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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: Li 2 O 2 Without oxygen Source - IBM 2010 3
4 Peter Bruce 2010
Na – air Li -air 5
Disadvantages of the lithium–air cell • Very low power* - about 0. 1 to 1 m. A/cm 2 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 2 nm). • Sensitive to water and CO 2 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, 100 k. Wh, 100 k. W stack, we obtain 1 W/ml and 1 Wh/ml. • Assuming 2 mm-thick cells (Vs. 0. 2 mm in Li ion batteries), at 2. 5 V, the current and charge densities are 80 m. A/cm 2 and 80 m. Ah/cm 2. • Assuming 1 Ah/g of carbon, we obtain 80 mg carbon/cm 2 and about 0. 8 mm-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 160 m. A/cm 2 and 160 m. Ah/cm 2, and about 1. 6 mm-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. 1 A/cm 2 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. 8 o. C). • The theoretical specific energy of the sodium–air cell, assuming Na 2 O 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/O 2 cell is 1980 Wh/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 Na 2 O), 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 100 o. C, little if any interference of atmospheric water is expected. • In addition, unlike lithium, sodium does not form a nitride in air. • The adsorption of CO 2 may be reversed by the oxidation of sodium carbonate to oxygen and CO 2 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. 4 V, lower than that for the lithium– oxygen cell (3 V). • It has a lower specific energy. • At present, the cycling (coulombic) efficiency of molten sodium, covered by an SEI at 110 o. C (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. cm 2). 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 Na 2 CO 3, Na. F and Na 2 O 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: Na 2 S 2 O 4, Na 2 S 2 O 3 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 100 o. C* *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
Discharge/charge curves of a Na-O 2 cell at 105 o. C Voltage range of 1. 5 V-3. 0 V (or 20 minutes operation time), discharge and charge currents are 50µA 16 100µA respectively, (FC hardware, electrode area – 1 cm 2, and ETEK cathode): The electrolyte is based on PEGDME 2000 and PC.
Oxygen starvation of a Na-O 2 cell at 105 o. C Voltage limits 1. 5 - 3 V and 1 min rest at OCV, charge and discharge at 100µA and 50µA, respectively. (FC hardware, electrode area – 1 cm 2, ETEK cathode): The 17 electrolyte is based on PEGDME 2000 and 10%PC.
Charge discharge cycles of sodium – air cell at 110 o. C Ch. at 100μA/cm 2, Dis. at 40μA/cm 2, Voltage limit 1 -4 V, time limit 0. 5 h (PE based) The problem: A rise of the charging voltage with cycle number.
Sodium plating and dissolution at above 100 o. C • In order to prove that sodium can be cycled in its molten state, we ran deposition–dissolution tests of sodium on aluminum at 110 o. C (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 1100 C (Na/SS cell, time range = 11350 min - 12000 min) =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/cm 2 following the use of thick cells, (Rcell = 1 to 10 Ohm. cm 2). Peroxide formation must be avoided (obtain a reversible 4 e ORR). Dendrite formation on charge must be avoided. Sensitivity to moist air and CO 2 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 100 o. C may solve or ease these problems: • • Accelerates sluggish cathode reactions and lowers cell impedance. Dendrites are not formed. It is easier to obtain a reversible 4 e ORR and avoid peroxide formation. Interference by water vapor and CO 2 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. 21
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 3 x 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
Sodium deposition-dissolution on Al at 105 o. C. Na/Na. Tf: PEO 6 + Methyl methanesulfonate 5%(wt)/Al cell. Discharge and charge rates are 50µA for 10 min and 25µA for 20 min, 25 respectively. Coin-cell hardware. Electrode area - 0. 57 cm 2.
Deposition and dissolution cycles of sodium on Al at 105 o. C Na/Na. Tf: PEO 6 + Methyl methanesulfonate 5%(wt)/Al coin cell. Discharge and charge rates are 50µA for 10 min and 25µA for 20 min, respectively. 26
SEI apparent thickness of 94Å is attributed to the second semi-circle of 1. 7µF capacitance and 330 Å to the first one. AC impedance spectra of Na/Na. Tf: PEO 6 + Methyl methanesulfonate 5%(wt)/Al cell 105 o. C. 27 After plating of Na on Al, frequency range - 10 MHz to 1 m. Hz. Electrode area - 0. 57 cm 2.
PEM FC stack 30
31 Honda FCX Clarity FC Stack 1. 75 k. W/l 1. 5 k. W/kg