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The urge for better batteries [part 2]

Part II: Active materials  

In Part 1 of this article series, we learned that there is huge potential for battery electric vehicles (BEVs) to take over a significant part of the passenger car industry soon. However, this revolution hinges on batteries becoming safer, more energy dense, more environmentally friendly and—also very important—less expensive. Here, we’ll highlight a number of revolutionary active materials which, in the near future, can replace the conventional materials currently used in lithium-ion batteries.

So-called ‘active materials’ are responsible for the actual charge storage, and are situated at the battery’s electrodes. Anode and cathode each use a different chemical compound as active material. In state-of-the-art lithium-ion batteries, for instance, graphite and nickel-cobalt-manganese-oxide (NMC) are the two most popular materials of choice for the anode and cathode, respectively.

Now, let’s take a look at the new materials in the pipeline.

Anode

Silicon: The appealing feature of silicon is its high theoretical capacity (~4000 mAh/g) compared to graphite (~380 mAh/g). This means that Si anodes could potentially weigh 10 times less than a graphite anode with the same capacity. In other words, the capacity of a graphite electrode is one tenth the capacity of a silicon anode of the same mass. Notwithstanding this latent benefit, pure silicon anodes are not commercialised yet due to their poor stability during long-term cycling. The good news, however, is that some graphite and silicon mixed anodes have recently been introduced to the market using <5%wt of silicon monoxide.

 

Lithium: Just like silicon, lithium metal has a very high capacity (~3860 mAh/g) but also a very low redox potential. A lower redox potential is a huge plus for an anode since the overall voltage of the cell is directly proportional to the difference between the redox potentials of the cathode and anode. A battery with a higher operating voltage can deliver a higher amount of energy during discharge (i.e., energy (Wh)=capacity(Ah)*voltage(V)). Safety is the biggest limitation to lithium anodes penetrating the market of high-energy rechargeable batteries. The growth of lithium dendrites is the main culprit of high safety threats during the charging/discharging of lithium-based batteries. Such sharp tips (dendrites) are formed, and continuously grow, due to a preferential deposition of lithium during charging events. Once a dendrite reaches the other side of the battery (i.e., the cathode), a short-circuit happens!    

Cathode

Sulfur: The low cost, high natural abundance, and high capacity (~1680 mAh/g) of elemental sulphur has inspired the battery community to replace state-of-the-art cathode materials with sulphur-based electrodes. The transition metal-based cathodes in state-of-the-art lithium-ion batteries contain expensive and scarce elements such as cobalt. Therefore, active materials such as LCO, NCA and NMC cost more than €20/kg, which is significantly more expensive than elemental sulphur, i.e., €0.20/kg. Life-cycle assessment studies predict a better environmental footprint during production and a lower waste outflow for lithium-sulphur batteries compared to lithium-ion technology, i.e., 9-90% lower impact. In order to leverage these merits, however, it is important to increase the lifetime of sulphur electrodes by at least a factor of 5.

Electrolyte

Solid electrolytes: The electrolyte in state-of-the-art lithium-ion batteries is liquid and has a potential stability window of approximately 1-4 V. However, the flammable nature of the liquid solvents used during the preparation of these electrolytes, such as ethylene carbonate, raises safety concerns. Additionally, there are usually high expenses in the production phase due to the sensitivity of such conventional liquid electrolytes to moisture: some production steps need to take place inside a dry room (i.e., H2O<<500ppm). The prospect of a solid electrolyte could mean a huge improvement for battery safety, since it does away with flammable liquid solvents. However, the search for a promising solid electrolyte candidate with a decent ionic conductivity at room temperature, and high contact area with electrodes, is still ongoing. Meanwhile, their application is limited to thin film batteries (with very low capacity) where a solid electrolyte, such as LiPON, with an ionic conductivity of 0.001 mS/cm is usually used.

Conclusion

As you can see, there is still a lot of work to be done. But there are also lots of opportunities for chemists and material researchers to come up with materials that tick off all the boxes, qualifying them to end up in a new generation of batteries. Once they do, however, it could mean a huge boost for the BEV market.

References:

  1. W. Choi & D. Aurbach, Nat. Reviews. (2016) 1.

 

Author

Mohammadhosein (Momo) Safari – Associate Professor, Department of Engineering Technology, Hasselt University & EnergyVille, Thor Park 8310, BE-3600 Genk, Belgium

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