Advanced Power Sources for EVs
E. Peled
School of Chemistry
Tel Aviv University, Tel Aviv, Israel
IFCBC 26.1.2011
http://www.tau.ac.il/institutes/ifcbc/presentations-2010.html
1
Issues
• Introduction
• Comparison between fuel cells and
lithium ion power sources for EVs
• Advantages and limitations of lithium
air battery (recently attracting a lot of
attention)
• Advantages and limitations of a novel
sodium air battery
• Preliminary performance of sodium
air battery
• Summary
2
Product:
Li2O
Li2O2
Without oxygen
3
Source - IBM 2010
4
Peter Bruce 2010
Na – air
Li -air
5
Disadvantages of the lithium–air cell
• Very low power* - about 0.1 to 1mA/cm2 mainly due
to a sluggish oxygen-reduction reaction (ORR).
• The oxygen-discharge product is lithium peroxide (a
very strong oxidizing agent) which is very reactive
toward the electrolyte solvents and the environment.
• In addition, it is an electrical insulator, thus a large
area of carbon substrate is required to
accommodate the solid peroxide at a thickness
lower than the tunneling range of electrons (about
2nm).
• Sensitive to water and CO2 penetration.
• Safety issues, especially due to lithium dendrite
formation.
6*
* K. Abraham, P. Bruce, S. Mukerjee
Dendrite Formation on Charge
• In all nonaqueous lithium batteries, the anode is covered
by a thin film called a Solid Electrolyte Interphase (SEI)*.
• As a result, on charge, lithium deposits through the SEI in
the form of lithium dendrites and mossy (sponge) lithium.
• This raises safety issues – the formation of internal short
circuits by lithium dendrites.
• For these reasons, efforts to develop rechargeable lithiummetal batteries have failed and today only rechargeable
lithium-ion batteries, which do not contain metallic lithium,
are in use.
* E. Peled
The Electrochemical Behavior
of Alkali and Alkaline Earth
Metals in
Nonaqueous Battery Systems
-The Solid Electrolyte
Interphase (SEI) Model.
J. Electrochem.
Soc. 126,
7
2047-2051 (1979).
Charge
Preliminary evaluation of energy- and power-density
constraints for a lithium – air battery
(for a bipolar-plate battery design)
• Assuming a 100 liter, 100kWh, 100kW stack, we obtain
1W/ml and 1Wh/ml.
• Assuming 2mm-thick cells (Vs. 0.2 mm in Li ion
batteries), at 2.5V, the current and charge densities are
80mA/cm2 and 80mAh/cm2.
• Assuming 1Ah/g of carbon, we obtain 80mg carbon/cm2 and
about 0.8mm-thick empty carbon electrode.
• For a 50 liter stack (a volume similar to that of the Honda
FCX Clarity FC stack (57 liter), we get 160mA/cm2 and
160mAh/cm2, and about 1.6mm-thick empty carbon
electrode.
• Conclusion: Due to a thick air electrode, the lithium-air
battery, having the BPP design, has to run at about
0.1A/cm2 in order to have a practical volume and weight.
8
Molten sodium–air nonaqueous battery
• We suggest here a novel concept, namely to replace
the metallic lithium anode by liquid sodium (absorbed in
a porous matrix) and to operate the sodium–air (oxygen)
cell above the sodium melting point (97.8oC).
• The theoretical specific energy of the sodium–air cell,
assuming Na2O as the discharge product and including
the weight of oxygen, is
•
1690 Wh/kg, about four times that of state-of-the-art
lithium-ion batteries.
• (The average specific energy density of the Na/O2 cell is
1980Wh/kg)
9
Advantages of molten sodium as an
anode for a rechargeable air cell
• Sodium is much cheaper and more abundant than
lithium.
• The surface tension of the liquid sodium anode is
expected to prevent the formation of sodium
dendrites on charge. Any sodium dendrites that might
be formed would be absorbed into the liquid phase.
• The higher operating temperature accelerates electrode
kinetics and reduces electrolyte resistance, thus
enabling running the cell at higher power.
• Sodium peroxide is less stable and more reactive
than lithium peroxide and can be decomposed by a
manganese dioxide.
10
Advantages of molten sodium as an
anode for a rechargeable air cell (cont.)
• At the higher operating temperature and with the use of a proper fourelectron ORR catalyst, it may be possible to reversibly reduce oxygen to
oxide (as Na2O), thus avoiding the accumulation of peroxide in the air
electrode.
• In contrast to lithium, sodium does not dissolve in aluminum (0.003%)
and this enables the use of thin aluminum foil as a light and low-cost
hardware material, especially for thin bipolar plates.
• By contrast, lithium cells require the use of copper or nickel as anode
current-collector materials, both of them heavier and much more
expensive than aluminum.
• At temperatures above 100oC, little if any interference of atmospheric
water is expected.
• In addition, unlike lithium, sodium does not form a nitride in air.
• The adsorption of CO2 may be reversed by the oxidation of sodium
carbonate to oxygen and CO2 on charge.
11
The disadvantages of the sodium-air battery
(in comparison to the room-temperature lithium-air battery)
• The open-circuit voltage of the sodium-oxygen
cell is 2.3-2.4V, lower than that for the lithium–
oxygen cell (3V).
• It has a lower specific energy.
• At present, the cycling (coulombic) efficiency of
molten sodium, covered by an SEI at 110oC (70
-90%) is not high enough, and increasing it to
nearly 100% presents a challenge (it is low for a
fresh cell and rises to over 95% during cycling) .
• At present, SEI resistance is too high (about 200
Ohm.cm2).
12
Sodium SEI Issues
• In order to create a protective SEI on alkali metal
anodes it is essential that the equivalent volumes of
the SEI materials be larger than that of the anode*.
• Only in this way, the SEI can completely cover the anode
surface and stop corrosion. If not, the anode will continue
to corrode.
• The equivalent volumes of Na2CO3, NaF and Na2O are
lower than that of sodium.Thus these cannot serve as
good SEI-building materials.
• On the other hand, the equivalent volumes of several
sodium oxosulfur materials including: Na2S2O4, Na2S2O3
are larger than that of sodium, thus they are suitable
candidates for use as sodium SEI-building materials.
• * E. Peled,
D. Golodnitsky, C. Menachem, and D. Bar Tow.
An Advanced Tool for the Selection of Electrolyte Components for Rechargeable Lithium
Batteries J. Electrochem. Soc., Vol. 145, No. 10, October 1998
13
Sodium - Air Battery – FC BPP Stack Design
Cell thickness (including a
cooling cell) is estimated to
be about 2 mm
14
Molten sodium–air cells, preliminary results
at above 100oC*
*Parameter analysis of a practical lithium-and sodium-air electric
vehicle battery E. Peled, D. Golodnitsky, H. Mazor, M.Goor, S.
Avshalomova;
Journal of Power Sources xxx (2010) xxx–xxx
15
-4
1.0x10
-4
3.5
3.0
charge
2.5
5.0x10
-5
2.0
discharge
1.5
0.0
1.0
-5.0x10
-1.0x10
Voltage (V)
Current (A)
1.5x10
-5
V
I
0.5
-4
2000
3000
4000
5000
0.0
6000
Test Time (s)
Discharge/charge curves of a Na-O2 cell at 105oC
Voltage range of 1.5V-3.0V (or 20 minutes operation time), discharge and charge
currents
are 50µA and 100µA respectively, (FC hardware, electrode area – 1cm2,
16
ETEK cathode): The electrolyte is based on PEGDME 2000 and PC.
-4
1.0x10
-4
Oxygen Starvation- Voltage/Current Profile
3.5
3.0
2.5
5.0x10
-5
2.0
1.5
0.0
1.0
-5.0x10
-1.0x10
Voltage (V)
Current (A)
1.5x10
-5
V
I
-4
6000
0.5
O2 flow off
7000
8000
O2 flow on
9000
0.0
10000
Test Time (s)
Oxygen starvation of a Na-O2 cell at 105oC
Voltage limits 1.5 - 3V and 1 min rest at OCV, charge and discharge at 100µA and
50µA, respectively. (FC hardware, electrode area – 1cm2, ETEK cathode): The
17
electrolyte
is based on PEGDME 2000 and 10%PC.
Charge discharge cycles of sodium – air cell at 110oC
Ch. at 100μA/cm2, Dis. at 40μA/cm2, Voltage limit 1-4V, time limit 0.5h (PE based)
The problem: A rise of
the charging voltage
with cycle number.
Sodium plating and dissolution at above
100oC
• In order to prove that sodium can be
cycled in its molten state, we ran
deposition–dissolution tests of sodium on
aluminum at 110oC (above the melting
point of sodium).
• We added methyl methanesulfonate as an
SEI precursor and obtained, after some
SEI building cycles, cycling current
efficiency of 70 to 90%.
19
Sodium cycling efficiency at 1100C
(Na/SS cell, time range = 11350min - 12000min)
=70 - 90%
No dendrites
formation
During
over 300
hours and
over 400
cycles!
Nonaqueous Alkali Metal-Air EV Battery - Summary
Issues that need to be addressed:
•
•
•
•
•
Power must be increased by two orders of magnitude (up to about 0.1
A/cm2 following the use of thick cells, (Rcell = 1 to 10 Ohm.cm2).
Peroxide formation must be avoided (obtain a reversible 4e ORR).
Dendrite formation on charge must be avoided.
Sensitivity to moist air and CO2 should be reduced, or use an efficient
water barrier.
The battery should be preferably assembled in the discharged state
(by charging the cathode with carbonate).
The use of a liquid sodium anode at above 100oC may solve or
ease these problems:
•
•
•
•
•
•
•
21
Accelerates sluggish cathode reactions and lowers cell impedance.
Dendrites are not formed.
It is easier to obtain a reversible 4e ORR and avoid peroxide formation.
Interference by water vapor and CO2 is minimized.
We found indications for carbonate decomposition on the first charge
and this may enable battery assembly in the discharge state.
In addition - lighter and lower-cost hardware material (aluminum) is
used.
Preliminary results show: (a) the functioning of the molten sodium-air
battery; (b) high faradaic efficiency of the sodium plating–dissolution
process and (c) possibility of oxidizing sodium carbonate.
Conclusions
•
In the near future all-electric battery-powered electric vehicles
will find niche applications as city cars and limited range
commuter cars.
•
Lithium and sodium – air batteries can make a major
breakthrough in battery technology and cost giving Evs, in the
long term, a driving range of 500 km.
• The fuel cell electric vehicle could provide the range and
refueling times at an affordable price demanded by modern
drivers for full function passenger vehicles.
• Market penetration will follow the order: HEV < PHEV < FCEV
< BEV
22
Secretary Chu's (US Secretery of Energy) addressed the United
Nations Climate Change Conference in Cancun (December 2010).
• "A rechargeable battery that can last for 5,000 deep discharges,
6-7 x higher storage capacity (1,000 Wh/Kg) at 3x lower price
will be competitive with internal combustion engines (400 - 500
mile range).“
• The only battery chemistries that have a chance of achieving
energy densities in the 1,000 wh/kg range are rechargeable
metal-air
Thank you for your attention
Acknowledgments
Prof. D. Golodnitsky, H. Mazur, M. Goor and S. Avshalomi
24
Na deposition
50A
0.06
1.0
0.04
0.5
0.0
0.02
-0.5
0.00
-0.02
615
I
630
-1.0
25A
o
Na dissolution
V
645
660
675
690
Potential [V]
Current [mA]
1.5
o
-1.5
705
time [min]
Sodium deposition-dissolution on Al at 105oC.
Na/NaTf:PEO6 + Methyl methanesulfonate 5%(wt)/Al cell. Discharge
25
and charge rates are 50µA for 10 min and 25µA for 20 min,
respectively. Coin-cell hardware. Electrode area - 0.57cm2.
Capacity [mAh]
0.015
o
dissolution of Na
o
deposition of Na
0.010
0.005
discharge=50A, 10min
charge=25A, 20min
0.000
0
20
40
60
80
100
120
140
Cycle No.
Deposition and dissolution cycles of sodium on Al at 105oC
Na/NaTf:PEO6 + Methyl methanesulfonate 5%(wt)/Al coin cell.
Discharge and charge rates are 50µA for 10 min and 25µA for 20 min, respectively.
26
-700
SEI apparent thickness
of 94Å is attributed to the
AC after discharge
second semi-circle of 1.7µF
capacitance and 330 Å to
the first one.
-600
Z''/ohm
-500
Rbulk=60
-400
RSEI,2=620
Capacity =1.7F
RSEI,1=540
Capacity=0.2F
-300
-200
-100
0
0
400
800
1200
1600
2000
Z'/ohm
AC impedance spectra of Na/NaTf:PEO6 + Methyl
methanesulfonate 5%(wt)/Al cell 105oC.
27
After plating of Na on Al, frequency range - 10MHz to 1mHz.
Electrode area - 0.57cm2 .
28
29
PEM FC stack
30
31
Honda FCX Clarity
FC Stack
1.75 kW/l
1.5 kW/kg