Physical Characteristics of New Potential Electrolytes for High Energy and Power Density Electrochemical Devices

A modern technological society demands the use and storage of energy, employing novel large and small scale systems. High power density and cyclability make electrical double layer capacitors (EDLCs), Li-ion and Na-ion capacitors/batteries desirable energy storage devices in many technological fields [1]. The properties of electrolyte and electrode materials used and overlapping of physical and electrochemical characteristics determine the parameters (electrical reversibility, coulombic efficiency, capacitance, relaxation time, specific power and energy densities) of EDLC, Li-ion (LiIB) and Na-ion batteries (NaIB). For that reason, the study and modeling of the microporous carbon electrode |electrolyte (including room-temperature ionic liquid and mixtures of ionic liquids) interface in EDLC, LiIB and NaIB is essential for development of systems with high energy and power densities [2-12].

Previous studies have shown that power and energy densities of the above mentioned systems are mainly determined by electrolyte properties (viscosity, conductivity, solvation energy, effective Debye screening length) in the microporous carbide derived carbon (CDC) electrode material and porous separator matrix [2]. The strong influence of molar volume and chemical composition of a cation on the EDLC, LiIB and NaIB behavior has been observed when the sizes of the partially desolvated ions used (tetraethylammonium, triethyl-methylammonium, substituted phosphonium, substituted imidazolium and Li⁺, Na⁺ and Cs⁺ cations and ClO₄^-, BF₄^- and PF₆^- anions) are comparable with the average pore diameter for the CDC electrode [2-12].

The main aim of this study was the comprehensive analysis of the electrochemical characteristics of EDLCs, LiIBs and NaIBs based on the CDC electrodes in various electrolytes, including 1 M LiClO₄, LiBF₄, LiPF₆, NaClO₄, NaBF₄, NaPF₆, 1-ethyl-3-methylimidazolium tetrafluoroborate, 0.4 M N,N-dimethyl-1,4-diazabicyclo[2,2,2]octanediium tetra-fluoroborate (DMDABCO(BF₄)₂), and 0.2 M in DMDABCO(BF₄)₂ + 0.2 M triethylmethylammonium tetrafluoroborate salts in g-butyrolactone and 1 M tetrakis(diethylamino)phosphonium tetra-fluoroborate, tetrakis(diethylamino)phosphonium hexa-fluoro-phosphate, tetrakis(dimethylamino)phosphonium tetrafluoroborate in acetonitrile. The electrochemical behavior and adsorption of doubly charged cations at microporous carbon electrode has been investigated in order to increase the electric charge density at the electrode surfaces [3-8].

The geometry of solvation shells around cations and anions were optimized based on molecular dynamics calculations (incl. Na⁺, Li⁺, Et₃MeN⁺, (Me₂N)₄P⁺, (Et₂N)₄P⁺ cations and, BF₄^- ClO₄^- and PF₆^- anions).

For electrochemical studies, the two-electrode HS-Test Cells (Hohsen Corp, Japan) were completed using 2 cm² (active volume ~0.045 cm³) microporous CDC as electrodes and cellulose membrane TF4425 (thickness 25 mm) from Nippon Kodoshi as a separator. All electrochemical experiments were carried out inside a glove box filled with Ar (O₂ and H₂O content < 0.1 ppm).

Influence of the electrolyte chemical composition and the solvent properties on the region of ideal capacitive behavior has been established at potential scan rates ν ≤ 100 mV/s and cell voltages ≤ 3.5 V at temperatures T ≤ 80 °C. For some ionic liquids and non-aqueous as well aqueous electrolytes the so-called distortion effects have been established in the region of potential switch over, which are caused by decomposition of very small amount of residual water in non-aqueous electrolyte or ionic liquids applied. Electrochemical reduction of O₂ and oxidation of carbon surface or surface functionalities, existing at CDC surface even after reduction of carbon with H₂ at T = 800 °C for 4 hours, has been established and discussed as well.

In situ synchrotron radiation based experiments [13] have been started with the main aim to establish the intermediates and final products formed at the extremely high cell potentials. It was demonstrated that in situ XPS method can be applied for analysis of ionic liquid|CDC interface with quite high sensitivity and resolution of binding energy values for intermediates formed and the results obtained are comparable with in situ FTIR data.

 

Acknowledgments: This work was supported by the Estonian Science Foundation under Projects Nos. 8172 and 9184, Estonian Ministry of Education and Research project SF0180002s08 and European Regional Development Fund Project SLOKT10209T, Estonian Centre of Excellence in Research Project TK117T "High-technology Materials for Sustainable Development".

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