Research at TU Delft found that introducing a little disorder in a battery’s electrolyte goes a long way in improving its lifetime.
Lithium-ion battery life improves significantly when a single-compound electrolyte is swapped for one consisting of five salts, researchers from Delft University of Technology found. Thanks to their cocktail of five lithium salts, a more stable ‘coating’ is formed on electrodes, allowing batteries to last up to twice as long. Other combinations of salts could work even better.
An electrolyte is an ion-containing organic liquid that shuttles lithium ions back and forth between electrodes. During charging and discharging some of its constituents decompose, forming a solid film on the electrodes’ surface. This so-called solid-electrolyte interface (SEI) has a profound impact on battery performance, including lifetime.
Initially, the SEI acts primarily as a protective barrier that allows lithium ions to pass through but stops electrons from taking a ‘shortcut’ between electrodes. As the electrolyte keeps decomposing during charging and discharging, however, SEI film thickness grows, trapping lithium ions. This increases electrical resistance and reduces battery capacity.
The mix of lithium salts investigated by Marnix Wagemaker’s team affects the decomposition process, changing the characteristics of the SEI that’s formed in beneficial ways. Through a series of measurements, the Delft researchers show that the more chaotic nature of the electrolyte mixture is responsible for the improved lifetime.
Added bonus
Very coarsely speaking, chaos – as in: the amount of randomness or disorder within a system – can be equated with the concept of entropy. In the case of the Delft study, the entropy that matters derives from so-called solvation structures, referring to solvent molecules surrounding positively charged lithium ions with their slightly negative side. Similarly, lithium’s nearby counterions (the anions) face the solvent molecules’ slightly positively charged sides. If you take a snapshot of the electrolyte on a molecular level, you’d see such structures dotted throughout the liquid.
Introducing several types of anion induces a larger diversity of solvation structures, which is basically the same as saying that the electrolyte is less ordered compared to the single-salt equivalent. The Delft researchers’ results indicate that as a result of this increased disorder, interactions between the constituents in the liquid change, affecting their movement and behavior in such a way that a more stable SEI is formed. As an added bonus, lithium ions move more easily in a less ordered medium, resulting in more power and faster charging.
Hurdle
Wagemaker believes that it should be relatively straightforward to apply the results in commercial production without adding cost, he told Tweakers. That said, many questions remain unanswered, particularly concerning compatibility with commonly used electrodes and safety. Additionally, through varying composition and concentration, a practically infinite amount of mixtures can be prepared and some of them may perform even better.
Leydenjar isn’t waiting for those results to come in and has started its own tests. The startup with a lab in Leiden and a pilot facility in Eindhoven has developed silicon anodes that enable 70 percent higher density compared to traditional graphite anodes. Increasing lifetime to meet requirements from the automotive industry is still a major hurdle for the company, and the high-entropy electrolyte could help with that.