Rechargeable nonaqueous lithium-air (Li-O₂) batteries are of great interest as a next-generation high-energy-density battery technology that could put EVs closer to combustion-engined cars in terms of range parity.

Typical Li-O₂ cells are built with a Li-metal negative electrode; a nonaqueous Li⁺ electrolyte, and a porous positive electrode. During discharge, O₂ is reduced and combines with Li⁺ at the positive electrode, forming insoluble discharge products (typically Li₂O₂) that fill up the porous electrode.

The porous electrode serves as a conductive stable framework that hosts the reaction products. During charge, the previously formed discharge products must be removed to prevent the electrode pores becoming rapidly clogged with discharge products and products from unwanted side reactions, thereby suffocating the cell after a few discharge/charge cycles.

To date, a number of challenges have precluded the practical realization of Li-O₂ batteries.

• Reversible capacity (and thus energy density). Reversible capacity is determined by the pore volume of the porous electrode, which limits both the total quantity of the discharge products and how large the discharge product crystals can grow. Theoretically, the ultimate capacity is achieved in the extreme case in which large single crystals of the discharge product grow to occupy the full geometric volume of the positive electrode.

  The commonly used mesoporous Super P (SP)/Ketjen carbon electrodes have relatively small pore sizes and volumes, with their crystalline discharge products typically less than 2 μm in size; this in turn limits the capacity to < 5000 milli–ampere⋅hour per gram of carbon (typically <1.5 mAh based on 1 mg of carbon and binder). In addition, uses of smaller pores tend to lead to pore clogging, hindering the diffusion of O₂ and Li+ and causing high overpotentials during cycling.

• Side reactions. Severe side reactions can occur on cycling, involving the electrode materials, electrolyte, and intermediate as well as final discharge products. Major causes of these decomposition reactions include the superoxide ion that forms as an intermediate on reduction of oxygen, which readily attacks most electrolytes, and the large overpotential on charge, often required to remove the insulating discharge products, which results in the oxidation of some of the cell components such as the host electrode. Other studies suggest that 3.5 V (versus Li/Li⁺) represents the maximum voltage that carbon-based electrodes can tolerate without significant side reactions.

• Hysteresis. The large hysteresis seen between charge and discharge (up to 2 V) results in extremely low energy efficiencies, limiting the use of this battery in practical applications.

• Moisture and CO₂. LiO₂ cells are very sensitive to moisture and carbon dioxide. The more stable LiOH and Li carbonate phases are formed, which gradually accumulate in the cell, resulting in battery failure. Moisture and CO₂ also have deleterious effects on the Li-metal anode.

A number of strategies have been proposed to reduce the voltage hysteresis, involving the use of electrocatalysts, porous electrode structures, and redox mediators. Soluble redox mediators, such as tetrathiafuvalene (TTF) and LiI, have been used to reduce the overpotential of the charge process, resulting in the overall voltage hysteresis dropping to around 0.5 V. Their operation relies on the electrochemical oxidation of the mediator, which itself then chemically decomposes the Li₂O₂. The charge voltage is thus tuned close to the redox potential of the mediator. For discharge, the ethyl viologen redox couple has also been used to reduce O₂ in the liquid electrolyte rather than on the solid electrode surface, again to help prevent rapid blocking of the solid electrode surface by Li₂O₂. We used the redox mediator LiI and report a Li-O₂ battery with an extremely high efficiency, large capacity, and a very low overpotential. This battery cycles via LiOH formation, not Li₂O₂, and is able to tolerate large quantities of water. This current work directly addresses a number of critical issues associated with this battery technology.

—Liu et al.

The additive LiI has three roles in the new battery:

• It operates as a redox mediator whose redox potential can be tuned by using different electrolyte solvents and electrode structures; this redox potential guides the charge voltage and thus affects the cycling stability of the cell.

• LiI, together with water, affects the chemical nature and physical morphology of the discharge products, inducing the growth of large LiOH crystals that efficiently take up the pore volume of macroporous rGO electrodes; this is the origin of the observed large capacity.

• Third, it enables a chemical pathway to remove LiOH at low overpotentials.

The hierarchically macroporous rGO electrode is also an important factor for the high efficiency and capacity. Not only does the rGO framework provide efficient diffusion pathways for all redox active species in the electrolyte and hence, a reduced cell polarization and flatter electrochemical profile, it also permits the growth of LiOH crystals of tens of micrometers in size, resulting in a capacity that is much closer to the theoretical value of Li-O₂ batteries. These desirable features were not observed for Li-O₂ cells with mesoporous SP electrodes, even when the same electrolyte was used. The combination of electrolyte additives, the porous electrode structure, and the electrolyte solvent, synergistically, not only determines the chemical nature of the discharge product but also governs the physical size and morphology of it, playing a decisive factor in the capacity and rechargeability of the resulting Li-O₂ battery. In a broader sense, this work can inspire ways to remove other stable, detrimental chemicals, such as Li₂CO₃, which is relevant to cycling Li-air batteries in real practical conditions.

—Liu et al.

Electrochemical performance of the Li-O2 battery. Discharge/charge curves for Li-O2 batteries using rGO electrodes and a 0.05 M LiI/0.25 M LiTFSI/DME electrolyte with capacity limits of 1000 mA⋅h/gc (A), 5000 mA⋅h/gc (B), and 8000 mA⋅h/gc (C), as a function of rate (D); three cycles were performed for each rate in (D). The cell cycle rate is based on the mass of rGO; i.e., 5 A/gc is equivalent to 0.1 mA/cm2. Credit: Liu et al.Click to enlarge.

By precisely engineering the structure of the electrode, changing it to a highly porous form of graphene, adding lithium iodide, and changing the chemical makeup of the electrolyte, the researchers were able to reduce the voltage gap between charge and discharge to 0.2 volts. A small voltage gap equals a more efficient battery—previous versions of a lithium-air battery have only managed to get the gap down to 0.5 – 1.0 volts, whereas 0.2 volts is closer to that of a Li-ion battery, and equates to an energy efficiency of 93%.

The highly porous graphene electrode also greatly increases the capacity of the demonstrator, although only at certain rates of charge and discharge. Other issues that still have to be addressed include finding a way to protect the metal electrode so that it doesn’t form dendrites, which can cause batteries to short-circuit the battery.

Additionally, the demonstrator can only be cycled in pure oxygen, while the air around us also contains carbon dioxide, nitrogen and moisture, all of which are generally harmful to the metal electrode.

What we’ve achieved is a significant advance for this technology and suggests whole new areas for research—we haven’t solved all the problems inherent to this chemistry, but our results do show routes forward towards a practical device.

—Professor Clare Grey, senior author

The authors acknowledge support from the US Department of Energy, the Engineering and Physical Sciences Research Council (EPSRC), Johnson Matthey and the European Union via Marie Curie Actions and the Graphene Flagship. The technology has been patented and is being commercialized through Cambridge Enterprise, the University’s commercialization arm.

Resources

• Liu, T et. al. (2015) “Cycling Li-O₂ Batteries via LiOH Formation and Decomposition.” Science doi: 10.1126/science.aac7730