The race to find better alternatives to lithium-ion batteries has taken an exciting turn. Researchers in Germany have discovered a new sodium-based material that could make rechargeable batteries more affordable and sustainable without sacrificing performance.

As our world becomes increasingly electrified, from smartphones to electric vehicles and grid-scale energy storage, the demand for battery materials continues to soar. While lithium-ion batteries have dominated the market for years, concerns about lithium's limited availability and rising costs have prompted scientists to search for alternatives. Sodium, which is far more abundant and cheaper than lithium, has emerged as a promising candidate.

The Sodium Advantage

Sodium is the sixth most abundant element on Earth and can be extracted from seawater or mined from vast salt deposits. Unlike lithium, which is concentrated in only a few regions worldwide, sodium is universally available. This abundance translates to potentially lower production costs and reduced geopolitical concerns about supply chains.

However, sodium has its challenges. It is about three times heavier than lithium and has a lower electrochemical potential, which means sodium-ion batteries typically cannot match lithium-ion batteries in energy density or specific capacity. Despite these limitations, sodium batteries hold tremendous promise for applications where weight is less critical, such as stationary energy storage systems that support renewable energy grids.

The real bottleneck in developing sodium-ion batteries has been finding materials that allow sodium ions to move quickly and efficiently through the battery. This movement, called ionic conductivity, is crucial for battery performance. Fast ion conductors enable batteries to charge quickly and deliver power effectively.

A New Material Enters the Scene

Scientists at the Technical University of Munich have developed a novel sodium-based material called Na8SnP4, composed of sodium, tin, and phosphorus. What makes this material remarkable is its ability to conduct sodium ions at room temperature with a conductivity of 0.53 milliSiemens per centimeter. While this might sound like technical jargon, it essentially means the material allows sodium ions to flow through it about twice as efficiently as earlier sodium phosphide materials.

The research team discovered that Na8SnP4 exists in two different structural forms, called polymorphs, depending on how the material is cooled during synthesis. The low temperature polymorph forms when the material cools slowly, while the high temperature polymorph results from rapid cooling. These two forms have dramatically different properties, providing valuable insights into how structure affects performance.

Understanding the Structure

The structure of Na8SnP4 is built around simple tetrahedral units where one tin atom sits at the center surrounded by four phosphorus atoms, creating what chemists call SnP4 tetrahedra. These building blocks are then surrounded by sodium ions, which are the charge carriers that move through the material during battery operation.

In the low temperature form, the sodium atoms are arranged in a highly ordered pattern, occupying specific positions within the structure. Some sites remain completely empty, while others are fully occupied. This ordered arrangement, while structurally neat, turns out to be detrimental for ion movement. The material essentially behaves as an electronic conductor rather than an ionic one.

The high temperature form tells a different story. Here, the sodium atoms are distributed over many more possible positions, with partial occupancy at most sites. This disorder, which might seem chaotic, actually creates favorable conditions for sodium ion movement. The ions can hop between neighboring positions more easily, leading to the high ionic conductivity observed at room temperature.

Think of it like the difference between a crowded subway car where everyone stands rigidly in assigned spots versus one where people can shift and move around each other. The latter allows for much easier movement, even though it seems less organized.

How the Material Works

Using advanced techniques including synchrotron X-ray diffraction (which uses extremely bright X-rays to determine atomic arrangements), nuclear magnetic resonance spectroscopy (which probes the local environment of atoms), and impedance spectroscopy (which measures how well ions and electrons flow through a material), the research team built a comprehensive picture of how Na8SnP4 works.

The phosphorus atoms in the structure arrange themselves in what is called a cubic close-packed arrangement, one of the most efficient ways to pack spheres in three-dimensional space. Within this framework, the tin atoms occupy some of the small tetrahedral spaces between phosphorus atoms, while sodium atoms fill various tetrahedral and octahedral voids.

The key to ionic conductivity lies in the partially occupied sites in the high temperature form. When a sodium position is only partially filled, it means that sometimes a sodium ion occupies that spot, and sometimes it doesn't. This creates a dynamic situation where ions can move from one position to another.

The researchers identified particularly interesting behavior around two specific sodium positions in the structure. One sodium site sits in a tetrahedral void while another sits in a neighboring octahedral void. These two sites share a triangular face, and the research suggests that sodium ions migrate between these sites by passing through this shared triangular opening.

This finding aligns with observations from similar lithium-based materials, where ion migration occurs preferentially through face-sharing tetrahedral and octahedral sites. The consistency of this mechanism across different materials provides confidence that scientists are beginning to understand the fundamental principles governing ion conductivity in these phosphide-based compounds.

Measuring Performance

The activation energy for sodium ion movement in the high temperature form of Na8SnP4 is remarkably low at about 36 kilojoules per mole. Activation energy represents the energy barrier that ions must overcome to move from one position to another. Lower activation energies mean ions can move more easily, even at room temperature.

This low activation energy was confirmed through two independent measurement techniques. Temperature-dependent impedance spectroscopy, which measures how conductivity changes with temperature, yielded an activation energy of 36.1 kilojoules per mole. Static sodium-23 nuclear magnetic resonance spectroscopy, which observes how sodium nuclei respond to magnetic fields at different temperatures, gave a consistent value of 34 kilojoules per mole.

The agreement between these two very different experimental approaches strengthens confidence in the results and confirms that the material genuinely exhibits fast sodium ion movement with a low energy barrier.

Comparing Performance

To put these numbers in perspective, the conductivity of 0.53 milliSiemens per centimeter achieved by Na8SnP4 surpasses the original sodium phosphide conductor Na3PS4, which showed a conductivity of 0.2 milliSiemens per centimeter when first reported in 1992. The new material also outperforms other sodium phosphide compounds that form complex three-dimensional networks, which typically show conductivities ranging from extremely low values up to about 0.4 milliSiemens per centimeter.

However, it is important to note that Na8SnP4 still lags behind the best performing sodium solid electrolytes, which are sulfur-based materials. Some modified sodium sulfide compounds have achieved conductivities exceeding 40 milliSiemens per centimeter at room temperature through careful chemical substitution and optimization.

The Path Forward

What makes Na8SnP4 particularly exciting is not just its current performance but its potential for improvement. The material's structure, based on simple tetrahedral building blocks, closely resembles that of successful sulfide-based conductors. These sulfide materials have been extensively optimized through a strategy called aliovalent substitution, where some atoms in the structure are replaced with different elements that have different numbers of valence electrons.

For example, in sodium sulfide conductors, replacing some antimony (which forms SbS4 units with a 3- charge) with tungsten (which forms WS4 units with a 2- charge) dramatically increases conductivity. This substitution creates vacancies or defects in the structure that facilitate ion movement.

The same strategy could potentially be applied to Na8SnP4. The tin atoms could be partially replaced with other elements, or the phosphorus framework could be modified, to create a more favorable environment for sodium ion movement. Given that sulfide conductors have been improved by more than two orders of magnitude through such modifications, there is reason to believe that significant improvements in Na8SnP4 are possible.

Moreover, the material's synthesis is relatively straightforward. The researchers produced it on a gram scale using a two-step process that involves ball milling the elements together and then heating the mixture. This simplicity, combined with the use of abundant elements (sodium, tin, and phosphorus), suggests that the material could be practical for large-scale production if its properties can be further enhanced.

Insights from Structure

One of the most valuable aspects of discovering both low and high temperature polymorphs is the opportunity to understand structure-property relationships. By comparing the two forms, researchers can identify exactly which structural features promote ionic conductivity and which hinder it.

The comparison reveals several important principles. First, having completely empty sites in the structure does not promote ionic conductivity. The low temperature form has fully vacant positions, yet shows no sodium ion movement in the temperature range studied. This suggests that ions need accessible neighboring sites to move into, not just empty spaces somewhere in the structure.

Second, partial occupancy of many sites appears crucial. The high temperature form, with its numerous partially occupied sodium positions, shows high conductivity. This partial occupancy creates an energy landscape where multiple positions have similar energies, making it easier for ions to hop between them.

Third, sites that show strong distortions or disorder are particularly important for conductivity. In the high temperature form, some octahedral sodium positions are significantly shifted toward the faces they share with neighboring tetrahedra. This displacement suggests that ions at these positions are mobile and frequently exchanging with neighboring sites.

Broader Implications

The development of Na8SnP4 contributes to a growing family of phosphide-based ion conductors. Over the past decade, researchers have developed numerous lithium phosphide compounds with high ionic conductivity, creating a rich library of materials with understood structure-property relationships.

These lithium phosphides include compounds like Li8SnP4, Li9AlP4, and Li14SnP6, all based on similar structural principles involving discrete tetrahedral building blocks and cubic close-packed phosphorus arrangements. The materials show ionic conductivities comparable to state-of-the-art lithium solid electrolytes and have revealed fundamental insights into how ions move through solid materials.

Extending this chemistry to sodium has been challenging. Early attempts produced materials that either conducted electrons rather than ions or showed no conductivity at all. Na8SnP4 represents a breakthrough in successfully transferring the favorable structural features from lithium systems to a sodium-based material while maintaining good ionic conductivity.

This success opens the door to developing a parallel family of sodium phosphide conductors. By systematically varying the elements and structures, researchers can explore a vast chemical space and potentially discover even better performing materials.

Environmental and Economic Considerations

Beyond the technical performance, sodium-based batteries offer significant environmental and economic advantages. Lithium extraction, particularly from brine deposits, requires large amounts of water and can take months to years. The process also leaves behind waste materials that can affect local ecosystems.

Sodium, in contrast, can be extracted from seawater or rock salt deposits using well-established, relatively low-impact processes. The abundance of sodium also means that increased demand would not lead to supply shortages or dramatic price increases, unlike lithium where supply constraints are a growing concern.

For applications like grid-scale energy storage, which will be critical for integrating renewable energy sources like solar and wind power, cost and sustainability matter as much as performance. A storage system that is slightly less energy dense but significantly cheaper and more sustainable could still be preferable for many applications.

Challenges Ahead

While Na8SnP4 represents an important advance, significant work remains before sodium phosphide batteries can become practical. The material's ionic conductivity, while good for a pristine phosphide compound, still needs improvement to match the best solid electrolytes.

The material must also prove stable in contact with electrode materials. Solid electrolytes need to maintain their properties when in contact with the negative and positive electrodes of a battery, neither reacting chemically nor degrading during repeated charging and discharging cycles.

Processing the material into forms suitable for battery manufacturing presents another challenge. Solid electrolytes typically need to be formed into thin, dense layers to minimize resistance while maintaining mechanical strength. Developing methods to fabricate Na8SnP4 into appropriate forms at scale will require additional research and development.

Safety testing will also be essential. While solid electrolytes generally offer safety advantages over liquid electrolytes, which can be flammable, each new material must be thoroughly characterized to ensure it performs safely under various conditions including elevated temperatures, mechanical stress, and electrical abuse scenarios.

Looking Toward the Future

The discovery of Na8SnP4 marks an important milestone in the development of sodium-ion battery technology. It demonstrates that phosphide-based sodium conductors can achieve performance levels relevant for practical applications and provides a platform for further optimization.

The research also contributes to fundamental scientific understanding of how ions move through solid materials. The detailed structural characterization, combined with measurements of ionic conductivity and activation energies, helps build a comprehensive picture of the relationship between atomic arrangement and macroscopic properties.

As the global demand for energy storage continues to grow, driven by the transition to renewable energy and electrification of transportation, the need for diverse battery technologies becomes more pressing. Lithium-ion batteries will undoubtedly continue to play a major role, particularly in applications where energy density is paramount.

However, for many applications, sodium-ion batteries could offer a compelling alternative. Grid storage, where space is less constrained and cost is critical, represents a particularly promising opportunity. Even lower-performance vehicles or consumer electronics where slightly larger battery packs are acceptable could benefit from the cost savings of sodium-based systems.

The path from laboratory discovery to commercial product is long and uncertain. Many promising materials never make it beyond the research stage due to unforeseen challenges in scaling, stability, or economics. However, each advance, such as the development of Na8SnP4, builds knowledge and moves the field forward.

As researchers continue to explore chemical modifications, optimize synthesis methods, and understand the fundamental science of ionic conduction, the performance of sodium phosphide conductors will likely continue to improve. The simple, well-defined structure of Na8SnP4 makes it an ideal platform for these optimization efforts.

The work exemplifies how basic materials research can contribute to solving pressing global challenges. By understanding the fundamental principles of how atoms arrange themselves and how this arrangement affects properties, scientists can design new materials with desired characteristics. This approach, combining synthesis, structural characterization, and property measurement, provides a roadmap for continued progress in energy storage and beyond.

Publication Details

Published: 2025 (Online)
Journal: Angewandte Chemie International Edition
Publisher: Wiley-VCH GmbH
DOI: https://doi.org/10.1002/anie.202419381

Credit and Disclaimer

This article is based on original research published in Angewandte Chemie International Edition. The content has been adapted for a broader audience while maintaining scientific accuracy. For complete details, comprehensive data, full methodology, and in-depth analysis, readers are strongly encouraged to access the original peer-reviewed research article through the DOI link provided above. All factual information, data interpretations, and scientific conclusions presented here are derived from the original publication, and full credit goes to the research team and their contributing institutions.