To create the perfect cappuccino, frothing the milk requires precise timing. Aerate it too long, and the foam becomes bubbly, unstable and much more like batter. Such is the case for the solid electrolyte interphase (SEI), the thin protective film that emerges when lithium-ion batteries are used. When SEI growth keeps going even after an ideal inner layer has already formed, the battery’s energy storage performance or initial Coulombic efficiency (ICE) drops.
Researchers at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and Nanyang Technological University, Singapore, observed this continuous SEI formation wile testing ultrafast-charging anodes like titanium dioxide (TiO2). The SEI’s repeated breakdown and regrowth irreversibly consumes lithium without improving the battery’s functionality.
“We realised that controlling when SEI growth stops is just as important as controlling how it begins. This shifted our focus from building a better SEI to ensuring that it stops evolving at the right time,” said Xian Jun Loh, Executive Director at A*STAR IMRE.
To disrupt the endless cycle, the team decided to target phosphorus pentafluoride (PF5), which is a byproduct derived from the decomposition of a common electrolyte salt. This PF5 reacts with hydroxyl (─OH) groups on the TiO2 surface, producing corrosive intermediates that help trigger SEI degradation and reformation. The continuous cycle sustains parasitic interfacial reactions that needlessly consume additional lithium.
By inducing a chemical attack known as nucleophilic fluorination, the researchers replaced the hydroxyl groups on the electrode surface with fluorine atoms before the electrochemical cycling reactions could unfold. At just the right time, the PF5 and OH interactions are suppressed, promoting earlier self-termination of SEI formation.
“Through chemical fluorination, we enable the interface to reach stability earlier and preserve the beneficial SEI inner layer,” said Qiang Zhu, Head of the Advanced Characterization and Instrumentation Department at A*STAR IMRE. The fluorinated TiO2 had an ICE above 90 percent, a marked improvement over the original anode’s 74 percent.
The team also tested fluoroethylene carbonate (FEC), a common additive used to enrich the SEI inner layer with inorganic compounds that are thought to improve cycling performance.
Instead of benefitting interfacial stability, however, incorporating FEC reduced ICE by around five percent.
“Once a thin SEI layer has been established, any further growth largely reflects parasitic reactions. Moreover, attempts to engineer the ideal composition of this inner layer may irreversibly consume more lithium in the process,” said Shengkai Cao, a Research Scientist at A*STAR IMRE.
The researchers next plan to apply their early termination strategy to aqueous lithium-ion batteries for safe, large-scale stationary energy storage. By understanding such surface chemistry across different electrolyte-based systems, they hope to establish early interfacial termination as a broader design principle for improving battery efficiency and durability.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
