Understanding the role of water-based electrolytes on magnesium-ion insertion/extraction into λ-MnO ₂ lattice structure

In rechargeable batteries, the different anionic groups of the electrolyte salt having same cation play an importance role in improving electrode material performance due to their intrinsic characteristics which affect ion migration process, structural stability and self-discharge. To this end, we investigate Mg ²⁺ ion insertion into λ-MnO ₂ structure with aqueous electrolytes that contain three different salts of Mg. Experimental study includes both material characterizations performed to describe the physical properties of the active material and battery performance tests of the working electrode in Mg-based aqueous mediums. Herein, λ-MnO ₂ that functions as an inorganic electroactive template is synthesized by extracting Li ⁺ ion from LiMn ₂ O ₄ through acid leaching method. Electrochemical performance of the spinel-type positive electrode material is evaluated by means of cyclic voltammetry (CV) and galvanostatic charge-discharge measurements. In charge-discharge tests conducted at C/10 current density, λ-MnO ₂ electrode deliver approximately a reversible discharge capacity of 70 mAh g ⁻¹ in 1 M MgSO ₄ , 20 mAh g ⁻¹ in both 1 M MgCI ₂ and 1 M Mg(NO ₃ ) ₂ after 100 cycles. C-rate performance tests are carried out for the samples in which the electrolyte contained MgSO ₄ salt, and the result is found to be around 220 mAh g ⁻¹ capacity at C/40 current rate. Concurrently, the insertion of Mg ²⁺ ion is confirmed by EDX-mapping and XPS analysis after the first reduction step.