(457c) Design and Characterization of a Neutron-Friendly Lithium-Ion Battery Coin Cell for Extreme Fast-Charging

Lithium-ion batteries (LIBs) have profoundly advanced the development of electric vehicles (EV). However, one of the remaining bottlenecks in the widespread deployment of EVs is the long charging time typically required for commercial LIBs. There is a global push towards extreme fast charging (XFC) of EV batteries to reduce their charging times to 10-15 minutes.¹However, XFC causes severe degradation in the electrochemical performance of LIBs, which is mainly attributed to irreversible Li plating on the surface of the graphite anode rather than intercalation of Li ions inside the graphite as the battery is fast-charged.^2-3 Irreversible Li plating during XFC has been characterized using a variety of techniques such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray micro-computed tomography (CT) amongst others.⁴ While valuable, these characterization techniques have limitations in terms of the sample preparation involved, spatial and temporal resolution required, and/or the necessary imaging contrast to distinguish between materials of similar atomic number (Z). In addition, understanding the origin and spatial heterogeneity of Li plating across three dimensions (3D) on porous graphite anodes as a function of time necessitates a non-destructive characterization platform to differentiate between graphite and Li.

Neutron-based imaging provides the sensitivity to differentiate graphite from Li due to the large difference in their total neutron (absorption, coherent, and incoherent scattering) cross-sections (Li:C ~ 73:11 barns)⁵ that are challenging to detect using X-ray imaging alone. However, high energy X-rays provide the benefit of penetrating the metallic battery components. e.g., Cu, and Al current collectors. Therefore, to test the feasibility of simultaneous neutron and X-ray tomography (NeXT), we performed proof-of-concept multi-modal imaging experiments at the BT-2 imaging beamline at National Institute of Standards and Technology Center for Neutron Research.^6-7 Our spatial resolution for both X-rays and neutrons was ~15-20 μm. For ex-situ Li plating detection, we imaged 1) pristine/uncycled and 2) a cycled graphite anode. For cycled graphite anode, we disassembled the battery pouch cells in their fully discharged condition and harvested the anode after 450 cycles of 9C charging and C/2 discharging. Thus, our cycled anodes contained regions of high and low Li plating. In addition, to determine NeXT feasibility for in-situ imaging, we characterized a fully assembled, uncycled single-layer battery pouch cell.

For data analysis, we used 2D bivariate histogram phase segmentation to differentiate different battery materials such as Cu, graphite and Li, Al, etc.⁸ However, too low of an X-ray energy (40keV) combined with the flat sample geometry of the graphite strips made it challenging to remove beam hardening artifacts from copper and, therefore, to segment lithium from graphite. For the fully assembled, uncycled pouch cell, we gently rolled the pouch into a ring to fit it into the field-of-view for the highest resolution settings of the system. However, the high neutron attenuation by the hydrogen in EC: EMC (3:7) electrolyte and hydrogenous polymeric materials in the Celgard separator made it particularly difficult to segment graphite anode from the separator and positively identify interfaces.

In this presentation, we will outline the design and characterization of a neutron-friendly LIB coin cell for extreme fast-charging. Our design is based on a standard 2032 format coin cell with custom modifications to address the dual criteria of electrochemistry and neutron imaging, i.e., good neutron transmission and relative contrast from a functional LIB at 6-C charging rate. We will present three advantages of coin cell geometry: 1) coin cells provide more uniform attenuation throughout the entire tomography scan thus reducing artifacts; 2) multiple coin cells can be stacked on top of each other for high throughput battery imaging experiments; 3) they reduce the amount of expensive deuterated materials needed while developing the optimum cell configurations. In addition, we will address the need of a deuterated EC: DMC electrolyte in a fluorinated separator for the neutron-friendly battery design (Figure 1). Since the total neutron cross-section of deuterium and fluorine are on the orders of magnitude lower than that of hydrogen, we anticipate that deuterated electrolyte and a fluorinated separator will help on several fronts. First, these modifications will increase neutron transmission through the battery, thus improving signal-to-noise ratio and reducing acquisition time especially at low neutron flux facilities. Secondly, they will result in a cleaner electrolyte-separator-anode interface for the detection of plated lithium because of absence of hydrogen which is a major source of incoherent scattering in the neutron histogram. Finally, we will show how these modifications affect the electrochemical performance of our LIB coin cells to determine the onset of Li plating at 6-C charging rate. i-e., capacity fade vs. number of cycles.

References:

1. Liu, Y., Zhu, Y., & Cui, Y. (2019). Challenges and opportunities towards fast-charging battery materials. Nature Energy, 1.
2. Tomaszewska, A., Chu, Z., Feng, X., O'Kane, S., Liu, X., Chen, J., ... & Li, Y. (2019). Lithium-ion battery fast charging: A review. eTransportation, 1, 100011.
3. Liu, Q., Du, C., Shen, B., Zuo, P., Cheng, X., Ma, Y., ... & Gao, Y. (2016). Understanding undesirable anode lithium plating issues in lithium-ion batteries. RSC advances, 6(91), 88683-88700.
4. Paul, P. P., McShane, E. J., Colclasure, A. M., Balsara, N., Brown, D. E., Cao, C., ... & Nelson Weker, J. (2021). A Review of Existing and Emerging Methods for Lithium Detection and Characterization in Li‐Ion and Li‐Metal Batteries. Advanced Energy Materials, 2100372.
5. https://www.nist.gov/ncnr/neutron-scattering-lengths-list .
6. LaManna, Jacob M., et al. "Neutron and X-ray Tomography (NeXT) system for simultaneous, dual modality tomography." Review of Scientific Instruments11 (2017): 113702.
7. Yusuf, Maha, et al. "Lithium Plating Detection in Extremely Fast-Charged Lithium-Ion Batteries Using Simultaneous Neutron and X-Ray Imaging." ECS Meeting Abstracts. No. 3. IOP Publishing, 2020.
8. LaManna, J. M., et al. "NIST NeXT: a system for truly simultaneous neutron and x-ray tomography." Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XXII. Vol. 11494. International Society for Optics and Photonics, 2020.