The characteristics of the interface between Ni-rich cathode materials and carbonate-based electrolytes are crucial for stability in charge-discharge cycles. In this study, we used tris(trimethylsilyl)phosphite (TMSPi) as an additive for film formation and investigated the properties of the layer formed on a LiNi0.6Co0.2Mn0.2O2 (NCM622) thin-film electrode. This layer can act as a barrier to mitigate the underlying attack at the oxygen sites of the layered Ni-rich structure. Moreover, this layer mainly consists of Li-rich organic components, leading to a decreased activation energy with a rapid Li-ion transfer process.
Renewable energy resources and rechargeable batteries are key to establishing a carbon-neutral society. Lithium-ion batteries (LIBs) have been widely used in portable electronic devices for the past 30 years. However, the further spread of large-scale batteries is essential in the household and industrial sectors, which drives the research and development of technologies beyond LIBs. Since ionic liquids are safe and confer unique physicochemical properties, several next-generation batteries utilizing ionic liquid electrolytes have been researched. Sodium-ion and potassium-ion batteries show promise in overcoming the potential problems of LIBs related to the uneven distribution of lithium and cobalt resources. Fluoride-shuttle batteries deliver significantly higher theoretical energy densities compared to current LIBs. Nevertheless, many issues remain unresolved for the practical application of these batteries. This comprehensive paper provides several research topics on next-generation rechargeable batteries utilizing ionic liquids and various charge carriers, unveiling their novelty, the issues to be solved, and future research directions.
Li salts and polar solvents form solvates, and certain solvates have low melting temperatures and remain in a liquid state at room temperature. Liquid-state solvates exhibit ionic conductivity and can be used as electrolytes in lithium batteries. The author and co-workers have systematically studied the interactions of Li+ ions with solvents and anions, Li+-coordination structures, thermal properties, transport properties, and electrochemical properties in molten-solvate electrolytes. In molten solvates, almost all solvent molecules are coordinated to Li+ ions, and uncoordinated (free) solvents are rare. Additionally, anions are involved in the coordination of the Li+ ions. The molten solvate electrolytes show non-flammability and negligible vapor pressure at room temperature because of the extremely low concentration (activity) of the free solvent, which can improve the thermal stability of Li batteries. The low activity of the free solvent results in a wide electrochemical window of the molten-solvate electrolytes, thereby suppressing undesired side reactions in Li batteries. The activity of the free solvent in the electrolytes significantly affects the electrochemical reaction processes, such as the reduction reaction of sulfur (S8) in a Li–S battery and the oxygen reduction reaction (ORR) in a Li–air battery. The solubility of the reaction intermediates of the S8 cathode and the ORR decreases with the decrease in solvent activity, which enables the highly efficient charge–discharge of Li–S and Li–air batteries. In molten solvates, Li+ ions diffuse and migrate by exchanging ligands (solvents and anions). Certain molten-solvate electrolytes show high Li+ ion transference numbers over 0.5, and these high transference numbers are useful in mitigating the concentration overpotential during the charging and discharging of Li batteries at high current densities.
The supporting electrolyte salt is an essential component of electrochemical reactions. Although there have been many reports on the influence of the type of electrolyte and its concentration on reaction efficiency in electrosynthesis, very few reports have systematically discussed the reasons for such effect. In several reaction systems, we have found that the coordination of anions from the supporting electrolyte to cationic organic species generated in electrochemical oxidation dramatically changes the reaction efficiency. In this comprehensive paper, we review these case studies, generalize the findings learned from them, and provide guidelines for strategic electrolyte design.
Boron-doped diamond (BDD) electrodes are next generation electrode materials and their electrochemical applications have been actively developed in recent years. They are expected to be useful electrode materials for improving the environment and for bio-medical applications. Here, examples of practical applications as electrochemical sensors, the development of in vivo real time measurements, and electrochemical organic synthesis using BDD electrodes are briefly introduced. In the second part, our recent work on the production of useful chemicals by means of the electrochemical reduction of CO2 using BDD electrodes is described. The work has attracted particular attention for its potential contribution to carbon neutrality and carbon recycling.