From electric vehicles to mobile devices, modern social infrastructure remains heavily dependent on lithium-ion batteries. However, lithium resources are highly concentrated geographically, posing constant risks of supply chain fragility and price volatility. As an alternative, "sodium metal batteries," which utilize sodium—an element abundant on Earth and inexpensive to procure—have long been anticipated as next-generation energy storage systems. Unlike mainstream sodium-ion batteries, sodium metal batteries use metallic sodium directly as the anode, offering the prospect of higher energy density.
However, a formidable technical barrier has stood in the way of practical implementation. In "quasi-solid-state electrolytes (QSE)," which hold the key to commercialization, achieving both fast charging speed and long battery cycle life simultaneously has proven extremely difficult.
An electrolyte, by definition, serves as the "pathway" for ions traveling between the cathode and anode. Liquid electrolytes, which have long been the standard in battery development, can transport ions rapidly, but they always carry risks of flammability and leakage. In recent years, efforts worldwide have shifted toward QSE—liquids gelled into a solid-like state—to enhance safety. However, this very approach of "solidifying" the electrolyte has created a new barrier.
In conventional QSE, negatively charged anions dominate the conduction process, which inevitably delays the transport of sodium ions (Na+) within the electrolyte, causing a phenomenon known as concentration polarization. Furthermore, since rigid electrolytes struggle to maintain close contact with electrodes, ion diffusion at the interface also becomes insufficient. When charging and discharging are forced to occur rapidly, sodium metal that cannot find a proper pathway grows into dendrites—branch-like structures—leading to cell short-circuits and rapid performance degradation. Making the electrolyte more rigid for safety, in turn, blocks the pathway for ions, reducing both lifespan and speed. This severe dilemma has long troubled developers in the field.
A Network of Conduction and Stability Bound by a "Double Interlock"
Taking on this challenge is a research team led by Professor Long Pan, Professor Yang Zhou, and Professor Zhengming Sun at Southeast University. They fundamentally reconsidered conventional approaches that relied on complex polymer modifications—often at the expense of ionic conductivity—and introduced a new strategy called "dual-interlock design." This method combines a salt containing positively charged tin ions () with negatively charged difluoro(oxalato)borate ().
This design operates in "two stages": during the electrolyte fabrication process and during battery operation. First, during electrolyte formation, initiates the polymerization reaction of the base material. Meanwhile, acts as a delaying agent to prevent this reaction from running out of control, forming a uniform and robust network structure. This is not simply a matter of mixing two salts together. According to molecular dynamics simulations, preferentially competes with Na+ for binding sites, weakening the interaction between Na+ and the polymer chains and thereby liberating free Na+ ions. As a result, a robust electrolyte was created with a puncture resistance strength reaching $8.5 \text{ kPa}$.
An even more counterintuitive phenomenon emerged when the battery was actually put into operation. Typically, electrolytes degrade most readily at the electrode interface. However, in this new electrolyte, —which has a favorable LUMO level for accepting energy—is preferentially reduced at the anode side, forming a robust solid electrolyte interphase (SEI) composed of a sodium-tin hybrid alloy. This alloy layer uniformizes the electric field, fundamentally suppressing dendrite formation at its root. At the same time, —which has a favorable HOMO level for releasing energy—is oxidized at the cathode side, creating an extremely robust cathode electrolyte interphase (CEI) that is only 14 nm thick, less than half the thickness of conventional systems. Rather than incorporating complex mechanisms separately, the team linked the opposing functions of the two salts together, simultaneously achieving rapid internal conduction and interfacial stability at both electrodes.
Remarkable Charging Speed and Cycle Life
This new approach recorded figures far surpassing conventional systems. The sodium-ion transference number (the fraction of current carried by ions)—which remained at only 0.4 to 0.7 in conventional quasi-solid-state electrolytes—reached 0.94, close to the ideal value. The sodium-ion diffusion coefficient also jumped to $16.8 \text{ \AA}^2\text{ ns}^{-1}$, six times that of conventional liquid electrolytes. Ionic conductivity was also secured at a high level of $1.3 \text{ mS cm}^{-1}$.
This strength was maintained in actual charge-discharge performance as well. In full-cell tests using sodium vanadium phosphate as the cathode, even under the harsh conditions of a 15C ultra-fast charging rate (equivalent to charging in approximately 4 minutes), the battery maintained a high capacity of $80.1 \text{ mAh g}^{-1}$. Under a practical 3C charge-discharge rate, the battery retained 90% of its initial capacity even after 2000 cycles. In basic experiments using symmetric cells with sodium electrodes facing each other, the cell operated stably for 6000 hours (approximately 8 months) at a current density of $0.1 \text{ mA cm}^{-2}$ without forming any dendrites. The cell polarization (voltage fluctuation) during this period was kept at an extremely low level of about 0.1 V.
A Realistic Choice for Next-Generation Energy Storage
This breakthrough demonstrates that sodium metal batteries have taken a major step forward from being a "laboratory future technology" to becoming a "real industrial option." The effectiveness of an approach that integrates bulk transport and interfacial stabilization—elements that have historically been optimized independently and were sometimes considered mutually incompatible—has been proven. The research team has also prototyped a 4×5 cm pouch-type battery capable of powering a smartphone, demonstrating flexibility and mechanical durability that allows it to continue operating even after repeated folding. Compatibility with high-loading cathode materials has also been confirmed.
Room for further verification remains before practical application and commercial adoption can be achieved. While this technology has demonstrated high performance in small-scale pouch cells, deploying it in electric vehicles or large-scale grid storage facilities will require establishing techniques to minimize performance variability in large-scale mass production lines. Additionally, to pursue even higher energy density, the long-term behavior when combined with new conversion-type cathode materials must also be verified. Whether this design approach—which links opposing challenges together through a "double chain"—will become the standard paradigm for next-generation batteries remains to be seen, and further verification toward practical application is anticipated.