Rechargeable lithium-ion battery packs are standard, powering everything from smartphones to tablets and laptops to electric cars. This is why it is a challenging design problem to make these batteries lighter, more compact, cheaper, and faster charging without compromising performance. Engineers and scientists are working together to develop new electrode materials that store more lithium in the same space.
Using an anode, or the negative electrode in a battery, made from alloy-type materials is a promising option. One pound of Silicon, which makes an “alloy type” anode, can store the same amount as ten pounds of graphite. This is the case with the “intercalation style” anodes currently used in commercial lithium-ion batteries. The result is that the anode could be 10x lighter and much smaller if replaced with the former.
However, anodes made from alloy-type Silicon have yet to be widely adopted despite their promise. The reason is that lithium ions can be inserted into the anode’s alloy-type silicon particles. This causes the battery to fail after only a few charging cycles. This kind of degradation can be mitigated by reducing the size of these particles to make their features at the nanoscale, such as in nanoporous Silicon. However, the exact mechanisms behind it still need to be fully understood.
In a new study, Penn Engineering researchers reveal the complex electrochemical process at the nanoscale that happens when anodes of the alloy type charge and discharge. This promising class of energy storage materials can become more efficient by understanding the degradation behavior.
Eric Detsi (Stephenson Term Assistant Professor in Materials Science and Engineering) and graduate research assistants John Corsi and Samuel Welborn conducted the study. Eric Stach, professor of MSE and director of the Laboratory for Research on the Structure of Matter, collaborated with them.
Lithium-ion batteries, which are also called anodes, store energy by an electrochemical reaction between the lithium from the positive electrode (also known as the anode) and the material in the anode. Lithium ions can physically enter and bond with the material in the anode’s lattice. When the battery is discharged, the lithium is removed to repeat the process. However, in the case of alloy-type anodes, the anode material will grow and eventually fall apart.
These processes involve multiple intermediate steps. Understanding the differences between dense and nanoporous Silicon may help us better understand why they resist degradation. Unfortunately, imaging of the appropriate silicon structures at small scales has made it difficult to examine these processes in action closely.
Detsi states, “to address this challenge, we used a unique combination transmission electron microscopy as well as X-ray scattering to study the degradation in lithium-ion anodes during charging or discharging.”
We chose to use gold over Silicon as it contrasts better in electron microscopy imaging than silica,” Welborn says. “This allows for clear detection and identification of the solid-electrolyte surface coating known as SEI that forms on the electrode during charging or discharging.” Also, gold scatters more Xrays than Silicon, making it easier for researchers to examine changes in the anode structure during these processes.
The electron microscopy equipment at the Singh Center of Nanotechnology was used in this study. Also, the Penn Dual Source and Environmental X-ray Scattering facility (DEXS) was used by the LRSM. These two techniques provided rich data that enabled researchers to revise the model of how this process happens.
The team identified the critical step in discharge using these instruments: forming a thick SEI layer over the porous gold surface.
Corsi explains that lithium stored in gold causes the volume of metallic gold ligaments within the nanoporous structure to expand rapidly and eventually break. These fragmented ligaments become trapped in the SEI layer. The process reverses, and the ligaments contract when lithium is removed. This volume change causes the SEI layer that contains trapped material to crackle and separate from the rest.
A new SEI layer forms on the surface of the battery as it is recharged again. This collects more fragmented parts of the electrode. Over repeated charging cycles, this damage can cause large electrode fragments to split off, leading to rapid battery failure.
Researchers believe the findings from nanoporous metal have broad implications for other promising alloy-type anode materials like Silicon and tin. Researchers will be able to develop long-lasting, high-energy-density batteries by understanding the mechanisms of how these anodes break down over time.
