Investigations on Interfacial Reactions of Electro-lytes on Carboneous and Magnesium Anodes for Rechargeable Lithium and Magnesium Batteries

Abstract

This study investigated the effects of HF, an impurity in LiPF6 electrolytes, on redox reactions of graphite anodes for the lithium-ion batteries (LIBs) and electrolyte solution for magnesium-ion batteries were developed, based on sulfone group solution, non-Grignard reagents. In the first place, it is found that pyrolytic graphite edge plane electrode (PGE) presents reversible Li+ transport behavior in LiClO4 solution, but suppressed intercalation/de-intercalation reaction in LiPF6 electro-lyte. The sluggish intercalation/de-intercalation reaction in LiPF6 is progressed by adding a HF scavenger, whereas the facile intercalation/de-intercalation reaction in LiClO4 is depressed by adding HF. In addition, the Li+ transport in LiPF6 is enhanced by decreasing electrolyte volume or by increasing PGE surface area. These behaviors are attributed to the HF-induced formation of LiF layer on graphite anode surface, which is facilitated at high ratio of electrolyte volume to electrode area (V/A ratio). The electrolyte-volume-to-electrode-area ratio affects the Li+ transport behavior of the graphite composite electrodes for commercial grade LIBs. This study explains on an age-old question: why Li+ transport behavior of graphite anode in LiPF6 is suppressed in flooded cells, but not in commercial LIBs. Areas of electrode surface were carried out by double-layer capacitances (Cdl) which related to a fraction of edge (fe). Lastly, it is necessary that new electrolyte of reversible magnesium system is developed instead of Grignard-based because of its unstable chemically and electrochemically. Sulfone-based solutions are one of good can-didates. Dialkyl sulfones (R1R2SO2) such as dipropyl sulfone (DPSO2), ethyl-methyl sulfone (EMSO2) and dibutyl sulfone (DBSO2) were performed with conventional magnesium salt (MgCl2) in CVs. Eutectic of sulfone-based electrolyte and co-solvent with sulfone electrolyte were performed. Especially, cell perfor-mances were enhanced when adding tetrahydrofuran (THF) in dialkyl sulfone electrolyte by volume ration one to one (1/1 = v/v). The high performance sulfone electrolyte such as DPSO2 and DPSO2/THF were com-pared with their characteristics such as ionic conductivity. Ionic conductivity is improved with adding THF in sulfone solution. Besides the study of cathode materials and current collectors, incompatibility between the electrolytes, which may be related to the chemical instability of the Grignard reagent-based electrolyte, is also critical issues for rechargeable Mg battery with a good performance. Therefore, understanding the electro-chemical behavior of current collectors and synthesis of stable electrolyte are critical for Mg batteries.

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