Degeneration of Key Structural Components Resulting in Ageing of Supercapacitors and the Related Chemical Ageing Mechanism

Abstract

The research on supercapacitors (SCs) is one of the hot topics in the field of energy storage, and the intrinsic ageing mechanism of SCs is significant from both the economic and the scientific point of view. In this paper, the negative effects of decay of the key structural components on ageing of SCs were investigated by factorial design and analysis of variance (ANOVA). The ANOVA results showed that the degree of the negative influence on ageing of SCs could be ranked in descending order as anode > separator > cathode. The ageing would be accelerated due to the interaction between the electrode and separator, especially at a high charge−discharge current density. Further, the intrinsic chemical ageing mechanism of SCs was revealed by the morphology, microstructure, and chemical composition analyses of the fresh and aged key components (the electrode carbon materials, current collectors, and separators) with scanning electron microscopy (SEM), Brunauer−Emmett−Teller (BET), X-ray photoelectron spectra (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), etc. Moreover, the minimum pore width of electrode carbon materials suitable for electrolyte ion diffusion was obtained by density functional theory (DFT) calculations, which corroborated the assumption that the pore structure deterioration was one of the direct causes of capacitance loss for aged SCs. Generally, the ageing mechanism of key components of SCs could be a reference to develop advanced electrode materials and separators for SCs.

Whole Landscape of the Origin and Evolution of Gassing in Supercapacitors at a High Voltage

“… The increase in the content of various C–F bonds after the floating charge indicates that C–F bonds are more susceptible to forming at positive electrodes and high voltages. Combined with the previous gas components results, it can be concluded that the residual water in the system is oxidized to produce O 2 and H + (Scheme S1) at the positive electrode, which is weak at 2.7 V due to the almost no Raman signal from O 2 (Figure a), as previously reported. , Furthermore, the anion BF 4 – undergoes light hydrolysis to produce H + and F – (Scheme S2), consistent with the previous understanding . The tendency of surface oxygen-containing functional groups to react with H + and F – can facilitate the transformation of C–O/CO into more C–F bonds through fluorination reactions (Schemes S3 and S4). …”

“…This difference can be ascribed to the oxidation reactions occurring at the positive electrode (Schemes S5 and S6). Conversely, reduction reactions take place at the negative electrode, converting O–CO/CO to C–CO/CH (Schemes S7 and S8). The absence of the CO 2 signal after the floating charge at 2.7 V can be explained by the occurrence of oxidation reactions at high voltages. Similar mechanisms of CO 2 generation have also been reported. ,, …”

“…The changes in the content of C–O/CN and CO/C–N bonds after floating demonstrate that the content of CN/C–N bonds also changed, with the N element primarily sourced from the AN and SBP + . Previous works − show the possibility of the oxidation and polymerization of AN. The results here confirm the conversion of AN into carbocation (Scheme S9) driven by Schemes S1 and S2 in the positive region, which then further initiates the polymerization of AN to generate poly acetonitrile (poly-AN) deposits (Scheme S10).…”

“…Similar mechanisms of CO 2 generation have also been reported. 14,15,33 The changes in the content of C−O/C�N and C�O/C−N bonds after floating demonstrate that the content of C�N/C− N bonds also changed, with the N element primarily sourced from the AN and SBP + . Previous works 15−17 show the possibility of the oxidation and polymerization of AN.…”

“…Erradi and co-workers proposed the generation of CO, CO 2 , and NH 3 from AN decomposition by analyzing the composition of crystalline salts using thermogravimetric analysis–infrared spectroscopy (TGA–IR). Additionally, other studies on the failure of key components (such as electrodes and electrolytes) have hypothesized possible side reactions involving gas generation based on their aging phenomena. − While the above-mentioned works have provided some insights, a comprehensive determination of the origins and evolution mechanisms of all gas species related to the active materials, current collectors, and electrolyte components is lacking. Therefore, the exact origins and causes of gas generation in AN-based SCs need to be further analyzed and elucidated.…”