Jakob Krummacher

A 7 kWh automotive battery module with 396 interconnected cells was tested with electrochemical impedance spectroscopy (EIS) and time‐domain pulsing over 260 charge‐discharge cycles. An EIS calibration workflow was developed for low complex impedance values in a frequency range of 1 kHz to 50 mHz. Significant corrections on the resistance and the reactance were obtained from the calibration, particularly at frequencies above 100 Hz. Equivalent circuit parameters were extracted from the EIS spectra and the pulse response and investigated with respect to the cycle number and state‐of‐charge (SoC). Fit parameters were robustly extracted including R_sol, R_ct, and L from EIS, and R₀, τ₁ and τ₂ from time‐domain pulsing. The ohmic resistance decreased over the cycling number indicating an enhanced wetting of the electrodes. Charge transfer resistance R_ct showed a monotonic increase over the cycles related to cell ageing. From the charge and discharge pulses, the ohmic resistance R₀ was determined from the instantaneous voltage step of the recovery pulse, while the two time constants τ₁ and τ₂ correspond to the slower exponential recovery phase. R₀ from the time‐domain showed a similar trend as R_sol plus a contribution of R_ct from EIS. Overall, we show that calibrated EIS and time‐domain pulsing are efficient methods to gain insights into the electrochemical processes related to different SoCs and the cycling ageing of battery modules and packs.

This study investigated the anodic dissolution of Al current collectors in unconventional electrolytes for high voltage electrochemical double‐layer capacitors (EDLCs) containing adiponitrile (ADN), 3‐cyanopropionic acid methyl ester (CPAME), 2‐methyl‐glutaronitrile (2‐MGN) as solvent, and tetraethylammonium tetrafluoroborate (Et₄NBF₄) and tetraethylammonium bis(trifluoromethanesulfonyl)imide (Et₄NTFSI) as conductive salts. To have a comparison with the state‐of‐the‐art electrolytes, the same salts were also used in combination with acetonitrile (ACN). The chemical–physical properties of the electrolytes were investigated. Furthermore, their impact on the anodic dissolution of Al was analyzed in detail as well as the influence of this process on the performance of high voltage EDLCs. The results of this study indicated that in the case of Et₄NBF₄‐based electrolytes, the use of an alternative solvent is very beneficial for the realization of stable devices. When Et₄NTFSI is used, the reduced solubility of the complex Al(TFSI)₃appears to be the key for the realization of advanced electrolytes.

Herein we report a systematic investigation of the chemical and physical properties of acetonitrile‐based electrolytes that contained the salts tetraethylammonium tetrafluroroborate (Et₄NBF₄), tetraethylammonium bis(trifluoromethanesulfonyl)‐ imide (Et₄NTFSI), 1‐butyl‐1‐methylpyrrolidinium tetrafluoroborate (Pyr₁₄BF₄), or 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI). The ionic conductivity, viscosity, density, and electrochemical stability windows of these electrolytic solutions have been considered in detail. From these results, the electrolyte Et₄NTFSI (1 mol dm⁻³) in ACN appeared to display a promising set of properties that allow the realization of ACN‐based electrochemical double‐layer capacitors able to display stable behavior at 3.1 V.