Shaping hard carbon electrodes for next-gen batteries

Lithium-ion batteries (LIBs) are a widely used type of rechargeable battery with a range of applications. However, lithium is a scarce resource and lithium extraction and improperly discarded LIBs pose huge environmental challenges as the liquid electrolytes used in them are toxic and flammable. Sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) are two emerging alternatives that are cost-efficient and sustainable. Unfortunately, the capacity of the electrode materials used in NIBs and KIBs still lags behind that of lithium-ion batteries.

Now, researchers led by Professor Shinichi Komaba from Tokyo University Science (TUS) Japan have developed high-capacity electrode materials for NIBs and KIBs. In their latest study, published in Advanced Energy Materials, the researchers reported a new synthesis strategy for nanostructured “hard carbon” (HC) electrodes that enhance performance.

Unlike other forms of carbon, such as graphene or diamond, hard carbon is amorphous (it lacks a well-defined crystalline structure). It is also strong and resistant. A previous study carried out by Komaba found a way to use magnesium oxide (MgO) as a template during the synthesis of HC electrodes for NIBs, thereby altering their final nanostructure. The process led to the formation of nanopores within the electrodes upon MgO removal, which, in turn, had increased their capacity to store Na⁺ ions.

Motivated by these findings, the researchers explored whether compounds made from zinc (Zn) and calcium (Ca) could also be useful as nano-templates for HC electrodes. They investigated different HC samples made using zinc oxide (ZnO) and calcium carbonate (CaCO₃) and compared their performance with the ones synthesised using magnesium oxide (MgO). Preliminary experiments showed that ZnO was promising for the negative electrode of NIBs. The researchers optimised the concentration of ZnO embedded in the HC matrix during synthesis, demonstrating a reversible capacity of 464 mAh g^–1 (corresponding to NaC_4.8) with a high initial Coulombic efficiency of 91.7% and a low average potential of 0.18 V vs. Na⁺/Na.

The researchers then incorporated this powerful electrode material into an actual battery. “The NIB fabricated using the optimised ZnO-templated HC as the negative electrode exhibited an energy density of 312 Wh kg^–1. This value is equivalent to the energy density of certain types of currently commercialised LIBs with LiFePO4 and graphite, and is more than 1.6 times the energy density of the first NIBs (192 Wh kg^–1), which our laboratory reported back in 2011,” Komaba said.

The ZnO-templated HC also exhibited a significant capacity of 381 mAh g^–1 when incorporated into a KIB, further showcasing its potential. The results of this study show that using inorganic nanoparticles as a template to control the pore structure may provide an effective guideline for the development of HC electrodes. “Our findings prove that HCs are promising candidates for negative electrodes as an alternative to graphite,” Komaba said.

This could make NIBs viable for practical applications, such as the development of sustainable consumer electronics and electric vehicles, and low carbon footprint energy storage systems for storing energy from solar and wind farms.

Image credit: iStock.com/Gerardo Carnero