Transcript Objectives

Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Gang Ning, Bala S. Haran, B. N. Popov
Department of Chemical Engineering
University of South Carolina
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Objectives
To determine the capacity fade of Li-ion cells cycled under
different discharge rates
To break down total capacity fade of Li-ion cells into
separate parts
To analyze the mechanism of the capacity fade
To provide experimental data for the capacity fade model
under high discharge rate
Department of Chemical Engineering
University of South Carolina
2
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Background
Capacity fade is a key factor in determining the life of the battery in a
specific application.
Generally there are two ways to analyze this phenomenon:
calendar/shelf life study ( under no applied current)
cycling study (under a specific charge&discharge protocol)
Many papers regarding charge protocols and the capacity fade can be
found in current literature. Performance of Li-ion cells cycled at higher
discharge rate is scarcely reported.
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Capacity fade as a function of cycle No.
16.9%
1C Discharge Rate
0.18
CC+CV charge: (1.0A+4.2
2C Discharge Rate
0.16
V+50 mV)
3C Discharge Rate
13.2%
Capacity Fade Percentage
0.14
Discharge Rates: 1C, 2C,
0.12
9.5%
3C
0.10
0.08
Frequency: once/50 cycles
0.06
0.04
Capacity Measurement
0.02
Rate: 0.7 A
0.00
0
30
60
90
120
150
Cycle No.
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180
210
240
270
300
Temperature: 25 0C
4
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Discharge Profile of fresh Li-ion cell and cells cycled after 300 times
4.1
3.9
Voltage (V)
3.7
Initial Discharge
2C Discharge
3.5
1C Discharge
3.3
3.1
3C Discharge
2.9
2.7
2.5
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0.1
0.3
0.5
0.7
0.9
1.1
1.3
5
Discharge Capacity (Ah)
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Rate capability study
Cells were fully charged
1.4
with CC-CV protocol and
1.3
Discharge Capacity (Ah)
1.2
discharged subsequently
1.1
1.0
with C/10, C/4, C/2, 1C, 2C
0.9
Battery_Fresh
0.8
and 3C rates
Battery_1C
Battery_2C
0.7
Battery_3C
0.6
0.5
0.00
0.42
0.84
Department of Chemical Engineering
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1.26
1.68
2.10
2.52
Discharge Current (A)
2.94
3.36
3.78
4.20
6
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
DC resistance Rdc as a function of depth of discharge (DOD)
Internal DC resistance of
3C 300 Cycles
330
the whole-cell was
320
DC resistance (m )
310
determined by
2C 30 Cycles
300
intermittently interrupting
290
the discharge current in the
280
process of discharge
1C 300 Cycles
270
Rdc = (Discharge Voltage –
260
Open Circuit Voltage (0.1
250
Initially
second after the pulse rest))/
240
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Depth of Discharge (DOD)
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0.8
0.9
1.0
Discharge Current (1A)
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Impedance Spectra of fresh cell and cells cycled up to 300 cycles
0.08
1C Discharge
1C Discharge
0.07
2C Discharge
0.025
2C Discharge
0.06
SOC: 100%
Fresh
0.030
SOC: 0%
Fresh
3C Discharge
3C Discharge
( 
Im
0.04
Z
Z
Im
( )
0.020
0.05
0.015
0.03
0.010
0.02
(a)
(b)
0.005
0.01
0.00
0.24
0.26
0.28
0.30
0.32
0.34
Z Re (  )
(a) 0% SOC
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0.36
0.38
0.000
0.25
0.26
0.27
0.28
0.29
0.30
0.31
Z Re (  
(b) 100% SOC
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Half Cell Study (T-cells)
2.2
5.0
2.0E-004
2.0E-004
Current
Current
2.0
Voltage
Voltage
4.3
1.8
1.6
1.0E-004
1.0E-004
0.8
0.6
3.0
0.0E+000
Delithiation
Lithiation
2.3
Delithiation
Lithiation
Voltage (V)
0.0E+000
1.0
Current (A)
Voltage (V)
1.2
-1.0E-004
-1.0E-004
0.4
1.7
0.2
0.0
0
20000
40000
60000
80000
Time (s)
Carbon Half-cell
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100000
120000
-2.0E-004
140000
1.0
0
20000
40000
60000
80000
100000
120000
-2.0E-004
140000
Time (s)
LiCoO2 Half-cell
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Current (A)
3.7
1.4
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Half-cell analysis of capacity fade (in percentage) of negative Carbon
electrode and positive LiCoO2 electrode
The percentage
Capacity Fade
(in percentage)
Fresh
1C 300
Cycles
2C 300
Cycles
3C 300
Cycles
loss of capacity
is calculated
based on the
Carbon
0.00%
2.77%
8.30%
10.59%
capacity of
fresh electrode
LiCoO2
0.00%
3.98%
4.38%
5.18%
material.
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10
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Breakdown of the total capacity fade of the whole lithium-ion battery
Q: total capacity loss of
Cell cycled at 1C
rate
Cell cycled at 2C
rate
Cell cycled at 3C
rate
the whole lithium-ion
cell
Total capacity
fade of Li-ion
Battery
9.5%
Q1
3.5%
13.2%
16.9%
Q1: capacity correction
due to rate capability
2.9%
2.8%
Q2: capacity fade due to
Q2 (Carbon)
Q2 (LiCoO2)
NA
8.4%
10.6%
3.8%
NA
NA
the loss of secondary
material (Carbon or
LiCoO2)
Q3
2.3%
2.0%
3.4%
Q3:capacity fade due to
Q:=Q1 + Q2 +Q3
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the loss of primary
material (Li+)
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Typical Nyquist plots of Carbon half-cell obtained at 25 0C
(a)
1000
900
11000
800
1 mHz
700
1.730 V
10000
Im
()
600
500
Z
9000
400
8000
1.154 V
300
200
Z Im (  )
0.992 V
7000
100
1.730 V
(b)
0.913 V
0
0
125
250
6000
375
500
625
750
875
1000
Z Re (   
5000
4000
3000
1.154 V
2000
(a)
0.992 V
1000
0.913 V
0
0
2000
4000
6000
8000
10000
12000
14000
16000
Z Re (  
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potential ranging from 0.913 to 1.730 V vs. Li+/Li
12
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Typical Nyquist plots of Carbon half-cell obtained at 25 0C
(b)
250
0.773 V
1 mHz
200
0.587 V
Z Im (  )
0.258 V
150
0.406 V
100
0.126 V
50
(c)
0
0
100
200
300
400
500
600
700
Z Re (  )
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potential ranging from 0.126 to 0.773 V vs. Li+/Li
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Equivalent circuit of the EIS spectra
Qf
Relect
Rf
Qct
Rct
Qe
Re
Zw
Cint
Relect: resistance of electrolyte
Re: resistance of bulk material
Rf: resistance of surface film
Zw: Resistance of Warburg
Rct: resistance of charge transfer
Diffusion
Cint:intercalation capacitance
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Q: constant phase elements
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Data Fitting
1500.0
Rf : 6.87 
1312.5
0.0001 Hz
1125.0
Re : 110 
plot by fitting
750.0
Rct :=40.37 
Z
im
( )
937.5
562.5
375.0
Cint := 1.5 F
0.001 Hz
187.5
plot by experiment
0.0
0.0
187.5
375.0
562.5
750.0
Z Re ( )
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937.5
1125.0
1312.5
1500.0
Log(D) := -9.7
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Parameter comparisons
2C
7.0
3C
6.0
100
1C
2C
R e ()
Rf ()
3C
2C
120
2C
5.0
4.0
1C
3.0
3C
140
3C
1C
80
60
1C
2.0
40
1.0
20
0
0.0
10% SOC
10% SOC
20% SOC
State of Charge
20% SOC
State of Charge
Rf
Re
3C
90
80
3C
R ct ()
70
60
Rct
2C
50
40
2C
30
20
1C
1C
10
0
10% SOC
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20% SOC
State of Charge
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
SEM images of the electrode surface
SEM (X1000/30 m)
of Carbon materials
cycled
at
different
discharge rates.
(A) : Carbon cycled
at 1C
A
B
(B) : Carbon cycled at
2C discharge rate
(C)+(D)
cycled
:
Carbon
at
3C
discharge rate
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C
D
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Mechanism of Property Changes
Initial SEI film
Thicker SEI film
Carbon Particles
Binder particles
Current collector
2Li+ + 2e- + 2(CH2O) CO (EC) → CH2 (OCO2Li) CH2OCO2Li ↓+ CH2CH2 ↑
2Li+ + 2e- + (CH2O) CO (EC) → Li2CO3 ↓ + C2H4 ↑
Li+ + e- + CH3OCH2CH3 (DMC) → CH3 OCO2Li ↓ + CH3•
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Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Conclusion
The negative Carbon electrode deteriorates much faster than the positive
LiCoO2 electrode when the Li-ion cell was cycled under higher CC
discharge rate.
Increase of the internal impedance, (predominantly resulting from the
thicker SEI film of carbon) is the primary cause of the capacity fade of the
whole Li-ion battery.
High internal temperature due to high discharge rates probably leads to
the cracks of initial SEI film and more electrolyte will take part in the side
reactions. As a consequence, the products of those side reactions will make
the SEI film become thicker and thicker.
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University of South Carolina
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