Transcript Development of Novel Charging Protocols for Li

Performance studies of a rapid
charging Protocol for Li-ion Cells
by
Godfrey Sikha, P.Ramadass,
Bala S. Haran, Ralph E. White, Branko N. Popov
Center for Electrochemical Engineering,
Department of Chemical Engineering,
University of South Carolina Columbia, SC 29208
Objectives
 To develop a new protocol for charging Li-ion cells.
 To reduce the total charging time.
 To reduce capacity fade due to over-charging.
 To develop a smart charger after optimizing the cell
capacity, charging time and capacity decay.
Need for a New Protocol…
 The charging time for CC-CV protocol is higher.
 Using higher DC currents during CC charge keeps the cell during most
part of the time in CV mode, but still there is no considerable decrease
in total charging time.
 Usage of CV mode for the entire charging time could decrease the total
charging time.
 But the CV charging needs a very high current during the earlier stages
of charging that makes the cost of the charger high.In addition its
capacity fade is more
 Thus, a new protocol that may be similar to CV charging, but not using
very high currents, could be ideal for a smart charger and could reduce
the charging time to a great extent.
Development of Current Decay Protocol
Our Approach..
 Charging the Li-ion cell with linearly descending current with time
reduces the charging time as compared to constant current charging*.
 Similar mode of charging was tested with commercial Sony 18650
cells.
 Based on the preliminary results of LCD charging, the protocol was
modified to avoid over charging and to decrease the charging time
further.
 A new protocol was developed that consists of initial high current DC
charging (~3C rate) followed by a current decay with the fixed total
charging time.
 Cycling studies were carried out with the new protocol and the
performance was compared with conventional CC-CV and CV mode
of charging.
*S.K.
Chang, A.A. Andriiko, A.P. Monko and S.H. Lee, Journal of Power Sources 79 (1999) 205-211.
Development of New protocol
Linear Current Decay Protocol (LCD)
I  I 0  k1t
Modified Linear Current Decay Protocol (MLCD)
I  I0  k1t  k2 t
The New Protocol
High Current DC charging followed by
a current decay
LCD & MLCD Protocols…
3.5
4.3
4.1
I  I 0  k1t
Cell Voltage (V)
3.9
2.1
3.7
1.4
3.5
0.7
3.1
300
800
1300
Charging T ime (sec)
LCD-Protocol
1800
4.3
2300
5
4.2
4.1
Charging Current (A)
0.0
-200
3.3
voltage
4
I  I0  k1t  k2 t
3
4.0
3.9
3.8
3.7
2
3.6
3.5
1
current
3.4
3.3
0
0
MLCD-Protocol
600
1200
1800
Charging T ime (sec)
2400
3.2
3000
Cell Voltage (V)
Charging Current (A)
2.8
Variation of Charging Current and Cell
Voltage for the New Protocol
6
4.30
voltage
5
3.92
3.54
3
current
2
3.16
Current
voltage
1
2.78
0
0
1000
2000
3000
4000
charging time (sec)
5000
2.40
6000
Volatge(V)
Current(A)
4
Experimental
Cycling Studies:
For the comparison of performance of the New Protocol the following
cycling studies were done.

CC-CV Protocol
0.9 A charging until 4.2V and a float at 4.2 V
until the utilization reached 98% for the first cycle(the corresponding time
was kept as a time limit for further cycling)and 1A discharge until the
potential drops to 2.5V.

CV Protocol
Potentiostatically controlled at 4.2V until the
utilization reached 98% for the first cycle(the corresponding time was kept as
a time limit for further cycling) and 1A discharge until the potential drops to
2.5 V

New Protocol
A short 5A pulse until the potential reaches
4.2 V followed by the current decay profile for a total time of 5400 seconds
which yielded an utilization ca 98 % and 1A discharge until the potential
drops to 2.5 V
Post Cycling studies

Rate Capability Studies were done after 150 cycles, where all cells
are charged using CC-CV protocol with 1A DC and discharged at
different rates namely C/8, C/4, C, 3/2C and 2C.

CV's were obtained at the scan rate of 0.05 mV/s within the voltage
range of 2.5-4.2 V at the end of 150cycles.

Impedance measurements were done at fully charged and fully
discharged states. (100 SOC & 0 SOC)for fresh and cycled full cells

T-cell studies: The cycled cells were cut open and individual
electrodes were cycled against excess lithium as counter and capacity of
individual electrodes were measured.

The individual half-cells were subjected to cyclic voltammogram and
ac-impedance studies
Comparison of Utilization of CV-charging,
CC-CV charging and new protocol (cycle 1)
100
4476 sec
2461 sec
60
2125 sec
Utilization (%)
80
40
CC-CV Protocol
CV Protocol
New Protocol
20
0
0
900
1800
2700
T ime(sec)
3600
4500
5400
Charge Curves of the CC-CV Protocol
and CV Protocol
15
1.0
cycle 1
cycle 50
cycle 100
cycle 150
0.8
Current (A)
Current (A)
10
0.6
0.4
cycle1
cycle 50
cycle 100
cycle 150
0.2
0
1000
2000
5
0
3000
4000
5000
Time(sec)
CC-CV Protocol
6000
7000
0
1000
2000
3000
4000
T ime(sec)
CV Protocol
5000
6000
Charge Curves of the New Protocol
6
6.0
5.5
Current (A)
Current (A)
5
4
5.0
4.5
4.0
cycle 1
cycle 50
cycle 100
cycle 150
3.5
3
3.0
0
50
100
150
T ime(sec)
200
250
300
cycle 1
cycle 50
cycle 100
cycle 150
2
1
0
0
1000
2000
3000
T ime(sec)
4000
5000
6000
100
80
80
% Utilization
100
60
40
cycle1
cycle50
cycle100
cycle150
20
60
40
cycle1
cycle50
cycle100
cycle150
20
0
0
0
1000
2000
3000
4000
5000
Time(sec)
6000
7000
0
1000
2000
CC-CV
3000
4000
T ime (sec)
100
CV
80
% Utilization
% Utilization
Charge Utilization Curves
60
40
cycle 1
cycle 50
cycle 100
cycle 150
20
0
0
1000
2000
3000
4000
T ime(sec)
New Protocol
5000
6000
5000
6000
Discharge Curves
4.30
4.30
cycle 1
cycle 50
cycle 100
cycle 150
Voltage(V)
3.92
3.54
3.16
2.40
0.0
3.54
3.16
2.78
2.78
0.3
0.6
0.9
1.2
2.40
0.0
1.5
0.3
CC-CV
0.6
CV
4.4
4.0
3.6
3.2
cycle1
cycle50
cycle100
cycle150
2.8
2.4
2.0
0.0
0.9
Discharge Capacity(Ah)
Discharge Capacity(Ah)
Voltage(V)
Voltage(V)
3.92
cycle 1
cycle 50
cycle 100
cycle 150
0.3
0.6
0.9
Discharge Capacity(Ah)
New protocol
1.2
1.5
1.2
1.5
Variation of Discharge Capacity with Cycling
100
98
Capacity Fade(%)
96
94
92
90
CC-CV protocol
New Protocol
CV Protocol
88
86
0
50
100
Cycle Number
150
200
Capacity Fade Comparison after
150 Cycles
% Capacity Fade after…
Mode of
Charging
50 cycles 100 cycles 150 cycles
CC-CV
4.06
5.61
6.64
CV
5.24
7.51
10.42
New
Protocol
5.37
7.54
9.506
Rate Capability after 150 cycles
Discharge capacity(Ah)
1.4
1.2
C/8
C/2
C
3/2C
1.0
0.8
0.6
0.100
2C
CC-CV charging
CV charging
Fresh Cell
New Protocol
0.825
1.550
2.275
Discharge Current (A)
3.000
Nyquist Plots for LiCoO2 half
cell(fully Lithiated and delithiated)
800
480
Fresh Cell
CC-CV
CV
New Protocol
600
400
200
ZIm ( cm2)
600
ZIm ( cm2 )
1000
Fresh
CC-CV
CV
NewProtocol
360
240
120
0
0
0
300
600
900
ZRe( cm)
lithiated
1200
1500
0
200
400
600
2
ZRe cm )
800
delithiated
1000
Nyquist Plots for Carbon half
cell(fully Lithiated and delithiated)
600
200
ZIm ( cm2)
120
80
480
ZIm ( cm2)
Fresh
CC-CV
CV
New Protocol
160
360
240
Fresh
CC-CV
CV
NewProtocol
120
40
0
0
0
80
160
240

ZRe( cm )
lithiated
320
400
0
200
400
600
2
ZRe cm )
delithiated
800
1000
Quantitative analysis of Capacity
fade from half cell measurements

The overall capacity fade (Q) is the result of two major
contributions one from the rate capability (Q1) and other from the
secondary material losses (Q2), either carbon on LiCoO2

A very low rate discharge(C/8) at the end of cycling will give the
maximum available capacity on the full cell. The difference between this
capacity and the measured capacity will account for the rate capability
losses.(Q1)

The T-cells(both LiCoO2 and Carbon) made from cycled full cells
are lithiated against excess Lithium as counter electrode to check the
maximum available capacity in the respective secondary material.

The same procedure is done for fresh cell and the degradation of
secondary material that is limiting can be found(Carbon showed more
degradation as compared to Lithium in all the three protocols)

A capacity balance is performed to find Q3 which is categorized
as other losses
Quantitative split of capacity fade
Q%
Q1%
Q2 %
Constant current
constant voltage
(CC-CV)
6.64
2.279
3.213
1.148
Constant
voltage(CV)
10.42
4.322
5.786
0.312
New Protocol
9.506
4.014
5.259
0.233
Protocol Type
(Carbon)
Q3%
Conclusions
 New charging protocol was developed for charging commercial 18650 Li-
ion cells.
 High utilizations can be achieved at short periods of time which is useful
for many applications
 New protocol shows better performance when compared with CV mode
,gains a lot of time despite a poor performance when compared to CC-CV
mode of charging.
 Effective control of overcharging can be ensured when much better
current profiles are chosen.
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