Effects of High-Temperature Preconditioning on Li Plating During Low-Temperature Cycling of the COTS LG Chem HG2 Cell

Spacecraft travel through environments that are hostile to almost everything we know. In the deep silence of space, temperatures can plummet far below the freezing point of water, and the batteries that power these missions must survive this cold without failing. For decades, engineers have relied on custom-built batteries designed specifically for these extreme conditions. However, a more practical and cost-effective solution has emerged: using the same high-quality lithium-ion batteries found in consumer electronics, known as commercial off-the-shelf cells. These batteries are mass-produced, highly consistent, and offer impressive energy density. Yet, they come with a catch. They are designed to be charged in mild temperatures, and when forced to operate in the deep cold of space, a dangerous chemical process can occur inside them. When a lithium-ion battery is charged in the cold, lithium ions can get stuck on the surface of the negative electrode instead of sinking into it. This creates a layer of metallic lithium, a phenomenon called lithium plating. This buildup is not just a sign of wear; it can permanently damage the battery's capacity and, in severe cases, lead to safety hazards like short circuits or fires.

A team of researchers set out to understand how to prevent this dangerous plating in a specific, widely used commercial battery, the LG Chem HG2, when it is subjected to the freezing conditions of space. They focused on a simple question: does how you treat a battery before it gets cold matter? Specifically, they wanted to know if warming the battery up and charging it repeatedly before sending it into the cold would make it more resistant to plating than simply leaving it sitting in the warmth or sending it straight into the cold. To find the answer, they combined real-world experiments with powerful computer simulations that could peer inside the battery's microscopic structure. The results revealed a surprising truth about how the battery's internal state, rather than just its surface condition, dictates its survival in the cold.

The researchers tested three different groups of these commercial batteries to see how they would behave at minus 20 degrees Celsius. The first group underwent a "preconditioning" phase where they were charged and discharged repeatedly for two weeks at a warm 30 degrees Celsius. The second group was simply stored at that same warm temperature for the same amount of time without being charged or discharged. The third group received no special treatment at all; they were taken directly from room temperature and put into the cold for testing. After these initial steps, all the batteries were subjected to the same rigorous cycling test at minus 20 degrees Celsius. The goal was to watch for signs of lithium plating, which often reveals itself through specific changes in the battery's voltage during discharge.

The results were clear and distinct. The batteries that had been stored in the warmth or sent directly into the cold showed strong signs of lithium plating. When these batteries were discharged, their voltage curves displayed specific plateaus, and analysis of their voltage changes indicated that metallic lithium had formed on the anode and was being stripped away. This is a sign of degradation and potential danger. In contrast, the batteries that had been preconditioned by cycling them at the warm temperature showed no such signs. They cycled more efficiently, retained their capacity better, and displayed no evidence of the dangerous lithium buildup. The data suggested that the act of charging and discharging the battery while it was warm fundamentally changed its behavior, making it much more resilient when it eventually faced the cold.

To understand why this happened, the researchers turned to a sophisticated computer model. They built a digital twin of the battery, using detailed 3D images of the electrode's microscopic structure taken with a high-resolution scanner. This allowed them to simulate the movement of ions and electrons inside the battery with incredible precision. The simulation tracked a key factor called overpotential, which is essentially the extra electrical pressure needed to push the chemical reactions forward. If this pressure drops too low, lithium ions will settle on the surface as metal instead of entering the electrode. The simulation confirmed that the preconditioned batteries had a higher resistance to this plating condition compared to the others.

The study then dug deeper to find the root cause of this difference. One might assume that the warm cycling changed the physical coating on the electrode, known as the solid-electrolyte interphase, making it tougher. However, the simulation showed that the thickness and composition of this coating were not the primary drivers of the difference. Instead, the key factor was the battery's state of charge—the amount of energy stored inside it—at the moment it entered the cold. The batteries that were preconditioned by cycling ended up in a different state of charge than the ones that were just stored or left untreated. When the preconditioned batteries were cooled down, they started their cold cycle with a higher level of stored energy. This higher starting point meant that during the first charge in the cold, the lithium ions were able to move more freely and settle correctly inside the electrode, rather than getting stuck on the surface. The batteries that started with a lower state of charge struggled more, leading to the conditions that favored plating.

The researchers also noted that while their computer models could explain why the preconditioned batteries performed better in the first few cycles, they could not fully explain why the other batteries continued to degrade over time. The models showed that the plating tendency should have stabilized, but the experiments showed that the damage actually grew worse with more cycles. This suggests that once the initial plating occurs, it triggers a chain reaction of physical changes—perhaps cracks in the protective coating or other structural shifts—that the current models do not yet capture. This gap between the simulation and the real-world data highlights the complexity of battery chemistry and the need for continued research.

Ultimately, this work provides a practical roadmap for using commercial batteries in space. It demonstrates that a simple, warm-up routine involving repeated charging and discharging can significantly improve a battery's ability to survive in freezing temperatures. By ensuring the battery starts its cold journey in the right state of charge, engineers can avoid the dangerous buildup of metallic lithium. This finding is crucial for future space missions, where the reliability of power systems is a matter of life and death. The study confirms that while these commercial cells were not originally designed for the deep cold, with the right preparation, they can be coaxed into performing safely and effectively in one of the harshest environments imaginable.