Research and Application Prospects of NZSP as an Electrolyte Material for Sodium-Ion Solid-State Batteries

The increasingly severe global climate and environmental issues have prompted more countries to accelerate the transition from traditional fossil fuels to clean renewable energy. However, the large-scale application of renewable energy faces challenges in energy storage technology.

 

The increasingly severe global climate and environmental issues have prompted more countries to accelerate the transition from traditional fossil fuels to clean renewable energy. However, the large-scale application of renewable energy faces challenges in energy storage technology. With the growing energy demand in human society, developing efficient, safe, and cost-effective energy storage technologies has become a research hotspot in academia.

 

Solid-state batteries are currently regarded as the next-generation electrochemical devices to achieve high energy density and safety. The key to developing solid-state battery technology lies in replacing traditional organic liquid electrolytes with solid-state electrolytes (SSE). Most current research focuses on solid-state lithium (Li) batteries. However, considering the availability of lithium resources triggered by the huge demand for lithium-based batteries—lithium is scarce, unevenly distributed, and costly, while the use of cobalt (Co) in cathode materials further increases costs—developing alternative rechargeable batteries, particularly solid-state sodium (Na) batteries, is advisable as a technical reserve.

 

 

As an emerging energy storage technology, sodium-ion batteries offer broad application prospects in fields such as smart grids and new energy vehicles due to their abundant resources and low cost.

 

In the research of sodium-ion batteries, the selection and performance of electrolyte materials are crucial. Solid-state electrolytes have gradually gained favor among researchers due to their high safety and long lifespan. Notably, NZSP (Na₃Zr₂Si₂PO₁₂) solid-state electrolyte, with its unique structural and performance characteristics, has become a research hotspot in the field of sodium-ion batteries.

 

In 1976, Good enough et al. first proposed the NASICON (Na super ionic conductor)-type sodium ion conductor. NASICON materials are another important class of sodium ion conductor materials, featuring a three-dimensional framework structure formed by corner-sharing ZrO₆ octahedrons and POâ‚„ or SiOâ‚„ tetrahedrons. Sodium ions are embedded in the gaps of the three-dimensional framework, which form three-dimensional isotropic sodium ion diffusion channels.

As sodium ions only occupy part of the available sites in these channels, NASICON materials exhibit high ionic conductivity, comparable to that of β″-Alâ‚‚O₃, one of the best-known sodium ion conductors. When x=2, Na₁₊ₓZrâ‚‚Siâ‚“P₃₋ₓO₁₂ demonstrates the highest ionic conductivity, making it the first reported sodium ion conductor with a NASICON structure. Therefore, researchers typically focus on doping modifications based on Na₃Zrâ‚‚Siâ‚‚PO₁₂. NZSP exhibits good room-temperature ionic conductivity (10⁻⁴ to 10⁻³ S cm⁻¹), which helps improve battery charging and discharging efficiency, providing favorable conditions for its application in sodium-ion batteries.

 

Additionally, NZSP electrolytes feature high chemical stability and a wide electrochemical window, enabling them to form good interfaces with positive and negative electrode materials, reduce interfacial resistance, and maintain stable operation in complex environments, thus enhancing battery performance and offering more possibilities for applications in solid-state sodium-ion batteries.

 

Higher Safety: As solid-state electrolytes, NZSP materials are non-leaking and non-flammable, significantly reducing safety risks during battery use compared to liquid electrolytes.

 

Good Mechanical Properties: Facilitates easy processing and packaging during battery manufacturing.

 

Excellent Thermal Stability: Maintains good performance in high-temperature environments.

However, solar panel are typically synthesized via high-temperature sintering, leading to a major issue: high contact resistance between electrode materials and brittle ceramic electrolytes.

 

During cycling, volume changes in electrode materials can cause structural defects and cracks in ceramic solid-state electrolytes, easily inducing sodium dendrite growth along the cracks. This increases interfacial resistance and reduces the reversible capacity and cycling stability of electrode materials. Composite electrolytes with other types of electrolytes can partially reduce interfacial resistance between electrodes and electrolytes. Polymer electrolyte components help lower interfacial resistance, improve ion mobility, and enhance ion migration and diffusion in organic phases, holding promising application prospects in future sodium-ion batteries.

 

Furthermore, poor wettability between NASICON ceramic solid-state electrolytes and metallic sodium leads to high interfacial contact resistance, and issues such as sodium dendrites piercing ceramic electrolyte grain boundaries significantly limit the application of ceramic electrolytes. Introducing a "polymer-Na₃Zr₂PO₄(SiO₄)₂-polymer" sandwich structure to avoid direct contact between solid ceramic electrolytes and metallic sodium electrodes can significantly improve the wettability of metallic sodium on ceramic electrolyte surfaces, reduce interfacial resistance, and inhibit sodium dendrite formation during cycling, achieving good interfacial stability between sodium electrodes and ceramic solid-state electrolytes.

 

Recently, domestic scholars developed a synchrotron-based in-situ X-ray imaging method to systematically study the origin of dendrites in NZSP-based SSE. The study found that dendrite growth essentially depends on the grain boundaries (GBs) in NZSP and the interfacial properties between NZSP and Na. It was confirmed that the wetting kinetic evolution of Na dendrites in NZSP is closely related to Na ion/electron conductivity and the Young's modulus of GBs.

As a polycrystalline material, NZSP is affected by inevitable microdefects such as voids, cracks, and grain boundaries. Even with identical compositions and crystal structures, these microdefects can cause conductivity differences of several orders of magnitude between intra-cellular and total conductivity in polycrystalline materials. Therefore, fine-sized and uniformly distributed NZSP nanopowders are more conducive to air evacuation during preparation, reducing voids and cracks and forming dense and uniform solid-state electrolytes.

 

With the rapid development of new energy vehicles, smart grids, and other fields, the demand for high-performance and high-safety energy storage technologies is increasingly urgent. As an excellent electrolyte material for sodium-ion batteries, NZSP solid-state electrolytes have broad development prospects.

 

The high safety and long lifespan of NZSP solid-state electrolytes offer great application potential. By reducing battery replacement costs, they further lower the usage costs of electric vehicles. Compared to traditional liquid electrolytes, their non-leaking and non-flammable properties significantly enhance battery safety. High ionic conductivity and stability also help improve energy density and cycle life, extending driving range and operational efficiency through high efficiency and energy density.

 

Large-scale energy storage systems based on NZSP solid-state electrolytes can ensure stable operation and dispatching of power grids. Their high energy density and fast charging/discharging capabilities enable grids to more efficiently absorb and release renewable energy, improving energy utilization.

 

As an excellent electrolyte material for sodium-ion batteries, NZSP solid-state electrolytes have broad application prospects in new energy vehicles, smart grids, and other fields. With technological advancements and industrial chain improvement, the production costs of NZSP solid-state electrolytes will continue to decrease, promising to bring more clean energy and sustainable development momentum to human society.