Key Takeaways & Executive Findings
- •• LiPF6 integration into sodium-ion battery electrolytes strengthens solid electrolyte interphase (SEI) film and stabilizes O3 electrode surfaces, enhancing cycleability with 92.7% at 400 cycles. • Li-based SEI exhibits reduced solubility, effectively suppressing sodium-ion and electron leakage, and mitigating electrolyte decomposition on hard carbon electrode. • The formation of Li-ion pillars on O3-type electrode surfaces significantly reduces oxygen release and electrolyte degradation, resulting in improved capacity retention. • Compared with conventional fluoroethylene carbonate additive, the dual-interface strengthening approach yields superior capacity retention and long-term stability.
Abstract
The robust respective formations of a solid electrolyte interphase (SEI) and pillar at the surfaces of hard carbon and O3-type positive electrodes are the consequences of integrating LiPF6 salt into a sodium-ion battery electrolyte that considerably strengthens both interfaces of positive and negative electrodes. The improvement of cycle performances due to the formation of highly passivating SEI on the hard carbon electrode is induced by the alternated solvation structure following the addition of Li salt, which inhibits sodium-ion and electron leakage from further electrolyte decomposition. The SEI with incorporated Li is less soluble than Na-based SEI, and the passivation ability of the initially formed SEI can thus be well preserved. Conversely, the gas evolution caused by oxygen release is reduced considerably by the marginal surface intercalation of Li ions at the surface of the O3-positive electrode. Additionally, the LiF layer that forms on the O3 surface diminishes additional deterioration of the electrolyte after formation. Compared with the fluoroethylene carbonate additive that is typically applied, a simultaneously strengthened interface yields major improvements in capacity retention.
1. Introduction
The substitution of charge carriers by sodium ions ensures the natural abundance of material resources. Hence, the supply of minerals can be stabilized, thereby delivering financially stable chemistry, resulting in large-scale energy storage and device-compatible, sodium-ion batteries (SIBs) [1–14]. The most widely used SIB chemistry is based on the hard carbon and layered transition metal oxide system [15–21]; therefore, the initially injected electrolyte is decomposed from the limited electrochemical stability window of the typical carbonate-based electrolytes [22–29]. From this decomposition, interphase formation occurs on the surface of the electrode in SIBs; however, the dominant decomposition characteristics of electrolytes in SIBs are considerably different than those of lithium-ion batteries (LIBs).
The standard reduction potential of the Na/Na+ redox couple is 0.33 V higher than that of Li/Li+ redox couple; thus, the overpotential necessary to deliver the reduction current of the electrolyte, which expresses the kinetics of the formation of the solid electrolyte interphase (SEI), is not sufficient at identical voltage swing range [25, 30]. This results the thermally vulnerable performances of SIBs due to the insufficient deposition of SEI film on hard carbon electrode at SIB than that of LIBs. Furthermore, the number of solvated solvents is higher in SIBs compared with LIBs, and the reducibility of the solvated cation cluster is thus highly decreased [30, 31]. It is because the positive charge density of solvation cluster is decreased with the increment of solvation, which leads less reducible environment of electrolyte. Consequently, the formation of SEI in the SIBs is thermodynamically less favorable than in LIBs from thermodynamically and kinetically [30]. Moreover, formed sodium-ion-based SEI is more soluble in electrolytes compared with the SEI in the LIBs, implying that the chemical stability of SEI is insufficient for long-term operations [25, 32–34]. In layered structures, intercalation and de-intercalation of sodium ions severely degrade the interface because of the larger ionic radius of the sodium ion compared with that of lithium ions.
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Jooeun Byun, Joon Ha Chang, Chihyun Hwang, Chae Rim Lee, Miseung Kim, Jun Ho Song, Boseong Heo, Sunghun Choi, Jong Hyeok Han, Hee-Jae Jeon, Beom Tak Na, Youngjin Kim, Ji-Sang Yu, Hyun-seung Kim (2025). Transformative Effect of Li Salt for Proactively Mitigating Interfacial Side Reactions in Sodium-Ion Batteries. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01742-z
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Frequently Asked Questions
What is the main finding of this study?
The study demonstrates that integrating LiPF6 salt into sodium-ion battery electrolytes significantly strengthens both the solid electrolyte interphase (SEI) on hard carbon anodes and the surface stability of O3-type cathodes, leading to enhanced cycle life and capacity retention.
How does Li salt improve the SEI in sodium-ion batteries?
Li salt alters the solvation structure, resulting in a Li-incorporated SEI that is less soluble than Na-based SEI, thereby better suppressing sodium-ion and electron leakage and preventing further electrolyte decomposition.
What is the effect of Li salt on the O3-type cathode?
Li ions intercalate marginally at the cathode surface, forming Li-ion pillars that reduce oxygen release and electrolyte degradation. Additionally, a LiF layer forms, further protecting the cathode and improving capacity retention.
How does this approach compare to using fluoroethylene carbonate (FEC) additive?
The dual-interface strengthening achieved with Li salt yields greater improvements in capacity retention compared to the conventional FEC additive, as it simultaneously enhances both anode and cathode interfaces.
What are the practical implications of this research?
This research provides a promising strategy for developing high-performance sodium-ion batteries with improved cycle stability, which is crucial for large-scale energy storage applications.
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