Performance studies of a rapid Performance studies of a rapid charging Protocol for Li-ion Cellscharging Protocol for Li-ion Cells

byGodfrey 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

ObjectivesObjectives

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…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 Development of Current Decay Protocol Our Approach..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 protocolDevelopment of New protocol

0 1I I k t

Linear Current Decay Protocol (LCD)

0 1 2I I k t k t Modified Linear Current Decay Protocol (MLCD)

The New ProtocolHigh Current DC charging followed by

a current decay

LCD & MLCD Protocols…LCD & MLCD Protocols…

-200 300 800 1300 1800 2300

Charging T ime (sec)

0.0

0.7

1.4

2.1

2.8

3.5

Cha

rgin

g C

urre

nt (A

)

Cel

l Vol

tage

(V)

3.1

3.3

3.5

3.7

3.9

4.1

4.3

0 1I I k t

LCD-Protocol

MLCD-Protocol0 600 1200 1800 2400 3000

Charging T ime (sec)

0

1

2

3

4

5

Cha

rgin

g C

urre

nt (A

)

Cel

l Vol

tage

(V)

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

voltage

current

0 1 2I I k t k t

Variation of Charging Current and Cell Variation of Charging Current and Cell Voltage for the New ProtocolVoltage for the New Protocol

0 1000 2000 3000 4000 5000 6000

charging time (sec)

0

1

2

3

4

5

6

Cur

rent

(A)

Vol

atge

(V)

2.40

2.78

3.16

3.54

3.92

4.30

Currentvoltage

voltage

current

ExperimentalExperimentalCycling 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, Comparison of Utilization of CV-charging,

CC-CV charging and new protocol (cycle 1)CC-CV charging and new protocol (cycle 1)

0 900 1800 2700 3600 4500 5400

Time(sec)

0

20

40

60

80

100

Uti

liza

tion

(%)

CC-CV ProtocolCV ProtocolNew Protocol

2125

sec 24

61 s

ec

4476

sec

Charge Curves of the CC-CV Protocol Charge Curves of the CC-CV Protocol and CV Protocoland CV Protocol

0 1000 2000 3000 4000 5000 6000 7000

Time(sec)

0.2

0.4

0.6

0.8

1.0

Cur

rent

(A

)

cycle1cycle 50cycle 100cycle 150

0 1000 2000 3000 4000 5000 6000

Time(sec)

0

5

10

15

Cur

rent

(A)

cycle 1cycle 50cycle 100cycle 150

CC-CV Protocol CV Protocol

0 1000 2000 3000 4000 5000 6000

Time(sec)

0

1

2

3

4

5

6

Cur

rent

(A)

cycle 1cycle 50cycle 100cycle 150

0 50 100 150 200 250 300Time(sec)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Cur

rent

(A)

cycle 1cycle 50cycle 100cycle 150

Charge Curves of the New ProtocolCharge Curves of the New Protocol

Charge Utilization CurvesCharge Utilization Curves

0 1000 2000 3000 4000 5000 6000 7000

Time(sec)

0

20

40

60

80

100

% U

tili

zati

on

cycle1cycle50cycle100cycle150

0 1000 2000 3000 4000 5000 6000

Time (sec)

0

20

40

60

80

100

% U

tili

zati

on

cycle1cycle50cycle100cycle150

0 1000 2000 3000 4000 5000 6000

Time(sec)

0

20

40

60

80

100

% U

tili

zati

on

cycle 1cycle 50cycle 100cycle 150

CC-CV CV

New Protocol

Discharge Curves Discharge Curves

0.0 0.3 0.6 0.9 1.2 1.5

Discharge Capacity(Ah)

2.40

2.78

3.16

3.54

3.92

4.30

Vol

tage

(V)

cycle 1cycle 50cycle 100cycle 150

0.0 0.3 0.6 0.9 1.2 1.5

Discharge Capacity(Ah)

2.40

2.78

3.16

3.54

3.92

4.30

Vol

tage

(V)

cycle 1cycle 50cycle 100cycle 150

0.0 0.3 0.6 0.9 1.2 1.5

Discharge Capacity(Ah)

2.0

2.4

2.8

3.2

3.6

4.0

4.4

Vol

tage

(V)

cycle1cycle50cycle100cycle150

CC-CV

New protocol

CV

Variation of Discharge Capacity with CyclingVariation of Discharge Capacity with Cycling

0 50 100 150 200

Cycle Number

86

88

90

92

94

96

98

100

Cap

acit

y Fa

de(%

)

CC-CV protocolNew ProtocolCV Protocol

Capacity Fade Comparison after Capacity Fade Comparison after 150 Cycles150 Cycles

Mode of Charging

% Capacity Fade after…

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

0.100 0.825 1.550 2.275 3.000

Discharge Current (A)

0.6

0.8

1.0

1.2

1.4

Dis

char

ge c

apac

ity(

Ah)

CC-CV chargingCV chargingFresh CellNew Protocol

Rate Capability after 150 cycles Rate Capability after 150 cycles

C/2C/8 C 3/2C 2C

Nyquist Plots for LiCoONyquist Plots for LiCoO22 half half

cell(fully Lithiated and delithiated)cell(fully Lithiated and delithiated)

0 300 600 900 1200 1500

ZRe(cm)

0

200

400

600

800

1000

ZIm

( c

m2)

Fresh CellCC-CVCVNew Protocol

0 200 400 600 800 1000

ZRe cm2)

0

120

240

360

480

600

ZIm

( c

m2)

FreshCC-CVCVNewProtocol

lithiated delithiated

Nyquist Plots for Carbon half Nyquist Plots for Carbon half cell(fully Lithiated and delithiated)cell(fully Lithiated and delithiated)

0 80 160 240 320 400

ZRe(cm)

0

40

80

120

160

200

ZIm

( c

m2)

FreshCC-CVCVNew Protocol

0 200 400 600 800 1000

ZRe cm2)

0

120

240

360

480

600

ZIm

( c

m2)

FreshCC-CVCVNewProtocol

lithiated delithiated

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 analysis of Capacity fade from half cell measurements

Quantitative split of capacity fade

Protocol Type Q% Q1% Q2 % (Carbon)

Q3%

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

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 EngineeringUniversity of South Carolina

Center for Electrochemical EngineeringUniversity of South Carolina