Significance Statement
Lithium-fluorinated carbon cells Li/CFX have various advantages in terms of high specific capacity and specific energy of about 250WhKg-1 and 600Whl-1 respectively with stable voltage and shelf-life up to 20 years. However, they don’t provide built-in power and are efficient only at minimum loads. This situation is worthy of study in cold regions such as Nordic countries (-200C to -500C).
Aprotic liquid electrolyte can however provide work of power sources in deep subzero temperatures. Aprotic liquid electrolytes for commercial Li/CFX-systems include LiBF4 salt dissolved in single organic solvent gamma-butyrolactone or in mixture of organic solvent such as propylene carbonate, ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate.
2 Comparative studies on behaviors of LiPF6 and LiBF4 salts can provide better possibilities to improve properties of electrolyte at low temperatures as they show different ionic conductivity in different solvent and temperatures. Another problem of cell performance is deceleration of electrode reactions as temperature decrease in magnitude of 300C.
In recent article of Ignatova et al. (2016) and published in Journal of Power Sources, investigations were made on effect of the 15-crown-5 additive in the liquid electrolyte on performance of Li/CFX– system at room and low temperatures (down to -500C) under varying the composition of the electrolyte and thickness of the electrode.
The researchers investigated the influence of crown-ether addition on performance of Li/CFX-system, two liquid electrolyte compositions with different solvent and lithium salt. First was 1M LiBF4 in gamma-butirolactone and 1M LiPF6 in EC/DMC/EMC in ratio of 1:1:3 by weight. Both electrolytes were studied in presence of 2 vol. % 15-crown-5 at room temperatures of -450C for first composition and -500C for second composition.
Current-voltage characteristics were measured in galvanostatic mode on the Potentiostat P-8, discharge characteristics were registered with resistive load of 910 Ohm. A galvanostatic mode with increment 0.5mA potential of the cell was measured in 20s. Differential scanning calorimetry was conducted by a device DSC 822 Mettler-Toledo with initial cooling of sample to -1500C with liquid nitrogen followed by heating with scanning rate of 50Cm-1.
Results from differential scanning calorimetry for electrolyte based on gamma-butirolactone showed that at -490C electrolyte freezes and becomes nonconductive while second electrolyte indicated homogeneity of a system keeping a liquid state to -1050C/-1130C. Addition of 2vol% of liquid 15-crown-5 under glass transition temperature shifted to 140C and 90C towards higher and lower temperatures for both electrolytes.
Crystallization temperature of first electrolyte increased on 110C and melting point does not change which is equal to -49.70C which indicates limitation of application of traditional electrolyte at low temperatures. It also showed that crown ether additive increases the current Isc in all cases, a slight decrease in melting pointing of first electrolyte with addition of crown ether allows the cell operate at -500C.
Current-voltage characteristics of a system with second electrolyte markedly improved as length of plateau at -500C is increased by 1.5 times as compared with traditional electrolyte based on gamma-butirolactone.
At room temperature, traditional electrolyte 1M LiBF4 in gamma-butirolactone, the charge capacity is 900mAhg-1 which is 1.5 times higher 600mAhg-1 for 1M LiPF6 in EC/DMC/EMC (1:1:3) electrolyte.
Results on discharge characteristics of cells demonstrated that crown ether additive improves significantly electrolyte performance at low temperatures. The cell with electrolyte based on gamma-butirolactone had the discharge plateau approximately by 25mAhg-1 longer than the cell with the electrolyte based on carbonate solvent.
According to quantum chemical modeling ordered layers of crown ether molecules are probably formed at low temperatures on the electrode surface that improves Li+ conductivity of the electrolyte-electrode interface.
Journal Reference
[expand title=”Show Affiliations”]
- The Institute of Problems of Chemical Physics of the Russian Academy of Sciences, Academician Semenov Avenue 1, Chernogolovka, Moscow, 142432, Russian Federation
- Ltd. Regional Consulting and Technical Center of autonomous Power sources “Firm Alfa-plus”, B.Semenovskaya St. 42 (Building 1, Room VI), Moscow, Russian Federation[/expand]