Abstract

A high-capacity anode active material was synthesized via a green hydrogel method using bio-derived agar as both an environmentally friendly gelling agent and a weak complexing matrix for metal ions. Hydrogels were prepared at three different pH values (3, 5, and 10) by incorporating nickel nitrate into the agar network under controlled conditions. After drying, all hydrogels were pre-treated at 180 degrees C in air. To assess the influence of the pre-treatment atmosphere, the hydrogel synthesized at pH 10 was additionally subjected to pre-treatment at 180 degrees C under vacuum, resulting in four distinct samples in total. Subsequently, all four samples were separately calcined at 600 degrees C for 5 h in air to complete crystallization and remove residual organics, then characterized structurally and morphologically. The gel precipitated at pH 10, then vacuum-pretreated (S10v) exhibited the best electrochemical performance, achieving a reversible capacity of 1107.81 mAh g-1 at 100 mA g-1 and maintaining 1531.16 mAh g-1 after 300 cycles at 0.1 A g-1, outperforming conventional NiO. This performance was attributed to its high specific surface area and the coexistence of nano-sized Ni along with NiO particles. The porous structure improved electrode-electrolyte interactions, while metallic Ni atoms provided catalytic activity and improved electron transport. In the synthesis, the partial reduction of NiO to Ni was likely driven by the carbonization of agar under oxygen-deficient conditions, which could produce reducing gases or localized carbon, alongside the exothermic decomposition of nickel nitrate. These findings highlight the promise of agar-assisted xerogel-derived Ni/NiO composites as high-performance anode materials for lithium-ion batteries.