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Electrochemical double-layer capacitors (EDLCs) possess extremely high-power density and a long cycle lifespan, but they have been long constrained by a low energy density. Since the electrochemical stability of electrolytes is essential to the operating voltage of EDLCs, and thus to their energy density, the tuning of electrolyte components towards a high-voltage window has been a research focus for a long time. Organic electrolytes based on ionic liquids (ILs) are recognized as the most commercially promising owing to their moderate operating voltage and excellent conductivity. Despite impressive progress, the working voltage of IL–solvent electrolytes needs to be improved to meet the growing demand. In this review, the recent progress in the tuning of IL- based organic electrolyte components for higher-voltage EDLCs is comprehensively summarized and the advantages and limitations of these innovative components are outlined. Furthermore, future trends of IL–solvent electrolytes in this field are highlighted. [ABSTRACT FROM AUTHOR]

Developing high‐voltage and environmentally friendly aqueous electrolytes have become a significant trend in supercapacitors (SCs). However, achieving this goal usually requires the use of "water‐in‐salt" (WIS) electrolytes, inevitably increasing manufacturing costs and sometimes posing problems such as environmental pollution. Here, we developed a water/organic solvent hybrid electrolyte (8 m NaClO4‐20 % H2O‐30 % ACN‐50 % PEG‐200) with low cost and high‐voltage capacity using the water‐miscible polymer polyethylene glycol (PEG) as a crowding agent to suppress water activity, and acetonitrile (ACN) as a co‐solvent to improve the ion transport properties. The synergistic advantages of the two co‐solvents allow SCs using this electrolyte to have a high operative voltage window of up to 2.6 V, currently the highest operating voltage for aqueous carbon‐based SCs, excellent capacity retention (over 11000 cycles) and high energy densities (31.2 Wh kg−1). This work provides a promising method for developing green and inexpensive high‐voltage window aqueous electrolytes. [ABSTRACT FROM AUTHOR]

Dual‐ion batteries (DIBs) have gained great attention due to their advantages of high operating voltage and low cost. However, the overall performance of the reported DIBs falls far below expectation. In particular, electrode materials with rapid‐charging kinetics and high volumetric packing densities are still in short supply. Herein, a holistic design of the DIB is proposed by using a Wadsley–Roth phase niobium tungsten oxide as the anode, which possesses facile ion transport tunnels and ultrahigh electrode density to deliver superior volumetric performance (521.5 Ah L−1 at 0.2 C and 154.3 Ah L−1 at 60 C), and choosing graphite as the cathode. Benefiting from the respective advantages of this novel anode and the graphite cathode, the as‐built DIB full cell exhibits high volumetric and excellent rate performance (137.7 Ah L−1 at 1 C and 79.2 Ah L−1 at 5 C), and long‐term cyclic stability (1000 cycles at 5 C with negligible capacity attenuation). More impressively, such cells can deliver a high volumetric energy density (422.7 Wh L−1) and power density (8.3 kW L−1). Furthermore, the comprehensive charge storage chemistry of this DIB is elaborated by operando electrochemical characterizations, and this assembly of flexible pouch cells shows its practical prospects. [ABSTRACT FROM AUTHOR]

Sodium-ion capacitors are a new type of energy storage devices that possess the respective advantages of both sodium-ion batteries and electrochemical capacitors. However, the low energy density of sodium-ion capacitors is the main reason restricting their real applications. The key point is to match the kinetic behaviors of the anode and cathode. In this study, similar pore microstructures for both anode and cathode are constructed by using bacterial cellulose as a precursor; the products exhibit identical hierarchical micropore–mesopore networks and consistent pore distributions of 1–3 nm. These similar pore microstructures can effectively improve the behaviors of charge transport and ion diffusion. Benefiting from the merits of the advanced structures and good performance of the anode and cathode materials, a constructed sodium-ion capacitor delivers a high energy density of 124 W h kg−1 at a power density of 210 W kg−1 and a power density of 15 500 W kg−1 at an energy density of 22 W h kg−1 within the potential range of 0–4.2 V; meanwhile, an outstanding cycle performance of 2000 cycles was obtained, with a capacity retention of 99.6%. This study provides a simple and promising method to match the capacity and kinetic behavior of the anode and cathode to build high-performance sodium-ion capacitors. [ABSTRACT FROM AUTHOR]

Abstract Improving the low-temperature tolerance and increasing the energy density of supercapacitors are currently desirable for real-life applications. Commercially available non-aqueous supercapacitors are typically limited in operation to −40 °C or higher, and they usually exhibit poor performance at lower than room temperature. In this work, we investigate symmetric supercapacitors using the 1-butyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4) ionic liquid modified with organic solvents as the electrolyte and activated polyaniline-derived carbon (APDC) as the active material. The symmetric coin cells tests are performed in the range of temperatures from −100 °C to 25 °C. Interestingly, with decreasing temperature, the working voltage window of these supercapacitors gradually increases from 2.5 V to 4.0 V, whereas the energy density initially increases and then decreases. The optimal combination of an APDC electrode and the EMIMBF 4 /acetonitrile/methyl acetate-based electrolyte enables such a supercapacitor to work with a high potential window of 3.5 V and deliver a maximum energy density of 80 Wh kg−1 (based on the total mass of the two-electrode material), a maximum power density of 26 kW kg−1, and a long-term cycle life of 10000 cycles at a low temperature of −50 °C. Highlights • The low-temperature tolerance of supercapacitors is improved. • The voltage window gradually increases with decreasing temperature. • The energy density initially increases and then decreases with decreasing temperature. • A maximum energy density of 80 Wh kg−1 can be obtained at −50 °C. [ABSTRACT FROM AUTHOR]

Carbon based materials as one promising cathode to accommodate the insoluble and insulating discharge products (Li2O2) for lithium oxygen (Li-O2) batteries have attracted great attention due to their large energy density store ability compared with the other carbon-free cathodes. However, the side reaction occurring at carbon/Li2O2 interfaces hinders their large-scale application in Li-O2 batteries. Herein, a simple and cost-effective strategy is developed for the growth of core-shell-like Co/CoO nanoparticles on 3D graphene-wrapped carbon foam using 3D melamine foam as the initial backbone. This unique 3D hierarchical carbonized melamine foam-graphene-Co/CoO hybrid (CMF-G-Co/CoO) with a continuous conductive network and elastic properties is used as binder-free oxygen electrode for Li-O2 batteries. Electrochemical and structural measurements show that a synergistic effect is observed between Co/CoO and graphene, where Li2O2 grows on the Co/CoO surfaces instead of the carbon surfaces at the initial discharge state (500 mAh ), indicating the reduced carbon/Li2O2 interfaces and alleviative side reactions during the electrochemical process. Importantly, the CMF-G-Co/CoO electrode can achieve greatly improved cycle life over the electrode without aid of the Co/CoO. Furthermore, it delivers a large capacity of ≈7800 mAh and outstanding rate capability, exhibiting the great potential for the application in Li-O2 batteries. [ABSTRACT FROM AUTHOR]

Owing to their unique nanosize effect and surface effect, pseudocapacitive quantum dots (QDs) hold considerable potential for high‐efficiency supercapacitors (SCs). However, their pseudocapacitive behavior is exploited in aqueous electrolytes with narrow potential windows, thereby leading to a low energy density of the SCs. Here, a film electrode based on dual‐confined FeOOH QDs (FQDs) with superior pseudocapacitive behavior in a high‐voltage ionic liquid (IL) electrolyte is put forward. In such a film electrode, FQDs are steadily dual‐confined in a 2D heterogeneous nanospace supported by graphite carbon nitride (g‐C3N4) and Ti‐MXene (Ti3C2). Probing of potential‐driven ion accumulation elucidates that strong adsorption occurs between the IL cation and the electrode surface with abundant active sites, providing sufficient redox reaction of FQDs in the film electrode. Furthermore, porous g‐C3N4 and conductive Ti3C2 act as ion‐accessible channels and charge‐transfer pathways, respectively, endowing the FQDs‐based film electrode with favorable electrochemical kinetics in the IL electrolyte. A high‐voltage flexible SC (FSC) based on an ionogel electrolyte is fabricated, exhibiting a high energy density (77.12 mWh cm−3), a high power density, a remarkable rate capability, and long‐term durability. Such an FSC can also be charged by harvesting sustainable energy and can effectively power various wearable and portable electronics. [ABSTRACT FROM AUTHOR]

Developing aqueous electrolyte with wide electrochemical stability window (ESW) is one of the key technologies to boosting the energy density of aqueous electrochemical double-layer capacitors (EDLCs). However, expanding the ESW using super-concentrated electrolytes comes at the expense of cost, ionic conductivity and the mass density of the device. Herein, we selected a cost-effective and environmentally friendly molecular crowding agent, polyethylene glycol dimethyl ether (PEGDME-250), as the electrolyte additive designed to achieve high ESW at low salt concentrations. First, PEGDME can reduces the activity of water and increases the ESW of the electrolyte by forming hydrogen bonds with free water molecule. In addition, PEGDME does not have the interaction between terminal groups compared to conventional polyethylene glycols (PEG), which helps to reduce the viscosity and improve the ionic conductivity of the electrolyte. By adjusting the composition of the electrolyte, the optimal electrolyte with a high ESW of 2.96 V was achieved with the formula of 3 m NaClO 4 -25 % H 2 O-75 % PEGDME. The EDLCs assembled with this diluted aqueous electrolyte and YP-50 F electrodes presents a high operational voltage window of 2.4 V and outstanding cycle stability (>10000 cycles), and maintains excellent rate performance in a temperature range of −10–35 ℃. • The addition of PEGDME reduces the activity of free water molecules. • Low concentration aqueous electrolyte realized electrochemical stabilization window of 2.96 V. • The EDLCs with optimized electrolyte and YP-50 F activated carbon were assembled. • The EDLCs were able to cycle 10,000 cycles at an operating voltage of 2.4 V. • The EDLCs were capable to maintain 72.3 % capacity when the current density increasing from 1 A g−1 to 10 A g−1. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel di-pyrrolidine based electrolyte salt C 4 di[mPyr].di[TFSI] developed for EDLCs. • 0.5 M C 4 di[mPyr].di[TFSI]/AN achieves a 3.4 V working voltage in EDLCs. • EDLCs with 0.5 M C 4 di[mPyr].di[TFSI]/AN sustain 94.92 % capacitance over 2000 cycles at 3.4 V. • GC–MS and XPS reveal the aging mechanisms during polarization. One of the main strategies to enhance the energy density of electrochemical double layer capacitors (EDLCs) is to surpass the limitations of traditional electrolyte operating voltages. In our study, a novel high-purity di-pyrrolidine based electrolyte salt, 1-butyl-3-dimethylpyrrolidinium di[bis(trifluoromethanesulfonyl)imide] (C 4 di[mPyr].di[TFSI]) is synthesized, and high voltage electrolyte using acetonitrile (AN) as the sole solvent is prepared for EDLCs. The 0.5 M C 4 di[mPyr].di[TFSI]/AN electrolyte demonstrates an impressive electrochemical stability window of 6.04 V and a high operating voltage of 3.4 V, which is almost the highest stability reported so far for low concentration AN-based electrolytes. The impedance and conductivity of the 0.5 M C 4 di[mPyr].di[TFSI]/AN electrolyte are highly comparable to commercial 1.0 M TEA.BF 4 /AN electrolyte. EDLCs equipped with the electrolyte can maintain specific capacitance retentions of 94.92 % and 77.78 % after 2000 cycles at 3.4 V and 3.6 V, respectively. Besides, the GC–MS and XPS further prove its stability and confirm the possible electrochemical reactions of the C 4 di[mPyr].di[TFSI]/AN electrolyte on the electrode surface under the polarized voltages. Furthermore, the XPS also appears to show a relatively weak interaction between TFSI− and the electrode, which may serve as a potential factor in inhibiting the hydrolysis of TFSI− and the polymerization of AN. [ABSTRACT FROM AUTHOR]

As an emerging multivalent-ion-based energy storage device, aqueous zinc-ion capacitors (AZICs) combine the merits of zinc-ion batteries with high energy density, excellent safety, low cost and environmental friendliness, and the advantages of supercapacitors with high power density and superior cycling performance. Therefore, AZICs have been considered as a new promising electrochemical energy storage system. Though great progress of AZICs has been made recently, they are still in the infant stage and face many challenges. Specially, the unsatisfactory energy density needs to be improved to realize their practical applications, which calls for high-capacity cathode materials. In this review, after a brief introduction of progress and mechanism for AZICs, we provide a systematical overview of the latest advances in the state-of-art cathode materials. The synthesis route, structure and morphology, electrochemical performance, energy storage mechanism as well as the merits and drawbacks of various cathode materials are comprehensively compared and discussed. Finally, a summary and outlook of the challenges and future perspectives of AZICs were presented. With these, this review might offer some guidance for the future design of dedicated novel cathode materials to realize the great potential of AZICs. In this review, we provide a systematical overview of the latest advances in the state-of-art electrode materials for aqueous zinc-ion capacitors (AZICs). The synthesis routes, structure and morphology, electrochemical performance, energy storage mechanism as well as the merits and drawbacks of various cathode materials are comprehensively compared and discussed. In the end, a summary and outlook of the challenges and future perspectives of AZICs are presented. [Display omitted] [ABSTRACT FROM AUTHOR]

Perovskite-based aqueous supercapacitors have important development prospects due to their advantages of high energy density, low cost, environmental benignity. Here, bimetallic Ni-Mn and trimetallic Ni-Co-Mn perovskite fluorides are prepared via facile solvothermal method and characterized as positive electrode materials for supercapacitors. The structure, composition, chemical states and the electrochemical properties of these samples are investigated in detail. Three electrode measurements indicate that the electrochemical properties of the perovskite fluorides KNiF 3 depend on the nature and amount of dopants. Partial doping of Ni by Mn increases the rate capability but decreases the specific capacity, while Co and Mn co-doping improve both the rate capability and specific capacity of perovskite fluoride KNiF 3. The K-Ni-Co-Mn-F (Ni/Co/Mn = 12:2:1) sample exhibits the maximum specific capacity of 211 mAh g−1, low internal resistance and a high rate capability (82% capacity retention from 1 to 16 A g−1). Furthermore, the AC//K-Ni-Co-Mn-F (Ni/Co/Mn = 12:2:1) asymmetric supercapacitor delivers a maximum energy density of 50.2 Wh kg−1. These results prove that Co and Mn co-doped KNiF 3 can be a promising material for supercapacitor. [ABSTRACT FROM AUTHOR]

Aqueous zinc ion chemistry is the hot topic of new energy storage technologies nowadays for its high intrinsic safety as well as its advantages in terms of environmental protection and price. However, the freezing of water at low temperatures limits the practical application to a broad range of temperatures. Herein, in view of the disturbing effect of Mg(ClO 4) 2 on hydrogen bond of water molecules, a novel anti-freezing electrolyte (0.4 m ZnSO 4 + 2.7 m Mg(ClO 4) 2) is developed, which features ultra-low freezing point (<−80 °C) and superior conductivity (7.50 mS cm−1 at −60 °C). Consequently, this organics-free hybrid electrolyte renders the as-built aqueous zinc ion hybrid capacitors (AZHCs) available to operate even at −60 °C with a high potential window of 2.4 V. Moreover, at −30 °C, such AZHC devices exhibit the highest specific capacity (123 mA h g−1), the maximum energy density (126 Wh kg−1 at 1050 W kg−1), an excellent rate performance (75.8 Wh kg−1 at 10495.4 W kg−1) and a stable working life over 32 000 cycles. This research points to a feasible strategy to improve the cryogenic performance of AZHCs with high energy density. • The addition of Mg(ClO 4) 2 can break the hydrogen bonding between water molecules. • The organics-free aqueous hybrid electrolyte possesses an ultra low freezing point. • The AZHCs exhibit excellent electrochemical performance at low temperatures. • The AZHCs possess a wider ESW at low temperatures. • Provide insights for other AZHCs to have superior low temperature performance. [ABSTRACT FROM AUTHOR]

• A polymeric ionic liquid is synsthesized and served as an electrolyte additive. • PVEIm.TFSI can improve the electrochemical stability of EMIm.TFSI/PC based electrolytes. • The operational voltage of EDLC with PIL additive increases from 2.8 V to 3.2 V. • Equivalent circle model and bode plots demonstrate changes in interface behaviors. • PIL additive reduces each element of impedance and improves the ions diffusion. Improving the energy density of electrochemical double layer capacitors (EDLC) is an important but challenging research topic. Electrolytes with wide operating potential windows are key materials for current EDLC technology. In this work, an ether branch polymeric imidazole ionic liquid (PVEIm.TFSI) is synsthesized and served as an electrolyte additive in common electrolyte to optimize the working potential ranges of carbon-based EDLC. The addition of PVEIm.TFSI can improve the electrochemical stability of electrolytes and effectively adjust the upper limit potential (P U), lower limit potential (P L). Under the optimal addition ratio of 10%, the operational voltage of EDLC increases from 2.8 V to 3.2 V, which lead to an approximately 41.5% energy density increase compared with EDLC without additive. Cycling stabibity of EDLC containing 10% PVEIm.TFSI is significantly enhanced under both 3.0 V and 3.2 V. Furthermore, an equivalent circuit model and Bode plots are proposed from the electrochemical impedance spectroscopy (EIS) measurements. Both simulated and measured date dedicate that PVEIm.TFSI reduces each element of resistance, which result in more ions movement and smaller energy barrier for the diffusion processes, thereby improving the electrochemical performance of cells. [Display omitted] [ABSTRACT FROM AUTHOR]

A series of KNiF 3 @carbon nanotubes (CNTs) are synthesized via a facile solvothermal method and applied as supercapacitor electrodes. The addition of carbon nanotubes can significantly overcome the rate capability defect of bare KNiF 3 and improve the specific capacity to a certain extent, which mainly owing to the increase of conductivity and electron/ion transports capacity. In addition, the cross-linked structure of carbon nanotubes and the evenly anchoring of KNiF 3 nanoparticles can increase the stability of the composites. As a result, the optimal candidate KNiF 3 @CNTs-8 (the content of carbon nanotubes is 8 mg and the mass fraction is 1.0%) shows highest specific capacity (821.4 C g−1 at 1 A g−1) and capacity retention (83.6% at 16 A g−1). The asymmetric supercapacitor AC//KNiF 3 @CNTs-8 delivers extremely high energy density of 63.7 Wh kg−1. More importantly, the ASC device shows ultra-long cycling stability, with 95.9% capacity retention after 5000 cycles. These results prove that KNiF 3 @CNTs can be a promising material for supercapacitor. Image 1 • KNiF 3 @CNTs-8 presented high specific capacity and excellent rate capability. • The optimal composite material exhibited high electrical conductivity. • An asymmetric supercapacitor AC//KNiF 3 @CNTs-8 was fabricated and investigated. • The ASC device exhibited ultra-high energy density and excellent cycling life. [ABSTRACT FROM AUTHOR]