Redox-Targeting Approach with Hard Carbon and Biphenyl Semi-Liquid Electrode for Enhancing Energy Density in Seawater Batteries

Renewable energy sources such as solar, wind, and wave power are growing in importance due to concerns about fossil fuels and climate change. To make these sources reliable, energy storage systems (ESSs) are essential. Among ESS designs, Seawater Batteries (SWB) stand out as the next generation of large-scale ESSs, utilizing the abundant Na+ ions in seawater and an open cathode design for high theoretical energy density. SWB requires specific anode materials and liquid electrolytes due to the presence of a solid electrolyte (NASICON) that separates the anode and cathode. Unlike traditional batteries, SWB's liquid electrolyte requires a narrow electrochemical stability window only for the anode, allowing electronic conductivity without the risk of short circuits. Previous studies have used Na metal, solid anodes, and liquid anode materials in SWB. While sodium metal can provide high volumetric energy density, it suffers from limited cycle life. Solid-state anodes face electrode stacking challenges that limit energy density improvements. Liquid anodes such as sodium-biphenyl fill the cell efficiently, but offer lower volumetric energy density. Nevertheless, the use of a liquid anode with high cyclability made it possible to realize the most reliable seawater battery and it became the most widely used. To address the energy density challenge, a concept called "Redox-targeting" from redox flow battery systems has been applied to SWB. This approach incorporates semi-liquid active materials in a powder state and redox-mediated electrolytes in a liquid state, forming a semi-liquid electrode (SLE) configuration. Sodium-biphenyl (Na-BP), with ionic and electronic conductivity, and hard carbon (HC) were used as active materials for the HCBP SLE electrode. For Na-BP and HC, the overlapping response voltage bands were confirmed by the dQ/dV plot to confirm the amount of available capacity. After confirming the reaction mechanism of the HC used by the galvanostatic intermittent titration (GITT) method, it was found that the intercalation reaction could be involved, which could be confirmed by ex-situ TEM images. The electrochemical properties were then evaluated in a seawater half-cell system using sodium ferrocyanide as the cathode material and were confirmed to be 11.1 mAh cm^-2 for over 500 cycles (5000 hours). Using this approach, successful mass production of HCBP SLE and its application in seawater batteries has achieved, indicating the potential for industrial utilization.