Comparative Investigation of Large-Scale Cells of Supercapacitors Using Low-Temperature Electrolytes
Authors: Tabarov Farrukh , Galimzyanov Ruslan , Nematov Makhsudsho , Kalashnik Anatoliy 
Published: August 2024

The implementation of the concept of sustainable development for humanity, the advancement of alternative energy, hybrid and electric transport, and backup power supply systems, necessitate the creation of efficient energy storage devices. Such systems include lithium-ion, nickel-metal hydride, and redox flow batteries, where Faraday processes play a crucial role in energy storage mechanisms. One notable advantage of double-layer supercapacitors (SCs) as energy storage devices is the absence of Faraday processes during charge and discharge [1,2]. Moreover, the absence of Faraday processes, the rate of which sharply decreases with decreasing temperature, makes SC cells practically the only devices where power and energy features can potentially be maintained at a sufficient level at extremely low temperatures, down to minus 70-80°C [3]. The demand for developing energy storage systems capable of operating at extreme temperatures without extensive thermal control, such as space avionics systems, electric aircraft, uncrewed aerial vehicles, and electric and hybrid electric vehicles, is growing.
Various methods have been adopted today to improve the performance of supercapacitors at low temperatures [4,5]. The central aspect in this direction is electrolytes, which have the most critical impact on the supercapacitor performance at these temperatures. While many investigations have been conducted on laboratory cells, a comparative analysis of the electrochemical characteristics of cells based on the developed electrolytes (non-aqueous electrolytes) was carried out to understand the correlation between laboratory and large-scale manufactured SC cells.
The electrolyte mixture was prepared using organic substances: acetonitrile (AN), ethyl acetate (EA), toluene (TL), diethyl ether (DE), and vinylene carbonate (VC). A 1.2 M solution of salt TEMA TFB was used to prepare the electrolyte. Test electrolyte mixtures were prepared by adding the test additive in the amount of 5, 10, 15, and 20 vol% for DE and TL and 0.1, 1, 3, and 5 vol% for VC to the AN: EA (3:1 by volume %) mixture. These co-solvents were chosen due to their low melting point, relatively low viscosity, and moderate dielectric constant values.
A comprehensive investigation of the physical-chemical and electrochemical properties demonstrates that additives to the electrolyte mixture can extend the lower boundary of the operating temperature range down to −68°C, providing good electrochemical characteristics of the cells: capacitance and stability during long-term cycling [6].To understand the behavior of developed electrolytes in large-scale SC cells, manufactured large-scale SC cells (LSC) with a standard initial capacitance of 1500F were used. Figure 1a shows the capacitance change of large-scale SC cells based on electrolyte mixture AN: EA (3:1), AN: EA (3:1) +3%VC, AN: EA (3:1) +10%DE, and AN: EA (3:1) +10%TL at +25°C. It is evident that the capacitance of LSC-based SC is nearly maintained for more than 1000 charge-discharge cycles at a temperature of +25°C, demonstrating better performance than laboratory cells. Equivalent measurements were also carried out on the self-discharge of LSC-based SC cells (Fig. 1d), showing only a slight voltage drop for just over 2 days.
This behaviour indicates high resistance to various electrochemical processes, such as

electrochemical decomposition of the activated electrode material and electrolyte compound,

corrosion of the current-collector material,

undesirable Faraday redox processes.

Notably, the leakage current for large-scale cells is lower than for small (laboratory) cells, enhancing the capacitance retention ability for large-scale cells.

Figure 1. Cycle-life stability (a, b, c) and self-discharge behavior (d) of the LSC cells based on electrolyte mixtures at temperatures of 25 °C (a, d), -60 (b) and -65 (c).
For all samples, an increase in capacitance is observed as the number of charge-discharge cycles increases (Fig.1). This is likely due to the high current value (50A) used for large-scale cells, causing Joule heating in the cell structures, and increasing the temperature of the electrolyte mixture by several degrees.
Tests of the developed electrolytes as part of laboratory SC cells and large-scale industrially produced SC cells confirmed their high resource stability during 10,000 cycles of continuous charge-discharge at a current density of 1.5 A/g, including at a temperature corresponding to the upper limit of the operating temperature range. Additionally, the resistance of SCs with these electrolytes to self-discharge was demonstrated, showcasing the possibility of a "cold start" of SC elements at a temperature of –60 °C.

References

1.M. Ue, K. Ida, S. Mori., J. Electrochemical Society 141 (1994) 2989-2996.
2.Conway, B.E., Springer Science & Business Media (2013).
3.C. Zhong, W. Hu, J. EESC, (2016) pp. 31-254.
4.H. Yong, H. Park, C., J. Power Sources 447 (2020) 227390.
5.E.J. Brandon, et.al., J. Power Sources 170 (2007) 225-232.
6.F.S.Tabarov, et al. J. Power Sources 560 (2023): 232658.

Figure 1