Novel Binder Elevates Hard Carbon Anodes for Sodium-Ion Batteries

Researchers at the Japan Advanced Institute of Science and Technology (JAIST), led by Professor Noriyoshi Matsumi and Doctoral Course Student Amarshi Patra, have developed a hard carbon (HC) anode using a poly(fumaric acid) (PFA) binder. This innovative approach addresses key challenges in the commercialization of sodium-ion batteries (SIBs), particularly issues related to ion diffusion kinetics, initial coulombic efficiency (ICE), and electrolyte decomposition, which traditionally result in sluggish rate capability and specific capacity for HC-based SIBs. The findings of this research were published in the Journal of Materials Chemistry A.

The benefits of PFA

The novel PFA binder distinguishes itself through its high functional density, featuring carboxylic acid groups on all carbon atoms of the main chain. This structure provides numerous ion hopping sites, thereby enhancing sodium ion diffusion and significantly improving the binder's adhesion to the electrode. Furthermore, PFA is environmentally friendly, water-soluble, and non-toxic and is derived from bio-based fumaric acid.

Explaining the benefits of PFA, Prof. Matsumi states, “Unlike conventional poly(acrylic acid) binders, PFA is a high-functional density polymer with carboxylic acid present on all the carbon atoms of the main chain. This enables PFA to improve Na ion diffusion due to the presence of highly concentrated ion hopping sites and to adhere to the electrode more strongly. Additionally, PFA binders offer water solubility and non-toxicity, and its precursor, fumaric acid, is a bio-based polymer.”

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Producing HC anodes

To synthesize the HC anode, the researchers hydrolyzed poly(fumarate ester)s to produce PFA, then combined HC, Super P carbon, and PFA in water to create an aqueous slurry. This slurry was coated onto copper foil and dried overnight to produce the HC anode. The anode was then assembled into an anode-type half-cell, using a sodium metal disc as the counter electrode and 1.0 M NaClO4 as the electrolyte.

The researchers conducted a peeling test to evaluate the adhesion between the electrode components and the copper current collector. Strong adhesion is crucial for the longevity of SIBs, and the PFA binder demonstrated superior performance with a peeling force of 12.5 N. This was notably higher than the 11.5 N observed for poly(acrylic acid)-HC electrodes and the 9.8 N for PVDF-HC electrodes.

Electrochemical tests revealed that the PFA-HC anode half-cell exhibited outstanding performance. It achieved specific capacities of 288 mAhg^-1 and 254 mAhg^-1 at current densities of 30 mAg^-1 and 60 mAg^-1, respectively, outperforming both PVDF and poly(acrylic acid)-based electrodes. Additionally, the anode demonstrated excellent long-cycle stability, retaining 85.4% of its capacity after 250 cycles. This stability is attributed to the formation of a thin, stable solid electrolyte interphase (SEI) that does not crack or exfoliate, enhancing the durability of the half-cell. Furthermore, the sodium ion diffusion coefficient for the PFA-HC electrode was found to be 1.9 × 10^-13 cm^2/s, higher than those of poly(acrylic acid)-HC and PVDF-HC electrodes.

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The research team plans to collaborate with industry partners to facilitate the commercial implementation of this technology. The water-soluble, non-toxic nature of the PFA binder, combined with its superior performance characteristics, makes it a promising candidate not only for SIBs but also for a wide range of energy storage devices.

This innovation represents a significant step towards developing low-cost, efficient energy storage solutions, contributing to a more energy-efficient and carbon-neutral society. The advancements in HC anodes utilizing PFA binders could lead to broader adoption and commercialization of SIBs, offering a sustainable alternative to lithium-ion batteries.

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