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The use of pseudocapacitive electrode materials can enable devices to store more energy than electrical double-layer capacitors (EDLCs). However, only a few pseudocapacitive materials can maintain excellent performance at low temperatures, which limits their application in harsh climate conditions. Here we demonstrate that a pseudocapacitor with two-dimensional transition metal carbide (MXene) electrode can exhibit excellent low-temperature performance like EDLC. The MXene electrodes contain electrolyte between 2D sheets, and the electrolyte ions can unimpededly reach redox-active sites and interact with surface oxygen groups rapidly, even at low temperatures. With a combination of 40 wt.% sulfuric acid solution as the electrolyte, the working temperature of the MXene electrode extends to -60 °C. The electrode exhibits temperature-insensitive performance at a low scan rate, and the capacity of MXene (88 mAh g−1 at 5 mV s−1) stays almost constant when the temperature decreases from 20 to -50 °C. Moreover, at -50 °C, MXene electrodes show a high capacity retention of > 75% at 100 mV s−1, indicating good low-temperature rate performance. Interestingly, a broad working potential window of 1.5 V is achieved at -60 °C. Such an excellent low-temperature performance demonstrates that MXene is a promising electrode candidate for low-temperature pseudocapacitive energy storage applications.

The development of new electrolyte and electrode designs and compositions has led to advances in electrochemical energy-storage (EES) devices over the past decade. However, focusing on either the electrode or electrolyte separately is insufficient for developing safer and more efficient EES devices in various working environments, as the energy-storage ability is determined by the ion arrangement and charge and/or electron transfer at the electrode–electrolyte interface. In this Review, we assess the fundamental physicochemical and electrochemical properties at the electrode–electrolyte interfaces in Li-ion batteries and supercapacitors using safe and electrochemically stable ionic-liquid electrolytes. Key reactions and interactions at the electrode–electrolyte interface, as well as geometric constraints and temperature effects, are highlighted. Building on the fundamental understanding of interfacial processes, we suggest potential strategies for designing stable and efficient ionic-liquid-based EES devices with emerging electrode materials. The development of efficient, high-energy and high-power electrochemical energy-storage devices requires a systems-level holistic approach, rather than focusing on the electrode or electrolyte separately. In this Review, we discuss the interfacial reactions and ion transport in ionic-liquid-based Li-ion batteries and supercapacitors, and summarize their impact on device performance.

Achieving high performance during low-temperature operation of lithium-ion (Li+) batteries (LIBs) remains a great challenge. In this work, we choose an electrolyte with low binding energy between Li+ and solvent molecule, such as 1,3-dioxolane-based electrolyte, to extend the low temperature operational limit of LIB. Further, to compensate the reduced diffusion coefficient of the electrode material at ultralow temperature, nanoscale lithium titanate is used as electrode material, which finally, we demonstrate a LIB with unprecedented low-temperature performance, delivering ∼60% of its room-temperature capacity (0.1 °C rate) at −80 °C. Though insufficient ionic conductivity of the electrolyte is generally considered as the main reason for the poor low-temperature performance in LIBs, we found that the sluggish desolvation of solvated Li+ at the liquid-solid interface might be the critical factor. These findings provide evidence for the effective design of robust LIBs for ultralow temperature applications.

Development of cost-effective electrocatalysts as an alternative to platinum for oxygen reduction reaction (ORR) is of great significance for boosting the applications of green energy devices such as fuel cells and metal-air batteries. Here we report a nitrogen and phosphorus tri-doped hierarchically porous carbon supported highly cost-effective, efficient and durable Fe single-site electrocatalyst derived from biomass. Combined aberration-corrected HAADF-STEM, XPS and XAFS measurements and theoretical calculations reveal the atomically dispersed Fe-N-P-C-O complex as the dominant active sites for ORR. This work also shows the design principle for enhancing the ORR activity of single Fe site catalysts with higher Fe charge, which can be manipulated by the coordinated structure in the active centre. Theoretical calculations reveal that the main effective sites are singleN-P-O-Fe-O centers, where the associated P-O-Fe bond can significantly lower the stability of strongly adsorbed O* and OH* on the catalytically active sites and thus give rise to enhanced ORR performance. The insights reported here open a new avenue for constructing highly efficient molecule-like heterogeneous catalysts in electrochemical energy technologies.

Authors acknowledge the Core Research Facilities (CRF) at Drexel University for providing access to characterization tools. J. L. acknowledges support through the Fundamental Research Funds for the Central Universities (CUSF-DH-D-2017028) and the Special Excellent Ph.D. International Visit Program by Donghua University. K. J. thanks Prof. Leslie Lamberson and Steve Pagano for organizing the IExE REU at Drexel. A. L. was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1646737. Research reported in this publication was supported by King Abdullah University of Science and Technology (KAUST) under the KAUST-Drexel Competitive Research Grant (URF/1/2963-01-01).

Increasing the mass loading of active material in a supercapacitor electrode improves the energy storage capability per electrode area. However, the increase of mass loading is accompanied by the increased electrode thickness, which often causes drops in specific capacitance and counteracts the potential increase in areal capacitance once the electrode thickness exceeds 100 μm. In our previous work, we showed high specific capacitance retention of coarse-grained carbide derived carbon (CDC) with electrode thicknesses up to 1000 μm in organic electrolytes. In this work, we report that this behavior can be extended to coarse-grained activated carbon (AC). AC is the most common commercial supercapacitor electrode material with a much broader pore size distribution and lower electric conductivity compared to CDC. The areal capacitance of the AC electrode is enhanced from 2.3 F/cm2 to 7.4 F/cm2 at 5 mV/s, as the electrode thickness increases from 200 to 800 μm. With the increased mass loading of the active electrode material, the mass and volume occupied by current collectors and separators are reduced in the electrode stack, which leads to an increase of the gravimetric and volumetric energy density of the device. By reporting on this advantageous behavior in thick electrodes using coarse-grained carbons, we hope to garner interest toward an unexplored method for improving the performance of porous carbon-based supercapacitors, without increasing the cost or changing the current used supercapacitor manufacturing process.

The impact of cell voltage on the capacitance of practical electrochemical supercapacitors is a phenomenon observed experimentally, which lacks a solid theoretical explanation. Herein, we provide combined experimental and molecular dynamics investigation of the relation between voltage and capacitance. We have studied this relation in supercapacitor cells comprising of activated carbon material as the active electrode material, and neat ionic liquids (ILs), and a mixture of ILs as the electrolyte. It has been observed that the increase of accumulative charge impacts the conformation and packing of the cations in the anode, which determines its nonlinear behavior with increasing voltage. It has also been shown that for the mixture IL with two types of cations, the contribution of each type of cation to the overall capacitance is highly dependent on the different pore sizes in the system. The smaller tetramethylammonium (TMA + ) shows tendency for more efficient adsorption in the mesopores, while 1-Ethyl-3-methylimidazolium (EMIM + ) is found to be present almost exclusively in the micropores where TMA + is present in small quantities. Such microscopic insights from computer simulations of the molecular phenomena affecting the overall performance in supercapacitors can help to design more efficient electrolytes and devices.

Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance. In particular, the kinetics of ion transport in organic electrolytes is slow, especially at low operating temperatures. Herein, we report a new type of MXene-carbon nanotube (CNT) composite electrode that maximizes ion accessibility resulting in exceptional rate performance at low temperatures. The improved ion transport at low temperatures is made possible by breaking the conventional horizontal alignment of the two-dimensional layers of the MXene Ti3C2 by using specially designed knotted CNTs. The large, knot-like structures in the knotted CNTs prevent the usual restacking of the Ti3C2 flakes and create fast ion transport pathways. The MXene-knotted CNT composite electrodes achieve high capacitance (up to 130 F g−1 (276 F cm−3)) in organic electrolytes with high capacitance retention over a wide scan rate range of 10 mV s−1 to 10 V s−1. This study is also the first report utilizing MXene-based supercapacitors at low temperatures (down to −60 °C)., Improving the accessibility of ions in the electrodes of electrochemical energy storage devices is vital for charge storage and rate performance. Here, the authors report a new type of MXene-carbon nanotube composite electrode that maximizes ion accessibility, resulting in high rate performance at low temperatures.

Electroactive yarns that are stretchable are desired for many electronic textile applications, including energy storage, soft robotics, and sensing. However, using current methods to produce these yarns, achieving high loadings of electroactive materials and simultaneously demonstrating stretchability is a critical challenge. Here, a one-step bath electrospinning technique is developed to effectively capture Ti3 C2 Tx MXene flakes throughout continuous nylon and polyurethane (PU) nanofiber yarns (nanoyarns). With up to ≈90 wt% MXene loading, the resulting MXene/nylon nanoyarns demonstrate high electrical conductivity (up to 1195 S cm-1 ). By varying the flake size and MXene concentration, nanoyarns achieve stretchability of up to 43% (MXene/nylon) and 263% (MXene/PU). MXene/nylon nanoyarn electrodes offer high specific capacitance in saturated LiClO4 electrolyte (440 F cm-3 at 5 mV s-1 ), with a wide voltage window of 1.25 V and high rate capability (72% between 5 and 500 mV s-1 ). As strain sensors, MXene/PU yarns demonstrate a wide sensing range (60% under cyclic stretching), high sensitivity (gauge factor of ≈17 in the range of 20-50% strain), and low drift. Utilizing the stretchability of polymer nanofibers and the electrical and electrochemical properties of MXene, MXene-based nanoyarns demonstrate potential in a wide range of applications, including stretchable electronics and body movement monitoring.

Using electrodes with subnanometer pores and ionic liquid electrolytes can improve the charge storage capacity at the expense of the charging rate. The fundamental understanding of the charging dynamics of nanoporous electrodes can help to avoid compromising the power density. In this work, we performed molecular dynamics simulations to reveal the charging mechanism of subnanometer pores in ionic liquids. Different from the traditional view that a smaller pore results in slower charging, a non-monotonic relation is found between the charging rate and pore size, in which the charging process is accelerated in some subnanometer pores. Our analysis uncovers that the mechanism of the charging enhancement can be attributed to the transition of in-pore ion structure.

Mg batteries are one of several new battery technologies expected to partially substitute lithium-based batteries in the future due to the lower cost and higher safety. However, the development of Mg batteries has been greatly hindered by the sluggish Mg migration kinetics in the solid state. Here, we exploit a high performance cathode for Mg battery based on a tailored nanocomposite, synthesized by in-situ growth of nanocrystalline Mn3O4 on graphene substrates, which provides high reversible capacities (~ 220 mA h g−1 at 15.4 mA g−1 and ~ 80 mA h g−1 at 1.54 A g−1), good rate performance (high reversibility at various current rates), and excellent cycling stability (no capacity decay after 700 hundred cycles). The magnesiation mechanism in our cell system has been identified as a combination of capacitive processes and diffusion-controlled reactions involving electrolyte solvents. Characterization is performed by ex-situ transmission electron microscopy (TEM)/scanning TEM (STEM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) in addition to quantitative kinetics analysis. Exploiting the high-performance capacitive-type electrodes, where the specific capacity is limited by the kinetics of surface processes and not by bulk Mg ion diffusion governing the properties of conventional intercalation-type electrodes, could reveal a new approach to developing commercially viable Mg batteries.

Supercapacitor (SC) with organic electrolyte or ionic liquid (IL) electrolyte can generally store/release higher energy than that with an aqueous electrolyte, due to a larger operating voltage window of a non-aqueous electrolytes. A carbon is synthesized by a facile impregnate-activation method from renewable woody biomass, which has twice of the specific surface area and pore volume than the sample synthesized by conventional KOH activation. Biomass-derived carbons with high ion accessible surface area and highly integrated micropores and mesopores provide superior capacitance, excellent rate capability and good stability in both organic electrolyte and IL electrolytes. Significant enhancement in the capacitance and rate capability were obtained by the generation of micropores similar to the ion size and better pore network through removal of impurities in the biomass. High specific capacitances of 146 F g−1 in the organic electrolyte and 224 F g−1 in the IL electrolyte at current density of 0.1 A g−1 are achieved. Highly integrated micro- and mesoporous structure leads to a good rate capability of 100% capacitance retention at current density up to 10 A g−1 in the organic electrolyte and 67% capacitance retention at current density up to 7 A g−1 in the IL. With the large voltage offered by the non-aqueous electrolyte, the material can store/release high maximum energy of 26 W h kg−1 and 92 W h kg−1 in the organic electrolyte and IL electrolyte, respectively. It reveals that the biomass derived carbon is a promising and cost effective candidate for electrodes in high performance SCs. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Pseudocapacitors or redox capacitors that synergize the merits of batteries and double-layer capacitors are among the most promising candidates for high-energy and high-power energy storage applications. 2D transition metal carbides (MXenes), an emerging family of pseudocapacitive materials with ultrahigh rate capability and volumetric capacitance, have attracted much interest in recent years. However, MXenes have only been used as negative electrodes as they are easily oxidized at positive (anodic) potential. To construct a high-performance MXene-based asymmetric device, a positive electrode with a compatible performance is highly desired. Herein, an ultrafast polyaniline@MXene cathode prepared by casting a homogenous polyaniline layer onto a 3D porous Ti3 C2 Tx MXene is reported, which enables the stable operation of MXene at positive potentials because of the enlarged work function after compositing with polyaniline, according to the first-principle calculations. The resulting flexible polyaniline@MXene positive electrode demonstrates a high volumetric capacitance of 1632 F cm-3 and an ultrahigh rate capability with 827 F cm-3 at 5000 mV s-1 , surpassing all reported positive electrodes. An asymmetric device is further fabricated with MXene as the anode and polyaniline@MXene as the cathode, which delivers a high energy density of 50.6 Wh L-1 and an ultrahigh power density of 127 kW L-1 .

International audience; Pseudocapacitive energy storage in supercapacitor electrodes differs significantly from the electrical double-layer mechanism of porous carbon materials, which requires a change from conventional thinking when choosing appropriate electrolytes. Here we show how simply changing the solvent of an electrolyte system can drastically influence the pseudocapacitive charge storage of the two-dimensional titanium carbide, Ti3C2 (a representative member of the MXene family). Measurements of the charge stored by Ti3C2 in lithium-containing electrolytes with nitrile-, carbonate- and sulfoxide-based solvents show that the use of a carbonate solvent doubles the charge stored by Ti3C2 when compared with the other solvent systems. We find that the chemical nature of the electrolyte solvent has a profound effect on the arrangement of molecules/ions in Ti3C2, which correlates directly to the total charge being stored. Having nearly completely desolvated lithium ions in Ti3C2 for the carbonate-based electrolyte leads to high volumetric capacitance at high charge–discharge rates, demonstrating the importance of considering all aspects of an electrochemical system during development.

International audience; We demonstrate an asymmetric supercapacitor in a potassium acetate-based water-in-salt electrolyte, where 2-D titanium carbide MXene and manganese oxide were used as negative and positive electrode materials, respectively. Use of water-in-salt electrolyte enables the assembled asymmetric device to be operated up to a cell voltage of 2.2 V, which overcomes the limited cell voltage issue in aqueous pseudocapacitors (1.2 - 1.4 V). This cell shows excellent rate capability (~48%) between 5 and 100 mV s-1 and good stability (~93%) throughout 10,000 charge-discharge cycles (at 1 A g-1) and 25 h voltage-hold at 2.2 V, which is competitive when compared with the performance of known asymmetric supercapacitors designed with activated carbon electrodes and fluorinated-imide based water-in-salt electrolytes. Moreover, our device shows slower self-discharge and ~32% higher volumetric energy density than activated carbon-based supercapacitors and is promising for applications where volumetric energy density is critical.

A supercapacitor, a safe and durable electrical energy storage device with fast charge–discharge capability, will achieve more widespread use if the specific energy can be improved. However, current understanding of pore characteristic effects on gravimetric capacitance has limited the development of electrode materials. We derive a model of ion packing in cylindrical nanopores, and it quantitatively reveals the significant impact of pore geometric characteristics on the gravimetric capacitance in neat ionic liquid, which is confirmed experimentally using a series of sponge-like carbons (carbon nanosponge). With the favourable ion packing proposed by the model, the electrode using the carbon nanosponge as an active material delivered double-layer capacitances of 290 F g−1 (20 °C) and 387 F g−1 (60 °C) with an operating cell voltage of 4 V. This study also provides systematical strategies for rational design of various carbon materials and ionic liquids by optimized ion packing for ultrahigh gravimetric capacitance.

With ever-increasing demands for high-energy and high-power operation of compact energy storage devices, asymmetric pseudocapacitors attract tremendous attention due to their high volumetric energy and fast charge/discharge capability. However, it is challenging to achieve compact structures with abundant ions access and fast charge transport for both positive and negative electrodes simultaneously. Herein, we designed a flexible and asymmetric pseudocapacitor using a Ti3C2Tx MXene film with wavy architecture as the negative electrode, and a graphene/carbon nanotube/polyaniline (rGO/CNT/PANI) ternary nanocomposite film resistive to anodic oxidation as the positive electrode, respectively. The wavy MXene facilitates ion transport and maintains its highly compact structure, resulting in an ultrahigh volumetric capacitance of 1277 F cm−3 and a significantly enhanced rate capability with 89% capacitance retention at 1000 mV s−1. The compatible metal-free rGO/CNT/PANI positive electrode with molecular-level integration of PANI on graphene and further insertion of CNT in between rGO/PANI sheets also sets an extremely high volumetric capacitance of 1038 F cm−3 and excellent rate performance. As a result, Ti3C2Tx//rGO/CNT/PANI asymmetric device outputs both high volumetric energy density of 70 Wh L−1 and high volumetric power density of 111 kW L−1 (per volume of active material), which surpasses most state-of-the-art aqueous asymmetric and symmetric devices.

Ion–ion interactions in supercapacitor (SC) electrolytes are considered to have significant influence over the charging process and therefore the overall performance of the SC system. Current strategies used to weaken ionic interactions can enhance the power of SCs, but consequently, the energy density will decrease due to the increased distance between adjacent electrolyte ions at the electrode surface. Herein, we report on the simultaneous enhancement of the power and energy densities of a SC using an ionic mixture electrolyte with different types of ionic interactions. Two types of cations with stronger ionic interactions can be packed in a denser arrangement in mesopores to increase the capacitance, whereas only cations with weaker ionic interactions are allowed to enter micropores without sacrificing the power density. This unique selective charging behavior in different confined porous structure was investigated by solid-state nuclear magnetic resonance experiments and further confirmed theoretically by both density functional theory and molecular dynamics simulations. Our results offer a distinct insight into pairing ionic mixture electrolytes with materials with confined porous characteristics and further propose that it is possible to control the charging process resulting in comprehensive enhancements in SC performance. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

International audience; Due to the tremendous importance of electrochemical energy storage, numerous new materials and electrode architectures for batteries and supercapacitors have emerged in recent years. Correctly characterizing these systems requires considerable time, effort, and experience to ensure proper metrics are reported. Many new nanomaterials show electrochemical behavior somewhere in between conventional double‐layer capacitor and battery electrode materials, making their characterization a non‐straightforward task. It is understandable that some researchers may be misinformed about how to rigorously characterize their materials and devices, which can result in inflation of their reported data. This is not uncommon considering the current state of the field nearly requires record breaking performance for publication in high‐impact journals. Incorrect characterization and data reporting misleads both the materials and device development communities, and it is the shared responsibility of the community to follow rigorous reporting methodologies to ensure published results are reliable to ensure constructive progress. This tutorial aims to clarify the main causes of inaccurate data reporting and to give examples of how researchers should proceed. The best practices for measuring and reporting metrics such as capacitance, capacity, coulombic and energy efficiencies, electrochemical impedance, and the energy and power densities of capacitive and pseudocapacitive materials are discussed.

The specific energy of a supercapacitor (SC) with an ionic liquid (IL)-based electrolyte is larger than that using an aqueous electrolyte owing to the wide operating voltage window provided by the IL. However, the wide-scale application of high-energy SCs using ILs is limited owing to a serious reduction of the energy with increasing power. The introduction of macropores to the porous material can mitigate the reduction in the gravimetric capacitance at high rates, but this lowers the volumetric capacitance. Synthetic polymers can be used to obtain macroporous frameworks with high apparent densities, but the preservation of the frameworks during activation is challenging. To simultaneously achieve high gravimetric capacitance, volumetric capacitance, and rate capability, a systematic strategy was used to synthesize a densely knitted carbon framework with a hierarchical pore structure by using a polymer. The energy of the SC using the hierarchically porous carbon was 160 Wh kg−1 and 85 Wh L−1 on an active material base at a power of 100 W kg−1 in an IL electrolyte, and 60 % of the energy was still retained at a power larger than 5000 W kg−1. To illustrate, a full-packaged SC with the material could store/release energy comparable to a Ni–metal hydride battery (gravimetrically) and one order of magnitude higher than a commercial carbon-based SC (volumetrically), within one minute.