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Sodium-Ion Batteries Get a Boost with Novel PFA Binder

Due to the rising demand for energy storage devices, researchers from the Japan Advanced Institute of Science and Technology sought more affordable and accessible materials for rechargeable batteries. They identified sodium-ion batteries (SIBs) as a promising alternative and developed a new binder material that enhances the performance of SIB anodes. Their findings were published in the Journal of Materials Chemistry A.

Significant research has been done on upgrading the materials for positive electrodes (cathodes), negative electrodes (anodes), and electrolytes to improve long-cycle stability and achieve a thin solid electrolyte interface (SEI) for SIBs.

To prevent the anode from deteriorating due to reactions with the electrolyte, a passive layer called a Surface Electrolyte Inhibitor (SEI) is created on the anode surface during the early charge/discharge cycles.

Battery performance is dependent on a well-formed SEI. Hard carbon (HC) has become a potential anode material in this context. However, commercialization has been challenging because of its uneven, thick, and weak SEI formed by increased electrolyte consumption, which reduces reaction rates and charging/discharging stability.

Binders such as poly(acrylic acid) derivatives, carboxymethyl cellulose salts, and poly(vinylidene fluoride) (PVDF) have been employed to solve these problems. Nevertheless, these binders result in a delayed diffusion of Na ions in the anode, which lowers the HC-based SIBs' rate capability.

Professor Noriyoshi Matsumi and Doctoral Course Student Amarshi Patra from the Japan Advanced Institute of Science and Technology (JAIST) created an HC anode with a poly (fumaric acid) (PFA) binder to overcome these drawbacks.

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.

Noriyoshi Matsumi, Professor, Japan Advanced Institute of Science and Technology

The researchers hydrolyzed the poly (fumarate ester)s to create PFA. They then combined HC, Super P carbon, and PFA with water to make an aqueous slurry, which was coated onto copper foil and allowed to dry for the entire night to form an HC anode. An anode-type half-cell was constructed using this anode, a sodium metal disk as the counter electrode, and 1.0 M NaClO4 as the electrolyte.

The researchers performed a peeling test to evaluate the impact of the binder on the adherence of electrode components to the copper current collector. Notably, the long life of SIBs depends on robust adherence.

The PFA-binder HC electrode was found to have a peeling force of 12.5 N, which was much higher than the peeling force of the poly (acrylic acid)-HC electrodes (11.5 N) and the PVDF-HC electrodes (9.8 N).

They conducted several electrochemical and battery performance studies on the anode half-cell. In charging/discharging cycle experiments, the anode half-cell outperformed PVDF and poly(acrylic acid)-type electrodes, exhibiting specific capacities of 288 mAhg-1 and 254 mAhg-1 at current densities of 30 mAg-1 and 60 mAg-1, respectively.

After 250 cycles, it maintained 85.4 % of its capacity, demonstrating good long-cycle stability. The anode produced a thin SEI and exhibited neither exfoliation nor crack formation, which added to the half-cell's increased durability. In addition, the PFA-HC electrode had a greater Na ion diffusion coefficient (1.9 × 10-13 cm2/s) than the PVDF-HC and poly (acrylic acid)-HC electrodes.

These findings can lead to the development of SIBs with improved electrochemical performance.

In this polymer material, various structural modifications are possible through different polymer reactions, which can further improve performance. In the future, we aim to conduct joint research with companies for its commercial implementation. Additionally, as a water-soluble and non-toxic binder material that improves durability, it can not only be applied in SIBs but also in a wide range of energy storage devices.

Noriyoshi Matsumi, Professor, Japan Advanced Institute of Science and Technology

This novel material may increase the use of inexpensive energy gadgets built on SIBs, resulting in a more carbon-neutral and energy-efficient world.

Journal Reference:

‌Patra, A., et al. (2024) Water-soluble densely functionalized poly(hydroxycarbonylmethylene) binder for higher-performance hard carbon anode-based sodium-ion batteries. Journal of Materials Chemistry. A.

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