The conventional LiPF₆/carbonate-based electrolytes have been widely used in graphite (Gr)-based lithium (Li) ion batteries (LIBs) for more than 30 years because a stable solid electrolyte interphase (SEI) layer forms on the graphite surface and enables its long-term cycling stability. However, few of these electrolytes are stable under the more stringent conditions needed with a Li metal anode (LMA) and other anodes, such as silicon (Si), which exhibit large volume changes during charge/discharge processes. Many different approaches have been developed lately to stabilize Li metal batteries (LMBs) and Si-based LIBs. From this aspect, localized high-concentration electrolytes (LHCEs) have unique advantages: not only are they stable in a wide electrochemical window, they can also form stable SEI layers on LMA and Si anode surfaces to enable their long-term cycling stability. The ultrathin SEI layer formed on a Gr anode can also improve the safety and high-rate operation of conventional LIBs. In this paper, we give a brief summary of our recent work on LHCEs, including their design principle and applications in both LMBs and LIBs. A perspective on the future development of LHCEs is also discussed.

Significance: Rechargeable lithium metal batteries are promising next-generation high-energy-density energy storage systems. However, their applications have been greatly impeded by the instabilities of electrolytes toward both highly reductive lithium metal anode and high-voltage cathodes. Here, we use localized high-concentration electrolytes to investigate the fundamental correlation between the battery performance and the electrolyte solvation chemistry and its profound electrode/electrolyte interphase properties. Experimental and theoretical analyses reveal an intriguing general rule that the strong interactions between the salt and the solvent in the inner solvation sheath within the salt–solvent complexes promote their intermolecular proton/charge-transfer reactions, dictate the properties of the electrode/electrolyte interphases, and then influence the battery performances. These findings can shed light for rational electrolyte design for various battery systems.

Electrolyte is very critical to the performance of the high-voltage lithium (Li) metal battery (LMB), which is one of the most attractive candidates for the next-generation high-density energy-storage systems. Electrolyte formulation and structure determine the physical properties of the electrolytes and their interfacial chemistries on the electrode surfaces. Localized high-concentration electrolytes (LHCEs) outperform state-of-the-art carbonate electrolytes in many aspects in LMBs due to their unique solvation structures. Types of fluorinated cosolvents used in LHCEs are investigated here in searching for the most suitable diluent for high-concentration electrolytes (HCEs). Nonsolvating solvents (including fluorinated ethers, fluorinated borate, and fluorinated orthoformate) added in HCEs enable the formation of LHCEs with high-concentration solvation structures. However, low-solvating fluorinated carbonate will coordinate with Li⁺ ions and form a second solvation shell or a pseudo-LHCE which diminishes the benefits of LHCE. In addition, it is evident that the diluent has significant influence on the electrode/electrolyte interphases (EEIs) beyond retaining the high-concentration solvation structures. Diluent molecules surrounding the high-concentration clusters could accelerate or decelerate the anion decomposition through coparticipation of diluent decomposition in the EEI formation. The varied interphase features lead to significantly different battery performance. This study points out the importance of diluents and their synergetic effects with the conductive salt and the solvating solvent in designing LHCEs. These systematic comparisons and fundamental insights into LHCEs using different types of fluorinated solvents can guide further development of advanced electrolytes for high-voltage LMBs. Significance Lithium (Li) metal battery (LMB) is a very promising candidate for the next-generation high-energy-density batteries. However, its practical applications have been impeded by the instability of metallic Li in the state-of-the-art carbonate electrolytes. Localized high-concentration electrolytes (LHCEs) shed light for practical use of LMBs by generating stable, robust solid electrolyte interphase on Li. Here we investigate various types of fluorinated cosolvents as diluents for LHCEs to understand the correlation between the battery performance and the solvation structure and electrode/electrolyte interfacial chemistry. It clearly reveals that the diluent surrounding the highly concentrated salt-solvating solvent clusters accelerates/decelerates anion decomposition, tunes the properties of interphases, and subsequently influences the battery performance. These results can guide further development of advanced electrolytes for LMBs.