Heterogeneous electron transfer of ferrocene in acetonitrile-LiTFSI highly concentrated electrolyte

Highly concentrated electrolytes (HCE) are mixtures of equivalent or near-equivalent amounts of salt and solvent displaying a liquid phase at room temperature. HCE have intensively studied for application in energy storage devices with a particular focus on batteries since the demonstration of the lack of reactivity of lithium metal in an acetonitrile HCE. The lack of "free" solvent molecules in HCE is responsible for their stability. This feature also suggests that heterogeneous electron transfer (ET) in HCE could be different from conventional electrolytes because of the importance of solvent reorganization during ET. Thus, we investigated the heterogeneous electron transfer of the ferrocenium/ferrocene (Fc+/Fc) redox couple as a function of concentration of the salt Li bis(trifluoromethanesulfonyl)imide in acetonitrile, a model system for HCE. We show that while the diffusivity of Fc (Shoup-Szabo) follows the trend with viscosity (η) expected from the Stokes-Einstein relation over the entire concentration range, the ET rate constant (k0) variation with η on the other hand diverges from ideality. Using Raman spectroscopy in the solution and on the surface (EC-SERS), we show that the most likely cause for the difference in ET rate constant between dilute and highly concentrated electrolytes involves a strong coordination of the ferrocenium with the complexes found in HCE. This new knowledge highlights the importance of increasing fundamental research on the topic of electrochemistry in HCE

Investigation of protic ionic liquid electrolytes for porous RuO2 micro-supercapacitors

International audienceThe rapid advancement of the Internet of things (IoT) with applications across various sectors urges the development of miniaturized energy-storage devices that can harvest or deliver energy with high power capabilities. While micro-supercapacitors can meet the high-power requirements of ubiquitous sensors connected to IoT networks, their low voltage and low energy density remain a major bottleneck preventing their wide-scale adoption. In this report, we develop micro-supercapacitors using RuO2 electrodes providing pseudocapacitive charge storage in protic ionic liquid-based non-aqueous electrolytes while enlarging their operational voltage. The triethylammonium bis(trifluoromethanesulfonyl)imide (TEAH-TFSI)-based interdigitated porous RuO2 micro-supercapacitors showed an extended cell voltage up to 2 V with 4 times more energy density compared with conventional H2SO4 electrolyte. We then developed an all-solid-state micro-supercapacitor using TEAH-TFSI-based ionogel electrolyte able to deliver high areal capacitance (78 mF cm-2 at 2 mV s-1) and long-term cycling stability that is superior to state-of-the-art ionogel-based micro-supercapacitors employing carbonbased or pseudocapacitive materials. This study gives a new perspective to develop all-solidstate micro-supercapacitors using pseudocapacitive active materials that can operate in ionicliquid-based non-aqueous electrolytes compatible with on-chip IoT-based device applications seeking high areal energy/ power performance

Electrochemistry and transport properties of electrolytes modified with ferrocene redox-active ionic liquid additives

Used in their pure, undiluted form, ionic liquids usually result in Li-ion battery electrolytes with inadequate performance due low Li+ transport numbers (tLi+). Alternatively, they can be used as additives dissolved in carbonates to maintain a high tLi+ while providing the electrolyte with additional properties such as resistance to combustion, current collector passivation, and decreased Li dendritic growth. Additional properties can be imparted to the ionic liquid via the modification of their structure. Ionic liquids modified with electroactive moieties such as ferrocene (Fc-IL) can be used as an additive in Li-ion battery (LiB) electrolytes to prevent cathode over-oxidation via the redox shuttle mechanism. The aim of the present work is to evaluate the properties of LiB electrolytes modified with such Fc-IL at different concentrations. At low concentrations (0.3–0.5 mol/L), the redox-active ionic liquid behaves as expected for a redox shuttle. We show that at 1 mol/L, however, the redox ionic liquid yields a different discharge behavior after the overcharging step, providing an increase in discharge capacity. This behavior is linked to the deposition of the ferrocenium-IL at the positive electrode. Such electrolyte is non-flammable and is highly efficient to achieve shuttling of excess charge. Based on this principle, it is expected that novel ionic liquids can be designed for development of other types of additives and contribute to developing safer battery electrolytes. As a part of this commemorative issue, this contribution highlights the type of collaborative research currently being done on energy storage devices at the Department of Chemistry at the Université de Montréal.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Redox shuttles for lithium-ion batteries at concentrations up to 1 M using an electroactive ionic liquid based on 2,5-di-tert-butyl-1,4-dimethoxybenzene

In this work, we designed two redox shuttles with high solubility (up to 1 M) in conventional carbonate-based lithium-ion battery (LIB) electrolytes. At this high concentration, redox shuttles ensure improved overcharge protection than lower concentrations. We developed electroactive imidazolium salts by modifying imidazolium with 2,5-di-tert-butyl-1,4-dimethoxybenzene. Two salts with the cation 1-(3-(2,5-di-tert-butyl-1,4-methoxyphenoxy)propyl)-3-methyl-1H-imidazol-3-ium (EMIm) were synthesized using either hexafluorophosphate (DDB-EMIm-PF<inf>6</inf>) or bis(trifluromethanesulfonyl)amide (DDB-EMIm-TFSI)) anions. The electrochemical properties of DDB-EMIm-PF<inf>6</inf> and DDB-EMIm-TFSI dissolved in ethylene carbonate: diethyl carbonate (EC:DEC), in the presence of either LiPF<inf>6</inf> or LiTFSI, were evaluated. Cyclic voltammetry showed a compatible potential ( 3c3.85 V vs. Li/Li+) for use in LIBs using LiFePO4 as cathodes. Electrolytes using 0.1 M of DDB-EMIm-PF<inf>6</inf> or 0.3, 0.7 and 1 M of DDB-EMIm-TFSI were prepared and evaluated in Li/LiFePO<inf>4</inf> (LFP) test cells to demonstrate overcharge protection. Electrochemical cycling at C/10 showed an overcharge protection for all concentrations of the redox ionic salts under 100% overcharge conditions. Among these salts, DDBEMIm-TFSI, at a concentration of 0.7 M, was effective in shuttling excess current for over 200 cycles, representing over 6000 operating hours, while maintaining nominal values for the discharge capacity of LiFePO<inf>4</inf>.Peer reviewed: YesNRC publication: Ye