Magneto-ionics for Accurate SOC Estimation in Lithium-ion Batteries

Lithium-ion batteries, used in many applications such as electric vehicles or grid storage, require a mechanism to measure the state of charge (SOC) of the battery cells. SOC is the available capacity and can be viewed as a thermodynamic quantity that can determine the potential energy of a battery. In other words, it is the measure of the amount of charge left in a rechargeable battery before it needs to recharge.

SOC estimation is a complex task depending on the type of battery and application. Accurate SOC estimation is essential for high system performance, reliability, and the long lifetime of the batteries. Much research and development are being done to improve SOC estimation accuracy. However, as charging and discharging in a cell involve complex chemical and physical processes, it is not trivial to estimate the SOC accurately under various operation conditions.

Researchers at the University of Buffalo show how a magnetic material can accurately and directly estimate the SOC of a rechargeable battery.

 

Why Accurate SOC Estimation?

Precise SOC estimation of a battery pack can prevent system disruptions, overcharging and over-discharging of the cells. Overcharging and over-discharging can permanently damage the internal structure of batteries. Therefore, accurate SOC estimation can improve the reliability of the battery cell in the application system and its lifetime. The advantages also include reduced battery maintenance costs.

Moreover, it enables the battery pack to be cycled at its full potential. This is because the battery pack has to be operated conservatively to avoid overcharging and over-discharging the cell due to low-reliability SOC estimation.

 

SOC Estimation Methods

One of the simplest methods to estimate SOC is open-circuit voltage (OCV) based. OCV is the electrode voltage when no current is flowing through it. The cell voltage shows a strong dependence on SoC in most batteries. However, this technique might be impractical for real-time systems as a long time is required for the electrode to reach the chemical equilibrium.

A more accurate method is to measure the coulombs and current flowing through the cells under all operating conditions and the individual cell voltages of each cell in the stack. This data is used with the previously loaded cell stack data for accurate estimation of SOC. Further data required for this method is cell temperature, state of the cell during the measurements, cell age, and other relevant cell data obtained from the cell manufacturer.

Currently, there is no direct way to measure the SoC of a battery cell. It must be estimated from measured current, voltage, and temperature.

 

Magnetic Material-based SOC Estimation

University of Buffalo researchers took advantage of ultralow-field sensor technologies based on magnetic materials for SOC estimation. They report in the paper published in PNAS how magneto-ionic material enables precise SOC sensor monitoring in Li-ion batteries using a molecular magnetic electrode.

As the battery gets charged and discharged, lithium ions flow from one side of the battery to the other. Researchers discovered that a magnetic material changes its magnetism as lithium enters or leaves the electrode. By monitoring the magnetism, one can then accurately and reliably measure the SOC of the battery.

 

Microwave-excited spin wave for SOC estimation. Image used courtesy of University of Buffalo

 

The magneto-ionic material is made from vanadium, chromium and cyanide with an aqua ligand. The new sensor demonstrated a more than 2,000% increase in accuracy and a more than 5,000% reduction in response time.

The researchers believe that their developed sensor is a promising solution for real-time monitoring of the state of charge.

Ren, PhD, UB professor of mechanical and aerospace engineering, chemistry, and a core faculty member in the UB RENEW Institute, summarized, "The main goal of this project was working on the magneto-ionics, which uses ions to control the magnetism of materials. As the lithium ions travel in or out of the material we are using, the material will change its magnetization. We can monitor the magnetism, and this enables us to indirectly monitor the lithium ions--the state of charge. We believe this is a new way to provide an accurate, fast, responsive sensing of state of charge."

 

Feature image used courtesy of University of Buffalo