Methodology for Identifying the Degradation Phenomena at the Electrode/Electrolyte Interface of Carbon-Based Supercapacitors Under High Voltage Operation
Authors: Ratajczak Paula , Pourhosseini S.E.M , Bothe Annika , Balducci Andrea , Beguin Francois 
Published: December 2020

The worsening of supercapacitors (SCs) performance under high voltage operation is generally revealed by a loss of capacitance, increase of resistance, internal pressure growth (related to the carbon surface functionalization and/or pore blockage by electrolyte decomposition products) and weakening of the adhesion between the active mass and the current collectors entailed essentially by gas evolution [1]. A part of the effects produced by high voltage performance of SCs, such as modification of the electrical (e.g., capacitance, equivalent series resistance (ESR)) properties of the cells and evolving gaseous products [2], is generally obtained from in-situ/operando methodologies. Besides, the origin of the ageing processes occurring at the electrode/electrolyte interface (e.g., modification of electrodes surface chemistry and porous texture) is disclosed from ex-situ experiments (so-called post-mortem analyzes) done on the already aged electrodes, using generally the electrolyte applied in commercial systems (i.e., TEABF4/ACN) [3]. However, the obtained data are always distorted by the presence of the binder, which blocks a part of the electrodes porosity and may also interfere and/or overlap with the detected chemical species/surface functionalities.

In this context, we propose to manufacture the electrodes with a water-washable carboxymethyl cellulose (CMC) binder to more accurately assess the ageing phenomena taking place at the electrode/electrolyte interface during floating of SCs at high voltage (2.7 – 3.3 V). The study is conducted in 1.75 mol L-1
N-butyl-N-methylpyrrolidinium tetrafluoroborate in adiponitrile (Pyr14BF4/ADN), yet it can be as well conducted with any organic or ionic liquid electrolyte. We show that washing of the electrodes via Soxhlet extraction allows the CMC binder to be removed and the initially blocked pore entrances to be recovered. The modifications of surface chemistry and porous texture of the aged electrodes – washed by water in a Soxhlet extractor, are analyzed by temperature programmed desorption (TPD), and gas (nitrogen) adsorption/desorption, respectively. The analyses confirm that the surface functionality and porous texture is more affected by ageing for the positive electrode (reduction of the micro- and mesopore volumes) than for the negative one. At the same time, some electrolytic species remain in the porosity of both electrodes and contribute to the blockage of the small carbon micropores, hence, reduce the extractable energy of the carbon-based supercapacitors. The elemental mapping by Energy-Dispersive Spectroscopy (EDS) on the aged electrodes demonstrates that the increase of the SCs internal resistance is also partly related to a partial destruction of the Al2O3 passive layer by HF, which entails the delamination of the carbon coating from the positive aluminum current collector after ageing.

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