Differentiating ion transport of water-in-salt electrolytes within micro- and meso-pores of a multiporous carbon electrode

Abstract

Understanding ion transport in porous carbon electrodes is crucial for enhancing the performance of electrochemical energy storage devices. However, for systems using carbon electrodes and water-in-salt electrolytes, this is not generally understood. Here, two salts with different ionic interactions in water, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or choline chloride (ChCl), were utilized at concentrations up to 20 mol kg⁻¹ to explore the ion transport behavior. We report a new method for calculating the ion diffusion coefficient in carbon pores, considering the diffusivity of the bulk electrolyte, as well as the tortuosity and porosity of the carbon electrode. Accuracy is validated by comparing data with bulk electrolyte diffusivity reported from PFG-NMR in the literature, which is further used together with porosity estimated with nitrogen gas adsorption and tortuosity from electrochemical impedance spectroscopy. This technique effectively distinguishes between tortuosity in micro- and meso-pores by considering their volume and surface area. The different ion hydration patterns of ChCl and LiTFSI at concentrations above 10 mol kg⁻¹ influence the ion transport and the tortuosity to different extents. This is confirmed by changes in hydrogen bonding observed in the Raman water bands. Lastly, we introduce a relationship between tortuosity, in-pore ion diffusivity and capacitance to distinguish the charge distribution within micro- and meso-pores at open circuit voltage as well as under applied bias voltages. Our findings reveal the degree of ion dissociation in concentrated aqueous electrolytes as a key parameter determining the charging/discharging rate performance of carbon electrode based capacitors. This study helps develop carbon materials and compatible electrolytes to ensure that the capacitor meets the desired performance criteria while being reliable and efficient.