Basic Concepts
What is an electrolyte
An electrolyte is an electronically 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 active material 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. Increasing active material loading (per electrode) is one of the few remaining routes with untapped potential for further reducing the cost of batteries.
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 PF6–. Then, 4 Li+ ions are supplied from the bulk while 6 PF6– 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 PF6– ions, or in short notation, 6 LiPF6 neutral salt molecules.
What does it take to overcome this depletion of salt? Ambipolar 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.
Our Research
Electrolyte figure-of-merit for supporting thicker electrodes
If we had to support thicker electrodes, exactly what electrolyte transport property should we improve? Total ionic conductivity is only a proxy. In our published work led by Yeonggeon Choi, we show that it is important to improve the product of “partial Li-ion conductivity” and “Thermodynamic factor (of the salt).” This work highlights the importance to accurately measure the Li-ion transference number of electrolytes (not just total ionic conductivity). It is also important to come up with better and easier methods to measure the thermodynamic factor of salt. In our paper, we also discuss the current limitations we face in measuring these properties.
Further Research Goals
Motivated by our work on identifying the electrolyte figure-of-merit, we are trying to develop new methods that could better measure electrolyte transport properties. This effort now targets electrolytes for both Li-ion and Na-ion batteries. We believe better measurements are needed to see how various chemical features impact the relevant transport properties. With more data, we should then be able to tap into AI-based methods for accelerated discovery and development.
Last modified: Jul. 30, 2026