Achieving Ultrahigh Energy Density and Long Durability in a Flexible Rechargeable Quasi‐Solid‐State Zn–MnO2 Battery

Advanced flexible batteries with high energy density and long cycle life are an important research target. Herein, the first paradigm of a high‐performance and stable flexible rechargeable quasi‐solid‐state Zn–MnO₂ battery is constructed by engineering MnO₂ electrodes and gel electrolyte. Benefiting from a poly(3,4‐ethylenedioxythiophene) (PEDOT) buffer layer and a Mn²⁺‐based neutral electrolyte, the fabricated Zn–MnO₂@PEDOT battery presents a remarkable capacity of 366.6 mA h g^?1 and good cycling performance (83.7% after 300 cycles) in aqueous electrolyte. More importantly, when using PVA/ZnCl₂/MnSO₄ gel as electrolyte, the as‐fabricated quasi‐solid‐state Zn–MnO₂@PEDOT battery remains highly rechargeable, maintaining more than 77.7% of its initial capacity and nearly 100% Coulombic efficiency after 300 cycles. Moreover, this flexible quasi‐solid‐state Zn–MnO₂ battery achieves an admirable energy density of 504.9 W h kg^?1 (33.95 mW h cm^?3), together with a peak power density of 8.6 kW kg^?1, substantially higher than most recently reported flexible energy‐storage devices. With the merits of impressive energy density and durability, this highly flexible rechargeable Zn–MnO₂ battery opens new opportunities for powering portable and wearable electronics.     

Bamboo‐Like Nitrogen‐Doped Carbon Nanotube Forests as Durable Metal‐Free Catalysts for Self‐Powered Flexible Li–CO2 Batteries

The Li–CO₂ battery is a promising energy storage device for wearable electronics due to its long discharge plateau, high energy density, and environmental friendliness. However, its utilization is largely hindered by poor cyclability and mechanical rigidity due to the lack of a flexible and durable catalyst electrode. Herein, flexible fiber‐shaped Li–CO₂ batteries with ultralong cycle‐life, high rate capability, and large specific capacity are fabricated, employing bamboo‐like N‐doped carbon nanotube fiber (B‐NCNT) as flexible, durable metal‐free catalysts for both CO₂ reduction and evolution reactions. Benefiting from high N‐doping with abundant pyridinic groups, rich defects, and active sites of the periodic bamboo‐like nodes, the fabricated Li–CO₂ battery shows outstanding electrochemical performance with high full‐discharge capacity of 23 328 mAh g^?1, high rate capability with a low potential gap up to 1.96 V at a current density of 1000 mA g^?1, stability over 360 cycles, and good flexibility. Meanwhile, the bifunctional B‐NCNT is used as the counter electrode for a fiber‐shaped dye‐sensitized solar cell to fabricate a self‐powered fiber‐shaped Li–CO₂ battery with overall photochemical–electric energy conversion efficiency of up to 4.6%. Along with a stable voltage output, this design demonstrates great adaptability and application potentiality in wearable electronics with a breath monitor as an example.     

Interfacial Engineering Coupled Valence Tuning of MoO3 Cathode for High‐Capacity and High‐Rate Fiber‐Shaped Zinc‐Ion Batteries

Aqueous Zn‐ion batteries (ZIBs) have garnered the researchers' spotlight owing to its high safety, cost effectiveness, and high theoretical capacity of Zn anode. However, the availability of cathode materials for Zn ions storage is limited. With unique layered structure along the [010] direction, α‐MoO₃ holds great promise as a cathode material for ZIBs, but its intrinsically poor conductivity severely restricts the capacity and rate capability. To circumvent this issue, an efficient surface engineering strategy is proposed to significantly improve the electric conductivity, Zn ion diffusion rate, and cycling stability of the MoO₃ cathode for ZIBs, thus drastically promoting its electrochemical properties. With the synergetic effect of Al₂O₃ coating and phosphating process, the constructed Zn//P‐MoO_3?x@Al₂O₃ battery delivers impressive capacity of 257.7 mAh g^?1 at 1 A g^?1 and superior rate capability (57% capacity retention at 20 A g^?1), dramatically surpassing the pristine Zn//MoO₃ battery (115.8 mAh g^?1; 19.7%). More importantly, capitalized on polyvinyl alcohol gel electrolyte, an admirable capacity (19.2 mAh cm^?3) as well as favorable energy density (14.4 mWh cm^?3; 240 Wh kg^?1) are both achieved by the fiber‐shaped quasi‐solid‐state ZIB. This work may be a great motivation for further research on molybdenum or other layered structure materials for high‐performance ZIBs.     

In Situ Activation of 3D Porous Bi/Carbon Architectures: Toward High‐Energy and Stable Nickel–Bismuth Batteries

To achieve high‐energy and stable aqueous rechargeable batteries, state‐of‐the art of anode materials are needed. Bismuth (Bi) has recently emerged as an attractive anode material due to its highly reversible redox reaction and suitable negative operating working window. However, the capacity and durability of currently reported Bi anodes are still far from satisfactory. Here, an in situ activation strategy is reported to prepare a 3D porous high‐density Bi nanoparticles/carbon architecture (P–Bi–C) as an efficient anode for nickel–bismuth batteries. Taking advantages of the fast channels for charge transfer and ion diffusion, enhanced wettability, and accessible surface area, the highly loaded P–Bi–C electrode delivers a remarkable capacity of 2.11 mA h cm^?2 as well as high rate capability (1.19 mA h cm^?2 at 120 mA cm^?2). To highlight, a robust aqueous rechargeable Ni//Bi battery based on the P–Bi–C anode is first constructed, achieving decent capacity (141 mA h g^?1), impressive durability (94% capacity retention after 5000 cycles), and admirable energy density (16.9 mW h cm^?3). This work paves the way for designing superfast nickel–bismuth batteries with high energy and long‐life and may inspire new development for aqueous rechargeable batteries.     

Exploration of Advanced Electrode Materials for Approaching High‐Performance Nickel‐Based Superbatteries

The surging interest in high performance, low‐cost, and safe energy storage devices has spurred tremendous research efforts in the development of advanced electrode active materials. Herein, the in situ growth of zinc–iron layered double hydroxide (Zn–Fe LDH) on graphene aerogel (GA) substrates through a facile, one‐pot hydrothermal method is reported. The strong interaction and efficient electronic coupling between LDH and graphene substantially improve interfacial charge transport properties of the resulting nanocomposite and provide more available redox active sites for faradaic reactions. An LDH–GA||Ni(OH)₂ device is also fabricated that results in greatly enhanced specific capacity (187 mAh g^?1 at 0.1 A g^?1), outstanding specific energy (147 Wh kg^?1), excellent specific power (16.7 kW kg^?1), along with 88% capacity retention after >10 000 cycles. This approach is further extended to Ni–MH and Ni–Cd batteries to demonstrate the feasibility of compositing with graphene for boosting the energy storage performance of other well‐known Ni‐based batteries. In contrast to conventional Ni‐based batteries, the nearly flat voltage plateau followed by a sloping potential profile of the integrated supercapacitor–battery enables it to be discharged down to 0 V without being damaged. These findings provide new prospects for the design of high‐performance and affordable superbatteries based on earth‐abundant elements.     

Co Nanoislands Rooted on Co–N–C Nanosheets as Efficient Oxygen Electrocatalyst for Zn–Air Batteries

Developing non‐precious‐metal bifunctional oxygen reduction and evolution reaction (ORR/OER) catalysts is a major task for promoting the reaction efficiency of Zn–air batteries. Co‐based catalysts have been regarded as promising ORR and OER catalysts owing to the multivalence characteristic of cobalt element. Herein, the synthesis of Co nanoislands rooted on Co–N–C nanosheets supported by carbon felts (Co/Co–N–C) is reported. Co nanosheets rooted on the carbon felt derived from electrodeposition are applied as the self‐template and cobalt source. The synergistic effect of metal Co islands with OER activity and Co–N–C nanosheets with superior ORR performance leads to good bifuctional catalytic performances. Wavelet transform extended X‐ray absorption fine spectroscopy and X‐ray photoelectron spectroscopy certify the formation of Co (mainly Co⁰) and the Co–N–C (mainly Co²⁺ and Co³⁺) structure. As the air‐cathode, the assembled aqueous Zn–air battery exhibits a small charge–discharge voltage gap (0.82 V@10 mA cm^?2) and high power density of 132 mW cm^?2, outperforming the commercial Pt/C catalyst. Additionally, the cable flexible rechargeable Zn–air battery exhibits excellent bendable and durability. Density functional theory calculation is combined with operando X‐ray absorption spectroscopy to further elucidate the active sites of oxygen reactions at the Co/Co–N–C cathode in Zn–air battery.     

Textile Inspired Lithium–Oxygen Battery Cathode with Decoupled Oxygen and Electrolyte Pathways

The lithium–air (Li–O₂) battery has been deemed one of the most promising next‐generation energy‐storage devices due to its ultrahigh energy density. However, in conventional porous carbon–air cathodes, the oxygen gas and electrolyte often compete for transport pathways, which limit battery performance. Here, a novel textile‐based air cathode is developed with a triple‐phase structure to improve overall battery performance. The hierarchical structure of the conductive textile network leads to decoupled pathways for oxygen gas and electrolyte: oxygen flows through the woven mesh while the electrolyte diffuses along the textile fibers. Due to noncompetitive transport, the textile‐based Li–O₂ cathode exhibits a high discharge capacity of 8.6 mAh cm^?2, a low overpotential of 1.15 V, and stable operation exceeding 50 cycles. The textile‐based structure can be applied to a range of applications (fuel cells, water splitting, and redox flow batteries) that involve multiple phase reactions. The reported decoupled transport pathway design also spurs potential toward flexible/wearable Li–O₂ batteries.     

Atomically Thin Mesoporous Co3O4 Layers Strongly Coupled with N‐rGO Nanosheets as High‐Performance Bifunctional Catalysts for 1D Knittable Zinc–Air Batteries

Under development for next‐generation wearable electronics are flexible, knittable, and wearable energy‐storage devices with high energy density that can be integrated into textiles. Herein, knittable fiber‐shaped zinc–air batteries with high volumetric energy density (36.1 mWh cm^?3) are fabricated via a facile and continuous method with low‐cost materials. Furthermore, a high‐yield method is developed to prepare the key component of the fiber‐shaped zinc–air battery, i.e., a bifunctional catalyst composed of atomically thin layer‐by‐layer mesoporous Co₃O₄/nitrogen‐doped reduced graphene oxide (N‐rGO) nanosheets. Benefiting from the high surface area, mesoporous structure, and strong synergetic effect between the Co₃O₄ and N‐rGO nanosheets, the bifunctional catalyst exhibits high activity and superior durability for oxygen reduction and evolution reactions. Compared to a fiber‐shaped zinc–air battery using state‐of‐the‐art Pt/C + RuO₂ catalysts, the battery based on these Co₃O₄/N‐rGO nanosheets demonstrates enhanced and stable electrochemical performance, even under severe deformation. Such batteries, for the first time, can be successfully knitted into clothes without short circuits under external forces and can power various electronic devices and even charge a cellphone.     

A Quasi‐Solid‐State Flexible Fiber‐Shaped Li–CO2 Battery with Low Overpotential and High Energy Efficiency

The rapid development of wearable electronics requires a revolution of power accessories regarding flexibility and energy density. The Li–CO₂ battery was recently proposed as a novel and promising candidate for next‐generation energy‐storage systems. However, the current Li–CO₂ batteries usually suffer from the difficulties of poor stability, low energy efficiency, and leakage of liquid electrolyte, and few flexible Li–CO₂ batteries for wearable electronics have been reported so far. Herein, a quasi‐solid‐state flexible fiber‐shaped Li–CO₂ battery with low overpotential and high energy efficiency, by employing ultrafine Mo₂C nanoparticles anchored on a carbon nanotube (CNT) cloth freestanding hybrid film as the cathode, is demonstrated. Due to the synergistic effects of the CNT substrate and Mo₂C catalyst, it achieves a low charge potential below 3.4 V, a high energy efficiency of ≈80%, and can be reversibly discharged and charged for 40 cycles. Experimental results and theoretical simulation show that the intermediate discharge product Li₂C₂O₄ stabilized by Mo₂C via coordinative electrons transfer should be responsible for the reduction of overpotential. The as‐fabricated quasi‐solid‐state flexible fiber‐shaped Li–CO₂ battery can also keep working normally even under various deformation conditions, giving it great potential of becoming an advanced energy accessory for wearable electronics.     

Transparent Flexible Heteroepitaxy of NiO Coated AZO Nanorods Arrays on Muscovites for Enhanced Energy Storage Application

Transparent flexible energy storage devices are considered as important chains in the next‐generation, which are able to store and supply energy for electronic devices. Here, aluminum‐doped zinc oxide (AZO) nanorods (NRs) and nickel oxide (NiO)‐coated AZO NRs on muscovites are fabricated by a radio frequency (RF) magnetron sputtering deposition method. Interestingly, AZO NRs and AZO/NiO NRs are excellent electrodes for energy storage application with high optical transparency, high conductivity, large surface area, stability under compressive and tensile strain down to a bending radius of 5 mm with 1000 bending cycles. The obtained symmetric solid‐state supercapacitors based on these electrodes exhibit good performance with a large areal specific capacitance of 3.4 mF cm^?2, long cycle life 1000 times, robust mechanical properties, and high chemical stability. Furthermore, an AZO/NiO//Zn battery based on these electrodes is demonstrated, yielding a discharge capacity of 195 mAh g^?1 at a current rate of 8 A g^?1 and a discharge capacity of over 1000 cycles with coulombic efficiency to 92%. These results deliver a concept of opening a new opportunity for future applications in transparent flexible energy storage.     

Compactly Coupled Nitrogen‐Doped Carbon Nanosheets/Molybdenum Phosphide Nanocrystal Hollow Nanospheres as Polysulfide Reservoirs for High‐Performance Lithium–Sulfur Chemistry

Lithium–sulfur (Li–S) batteries have been disclosed as one of the most promising energy storage systems. However, the low utilization of sulfur, the detrimental shuttling behavior of polysulfides, and the sluggish kinetics in electrochemical processes, severely impede their application. Herein, 3D hierarchical nitrogen‐doped carbon nanosheets/molybdenum phosphide nanocrystal hollow nanospheres (MoP@C/N HCSs) are introduced to Li–S batteries via decorating commercial separators to inhibit polysulfides diffusion. It acts not only as a polysulfides immobilizer to provide strong physical trapping and chemical anchoring toward polysulfides, but also as an electrocatalyst to accelerate the kinetics of the polysulfides redox reaction, and to lower the Li₂S nucleation/dissolution interfacial energy barrier and self‐discharge capacity loss in working Li–S batteries, simultaneously. As a result, the Li–S batteries with MoP@C/N HCS‐modified separators show superior rate capability (920 mAh g^?1 at 2 C) and stable cycling life with only 0.04% capacity decay per cycle over 500 cycles at 1 C with nearly 100% Coulombic efficiency. Furthermore, the Li–S battery can achieve a high area capacity of 5.1 mAh cm^?2 with satisfied capacity retention when the cathode loading reaches 5.5 mg cm^?2. This work offers a brand new guidance for rational separator design into the energy chemistry of high‐stable Li–S batteries.     

Oxygen‐Deficient Ferric Oxide as an Electrochemical Cathode Catalyst for High‐Energy Lithium–Sulfur Batteries

Lithium–sulfur batteries, as one of promising next‐generation energy storage devices, hold great potential to meet the demands of electric vehicles and grids due to their high specific energy. However, the sluggish kinetics and the inevitable “shuttle effect” severely limit the practical application of this technology. Recently, design of composite cathode with effective catalysts has been reported as an essential way to overcome these issues. In this work, oxygen‐deficient ferric oxide (Fe₂O_3?x), prepared by lithiothermic reduction, is used as a low‐cost and effective cathodic catalyst. By introducing a small amount of Fe₂O_3?x into the cathode, the battery can deliver a high capacity of 512 mAh g^?1 over 500 cycles at 4 C, with a capacity fade rate of 0.049% per cycle. In addition, a self‐supporting porous S@KB/Fe₂O_3?x cathode with a high sulfur loading of 12.73 mg cm^?2 is prepared by freeze‐drying, which can achieve a high areal capacity of 12.24 mAh cm^?2 at 0.05 C. Both the calculative and experimental results demonstrate that the Fe₂O_3?x has a strong adsorption toward soluble polysulfides and can accelerate their subsequent conversion to insoluble products. As a result, this work provides a low‐cost and effective catalyst candidate for the practical application of lithium–sulfur batteries.     

Boosting the Cycling Stability of Aqueous Flexible Zn Batteries via F Doping in Nickel–Cobalt Carbonate Hydroxide Cathode

Cathodes of rechargeable Zn batteries typically face the issues of irreversible phase transformation, structure collapse, and volume expansion during repeated charge/discharge cycles, which result in an increased transfer resistance and poor long‐term cycling stability. Herein, a facile F doping strategy is developed to boost the cycling stability of nickel cobalt carbonate hydroxide (NiCo–CH) cathode. Benefiting from the extremely high electronegativity, the phase and morphology stabilities as well as the electrical conductivity of NiCo–CH are remarkably enhanced by F incorporation (NiCo–CH–F). Phase interface and amorphous microdomains are also introduced, which are favorable for the electrochemical performance of cathode. Benefiting from these features, NiCo–CH–F delivers a high capacity (245 mA h g^?1), excellent rate capability (64% retention at 8 A g^?1), and outstanding cycling stability (maintains 90% after 10 000 cycles). Moreover, the quasi‐solid‐state battery also manifests superior cycling stability (maintains 90% after 7200 cycles) and desirable flexibility. This work offers a general strategy to boost the cycling stability of cathode materials for aqueous Zn batteries.     

Graphene Scroll‐Coated α‐MnO2 Nanowires as High‐Performance Cathode Materials for Aqueous Zn‐Ion Battery

The development of manganese dioxide as the cathode for aqueous Zn‐ion battery (ZIB) is limited by the rapid capacity fading and material dissolution. Here, a highly reversible aqueous ZIB using graphene scroll‐coated α‐MnO₂ as the cathode is proposed. The graphene scroll is uniformly coated on the MnO₂ nanowire with an average width of 5 nm, which increases the electrical conductivity of the MnO₂ nanowire and relieves the dissolution of the cathode material during cycling. An energy density of 406.6 Wh kg^?1 (382.2 mA h g^?1) at 0.3 A g^?1 can be reached, which is the highest specific energy value among all the cathode materials for aqueous Zn‐ion battery so far, and good long‐term cycling stability with 94% capacity retention after 3000 cycles at 3 A g^?1 are achieved. Meanwhile, a two‐step intercalation mechanism that Zn ions first insert into the layers and then the tunnels of MnO₂ framework is proved by in situ X‐ray diffraction, galvanostatic intermittent titration technique, and X‐ray photoelectron spectroscopy characterizations. The graphene scroll‐coated metallic oxide strategy can also bring intensive interests for other energy storage systems.     

Covalent‐Organic‐Framework‐Based Li–CO2 Batteries

Covalent organic frameworks (COFs) are an emerging class of porous crystalline materials constructed from designer molecular building blocks that are linked and extended periodically via covalent bonds. Their high stability, open channels, and ease of functionalization suggest that they can function as a useful cathode material in reversible lithium batteries. Here, a COF constructed from hydrazone/hydrazide‐containing molecular units, which shows good CO₂ sequestration properties, is reported. The COF is hybridized to Ru‐nanoparticle‐coated carbon nanotubes, and the composite is found to function as highly efficient cathode in a Li–CO₂ battery. The robust 1D channels in the COF serve as CO₂^– and lithium‐ion‐diffusion channels and improve the kinetics of electrochemical reactions. The COF‐based Li–CO₂ battery exhibits an ultrahigh capacity of 27 348 mAh g^?1 at a current density of 200 mA g^?1, and a low cut‐off overpotential of 1.24 V within a limiting capacity of 1000 mAh g^?1. The rate performance of the battery is improved considerably with the use of the COF at the cathode, where the battery shows a slow decay of discharge voltage from a current density of 0.1 to 4 A g^?1. The COF‐based battery runs for 200 cycles when discharged/charged at a high current density of 1 A g^?1.     

High‐Performance Integrated Self‐Package Flexible Li–O2 Battery Based on Stable Composite Anode and Flexible Gas Diffusion Layer

With the rising development of flexible and wearable electronics, corresponding flexible energy storage devices with high energy density are required to provide a sustainable energy supply. Theoretically, rechargeable flexible Li–O₂ batteries can provide high specific energy density; however, there are only a few reports on the construction of flexible Li–O₂ batteries. Conventional flexible Li–O₂ batteries possess a loose battery structure, which prevents flexibility and stability. The low mechanical strength of the gas diffusion layer and anode also lead to a flexible Li–O₂ battery with poor mechanical properties. All these attributes limit their practical applications. Herein, the authors develop an integrated flexible Li–O₂ battery based on a high‐fatigue‐resistance anode and a novel flexible stretchable gas diffusion layer. Owing to the synergistic effect of the stable electrocatalytic activity and hierarchical 3D interconnected network structure of the free‐standing cathode, the obtained flexible Li–O₂ batteries exhibit superior electrochemical performance, including a high specific capacity, an excellent rate capability, and exceptional cycle stability. Furthermore, benefitting from the above advantages, the as‐fabricated flexible batteries can realize excellent mechanical and electrochemical stability. Even after a thousand cycles of the bending process, the flexible Li–O₂ battery can still possess a stable open‐circuit voltage, a high specific capacity, and a durable cycle performance.     

A Nonaqueous Potassium‐Based Battery–Supercapacitor Hybrid Device

A low cost nonaqueous potassium‐based battery–supercapacitor hybrid device (BSH) is successfully established for the first time with soft carbon as the anode, commercialized activated carbon as the cathode, and potassium bis(fluoro‐slufonyl)imide in dimethyl ether as the electrolyte. This BSH reconciles the advantages of potassium ion batteries and supercapacitors, achieving a high energy density of 120 W h kg^?1, a high power density of 599 W kg^?1, a long cycle life of 1500 cycles, and an ultrafast charge/slow discharge performance (energy density and power density are calculated based on the total mass of active materials in the anode and cathode). This work demonstrates a great potential of applying the nonaqueous BSH for low cost electric energy storage systems.     

A Freestanding and Long‐Life Sodium–Selenium Cathode by Encapsulation of Selenium into Microporous Multichannel Carbon Nanofibers

Selenium cathode has attracted more and more attention because of its comparable volumetric capacity but much higher electrical conductivity than sulfur cathode. Compared to Li–Se batteries, Na–Se batteries show many advantages, including the low cost of sodium resources and high volumetric capacity. However, Na–Se batteries still suffer from the shuttle effect of polyselenides and high volumetric expansion, resulting in the poor electrochemical performance. Herein, Se is impregnated into microporous multichannel carbon nanofibers (Se@MCNFs) thin film with high flexibility as a binder‐free cathode material for Na–Se batteries. The fibrous unique structure of the Se@MCNFs is beneficial to alleviate the volume change of Se during cycling, improve the utilization of active material, and suppress the dissolution of polyselenides into electrolyte. The freestanding Se@MCNF thin‐film electrode exhibits high discharge capacity (596 mA h g^?1 at the 100th cycle at 0.1 A g^?1) and excellent rate capability (379 mA h g^?1 at 2 A g^?1) for Na–Se batteries. In addition, it also shows long cycle life with a negligible capacity decay of 0.067% per cycle over 300 cycles at 0.5 A g^?1. This work demonstrates the possibility to develop high performance Na–Se batteries and flexible energy storage devices.     

Oxygen Vacancy–Rich In‐Doped CoO/CoP Heterostructure as an Effective Air Cathode for Rechargeable Zn–Air Batteries

An efficient and low‐cost electrocatalyst for reversible oxygen electrocatalysis is crucial for improving the performance of rechargeable metal?air batteries. Herein, a novel oxygen vacancy–rich 2D porous In‐doped CoO/CoP heterostructure (In‐CoO/CoP FNS) is designed and developed by a facile free radicals–induced strategy as an effective bifunctional electrocatalyst for rechargeable Zn–air batteries. The electron spin resonance and X‐ray absorption near edge spectroscopy provide clear evidence that abundant oxygen vacancies are formed in the interface of In‐CoO/CoP FNS. Owing to abundant oxygen vacancies, porous heterostructure, and multiple components, In‐CoO/CoP FNS exhibits excellent oxygen reduction reaction activity with a positive half‐wave potential of 0.81 V and superior oxygen evolution reaction activity with a low overpotential of 365 mV at 10 mA cm^?2. Moreover, a home‐made Zn–air battery with In‐CoO/CoP FNS as an air cathode delivers a large power density of 139.4 mW cm^?2, a high energy density of 938 Wh kg_Zn^?1, and can be steadily cycled over 130 h at 10 mA cm^?2, demonstrating great application potential in rechargeable metal–air batteries.     

A Novel Phase‐Transformation Activation Process toward Ni–Mn–O Nanoprism Arrays for 2.4 V Ultrahigh‐Voltage Aqueous Supercapacitors

One of the key challenges of aqueous supercapacitors is the relatively low voltage (0.8–2.0 V), which significantly limits the energy density and feasibility of practical applications of the device. Herein, this study reports a novel Ni–Mn–O solid‐solution cathode to widen the supercapacitor device voltage, which can potentially suppress the oxygen evolution reaction and thus be operated stably within a quite wide potential window of 0–1.4 V (vs saturated calomel electrode) after a simple but unique phase‐transformation electrochemical activation. The solid‐solution structure is designed with an ordered array architecture and in situ nanocarbon modification to promote the charge/mass transfer kinetics. By paring with commercial activated carbon anode, an ultrahigh voltage asymmetric supercapacitor in neutral aqueous LiCl electrolyte is assembled (2.4 V; among the highest for single‐cell supercapacitors). Moreover, by using a polyvinyl alcohol (PVA)–LiCl electrolyte, a 2.4 V hydrogel supercapacitor is further developed with an excellent Coulombic efficiency, good rate capability, and remarkable cycle life (>5000 cycles; 95.5% capacity retention). Only one cell can power the light‐emitting diode indicator brightly. The resulting maximum volumetric energy density is 4.72 mWh cm^?3, which is much superior to previous thin‐film manganese‐oxide‐based supercapacitors and even battery–supercapacitor hybrid devices.