Linear sweep voltammetry takes more than a multimeter.
Who knows what will happen in the future. With all the focus on alternative energy and climate change, battery research and power electronics for energy storage tend to take a back seat. If you’re designing power electronics for energy storage systems and regulation/charging systems for batteries, you’ll need some data from your target battery as you begin to design your electronics.
One of these fundamental measurements is linear sweep voltammetry, and a related measurement technique, cyclic voltammetry. This measurement is intended to examine how the current produced in an electrolyte depends on the voltage drop applied across the electrolyte. These measurements are very easy to gather, but they can be difficult to interpret if you’re not a chemist. Once you understand the meaning of these measurements, it becomes easier to design a charger or regulator for your battery system.
Linear Sweep Voltammetry Analysis
Batteries contain an electrochemical solution in contact with electrodes, and flow of current between the positive and negative electrodes in a battery is driven by an electrochemical reaction. This electrochemical reaction is a redox reaction, where electrons leave one electrode (the cathode) and enter the other electrode (the anode). The redox potential difference between these two solutions and the terminal voltage on the battery electrodes will determine the output voltage from the battery.
Electrochemical reactions that power rechargeable batteries are reversible, just like many other chemical reactions. When given a sufficiently strong voltage, the redox reaction can be reversed and the battery can be recharged. The central idea in building a battery charger is to determine the charging current that should be used in the system. By pairing the correct charging current for your particular battery chemistry, you can determine the correct charging time, open circuit voltage, and even the useful lifetime for your battery.
The graphs below show results from a linear sweep voltammetry measurement. In this measurement, the voltage applied to a 3-electrode electrochemical cell is swept in a linear fashion over time. The sweep rate should generally be slow enough to gather stable measurements, but not so slow that the current begins to decrease as the available reactants in the redox reaction are consumed. The current in the electrochemical cell is measured as the voltage is swept, and the current vs. voltage data is plotted on a graph.
Linear sweep voltammetry curve
In the above graph, there is only a single peak, which corresponds to progress of the redox reactions at the electrode. In a real linear sweep voltammetry graph, you’ll often see a series of peaks corresponding to different chemical reactions that occur with high rate at different voltages. One common set of reactions is adsorption/desorption reactions at one of the electrodes. Interpreting this type of graph requires thoroughly understanding the chemistry in the battery.
Linear Sweep Voltammetry vs. Cyclic Voltammetry
A related measurement technique for examining battery behavior is cyclic voltammetry. In this measurement, the voltage is swept linearly back-and-forth between two values, yielding two current vs. voltage curves with hysteresis. One of these curves corresponds to a reduction at the cathode, and the other corresponds to oxidation at the anode. This curve is one of many examples of a curve with hysteresis. Note that you’ll need to focus on the range of applied voltages that drive the fundamental redox reaction, rather than any adsorption/desorption reactions or side reactions.
Cyclic sweep voltammetry curves plotted with US and IUPAC conventions. The arrow shows the voltage sweep direction in the experiment.
The applied voltage at the peaks in these plots is the standard electrode potential for the electrochemical cell. From this you can determine the Nernst potential (i.e., open-circuit voltage) for each electrode using the activities for the oxidized and reduced species in the cell:
Nernst equation for the open-circuit voltage from cyclic voltammetry data.
Here, each ai is the activity for an anode or cathode reaction, and Ec and Ea are respectively the anode and cathode potentials from a cyclic voltammetry graph. The remaining symbols have their usual meanings in the electrochemistry literature.
Determining Charging Rates and Voltage
The idea behind a charging system is to regulate the charging current flowing into a battery while maintaining voltage at a fixed value. The idea is to drive the spontaneous redox reaction into reverse inside the battery, which requires applying a voltage slightly above the open-circuit voltage. Battery voltages will drop off nonlinearly as the concentration of reactants in the battery’s redox reaction are consumed, thus the open-circuit voltage is also nonlinear.
Rather than tracking the open-circuit voltage during charging and applying a fixed voltage above open circuit, the typical method is to simply set the applied voltage a few V above the maximum open-circuit voltage. To determine the charging rate for a battery regulation system, the typical method is to use a current ranging from 20% to 25% of the theoretical capacity, i.e., to use a charging rate that will cause the battery to charge within 4 to 5 hours.
Preventing Side Reactions During Charging
This is a simple, yet widely used way to determine the appropriate current and voltage output from a charging regulator for use in battery charging. When working with newer battery materials (e.g., unique porous separators for advanced Li-ion batteries), great care should be taken so that an unintended side reaction is not induced, which can occur when the applied voltage and/or current are too large.
Some side reactions can cause morphology changes in separator materials or electrolytes, which can decrease total capacity, maximum discharge rates, and other important battery metrics. For very sensitive batteries, you may not have any choice but to build a control system that tracks the battery voltage during charging. This can help prevent any side reactions that may occur during charging.
Development of new battery materials and control systems for these batteries is an ongoing area of materials science and engineering research. Linear sweep voltammetry and related measurements will continue to be fundamental for developing these systems. If you have access to the right PCB layout and design software, you can design the important electronic charging and regulation components these systems need. Allegro PCB Designer and Cadence’s full suite of design tools are ideal for designing these systems and simulating their behavior.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.