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The stability of the solid electrolyte interphase (SEI) layer during the charging-discharging cycles is reasonably related to its microscopic elasticity. For the first time, it was theoretically revealed that each component of the elastic moduli takes a maximum at an optimal concentration of 1.0 vol % of fluoroethylene carbonate (FEC) for the SEI layer formed in the FEC-added NaPF 6 /PC-based electrolyte. The elastic constants indicated that the SEI layer formed at lower FEC concentrations is more resistant to tensile and shear deformations. The optimal hardness is sensitive in the lower FEC concentrations although it simply decreases as the FEC concentration increases. This is due to the formation of a denser SEI structure with small cavities in the lower concentrations. The results are excellently consistent with the experimental one, justifying the microscopic understanding of the FEC additive effect on the mechanical stability of the SEI layers designed through the Red Moon simulation., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs). Recent investigations were summarized on NaPF 6 salt and fluoroethylene carbonate (FEC) additives in propylene carbonate (PC)-based electrolyte solution, as one of the best electrolytes to effectively passivate the hard-carbon electrode with higher cycling performance for next-generation NIBs. The FEC additive showed high efficiency to significantly enhance the capacity and cyclability of NIBs, with an optimal performance that is sensitive at low concentration. Computationally, both microscopic effects, positive and negative, were revealed at low and high concentrations of FEC, respectively. In addition to the role of FEC decomposition to form a NaF-rich solid electrolyte interphase (SEI) film, intact FECs play a role in suppressing the dissolution to form a compact and stable SEI film. However, the increase in FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. In addition, this review introduces the Red Moon (RM) methodology, recent computational battery technology, which has shown a high efficiency to bridge the gap between the conventional theoretical results and experimental ones through a number of successful applications in NIBs., Competing Interests: The authors declare that there are no conflicts of interest regarding the publication of this article., (This journal is © The Royal Society of Chemistry.)

Solid electrolyte interphase (SEI) is an ion conductive yet electron-insulating layer on battery electrodes, which is formed by the reductive decomposition of electrolytes during the initial charge. The nature of the SEI significantly impacts the safety, power, and lifetime of the batteries. Hence, elucidating the formation mechanism of the SEI layer has become a top priority. Conventional theoretical calculations reveal initial elementary steps of electrolyte reductive decomposition, whereas experimental approaches mainly focus on the characterization of the formed SEI in the final form. Moreover, both theoretical and experimental methodologies could not approach intermediate or transient steps of SEI growth. A major breakthrough has recently been achieved through a novel multiscale simulation method, which has enriched the understanding of how the reduction products are aggregated near the electrode and influence the SEI morphologies. This review highlights recent theoretical achievements to reveal the growth mechanism and provides a clear guideline for designing a stable SEI layer for advanced batteries., (© 2021 Wiley-VCH GmbH.)

The practical application of nonflammable highly salt-concentrated (HC) electrolyte is strongly desired for safe Li-ion batteries. Not only experimentalists but also theoreticians are extensively focusing on the dilution approach to address the limitations of HC electrolyte such as low ionic conductivity and high viscosity. This study suggests promising highly-fluorinated ethers to dilute the HC electrolyte based on non-flammable trimethyl phosphate (TMP) solvent. According to the quantum mechanical and molecular dynamics calculations, the fluorinated ether diluents showed a miscibility behavior in HC TMP-based electrolyte. While such miscibility behavior of the diluent with TMP solvent has been significantly enhanced by increasing its degree of fluorination, i.e., the "fluorous effect", it is remarkable that the self-diffusion constant of Li + and the ionic conductivity should be significantly improved by dilution with bis(1,1,2,2-tetrafluoro ethyl) ether (B2E) and bis(pentafluoro ethyl) ether (BPE) compared to other common hydrofluoroether diluents. In addition, the fluorinated-ether diluents have high ability to form a localized-concentrated electrolyte in HC TMP-based solution, leading to high expectation for the formation of a stable and a compact inorganic SEI film.

A highly salt-concentrated (HC) electrolyte based on the nonflammable solvent trimethyl-phosphate (TMP) has recently shown an attractive self-extinguishing property in addition to an excellent charge-discharge performance. However, the microscopic understanding of its solid electrolyte interphase (SEI) layer remains an open question. In this Letter, the red moon (RM) method was used to investigate the molecular mechanism of SEI layer formation depending on lithium bis(fluorosulfonyl)amide (LiFSA) salt concentration in a TMP-based electrolyte and was able to reproduce successfully the experimental observations, i.e., the "bottom-up" formation mechanism with a thinner and denser SEI layer mainly based on salt reduction in the HC electrolyte. The results showed that a pure dense inorganic layer is formed in the HC electrolyte, which should considerably improve the SEI layer stability leading to a longer lifetime in charge-discharge performance. This new microscopic finding should provide an important guide in designing an effective nonflammable electrolyte to develop advanced, safe secondary batteries.

Fluoroethylene carbonate (FEC) is an effective additive to improve the performance of Na-ion batteries (NIB). A recent experimental study has shown that a small amount of FEC enhances the NIB performance, whereas increasing the FEC amount deteriorates the performance. Toward understanding the microscopic mechanism of this observation, the dependency of the solid electrolyte interphase (SEI) film formation on the FEC concentration has been investigated in a propylene carbonate (PC)-based electrolyte solution by using the Red Moon method. This method was able to reproduce successfully the experimental observations where a small amount of FEC makes SEI film stable. Further, the increase in FEC amounts decreased the stability of the SEI film and should lead to the decrease in the NIB lifetime during charge-discharge cycles. It was revealed that this is because of the insufficient organic dimer formation between the monomer products at the higher FEC concentration. Finally, it was reconfirmed theoretically that the appropriate adjustment of FEC additive amount is essential to develop the high-performance of NIB.