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On the reversible homogenization effect in high energy lithium-ion batteries caused by the graphite anode

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Abstract and Figures

The importance of the graphite anode in lithium-ion batteries is recently recognized. It is used as an indicator of the state of health for the full cell [1,2]. The behavior of the anode can easily be observed because of its characteristic potential curve against lithium. This is due to the large plateau on the left hand and right hand side of graphite’s stage 2. This stage 2 solid regime causes a voltage change of around 40 mV and thus is easily recognized, see figure below. In this work the transition to and from graphite’s stage 2 during lithiation and delithiation is analyzed. It was found out that the relaxation that occurs after applying a relative strong pulse was intensely influenced by the stage. To analyze this behavior a voltage transform is introduced to make the relaxation curves at different working points (SOC, lithium concentrations) comparable. Also a relaxation time constant is introduced. This relaxation time constant is investigated for the graphite half-cell against metallic lithium and the results are compared with relaxation behavior of the full cell. It was observed, that the relaxation behavior of the full cell is nearly solely dependent on the anode and the different stages of the graphite. However the relaxation behavior of the cathode did not change significantly. In this work a hypothesis for the behavior of the graphite electrode is introduced and the relaxation voltage transformation is explained. Furthermore the conducted experiments on the full cell and the half cells will be shown and interpreted. This research is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) by resolution of the German Federal Parliament under the project number 0325905.
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Lehrstuhl für Elektrochemische Energiewandlung
und Speichersystemtechnik
On the reversible homogenization effect in high energy
lithium-ion batteries caused by the graphite anode
Friedrich Emanuel Hust, Dirk Uwe Sauer
12.04.2018
1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Motivation
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer2
Understanding what is happening inside a high energy-cell during excitation, relaxation and its
implication on the homogeneity of lithium distribution
shoulder
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Structure of the talk
1. Definition of homogeneity of lithium distribution
2. Experimental results on half-cells
3. Simple mathematical model for a high energy graphite anode
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer3
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
Homogeneity of lithium distribution:
Is a metric for the distribution of lithium on the
electrode
A homogenous distribution is well defined:
Every particle of an electrode has the same
lithium concentration
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer4
Particle Particle
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
Homogeneity of lithium distribution:
Is a metric for the distribution of lithium on the
electrode
A homogenous distribution is well defined:
Every particle of an electrode has the same
lithium concentration
An inhomogeneous distribution is not well defined.
It might be:
Discrete
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer5
Particle Particle
|
Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
Homogeneity of lithium distribution:
Is a metric for the distribution of lithium on the
electrode
A homogenous distribution is well defined:
Every particle of an electrode has the same
lithium concentration
An inhomogeneous distribution is not well defined.
It might be:
Discrete
A distribution
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer6
|
Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
Homogeneity of lithium distribution:
Is a metric for the distribution of lithium on the
electrode
A homogenous distribution is well defined:
Every particle of an electrode has the same
lithium concentration
An inhomogeneous distribution is not well defined.
It might be:
Discrete
A distribution
It is not possible to differentiate the homogeneity of
lithium distribution via OCV-measurement
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer7
|
Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
Homogeneity of lithium distribution:
Is a metric for the distribution of lithium on the
electrode
A homogenous distribution is well defined:
Every particle of an electrode has the same
lithium concentration
An inhomogeneous distribution is not well defined.
It might be:
Discrete
A distribution
It is not possible to differentiate the homogeneity of
lithium distribution via OCV-measurement
However when charging or discharging, the shape
of the distribution will change and this can be
measured via the voltage
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer8
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Experimental
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer9
Specimen Tesla model S cell
3.0 Ah cell
Voltage range 2.5 V to 4.2 V
Half-cells vs lithium were build:
Graphite (anode)
And evaluated by 2.5 % dis-/charging pulses:
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Compensating change in OCV
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer10
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Compensating change in OCV
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer11
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
12
stage
2L - 2
stage 2
stage
2 -1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
13
stage
2L - 2
stage 2
stage
2 -1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
14
stage
2L - 2
stage 2
stage
2 -1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
15
stage
2L - 2
stage 2
stage
2 -1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
16
stage
2L - 2
stage 2
stage
2 -1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
17
stage
2L - 2
stage 2
stage
2 -1
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
18
stage
2L - 2
stage 2
stage
2 -1
stage
2L - 2
stage 2
V 40 mV
stage 2
transition
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
19
stage
2L - 2
stage 2
stage
2 -1
stage
2L - 2
stage 2
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
12.04.2018
Friedrich Emanuel Hust, Dirk Uwe Sauer
20
stage
2L - 2
stage 2
stage
2 -1
stage
2L - 2
stage 2
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Mathematical Model
Goal:
Describe the terminal voltage behavior with potential driven relaxation
Without:
Internal pressure change of the cell
Diffusion
Etc.
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer21
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Hypothesis:
Anode as a cause of homogenization
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer22
Fast particle
Slow particle
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Hypothesis:
Anode as a cause of homogenization
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer23
Fast particle
Slow particle
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Hypothesis:
Anode as a cause of homogenization
In plateau 
  
In solid-solution phase 
  
  := const
  
    
=
>
After transiting into stage 2:
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer24
Fast particle
Slow particle
Slow
Fast
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Hypothesis:
Anode as a cause of homogenization
In plateau 
  
In solid-solution phase 
  
  := const
  
    
=
>
After transiting into stage 2:
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer25
Fast particle
Slow particle
Slow
Fast
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Hypothesis:
Anode as a cause of homogenization
   
  
    
Until
=
a current from 1 to 2 will flow
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer26
Fast particle
Slow particle
Slow
Fast
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Hypothesis:
Anode as a cause of homogenization
   
  
    
Until
=
a current from 1 to 2 will flow
This will lead to three results:
1. The fast reacting particle will rebound and stall before stage 2
2. The slow reacting particle will be lithiated by the fast particle
3. The voltage at terminal will increase by around 40 mV
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer27
Fast particle
Slow particle
Slow
Fast
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
When charging the shape of the distribution will
change
Redistribution of lithium via potential
homogeneity of lithium distribution increases
Lithium distribution will peak before stage 2
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer28
Instable
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
When charging the shape of the distribution will
change
Redistribution of lithium via potential
homogeneity of lithium distribution increases
Lithium distribution will peak before stage 2
Every concurrent pulse will increase
homogeneity
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer29
|
Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
When charging the shape of the distribution will
change
Redistribution of lithium via potential
homogeneity of lithium distribution increases
Lithium distribution will peak before stage 2
Every concurrent pulse will increase
homogeneity
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer30
|
Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Homogeneity of lithium distribution on the anode
When charging the shape of the distribution will
change
Redistribution of lithium via potential
homogeneity of lithium distribution increases
Lithium distribution will peak before stage 2
Every concurrent pulse will increase
homogeneity
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer31
Until after stage 2
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Questions and answers
Does this occur in the full cell?
Yes it does!
Does this occur during charge and discharge?
Yes it does!
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer32
Charge
Discharge
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Questions and answers
Does this occur in the full cell?
Yes it does!
Does this occur during charge and discharge?
Yes it does!
What about the cathode?
Smaller particles
Usually no significant plateaus e.g. NCA
higher homogeneity of lithium distribution
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer33
140 μm x 105 μm
Anode
Cathode
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Summary
Theory for homogeneity of lithium distribution
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer34
Instable
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Summary
Theory for homogeneity of lithium distribution
Experimental results of a high energy anode
were shown
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer35
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Lehrstuhl für Elektrochemische Energiewandlung und Speichersystemtechnik
Summary
Theory for homogeneity of lithium distribution
Experimental results of a high energy anode
were shown
Simple mathematical model for the anode was
introduced to explain
12.04.2018 Friedrich Emanuel Hust, Dirk Uwe Sauer36
Thank you
for your attention
Contact
Chair for Electrochemical Energy Conversion
and Storage Systems
Univ.-Prof. Dr. rer. nat. Dirk Uwe Sauer
RWTH Aachen University
Jaegerstrasse 17/19
52066 Aachen
GERMANY
www.isea.rwth-aachen.de
We thank
37
Friedrich.Hust@isea.rwth-aachen.de
Friedrich Emanuel Hust, Dirk Uwe Sauer
12.04.2018
Please also check the poster P3-75 for life time implications:
Sudden slope increase of capacity and resistance aging
characteristics nearing end of life explained by cover layer
formation
and the talk:
Irreversible calendaric aging and quantification of the reversible
capacity loss caused by anode overhang
for funding the openbat
project:
https://www.openbat.de
Chair for Electrochemical Energy Conversion
and Storage Systems
On the reversible homogenization effect in high energy
lithium-ion batteries caused by the graphite anode
Friedrich Emanuel Hust, Dirk Uwe Sauer
12.04.2018
38
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