Why are sodium ion batteries so important?

Author: Eshaal Alavi 

Lithium ion batteries – the same ones we use in our phones – have been crucial in the transition towards cleaner energy. So far, they have been the best battery technology to store energy for electric vehicles (EVs), and for renewable energy (Trotta, 2022). However, to truly decarbonise our future, we would need a few hundred terawatt-hours of energy storage, worldwide (TEDx Talks, 2024). To put this into perspective, the US uses 1 TWh of energy in 1 hour. And, globally, we only have around 1 TWh of energy storage capacity – this stresses the need for advancements in battery technology. Future batteries must be scalable, sustainable and have longer lifetimes. 

Unfortunately, Lithium ion batteries (LIBs) include two major components which are on the list of EU 2020 critical materials – graphite and Lithium. Not only are these material scarce, but mining for these materials (when using raw graphite) is extremely damaging for the environment and human health (FutureBatt, 2024). In addition, LIBs can be quite dangerous to use. In recent years, most fires within energy storage power systems have been caused by LIB explosions (DNK, 2024). This makes these batteries, at times, unsafe, as well as unsustainable to continue using.

This has led to the emergence of alternative battery chemistries such as Sodium ion batteries (SIBs). Sodium, being an abundant material, is cheaper and is also more evenly distributed around the world. It is also safer to use because it is less reactive and therefore, less prone to fires or explosions. There have been a multitude of materials researched for LIB anodes, however, one that stands out in research is hard carbon (HC) (Hwang, 2017). The reason for this is that hard carbon can be made from a variety of precursors, including waste plastic and biomass, which can facilitate waste management (FutureBatt, 2024)

How is hard carbon made?

Hard carbon, unlike graphite, is an amorphous solid – it is disordered. Instead of neat layers of graphene stacked on top of one another (like a stack of paper), the layers are misaligned, folded and all over the place (like uneven, crumpled pages, stacked haphazardly(Meysami, 2022). The structure also contains a lot of defects (irregularities in the structure), which are crucial for the battery’s electrochemistry.

So how are these hard carbon anodes made? Most research so far has been focused on using biomass precursors to produce HC. For example, plant peels, coconut shells, apples, cotton, oatmeal, old furniture, textiles and even sewage sludge (Meysami, 2022; Bai, 2023)! Moreover, there are many different ways of synthesising HC (many of which are patented by companies), which result in different HC structures. One method that is often used is to first, heat the feedstock material (e.g. banana peel) in water, at a high pressure and temperature for a given duration of time – this is called hydrothermal carbonisation (HTC). After this, the material is heated in the absence of oxygen – this is called pyrolysis. Pyrolysis allows us to fine-tune the properties of the hard carbon.

In one study, it was found that increasing the pyrolysis temperature from 1000°C to 1900°C resulted in a more ordered HC structure, with more closely-packed, flatter layers (shorter interlayer distance) (Guo, 2023). It also affected the types of defects and the size of pores (holes), which were in the material. The overall balance of the interlayer distance, defects and porosity is what determined how well the sodium ions could move and be stored in the material. Now, even though the structure of the HC anode varied at different pyrolysis temperatures, it is still hard carbon. You can think of each element of the structure as a key ingredient of a particular dish. Depending on how much of each ingredient you add, you get the same dish, with a slight twist. All these ‘ingredients’ contribute to the hard carbon’s properties, such as its capacity to store sodium, its charge time and cycle life. Therefore, the microstructure of the HC is very important and it is determined by both the precursor used, as well as the method of synthesis. 

Another ‘ingredient’ that can adjust the HC’s structure is doping (Bai, 2023). This is a type of defect where a few Carbon atoms in the material are swapped with another light atom, such as Nitrogen or Sulfur. The exact effects of doping depend on the atom being used but it has been shown to enhance storage capacity, increase stability of the anode and increase charging rates.

However, the exact functions of different defects, type of pore and interlayer distance in affecting sodium ion storage and movement is still being investigated. It is still unclear the mechanisms through which sodium ions are stored and move through hard carbon, and why hard carbon is such an effective anode for sodium, compared to graphite.

 

Setbacks

Despite the numerous advantages of sodium-ion batteries, they face three significant challenges in replacing LIBs: low energy density, smaller charge capacity, and shorter cycle life (DNK, 2024). To put this into perspective, the energy density (energy stored per gram or kilogram) of SIBs ranges from approximately 100-150 kWh/kg, compared to the 120-180 kWh/kg typical of LIBs. Moreover, SIBs have a lower specific charge (charge stored per gram or kilogram – also referred to as charge capacity) of around 300 mAh/g compared to 372 mAh/g for LIBs. These two factors, result in more material used, to achieve the same energy output and capacity. Not only does this lead to higher long-term costs, but these batteries also demand larger space and weight capacity (which is especially unfavourable for EVs).

Another major disadvantage is that SIBs have a much shorter cycle life of around 2000 charge-discharge cycles before their charge capacity drops below a given threshold, whereas LIBs can last for around 3000 cycles (DNK, 2024). As a result, SIBs have to be replaced more often. This also leads to higher long-term costs and can lead to greater environmental harm (in producing the larger number of SIBs compared to LIBs, for the same duration).

The main reason for these setbacks is due to a gap in the volume of research between LIBs and SIBs. Although both SIBs and LIBs were being developed at around the same time (in the 1980s), the superior performance of LIBs stunted the research and development of SIBs. In fact, between 1990 and 2010, there was almost no research funding for sodium ion batteries (The Limiting Factor, 2024). However, the issues raised earlier with regards to Lithium ion batteries have allowed research to propel towards SIBs and HC anodes, in recent years. 

In my upcoming blog post, I’ll dive deeper into an environmental comparison between SIBs and LIBs.

 

 

References: 

  • FutureBatt (2024). New Battery Materials. Available at: https://www.futurebatt.org/new-battery-materials  (Accessed: 21 September 2024).
  • Guo, Z. et al. (2023) ‘Investigating the superior performance of hard carbon anodes in sodium-ion compared with lithium- and potassium-ion batteries’, Advanced Materials, 35(42), 2304091. doi: 10.1002/adma.202304091.
  • Hwang, J.-Y. et al. (2017) ‘Sodium-ion batteries: present and future’, Chemical Society Reviews, 46(12), pp. 3529–3614. doi: 10.1039/c6cs00776g.
  • Bai, X. et al. (2023) ‘Recent advances in anode materials for sodium-ion batteries’, Inorganics, 11(7), 289. doi: 10.3390/inorganics11070289. doi: 10.1002/adsu.202200047.
  • Trotta, F. et al. (2022) ‘A comparative techno‐economic and lifecycle analysis of biomass‐derived anode materials for lithium‐ and sodium‐ion batteries’, Advanced Sustainable Systems, 6, 2200047. doi: 10.1002/adsu.202100506.
  • Sheyan Meysami (2022). An overview of hard carbon as anode materials for sodium-ion batteries. Available at: https://medium.com/batterybits/an-overview-of-hard-carbon-as-anode-materials-for-sodium-ion-batteries-8db98fd60965 (Accessed: 21 September 2024).
  • DNK (2024). Will Sodium Batteries Replace Lithium Batteries. Available at: https://www.dnkpower.com/will-sodium-batteries-replace-lithium-batteries/  (Accessed: 21 September 2024).
  • TEDx Talks (2024). How can we make better batteries? |Dr. Shirley Meng |TedxChicago. [Online video]. Available at: How can we make better batteries? | Dr. Shirley Meng | TEDxChicago (Accessed: 22 June 2024)
  • The Limiting Factor (2024). Professor Shirley Meng: Sodium Ion Batteries // Deep DIve. [Online video]. Available at: Professor Shirley Meng:  Sodium Ion Batteries // Deep Dive .(Accessed: 22 June 2024)

 

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