Research progress on diluents of localized high-concentration electrolytes for high performance lithium metal batteries

Recently, localized high-concentration electrolytes (LHCEs) has been emerged as the next-generation electrolytes for higher power and energy density rechargeable batteries. Except for reduced viscosity and cost, LHCEs maintain the unique solvation structure and good interfacial compatibility of high-concentration electrolytes. The diluent, which is miscible with the parent concentrated electrolyte but does not coordinate with lithium cations, can not only optimize the physicochemical properties but also regulate the solvation structures of LHCEs. In this review, the recent research progress on diluents is comprehensive reviewed, focusing on their basic parameters and design principles, their molecular structure and application, their structure-function relationship, and future direction. Basic theory and principles of design novel diluent involve Kamlet-Taft Lewis basicity (β), electronic transition energy (ETN), donor number (DN) and maximum electrostatic potential (ESPmax), which are summarized in detail. Subsequently, according to the molecular structure of diluent, fluorinated linear ethers, fluorinated cyclic ethers, fluoroaromatic hydrocarbons, fluorinated esters, non-fluorinated diluents are systematically discussed, with a focus on the relationship between the molecular structures and cell performances. At last, this review is concluded with a proposal of the future direction on diluents for advanced LHCEs.