Precise Characterization of Viscosity of Rechargeable Batteries

The type of electrolyte used in Li-ion batteries governs the power and lifetime capabilities. While developing new kinds of electrolytes, factors like conductivity, viscosity, and dielectric constant of the blend have to be carefully taken into consideration.

Conductivity and viscosity are known to be interdependent and associated with the Stokes model. Viscosity is a parameter that directly impacts ionic conductivity and establishes how fast a cell can be charged and discharged; therefore, it should be carefully regulated. Battery electrolytes are a combination of various solvents, and therefore, viscosity measurements of these mixtures can present certain challenges with regards to the low volatility and viscosity of the products.

Method & Technique

FluidicamRHEO is capable of measuring viscosity at high shear rates with high precision requiring only a small volume of sample. A reference solution and the sample are introduced into the microfluidic channel at regulated flow rates. This leads to a laminar co-flow where images of the interface, captured at the time of the measurement, enable to determine the position of the interface and to calculate the viscosity of the sample to directly plot an interactive flow curve.

Conventional solvent base meant for Li-ion batteries is a combination of solvents (EC, DMC, and EMC). While EC exhibits a high dielectric constant, assisting with the dissociation of lithium salts, DMC and EMC are characterized by lower melting point and viscosity: When these are combined together, it results in a good compromise between required electrochemical properties, low viscosity (η), and high dielectric constant (ε). Fluidicam Rheo allows precise measurement of volatile organic solvents and blends with fast temperature screening enabling quick characterization of multiple mixtures in a matter of minutes.

The viscosity of pure solvents as well as the varied mixtures of EMC: DMC: EC (different w/w EC ratios) were initially examined at different temperatures. This was followed by studying the viscosity of electrolyte solutions at various concentrations.

Temperature Relation to the Viscosity of the Blends

Figure 1 demonstrates how the viscosity of the analyzed mixtures of EMC:DMC:EC and EMC:DMC lowers with the temperature (range between 15 °C and 65 °C).

Viscosity-temperature dependence of the two solvent mixtures.

Figure 1. Viscosity-temperature dependence of the two solvent mixtures.

Effect of EC Concentration on the Blend Viscosity

Next, the viscosity of the solvent base EMC:DMC:EC was determined at different concentrations of EC (see Table 1).

Table 1. Viscosity as a function of EC concentration

EMC DMC EC Viscosity [mPa.s]
1 1 0 0.614 ± 0.007
1 1 0.245 0.726 ± 0.007
1 1 0.497 0.806 ± 0.001
1 1 0.724 0.908 ± 0.012
1 1 1 0.965 ± 0.008

Results demonstrate that the viscosity of the blends increases with higher EC concentration. Moreover, higher viscosities of the electrolytes result in lower ion conductivity, and therefore may affect the performance of batteries.

Electrolyte Concentration and Temperature Influence on Viscosity — Case of LiClO4

A model solution of EMC:DMC, in 1:1 weight ratio, was first prepared and supplemented with varying quantities of dissolved Lithium salt. The viscosity of the blends at varied temperatures, representative of the usage conditions of the battery, is shown in Table 2 and plotted in Figure 2.

A total analysis time of 1 hour and 20 minutes was taken to complete the 17 tests, including the sampling time where it would have taken approximately a day with a traditional capillary viscometer.

Table 2. Viscosity of blends at 25 °C, 35 °C, 45 °C

EMC: DMC:Li
(w/w)
Viscosity
[mPa.s]
25 °C
Viscosity
[mPa.s]
35 °C
Viscosity
[mPa.s]
45 °C
1: 1: 0 0.614±0.002 0.534±0.002 0.461±0.002
1: 1: 0.0286 0.684±0.001 0.562±0.004 0.512±0.004
1: 1: 0.056 0.791±0.006 0.598±0.003 0.588±0.003
1: 1: 0.113 0.893±0.003 0.723±0.001 0.662±0.001
1: 1: 0.216 1.397±0.008 1.005±0.002 0.994±0.002

Viscosity as a function of Li+ concentration and temperature.

Figure 2. Viscosity as a function of Li+ concentration and temperature.

As demonstrated, the viscosity is associated with the concentration of Lithium salts but inversely associated with temperature. When the temperature increases, the viscosity decreases and the higher the concentration the higher the viscosity is..

With regards to the solvents’ physical properties, that is, low boiling points and low viscosity values, the  FluidicamRHEO is more appropriate for viscosity measurements when compared to traditional viscometers due to its confined microfluidic system that prevents risks of volatility and drying.

Conclusion

This technique is highly sensitive to small viscosity differences over the range of shear rate, and as a result, the viscosity measurements achieved with FluidicamRHEO are highly precise and accurate. On the whole, FluidicamRHEO has been shown to be a dependable tool for the characterization of electrolyte viscosity, and also, its confined geometry makes it possible to work with volatile products. Robust measurement requirements for identifying optimum mixture parameters for the best battery performances are also met.

References

  1. Logan, E. R. et al. A Study of the Physical Properties of Li-Ion Battery Electrolytes Containing Esters. Journal of The Electrochemical Society 165, A21–A30 (2018).
  2. Ue, M., Sasaki, Y., Tanaka, Y. & Morita, M. Nonaqueous Electrolytes with Advances in Solvents. in Electrolytes for Lithium and Lithium-Ion Batteries (eds. Jow, T. R., Xu, K., Borodin, O. & Ue, M.) 58, 93–165 (Springer New York, 2014).

This information has been sourced, reviewed and adapted from materials provided by Formulaction.

For more information on this source, please visit Formulaction.

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