Geometry and Size Dependence of Localized Plating in Lithium-Ion Batteries
Authors: Liu Xinyi M , Fang Alta , Haataja Mikko , Arnold Craig B. 
Published: February 2020

Lithium plating is an important degradation mechanism in Li-ion batteries that not only reduces battery capacity, but also can lead to catastrophic failures. Among all the side reactions, lithium plating, which is the deposition of metallic lithium on the electrode surface, is one of the most dangerous. The plated lithium can turn into dendrites and gradually grow through the separator, leading to a short circuit. Preventing metallic lithium from forming is challenging, as the heterogeneity of materials typically used in batteries can create transport non-uniformities, which can lead to unanticipated local plating. Therefore, being able to predict the occurrence of plating given a defect with a certain pattern becomes important. In previous work, we showed that non-uniform ionic transport inside a battery can lead to places with locally high current densities and consequently induce plating [1]. In this study, we probe into the importance of the size scale and geometry on localized plating using both numerical simulations and experiments.
We create transport non-uniformities through modifying the porosity of separators under mechanical compression [2]. Separators were mechanically compressed to close all the pores and then cut into specific shapes and sizes using a laser. The compressed separator was then placed along with a pristine separator inside a coin cell. After a number of cycles, the cell was disassembled to observe any localized plating on the electrode.
In this study, we show that certain geometric features lead to more vulnerability to plating, and localization strongly depends on size [3]. Different geometries create concentrated electrochemical activities at different regions and those regions correspond to places where metallic lithium is most often observed. Figure 1 shows statistically averaged images of plating due to two different geometries. Plating is more likely to occur at corners than tips. We correlate the propensity of plating with a simple ion to exit ratio (IE ratio), which is a characteristic of the defect geometry and size. The higher the ratio is, the higher probability of observed plating. We also experimentally demonstrate that there exists a critical size below which plating is unlikely to occur, as shown in Figure 2. A single large feature in a separator induces more plating than a collection of smaller features with same total area. Finally, we look into the interactions between multiple features that are spaced at various distances. Our findings help elucidate the fundamentals behind heterogeneous plating, which can provide practical insights into battery safety and product control.

References:

1. Cannarella, John, and Craig B. Arnold. "The effects of defects on localized plating in lithium-ion batteries." Journal of The Electrochemical Society 162.7 (2015): A1365-A1373.
2. Cannarella, John, and Craig B. Arnold. "Ion transport restriction in mechanically strained separator membranes." Journal of Power Sources 226 (2013): 149-155.
3. Liu, Xinyi M., Alta Fang, Mikko P. Haataja, and Craig B. Arnold. "Geometry and size dependence of localized plating in lithium-ion batteries. ’’ Journal of The Electrochemical Society. Submitted

Figure Captions:

Figure 1. Average of electrodes from 30 different cells showing localized plating due to a triangular-shaped and M-shaped defect.
Figure 2. Probability of localized plating as a function of defect size after 10, 20, and 50 cycles.

Figure 1