Visualization of Charge Distribution in a Lithium Battery Electrode Page: 4 of 13
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composite electrodes in different cell configurations were studied. One was a circular electrode
(diameter is 13 mm) in a Swagelok-type cell (equivalent to a coin cell), in which the charge
distribution is non-uniform in cross section at high current density. The other was a larger
rectangular electrode (40 mm x 45 mm) with a tab centered at one end in a "pouch cell"
configuration, in which the charge distribution varies in-plane.
The cross section of the circular LiFePO4 electrode is shown in the scanning electron
microscope (SEM) image in Figure 1 a. Due to its small size, an Fe x-ray fluorescence image
was used to locate the area of interest, and the approximate locations of vertical and horizontal x-
ray microdiffraction (pXRD) scans are shown in Figure 1 b. In an electrode charged to 50 %
overall SOC (state of charge) at a current density of 20 mA g-i of active material (a rate of 0.11C,
where C is the rate at which the full charge capacity is delivered in one hour), the FePO4 phase
concentration, which reflects the local SOC, was nearly constant in both the vertical
(perpendicular to the current collector, Figure 1 c) and horizontal (in-plane, Figure 1 d)
directions. When the charging rate was increased to 3 A g1 (18 C), the SOC was high at the top
surface of the electrode and decreased steadily as it neared the current collector (Figure 1 e). The
in-plane distribution (Figure 1 f) remained constant. During charging, lithium ions are extracted
from the cathode and diffuse toward the anode, producing a concentration gradient within the
electrolyte and drawing anions toward the cathode. The reverse is true on discharging. Non-
aqueous lithium electrolytes have limited ion conductivities and the ion diffusion paths in the
porous electrode are narrow and tortuous. At low charge/discharge rates, diffusion in the
electrolyte phase is sufficient to maintain uniform charging at all depths in the porous electrode.
If the charging rate is high, however, the electrolyte at the greatest distance from the anode may
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Liu, Jun; Kunz, Martin; Chen, Kai; Tamura, Nobumichi & Richardson, Thomas J. Visualization of Charge Distribution in a Lithium Battery Electrode, article, July 2, 2010; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1014157/m1/4/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.