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With the skyrocketing demand of lithium-ion batteries driven by global net zero policies and the shift towards electric vehicles and renewable energies, the pressure on battery raw materials has significantly increased. Supply shortages in the coming decades for battery materials such as lithium, cobalt, and nickel have been predicted. Technological diversification is key to counteract these risks. Are sodium-ion batteries the solution?
Sodium-ion batteries are a type of rechargeable battery just like the commonly known lithium-ion battery, and their overall working principle, set-up and materials are similar to lithium-ion batteries. Instead of reversibly intercalating lithium in the electrodes, sodium-ion batteries rely on sodium as the mobile charge carrier. The graphite anode known in lithium-ion batteries is a hard carbon anode in sodium-ion batteries. And instead of cathode materials such as LiFePO4 and LiNiMnCoO2 used in lithium-ion batteries, sodium-ion batteries use, for example, Na3V2(PO4)2F3 and NaNiMnMgTiO2.
Sodium-ion batteries might not replace all lithium-ion batteries on the market, but will be applied for certain applications, such as large-scale grid or smaller transportation modes, where high energy densities are not necessarily required. Nevertheless, they will alleviate some of the raw materials pressures and supply chain risks that come with lithium-ion batteries. This means sodium-ion technology will have a lasting place in the energy transition, and we need to handle it accordingly. Besides materials, cost, lifetime and safety, we need to take a closer look at the sustainability of sodium-ion batteries.
Sodium, for example, is ubiquitously available and will resolve the dependence on a small number of lithium markets currently experienced by battery OEMs. However, sodium-ion batteries also contain critical and toxic materials such as vanadium in Na3V2(PO4)2F3 cathodes. Analyses of the future sodium-ion battery market can help predict potential shortfalls and supply chain risks and help prepare for these.
The cost of sodium-ion batteries is another key point. It would be lower on a per kilogram (kg) basis compared to lithium-based chemistries, mostly due to avoiding lithium and cobalt. Whilst this is indeed advantageous from an OEM and consumer perspective, it raises questions about the end-of-life treatment. If the materials in sodium-ion batteries have only little value, would it be profitable to recycle them? These concerns would need to be addressed in the near future and legislature similar to lithium-ion battery recycling policies would need to be put in place to ensure a safe and eco-friendly end-of-life treatment.
Similar to the early days of lithium-ion batteries, the answer to this question is not simple. Sodium-ion batteries are just at the brink of commercialization – Faradion has been ahead of the game, and other companies such as CATL and Tiamat are following. However, large amounts of data are needed to accurately and precisely evaluate the battery lifetime under various conditions, applications, and use patterns in the same way big battery data is collected and analyzed for lithium-ion batteries. Although knowledge transfer from lithium-ion technologies could be helpful, in the end, sodium-ion batteries rely on different materials, which impact chemical reactions and degradation patterns.
Sophisticated battery degradation models already applied to lithium-ion batteries, such as those developed by ACCURE, and early-on data collection of the cycling behavior of batteries in use will accelerate the understanding and prediction of the lifetime of sodium-ion batteries. The resulting knowledge can then be used by battery developers to develop rational design strategies for sodium-ion materials with high cycle lifetimes.
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While it is often claimed that sodium-ion batteries are safer than lithium-ion batteries, only little data is currently available on the potential safety risks of sodium-ion batteries. The use of flammable organic electrolytes carries the same safety risk as lithium-ion batteries. Some research groups have started looking at the effect of thermal abuse of sodium-ion batteries, and it has been shown that during a thermal runaway, flammable gases from the electrolyte are emitted. However, the use of different cathode and anode materials in sodium-ion batteries might also allow for the use of different electrolyte solvents with higher safety.
It also needs to be differentiated between the various cathode materials used in sodium-ion batteries. Polyanionic materials might behave differently to the layered oxides - similarly to the observations made for lithium-ion batteries, where, for instance, LiFePO4 (LFP) presents higher thermal stability than LiNiMnCoO2 (NMC) materials. More in-depth studies are needed to understand the effect of the various failure mechanisms (thermal, electrical, mechanical) on chemical reactions happening in sodium-ion batteries and the related safety implications.
A potential solution to safety concerns could be the development of solid-state sodium-ion batteries, similar to efforts in the lithium-ion field, where the flammable organic electrolyte is replaced with a solid electrolyte to improve overall battery safety. This could further increase the battery energy density since, for example, a metallic sodium-ion anode could be used. However, performance challenges related to ionic conductivities, electrode/electrolyte interfaces, and dendrite growth will need to be solved first, before a commercialization of this battery system can be envisaged. Given the long development times for lithium-based solid-state batteries, this might not happen in the next decade.
Lithium-ion batteries have come under heavy scrutiny due to their high environmental impacts. These include energy and water-intense mining activities – which further cause damage to the local biodiversity – high energy consumption, and the use of harsh and toxic chemicals throughout the manufacturing and recycling processes.
The question is, if these impacts can be avoided by sodium-ion batteries. For example, the indirect emissions in the value chain caused by the electricity production needed in the various steps are eliminated by decarbonizing the electricity grid, not by switching the battery system.
Also, most of the manufacturing steps of sodium-based materials are similar to that of lithium-ion batteries and, therefore, might not impact on the overall energy requirements. As mentioned above, recycling is as necessary yet complex for sodium-ion batteries as for lithium-ion batteries, which means that no benefits are made by switching.
As of now, no definitive answer about the environmental sustainability of sodium-ion batteries compared to lithium-ion batteries has been provided. More in-depth lifecycle studies are needed. However, what is clear is that we should not assume sodium-ion batteries will solve all the issues we currently face with lithium-ion batteries. They carry similar risks and if we don't start solving for these risks in the near future, we might repeat the mistakes made with lithium-ion batteries.
For the future development and commercialization steps of sodium-ion batteries, it is crucial that industry and academia work closely together and, most importantly, share data and insights with each other. This will accelerate technological progress and help solve challenges such as recycling, sustainability, lifetime prediction and safety more efficiently.
Laura is a lecturer in the Engineering Department at King'sCollege London. Her research aims to solve urgent issues around green energyand energy storage technologies. She focuses on the development of sustainablefuture battery technologies, their end-of-life treatment as well as theirenvironmental and economic footprint. In her free time, Laura enjoys art, literature, movies, and dancing.