...the requirements for large capacity automotive propulsion batteries are extensive, but quite well defined. They will serve as guidelines for the research to be carried out on Li-air systems. At present, automotive propulsion batteries are just beginning the transition from nickel metal hydride to Li-ion batteries, after nearly 35 years of research and development on the latter. The transition to Li-air batteries (if successful) should be viewed in terms of a similar development cycle.

—Girishkumar et al.

Basics. The energy density of gasoline is approximately 13,000 Wh/kg. With a current average tank-to-wheel efficiency of 12.6%, the usable energy density of gasoline in an automotive application is about 1,700 Wh/kg.

Since the efficiency of electric propulsion systems (battery-to-wheels) are about 90%, a 10-fold improvement of the current energy densities of Li-ion batteries, which are typically between 100 and 200 Wh/kg (cell level), would bring electric propulsion systems on-par with gasoline, at least as measured by gravimetric energy density. However, there is no expectation that current batteries such as Li-ion will ever come close to the target of 1700 Wh/kg. New chemistries are required to achieve this goal.

The oxidation of 1 kg of lithium metal releases 11,680 Wh/kg, not much lower than that of gasoline...However, practical energy densities for Li-air batteries will be far less. Existing metal-air batteries, such as Zn/air, typically have a practical energy density of about 40-50% of their theoretical density. However, one can safely assume that even fully developed Li-air cells will never achieve such an excellent ratio, because lithium is very light, and therefore the overhead of the battery structure, electrolytes, and so forth will have a much larger impact.

Fortunately, an energy density of 1700 Wh/kg for the fully charged battery corresponds only to 14.5% of the theoretical energy content of lithium metal. It is not inconceivable that such an energy density, at the cell level, may be achievable, given intensive and long-term development work. The energy density of the complete battery system may be only half of the density realized at the cell level.

—Girishkumar et al.

Considerations for Li-air systems include:

• Power density and cost. While Li-air systems offer the promise of very high energy densities, their power density is currently very low.

  Prototype aprotic Li-air cells deliver current densities in the order of 1 mA/cm². It will be critical to increase this current density by at least 1 order of magnitude. Even then, the macroscopic surface area to supply the total power for a propulsion battery is very large. For example, a battery with 100 kW power output at a cell voltage of 2.5 V and a current density of 25mA/cm² will require a total internal surface area of 160 m², equal to the internal surface of the human lung.

  —Girishkumar et al.

  One way around the power issue would be to utilize a hybrid system where a small capacity but high power battery, for example, provides power for short periods of high demand, such as during acceleration, the authors suggest.

• Electrical energy efficiency. Current Li-air cells have a charging voltage that is considerably higher than the discharge voltage (overvoltage). This corresponds to a low cycle electrical energy efficiency, currently on the order of 60-70%, the authors note. Practical propulsion batteries should exhibit “round-trip” energy efficiencies of 90%. The detailed mechanisms underlying these high over voltages are currently not understood.

• Lifetime and Cyclability. Current Li-air cells have been demonstrated with up to about 50 cycles with only moderate loss in capacity. Therefore, the authors suggest, future research efforts need to focus on improving the capacity retention during cycling.

• Safety. Typical thermal runaway of a Li-ion battery due to overcharging or internal shorts is not a possibility in Li-air batteries because of the rate-limited surface nature of the reaction, i.e., the reactant O₂ is not stored in the battery.

  However, there are two other safety concerns to be considered. First, the desired, though not mandatory, use of lithium metal anodes is a well-known safety problem, since lithium metal tends to form dendrites, which can short-circuit the battery and react aggressively with many contaminants. Second, the presumed dominant reaction product of aprotic cells is Li₂O₂, which is a strong oxidizer. Combined with an organic electrolyte, this could lead to safety issues in an accident. However, preliminary experiments at IBM indicate that no thermal exothermic reactions between Li₂O₂ and common electrolytes occur at temperatures below the melting point of lithium metal (180 °C). This safety concern does not exist in aqueous cells.

  —Girishkumar et al.

Architectures. There are currently four chemical architectures for Li-air batteries under investigation globally, including three versions with liquid electrolytes—a fully aprotic liquid electrolyte; an aqueous electrolyte; and a mixed system with an aqueous electrolyte immersing the cathode and an aprotic electrolyte immersing the anode—and an all-solid-state battery with a solid electrolyte. Only the aprotic configuration of a Li-air battery has shown any promise of electrical rechargeability; hence, this configuration is attracting the most effort to date, according to the authors.

The fundamental electrochemistry—which is not fully understood in detail—depends upon the electrolyte around the cathode, the authors note.

Schematic operation proposed for the rechargeable aprotic Li-air battery. During discharge, the spontaneous electrochemical reaction 2Li+O2→Li2O2 generates a voltage of 2.96 V at equilibrium (but practically somewhat less due to overpotentials). During charge, an applied voltage larger than 2.96 V (~4 V is required due to overpotentials) drives the reverse electrochemical reaction Li2O2→2Li + O2. Credit: ACS, Girishkumar et al. Click to enlarge.

Aprotic Li-air battery. A “typical” aprotic design would consist of a metallic lithium anode, an electrolyte comprising a dissolved lithium salt in an aprotic solvent and a porous O₂-breathing cathode composed of large surface area carbon particles and catalyst particles, bound to a mesh using a binder.

In addition to a detailed discussion of the possible dynamics of the electrochemistry of the cell, the authors note that there are other issues that need to be addressed, including the lithium anode, as well as the question of Li-air or Li-O₂ batteries—e.g., whether or not to deploy a membrane that selectively permeates O₂ to avoid unwanted parasitic reactions with components such as water, carbon dioxide, carbon monoxide and nitrogen in ambient air.

Recommended key research. To develop and commercialize a practical, rechargeable Li-air battery, the authors recommend research in the following key areas:

1. Quantitative understanding of the electrochemical reactions and their relationship to the discharge/charge currents. This is the key to quantitatively demonstrating chemical reversibility and understanding Coulombic efficiency of the battery in cycling.

2. Development of oxidation-resistant electrolytes and cathodes that can withstand high oxidation potentials in the presence of O₂. This is also essential for chemical reversibility and Coulombic efficiency in the battery cycling.

3. Understanding the nature of electrocatalysis for Li-air batteries where insoluble products are formed and the development of cost-effective catalysts to reduce overpotentials for the discharge and charge reactions. This is key to enhancing power density in discharge, electrical efficiency in a discharge-charge cycle, and ultimately in cycle life (due to possible electrolyte oxidation).

4. Development of new nanostructured air cathodes that optimize transport of all reactants (O₂, Li⁺, and electrons) to the active catalyst surfaces and provide appropriate space for solid lithium oxide products. This is required to maintain capacity at higher power densities. A new realization is that minimizing difficulties due to electron transport through the lithium oxide solid products in the cathode is important.

5. Development of a robust lithium metal or lithium composite electrode capable of repeated cycling at higher current densities. This will most likely require development of a protective layer that limits the deleterious effects of environmental contamination on the lithium and inhibits dendrite growth.

6. Development of high throughput air-breathing membranes (or other mechanisms) that separate O₂ from ambient air in order to avoid H₂O, CO₂, and other environmental contaminants from limiting the lifetime of Li-air batteries.

7. Understanding the origin of the temperature dependencies in Li-air batteries and minimizing their adverse effects.

Resources

• G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson and W. Wilcke (2010) Lithium-Air Battery: Promise and Challenges. J. Phys. Chem. Lett., doi: 10.1021/jz1005384