Electrochemical Film Formation on Magnesium Metal in an Ionic Liquid That Dissolves Metal Triflate and Its Application to an Active Material with Anion Charge Carrier

Irregular metallic growth at the anode during recharging of batteries can seriously influence the safety of batteries. To address this problem, we have attempted to design active anode materials with anion charge carriers and recently observed the formation and dissolution of an electrochemical film by triflate anions (CF₃SO₃^–) at the surface of magnesium in an ionic liquid (IL) electrolyte of Mg­(CF₃SO₃)₂, which represents a rare anode material. The effect of heterogeneous cations on film formation was examined in this work. In an IL that dissolves NaCF₃SO₃, sodium ions with a lower reduction potential than Mg²⁺/Mg would not be expected to assist film formation. However, to our surprise, we discovered that some sodium ions are involved in film formation. The sodium ions are believed to act as a cross-linking point for the formation of a film network, which resulted in fairly good reversibility for film formation. In a Ce­(CF₃SO₃)₃-IL electrolyte, an electrochemically formed film free of Ce³⁺ was obtained. The trivalent cerium cations were deactivated and transformed to an oxide on Mg metal. However, the reversibility of film formation in the Ce­(CF₃SO₃)₃ system did not meet the expected level. By coupling the film formation and dissolution behavior with a V₂O₅ cathode, a rechargeable battery was fabricated with dual ion transport species of Na⁺ or Ce³⁺ for the cathode and CF₃SO₃^– for the anode. The unique battery with NaCF₃SO₃ is demonstrated to exhibit good discharge/charge performance with long-term cyclability

Insertion of Calcium Ion into Prussian Blue Analogue in Nonaqueous Solutions and Its Application to a Rechargeable Battery with Dual Carriers

We observed the first electrochemical insertion of Ca²⁺ into Prussian blue analogue, MnFe­(CN)₆, in nonaqueous solutions of Ca­(CF₃SO₃)₂ and various solvents including ionic liquid at 60 °C. The kinetics for the Ca²⁺ insertion reaction were studied by cyclic voltammetry, and were compared to those of Na⁺ intercalation. By coupling this phenomenon with metallic anodes, two energy storage devices were made. Ca anode produced a primary cell that operated at a voltage of around 2.0 V. When Mg plate was used as an anode, the negative active material associated with CF₃SO₃^–, which we have already reported was newly formed at the surface of Mg plate. By combining the negative active material, we have fabricated a novel rechargeable battery using dual ion transport species of Ca²⁺ for the cathode and CF₃SO₃^– for the anode, and demonstrated that the battery showed repeated discharge/charge performance

Anode Material Associated with Polymeric Networking of Triflate Ions Formed on Mg

We have examined anode materials with anions as an ion transport species to solve metal deposition in rechargeable batteries intrinsically. Mg deposition-dissolution tests were conducted using some ionic liquid electrolytes at 60 °C, and a new electrochemical reaction was observed in addition to Mg deposition and dissolution in an electrolyte of Mg­(CF₃SO₃)₂ and <i>N</i>-methyl-N-propylpiperidinium bis­(trifluoromethane sulfonyl)­amide. The reaction was determined to be based on the formation and release of a polymeric network of triflate ions (CF₃SO₃^–) on the Mg metal surface, which suggests a novel anode material with anion carriers. An anion battery is also demonstrated using this phenomenon and incorporating an acrylate polymer with 2,2,6,6-tetramethylpiperidine-oxyl side units in the cathode to provide evidence of the rechargeability by the intermediary anion carrier