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.

Lithium metal batteries (LMBs) are one of the most promising candidates for next‐generation high‐energy‐density rechargeable batteries. Solid electrolyte interphase (SEI) on Li metal anodes plays a significant role in influencing the Li deposition morphology and the cycle life of LMBs. However, a thorough understanding on the mechanisms of SEI formation and evolution is still inadequate. In this review, the progress in understanding structures, properties, and influencing factors of SEI, as well as efficient strategies of tailoring SEI are focused upon. First, the compositions, models, and recent progress in characterizing atomic structures of SEI are summarized. Second, the properties of SEI, including electronic conduction, ionic conduction, stability, and mechanical properties are elucidated. Structures and properties of SEI are greatly affected by multiple factors, thus interactions between these factors and SEI are systematically discussed. Correlations of SEI with Li deposition morphology, rate capability, and cycle life are further summarized. Moreover, efficient strategies of tailoring SEI with desired properties, including in situ SEI and ex situ SEI, are also reviewed. Finally, future directions, including in‐operando techniques, multi‐modality approaches for characterization of SEI, and artificial intelligence assisted understanding of correlations between electrolyte components and SEI properties are proposed.

Functional electrolyte is the key to stabilize the highly reductive lithium (Li) metal anode and the high-voltage cathode for long-life, high-energy-density rechargeable Li metal batteries (LMBs). However, fundamental mechanisms on the interactions between reactive electrodes and electrolytes are still not well understood. Recently localized high-concentration electrolytes (LHCEs) are emerging as a promising electrolyte design strategy for LMBs. Here, we use LHCEs as an ideal platform to investigate the fundamental correlation between the reactive characteristics of the inner solvation sheath on electrode surfaces due to their unique solvation structures. The effects of a series of LHCEs with model electrolyte solvents (carbonate, sulfone, phosphate, and ether) on the stability of high-voltage LMBs are systematically studied. The stabilities of electrodes in different LHCEs indicate the intrinsic synergistic effects between the salt and the solvent when they coexist on electrode surfaces. Experimental and theoretical analyses reveal an intriguing general rule that the strong interactions between the salt and the solvent in the inner solvation sheath promote their intermolecular proton/charge transfer reactions, which dictates the properties of the electrode/electrolyte interphases and thus the battery performances.

Perspective on recent improvements in experiment and theory towards realizing lithium metal electrodes with liquid electrolytes.

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IntroductionLithium (Li)-ion batteries (LIBs) have now been the indispensable power sources for portable electronic devices, electric vehicles, stationary or grid applications, etc. [1] However, further efforts on extending the cycle life, rate capability, energy density and working temperature range and improving the safety of LIBs are still facing significant challenges for their large-scale applications. Focusing on the increase in energy density of a battery, the possible approach is to use the high capacity electrode (cathode or anode) material and the high voltage cathode material. Ni-rich layered oxides LiNi x Mn y Co 1−x−y O 2 (NMC) with Ni content ≥80% (e.g., NMC811) are regarded as one of the most potential candidates to usher in the new stage of ultra-high energy density LIBs due to their increased specific capacities at higher voltages and the low cost with less Co content. However, the practical applications of these Ni-rich NMC cathode materials are greatly hindered by the poor cathode-electrolyte interface (CEI) layer formed on such cathode surface in the state-of-the-art electrolytes comprised of lithium hexafluorophosphate (LiPF 6 ) in carbonate solvents, especially at voltages higher than 4.3 V versus Li/Li + , [2] causing continuous electrolyte oxidative decomposition and other related side reactions such as transition metal dissolution from the cathode surface, thus leading to poor cycling stability, especially at elevated temperatures and high operating voltages. [3] Therefore, advanced electrolytes with better oxidative protection to Ni-rich NMC cathode materials, especially under high voltages are critically important for enabling application of Ni-rich NMCs in LIBs.Significant efforts have been made to develop novel electrolytes for high voltage cathode materials, mainly through using high anodic solvents to substitute carbonate solvents, the increase of salt concentration and the utilization of film-forming additives. Fan et al. developed an all-fluorinated electrolyte of 1 m LiPF 6 in fluoroethylene carbonate/3,3,3-fluoroethylmethyl carbonate/1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether (FEC/FEMC/HFE, 2:6:2 by wt), which significantly enhanced the cycling stability of Li||NMC811 (2.7-4.4 V) and Li||LiCoPO 4 (3.5-5.0 V) in high voltages by effectively inhibiting electrolyte LiNi x Mn y Co 1−x−y O 2 (NMC) cathode materials with Ni ≥ 0.8 have attracted great interest for high energy-density lithium-ion batteries (LIBs) but their practical applications under high charge voltages (e.g., 4.4 V and above) still face significant challenges due to severe capacity fading by the unstable cathode/electrolyte interface. Here, an advanced electrolyte is developed that has a high oxidation potential over 4.9 V and enables NMC811-based LIBs to achieve excellent cycling stability in 2.5-4.4 V at room temperature and 60 °C, good rate capabilities under fast charging and discharging up to 3C rate (1C = 2.8 mA cm −2 ), and superior low-temperature discharge performance down to −30 °C with a ca...

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Figure 4. The CEI structures on cycled LCO cathodes in different electrolytes by HR-STEM characterizations. a,b) After 100 cycles in the BL-carbonate electrolyte. c,d) After 300 cycles in the AD-ether electrolyte. a,c) The ABF-STEM images. b,d) The HAADF-STEM images.