Redox Mediator-Based Anode System for High-Energy-Density Seawater Batteries

Abstract

The increasing demand for energy storage systems (ESS) and electric vehicles (EVs), driven by global energy trends and the aggressive market expansion of companies such as Tesla, has led to rapid growth in the secondary battery market. However, the reliance on limited raw materials, particularly lithium, poses a significant challenge to the sustainable expansion of conventional lithium-ion batteries (LIBs). To address this issue, seawater batteries (SWBs) have emerged as a promising next-generation energy storage system, using seawater as a cathode material to enhance cost-effectiveness and sustainability. Unlike LIBs, SWBs utilize a NASICON solid electrolyte to physically separate the cathode and anode compartments. Since the cathode side is composed of open seawater, which serves as an unlimited sodium reservoir, the anode becomes the primary determinant of energy density and overall performance. Therefore, the development of an efficient anode design is crucial for advancing SWB technology. Conventional strategies for enhancing battery energy density, such as the jelly-roll structure used in LIBs, are not directly applicable to SWBs due to the rigidity of NASICON solid electrolytes and potential electron-blocking issues in stacked electrodes. Consequently, specialized anode designs are required to optimize SWB energy density. While various anode materials have been explored, most exhibit limitations. Among them, sodium biphenyl anolytes have demonstrated excellent reversibility and remain the most commonly employed system. However, the practical energy density of SWBs remains far below their theoretical potential, comparable only to vanadium redox flow batteries. Therefore, this study investigates novel anode designs incorporating redox mediators to enhance SWB energy density. By focusing on both battery chemistry (material selection) and battery architecture (structural optimization), this research aims to bridge the gap between theoretical and practical performance, facilitating the realization of high-energy-density SWBs for real-world applications. 1. Sodium biphenyl redox mediator with hard carbon powder electrode To enhance the energy density of SWBs, this study introduces a sodium biphenyl (Na-Bp) redox mediator with a hard carbon (HC) powder electrode. Inspired by redox targeting concepts in redox flow batteries, this system employs Na-Bp for charge transfer and HC for high-capacity energy storage. A semi-liquid anode structure ensures efficient sodiation/desodiation without electron pathway obstructions. GITT and TEM analyses confirm Na intercalation in HC, with interlayer spacing expanding from 3.7 Å to 4.3 Å upon sodiation and reversing upon desodiation. Electrochemical evaluation in an SWB half-cell demonstrates stable operation over 500 cycles at SOC 33%. Full-cell testing achieves an energy density of 74.8 Wh L-1, a 284% improvement over previous SWBs, comparable to lead-acid batteries. This study demonstrates that integrating a Na-Bp redox mediator and HC powder electrode significantly enhances anode performance in SWBs, providing a feasible approach to increase energy density density for next-generation energy storage. 2. Sodium pyrene redox mediator with tin vertical electrode To further enhance the energy density of SWBs, this study introduces a sodium pyrene (Na-Pyr) redox mediator with a tin vertical electrode. While the Na-Bp–HC system in Topic 1 significantly improved energy density, its enhancement was limited by the low intrinsic capacity of HC. In contrast, tin offers a threefold increase in capacity, but its structural degradation during sodiation/desodiation leads to pulverization and electrical disconnection, making it difficult to utilize its full capacity. This issue is particularly severe in vertical electrode designs, where detached tin particles become electrically isolated. The Na-Pyr redox mediator addresses this problem by facilitating charge transfer from isolated tin particles. SEM imaging confirms that Na-Pyr reduces electrode pulverization, maintaining a smoother surface after cycling. XRD analysis further reveals complete desodiation under Na-Pyr, while Nyquist plots show significantly lower charge transfer resistance, confirming the redox mediator’s effectiveness in prolonged cycling. As a result, the system achieves an energy density of 133 Wh L-1, a 179% improvement over Topic 1. The stacking potential of the vertical electrode design suggests further scalability, marking a promising advancement in high-energy-density SWBs.