The effects of Viscosity on battery performance

The previous article addressed the importance of thermal management in batteries and how thermal conductivity measurements can be utilised to ensure that the materials used are optimal. As well as thermal properties, properties of the electrolyte within the batteries should also be characterised. Numerous characteristics of the electrolyte solution, including viscosity, salt concentration, composition and concentration of the electrolyte, and temperature, can affect how effectively the battery performs.  

The electrolyte in an electrochemical cell is used to transport ions between the electrodes. Its viscosity is a crucial factor that can impact a battery’s efficiency as viscosity is defined as a fluid’s resistance to flow. The higher the viscosity, the more resistance the ions will experience when moving between the electrodes, which will result in a lower efficiency, poorer battery performance, and poorer conductivity.

The ion conductivity is directly affected by changes in the electrolyte’s viscosity, which can be influenced by a variety of elements including ion size, solvent type, and ion solvation. Wettability is one attribute that can be influenced by the electrolyte’s viscosity, therefore, even minor variations in an electrolyte’s viscosity can have a significant impact on the battery system’s overall performance. As well as the viscosity, it is all affected by the dielectric constant.

When considering the composition of an electrolyte, a compromise must be made between the viscosity and the dielectric constant. There are both parameters that can affect the ion mobility (defined below) and can affect the overall performance.

ion mobility = 1 / 6πη ri

Where η is the viscosity and ri is the radius of the solvated ion.

As can be seen from the equation above, the ion mobility and viscosity are inversely related, which indicates that the higher the viscosity, the lower the ion conductivity in the electrolyte. The other factor that often impacts conductivity is the dielectric constant, which measures how easily the electrolyte can dissociate the salt into ions, leaving a larger concentration of ions in the electrolyte. The conductivity increases in proportion to the concentration.

Viscosity plays a crucial role in slurries, which are used in secondary batteries like lithium ion. In the process of making electrodes, slurries – typically non-Newtonian mixes of solid particles and a liquid binder – are frequently utilised as a coating. Viscosity control is vital to achieve a homogeneous coating thickness and little variation between cells. Problems can arise if the slurry is not of the optimal viscosity. Too low a viscosity can lead to pooling issues, where the electrode is not sufficiently covered, leading to a lower battery efficiency. Likewise, if the slurry is too viscous, it can lead to clumping of the solid particles, which can cause hot spots on the electrodes during battery operation, also leading to a less efficient cell (Cushing et al., 2021).

In contrast to other traditional viscometers, Rheosense’s VROC™ technology has been shown to have the accuracy and precision needed to reliably record the viscosities of electrolytes at low viscosities (usually between 1 and 3 cP). Most battery solutions are volatile, this leads to issues when measuring viscosities due to the sample partially evaporating, leading to incorrect data. Rheosense’s systems are closed systems, which prevents any evaporation with volatile samples, ensuring that the viscosity recorded is that of the sample and not the evaporated mixture.

Using a combination of microfluidics and MEMS technology, Rheosense have developed systems that make it feasible to measure low viscosities of Newtonian and non-Newtonian samples, as well as small sample sizes, all with high accuracy and precision, making VROC™ technology distinct from conventional rotational viscometers.

For more information about the Rheosense viscometers with battery applications, visit the Rheosense website, or watch their webinar about temperature dependent viscosities of mixtures.

For any enquiries or contract testing using the Rheosense systems, contact us.