image: The antisolvent in the electrolyte of lithium metal batteries is widely involved in modulating the solvation structure of the electrolyte, the formation of the SEI film and the lithium metal deposition behavior on the anode.
Credit: ©Science China Press
In response to the unclear modulation mechanism of antisolvents, the scientific team of Haoshen Zhou & Shaohua Guo from Nanjing University focused on the antisolvent polarity, and ingeniously constructed a series of ester-based localized high-concentration electrolytes (LHCEs) by using trifluorobenzene allotropes antisolvents with similar structures. They systematically studied the regulation mechanism of antisolvents in lithium metal batteries. They found that the antisolvent polarity widely affects the solvation structure, the solid electrolyte interphase (SEI) film and the lithium deposition behavior. The specific findings are briefly described as follows:
1) The antisolvent plays a "drag effect" in the solvation structure. Research has found that in LHCEs, highly polar antisolvents can interact with anions in the primary solvation sheath rather than the main solvent, which directly leads to a weakening of the interaction between cations and anions. Although this weak dipole-anion interaction can only play a "fine-tuning" role in the solvation structure, its accumulation throughout the battery life cycle will amplify its regulatory effect. More importantly, this discovery further corrects the existing micellar solvation structure model.
2) The antisolvent participates in the formation of the SEI film. Research has found that the decomposition of antisolvents will generate organic components at the electrode-electrolyte interface, which is not conducive to ion transport. And the antisolvent decomposition degree is determined by the quality of the anion derived SEI film in the early stage and the polarity of the antisolvent. Electrolytes containing highly polar antisolvents are not conducive to the formation of thin and highly ionic conductivity SEI films at the interface.
3) The interfacial adsorption of antisolvents affects the lithium deposition behavior. Research has found that highly polar antisolvents that are hydrophobic to lithium ions are more likely to adsorb on the surface of lithium metal electrodes, which hinders the transmission of lithium ions during the transport process and is not conducive to the uniform deposition of lithium metal.
By optimizing the antisolvent polarity, the research team has developed an ester-based LHCE with high lithium metal compatibility, which can support the stable cycling of lithium metal full batteries. More importantly, the research team established for the first time the structure-activity relationship between antisolvent polarity and solvation structure, interfacial chemistry and Li deposition behavior, successfully filling the theoretical gap in the design of antisolvent molecular structure. This work not only corrects the solvation structure model of LHCEs, further perfecting the solvation chemistry theory, but also provides a classic paradigm for the systematic research of advanced secondary battery electrolytes.