Imagine if Elon Musk met a genie who could grant him any one wish, what would he wish for; especially when even genie can’t take him to Mars; then definitely he would wish to have a battery pack with energy density similar to that of gasoline; because when more than perfectly designed Tesla cars are equipped with an ideal battery pack which could store huge amount of energy it would completely change the way we see electric vehicles today.
Battery technologies have undoubtedly become the Holy Grail of the EV industry, just as energy storage is the Holy Grail of renewable power generation/Integration. There are different types of batteries but Lithium-ion batteries are the most popular batteries existing in the market powering our consumer electronics and EVs successfully. The specific energy of a modern lithium-ion battery is about 220-240 Wh/kg which is still not good enough to compete with gasoline powered cars making EVs less viable option for long distance travels. Engineering lighter, more powerful batteries with higher energy density would definitely be a game changing innovation in battery technology systems. One such possibility is to replace modern lithium-ion batteries with so-called lithium-air batteries. Lithium-air battery would be able to accumulate five times more power than lithium-ion technology. This could have a major impact on electric cars market, which now uses lithium-ion batteries.
A new kind of battery systems called Lithium-air battery has created lot of excitement among the battery technology researchers because if Lithium-air battery works then energy density close to that of gasoline could be achieved. The idea of Lithium-air battery occurred accidently to chemist K M Abraham while testing a battery cell having a small leak in his laboratory in 1995, which provided the cell with far higher energy content than expected. Rather than try to fix the leak, he investigated and discovered the first rechargeable lithium-air (Li-air) battery. Lithium-air battery comprises of a metal electrode which is lithium in this case, electrolyte which could be either aqueous or non-aqueous, and a bi-functional air electrode. Lithium-air Battery works by electrochemically reducing O2 from air and oxidizing the metal electrode to reversibly form solid lithium-oxides. In this way, both the volume and the weight of the battery can be significantly reduced compared to Li-ion systems.
So far, we have understood how Lithium-air battery could work and yield five times more energy density than commercial lithium-ion battery technology, but at the moment it’s quite challenging to recharge a lithium-air battery more than a few times due to the oxidation of the lithium anode and production of undesirable byproducts on the cathode that result from lithium ions combining with carbon dioxide and water vapor in the air. These byproducts gum up the cathode, which eventually becomes completely coated and unable to function. These experimental batteries have relied on tanks of pure oxygen — which limits their practicality and poses serious safety risks due to the flammability of oxygen.
Successful Customized Lithium-air Cell at University of Illinois at Chicago
Researchers at University of Illinois at Chicago developed a custom-made cell using a MoS2 cathode, a protected lithium anode and an EMIM-BF4/DMSO (25%/75%) electrolyte in the lithium–air experiments. This electrolyte composition provides the maximum oxygen reduction and evolution in a three-electrode electrochemical cell. A custom-made simulated air stream of around 79% N2, around 21% O2, 500 p.p.m. CO2, and a relative humidity of 45% at 25 °C was used for the battery experiments. After longterm discharging and charging profiles up to a capacity of 500 mAh g−1 with a constant current density of 500 mA g−1 were observed. The charge at the first cycle began at 2.92 V, which is very close to the reversible thermodynamic potential of Li2O2 formation (2.96 V versus Li/Li+) and reached a potential of 3.75 V at a capacity of 500 mA g−1. The potential gap for the first cycle of the lithium–air system is 0.88 V, increasing to 1.3 V after 50 cycles, followed by a gradual increase to 1.62 V after 550 cycles. The increase in the potential gap during cycling may be due to slow degradation of the protective anode coating and/or the MoS2 cathode. However, no failure was observed in the battery during testing up to 700 cycles. The results indicate a substantial increase in the number of lithium–air cycles achieved when the anode is protected compared with when it is not; with no coating, the lithium–air cell fails after 11 cycles, whereas up to 700 cycles can be achieved with an anode-protection layer .
Extensive R&D in following areas could make Lithium Air battery a practical reality:
- Optimization of Air Cathode Structure
- Selection of Appropriate Electrolyte
- Construction of Laboratory Cell Prototype
Optimization of Air Cathode Structure
The limiting factor in this system, and nearly all of the metal air batteries, is the air cathode. The performance of the lithium-air battery has been limited by a low rate of oxygen diffusion in the porous cathode. Recognizing that improving the cathode structure is the key to increasing the energy density of the Lithium-air battery.
An effective air electrode would need to present a much shorter diffusion path for oxygen and offer the largest possible surface area for Li2O2 deposition. These requirements call for an open porous conductor structure using nanostructured materials. Materials used as cathode supports comprise porous carbon, graphene, carbon nanotubes (CNT) or carbon nanofibers (CNF) with catalysts such as metal oxides.
Selection of Appropriate Electrolyte
The requirements for the electrolyte in the lithium– air system are as follows:
- Stable with lithium metal
- A high oxidation potential
- A low vapor pressure and high boiling point
- A high lithium salt solubility and a good chemical stability.
Construction of Laboratory Cell Prototype
The main problem lies in making a system that is sufficiently shielded from the external while ensuring an oxygen flow in and out of the device at the same time.
The first design by Abraham and Jiang consisted of a pouch cell where a small aperture on the cathode side allowed for uniform oxygen flow. Others have resorted to a modified CR coin-cell design, by perforating the cathode metallic cover with a series of pinholes and then enclosing the cell in an oxygen-filled plastic bag. A similar concept has been applied to Swagelok cells, where oxygen is provided into the system by flowing it through a perforated cathode. The first problem with such designs is the volatility of the electrolyte, which is free to evaporate during cycling and storage. Secondly, the use of such electrolytes brings up the safety hazard posed by the coexistence of reactive lithium and a pure oxygen atmosphere in contact with a flammable organic electrolyte.
There is no doubt that the Lithium-air battery technology is quite promising but at the same time this technology has lot of challenges which need to be overcome before it becomes a viable and commercial reality. It would be too early to say now whether lithium-air batteries will be cheaper or more expensive than lithium-ion ones. We could assume that they will be cheaper. But the problem is often wrapped in details. It could be possible that in order to solve rechargeability problem, we’d have to add some very expensive additives. Many experts believe that we won’t get any prototypes till between 2020 and 2025.
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