Okay, not quite sea salt. But sodium, the 6th most common element on Earth, and regular feature at the dinner table (as sodium chloride table salt, don’t eat sodium metal!), could soon make batteries cheaper, and more sustainable.1–3
In modern batteries, a positive ion (an atom with less electrons than it would like, such as Li+ lithium, or Na+ sodium) moves backwards and forwards between the positive and negative ends of the battery (the electrodes) like a rocking-chair.4 When this happens, an electron also moves across the battery, but through an external circuit. The energy carried by these electrons is what powers anything that uses batteries, from smart phones, to electric cars, and even parts of the power grid!
So, if lithium is already good enough for all of this, you might ask “why bother with sodium?”. After all, if it’s not broke…
As of March 2022, the average ‘affordable’ electric vehicle cost £27,000, with the battery alone making up 30% (~£8000) of this.5 In the UK, the sale of new petrol and diesel vehicles will be banned from 2030, putting a ticking timer on cost-reducing developments, or risking that many cannot afford to travel.6 Recent events have unfortunately also demonstrated the impact on global supply lines that can result from resources with poor geographical distribution. Given that almost half of the world’s lithium comes from a single country, it’s easy to see how sodium (which is around 23,000 times more abundant) may be more reliable in the long-term transition to a fossil fuel-free economy.1,7
Why don’t we switch to sodium today? As anyone with an old smartphone will know, as batteries get older their life gets shorter and shorter, until they die (reach the end of useful life). Whilst this is inconvenient for smartphones and laptops, the problem becomes even more pronounced for electric vehicles (less miles per charge) or the grid, where failure of the battery could lead to power outages. The same ‘rocking-chair’ process that enables modern batteries to work, also causes the electrodes to break down over time due to swelling, cracking, and side-reactions. Sodium is larger, and more reactive than lithium, making these effects more severe; currently, sodium-ion batteries degrade too quickly to be useful.
That’s where we come in! Sodium cathodes (positive electrodes) closest to commercial viability rely on complex mixtures of transition metals in layered oxides, like Na0.95Ni0.317Mn0.317Mg0.158Ti0.208O2 (I know…).3 The exact compositions of these are (arguably) the result of trial and error (and countless hours of work by countless scientists!). In the Grey and Dutton groups, we’re investigating the nano-scale (local), and large-scale (bulk) crystal structure of layered sodium transition metal oxides, using techniques like: nuclear magnetic resonance, X-ray diffraction, and SQUID magnetometry. We watch (sometimes in real time) how the material changes as the battery is charged/discharge, letting us understand how tiny structural changes influence electronic and magnetic properties.
Through linking nano/microscopic material changes, to macroscopic charging characteristics, the group hopes to help develop the next generation of batteries, to power a sustainable future.
References:
1 J. Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, Oxford, New York, Second Edition., 2011.
2 A. M. March 2022, Battery boom time, https://www.chemistryworld.com/news/battery-boom-time/4015234.article, (accessed 10 June 2022).
3 A. Rudola, A. J. R. Rennie, R. Heap, S. S. Meysami, A. Lowbridge, F. Mazzali, R. Sayers, C. J. Wright and J. Barker, J. Mater. Chem. A, 2021, 9, 8279–8302.
4 A. Mauger, C. M. Julien, J. B. Goodenough and K. Zaghib, J. Electrochem. Soc., 2019, 167, 070507.
5 Average Cost of an Electric Car UK 2022, https://www.nimblefins.co.uk/average-cost-electric-car-uk, (accessed 10 June 2022).
6 Government takes historic step towards net-zero with end of sale of new petrol and diesel cars by 2030, https://www.gov.uk/government/news/government-takes-historic-step-towards-net-zero-with-end-of-sale-of-new-petrol-and-diesel-cars-by-2030, (accessed 10 June 2022).
7 Worldwide number of electric cars, https://www.statista.com/statistics/270603/worldwide-number-of-hybrid-and-electric-vehicles-since-2009/, (accessed 7 November 2021).
NanoDTC PhD Student, c2021