.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.

Interfacial Control of NiCoP@NiCoP Core-Shell Nanoflake Arrays as Advanced Cathodes for Ultrahigh-Energy-Density Fiber-Shaped Asymmetric Supercapacitors

Efficient improvement of the energy density and overall electrochemical performance of fiber-shaped asymmetric supercapacitors (FASCs) for practical applications in portable and wearable electronics requires highly electrochemically active materials and a rational design. Herein, two-step phosphorization (TSP) processes are performed to directly grow 3D well-aligned NiCoP@NiCoP (NCP@NCP TSP) nanoflake arrays (NFAs) on carbon nanotube fibers (CNTFs). Profiting from the metallic characteristics and excellent electrochemical performance of NiCoP and the hierarchical design of the core-shell heterostructure, the NCP@NCP TSP NFAs/CNTF hybrid electrode exhibits significantly improved electrochemical performance. The as-fabricated NCP@NCP TSP NFAs/CNTF electrode possesses an ultrahigh areal capacitance of 10 035 mF cm^-2 at a current density of 1 mA cm^-2 , with excellent rate capability and cycling stability. Furthermore, an FASC device with a maximum operating voltage of 1.6 V is assembled by adopting NCP@NCP TSP NFAs/CNTF as a positive electrode, hierarchical TiN@VN core-shell heterostructure nanowire arrays (NWAs)/CNTF as negative electrode, and KOH-PVA as a gel electrolyte. The FASC device exhibits a high areal capacitance of 430.4 mF cm^-2 and an ultrahigh energy density of 51.02 mWh cm^-3 . Thus, the rationally designed NiCoP@NiCoP electrode is a promising candidate for incorporation into next-generation wearable and portable energy-storage devices.

High mass loading flower-like MnO2 on NiCo2O4 deposited graphene/nickel foam as high-performance electrodes for asymmetric supercapacitors

The implementation of high mass loading MnO₂ on electrochemical electrodes of supercapacitors is currently challenging due to the poor electrical conductivity and elongated electron/ion transport distance. In this paper, a NiCo₂O₄/MnO₂ heterostructure was built on the surface of three-dimensional graphene/nickel foam (GNF) by a hydrothermal method. The petal structured NiCo₂O₄ loaded on graphene played a wonderful role as a supporting framework, which provided more space for the growth of high mass loading MnO₂ microflowers, thereby increasing the utilization rate of the active material MnO₂. The GNF@NiCo₂O₄/MnO₂ composite was used as a positive electrode and achieved a high areal capacitance of 1630.5 mF cm^-2 at 2 mA cm^-2 in the neutral Na₂SO₄ solution. The asymmetric supercapacitor assembled with the GNF@NiCo₂O₄/MnO₂ positive electrode and activated carbon negative electrode possessed a wide voltage window (2.1 V) and splendid energy density (45.9 Wh kg^-1), which was attributed to the satisfactory electroactive area, low resistance, quick mass diffusion and ion transport caused by high mass loading MnO₂.

Aligned carbon nanotube fibers for fiber-shaped solar cells, supercapacitors and batteries

Aligned carbon nanotube (CNT) fibers have been considered as one of the ideal candidate electrodes for fiber-shaped energy harvesting and storage devices, due to their merits of flexibility, lightweight, desirable mechanical property, outstanding electrical conductivity as well as high specific surface area. Herein, the recent advancements on the aligned CNT fibers for energy harvesting and storage devices are reviewed. The synthesis, structure, and properties of aligned carbon nanotube fibers are briefly summarized. Then, their applications in fiber-shaped energy harvesting and storage devices (i.e., solar cells, supercapacitors, and batteries) are demonstrated. The remaining challenges are finally discussed to highlight the future research direction in the development of aligned CNT fibers for fiber-shaped energy devices.

Ultra-High Energy Density Hybrid Supercapacitors Using MnO2/Reduced Graphene Oxide Hybrid Nanoscrolls

Manganese oxide (MnO2) is a promising material for supercapacitor applications, with a theoretical ultra-high energy density of 308 Wh/kg. However, such ultra-high energy density has not been achieved experimentally in MnO2-based supercapacitors because of several practical issues, such as low electrical conductivity of MnO2, incomplete utilization of MnO2, and dissolution of MnO2. The present study investigates the potential of MnO2/reduced graphene oxide (rGO) hybrid nanoscroll (GMS) structures as electrode material for overcoming the difficulties and for developing ultra-high-energy storage systems. A hybrid supercapacitor, comprising MnO2/rGO nanoscrolls as anode material and activated carbon (AC) as a cathode, is fabricated. The GMS/AC hybrid supercapacitor exhibited enhanced energy density, superior rate performance, and promising Li storage capability that bridged the energy-density gap between conventional Li-ion batteries (LIBs) and supercapacitors. The fabricated GMS/AC hybrid supercapacitor demonstrates an ultra-high lithium discharge capacity of 2040 mAh/g. The GMS/AC cell delivered a maximum energy density of 105.3 Wh/kg and a corresponding power density of 308.1 W/kg. It also delivered an energy density of 42.77 Wh/kg at a power density as high as 30,800 W/kg. Our GMS/AC cell's energy density values are very high compared with those of other reported values of graphene-based hybrid structures. The GMS structures offer significant potential as an electrode material for energy-storage systems and can also enhance the performance of the other electrode materials for LIBs and hybrid supercapacitors.

True Meaning of Pseudocapacitors and Their Performance Metrics: Asymmetric versus Hybrid Supercapacitors

The development of pseudocapacitive materials for energy-oriented applications has stimulated considerable interest in recent years due to their high energy-storing capacity with high power outputs. Nevertheless, the utilization of nanosized active materials in batteries leads to fast redox kinetics due to the improved surface area and short diffusion pathways, which shifts their electrochemical signatures from battery-like to the pseudocapacitive-like behavior. As a result, it becomes challenging to distinguish "pseudocapacitive" and "battery" materials. Such misconceptions have further impacted on the final device configurations. This Review is an earnest effort to clarify the confusion between the battery and pseudocapacitive materials by providing their true meanings and correct performance metrics. A method to distinguish battery-type and pseudocapacitive materials using the electrochemical signatures and quantitative kinetics analysis is outlined. Taking solid-state supercapacitors (SSCs, only polymer gel electrolytes) as an example, the distinction between asymmetric and hybrid supercapacitors is discussed. The state-of-the-art progress in the engineering of active materials is summarized, which will guide for the development of real-pseudocapacitive energy storage systems.

Defect Promoted Capacity and Durability of N-MnO2- x Branch Arrays via Low-Temperature NH3 Treatment for Advanced Aqueous Zinc Ion Batteries

Defect engineering (doping and vacancy) has emerged as a positive strategy to boost the intrinsic electrochemical reactivity and structural stability of MnO₂ -based cathodes of rechargeable aqueous zinc ion batteries (RAZIBs). Currently, there is no report on the nonmetal element doped MnO₂ cathode with concomitant oxygen vacancies, because of its low thermal stability with easy phase transformation from MnO₂ to Mn₃ O₄ (≥300 °C). Herein, for the first time, novel N-doped MnO_{2- x} (N-MnO_{2- x} ) branch arrays with abundant oxygen vacancies fabricated by a facile low-temperature (200 °C) NH₃ treatment technology are reported. Meanwhile, to further enhance the high-rate capability, highly conductive TiC/C nanorods are used as the core support for a N-MnO_{2- x} branch, forming high-quality N-MnO_{2- x} @TiC/C core/branch arrays. The introduced N dopants and oxygen vacancies in MnO₂ are demonstrated by synchrotron radiation technology. By virtue of an integrated conductive framework, enhanced electron density, and increased surface capacitive contribution, the designed N-MnO_{2- x} @TiC/C arrays are endowed with faster reaction kinetics, higher capacity (285 mAh g^-1 at 0.2 A g^-1 ) and better long-term cycles (85.7% retention after 1000 cycles at 1 A g^-1 ) than other MnO₂ -based counterparts (55.6%). The low-temperature defect engineering sheds light on construction of advanced cathodes for aqueous RAZIBs.

Experimental and Theoretical Investigation of the Effect of Oxygen Vacancies on the Electronic Structure and Pseudocapacitance of MnO2

Defect engineering is an effective way to modulate the intrinsic physicochemical properties of materials. In this work, δ-MnO₂ with oxygen vacancies is fabricated by a simple oxidation or reduction process, and the relationship between the electronic structure and pseudocapacitance is systematically studied through experimental analysis and theoretical calculations. The peaks in the Raman spectra of the as-prepared samples are shifted compared with those of pure MnO₂ and the Mn³⁺ /Mn⁴⁺ ratio and O species content also change after the introduction of oxygen vacancies. The optimized samples exhibit a better specific capacitance of 207 F g^-1 after the oxidation process and 181.4 F g^-1 after the reduction treatment compared with only 143.9 F g^-1 for the pure MnO₂ . The samples obtained through the oxidation or reduction process also retain 93.3 or 86.4 % of the initial capacity after 5000 cycles. The excellent properties are attributed to the enhanced conductivity and increased surface reactivity or electrochemically active sites. Theoretical calculations demonstrate that the presence of oxygen vacancies leads to an increase in the density of states, which improves the redox reaction of MnO₂ . This study will provide a reference for exploring and designing highperformance pseudocapacitive materials.

Electrodeposition of α-MnO2/γ-MnO2 on Carbon Nanotube for Yarn Supercapacitor

Yarn supercapacitors have attracted renewed interest as promising energy storage for wearable devices due to their lightweight, long cycling lifetime and excellent weavability. There has been much effort to fabricate high performance yarn supercapacitor by depositing pseudo-capacitive materials on the outer surface of the carbon fibers. However, a key challenge still remains to achieve high capacitance and high mass loading without sacrificing the cycling stability. Herein, we perform a phase-controlled of MnO₂ at various deposition temperatures with ultrahigh mass loading of 11 mg/cm² on a MWNT sheets and fabricate it to yarn structure to achieve high capacitance without decreasing in the electrochemical performance. The structure of optimized sample (MnO₂/CNTs-60, deposition at 60 °C) consists of the composite of primary α-MnO₂ nanosheets and secondary γ-MnO₂ nanoparticles. The heteronanostructures of MnO₂ provide facile ionic and electric transport in the yarn electrode, resulting in improvement of electrochemical performance and cycling stability. The MnO₂/CNTs-60 yarn electrode with ultrahigh mass loading delivers a high areal capacitance of 3.54 F/cm² at 1 mA/cm² and an excellent rate capability. Finally, the MnO₂/CNTs-60 device exhibits an outstanding high areal energy density of 93.8 μWh/cm² at the power density of 193 μW/cm², which is superior to previously reported symmetric yarn supercapacitors.

Greatly Enhanced Faradic Capacities of 3D Porous Mn3O4/G Composites as Lithium-Ion Anodes and Supercapacitors by C-O-Mn Bonding

Through C-O-Mn bonding, graphene nanosheets are homogeneously dispersed in porous Mn₃O₄ to take full advantages of porous Mn₃O₄ and graphene nanosheets, making the as-formed three-dimensional porous Mn₃O₄/reduced graphene oxide (rGO) composite exhibit good electrochemical performance. Besides, C-O-Mn bonding is demonstrated to greatly promote the Faradic reactions of the composite, resulting in the enhancement of its real capacity in supercapacitor (SC) electrodes as well as lithium-ion battery (LIB) anodes. By simply fine-tuning the content of graphene (<7 wt %), the composite with 2.8 wt % of rGO delivers a high capacitance of 315 F g^-1 at 0.5 A g^-1 with a high rate capability of 64.7% at 30 A g^-1 and an excellent cycling stability of 105% (5 A g^-1, 5000 cycles) as an SC electrode. Also, the one with 6.9 wt % rGO can present a reversible capacity of more than 1500 mAh g^-1 at 0.05 A g^-1 as the LIB anode, the highest value reported to date, which remains 561 mAh g^-1 at 1 A g^-1.