Sodium-ion batteries: The next revolution in energy storage?

This story is contributed by Milan Sadan, Research Institute of Green Energy and Technology (RIGET)

  • While the production of lithium-ion batteries (LIBs) will be subject to supply chain issues with regard to lithium, nickel, and cobalt, sodium-ion batteries (SIBs) have the potential to provide energy storage at scale using relatively abundant and inexpensive materials.
  • Most companies working on commercializing SIBs are using hard carbon as the anode and Prussian blue analogues or polyanionic materials as the cathode, but prices have yet to reach parity with LIBs.
  • In addition, SIBs also boast faster charge rates, longer cycle life, and improved safety.

The lithium-ion battery (LIB) market has become one of the hottest topics of the decade due to the surge in demand for energy storage. The evolution of LIBs from applications in small implantable electronic devices to large electric vehicles has proven their success in the consumer market, and their prospects have fueled the development of multiple gigafactories across Europe and Asia [1]. With the LIB market on the brink of reaching its full potential, should we even be thinking about another battery chemistry? The answer to this question is fundamental yet complex.

Emerging LIB chemistries may still lead to safer batteries with fast charging rates and long cycle life, but supply chain issues remain. Specifically, lithium is sometimes referred to as “white petroleum” [2] due to the unfavorable geographical locations of lithium mines and ever-fluctuating costs. Safety concerns on top of cobalt and nickel usage in current LIB chemistries provide even more reason for another revolution in energy storage.

Background

Although sodium-ion batteries (SIBs) have been studied since the early 1970s [3], they fell out of favor in the research community with the emergence and commercialization of other battery technologies. Due to the widespread success of LIBs and, at the same time, concerns regarding the cost of lithium and other raw materials, however, SIB research was revived in the late 90s [4]. Initially, SIBs were investigated as stationary energy storage devices by exploiting high-temperature Na-sulfur and Na/NiCl2 battery chemistries [5]. Interest in commercialization of SIBs for utilities other than stationary storage, however, demanded the development of room-temperature SIBs, and the past decade has witnessed its slow but steady evolution as a competent energy storage chemistry [5].

SIBs have similar electrochemical properties to LIBs, and Table 1 compares lithium and sodium ions. While Na+ has a similar standard potential to Li+ and is relatively inexpensive, additional issues arise due to the larger size and bulky nature of the Na+ ions.

Table 1. Comparison between Lithium and Sodium [6].

SIB’s have a faster charge rate and longer cycle life compared to LIBs. For instance, Natron Energy claims batteries that can charge within 8 minutes with a cycle life of 50,000 cycles based on Prussian blue cathodes and Tiamat Energy has developed polyanionic cathodes that can charge within 5 minutes with a cycle life of more than 5000 cycles. Faradion has developed pouch cells with an energy density of 160 Wh/kg, comparable to LIBs. The lower desolvation energy of sodium, 157 kJ/mol for Na+ compared to 218 kJ/mol for Li+, enhances SIBs kinetics and allows for fast charging.

In terms of cost, SIBs use relatively inexpensive materials (SIBs are estimated to be 12.5% cheaper than the present LIBs) (7) and sodium resources are widely available. In addition, aluminum does not react with sodium at low potentials, allowing for Al to be used as the anode current collector, unlike in LIBs where Cu is used instead. For comparison, the price of Al foil is $0.3/m2 and the price of Cu foil is $1.2/m2 (7). Although these factors contribute tremendously to reduce the cost, other hurdles, like atomic weight and standard potential, must be overcome to improve the overall energy density of SIBs.

State of the Art

SIBs are currently in the research and development stage and offer a plethora of opportunities in terms of safe and sustainable energy storage. Figure 1 shows the number of academic articles on SIBs since 2000.

Figure 1. The number of articles published in the field of sodium ion batteries. Although the 2021 point represents only six months, this year is already on track to reaching over 3000 publications.

The exponential increase in research articles on SIBs in recent years is very encouraging and suggests the potential for widespread commercialization in the near future.

Electrode materials for SIBs can be classified into three basic categories based on the reaction mechanism: insertion, alloying, and conversion, similar to that of LIBs. Insertion materials are those that insert the host ion (Na+) into the interstitial space, alloying materials form an alloy with the host material during the electrochemical reaction, and conversion materials react with host ions to form another compound that can be recovered upon reverse reaction.

Anode Materials

SIB anode materials research to date covers all three electrode chemistries (Figure 2). The larger Na+ ions pose a challenge to the development of SIBs. For example, due to the size of Na+, the common LIB insertion anode graphite does not work in SIBs with traditional carbonate-based electrolytes. An ether electrolyte can be used with graphite, but at significantly reduced potentials and only by co-intercalating ether molecules alongside the Na+ ions.

Figure 2. Anode materials studied for sodium ion batteries (Reproduced from Chem. Soc. Rev., 2017, 46, 3529. with permission from the Royal Society of Chemistry).

Hard carbon is widely studied and currently the only anode material in commercialization for SIBs, but it has a limited gravimetric capacity of around 200 mAh/g. Other insertion anode materials, such as sodium titanate and sodium titanium phosphate, are also being studied extensively. The fact that theoretical capacity is limited in insertion materials, however, poses serious concerns. One solution is to employ alloying anodes, such as Sn and Sb, to explore higher specific capacity. Unlike in LIBs, silicon has poor reversible capacity in SIBs. The volume change that occurs during the alloying reaction is even more severe due to the huge size of Na+ and ultimately leads to cell failure.

Recently, research has shifted from alloying and insertion materials to conversion materials, such as metal oxides, metal sulfides, and metal selenides. Conversion materials facilitate fast kinetics with a higher potential (lower overall voltage) compared to alloying and insertion anodes. As a general rule, conversion materials have the highest power density, followed by alloying and insertion materials, whereas insertion materials are the most energy dense, followed by alloying and conversion materials. The fast reaction in conversion materials favors power density while the lower potential (higher overall voltage) of insertion anode materials favors energy density. Hence, it is imperative to find a balance between power density and energy density in these electrode chemistries. Research into organic anode materials, yet another class of advanced anodes, are still in their infancy, and require significant investigation to elucidate the reaction mechanisms.

Cathode Materials

Figure 3. Cathode materials studied for sodium ion batteries (Reproduced from Chem. Soc. Rev., 2017, 46, 3529. with permission from the Royal Society of Chemistry).

Initial studies of SIB cathode materials have focused on insertion-type Na analogues of LIB cathodes. Specifically, metal oxides and polyanionic cathodes have garnered wide attention in the early days of SIB research, and most of them perform adequately, with energy densities above 100 Wh/kg and voltages over 3.0 V, proving that the insertion mechanism is indeed feasible in SIBs. The four main types of SIB insertion cathodes are categorized by electrode structure: sodium transition metal oxide, polyanionic, Prussian blue analogue, and organic-based cathodes. Among these cathode types, all except the organic compounds have some potential for practical application. A wide variety of materials are currently being explored. Among them, sodium iron hexacyanoferrate is promising in terms of energy density and sodium vanadium phosphate-based cathode materials are promising in terms of fast charging. Many startups are working on Prussian blue analogues (e.g., NaxFe[Fe(CN)6], Na2MnFe(CN)6H2O, etc.) and polyanionic cathode materials (e.g., NaV3(PO4)3, Na3V2(PO4)2F3, etc.), but the industry leader Faradion is currently developing NaxNi1-x-y-zMgxMnyTizO2. All companies are avoiding the use of cobalt in their cathodes.

Figure 4. Major companies in the field of sodium ion batteries.

Even though SIBs are still in the development stage, they have already received a lot of commercial attention. A few companies have recently started developing SIBs in the USA, Europe, and Asia, including Faradion, AGM batteries, TIAMAT Energy, HiNa Battery Technology, NGK Insulators, Natron Energy, Altris, Indienergy, and Energy11. While NGK Insulators is developing high-temperature sodium-sulfur batteries for stationary energy storage, all other companies are working on room temperature SIBs based on low-cost anode materials like hard carbon and cathode materials like Prussian blue analogues and polyanionic cathodes.

Outlook

SIBs still face many challenges and opportunities in their path towards commercialization. The most exciting features of SIBs include fast charging capability, safety, lower cost, and a relatively long cycle life compared to commercial LIBs. The non-toxic, cheap, and widely available electrode materials are another big advantage. Despite only a short period of academic research on room temperature SIBs, world-class companies have already demonstrated prototype batteries with impressive results, potentially marking the start of yet another revolution in the energy storage sector. Concerns remain for the commercialization of SIBs, however, in terms of cost ($263/kWh for SIBs vs. $198/kWh for NMC-based LIBs) [8], lower energy density, and volume expansion of the electrode materials. With plenty of opportunities and challenges for both academic research and industry development, SIBs hold a lot of promise for bringing about revolutionary change in the way we store energy.

References

[1] https://cleantechnica.com/2021/07/03/europe-planning-38-new-ev-battery-gigafactories

[2] https://www.theguardian.com/news/2020/dec/08/the-curse-of-white-oil-electric-vehicles-dirty-secret-lithium

[3] Research Development on Sodium-Ion Batteries, Chemical Reviews 2014, 114, 11636−11682.

[4] Commercialisation of high energy density sodium ion batteries: Faradion’s journey and outlook, Journal of Material Chemistry A, 2021, 9, 8279–8302.

[5] Challenges of today for Na-based batteries of the future: From materials to cell metrics, Journal of Power Sources, 2021, 482, 228872.

[6] Sodium-Ion Batteries, Advanced Functional Materials, 2013, 23, 947–958.

[7] A cost and resource analysis of sodium-ion batteries, Nature Reviews Materials, 2018, 3, 18013.

[8] Exploring the Economic Potential of Sodium-Ion Batteries, Batteries, 2019, 5, 10.

Milan K. Sadan is currently working as a senior researcher at the Research Institute of Green Energy and Technology (RIGET), South Korea. He has been associated with battery research since 2012 and holds a PhD from the Materials Engineering Department at Gyeongsang National University. He is currently working on improving the performance of next generation batteries, including sodium-ion batteries, potassium-ion batteries and room temperature sodium-sulfur batteries.

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