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Yan et al. Energy Mater 2023;3:300002 https://dx.doi.org/10.20517/energymater.2022.60 Page 5 of 32

Unstable formation of SEIs on Li surface
The SEI has become a critical research topic since the pioneering work of Aurbach and Peled [12-15] . Different
models, including mosaics (polyhetero microphase), vacancies (Schottky lattice defects), double-layer
capacitors, multilayers and monolithic SEIs, can be used to explain the chemical and physical properties of
SEIs [Figure 2A-E] [12,13,16,17] . Among them, the mosaic model with various components and a dual-layer
architecture is commonly accepted. The inner layer, which is close to the anode, consists of species with low
oxidation states, including LiF, Li N, Li O, Li CO and LiOH, and is labeled as an inorganic layer. In
3
2
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contrast, the outer layer of the SEI is composed of species with high oxidation states, such as RCOO Li,
2
ROLi and ROCO Li, and is labeled as the organic layer.
2
Due to the significantly negative electrochemical potential of Li /Li, virtually any available electrolyte can be
+
reduced by Li metal to form electrically insulating and ionically conductive species on the electrode surface
[Figure 3A]. Goodenough et al. explained the SEI formation mechanism based on the frontier molecular
[18]
orbital theory . As shown in Figure 3B, μ and μ refer to the electrochemical potentials of the anode and
A
C
cathode, respectively. Accordingly, E LUMO and E HOMO are representative of the voltage that corresponds to the
lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the
electrolytes, respectively. If μ > E LUMO , electrons tend to transfer from the anode surface to the unoccupied
A
orbital of the electrolyte, triggering the reduction reaction of the electrolyte. The generated SEI serves as a
barrier, preventing further decomposition of the electrolyte. Analogously, redox reactions facilitate the
formation of the SEI between the electrode and electrolyte in the case of μ < E HOMO .
C
The component and structure of the SEI film are strongly dependent on the composition of the electrolyte,
mainly organic carbonate- and ether-based electrolytes, which determine the primary properties of the SEI
film. Organic carbonates are the most commonly used electrolyte solvents for LMBs. The preliminary
composition of the SEI in a carbonate-based electrolyte system is mainly made up of Li alkyl carbonates
(ROCOOLi, where R is an organic group related to the solvent), which are generated by the one-electron
reduction of organic carbonates . More stable components of Li CO , Li O and Li halides can be presented
[14]
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in the SEI depending on the used salts . However, the SEIs formed in organic carbonate-based electrolytes
[15]
lack flexibility, which makes them susceptible to interfacial fluctuations. With significant dendrite
suppression capacity, ethers are considered as strong candidates for electrolyte solvents for LMBs in
multiple systems, which can be ascribed to the formation of oligomers with strong affinity to the Li surface
and excellent flexibility . For example, 1,3-dioxolane (DOL), as a widely used ether solvent in LMBs, can
[19]
form oligomeric SEI films with superior flexibility and strong binding affinity to Li metal. As the Li metal
anode is cycled in the DOL-based electrolyte, DOL is reduced on the Li metal surface to form different
alkoxy species, leading to its anionic partial polymerization to form LiO-(CH CH OCH O) Li type species.
2
2
2
n
A surface film is thus formed by these short-chain oligomers of poly(ethylene oxide) that is elastomeric and
can accommodate the morphological change of the anode surface upon long-term cycling.
+
The SEI film is an essential path for the desolvation and reduction of Li ions on the electrode, where the
diffusion behavior of Li ions crucially affects the Li deposition morphology. Specifically, once solvated Li
+
+
ions are transported to the SEI surface under the electric field, the SEI is capable of desolvating Li ions and
+
the naked Li ions are then transported to the current collector where they are reduced. Furthermore, robust
+
and dense SEI can prevent Li exposure, which directly causes the Li corrosion. A desired SEI for efficient
and safe LMBs should possess the following features: (1) excellently ionic conductivity; (2) proper thickness,
which is sufficient to prevent electron penetration to electrolyte completely and appropriate for the
diffusion resistance of Li ions; (3) extraordinary chemical and electrochemical stability in composition
+