Golden Boost: Nanoporous Gold Boosts Performance of Lithium-air-breathing Battery

Imagine a fully electric car running several hundred miles with a single charge and which does not require a new battery for several hundred recharges! It's still a dream, but scientists at University of St. Andrews in Scotland may have found just the thing, which can take us one step forward to achieving this. Here's an overview of their research that was recently published in the Science magazine [ref. 1].

Basics of Battery: A Quick Review

Figure 1: Block-diagram of a basic battery

Let's first recall the basics of an electric battery. A battery has a cathode (positive electrode) and an anode (negative electrode) separated by an electrolyte. When the battery discharges, a certain chemical reaction takes place. This reaction can be different for different types of batteries. As a result of this reaction the ions with positive electric charge from the electrolyte move towards the cathode or the ions with negative charge move to the anode or both. Thus we can get a current from the battery. When the rechargeable batteries are charged another chemical reaction takes place, which is generally a 'reverse' reaction of the one during the discharging process. This time the flow of the ions is also reversed and the battery regains its voltage. Many times a catalyst is also used to facilitate these chemical reactionsIn reality, during discharging and charging some unintended 'side-reactions' also take place, which reduce voltage of the battery after every discharge-charge cycle. This limits the number of times a battery can be recharged.

While designing a battery we intend to:

• Maximize the energy stored in a battery (measured in Watt-hour (Wh)) so that it can be used for the longest posible time;
• Minimize the side-reactions so that the battery can be recharged as many times as possible;
• Be able to recharge the battery fast i.e. in time much shorter than it takes for the battery to discharge. This is mainly a matter of convenience.

A particularly appealing rechargeable batteries are those having high energy storage capacity for the given voltage, large number of recharging cycles without reducing the capacity of the battery, high safety and low cost [ref. 2].

What's the presently-used technology?

Before getting to the recent results let's glance at the technology that is used at present in Hybrid Vehicles (HEVs) and Electric Vehicles (EVs). Ever since the research and development to manufacture HEVs and EVs began Lithium batteries have been the winning candidate having energy-storage capacity to power these vehicles to run long enough. Lithium batteries are not of one type, but of several. The technology that is used in HEVs and EVs at present are the Lithium-ion (Li⁺) batteries. These batteries use cathodes with metal-oxide or metal phosphates (manganese, iron or cobalt-based materials) and carbon-based anodes and a lithium-conducting electrolyte. During the discharging process the lithium ions flow through the electrolyte from the anode to the cathode forming a lithium compound. Charging the battery causes the reverse-reaction and hence reverses the ion-flow. Amount of energy that can be stored in the battery depends on the number of ions that can be stored. Li⁺ batteries can store a limited number of ions. Also during the charging process a number of side-reactions take place, which reduces the energy storage capacity of the battery thus limiting the number of times it can be recharged and reused. This prompted the scientists to look for new technologies to overcome the limitations of Li⁺ batteries.

What's the presently-researched technology?
The lithium-air (Li-O₂) batteries are found to outperform Li⁺ batteries on many of these grounds. Hence, they are a major candidate being researched for the next generation of HEV/ EV batteries.

Figure 2: A typical Lithium-oxygen battery. Researchers
need to look for the best combination of materials to enhance
the performance of the battery, at the same time, also
making it feasible for consumer-level production.
Image courtesy: University of Saint Andrews

A typical rechargeable non-aqueous (meaning it doesn't contain water) Li-O₂ cell is composed of a Li metal anode, a non-aqueous Li⁺ conducting electrolyte and a porous cathode. When the cell is used i.e. discharged, oxygen (O₂) from air is reduced at the cathode to oxygen ions (O₂^-) to combine with Li⁺ conducted through the electrolyte to form lithium peroxide (Li₂O₂). The reverse reaction takes place, when the cell is being charged, liberating O₂ into the air [ref. 1]. Hence, the name 'Lithium-air battery' or 'Lithium-air-breathing battery'.

Thus, if during discharge or charge, if any compounds other than lithium peroxide is formed then the capacity of the battery (time for which the battery can supply power) reduces. More are these side reactions, faster the battery will become useless.

Early research on Li-air batteries was focused on carbonate-based electrolytes, which also produce lithium formate, lithium acetate, etc. These undesired side-products cannot be converted back into lithium peroxide and so these batteries are of not much use for our purpose. Later ether-based electrolytes were tested, which were found to lead to little side-reactions. Although, the proportion of side-products formed due to ethers increased fast with the recharging cycles. Even when Nano-Porous Gold (NPG) electrode was used, only after 10 recharging cycles 20% of the products formed after discharging the cell were side-products formed due to decomposition of the electrolyte. This proportionally reduced the rechargeability of the cell.

The Latest Results
In the search of a better option a team of scientists at the University of St. Andrews, Scotland lead by Peter Bruce constructed a Li-O₂ cell with an electrolyte composed of 0.1 M LiClO₄ in dimethyl sulfoxide (DMSO) and NPG cathode. When the cell was operated at atmospheric pressure (so that the use of O₂ by the cell can be tested in natural environment), they observed excitingly significant improvement in the performance of the cell as compared to previously described cell-configurations.

The cell retained 95% of its initial capacity even after 100 recharging cycles. Also, when the products of the reactions in the cell were tested using techniques like Raman Spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray Diffraction (XRD) analysis it was found that more than 99% of the products after the discharge was lithium peroxide and less than 1% of lithium carbonate and lithium formate were created. Even after multiple recharging cycles no evidence of change in these proportions was observed. There was another possibility that is the side-products formed were gases and that's why they were not observed during test 'after' the discharge. So an analysis was doen using differential electrochemical mass spectrometry and no traces of any gases other than O₂, like carbon dioxide, sulfer dioxide or sulfer trioxide, were found to be involved with the discharge process. This test was also important to test the safety of this cell as the aforesaid gases can be hazardous to environment and health. When the same analysis was repeated during the charging process, only O₂ was found to be liberated. Also, Li₂O₂ was not observed in the products of the reaction during the charging process. This implies that the cell can regain all of its initial capacity on recharging and hence can be recharged without reduction in the capacity of the cell.

Hurray! A drastic enhancement in the performance of the battery over today's Li⁺ is achieved. But which of the materials used is responsible for this boost? The electrolyte or the NPG electrode? To find out answer to this question the scientists replaced just the LiClO₄ in the electrolyte by another compound of lithium represented by LiTFSI. There was no change in the performance. But when the nanoporous gold electrode was replaces by a carbon black electrode, the proportion of the undesired side-products formed increased from less than 1% to about 15%! Even a solid gold electrode gave inferior results than NPG electrode. So it's evident that the NPG electrode is mainly responsible for this  'golden boost' in the cell-performance. It was also found that the same size of this battery has a capacity to power a device 10 times longer than a battery with carbon-based electrode that is presently used in Li⁺ batteries (rate of 5000 mA/gm with NPG as compared to 500 mA/gm for carbon electrode with the same volume). This configuration of the battery not only has larger capacity, but it also requires much less energy to recharge than a battery with a carbon-based electrode.

Is this technology ready for use in cars?
As fascinating as these results are, this battery is cannot come out lab right now and fit into HEVs and EVs in the showrooms! Why? Simply because it uses 'gold' as an electrode. It's not just gold that makes it expensive, but also process of getting it into 'nano-porous' form adds to its cost. Also gold is almost 10 times heavier than carbon, meaning that the weight of this battery is 10 times that of a similar battery with carbon cathode! Although scientists acknowledged that this technology is not yet ready for consumer level production, they also said that if a similar enhancement can be achieved with e.g. a carbon electrode coated with a NPG, then the technology can be brought to use at the consumer level.

Along with this group at University of St. Andrews, several other places, like Massachusetts Institute of Technology, IBM, Argonne National Laboratory, University of Rome Sapienza and many more, are pursuing research on the lithium-air-battery technology.

Why is this research important?
The source of petroleum-based fuels is getting used-up fast. These fuels also add to the pollution, not to forget, to the global warming. In the light of this we are exploring the prospects of using renewable, clean energies like solar, wind, biofuels etc. Equally important is to develop the technology to store this energy more and more efficiently. If we can't store this energy in portable and efficient manner then it will not be possible to lessen the load on the petroleum-based fuels. Minimizing our dependency on the petroleum-based fuels is not something that can happen overnight or even over few months. It's a journey, in which the whole world needs to walk together and which can take decades before we reach our goal and hence every step, however small it is, like the one described in this post, has importance of a giant leap.

References:

1. Zhangquan Peng, Stefan A. Freunberger, Yuhui Chen, Peter G. Bruce: A Reversible and Higher-Rate Li-O₂ Battery, Science, July 19, 2012 (online) [DOI: 10.1126/science.1223985]
2. Bruno Scrosati, Jusef Hassoun, Yang-Kook Sun: Lithium-ion Batteries. A Look into The Future, Energy and Environmental Science, 2011, issue 4, pages 3287-3295 [DOI: 10.1039/C1EE01388B]