“Hunting” Na Dendrites by Self-Formed Fluorinated Fe Valence Gradient Interphase: A Step towards Practical Solid-State Sodium-Metal Batteries


Solid-state sodium-metal batteries (SSSMBs) is a promising energy storage technology complementary to traditional lithium-ion batteries (LIBs). The drive for SSSMBs is fueled by the urgent need for higher energy densities, improved safety, and the use of more abundant and less expensive materials. Despite these benefits, the practical application of SSSMBs is limited by several challenges, particularly related to the anode/electrolyte interface and dendrite growth. The compatibility between the sodium metal anode and the solid electrolyte is a major challenge because poor interfacial contact can lead to uneven sodium deposition and stripping during charge/discharge cycles. This uneven deposition results in the formation of sodium dendrites, needle-like structures that can grow through the electrolyte, potentially causing short circuits and catastrophic battery failure. This issue is exacerbated by the highly reactive nature of sodium, which tends to form unstable interfaces with many solid electrolytes. Among the various solid electrolytes explored, NASICON-type Na1+xZr2SixP3-xO12 (NZSP) has emerged as a leading candidate due to its high ionic conductivity, good thermal and mechanical stability, and broad electrochemical window. However, NZSP also suffers from poor interfacial contact with sodium metal, leading to rapid dendrite growth and high interfacial resistance. This has highlighted the need for the development of innovative strategies to stabilize the NZSP/Na interface and suppress dendrite formation.

New study published in Advanced Functional Materials and conducted by a research group led by Professor Xiaowen Zhan from the Anhui University designed a novel self-formed interphase with a unique Fe valence gradient which can stabilize the anode interface of NZSP solid electrolyte and enhance the overall performance and longevity of SSSMBs. The researchers employed a sodiophilic α-Fe2O3-xFx interlayer, which reacts in situ with sodium to form a stable, low-impedance, and dendrite-free interface.

In their experiments, the average ionic conductivity of their NZSP pellets prepared via a conventional solid-state reaction route was measured at 9.0 × 10-4 S cm-1 at 25 °C with a low activation barrier of 0.16 eV, comparable to the best values reported in the literature. The team developed a fluorinated iron oxide (α-Fe2O3-xFx, or α-FeOF) coating to enhance the interfacial properties of NZSP. This was achieved by drop-casting a FeF3·3H2O solution on the NZSP substrate followed by annealing at 550 °C in an Ar atmosphere.

“The conventional synthesis of fluorinated Fe2O3 involved either heating Fe2O3 and complex F-based precursors or annealing Fe2O3 in toxic fluorine or mixed fluorine/oxygen gas, the method we accidentally discovered in this work is very simple and environmentally friendly,” said first author Le Xiang, a graduate student in Prof. Zhan’s group.

The authors performed contact angle measurements to assess the Na wettability of the α-FeOF@NZSP surface. Molten Na spread quickly over the α-FeOF-coated NZSP, showing a significantly smaller contact angle compared to pristine NZSP, which indicated poor Na wettability. Cross-sectional SEM images further showed a tight interfacial contact between Na and α-FeOF@NZSP, contrasting with the apparent gap at the Na/NZSP interface. These results demonstrated that the α-FeOF coating substantially improved the wettability and interfacial contact with sodium metal.  Moreover, density functional theory calculations provided insights into the enhanced performance of the α-FeOF interlayer. The calculated work of adhesion (Wad) for the Na(001)/α-FeOF(110) interface was 1.10 J m⁻², significantly higher than for the Na(001)/Na2CO3(001) interface (0.12 J m⁻²). This indicated a stronger interaction and better wettability, corroborating the experimental contact angle measurements. Differential charge density analysis showed substantial charge transfer from Na to α-FeOF, further supporting the formation of a stable and intimate interphase.

They also tested interfacial resistance and critical current density (CCD) by assembling symmetric cells (Na|α-FeOF@NZSP|Na and Na|NZSP|Na). They showed that electrochemical impedance spectroscopy for Na|α-FeOF@NZS80P|Na cells to have a much lower interfacial resistance (31.4 Ω cm2) compared to Na|NZSP|Na cells (626.1 Ω cm2). CCD measurements revealed that Na|α-FeOF@NZSP|Na cells could achieve a remarkably high CCD of 1.9 mA cm-2 at 80 °C. Long-term cycling tests demonstrated that Na|α-FeOF@NZSP|Na cells maintained stability over 1000 hours at 1 mA cm-2, far exceeding the performance of cells without the α-FeOF interlayer.

“Increasing the CCD and prolonging the anode interface stability at high current densities are critical for developing practical SSSMBs capable of offering competitive energy and power density. That is why we’ve tried best to push the symmetric cells to endure long cycling at a relatively high current density (i.e., 1 mA cm-2) that is much more demanding than those used in most literatures on SSSMBs,” said corresponding author Xiaowen Zhan, who led the study.

To understand the stability and composition of the interphase during cycling, they conducted XPS depth profiling on a cycled Na|α-FeOF@NZSP|Na cell. The analysis revealed an outer layer rich in NaF and Fe, with a gradient in Fe valence from Fe2+ to Fe3+ as the distance from the Na interface increased. This composition prevented continuous reduction reactions and homogenized charge distribution, effectively suppressing dendrite growth. Cross-sectional SEM images of the cycled interphase confirmed the absence of dendrites and a stable, intimate interface. Finally, the researchers assembled full SSSMBs using Na3V2(PO4)3 (NVP) cathodes and evaluated their performance. The NVP|α-FeOF@NZSP|Na cells showed excellent rate performance, delivering 94.5 mAh g-1 at 0.2 C and 91.1 mAh g-1 at 1 C. The produced cells maintained a capacity retention of 96% over 120 cycles at 1 C, with high coulombic efficiency. In contrast, NVP|NZSP|Na cells without the α-FeOF interlayer experienced rapid capacity decline and eventual failure and highlights the effectiveness of the α-FeOF interphase in ensuring long-term stability and high performance.

In conclusion, Professor Xiaowen Zhan and colleagues successfully resolved the issues of poor anode/electrolyte compatibility and dendrite growth by designing a self-formed interphase with a unique Fe valence gradient. The reported improved interfacial stability and electrochemical performance demonstrated in the new study have several practical implications, for instance, it reduces the risk of dendrite formation, which is a primary cause of short circuits and battery failure and enhances the safety profile of SSSMBs, which make it more reliable for consumer and industrial applications. Moreover, the robust interphase design leads to longer cycle life and higher capacity retention makes SSSMBs more viable for long-term energy storage solutions and reduces the need for frequent battery replacements and lowers the overall cost of ownership. Furthermore, the excellent achievement of a high CCD demonstrates the capability of these batteries to operate efficiently at high power demands and this relevant for applications requiring rapid charge and discharge cycles, such as electric vehicles and grid energy storage. Additionally, sodium is an abundant and low-cost material compared to lithium which make the development of efficient SSSMBs using sodium reduce the dependency on lithium resources and can result in more sustainable and cost-effective energy storage solutions.

“Hunting” Na Dendrites by Self-Formed Fluorinated Fe Valence Gradient Interphase: A Step towards Practical Solid-State Sodium-Metal Batteries - Advances in Engineering

About the author

Xiaowen Zhan received his B.S. degree in University of Sci. & Tech. Beijing and Ph.D. degree in University of Kentucky. He started his postdoctoral researches in Pacific Northwest National Laboratory (2018–2020) and joined School of Materials Science and Engneering in Anhui university as a professor (2020). His current research interests include defect chemistry and solid state ionics of advanced ceramic materials and their applications in metal-ion and all-solid-state batteries.

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L. Xiang, D. Jiang, Y. Gao, C. Zhang, X. Ren, L. Zhu, S. Gao, X. Zhan, Self-Formed Fluorinated Interphase with Fe Valence Gradient for Dendrite-Free Solid-State Sodium-Metal Batteries. Adv. Funct. Mater. 2024, 34, 2301670. https://doi.org/10.1002/adfm.202301670

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