Search

In situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is used to understand the mixed lithiation/deposition behavior on graphite anodes during the charging process. The conductivity, degree of lithiation, and the deposition process of the graphite are reflected by the EPR spectroscopic quality factor, the spin density, and the EPR spectral change, respectively. Classical over‐charging (normally associated with potentials ≤0 V vs. Li+/Li) are not required for Li metal deposition onto the graphite anode: Li deposition initiates at ca. +0.04 V (vs. Li+/Li) when the scan rate is lowered to 0.04 mV s−1. The inhibition of Li deposition by vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, attributed to a more mechanically flexible and polymeric SEI layer with higher ionic conductivity. A safe cut‐off potential limit of +0.05 V for the anode is suggested for high rate cycling, confirmed by the EPR response over prolonged cycling. [ABSTRACT FROM AUTHOR]

In situ electrochemical electron paramagnetic resonance (EPR) spectroscopy is used to understand the mixed lithiation/deposition behavior on graphite anodes during the charging process. The conductivity, degree of lithiation, and the deposition process of the graphite are reflected by the EPR spectroscopic quality factor, the spin density, and the EPR spectral change, respectively. Classical over‐charging (normally associated with potentials ≤0 V vs. Li+/Li) are not required for Li metal deposition onto the graphite anode: Li deposition initiates at ca. +0.04 V (vs. Li+/Li) when the scan rate is lowered to 0.04 mV s−1. The inhibition of Li deposition by vinylene carbonate (VC) additive is highlighted by the EPR results during cycling, attributed to a more mechanically flexible and polymeric SEI layer with higher ionic conductivity. A safe cut‐off potential limit of +0.05 V for the anode is suggested for high rate cycling, confirmed by the EPR response over prolonged cycling. [ABSTRACT FROM AUTHOR]

The high temperature performance of water-in-salt electrolytes was investigated using a carbon-based electrode with commercial cell components. Supercapacitors using 21 m Li bis(trifluoromethylsulphonyl)imide (TFSI) and 21 m LiTFSI + 7 m Li trifluoromethanesulphonyl electrolytes are shown to operate at a voltage of 2 V at 100 °C and 120 °C, respectively, with gravimetric capacitances exceeding 100 F g−1. [ABSTRACT FROM AUTHOR]

A new criterion and potentiostatic method is demonstrated to determine the electrochemically stable potential window (ESPW) of supercapacitors, enabling the calculation of the optimal electrode mass ratio for two different electrode-electrolyte pairings. The criterion uses the decay of electrode current in a three-electrode test cell during potentiostatic ageing at progressively higher voltages to determine the ESPW. To perform these measurements a quasi-reference electrode (QRE) was incorporated within a CR2032 coin cell with a small and simple modification. This enabled three-electrode measurements to be performed inside the coin cell using organic electrolytes whilst outside a glovebox. The voltage-optimised mass ratio (m+/m−) for activated carbon electrodes in 1 M tetraethylammonium tetrafluoroborate (TEABF 4) in acetonitrile (ACN) and propylene carbonate (PC) electrolytes was shown to be 2.38 and 0.61 respectively. The validity of the technique was verified by full-cell potentiostatic ageing experiments on mass-balanced and mass-symmetric supercapacitors. For both electrolytes the maximum operating voltage, energy storage capacity and cycle lifetime were improved significantly with mass balancing. The maximum voltage increased from 2.0 V to 2.9 V for ACN and from 2.3 V to 2.7 V for PC, whilst the energy density increased by ∼90% for ACN and by ∼40% for PC. • Introduction of a reference electrode into a standard coin cell. • New criterion for determining stable voltage range. • Increases voltage to 2.9 V. [ABSTRACT FROM AUTHOR]

A comparison of the performance of graphene‐based supercapacitors is difficult, owing to the variety of production methods used to prepare the materials. To the best of our knowledge, there has been no systematic investigation into the effect of the graphene production method on the supercapacitor performance. In this work, we compare graphene produced through several routes. This includes anodic and cathodic electrochemically exfoliated graphene, liquid phase exfoliated graphene, graphene oxide, reduced graphene oxide, and graphene nanoribbons. Graphene oxide exhibited the highest capacitance of approximately 154 F g−1 in 6 M KOH at 0.5 A g−1 attributed to oxygen functional groups giving an additional pseudocapacitance and preventing significant restacking; however, the capacitance retention was poor, owing to the low conductivity. In comparison, the anodic electrochemically exfoliated graphene exhibited a capacitance of approximately 44 F g−1, the highest of the 'pure' graphene materials, which all exhibited superior capacitance retention, owing to their higher conductivity. The cyclability of all of the materials, with the exception of reduced graphene oxide (70 %), was found to be greater than 95 % after 10 000 cycles. These results highlight the importance of matching the graphene production method with a specific application; for example, graphene oxide and anodic electrochemically exfoliated graphene would be best suited for high energy and power applications, respectively. [ABSTRACT FROM AUTHOR]

A scale-up process of high-rate-capability supercapacitors based on electrochemically exfoliated graphene (EEG) and hybrid activated carbon (AC)/EEG are studied in this work. A comparison of the rate capabilities of large-scale EEG and AC/EEG-based pouch cell and commercial high-power supercapacitors are also presented in this paper. The oxygen content of the EEG used in this work is 9.6 at%, with a C/O ratio of 9.36, and the electrical conductivity is 2.68 × 10 4 Sm −1 . The specific capacitance (59 Fg −1 ) of the EEG-based supercapacitors remained above 80% of the maximum value as the scan rate was increased from 5 mVs −1 to 1 Vs −1 . Furthermore, our study reveals how the rate capability of activated carbon (AC) based supercapacitors can be improved by adding EEG into the electrodes to form a hybrid AC/EEG supercapacitor. Both the EEG-based and AC/EEG supercapacitors were scaled-up to pouch cells with capacitances of tens of farads. The electrochemical response was unchanged when scaling up from a coin cell to a pouch cell, although the specific capacitance fell slightly. The cycle performance of the AC/EEG pouch cell showed good long-term stability, with better than 95% capacitance retention after 10,000 cycles. Both the EEG and AC/EEG (with 1:1 mass ratio) pouch cells had rate capabilities that compared favourably with the commercial high-power supercapacitors. [ABSTRACT FROM AUTHOR]