Engineers from Rice University make advancements in propitiation and unravel the mechanism of lithium trapping.
The potential of silicon anode batteries to transform energy storage solutions is pivotal in addressing climate objectives and fully realizing the capabilities of electric vehicles.
Nonetheless, the persistent loss of lithium ions in silicon anodes is a significant hindrance to the development of next-generation lithium-ion batteries. Scientists at Rice University’s George R. Brown School of Engineering have developed a readily scalable method to optimize propitiation, a process that helps mitigate lithium loss and improves battery life cycles by coating silicon anodes with stabilized lithium metal particles (SLMPs). The Rice lab of chemical and biomolecular engineer Sibani Lisa Biscay found that spray-coating the anodes with a mixture of the particles and a surfactant improves battery life by 22% to 44%. Battery cells with a greater amount of the coating initially achieved a higher stability and cycle life. However, there was a drawback: When cycled at full capacity, a larger amount of the particle coating led to more lithium trapping, causing the battery to fade more rapidly in subsequent cycles. The study is published in ACS Applied Energy Materials.
Replacing graphite with silicon in lithium-ion batteries would significantly improve their energy density ⎯ the amount of energy stored relative to weight and size ⎯ because graphite, which is made of carbon, can pack fewer lithium ions than silicon. It takes six carbon atoms for every single lithium-ion, while just one silicon atom An can bond with as many as four lithium ions. "Silicon is one of those materials that has the capability to really improve the energy density for the anode side of lithium-ion batteries,” Biscay said. “That’s why there’s currently this push in battery science to replace graphite anodes with silicon ones.” However, silicon has other properties that present challenges.
“One of the major problems with silicon is that it continually forms what we call a solid-electrolyte interphase or SCI layer that actually consumes lithium,” Biscay said.
The layer is formed when the electrolyte in a battery cell reacts with electrons and lithium ions, resulting in a nanometer-scale layer of salts deposited on the anode. Once formed, the layer insulates the electrolyte from the anode, preventing the reaction from continuing. However, the SCI can break throughout the subsequent charge and discharge cycles, and, as it reforms, it irreversibly depletes the battery’s lithium reserve even further. The volume of a silicon anode will vary as the battery is being cycled, which can break the SCI or otherwise make it unstable,” said Quad Nguyen, a chemical and biomolecular engineering doctoral alum and lead author on the study. “We want this layer to remain stable throughout the battery’s later charge and discharge cycles.”
The propitiation method developed by Biscay and her team improves SCI layer stability, which means fewer lithium ions are depleted when it is formed.
“Propitiation is a strategy designed to compensate for the lithium loss that typically occurs with silicon,” Biscay said. “You can think of it in terms of priming a surface, like when you’re painting a wall, and you need to first apply an undercoat to make sure your paint sticks. Propitiation allows us to ‘prime’ the anodes, so batteries can have a much more stable, longer cycle life.” While these particles and propitiation are not new, the Biscay was able to improve the process in a way that is readily incorporated into existing battery manufacturing processes.
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