Note

This notebook is already available in your BattMo installation. In Matlab, run

open tutorial_3_modify_structural_parameters_live

Tutorial 3 - Modify Structural Parameters

Introduction

In this tutorial, we will use a P2D model to simulate the effects of changing structural parameters (like electrode thickness or porosity) on cell discharge. After completing this tutorial, you should have a working knowledge of:

• Basics of how structural properties are defined in BattMo

• How changing the thickness of one electrode can affect the overall capacity of the cell

• How to setup and execute a parameter sweep

We’ll use the same model from Tutorial 1.

jsonstruct = parseBattmoJson('Examples/jsondatafiles/sample_input.json');

Explore the Structural Parameters

Structural parameters are defined in the JSON parameter file and parsed into the MATLAB structure. Once the JSON parameter file has been read into MATLAB as a jsonstruct, its properties can be modified programmatically.

Let’s begin by exploring the parameters of the negative electrode coating with the following command:

disp(jsonstruct.NegativeElectrode.Coating)

               thickness: 6.4000e-05
                       N: 10
        effectiveDensity: 1900
    bruggemanCoefficient: 1.5000
          ActiveMaterial: [1x1 struct]
                  Binder: [1x1 struct]
      ConductingAdditive: [1x1 struct]

Here, we can see that the structural and material properties of the coating are defined. Let’s try increasing the thickness of the negative electrode from 64 µm to 72 µm.

jsonstruct.NegativeElectrode.Coating.thickness = 72*micro;

Now we can run the simulation and plot the discharge curve against the discharged capacity of the cell.

% instantiate an empty figure
figure()
output = runBatteryJson(jsonstruct);

retrieve the states from the simulation result

states = output.states;

% extract the time, voltage, and current quantities
time = cellfun(@(state) state.time, states);
voltage = cellfun(@(state) state.('Control').E, states);
current = cellfun(@(state) state.('Control').I, states);

% calculate the capacity
capacity = time .* current;

% plot the discharge curve in the figure
plot((capacity/(hour*milli)), voltage, '-', 'linewidth', 3)
xlabel('Capacity  /  mA \cdot h')
ylabel('Voltage  /  V')
legend('t_{NE} = 72 µm')

Setup and Run a Parameter Sweep

Now we can setup another parameter sweep to explore the effects of reducing the thickness of the negative electrode coating. Let’s try simuating the cell performance with coating thickness values of 16 µm, 32 µm, and 64 µm. As in the previous tutorial, we will first create a vector contianing the desired thickness values. Then we will use a for-loop to iterate through the thickness values, modify the value in the jsonstruct, and run the simulation. We will then plot the results together for comparison.

% create a vector of diffent thickness values
thickness = [16, 32, 64].*micro;

% instantiate and empty cell array to store the outputs of the simulations
output = cell(size(thickness));

% instantiate an empty figure
figure()

% use a for-loop to iterate through the vector of c-rates
for i = 1 : numel(thickness)
    % modify the value for the c-rate in the control definition and update
    % the total duration of the simulation accordingly
    jsonstruct.NegativeElectrode.Coating.thickness = thickness(i);

    % run the simulation and store the results in the output cell array
    output{i} = runBatteryJson(jsonstruct);

    % retrieve the states from the simulation result
    states = output{i}.states;

    % extract the time and voltage quantities
    time = cellfun(@(state) state.time, states);
    voltage = cellfun(@(state) state.('Control').E, states);
    current = cellfun(@(state) state.('Control').I, states);

    % calculate the capacity
    capacity = time .* current;

    % plot the discharge curve in the figure
    plot((capacity/(hour*milli)), voltage, '-', 'linewidth', 3)
    hold on
end

hold off
xlabel('Capacity  /  mA \cdot h')
ylabel('Voltage  /  V')
legend('t_{NE} = 16 µm', 't_{NE} = 32 µm', 't_{NE} = 64 µm')

The results of this parameter sweep show that reducing the thickness of the negative electrode to 32 µm and 16 µm reduces its capacity such that it becomes the limiting factor in the overall capacity of the cell. Note that in real Li-ion batteries, the capacity of the negative electrode is usually oversized with respect to the capacity of the positive electrode to avoid dangerous lithium plating.

Summary

In this tutorial, we explored how to modify structural parameters in BattMo. We first learned that structural properties of the electrodes are defined in the structure (e.g. NegativeElectrode.Coating). From there, it is possible to change individual parameter values and simulate the effects on the cell discharge. We then reviewed how to setup a parameter sweep and explored how changing the thickness of the negative electrode coating affects the overall capacity of the cell.