Batteries: Electrolyte

Basic Concepts

What is an electrolyte

An electrolyte is an electrically insulating but ionically conductive material, where the ionic conductivity is (partially) provided by the target ion of interest. For example, a Li-ion electrolyte can transport Li+ ions while blocking electrons. Electrolytes are essential in electrochemical cells because the electrical driving force in a cell (voltage) is achieved by allowing target ions to equilibrate while preventing electrons to equilibrate.

Why are electrolytes important

One of the many reasons why electrolytes are important is because it determines how much porous electrode can be packed into a cell, and thus the energy density of a cell; if the transference number, chemical diffusivity, and partial ionic conductivity of an electrolyte could be increased, it would allow higher loadings of the energy storage material in a battery.

Electrolytes also impact how well an electrochemical reaction can happen, what type of side reactions could occur in a cell, what type of intentional passivation layers form on active materials, just to name a few. These factors all heavily influence the quality of batteries, including power, efficiency, stability, and, longevity.

Transport in electrolytes

The transport capability of an electrolyte is often estimated simply based on its conductivity. However, conductivity represents only a small facet of the picture. In any electrolyte, positive ions need to be balanced with negative ions for charge neutrality. Both of these ions are mobile in general (exception: solid electrolytes), so the mechanism of electrolyte transport is far more involved than simple conduction.

Let's think about a simple example to understand the consequence of having multiple mobile ionic species. Consider electrochemical Li intercalation into a host. “Electrochemical” means we deliver ions and electrons separately, where ions come from the electrolyte and electrons come from the external circuit of the cell. Assume we are intercalating 10 Li atoms, as shown in Fig. 1a. What happens to the electrolyte adjacent to the host material? As illustrated in Fig. 1b, it loses 10 Li+ ions at the host interface. Meanwhile, the current is delivered from the bulk electrolyte, but we have more than one ionic species that can conduct electricity. In this example, assume 40% of current is carried by Li+ ions and the other 60% is carried by counter ions such as LiPF6. Then, 4 Li+ ions are supplied from the bulk while 6 LiPF6 ions move out to the bulk, netting a total flow of current equal to the flow of 10 charges. As a result, we have depletion in the electrolyte box amounting to a total of 6 Li+ ions and 6 LiPF6 ions, or in short notation, 6 LiPF6 neutral salt molecules.

Figure 1: Salt depletion from a Li-ion transference number less than 1
Figure 1: Salt depletion from a Li-ion transference number less than 1.

What does it take to overcome this depletion of salt? Bipolar chemical diffusion: the concurrent diffusion of both positive and negative ions, rather than just one of them. In other words, it is the chemical diffusivity of salt that determines how quickly the concentration gradients inside the electrolyte can be homogenized.

Research goals

One of our initial goals is to either improve conventional measurement techniques or develop new methods that could be more inquisitive than conventional techniques. Battery electrolytes are very complex systems because they are highly concentrated, include multiple solvents and additives, and often show property drift over time. Many fundamentally important properties are still not well-characterized, limiting our understanding on how ions interact with each other and the solvent. We are starting our quest for understanding electrolytes by equipping ourselves with better measurement methods.

Last modified: Oct. 15, 2023