Optimizing Ionic-Electronic Pathways Via Bioinspired Self-Polymerized Dopamine in All-Solid-State Composite Cathode
Authors: Vuong Thuy Hoai Linh , Nam Jae Seok , To A Ran Weerawat , Ha Chaeyeon , Kim Young-Jun 
Published: February 2024

The research of all-solid-state batteries (ASSBs) has been the principal emphasis in recent years. By utilizing the capabilities of advanced materials science and innovative engineering, ASSBs have the potential to revolutionize industries as diverse as consumer electronics, electric vehicles, aviation, and renewable energy. Solid-state batteries with their outstanding energy density, fast charging capabilities, and resistance to thermal runaway are anticipated to revolutionize the future of energy storage and allow a new era of eco-friendly, high-performance applications.

Nonetheless, the point-of-contact resistance of ASSBs is high owing to the diverse chemical and physical characteristics of the individual solid components and the nature of solid-solid contact. To further enhance the electrochemical performance of ASSBs, especially their high-rate capabilities, it is crucial to raise the electronic conductivity of the cathode composites. Improving the electrical conductivity of electrodes to achieve high-rate capacity requires the addition of carbon. Utilizing common carbon materials, such as vapor-grown carbon fibers (VGCF), enables homogeneous current distribution in the cathode layer. Nevertheless, the use of carbon additives in sulfide-based ASSBs is inhibited by significant challenges. Carbon additives provide enough electronic percolation pathways in the composite cathodes, speeding the decomposition of sulfide SSEs during the charging process. Consequently, even with minimal amounts of carbon additives, rapid deterioration of sulfide SSEs continues to occur at the electrolyte/carbon interface, causing severe side reactions and the creation of an unfavorable interfacial layer between the carbon additives and SSEs.

To achieve high cycle performance, ASSBs included conductive agents in the composite cathode to promote solid contact between the active material and sulfide solid electrolyte Li6PS5Cl; nevertheless, a significant side reaction was observed between the conductive agents and solid electrolyte. To prevent a substantial side reaction and achieve improved electrochemical performance with high-rate capacity, self-polymerized dopamine (PDA) thin film was coated on the VGCF’s surface in this investigation. Dopamine hydrochloride would be oxidized and self-polymerized into polydopamine under alkaline environments (pH 8.5) without the necessity for severe reaction conditions, sophisticated equipment, costly catalysts, or hazardous organic solvents.

During the electrochemical test, the improved lithium conductivity of polydopamine-coated VGCFs after cycling at 0.1 C-rate was confirmed by galvanostatic intermittent titration technique (GITT) data and electronic-blocking cells. After polydopamine coating, the electrochemical performance of VGCFs dramatically improved with better reaction reversibility, enhanced specific capacity with high C-rate, and extended cycle life, as evidenced by polydopamine-coated VGCFs achieving ~165.5 mAh.g-1 after 100 cycles, compared to ~118.7 mAh.g-1 for unmodified VGCFs. Owing to the presence of two-electron transfer redox processes per benzoquinone skeleton, which reduced to catecholate and coordinate with lithium-ion, polydopamine facilitated a shorter ion diffusion path for a more rapid and straightforward reduction reaction even if compare with non-VGCFs composite cathode.

In composite cathode, electronegative elements with lone-pair electrons, such as O (oxygen functional groups on VGCFs), react with electrophilic species, such as P5+ in Li6PS5Cl (PS4
3-), resulting in the formation of P-O bonding through nucleophilic reaction on oxygen-substituted thiophosphate and thioethers. In addition, the S atom of PS4
3- can attack the electrophilic C atom in the carbonyl group on the surface of VGCFs. Intense interactions between P5+ and the electronegative element O, as well as S2 and the electrophilic atom C, thus damage the crystal structure of SSEs and contribute to the reduction in the specific capacity of the composite cathode. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were then utilized to confirm the existence of by-products such as P-S-P, S-S, Li2S, and P-O; the presence of polydopamine demonstrated its ability to minimize side reactions and preserve the solid-state electrolyte.

In addition, polydopamine's excellent adhesive capabilities enable it to build a stable and homogenous coating on the VGCF's surface. This coating may enhance the adherence of the conductive agents to the electrode's components. As polydopamine-coated VGCFs exhibited low and homogeneous resistivity in the cathode's elements, scanning spreading resistance microscopy (SSRM) was used to highlight the enhancement of particle distribution to optimize the ionic and electronic pathways.

This study examines, in summary, the effect of a polydopamine coating layer on the conductivity of solid-state battery conductive agents. Even though polydopamine is recognized for its insulating characteristic, it may also display strong electrical conductivity and adhesion under certain conditions. Thus, we observed that VGCFs coated with a suitable PDA thin film eliminate unneeded kinetic energy for side reactions and improve the ionic-electronic route in composite electrodes. The discovered principles and mechanism of polydopamine coated on conductive agents are applicable to a polymer manufacturing method or modification that is coated on conductive agents, therefore enhancing the electrochemical performance of ASSBs.

Figure 1