Tech Briefs Magazine - February 2022 - BT-6

Battery Modeling
Positive electrode current collector
inlet
Separator
y
X
inlet
Flow path
Negative electrode current collector
Width of anodic
compartment
Flow path
Width of cathodic
compartment
Particles inside the cell/
simulation domain
Particles after
leaving the cell
Particles before
entering the cell
Z
Flow path
y
X
Figure 2. The P3D model of the SSFB.
highest energy density theoretically; however,
there are some drawbacks including
a high cost to make and an increased risk
of toxicity. SSFBs made with a nickelmetal-hydride
(NiMH) material include an
aqueous electrolyte of potassium hydroxide
to sidestep these issues.
No matter the type of SSFB, there is a
major design challenge to address:
Researchers need an electrochemical
model that accurately describes the
kinetic and transport processes occurring
within the design. That is where the
research group - and their novel modeling
approach - comes into play.
Battery Modeling: Now in
Pseudo 3D
The researchers realized that to successfully
model SSFBs, they needed to be
able to correctly account for the interactions
between macroscale and microscale
domains, as well as multiple physical
6
processes, simultaneously. " SSFBs are very
complex systems compared to other batteries.
For instance, you need the right viscosity
for the slurries, " said Dominguez.
" To predict what's happening, you need to
model it. Experiments would take too
much time and be too complex. "
The group found that the COMSOL
Multiphysics® software offers the multiphysics
and multiscale capabilities their
research calls for. In addition, the accurate
and efficient electrochemical modeling
that is possible in the software facilitates
the optimization and scaling up
of NiMH SSFB systems. " Simulations
like this are really only possible with
COMSOL®, " said Mulder.
Aside from the need for both multiphysics
and multiscale modeling, SSFBs
present another unique modeling challenge.
Because of the active particles
involved in the battery, the model needs
to include particle tracing. However, you
cannot couple a hydrodynamics analysis
with a full particle-tracing approach,
because the two studies are not compatible.
The researchers tackled this problem
in a two-step approach. First, they
modeled the electrode movement of a
nonflowing SSFB system in 2D (Figure 2).
The 2D model acted as a first approximation
where they could select optimized
parameters, such as the concentrated and
dilute solution theories for the electrolyte,
material balance in the solid
active particles, current balance, reaction
rate, and model geometry.
Next, the researchers extended the 2D
model into the pseudo 3D (P3D) model
for flowing SSFB systems. " We wanted to
make a nearly particle tracing model that
includes physics discretized in the time
domain; then stop and solve, update
the position of the particles, and start
again to generate proper results, " said
Chayambuka. " We needed a P3D geometry
to model the entire flow of the battery. "
To do so, the team determined all the
dependent variables in separate domains
as well as the associated variables that
needed to be made available in the different
geometries at their corresponding
coordinates. " The extrusion operator feature
in COMSOL Multiphysics® made it
simple to link the 2D and 3D domains, " he
said. The extrusion coupling functionality
also enabled them to map the variables
between separate geometries at every
time step in the simulation.
Using the P3D model, the team was
able to account for hydrodynamic effects
in the SSFB, such as transport in the electrolyte
through the Navier-Stokes equations
for incompressible Newtonian fluid,
as well as the transport in the solid phases,
including the hydrogen intercalation
process modeled by pure diffusion. The
team solved the time-dependent diffusion
equation inside the active particles using
partial differential equations (PDEs).
The researchers also found the
LiveLink™ for MATLAB® interfacing product
particularly helpful. Before introducing
LiveLink™ for MATLAB® into their
modeling workflow, the researchers did
not have an automated P3D process. This
meant that they had to repeatedly run
the simulation, change the particle position,
and then start all over again. This
process was fine when they were initially
Battery & Electrification Technology, February 2022
http://info.hotims.com/82318-801
Tech Briefs Magazine - February 2022

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