Transcript Charge Curves at Various Cycles

Quantitative Estimation of Capacity Fade of
Sony 18650 cells Cycled at Elevated
Temperatures
by
Branko N. Popov, P.Ramadass and Bala S. Haran
Center for Electrochemical Engineering
Department of Chemical Engineering
University of South Carolina Columbia, SC 29208
Center for Electrochemical Engineering
University of South Carolina
Objectives
 Develop a methodology to determine the cause of capacity fade in Li-ion cells:
 Primary Active Material (Li+) loss
 Secondary Active Material (LiCoO2/Carbon) losses
 Rate Capability loss
 Factors that control the capacity loss:
 Charging protocol
 Cycling Temperature
 Charge and discharge rates
 The depth of discharge (DOD)
 Quantify capacity fade using experimental data.
 Develop a capacity fade model that would predict cycle life of a Li-ion cell
Experimental – Cycling Studies
Studies of 18650 Li-ion Cell:
 Cells cycled using Constant Current-Constant Potential (CC-CV) protocol.
 Charged at 1A current till potential reaches 4.2 V
 Hold potential at 4.2V till current decays to 50 mA.
 Cells were discharged at a constant current of 1 A.
 Batteries were cycled at four temperatures: RT(25oC), 45oC, 50oC and 55oC.
 Rate capability studies done after 150, 300 and 800 cycles
 Cells charged at 1 A and discharged at different rates (C/9 to 1C).
 EIS measurements were done for fresh and cycled cells. (100 kHz ~ 1 mHz ±5
mV)
Studies of fresh and cycled electrode materials were carried out using a T-Cell
assembly with Li metal being the counter and reference electrode.
4.20
4.20
3.76
3.76
1
1
Voltage (V)
Voltage (V)
Discharge Curves at Various Cycles
3.32
800
2.88
150
500
2.44
0.4
25 deg C
0.8
1.2
1.6
2.88
800
300
2.00
0.0
2.0
150
500
2.44
300
2.00
0.0
3.32
0.4
Capacity (Ah)
0.8
1.2
1.6
45 deg C
Capacity (Ah)
4.20
2.0
4 .2 0
45 degree-Dicharge
RT -Dicharge
3.76
3 .7 6
3.32
2.88
300
600
150
2.44
500
2.00
0.0
0.4
50 deg C
Vo lta g e (V)
Voltage (V)
1
2 .8 8
300
2 .4 4
0.8
1.2
Capacity (Ah)
1.6
2.0
1
3 .3 2
2 .0 0
0 .0
150
490
0 .4
0 .8
1 .2
Ca p a city (Ah )
1 .6
2 .0
55 deg C
Capacity Fade as a Function of Cycle Life
Temperature
% Capacity Fade (cycle number)
50
100
150
300
500
800
25oC
3.8 5.11
6.06
10.29
22.5
30.63
45oC
3.8 5.46
7
11.75
26.46
36.21
50oC
2.4
5.1
7.9
23.9
43.21
-
55oC
4.3
6.4
9.4
27
70.56
-
Change in Charging Times with Cycling
1
1.0
1
1
150
150
150
300
150
300
300
300
800
800
0.5
500
300
490
3
0.0
RT
45
Constant Current
50
55
CV Time (h)
CC Time (h)
1.5
1
2
800
800
300
150
1
300
500
300
300
150
150
1
150
1
1
1
0
RT
45
Constant Voltage
50
55
Rate Capability with Cycling
2.00
2.00
Discharge Capacity (Ah)
Discharge Capacity (Ah)
Fresh
1.75
150
1.50
300
1.25
800
1.00
Fresh
1.75
150
1.50
300
1.25
1.00
800
0.75
0.0
0.4
25 deg C
0.8
1.2
1.6
0.75
0.0
2.0
0.8
1.2
1.6
2.0
45 deg C
Applied Current (A)
Applied Current (A)
2.00
2.00
Rate Capability comparison after 150 and 800 cycles
Fresh
Rate Capability comparison after 150 and 800 cycles
1.75
Discharge Capacity (Ah)
Discharge Capacity (Ah)
0.4
150
1.50
1.25
300
1.00
Fresh
1.75
150
1.50
1.25
300
1.00
500
0.75
0.0
50 deg C
0.4
0.8
1.2
Applied Current (A)
1.6
2.0
0.75
0.0
0.4
0.8
1.2
Applied Current (A)
1.6
2.0
55 deg C
Variation of Cell Impedance with Cycling
0.8
600
Overall Cell Resistance ()
0.7
800
800
0.6
0.5
0.4
1
300
150
300
1 150
300
1 150
300
1
150
0.3
0.2
0.1
0.0
RT
45
50
55
Comparison of Electrode Resistances
1200
Electrode Resistance (-cm2)
LiCoO2
Carbon
1000
800
600
400
200
LiCoO2
Carbon
1000
800
600
400
200
0
0
RT
45
50
150 Cycles
55
RT
LiCoO2
Carbon
1000
Electrode resistance after 150 cycles for a fully charged cell
800 Cycles
800
600
45
50
55
300 Cycles
1200
Electrode Resistance (-cm2)
Electrode Resistance (-cm2)
1200
600 cycles
Electrode resistance after 300 cycles foa a fully charged cell
800 cycles
800 cycles
400
200
0
RT
45
50
Specific Capacity of Positive and Negative Electrodes at Various
Cycles and Temperature
Temperature
Cycle
Number
LiCoO2
Carbon
148.132
339.896
150
145.61
334.03
300
141.07
325.04
800
122.14
271.10
150
143.74
332.84
300
139.26
325.71
800
118.43
264.56
150
143.28
331.20
300
133.56
293.91
600
94.05
202.53
150
142.84
328.90
300
131.13
286.92
Fresh
25oC
45oC
50oC
55oC
Specific Capacity (mAh/g)
Capacity Fade Balance
Q = Q1 + Q2 + Q3
Q: Total Capacity Loss
Q1:
Capacity Fade due to rate capability loss
Difference in capacity between C/9 and C/2 rate discharges.
Q2:
Capacity Fade due to loss of secondary
active material (LiCoO2 and Carbon)
Measurement done on T-cells
Q3:
Capacity Fade due to loss of primary
active material (Li+) and other losses.
Capacity Fade - Quantified
80
100
Q1
Q2
Q3
80
% of Total Capacity Loss
% of Total Capacity Loss
100
60
40
20
0
60
40
20
0
150
25oC
Q1
Q2
Q3
300
Cycle Number
Electrode resistance after 150 cycles
800
150
50oC
300
Cycle Number
Electrode resistance after 150 cycles
800
Simplified Diffusion Model
 Concentration variations in the solution phase can be
neglected for low to medium discharge currents.
 Solid phase potential drop is negligible as compared to
kinetic and concentration over-potentials.
 Eliminating the above two results in a simple diffusion
model which can be used to simulate the performance of
the Li-ion Battery.
 Model considers Li+ reaction at carbon/LiCoO2 particle
interface and subsequent transport in these materials.
Governing Equations: Diffusion Model
Fickian diffusion in spherical coordinates in carbon
and LiCoO2
c Li D eff
  2 c Li 
 Li
r

2
t
r 
r 
r
Initial Condition
t  0, cLi  coLi
Boundary Conditions
r  0,
r  Rp,
D
eff
Li
 c Li
 0
r
 c Li
j
 rate Li 
 rˆ
F
Electrode Reaction Rates

 j e
Lithium intercalation/deintercalation reaction:
j  j e
Li,in
j
Li,de
a1F 
ΦP U Pref  jR f 


RT
o,1
 a2 F 
Φ N U Nref  jR f 


RT
o,2
e
 c1F
ΦP U Pref  jR f 

RT 
e
V   
P


  c2 F
 Φ N U Nref  jR f 

RT 
N
Rf refer to total resistance that includes ohmic R and polarization
RP resistances. (Rf=R+RP)
Concentration dependent exchange current density:

jo, j  k j c
max
Li , j
c
s
Li , j
 c 
a , j
s
Li , j
c , j
, j  1, 2
Uref
A Function of SOC
U =fn( SOC )
ref
n
n
Urefp
Urefn
LiCoO2
SOCn
SOCp
U  fn( SOC )
ref
p
P
Carbon
Parameters Considered for Diffusion Model
 State of Charge of the electrode limited by capacity
To account for capacity loss due to primary and
secondary active materials.
 Solid Phase Diffusion Coefficient
To account for capacity loss due to rate capability.
 Film Resistance
To account for the drop in cell voltage due to increase
in ohmic resistance and polarization losses.
Incorporation of Q2 and Q3 in Diffusion Model
 SOC of the electrode material could be estimated
from active material losses.
 Calculate SOC of the negative electrode based on
the capacity loss (Q2 + Q3).
 Develop a correlation for variation of SOC of
negative electrode with cycle number.
 Using this capacity fade due to active material losses
could be incorporated in the diffusion model.
Comparison of Diffusion Model and RT
Experimental Data
Parameter
Property
SOC
Active material
losses
Dns
Rate Capability
losses
Rf
Drop in cell voltage
with cycling
Comparison of Diffusion Model and RT
Experimental Data for Utilization (Rate Capability)
1.8
Fresh
Capacity (Ah)
150 Cycles
1.6
300 Cycles
1.4
1.2
1.0
0.00
800 Cycles
0.25
0.50
0.75
1.00
Current (A)
1.25
1.50
1.75
Conclusions
 A diffusion model was developed to simulate the
discharge curves of Li-ion cell.
 Empirical correlations have been developed for variation
of SOC of Li-ion cell with continuous cycling.
 Active material losses have been accounted through the
variation of negative electrode SOC in the diffusion
model.
 Rate capability losses and Polarization resistance increase
have been accounted through varying the diffusion
coefficient and exchange current density respectively.
 Inclusion of effect of charging and discharge rates and
DOD into diffusion model is currently in progress.
Acknowledgements
This work was carried out under a contract
with the
National Reconnaissance Office
for
Hybrid Advanced Power Sources
# NRO-00-C-1034.
Center for Electrochemical Engineering
University of South Carolina