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| Fig. 1: Sodium Metal. (Source: Wikimedia Commons) |
Sodium-ion batteries have been at the forefront of establishing an alternative to consistently used Lithium-ion batteries as costs and environmental concerns have been raised in recent decades. Thus, research in the electrochemical field has prospered in seeking out possible, reliable options. As of now, sodium batteries have a high cost and lower energy density due to a lack of utilization and research, but sodium (see Fig. 1) is more abundant and cost-effective than lithium, making it an attractive contender in the race to find better and cleaner energy storage. [1]
There are numerous differences between Sodium and Lithium atoms that make each advantageous or disadvantageous. These range from differences in ion size (0.76 Angstroms [Lithium] vs. 1.02 Angstroms [Sodium]), to atomic mass (7 u [Lithium] vs. 23 u [Sodium]), ionization potential limit, electrochemical potential, and energy density. [2] Let's dive into current research that's happening in the field to understand the progress of sodium-ion batteries.
Lithium-ion batteries currently require a capacity between 200-300 mAh/g with over 80% capacity retention of over 200 cycles. [3] Pandit et al., for example, conducted sodium-ion battery experiments using manganese oxide nanorods for higher electrochemical performance. These cells, when tested, achieved 109 mAh/g with 58.6% retention after 800 cycles. When tested with a different electrolyte, the cells reached 181 mAh/g with 11.5% retention after 800 cycles. [2] Thus, the retention capacity suffers significantly with the second electrolyte, even with a higher initial specific capacity.
Although this is only one research paper, the practical viability of sodium-ion batteries is still under question as they will have to reach at least 200 mAh/g and sustain that capacity over many cycles to remain competitive. Beyond the research being done, we can compare the theoretical capacity of each material to assess the possibilities of what each material can reach under perfect conditions. Lithium has a theoretical specific capacity of 3860 mAh/g-Li, while sodium has 1165 mAh/g-Na. [2,4] This translates to how Lithium can deliver more electric charge per gram of Lithium than sodium can per gram of sodium. Thus, the limits of lithium may be higher than what can be practically achieved for sodium.
Although commercial sodium batteries do not exist yet as they strive to reach lithium battery standards, research is bound to continue to help them reach their goals. Bringing sodium batteries up to scale will be a difficult challenge as there are chemical restraints that need to be overcome. From ongoing research, it seems that the significance of electrolytes and electrode selection will bring the competitive advantage of sodium cells in the vast world of lithium batteries.
© Layaa Amirthalingam. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. Che et al., "Engineering Optimization Approach of Nonaqueous Electrolyte For Sodium Ion Battery With Long Cycle Life and Safety," Green Energy Environ. 6, 212 (2021).
[2] B. Pandit et al., "High Stability and Long Cycle Life of Rechargeable Sodium-Ion Battery Using Manganese Oxide Cathode: A Combined Density Functional Theory (DFT) and Experimental Study," ACS Appl. Mater. Interfaces 13, 11433 (2021).
[3] A.-G. Olabi, ed., Encyclopedia of Smart Materials, 1st Ed. (Elsevier, 2021).
[4] W. Zhanget al., "Sodium-Ion Battery Anodes: Status and Future Trends," EnergyChem 1, 100012 (2019).
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