In order to enable fast-charging ability in batteries, researchers have attempted to enhance the mass transfer of electrolytes and charge transfer in electrodes, with extensive research carried out on the former compared to the latter. Now, a study by a team of researchers, led by Professor Noriyoshi Matsumi from Japan Advanced Institute of Science and Technology (JAIST), showcases a new approach to facilitate fast charging using a binder material which promotes Li⁺-ion intercalation of active material.

The binder material—a novel aqueous borate type bio-based polymer with inherent Li⁺ ions designed as an SEI forming binder for graphite—leads to improved diffusion of desolvated Li⁺ ions across the solid electrolyte interface (SEI) and within the anode material and yields high conductivity, low impedance, and good stability. A paper on their work is published in ACS Materials Letters.

The low lying LUMO energy level enabled the preferential reduction of the binder prior to the degradation of the electrolyte or salt to form a thinner and highly conducting borate rich SEI. A robust boron rich SEI and a binder with inherent Li⁺ ions improved the kinetics with low activation energy for lithiation/desolvation (22.56 kJ/mol), lower SEI resistance, and a high Li⁺ diffusion coefficient across the graphite galleries (7.24 × 10^–9 cm² s^–1).

Anodic half-cells with the novel binder delivered a discharge capacity of 73 mAh/g at 10 C, which is three times higher than the those of the polyvinylidene fluoride (PVDF) and sodium carboxymethyl cellulose/polystyrene-polybutadiene rubber (CMC-SBR) counterparts, with a high capacity retention for more than 1000 cycles.

—Pradhan et al.

While most research on batteries is focused on the design of active materials and improved mass transfer of electrolytes, the current study provides a different approach via the design of specific binder material which promotes lithium-ion intercalation of the active material.

The role of boron compounds (such as the tetracordinate boron in the binder and the boron-rich SEI) is to aid in the desolvation of Li⁺ ions by decreasing the activation energy of desolvation of Li⁺ from the solvent sheath at the SEI. Also, with high diffusion and low impedance, the overpotential related to charge transfer at the interface is reduced.

Generally, when charging surpasses rate of intercalation, Li⁺ plating occurs on graphite electrodes. It is an undesired process leading to reduced battery life and limiting fast charge capability. In this study, the improved diffusion of ions across the SEI and within the electrodes limits the concentration polarization of Li⁺ ions—leading to the absence of plating on graphite.

In their study, not only do the researchers present a novel strategy for extremely high-rate chargeable batteries and reduced interfacial resistance, but they also used a biopolymer derived from caffeic acid. A plant-based organic compound, caffeic acid is a sustainable and environmentally safe source of material. Thus, while the market for these batteries grows tremendously, the use of bio-based resources in these batteries will also reduce carbon dioxide emissions.

Resources

• Anusha Pradhan, Rajashekar Badam, Ryoya Miyairi, Noriyuki Takamori, and Noriyoshi Matsumi (2023) “Extreme Fast Charging Capability in Graphite Anode via a Lithium Borate Type Biobased Polymer as Aqueous Polyelectrolyte Binder” ACS Materials Letters 5 (2), 413-420 doi: 10.1021/acsmaterialslett.2c00999