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3D-Electrochemistry Designing and resolving the
microstructure of an electrode Gaëtan Damblanc
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Addition of a 3D electrochemistry model
Applies to idealised or ‘real’ geometries
Battery Cell Battery modules/packs In Situ
Increasing Length Scale
Extension to the Development
MicroStructure
Modeling
Battery Simulation Module
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Motivations
What is a Li-ion Battery?
Designing the microstructure: The 3D approach
STAR-CCM+ Li-ion Battery Cell Model
Example of a 3D Numerical model
Future developments and conclusion
Agenda
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Cost reduction for the design
– Minimise the number of tests and experiments
– Identify at the early stages of the development potential problems
Process and research speed up
– Parameterisation and optimisation methods
Improvement of the understanding of the phenomena taking place
– Performance
– Ionic and electronic transport
– Ageing
– Short circuit
Motivations
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What is a Lithium -ion Battery?
𝑳𝒊𝑪𝒐𝑶𝟐 𝑳𝒊𝟏−𝒙 𝑪𝒐𝑶𝟐 + 𝒙𝑳𝒊+ + 𝒙𝒆−
𝒙𝑳𝒊+ + 𝒙𝒆− + 𝟔𝐂 𝑳𝒊𝒙𝑪𝟔
Positive and negative half reactions for a LiCoO2 cathode:
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The electrodes are made of porous active materials placed in a liquid
non-aqueous electrolyte
What is inside a Li-ion Electrode?
+ -
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Geometrically resolved electrode – Interfacial surface area
– Volume/porosity fraction
– Tortuosity
Local interactions – Voltage distribution
– Li-ion pathways
– Li-ion concentration
– Short circuiting
– Thermodynamic effects
– SEI growth
Material interactions – Active material
– Liquid or Solid Electrolyte
– Binders, conductive additives
Contraction and Expansion – During the intercalation process
Advantages of the 3D approach
SEI
Tortuosity
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The Li-ion Battery Cell model
Section across a typical Li-ion cell
Solid particles structure is to be resolved or
represented by a simpler, regular structure (e.g.
cylinders, grid etc.)
Geometry-resolved model
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We suggest to discretize separately solid and fluid regions within an electrode, with the desired level of complexity Within solid structure, one can account for active and passive materials (e.g. active material and conducting aid).
Chemical reactions take place at the solid-electrolyte interphase (SEI): standard form of equations can be used, with special conditions at interfaces.
The model of a Li-ion battery requires solution of the following equations: – Salt concentration in electrolyte;
– Concentration of Li in solid part of electrodes;
– Potential in solid;
– Potential in electrolyte;
– Thermal energy.
Flow will initially be neglected, but may be included in the future (as well as volume change during charging and discharging to account for expansion/contraction).
Crucial: conditions at the interfaces...
The Li-ion Battery Cell model
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Lithium (Li+) is transported into and out of particles by diffusion:
We assume here a binary electrolyte and express the
conservation equation for Li-salt in the liquid phase as follows
The potential in the solid phase, Φ1, is computed from the
following equation:
The potential in liquid, Φ2, can be computed from the following equation:
The main equations
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Interface Conditions
Local current density at solid active surface is modelled as (Buttler-Volmer relation):
cs – Site concentration of solid phase
(maximum possible value of c1).
k – Rate constant
Rsei
– Solid-electrolyte-interface resistance
Ueq
– Equilibrium potential of the active material
with
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The 3D Numerical Model Definition
• Cathode Collector – Aluminum foil 10µm thick (δkp),
• Cathode active material – LiMn2O4 80µm thick (δp), particle diameter 10µm, target porosity 40%
• Separator - 10µm thick (δs), porosity 40%, MacMullin number 5
• Anode active material – Graphite 96µm thick (δn), particle diameter 20µm, target porosity 40%
• Anode Collector – Copper foil 10µm thick (δkn)
• Electrolyte – ethylene carbonate/ethyl methyl carbonate 50:50 mix, salt - LiFP6
• Overall unit cell dimensions - 25 µm by 25 µm by 206 µm
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Model Definition - Cathode
Polyhedral mesh
2.8 million cells
Solid & electrolyte resolved
STAR-CCM+ CAD tool
40% Porosity
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Model Definition - Anode
Polyhedral mesh
1.2 million cells
Solid & electrolyte resolved
STAR-CCM+ CAD tool
40% Porosity
Anode Active Material - Graphite
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Mesh Details
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Conformal Mesh
Prism Layer at the SEI
Active Material
Electrolyte
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The full electrode to be resolved – Mesh view
Symmetrical boundaries on all external walls
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The properties of the Active Material are
defined in the physics continua
– Electrical conductivity
– Diffusion coefficient
– The parameter can also account for
a dependence on the Temperature and
Li-Ion Concentration
The Butler-Volmer relations parameters
are defined in a panel under the relevant
Liquid/Solid phases interfaces
The Physics set-up and Butler-Volmer relation
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Open questions:
• After setup: cell capacity (Ah / coulomb)
• During simulation: State Of Charge (SOC) and
corresponding Open Circuit Voltage (OCV)
• Additional: average electrode concentrations to reinitialize at different SOC / OCV
Provide only:
• Initial Setup (electrode regions & initial conditions)
• (Open Circuit) Voltages where cell is defined as fully
charged / discharged
Report computes amount of Li+ which can be shuttled
between electrodes until upper / lower voltage attained
Report for Battery Cell state
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Report for Battery Cell state
Results:
C/10 full discharge rest
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Results during Charge
Lithium salt concentration at 3 transient points through a charge
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3D model
1D model
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Solid Phase Concentration – Liquid Phase Electric
Potential
1 min 2 min 3 min
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Li-ion Concentration diffusion in solid phase
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Electric Potential in Electrolyte
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Next Steps and Conclusion
Initial results presented
Publication of validation paper
Begin working with external users
Available in STAR-CCM+ 7.06
Future developments
• Improve the model build process
• Extend work to “real” geometries
• Model half-electrode to focus on the Cathode or Anode design
• Measure SEI overpotential
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Many thanks to my colleagues who actively work on this innovative topic
– Dr Robert Spotnitz from Battery Design LLC
– Milovan Peric
– Steve Hartridge
– Boris Kaludercic
– Christian Walchshofer
THANK YOU!
Aknowledgement