A research team led by Prof. CHEN Wei from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) revealed for the first time the important role of halogen-mediated solvation structure in the de-solvation process of multivalent ions. The study was published in Joule. 

Researchers managed to use manganese metal batteries (MnMBs) as the research platform to demonstrate the important role of halogen-mediated (with Cl as the main research object) mechanism in lowering the overpotential of the multivalent metal ions deposition and enhancing the Coulombic and the dissolution/deposition efficiencies. With theoretical calculations and experiments, they verified that Cl was involved in the solvation of Mn²⁺ in the designed electrolyte, transforming [Mn(O_sol)₆]²⁺ solvated structure into [Mn(O_sol)₃Cl₃]²⁺.  

Compared with other atoms, Cl atoms have a larger radius and smaller charge density, thus the solvated Mn-Cl bond is weaker than the Mn-O bond, greatly reducing the de-solvation energies during deposition, lowering the deposition overpotential of the manganese metal anode and significantly enhancing the Coulombic and Faraday efficiencies. 

Researchers assembled symmetric and asymmetric cells to demonstrate the halogen-mediated electrolyte's reliability. The experimental data showed that the electrolyte can support stable cycling of symmetric cells for more than 700 h at a current density of 0.1mA cm^-2, which is far beyond the performance of the reported manganese-metal battery electrolytes. 

The symmetric cells showed steady polarization values at different current densities, which demonstrates the excellent multiplicity performance of the electrolyte. In addition, the electrolyte provided a Coulombic efficiency close to 100% and deposition/dissolution overpotentials of <200 mV even on different metallic or nonmetallic collectors. An asymmetric cell assembled from this electrolyte can be stably cycled for more than 1000h when applying Ketjenblack (KB) as the current collector. 

Remarkably, the manganese deposition/dissolution efficiency remained as high as 96.8% even at a higher surface capacity (5 mAh cm^-2). 

To further validate the value of the designed halogen-mediated electrolyte, researchers developed a halogen-meditated non-aqueous manganese metal full cell. The cell was able to cycle stably for nearly 600 cycles in the designed halogen-mediated electrolyte and exhibited satisfying multiplicity performance and two well-defined plateaus.  

The study provides a new way to develop rechargeable non-aqueous MnMBs enabled by electrolyte engineering, which is expected to benefit other multivalent metal batteries and electroplating industries.