Polymers from biomass waste including plant/forest waste, biological industrial process waste, municipal solid waste, algae, and livestock are potential sources for renewable and sustainable resources. Converting biomass-derived polymers to functional biochar materials via pyrolysis is a mature and promising approach as these products can be widely utilized in many areas such as carbon sequestration, power production, environmental remediation, and energy storage. With abundant sources, low cost, and special features, the biochar derived from biological polymeric substances exhibits great potential to be an alternative electrode material of high-performance supercapacitors. To extend this scope of application, synthesis of high-quality biochar will be a key issue. This work systematically reviews the char formation mechanisms and technologies from polymeric substances in biomass waste and introduces energy storage mechanisms of supercapacitors to provide overall insight into the biological polymer-based char material for electrochemical energy storage. Aiming to enhance the capacitance of biochar-derived supercapacitor, recent progress in biochar modification approaches including surface activation, doping, and recombination is also summarized. This review can provide guidance for valorizing biomass waste to functional biochar materials for supercapacitor to meet future needs.

Aqueous zinc‐iodine (Zn‐I ₂ ) batteries are promising energy storage devices; however, the conventional single‐electron reaction potential and energy density of iodine cathode are inadequate for practical applications. Activation of high‐valence iodine cathode reactions has evoked a compelling direction to developing high‐voltage zinc‐iodine batteries. Herein, ethylene glycol (EG) is proposed as a co‐solvent in a water‐in‐deep eutectic solvent (WiDES) electrolyte, enabling significant utilization of two‐electron‐transfer I ⁺ /I ⁰ /I ⁻ reactions and facilitating an additional reversibility of Cl ⁰ /Cl ⁻ redox reaction. Spectroscopic characterizations and calculations analyses reveal that EG integrates into the Zn ²⁺ solvation structure as a hydrogen‐bond donor, competitively binding O atoms in H ₂ O, which triggers a transition from water‐rich to water‐poor clusters of Zn ²⁺ , effectively disrupting the H ₂ O hydrogen‐bond network. Consequently, the aqueous Zn‐I ₂ cell achieves an exceptional capacity of 987 mAh g _I2 ⁻¹ with an energy density of 1278 Wh kg _I2 ⁻¹ , marking an enhancement of ≈300 mAh g ⁻¹ compared to electrolyte devoid of EG, and enhancing the Coulombic efficiency (CE) from 68.2% to 98.7%. Moreover, the pouch cell exhibits 3.72 mAh cm ⁻² capacity with an energy density of 4.52 mWh cm ⁻² , exhibiting robust cycling stability. Overall, this work contributes to the further development of high‐valence and high‐capacity aqueous Zn‐I ₂ batteries.

This review explores the current research progress on aluminum-ion batteries, focusing on cathode materials according to their mechanisms, as well as anodes and electrolytes, while discussing current challenges and modification strategies.

Rechargeable aqueous zinc batteries are promising for large‐scale energy storage due to their low cost and high safety; however, their energy density has reached the ceiling based on conventional cathodes with a single cationic redox reaction mechanism. Herein, a highly reversible cathode of typical layered vanadium oxide is reported, which operates on both the cationic redox couple of V⁵⁺/V³⁺ accompanied by the Zn²⁺ storage and the anionic O^–/O^2– redox couple by anion hosting in an aqueous deep eutectic solvent electrolyte. The reversible oxygen redox delivers an additional capacity of ∼100 mAh g^–1 at an operating voltage of ∼1.80 V, which increases the energy density of the cathode by ∼36%, endowing the cathode system a record high energy density of ∼506 Wh kg^–1. The findings highlight new opportunities for the design of high‐energy zinc batteries with both Zn²⁺ and anions as charge carriers.