modelling of charge/discharge cycle of lithium ion battery system
Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
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1 Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Gang Ning, Bala S. Haran, B. N. Popov
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells. Gang Ning, Bala S. Haran, B. N. Popov. 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 - PowerPoint PPT Presentation
Transcript of Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Effects of Discharge Rates on the
Capacity Fade of Li-ion Cells
Gang Ning, Bala S. Haran, B. N. Popov
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
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
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
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
Department of Chemical Engineering
University of South Carolina
Capacity fade as a function of cycle No.
0 30 60 90 120 150 180 210 240 270 300
Cycle No.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
Cap
acit
y Fa
de P
erce
ntag
e
1C Discharge Rate
2C Discharge Rate
3C Discharge Rate
16.9%
13.2%
9.5%
CC+CV charge: (1.0A+4.2
V+50 mV)
Discharge Rates: 1C, 2C,
3C
Frequency: once/50 cycles
Capacity Measurement
Rate: 0.7 A
Temperature: 25 0C
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Discharge Profile of fresh Li-ion cell and cells cycled after 300 times
0.1 0.3 0.5 0.7 0.9 1.1 1.3
Discharge Capacity (Ah)
2.5
2.7
2.9
3.1
3.3
3.5
3.7
3.9
4.1Vo
ltage
(V)
Initial Discharge
3C Discharge
2C Discharge
1C Discharge
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Rate capability study
0.00 0.42 0.84 1.26 1.68 2.10 2.52 2.94 3.36 3.78 4.20
Discharge Current (A)
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Dis
char
ge C
apac
ity (A
h)
Battery_Fresh
Battery_1C
Battery_2C
Battery_3C
Cells were fully charged with
CC-CV protocol and
discharged subsequently
with C/10, C/4, C/2, 1C, 2C
and 3C rates
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
DC resistance Rdc as a function of depth of discharge (DOD)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Depth of Discharge (DOD)
240
250
260
270
280
290
300
310
320
330
DC re
sist
ance
(m
)
3C 300 Cycles
1C 300 Cycles
2C 30 Cycles
Initially
Internal DC resistance of
the whole-cell was
determined by
intermittently interrupting
the discharge current in the
process of discharge
Rdc = (Discharge Voltage –
Open Circuit Voltage (0.1
second after the pulse rest))/
Discharge Current (1A)
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Impedance Spectra of fresh cell and cells cycled up to 300 cycles
0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38
Z Re ( )
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Z Im
(
)
Fresh
1C Discharge
2C Discharge
3C Discharge
SOC: 0%
(a)
0.25 0.26 0.27 0.28 0.29 0.30 0.31
Z Re (
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Z Im
(
SOC: 100%Fresh
1C Discharge
2C Discharge
3C Discharge
(b)
(a) 0% SOC (b) 100% SOC
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Half Cell Study (T-cells)
Carbon Half-cell LiCoO2 Half-cell
0 20000 40000 60000 80000 100000 120000 140000
Time (s)
1.0
1.7
2.3
3.0
3.7
4.3
5.0
Vol
tage
(V
)-2.0E-004
-1.0E-004
0.0E+000
1.0E-004
2.0E-004
Cur
rent
(A
)
Current
Voltage
Delithiation
Lithiation
0 20000 40000 60000 80000 100000 120000 140000
Time (s)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Vol
tage
(V)
-2.0E-004
-1.0E-004
0.0E+000
1.0E-004
2.0E-004
Cur
rent
(A)
Current
Voltage
Lithiation Delithiation
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Half-cell analysis of capacity fade (in percentage) of negative Carbon electrode and positive LiCoO2 electrode
Capacity Fade (in
percentage)Fresh
1C 300 Cycles
2C 300 Cycles
3C 300 Cycles
Carbon 0.00% 2.77% 8.30% 10.59%
LiCoO2 0.00% 3.98% 4.38% 5.18%
The percentage
loss of capacity
is calculated
based on the
capacity of
fresh electrode
material.
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Breakdown of the total capacity fade of the whole lithium-ion battery
Q: total capacity loss of
the whole lithium-ion
cell
Q1: capacity correction
due to rate capability
Q2: capacity fade due to
the loss of secondary
material (Carbon or
LiCoO2)
Q3:capacity fade due to
the loss of primary
material (Li+)
Cell cycled at 1C rate
Cell cycled at 2C rate
Cell cycled at 3C rate
Total capacity fade of Li-ion
Battery9.5% 13.2% 16.9%
Q1 3.5% 2.9% 2.8%
Q2 (Carbon)NA 8.4% 10.6%
Q2 (LiCoO2) 3.8% NA NA
Q3 2.3% 2.0% 3.4%
Q:=Q1 + Q2 +Q3
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
0 2000 4000 6000 8000 10000 12000 14000 16000
Z Re (
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
Z Im
(
)
1.154 V
1.730 V
0.992 V
0.913 V
1 mHz
(a)
0 125 250 375 500 625 750 875 1000
Z Re (
0
100
200
300
400
500
600
700
800
900
1000
Z Im
(
)
1.154 V
1.730 V
0.992 V
0.913 V(b)
Typical Nyquist plots of Carbon half-cell obtained at 25 0C (a)
potential ranging from 0.913 to 1.730 V vs. Li+/Li
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Typical Nyquist plots of Carbon half-cell obtained at 25 0C (b)
potential ranging from 0.126 to 0.773 V vs. Li+/Li
0 100 200 300 400 500 600 700
Z Re ( )
0
50
100
150
200
250
Z Im
(
)
0.126 V
0.406 V
0.258 V0.587 V
0.773 V 1 mHz
(c)
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Equivalent circuit of the EIS spectra
Relect Rf Rct Re Zw Cint
Qf Qct Qe
Re: resistance of bulk material
Zw: Resistance of Warburg
Diffusion
Cint:intercalation capacitance
Q: constant phase elements
Relect: resistance of electrolyte
Rf: resistance of surface film
Rct: resistance of charge transfer
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Data Fitting
0.0 187.5 375.0 562.5 750.0 937.5 1125.0 1312.5 1500.0
Z Re ()
0.0
187.5
375.0
562.5
750.0
937.5
1125.0
1312.5
1500.0
Z im
(
)
plot by fitting
plot by experiment
0.001 Hz
0.0001 Hz
Rf : 6.87
Re : 110
Rct :=40.37
Cint := 1.5 F
Log(D) := -9.7
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
10% SOC 20% SOC
State of Charge
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Rf
()
2C
1C
3C
1C
2C
3C
10% SOC 20% SOC
State of Charge
0
20
40
60
80
100
120
140
R e
(
)
1C
2C
3C
1C
2C
3C
10% SOC 20% SOC
State of Charge
0
10
20
30
40
50
60
70
80
90
R c
t (
)
3C
2C
1C 1C
2C
3C
Parameter comparisons
Rf Re
Rct
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
SEM images of the electrode surface
• SEM (X1000/30 m)
of Carbon materials
cycled at different
discharge rates.
• (A) : Carbon cycled
at 1C
• (B) : Carbon cycled at
2C discharge rate
• (C)+(D) : Carbon
cycled at 3C
discharge rate
A B
DC
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
Mechanism of Property Changes
Carbon Particles
Initial SEI film
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•
Thicker SEI film
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Effects of Discharge Rates on the Capacity Fade of Li-ion Cells
Department of Chemical Engineering
University of South Carolina
ConclusionThe 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.
