Thank You - Lithium Hexafluorophosphate Battery Electrolytes

Lithium-ion battery electrolytes are typically composed of lithium salts and a mixture of cyclic carbonates, e.g., ethylene carbonate (EC), and linear carbonates, e.g., dimethyl carbonate (DMC) and/or ethyl methyl carbonate (EMC). First commercialized in the early 1990’s, lithium-ion batteries have shown they can possess exceptional cycle durability and storage capacity. Lithium hexafluorophosphate (LiPF₆) is the most common lithium salt used and is gaining popularity due to the rising demand for electric vehicles and interest in renewable energy storage systems.

Electrolyte viscosity is a critical factor for the functioning of lithium-ion batteries, as it influences the mobility of ions between battery electrodes during charging & discharging cycles. Since temperature changes can affect the electrolyte viscosity and, hence, the battery’s performance, it is important to design electrolytes with appropriate viscosity characteristics over wide temperature ranges relevant to battery applications, like arctic to tropical hot conditions. To design better batteries, it is of interest to probe the viscosity of battery electrolyte solutions accurately and reliably over wide temperature ranges.

For this app note, we used the VROC^® initium one plus automated & high-throughput viscometer to measure the viscosity of small-volume (90 μL) LiPF₆ battery electrolyte solutions with high resolution, high accuracy, and minimal evaporation for a wide temperature range (with built-in Peltier temperature control).

Experimental

The solvent mixture was prepared at room temperature to have a mass ratio of 1:1 ethylene carbonate (EC) to dimethyl carbonate (DMC). The electrolyte was prepared by adding lithium hexafluorophosphate (LiPF₆) to the 1:1 EC:DMC solvent mixture. Both solvents and the electrolyte were purchased from Sigma Aldrich. The viscosity was measured using the level generator mode (25 & 95 % full-scale pressure, 5 repeats per level, T = 4, 15, 26, 37, 48, 59, 70 °C) with the VROC^® initium one plus using an A05 chip (flow channel depth = 50 μm, P_max = 12 kPa). Dry air (nitrogen) was supplied to the chip, test syringe, and sample tray via a port on the backside of the unit when T < 18 °C. Approximately 70 μL of the sample was loaded with the sample retrieval feature activated so that all temperature sweep measurements were performed with only one loaded volume. Cleaning of the loading & test syringes, as well as all relevant flow paths, was performed with DMC and acetone supplied by Sigma Aldrich. During measurements, the sample experienced minimal evaporation due to the absence of an air-liquid interface within the microfluidic channel.

Results & Discussion

Figure 1 displays the measured viscosity for each temperature for solutions consisting of 0 and 5 wt.% LiPF₆ in 1:1 EC:DMC. For both samples, the viscosity drops with temperature, while being relatively higher for the sample with added LiPF₆. The error bars, three times the standard deviation, are similar in size or smaller than the symbols, showing the high repeatability for such low viscosity samples (η ~ 1 mPa∙s). The viscosity is within 5 – 13% of that reported in the literature (Wu, et al., 2004; Dougassa, et al., 2014) for samples with ~ 5 wt.% LiPF₆ in 1:1 EC:DMC. The dashed lines are fits based on the following Arrhenius-like equation giving the dependence of viscosity η on temperature T as

                                                       η = A · e^{Ea/[R·(T+273.15)]}

In this equation, A is an exponential prefactor, E_a is the activation energy for viscous flow, and R is the universal gas constant. The fit is a good approximation of the results for 0 wt.% LiPF₆ in 1:1 EC:DMC, indicating this solution displays an Arrhenius-like behavior over the temperature range studied. The viscosity can be calculated for lower and higher temperatures with extrapolation of the fit, provided the solution will exhibit Arrhenius-like behavior. On the other hand, the Arrhenius-like fit is not appropriate for 5 wt.% LiPF₆ in 1:1 EC:DMC for all temperatures.

Concluding Remarks

For this app note, we used our VROC^® technology to measure the viscosity of LiPF₆ battery electrolyte solutions of small volume (90 μL) with high resolution, high accuracy, and minimal evaporation for a wide temperature range. For the samples studied, the viscosity drops with temperature while being relatively higher for the sample with added LiPF₆. An Arrhenius-like fit of viscosity vs temperature is a good approximation to the results of the electrolyte solvent mixture of 1:1 EC:DMC. With such accurate measurements and provided the sample viscosity vs temperature behavior is Arrhenius-like, we can extrapolate the viscosity to temperatures T < 4 °C and T > 70 °C.

References

Wu, M. S., Liao, T. L., Wang, Y. Y., & Wan, C. C. (2004). Assessment of the wettability of porous electrodes for lithium-ion batteries. Journal of applied electrochemistry, 34, 797-805.

Dougassa, Y. R., Jacquemin, J., El Ouatani, L., Tessier, C., & Anouti, M. (2014). Viscosity and carbon dioxide solubility for LiPF6, LiTFSI, and LiFAP in alkyl carbonates: lithium salt nature and concentration effect. The Journal of Physical Chemistry B, 118(14), 3973-3980.

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