High Spatial Resolution Neutron Imaging of Lithium-Ion Batteries: Correlating Microstructure and Lithium Transport

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With the increasing desire for fast charging capabilities in lithium-ion batteries due to the popularity of electric vehicles, research has recently been focused on increasing energy and power density while simultaneously revealing the need for improved safety mechanisms and decreased overall cost. In response to this need, interest in thick battery electrodes has increased. By increasing the active material per unit area within the electrode, the energy density is increased due to minimization of current collectors, separators, and other packaging components. However, increasing the thickness of electrodes introduces new challenges, including charge-transport limitations.To successfully address the desired increase of energy density and capability of fast charging, high spatial resolution in operando neutron radiography (nR) was utilized to better understand the correlation between microstructure and lithium transport of ultra-thick electrodes. This allows for a comparison between the electrochemical processes occurring during cycling and what is seen in neutron imaging, bridging the gap between microstructural observation and electrochemical testing. Two differing fabrication methods were used to prepare the cathodes and anodes: a solvent free method using a hydraulic press and a wet cast method using N-methyl-pyrrolidone solvent (NMP). The fabrication methods were purposefully chosen to yield differing electrode microstructures. The lithium-ion batteries used in this work were fabricated using graphite (graphite (90 wt%): carbon black (5 wt%): polyvinylidene fluoride (5 wt%)) anodes and NMC (Li(Ni0.8Mn0.1Co0.1)O2) (80 wt%): carbon black (10 wt%): polyvinylidene fluoride (10 wt%)) cathodes with a thickness of ~2.5mm and diameter of 10mm. A deuterated electrolyte (1M LiPF6 EC/DMC=30/70 (v/v)) was used in order to improve the attenuation of lithium within the electrodes. Custom cells were fabricated using PFA Swagelok components to avoid any unnecessary neutron attenuation during neutron radiography. High-resolution (7.5 µm pixel size) in operando nR was performed on these cells using the Multimodal Advanced Radiography Station (MARS) beamline at the Oak National Laboratory High Flux Isotope Reactor. High spatial resolution neutron tomography (nCT) was performed on a cycled battery following in operando nR.Comparing changes in the attenuation coefficient for both a solvent free anode and wet cast anode reveals distinct lithiation patterns in each electrode. In the solvent free sample, there is a dense, but thin, lithium rich region near the separator. In the wet cast sample, the lithium distribution is more evenly dispersed through the thickness of the electrode. The difference in distribution is likely due to the different average pore size of the samples, as represented by the size distribution of macroscopic porosity (pore radius > 20 µm) observed from high resolution nCT. From a comparison of the macroscopic pore radius for each fabrication method it is clear that the solvent free method yields smaller pores and an increased number of pores, whereas the wet cast method yields larger pores and a decreased number of pores. Results from in operando nR during electrochemical cycling and ex situ high spatial resolution nCT on cycled electrodes show that lithium distributions and plating risks are shown to be controlled by both the macroscopic structure of the electrodes and the network of pores and microscale active areas that support electrochemical reactions. This research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.