To increase the energy density of a secondary battery, high performance and safety of an electrolyte and high stabilization of an electrode-electrolyte interface are major problems. Therefore, for innovative electrolytes such as high concentration electrolytes that have high electrochemical stability and can reduce the risk of combustion, efforts will be made to elucidate the microscopic mechanisms related to the electrolyte structure and electrochemical reactivity, and to promote the acquisition of guidelines for new electrolyte materials.