Lingyang Liu

A high-temperature flexible supercapacitor based on pseudocapacitive behavior of FeOOH in an ionic liquid electrolyte

Although flexible all-solid-state supercapacitors (f-SSCs) have been receiving much attention as promising flexible energy storage devices, most of them cannot operate at high temperatures due to the volatility or flammability of currently used aqueous and organic electrolytes. Here, we report an ionic liquid (IL) gel-based asymmetric supercapacitor having excellent heat-resistant performance and flexibility. To this end, low-cost γ-FeOOH is firstly electrodeposited on carbon cloth, and its pseudocapacitive behavior in a typical IL is investigated through an electrochemical quartz crystal microbalance (EQCM) for the first time. The results show that the pseudocapacitance mainly originates from a diffusion-controlled insertion process of the cations. By taking advantage of the prominent pseudocapacitance of γ-FeOOH, as well as excellent characteristics of IL gel electrolytes (thermostability, non-flammability, chemical inertness and wide potential), an advanced high-temperature f-SSC is fabricated by using γ-FeOOH as the anode and porous N-doped activated carbon as the cathode. The f-SSC exhibits outstanding electrochemical performance at elevated temperatures, and can achieve a maximum volumetric energy density of 1.44 mW h cm−3 (based on the whole device volume) at 200 °C. Moreover, it is able to maintain a stable energy-storage ability during the bending process even at 180 °C, providing the highest reported temperature for flexibility tests in f-SSCs to date.

Silica-grafted ionic liquids for revealing the respective charging behaviors of cations and anions in supercapacitors

Supercapacitors based on activated carbon electrodes and ionic liquids as electrolytes are capable of storing charge through the electrosorption of ions on porous carbons and represent important energy storage devices with high power delivery/uptake. Various computational and instrumental methods have been developed to understand the ion storage behavior, however, techniques that can probe various cations and anions of ionic liquids separately remain lacking. Here, we report an approach to monitoring cations and anions independently by using silica nanoparticle-grafted ionic liquids, in which ions attaching to silica nanoparticle cannot access activated carbon pores upon charging, whereas free counter-ions can. Aided by this strategy, conventional electrochemical characterizations allow the direct measurement of the respective capacitance contributions and acting potential windows of different ions. Moreover, coupled with electrochemical quartz crystal microbalance, this method can provide unprecedented insight into the underlying electrochemistry.Quantifying the individual capacitance contributions of in-pore ions during charging remains a challenge. Here the authors design silica-grafted ionic liquids to reveal the charging behaviors of cations and anions separately, providing fresh insight into the storage mechanism of supercapacitors.

Opening Magnesium Storage Capability of Two-Dimensional MXene by Intercalation of Cationic Surfactant

Two-dimensional (2D) Ti3C2 MXene has attracted great attention in electrochemical energy storage devices (supercapacitors and lithium-ion and sodium-ion batteries) due to its excellent electrical conductivity as well as high volumetric capacity. Nevertheless, a previous study showed that multivalent Mg2+ ions cannot reversibly insert into MXene, resulting in a negligible capacity. Here, we demonstrate a simple strategy to achieve high magnesium storage capability for Ti3C2 MXene by preintercalating a cationic surfactant, cetyltrimethylammonium bromide (CTAB). Density functional theory simulations verify that intercalated CTA+ cations reduce the diffusion barrier of Mg2+ on the MXene surface, resulting in the significant improvement of the reversible insertion/deinsertion of Mg2+ ions between MXene layers. Consequently, the MXene electrode exhibits a desirable volumetric specific capacity of 300 mAh cm-3 at 50 mA g-1 as well as outstanding rate performance. This work endows MXene material with an application in electrochemical energy storage and, simultaneously, introduces magnesium battery materials as a member.

A Dual Carbon‐Based Potassium Dual Ion Battery with Robust Comprehensive Performance

Dual carbon-based potassium dual ion batteries (K-DCBs) have recently attracted ever-increasing attention owing to the potential advantages of high performance-to-cost ratio, good safety, and environmental friendliness. However, the reported K-DCBs still cannot simultaneously meet the requirements of high capacity, long cycling stability, and low cost, which are necessary for practical applications. In this study, a K-DCB with good comprehensive performance including capacity, cycling stability, medium discharge voltage, and energy density is developed by introducing the optimal cathode and anode materials, i.e., KS6 and natural graphite, respectively. An initial capacity of ≈54.6 mAh g-1 and 92.5% capacity retention after 400 cycles can be delivered in a wide voltage window of 2.4-5.4 V at the current density of 100 mA g-1 . A high medium discharge voltage around 4.2 V and an energy density up to 158.3 Wh kg-1 are meanwhile delivered by the K-DCB. In addition, the working mechanism of the devices is understood in detail. It is believed that valuable contributions to the electrochemical performance improvement of the related devices toward practical applications can be provided by this study.

Sprinkling MnFe2O4 quantum dots on nitrogen-doped graphene sheets: the formation mechanism and application for high-performance supercapacitor electrodes

Quantum dots (QDs)/graphene composites are interesting as promising electrode materials for high-performance supercapacitors because they can well integrate the complementary features of QDs and graphene. Herein, we demonstrate a MnFe2O4 QDs/nitrogen-doped graphene (NG) material prepared by a controllable solvothermal synthesis, in which ultra-small MnFe2O4 QDs are uniformly anchored on NG surfaces. First-principles calculations elucidate that the oxygen-containing groups of graphene oxide play a crucial role in generating such a structure, and a ferrite octahedral skeleton is firstly formed followed by Mn atom insertion. Powdery MnFe2O4 QDs/NG exhibits a high specific capacitance of 517 F g−2 within a negative potential window (−1 ∼ 0 V) in KOH electrolyte. When the lower cut off voltage is extended to −1.2 V, the specific capacitance can be increased to 905 F g−1. And the condensed MnFe2O4 QDs@NG electrode (forming a similar structure to pitaya slices) with an astonishing loading mass of 18 mg cm−2 can achieve high areal and volumetric capacitances (5.3 F cm−2 and 277.6 F cm−3). Moreover, carbon encapsulation is favorable for the improvement of rate and cycling performance, allowing a satisfactory capacitance of 150 F g−1 even at 200 A g−1 as well as a superior lifetime up to 65 000 cycles. These results make such materials competitive with supercapacitor electrodes and may speed up the development of QD-based electrodes for energy storage applications.