Fat-Soluble Vitamin K2 as a Biologically Safe and Sustainable Electrode Material with High Cycle Stability for Aqueous Zinc-Ion Batteries

Conventional Li-ion batteries (LIBs) are unsuitable for biorelated applications and produce environmentally harmful wastes upon disposal due to the high toxicity of their organic electrolytes and transition-metal oxide electrodes. Although several nature- or bioderived organic electrode materials such as riboflavin (i.e., Vitamin B2) have been examined for electrode applications, their high solubility in electrolytes leads to rapid capacity fading, and chemical modifications often increased their toxicity. In this study, we propose using fat-soluble Vitamin K2 (VK2) as a biologically safe cathode material for aqueous zinc-ion batteries. Owing to its poor solubility in the aqueous electrolytes, the VK2 electrode delivers considerably improved cycle stability compared with a water-soluble riboflavin electrode. To further minimize environmental impacts, we employed aqueous-processable binders, including carboxymethyl cellulose (CMC) and sodium alginate (SA), instead of the poly(vinylidene fluoride) (PVDF) binder that requires the use of toxic organic solvents. Notably, the VK2 electrodes with the CMC and SA binders achieved superior rate performance compared with their PVDF-based counterparts. It should also be noted that all VK2 electrodes demonstrated outstanding operational stability over 500 cycles regardless of binders used. Finally, the cell viability tests for the electrode slurries highlight the promising potential of VK2 as a biologically safe and sustainable electrode for future battery applications.