Gang Ning, Bala S. Haran, Branko N. Popov
Department of Chemical Engineering
University of South Carolina, Columbia, SC
Introduction:
Lithium-ion rechargeable batteries with LiCoO2
cathode and carbon anodes are rapidly replacing other
battery systems due to their high energy and power
densities. While the discharge properties and safety issues
with these batteries have been studied in details, not much
attention has been placed on the capacity fade due to
cycling, especially under different discharge rates. This
capacity fade is caused by various mechanisms, which
depend on not only the electrode materials but also on the
charge and discharge protocols [1, 2, 3, 4].
Commercial Li-ion cells lose capacity
continuously. This capacity fade is accompanied by an
increase in the internal impedance of the battery upon
cycling. Therefore, it is critical to clarify the contribution
of each of the components in the cell to the internal
impedance increase of the battery at different discharge
rates.
Objectives of this study were to determine the
mechanism of the capacity fade of Sony lithium-ion
batteries during continuous cycling under different
discharge rates as well as to determine if the battery
follows the same mechanism of capacity fade under
different discharge rates. The Half-cell analysis provides
an accurate way of determining the capacity fade resulting
from LiCoO2 and carbon respectively. Using EIS, the
change of impedance at the positive and negative
electrodes was estimated at different cycle numbers.
Data for high discharge rates will also be used as
baseline for the future capacity fade analysis of hybrid
system.
Experimental:
All experimental studies were done on Sony
Sony 18650 commercial cells. Constant current (CC) and
constant voltage (CV) protocol was utilized in charging
the batteries. The cell was charged at a constant current of
1 A until the potential reached 4.2 V. Subsequently, the
voltage was held constant at 4.2 V until the charge current
decayed to 50 mA. Discharge was carried out at different
rates of 1C, 2C and 3C within the voltage window from
4.2 to 2.5 V. Discharge capacity of cells was determined
by C/2 discharge rate. Cycling studies were carried out
using Arbin Battery Test (BT-2000) System.
Potentiostat/Galvanostat Model 273A were used for the
electrochemical characterization of Sony 18650 cells. The
impedance studies were carried out on cells that were
previously kept at open circuit for 3 hours in order to
stabilize the open cell voltage. The cell voltage changed
less than 1 mV during the experiments. EIS
measurements were done on the Sony cells as well as on
individual T-Cells initially and after 300 cycles.
Impedance studies were done on the cells at both charged
and discharged states. The frequency range for the whole
battery in the EIS test is from 0.01 Hz to 10000 Hz while
for the LiCoO2 T-cell is from 0.01 to 100000 Hz and for
the carbon T-cell is from 0.001 Hz to 100000 Hz. The
amplitude of the AC signal is 10 mV.
In order to identify the contribution of the
positive and negative electrodes to the total cell
impedance, studies were done on the individual
electrodes. Sony 18650 batteries were stripped off at fully
discharged state in a glove box filled with ultra pure argon
(National Gas and Welders). Both the positive (LiCoO2)
and negative (carbon) electrodes were carfully removed
from the cell. Pellets with a diameter of 1.2 cm were
punched from the removed electrodes. To ensure the best
contact between pellet and current collector of the T-cell
(made of stainless steel), the material on one side of the
pellet was scraped to the extent that the original fresh
copper or aluminum (collectors in the commercial
battery) would appear. Dimethyl carbonate (DMC) is used
to clean the surface of the pellets. Electrochemical
characterization of these individual pellets was done in Tcell using a three-electrode setup. Li/Li+ was used as both
reference electrode and counter electrode. The working
electrode was the well-prepared pellet.
Electrolyte used was 1 M LiPF6 in a 1:1 mixture
of ethylene carbonate (EC), and dimethyl carbonate
(DMC).
Results and Discussions:
Fig 1 shows the discharge capacity change
among initially, after 300 cycles under 1C discharge rate,
after 300 cycles under 2C discharge rate, and after 300
cycles under 3C discharge rate. It is clear that due to the
ohmic resistance increase in the internal battery that the
flat plat form voltage region is shortened. Specific
analyses of individual contribution to the resistance
increase and capacity fade are currently in progress.
Acknowledgment:
Financial support provided by National
Reconnaissance Office for Hybrid Advanced Power
Sources # NRO-00-C-1034 is acknowledged gratefully.
References
1. B. Johnson and R.E. White. J. Power Sources 70
(1998), p. 48.
2. D. Linden, Editor, Handbook of Batteries (2nd edn.
ed.), McGraw-Hill, New York (1995), pp. 36.44 -36.48.
3. P. Arora, R.E. White and M. Doyle. J. Electrochem.
Soc. 145 (1998), p. 3647.
4. D. Zhang, B.N.Popov. J. Power Sources 91 (2000), p.
122.
4.1
3.9
3.7
Voltage (V)
Study on the Effects of Discharge Rates on
the Capacity Fade of Lithium-ion Battery
Initial Discharge
2C Discharge
3.5
1C Discharge
3.3
3.1
3C Discharge
2.9
2.7
2.5
0.1
0.3
0.5
0.7
0.9
1.1
1.3
Discharge Capacity (Ah)
Fig1: Discharge capacity vs. voltage, initially and after
300 cycled
under different discharge rates