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An overview of hard carbon as anode materials for sodium-ion batteries

This article is contributed by Shayan Meysami

  • Sodium-ion (Na-ion) batteries have attracted significant interest from a wide spectrum of commercial applications, including home storage, uninterrupted power supply, and low-speed electric vehicles (EVs) ranging from electric three-wheelers to trucks.
  • The lower cost, improved sustainability, ability to be completely discharged to zero volts during transportation and drop-in nature of manufacturing (fully compatible with existing Li-ion cell manufacturing lines) are among the key features that make this chemistry an excellent choice to fill the longstanding gap between lead-acid and Li-ion batteries on the cost/performance curve [1].
  • Establishing a sustainable supply chain of Na-ion battery materials is of paramount importance given recent industry developments, such as the acquisition of Faradion by Reliance New Energy Solar Ltd for an enterprise value of £100 million plus £25 million of growth capital [2], as well as CATL’s plans to enter production by 2023 [3].

Introduction

To better understand Na-ion batteries’ position in the energy density spectrum of secondary batteries, it is helpful to analyze the performance of Faradion’s most recent commercialized 140 Wh pouch cells. With a capacity of 45 Ah and a nominal voltage of 3.1 V, the cell has a specific energy in the range of 150 Wh/kg. To put this in perspective, a recent study by Kampker’s Group [4] concluded that the specific energy of Li-ion pouch cells employed in EVs between 2010 and 2019 ranged from roughly 150 to 250 Wh/kg. This means that the demonstrated specific energy of Na-ion cells — in formats and capacities comparable to those employed in EVs — is in the range of the specific energies achieved by fully mature Li-ion chemistries. It should be noted however, that more energy-dense Li-ion chemistries (>200 Wh/kg) often use cathode compositions rich in nickel and cobalt. In contrast, Faradion’s proprietary cathode (Na, Ni, Mn, Mg, and Ti) contains no cobalt, which is associated with child labor and dangerous mines. When compared with lithium iron phosphate, the primary advantage of Na-ion chemistries is a 25–30% reduction in the bill of materials. In addition to the savings on cathode active materials, a cheaper anode current collector (Al instead of Cu) and cheaper electrolyte systems are major contributors to the overall cost reduction.

Figure 1. Industrialization of Na-ion batteries.

In the latest high-energy Li-ion cathode chemistries, much of the cobalt is substituted with nickel, which is cheaper and has a more reliable supply chain. As a result, modern Li-ion cells contain roughly 2.5 times the amount of nickel per kilogram of cathode active material compared to Faradion’s Na-ion cathode chemistry. This puts Na-ion technology in a unique performance/cost/sustainability position.

The following cycle life performance has been demonstrated by Faradion:

  • 1000 cycles at 1C in 1.0–4.2 voltage window
  • 3000 cycles at 1C in 1.0–4.0 voltage window (historic 2020 data; for more details refer to the publicly available supplementary information of reference [1])

Anode materials: opportunities, challenges and outlook

Cathode aside, the other two critical components of any commercially feasible metal-ion cell with the largest impact on cost and performance are the anode and electrolyte. Since the early days of Na-ion, hard carbon has been successfully employed as a reliable and cost-effective anode active material [5]. In conventional electrolytes, graphite is electrochemically inactive towards sodiation. What makes hard carbon different from more familiar forms of carbon such as the graphite in Li-ion batteries, is its flexibility in material design. Hard carbon is not crystallographically well defined and consists mostly of strongly cross-linked and highly immobilized small crystalline domains that cannot be reshuffled into well aligned graphitic structures. The mechanism of storing Na+ within the structure of hard carbon is highly diverse, as schematically depicted in Figure 2. There are many possible sites to store and reversibly release the charge carriers. From a materials engineering point of view, this flexibility in design at the atomic/microscopic levels allows for many aspects of cell performance to be finely tuned. Cycle life, charge acceptance, energy density and efficiency of the cell chemistry can be balanced to fit the requirements of the target application.

Hard carbon possesses the unique ability to alter the shape of the sodiation/desodiation profile to favor certain cell parameters such as charge acceptance and cell voltage over others. Hard carbon’s sodiation profile consists of two regions: sloping (high potential) and plateau (low potential). By engineering the pore structure, extent of crystallization and other characteristics (as summarized in Figure 2), it is possible to precisely control the portion of the capacity originating from each of the regions. For faster charge acceptance and higher cycling stability, the portion of the capacity originating from the sloping region should be increased at the expense of lower overall specific capacity and lower cell voltage. On the other hand, reducing the portion of the capacity originating from the sloping region and extending the low-potential plateau region of the profile provides a larger overall capacity and higher cell voltage at the expense of slower charge acceptance and stricter requirements for low-temperature charging.

Figure 2. Storage sites of hard carbon.

From the manufacturing point of view, synthesis of hard carbon is fundamentally different from that of graphite. First, hard carbon cannot be mined. Second, while synthetic graphite is produced via high-temperature graphitization of soft (graphitizable) carbon precursors such as pitch, hard carbon requires non-graphitizable precursors. This allows for the use of a variety of renewable resources, such as animal waste and sewage sludge [6], as well as coal and petroleum derivatives, which synthetic graphite production relies almost exclusively on. Producing hard carbon from more sustainable resources is currently more costly than utilizing fossil fuels derivatives. The former often requires more aggressive demineralization to yield sufficiently high carbon purity while the latter relies on the very supply chain that green energy solutions are meant to disrupt and ultimately phase out.

With hard carbon specific capacities now approaching those of graphite, combined with the accelerated commercialization of Na-ion chemistries in Europe, US, India and East Asia, the demand for hard carbon as an anode active material for metal-ion batteries is likely to grow rapidly. The adoption of Na-ion in particular will depend in part on the success of current hard carbon scale-up efforts, such as the recent technical collaboration between Faradion and Phillips 66 [6], which aims to further accelerate the large-scale industrialization of Faradion’s safe, low-cost sodium-ion energy technology.

Conclusion

Moving forward, it is anticipated that hard carbon will follow a similar pathway as that of high-capacity anode composites, such as graphite-silicon compounds. Again, this is where the design flexibility of hard carbon provides a unique advantage, allowing for the engineering of a material “scaffold” with a more chemically diverse composition [7].

Shayan Meysami is a Senior Scientist at Faradion Ltd. Prior to his current appointment, Shayan held various positions at the University of Oxford for 8 years as a Senior Demonstrator, Postdoctoral Researcher, and Marie Skłodowska-Curie Early-Stage Researcher. He was a Research Associate and a Member of the Senior Common Room at the Corpus Christi College. He completed his graduate studies at Technische Universität Darmstadt, Institut Polytechnique de Grenoble and École Polytechnique Fédérale de Lausanne before obtaining his DPhil from Oxford.

References

[1] Journal of Materials Chemistry A, 2021, 9, 8279; https://doi.org/10.1039/D1TA00376C.

[2] https://faradion.co.uk/reliance-new-energy-solar-to-acquire-faradion-limited/.

[3] https://www.catl.com/en/news/665.html.

[4] World Electric Vehicle Journal, 2020, 11(4), 77; https://doi.org/10.3390/wevj11040077.

[5] Journal of The Electrochemical Society, 2000, 147(4), 1271; https://doi.org/10.1149/1.1393348. [4] International Patent Application Number WO2020208341A1.

[6 https://www.faradion.co.uk/phillips-66-faradion-developing-sodium-ion-battery-materials/.

[7] International Patent Application Number WO2017060718A1.

Dr Seyyed Shayan Meysami

Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield S1 4DP, UK

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