Designing a Quasi-Liquid Alloy Interface for Solid Na-Ion Battery

Solid-state sodium-ion batteries is emerging as a promising alternative to traditional lithium-ion batteries. Similar to lithium-ion batteries, sodium-ion batteries utilize sodium ions (Na⁺) as charge carriers. Sodium is more abundant and cheaper than lithium. However, unlike traditional batteries that use liquid electrolytes, solid-state batteries use a solid electrolyte. This can be made of various materials, including ceramics or solid polymers. During charging and discharging, sodium ions move between the cathode and anode through the solid electrolyte. The solid-state electrolyte is typically more stable and less flammable than liquid electrolytes, reducing the risk of battery fires and improving overall safety. They are considered for various potential applications, from electric vehicles to grid storage, especially in contexts where battery cost, safety, and material availability are critical. However, there are ongoing technical challenges in improving the energy density, cycle life, and performance consistency of sodium-ion batteries and there is currently significant research is being invested in overcoming challenges such as growth of sodium dendrites and suboptimal wettability between sodium and solid electrolytes and making solid-state sodium-ion batteries a viable commercial technology.

In a new study published in ACS Nano led by Professor Shan Liu, Jing Suo, Qianqian Zhao, Haoqing Tian, Ling Wang, Lei Dai, and Jiayan Luo from the School of Chemical Engineering at North China University of Science and Technology, focused on improving the wettability between the sodium anode and the solid electrolyte and suppress dendrite growth, which are critical factors limiting the performance and safety of SSIBs. At the core of this innovation lies the quasi-liquid alloy interface, specifically a carbon-coated sodium-potassium (C@Na−K) interface. This interface is designed to mitigate dendrite growth and enhance the wettability between the sodium anode and the solid electrolyte. The quasi-liquid nature of the C@Na−K interface thickness allows for dynamic adjustments in response to the thermal changes during battery operation, thereby enhancing the battery’s rate performance and stability. The researchers developed a C@Na−K interface by mixing a sodium-potassium (Na−K) alloy with carbon black (Super P). This mixture resulted in a quasi-liquid state that could be applied to the solid electrolyte surface. The Na−K alloy was chosen due to its low eutectic temperature, which allows it to remain in a liquid state at room temperature. The C@Na−K composite was applied to the surface of a NASICON-type Na₃Hf₂Si₂PO₁₂ electrolyte, forming an interface layer. The authors subjected the batteries with this interface to various electrochemical tests to assess their performance, including cycling stability and charge transfer capabilities.

The authors showed that batteries equipped with the C@Na−K interface showed significantly improved electrochemical performance, characterized by better wettability and accelerated charge transfer. The quasi-liquid nature of the C@Na−K interface effectively suppressed the growth of Na dendrites, a critical issue in SSIBs. Symmetrical cells with the C@Na−K interface demonstrated stable cycling over 3500 hours at room temperature. The critical current density reached up to 2.6 mA/cm² at 40 °C. Full cells with the quasi-liquid alloy interface exhibited excellent capacity retention (97.1%) and Coulombic efficiency (99.6%) even after 300 cycles at 0.5 C. The thickness of the liquid phase alloy interface adjusted dynamically with the exotherm of the cell cycling process, leading to better rate performance.

According to the authors, the C@Na−K interface is not only electrochemically advantageous but also mechanically and chemically stable. The presence of carbon in the interface enhances its adhesion to the solid electrolyte, which in turn better processability. This stability is crucial in ensuring the long-term viability of the batteries, especially under the stress of repeated cycling. The team’s approach to creating the quasi-liquid alloy interface involved a careful selection of materials and innovative methodologies. The Na−K alloy used has a low eutectic temperature, allowing it to exist in a liquid state at room temperature. By combining this alloy with carbon black (Super P), the team was able to create a non-Newtonian fluid state composite that exhibits excellent mechanical properties and can be evenly applied to the solid electrolyte surface.

Despite its many advantages, the C@Na−K interface is not without its challenges. Compared with sodium, liquid sodium potassium alloy has higher reactivity, which will bring certain safety risks. The liquid nature of the alloy necessitates careful handling and precise control during the battery assembly process. Moreover, there is a need for further research to prevent the penetration of the quasi-liquid metal into the electrolyte, which can affect the battery’s long-term stability and performance.

In conclusion, the new study by Professor Shan Liu and colleagues introduced an innovative approach to enhance the performance of SSIBs by addressing critical issues like dendrite growth and poor wettability. The quasi-liquid alloy interface, particularly the C@Na−K, not only offers a stable and dendrite-suppressed environment but also improves the overall electrochemical performance of SSIBs. These findings hold significant potential for advancing the development of high-energy, safe, and efficient solid-state batteries. The study provides a robust foundation for future research in the field, potentially leading to more sustainable and reliable energy storage solutions.