Deep eutectic solvents (DESs) have attracted extensive research for their potential applications as leaching solvent to recycle valuable metal elements from spent lithium ion batteries (LIBs). Despite various advantages like being economical and green, the full potential of conventional binary DES has not yet been harnessed because of the kinetics during leaching. Herein, we consider the fundamental rate-determining-step (RDS) in conventional binary DES and attempt to design ternary DES, within which the chemical reaction kinetics and diffusion kinetics can be regulated to maximize the overall leaching rate. As a proof of concept, we show that the ternary choline chloride/succinic acid/ethylene glycol (ChCl/SA/EG) type ternary DES can completely dissolve LCO powder at 140 °C in 16 h. By systematically studying the leaching process at various conditions, the energy barrier during leaching can be calculated to be 11.77 kJ/mol. Furthermore, we demonstrate that the extraction of the cobalt ions from the leaching solution can be directly achieved by adding oxalic ions without neutralizing the solution. The precipitate can be used to regenerate LCO with high purity. The recycled materials show comparable electrochemical performance with commercial LCO. Our design strategy of ternary DES with regulated RDS is expected to have both scientific and technological significance in the field of hydrometallurgical recycling of LIBs.

The wider application of conventional electrochemical double-layer supercapacitors is limited by severe self-discharge. Current strategies for tackling self-discharge are mainly focused on each specific component of the device, while a general approach based on configuration design from the device level is largely unknown. Herein, we report a conjugated supercapacitor based on pairs of presodiated manganese oxides with the unique working mechanism of redox pseudocapacitance. Upon aging, the holding time for voltage dropping from 1.0 to 0.4 V for the conjugated device based on Na x δ-MnO2 vs Na1–x δ-MnO2 is four times longer than that of an activated carbon (AC) vs AC device. The successful suppression of self-discharge based on designing device configuration is expected to be universally viable for other capacitive materials.

The realization of carbon neutralization requires further development of highperformance supercapacitors, while their wider applications are limited by serious self-discharge. This work proposes a conjugately configured supercapacitor, as well as the strategy for optimizing the structure of active materials specifically for such supercapacitor. The conjugated supercapacitor consists of pairs of prelithiated T-Nb 2 O 5 @C and forms a Li x Nb 2 O 5 @C versus Li 2−x Nb 2 O 5 @C configuration. We demonstrate that the elegant control of the coated carbon shell has a significant effect on the self-discharge behavior. An optimized annealing temperature of 300°C is beneficial for reducing the elemental impurity while not introducing microregional phase impurity, thus leading the best resistance towards self-discharge. When the conjugated supercapacitor is charged to an initial voltage of 1.0 V, the voltage could retain for more than 3000 min before dropping to 0.5 V. This work is expected to provide general guidance for optimizing the active materials for conjugated supercapacitors.

Inside front cover image: The ambitious goal of cabron neutralization requires the further development of fast energy storage devices like supercapacitors, while the severe self‐discharge of conventional electrochemical double‐layer capacitors has greatly limited their development. The recent emergence of conjugately configured supercapacitor has brought a new frontier for alleviating self‐discharge. In article number https://doi.org/10.1002/cnl2.55, The researchers explored how the carbon coating strategy, which has already been widely used to modify active materials to enhance conductivity and cycling stability, could introduce profound and complex influence to self‐discharge property for active materials specifically used for conjugatedly configured supercapacitors. The proposed conjugated configuration as well as the optimization strategy specifically for such device is expected to provide a general and powerful approach for alleviating self‐discharge.