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Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 19 of 32

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of Li ions beneath the SEI, thereby making it a diffusion-controlled reaction. Reaction-controlled rather
[144]
than diffusion-controlled conditions are favorable for compact and stable deposition . Lithiated
NafionTM was reported as a single ion-conducting polymer to fabricate an ASEI [145,146] . Crosslinked ionic
networks offer the multifunctionality of both electrolyte blocking and single ion conduction . Xu et al.
[147]
proposed that coating an ion-transport rectifier on the Li metal anode allows the anions (e.g., PF ) to
6-
+
complex with the Lewis acid sites and the solvated cations (Li ) to effectively translocate within the pore
channels with both high σ and t [Figure 7F] . Apart from the diffusivity of ions within the interphase,
[148]
Li+
Li+
the surface diffusivity also has a significant influence on the Li deposition morphology. High surface
diffusivity is generally enabled by halides, such as LiF, LiBr and LiI . Gao et al. reported a self-assembled
[149]
monolayer of electrochemically active molecules that regulates the composition and nanostructure of the
SEI [Figure 7G] . A multilayer SEI that contains an amorphous outer layer and a LiF-rich inner phase
[150]
seals the Li anode effectively. Lin et al. also developed a conformal LiF coating technique on Li surface with
[151]
commercial Freon R134a as the reagent [Figure 7H] .
Electrolyte modification
In general, the interaction between electrolyte solvent molecules plays a critical role in deciding the bulk
properties of an electrolyte, such as the dielectric constant, boiling and melting points and viscosity, which
is largely driven by van der Waals forces and tends to be much weaker than the other interactions among
the solvent, cations and anions. As shown in Figure 8A, the strategies of electrolyte modification can be
divided into three classes based on the regulation of the interactions in the electrolyte system: (1) regulating
the interaction between the cations and solvent to modulate the redox stability of solvent molecules;
(2) regulating the interaction between cations and anions to optimize the formation of solvent separated ion
pairs (SSIPs), contact ion pairs (CIPs) and aggregates (AGGs) and salt solubility; and (3) regulating the
interaction between the anions and solvent to adjust the anion solvation behavior .
[152]
+
In electrolytes, especially in non-aqueous systems, Li ions are generally solvated by the surrounding solvent
molecules. Thus, the cation-solvent interactions have an important impact on guiding the cation
solvation/desolvation and transport behavior. For example, the Li solvation structure can be intuitively
+
tuned by adjusting the electrolyte compositions. The components featuring a high Gutmann donor number
are more favorable to be recruited into the inner solvation shell. Frontier molecular theory can be employed
to uncover the redox stability of the modified solvation shell. The coordinated Li ions attract electrons from
+
[153]
the solvent molecules and correspondingly lower their LUMO . In the electrolyte, the solvent molecules
+
coordinated with Li ions can obtain electrons much more easily from the anode than free solvent
molecules, leading to electrolyte decomposition. Therefore, according to the different affinities of solvents
+
toward Li ions, the chemical and electrochemical stabilities of electrolytes can be regulated and designed.
For example, fluoroethylene carbonate (FEC) possesses a lower LUMO energy level than DEC and EC and
can more easily obtain electrons from the electrode due to the strong electron-withdrawing effect of the F
functional group . Consequently, FEC preferentially reacts with the Li anode to induce a LiF-rich SEI.
[154]
Zhang et al. created a highly stable organic interphase with a well-tuned LUMO energy to improve the anti-
reduction ability of SEI components and enhance the long-term cyclability of LMBs [155,156] . Introducing
trifluoromethyl functional groups (-CF ) to the molecular structure in the SEI can significantly tune the
3
orbital energies and the HOMO/LUMO gap due to the strong electron-withdrawing property of -CF
3
functional groups [Figure 8B] [155,156] .

In contrast to aqueous electrolytes, where anions and cations are completely separated by the solvent
molecules, organic solvents tend to possess much lower dielectric constants than water and the cation-anion
interaction cannot be weakened effectively. Consequently, Li salts cannot be dissociated completely, and
+
therefore the Li -anion interaction significantly affects the coordination of SSIP, CIP and AGGs, further