RESUMEN
The solid electrolyte interphase (SEI) film, which consists of the products of reduction reaction of the electrolyte, has a strong influence on the lifetime and safety of Li-ion batteries. Of particular importance when designing SEI films is its strong dependence on the electrolyte solvent. In this study, we focused on geometric isomers cis- and trans-2,3-butylene carbonates ( c/ t-BC) as model electrolytes. Despite their similar structures and chemical properties, t-BC-based electrolytes have been reported to enable the reversible reaction of graphite anodes [as in ethylene carbonate (EC)], whereas c-BC-based electrolytes cause the exfoliation of graphite [as in propylene carbonate (PC)]. To understand the microscopic origin of the different electrochemical behaviors of t-BC and c-BC, we applied Red Moon simulation to elucidate the microscopic SEI film formation processes. The results revealed that the SEI film formed in c-BC-based electrolytes contains fewer dimerized products, which are primary components of a good SEI film; this lower number of dimerized products can cause reduced film stability. As one of the origins of the decreased dimerization in c-BC, we identified the larger solvation energy of c-BC for the intermediate species and its smaller diffusion constant, which largely diminishes the dimerization. Moreover, the correlation among the Li+ intercalation behavior, nature of the SEI film, and strength of solvation was found to be common for EC/PC and t-BC/ c-BC electrolytes, confirming the importance of solvation of the intermediates in the stability of the SEI film. These results suggest that weakening the solvation of the intermediates is one possible way to stabilize the SEI film for better charge-discharge performance.
RESUMEN
Secondary batteries such as Li-ion battery are expected to be utilized as not only ubiquitous electric power sources such as mobile phones but also large-scale electricity storage devices. Therefore, it is urgent to develop the higher performance secondary batteries. Their lifetime and stability are found to be strongly dependent on the nature of passivation film called solid electrolyte interphase (SEI) film formed on the anode surface in the initial charge-discharge cycle. However, since it is difficult to directly observe the film formation processes in experiment, its microscopic mechanism is still not found. On the other hand, although the theoretical methods are useful complement to the experiment, some new methodologies are necessary to understand the long-term processes of SEI film, which is produced as a result of that a lot of chemical reactions proceed simultaneously. Under the circumstances, we have developed Red Moon method that can simulate such complex chemical reaction systems, and were able to analyze for the first time the SEI film formation processes on the anode surface at the atomistic level. Then, we clarified theoretically the microscopic mechanism of the additive effect which is essential to improve the Na-ion battery performance so as to enhance the SEI film formation. This new microscopic insight must provide an important guiding principle for use in designing the most suitable electrolytes for developing high-performance secondary batteries.
RESUMEN
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.