ANN ARBOR, MI—Engineers at the University of Michigan have developed a method for predicting how changes to manufacturing processes and materials can impact battery life. They have discovered that internal resistance, measured immediately after cells are made, are a key indicator of how long a battery will last. The measurements can be done in just seconds at the tail end of the production process at little to no additional cost.
Previous research has shown lifespan prediction is possible, but it requires repeated cycling—charging, discharging and recharging—in order to gather data needed to train the algorithm. Aging tests needed to determine lifespan can also take weeks to months to complete. For this reason, the tests are typically performed on a limited basis.
But, accurate battery lifetimes can be predicted with the help of one resistance measurement, or a measure of how much the battery fights the flow of current inside it. That resistance can come from the materials used for internal components, or electrochemical factors that affect how well ions move between the battery electrodes.
Measuring resistance at low levels of charge is the key. The resistance measurement, at low state of charge, can, in principle, be obtained without any cycling, making the model training process much faster.
“It can give an indication of how much lithium merged with the liquid electrolyte that ferries ions from one electrode to the other inside the battery,” says Anna Stefanopoulou, Ph.D., a professor of technology who is leading the research effort at the University of Michigan Battery Lab. “That combination layer, known as the solid electrolyte interphase, can protect the surface of the electrode and enable longer lifetimes.
“The amount of lithium that goes into the solid electrolyte interphase is usually difficult to measure,” notes Stefanopoulou. “But, at a low state of charge, the battery’s internal resistance is closely related to how much lithium went into the solid electrolyte interphase. This provides a quick measure of that protective layer, as well as the operating capacity of the battery, with ordinary equipment.
“Automakers are always trying to decrease the cost of producing cars, and right now, they’re looking to make EV batteries as cost-effective as possible,” explains Stefanopoulou. “So the question we’ve tried to answer is ‘How fast can you learn about battery lifetime during the manufacturing process itself?’
“It turns out that the answer is, ‘Immediately, if you know the critical signal that can be acquired in high-throughput testing,’” says Stefanopoulou. “Finding such key measurable features can be used for continuous improvements and scaling up domestic battery manufacturing.”