Visualization of Charge Distribution in a Lithium Battery Electrode
Jun Liu,
a
Martin Kunz,
b
Kai Chen,
b
Nobumichi Tamura,
b
and Thomas J. Richardson
*a
a
Environmental Energy Technologies Division and
b
Advanced Light Source
Lawrence Berkeley National Laboratory
Berkeley, California 94720, USA
*
To whom correspondence should be addressed. E-mail: tjrichardson@lbl.gov. Tel.: +1-510-
486-8619. Fax: +1-510-486-8609.
ABSTRACT:
We describe a method for direct determination and visualization of the distribution of charge
in a composite electrode. Using synchrotron x-ray microdiffraction, state-of-charge profiles in-
plane and normal to the current collector were measured. In electrodes charged at high rate, the
signatures of non-uniform current distribution were evident. The portion of a prismatic cell
electrode closest to the current collector tab had the highest state of charge, due to electronic
resistance in the composite electrode and supporting foil. In a coin cell electrode, the active
material at the electrode surface was more fully charged than that close to the current collector
because the limiting factor in this case is ion conduction in the electrolyte contained within the
porous electrode.
TOC Graphic:
Keywords: Energy storage, batteries, charge distribution, polarization, electrochemical
characterization, microdiffraction.
Lithium ion batteries have made a great impact on consumer electronics and are about to
revolutionize the transportation area.
1
Although they were introduced almost two decades ago,
there are still some “black boxes”, such as charge distribution within battery electrodes. Non-
uniform charge distribution within battery electrodes may impact performance in a variety of
ways, including reduced energy and power, underutilization of capacity, localized heat
generation, and overcharge or overdischarge. While several current distribution models have
been developed,
2-10
there has been no simple way to obtain experimental data on composite
electrodes such as those used in lithium-ion batteries. Dynamic potentiometric methods such as
those employed by Díéz-Pérez et al.
11
are not readily applicable to the thin composite electrodes
of lithium-ion cells. Sever Skapin et al.
12
measured local conductivities in composite electrodes
using microcontact impedance spectroscopy, but the method does not address the current
distribution in the electrode as a whole. Kim et al.
9,10
measured in-plane thermal profiles of thin
Li(NiCoMn)O
2
batteries, which reflect the current distribution during discharge. When an active
material undergoes charge and discharge via a two-phase mechanism, as is the case with
LiFePO
4
, although there is minor relaxation of concentration gradients in the electrolyte upon
interrupting the charge or discharge current, redistribution of charge within the active material is
negligible. Here we present a novel experimental method for direct determination and
visualization of the charge distribution in a composite electrode using synchrotron x-ray
microdiffraction.
13
LiFePO
4
was chosen as active material for this study, because (i) it is a safe
and promising cathode material for plug-in hybrid electric vehicle application;
14
(ii) its two-
phase charge-discharge reaction produces a charge distribution that is “frozen” when the current
is interrupted.
15
Since non-uniform charge distribution may develop either in the cross section or
in the plane of the electrode depending on cell configuration and current density, two LiFePO
4
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 × 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 LiFePO
4
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 (µXRD) 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
-1
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 FePO
4
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 g
-1
(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
becomes sufficiently polarized that charging is impeded, and only the portions near the separator
can participate fully in the charging process.
