Lithium-Air...the ‘Ultimate’ Battery?
by Donald Georgi
Life’s quest for the technological holy grail takes on many different aspects. To fly... to eliminate polio...to go to the moon, some of these pursuits have been achieved. But people are never satisfied and the quest continues. Battery people, utilizing technology, have their moments to hope, dream and pursue greater energy densities. Having been brought up in the era of nuclear energy, the technologist envisions the day when a battery the size of a pack of chewing gum will power a computer for years. While batteries do make modest gains in energy stored, technologies such as cell phones expand to capture the dollar of the consumer by adding features which require greater power, but result in devices with micro-sized, dim displays to match almost unuseable, tiny keyboards.
Thinking beyond the device of today, some see an ‘always-connected’ person who has the combination of computer, cell phone, PDA and entertainment center in a single device ready to make the business deal, cure the illness and make the next hotel reservation while living in a virtual reality of a galaxy far, far away. Forgetting the electronic challenges of this magic module, the energy storage device necessary to provide this power, with small package size to satisfy the user, lies within this holy grail.
As we are jolted back into the real world bounded by physical laws which encompass electrochemistry, battery people have to ask, “Where is the upper limit of energy density?” Despite extensive knowledge of chemical fundamentals, that question has not been, and may never be, completely answered because the realm of practicality must be included. The practical answer includes acceptable safety, a dependence on the application, the environment of use, the economic factors and the admission that the battery for one application may not be the best choice for another.
Just as the mountain climber would look to Mt. Everest as the pinnacle of peaks, an electrochemist, browsing the list of metals’ reduction potentials and electrochemical equivalents, sees one possibility in the combination of lithium with oxygen. Lithium metal, excluding oxygen, has the highest standard potential and electrochemical equivalence of all metals.
Between theory and practice is a reduction of that value which can be anywhere from 2:1 for Lithium-thionyl chloride to 7:1 for Lead-acid. Stated in another way, only a part of the theoretical maximum energy density can be recovered in a practical manufactured battery. The challenge is to get the most practical energy from the highest theoretical combination.
To that end, using a lithium anode with an air cathode to supply the oxygen (as is commonly done with the very popular Zinc-air hearing aid batteries) may result in the highest practical energy density possible in a metal-based battery which has an abundant air supply, environmental friendliness, and reasonable safety. Since the anode is lithium metal which reacts aggressively with water, a nonaqueous electrolyte is used with an organic polymer film separator to facilitate the supply of oxygen from the air. The cathode consists of a metal current collector surrounded by a layer of carbon which provides the platform for combining the oxygen with the lithium ion which moves from the electrolyte to form lithium peroxide or lithium oxide. Electrolytes can either be non-aqueous liquid or polymer electrolyte.3
The description of Lithium-air sounds so ideal that we might wonder why we are still stuck with the myriad of chemistries with ‘inferior’ energy densities. To better understand why it is not universally used, an understanding of the previously mentioned suitability criteria which includes the application, the environment of use, economics and practicality is needed. Much has been learned in how to build Zinc-air cells; some of this knowledge can be applied to the design of Lithium-air chemistry.
Reversability of the reaction to allow electrical recharge of Lithium-air is possible. Despite classifying the Lithium-air cell as a primary battery, the literature does include data on the performance of a rechargeable form, researched by Abraham, et. al.2 When the liquid electrolyte is replaced with a polymer electrolyte, the reaction of the lithium directly with oxygen forms lithium peroxide which can reoxidized to oxygen with externally applied current. This has been found to have major barriers in Zinc-air, so starting with the primary version of Lithium-air simplifies the goal tremendously.
As noted in the Zinc-air experience, a virtually unlimited amount of ambient air can be used to supply the oxygen, but as a result, it also adds the limitation of convenience limiting the operating life of about two weeks after exposing the cathode material to the air. Unlike Alkalines, which just ‘sit there’ when not used for days weeks or years, the Lithium-air battery cannot be put into a standby mode conveniently. The solution here is to choose the application which properly suits the continuous period after activation.
Very low power density is another constraint of the Lithium-air battery. Unlike the high power providers of chemistries such as Lead-acid, current densities of Lithium-air can be as much as 1,000 times lower in order to extract the maximum amount of energy. Low current may not be a problem if the application is tailored to the capability, but one does not look upon Lithium-air as a replacement car starting battery.
The problem of temperature range must be considered again because the performance of Lithium-air varies by a factor of 5 over the -20 0C to +40 0C range. It is important to note that the battery must be tuned to the application because Lithium-air batteries are not going to start Minnesota autos in January.
Rather than speculate on the future of Lithium-air batteries, the work being done now is a good indicator of where the technology stands and may suggest when a practical version may be built. The two presentations which describe this work were given at the 41st Power Sources Conference and are highlighted as follows:
Session 4.3 Non-aqueous Lithium-Air Batteries with an Advanced Cathode Structure. Yardney Technical Products, Inc./Lithion, Inc.
Using non-aqueous electrolytes removes the problem of self-discharge parasitic corrosion due to lithium interaction with water. Non-aqueous electrolytes also remove the production of hydrogen, thus yielding a safer cell. The next barrier to maximizing performance is the low rate of oxygen diffusion in the air cathode. Taking experience from zinc and aluminum metal air batteries, new cathode structures were fabricated using a variety of metal catalysts added to a carbon with binder and deposited on a nickel current collector.4 A teflon film was placed over the structure to repel atmospheric water but to allow oxygen diffusion. Of all the catalysts, manganese was noticeably superior, extending the specific capacity of the air cathode to 3471 mAh/g. (Note: this is for the cathode only.) The carbon material provides the reactive surface for the reduction of the oxygen gas and must have a porous structure with highly distributed catalyst materials.
Several cells were constructed and tested at constant currents at 20 0C, yielding specific capacity of 3471 mAh/g. (Ed. note. With an average potential of 2.47 Volts, this would equate to 8573 Wh/kg. Assuming practical packaging requirements to triple the mass, a real battery might produce 1157 Wh/kg, which would be a wonderful achievement.)
4.4 Temperature Performance of the Non-aqueous Lithium/Air Battery. U.S. Army Research Laboratory, Adelphi, MD
Seeking combinations of battery support for the Land Warrior has opened up the possibility of including Lithium-air batteries. If radio batteries carried by the troops have rechargeable Lithium-ion batteries, they can be recharged for the next mission by a variety of sources which might not always be a large, heavy and noisy power source.
If Lithium-ion cells had energy densities approaching 200 Wh/kg, and Lithium-air cells had energy densities of 1000 Wh/kg, then 5 field packs could be recharged from a single Lithium-air battery weighing the same as one radio battery. Extending the concept, a combination (hybrid) Lithium-air and Lithium-ion might work together so that the Lithium-ion provided the talk power, and the Lithium-air constantly kept topping off the Lithium-ion resulting in total usability time increases and reduction in pack weight to carry.
But the Army realizes that major obstacles exist for Lithium-air, especially in the area of temperature range. The present study looks at liquid electrolyte and the carbon black coated anode current collector. Over temperature ranges from -30 0C to +40 0C, the cells were discharged at constant currents from 0.05 to 0.5 mA/cm2. Cells operated at +40 0C gave nominally 10 times more specific capacity than those at -30 0C.
It is projected that the temperature affects the diffusion of oxygen through the electrolyte. It is anticipated that future work should separate the effects of oxygen solubility from oxygen diffusion. The reformulation of the electrolyte may be the way to improve low temperature operation. Until then, Lithium-air will remain in the study category for the Army.
1. Handbook of Batteries, Third Edition. pp. 1.12-1.13
2. Handbook of Batteries, Third Edition. pp.38.49-38.51
3. “A Polymer Electrolyte-Based Rechargeable Lithium/Oxygen Battery,” Abraham, K.M. J. Electrochem Soc., 1996, 143. 1-5
4. “High Capacity Cathodes for Lithium-air Batteries,” Yardney TechnicalProducts.