image: Schematic illustration of the failure mechanisms of quasi Na-NASICON solid-state batteries.
Credit: Wenwen Sun and Yongjie Zhao from the Beijing Institute of Technology
Solid-state sodium batteries (SSSBs) are emerging as a promising alternative to conventional lithium-ion batteries, owing to their enhanced safety, cost-effectiveness as well as the abundance of sodium resources. However, despite their conceptual advantages, significant performance degradation, mainly associated to the electrode-electrolyte interfaces, has hindered their widespread application. A recent study led by researchers from the Beijing Institute of Technology provides a comprehensive mechanistic understanding of interfacial degradation in NASICON-type electrolyte-based solid-state sodium metal batteries. Their work focuses on Na₃Zr₂Si₂PO₁₂ (NZSP), a widely studied ceramic electrolyte known for its robust thermal stability and competitive ionic conductivity, yet plagued by poor long-term interfacial performance.
Solid-state sodium metal batteries (SSMBs) have attracted considerable attention due to their high energy density and inherent safety advantages over traditional liquid electrolyte systems, making them promising candidates for next-generation energy storage. Among the various electrolyte materials, NASICON-type ceramics like Na3Zr2Si2PO12 (or NZSP) stand out because of their excellent ionic conductivity and mechanical strength. However, the widespread practical application of these batteries remains hindered by persistent challenges related to interfacial instability, which leads to capacity fading, dendrite growth, and short-circuiting during cycling. The core limitation lies in the complex failure mechanisms occurring at the interfaces between the solid electrolyte, sodium metal anode, and cathode, driven by chemical reactions, mechanical degradation, and transport disruptions. Despite extensive research, a comprehensive understanding of these interfacial failure processes has remained elusive, impeding the development of strategies to enhance battery lifespan and safety.
A team of researchers from Beijing Institute of Technology have advanced the field by systematically investigating these electrochemical-mechanical failure mechanisms, revealing how interfacial reactions, including the formation of reaction products and voids, contribute to battery degradation. The research reveals that when sodium metal contacts with NZSP, an interfacial reaction leads to the formation of a sodium-rich phase, Na₃.₂₉₄Zr₁.₉₃₆Si₂PO₁₂. This new phase exhibits what the authors describe as a "dual-blocking" effect, significantly impeding sodium-ion migration at the interface. Such interfacial blocking causes inhomogeneous ion transport and the buildup of localized current density, and ultimately promotes the formation of voids and sodium dendrites. These phenomena, in turn, degrade the physical contact between the electrode and oxide electrolyte, reducing cycling stability and increasing the risk of short circuits.
Moreover, the study establishes a mechanistic correlation between the transport rate of Na⁺ ions (vₜ) and the self-diffusion rate of metallic sodium atoms (vₛ). When vₜ exceeds vₛ, especially under high current density, a mismatch arises that fosters void nucleation and dendritic growth. This imbalance is identified as a fundamental cause of interface failure in NZSP-based SSSBs.
Liquid Electrolyte Interphases: A Critical Yet Overlooked Factor
To further elucidate the electrochemical behavior of NZSP-based cells, the team investigated hybrid configurations incorporating small amounts of liquid electrolytes (LEs) between the solid electrolyte and cathode. Two commonly used sodium salts—NaPF₆ and NaClO₄—served as representative systems.
The results demonstrate stark contrasts in interfacial stability. The NaPF₆-based LE generates a relatively uniform and thin solid-liquid electrolyte interphase (SLEI), which improves ionic transport and supports stable cycling. Conversely, the NaClO₄-based LE produces a more heterogeneous and irregular SLEI, leading to increased impedance, non-uniform ion flux, and premature capacity fading.
Electrochemical impedance spectroscopy and distribution of relaxation time (DRT) analyses revealed that NaPF₆-based systems exhibit lower interfacial resistance and better cycle stability, retaining 63% of their initial capacity after 100 cycles. In contrast, cells utilizing NaClO₄ showed only 24% capacity retention under identical conditions.
Surface Chemistry and Structural Insights
Post-cycling surface analyses provide further insights into the interfacial chemistry. X-ray photoelectron spectroscopy (XPS) analysis confirms the presence of NaF and NaCl as major inorganic degradation products from NaPF₆ and NaClO₄, respectively. While NaF contributes to the formation of a stable SLEI, NaCl deposits tend to form discontinuous layers that increase resistance and compromise interfacial integrity.
Additionally, trace amounts of HF—formed via hydrolysis of NaPF₆—were shown to react with hydroxyl groups on the NZSP surface, inducing localized corrosion. These chemical alterations, while subtle, significantly affect long-term battery performance and must be addressed through optimized LE formulations or interface engineering strategies.
Toward Long-Life, High-Safety SSSBs
By dissecting the intricate electrochemical-mechanical coupling at both anode and cathode interfaces, this study advances the fundamental understanding of failure pathways in Na-NASICON solid batteries. The concept of “dual-blocking interphases” and the vₜ–vₛ mismatch offer a new perspective for designing future interfaces with improved mechanical stability and ion transport properties.
Looking forward, the authors emphasize the importance of further developing interface-tolerant electrode materials, refining synthesis methods of solid electrolyte, and exploring advanced interlayer designs. These approaches are essential for realizing the full potential of solid-state sodium metal batteries toward the application in large-scale energy storage and electric vehicles.
Reference: Wenwen Sun, Yang Li, Chen Sun, Xuanyi Yuan, Haibo Jin, Yongjie Zhao. Deciphering the electrochemical-mechanical coupling failure mechanism of Na-NASICON solid-state batteries[J]. Materials Futures, 2025, 4(3): 035102. DOI: 10.1088/2752-5724/adeff9
Journal
Materials Futures