Ultra-fast charging sodium metal battery

Chinese researchers built a sodium battery with ultra-fast charging and 90% capacity even after 2,000 cycles        

Researchers in China have developed a new quasi-solid-state electrolyte which could help sodium metal batteries charge faster, last longer and operate more safely. The team from Southeast University, working with HiNa Battery Technology and Yangzhou University, designed a dual-mediator electrolyte which addresses two major challenges in sodium metal batteries: slow sodium-ion transport and unstable interfaces that can lead to dendrite growth and battery failure. Quasi-solid-state electrolytes (QSEs) are critical for ultrafast-charging yet high-safety sodium metal batteries (SMBs), yet their implementation is hindered by sluggish Na+ transport in bulk and at interfaces. Dual interlocked mediator engineering which transcends conventional independent approaches by coupling cationic Sn2+ salt with anionic difluoro(oxalato)borate (DFOB⁻) salts to simultaneously regulating bulk ion transport and bilateral interface chemistry. During QSE preparation, Sn2+ initiates in situ cationic polymerization, while DFOB⁻ acts as a retarding agent to suppress runaway polymerization. The first interlocking effect in the Sn-FB QSE bulk builds a uniform network, enabling near-unity Na+ transference number (0.94) and robust puncture strength (8.5 kPa). During cell operation, Sn2+ is reduced to form a hybrid NaSn alloy-based solid-electrolyte interphase, while DFOB⁻ oxidizes to generate a robust yet thin cathode–electrolyte interphase, respectively. This second interlocking effect creates adaptable bilateral interphases that facilitate Na+ diffusion and mitigate interfacial degradation. As a result, the symmetric cells exhibit 6000 h stability, and full cells retain 80.1 mAh g–1 at an ultrafast-charging rate of 15C and retain 90% capacity at 3C over 2000 cycles. Furthermore, high-mass-loading full cells and pressure-free pouch cells are demonstrated, underscoring the potential of dual interlocked mediator engineering for practical SMBs.

Sodium batteries are attracting growing attention as a lower-cost alternative to lithium-ion systems because sodium is widely available and less vulnerable to supply chain constraints. However, achieving fast charging without sacrificing cycle life has remained difficult. The researchers report that their new electrolyte delivers a sodium-ion transference number of 0.94 while maintaining ionic conductivity of 1.3 mS cm⁻¹. Conventional quasi-solid-state electrolytes typically record transference numbers between 0.4 and 0.7, limiting charging performance. Realizing ultrafast charge (≥ 3C), however, hinges on electrolytes that concurrently provide high Na conductivity (σ) and Na+ transference number. Although conventional liquid electrolytes are effective, they amplify safety risks and promote inhomogeneous Na+ flux, aggravating dendritic failure, thus motivating quasi-solid-state electrolyte (QSE) designs that retain excellent interfacial wetting and scalability yet enhance intrinsic safety. Despite these advantages, the reported state-of-the-art QSEs remain two challenges under ultrafast-charging conditions. 

Limited ion transport in bulk: the strong Na⁺-polymer coordination and anion-dominated conduction depress and slow down bulk Na+ migration. 

Poor ion diffusion at bilateral interfaces: the compositionally mismatched solid-electrolyte interphase (SEI) and cathode–electrolyte interphase (CEI) layers fail to accommodate high Na⁺ fluxes, leading to heterogeneous electric fields, dendrite formation, and accelerated QSE degradation. Therefore, addressing these coupled Na+ transport in bulk and at bilateral interfaces challenges is pivotal to unlocking practical ultrafast-charging SMBs.

The electrolyte uses a combination of tin ions (Sn²⁺) and difluoro(oxalato)borate (DFOB⁻) ions. Together, they regulate both the electrolyte structure and the movement of sodium ions. According to the researchers, DFOB⁻ weakens interactions between sodium ions and the polymer network, freeing more sodium ions to move through the electrolyte. Simulations showed sodium-ion diffusion rates reaching 16.8 Ų ns⁻¹, roughly six times faster than those seen in conventional liquid electrolytes. The material also forms protective layers on both battery electrodes during operation. At the sodium metal anode, tin ions create a sodium-tin alloy-rich interface which promotes uniform sodium deposition. At the cathode, DFOB⁻ helps form a thin and mechanically robust protective layer that reduces electrolyte degradation. The dual-interlocked design also improves overall electrolyte stability by balancing ion coordination in the bulk and at interfaces, ensuring smoother sodium transport under high current conditions. This reduces concentration polarization and helps maintain consistent performance during fast charging cycles in both symmetric and full-cell configurations. These interfacial layers are designed to suppress dendrites, needle-like metallic structures that can trigger short circuits and shorten battery life. The performance results were among the strongest reported for sodium metal battery systems.

To synthesize a liquid sample (FB LE), 1.5 g of sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) and 0.1 g of sodium difluoro(oxalato)borate (NaDFOB) were dissolved in 10 mL of 1,3-dioxolane (DOL) in turn, followed by 1.0 mL of fluoroethylene carbonate (FEC) injected. On this base, a quasi-solid-state sample (Sn-FB QSE) was synthesized by in situ polymerization of LE with tin(II) trifluoromethanesulfonate (Sn(OTf)2) as the Lewis acid initiator while Sn QSE was polymerized without NaDFOB. All samples were kept at 25 °C for 48 h to ensure complete polymerization. In laboratory testing, sodium symmetric cells operated for 6,000 hours without dendrite-related failure at a current density of 0.1 mA cm⁻². The system also reached a critical current density of 3.0 mA cm⁻². When paired with sodium vanadium phosphate cathodes, the batteries still delivered 80.1 mAh g⁻¹ even at an ultra-fast charging rate equivalent to full charge in about four minutes. The cells also retained 90% of their capacity after 2,000 charge-discharge cycles at a high charging rate of 3C. The electrolyte remained stable up to 4.7 volts, potentially expanding its compatibility with higher-voltage cathode materials.

The Sn-FB QSE was synthesized via in situ polymerization of DOL, coupling Sn(OTf)2 as the Lewis acid initiator, and NaDFOB as the runaway polymerization retarder. For comparison, FB LE was synthesized without Sn(OTf), and Sn QSE was synthesized without NaDFOB. Vividly, a uniform polymerization product is presented in Sn-FB QSE. In contrast, FB LE presents its liquidity due to the lack of Sn2+ while Sn QSE presents its ununiform product after runaway polymerization due to the lack DFOB– and brings the uneven molecular weight distribution of PDOL. Electrostatic potential (ESP) distribution was applied to validate DFOB– works as polymerization retarder. Upon coordination with DFOB⁻, the blue coloration surrounding the Sn2+ center diminishes significantly, while the negative potential of DFOB⁻ shifts toward Sn2+. This visualizes the electron transfer occurs from the oxalate and fluorine atoms toward the Sn center, effectively dispersing the excess positive charge of the Sn2+ and thereby reducing its electrophilicity. Subsequently, 50 μL of the precursor solution for each sample was injected into the prepared cells and the obtained composite Sn-FB QSE/separator was only 27 μm. The more homogenous polymerization also contributes to a more robust structure in Sn-FB QSE. Therefore, puncture strength–deflection curves for both Sn QSE and Sn-FB QSE were recorded. The puncture strength in Sn-FB QSE reached approximately 8.5 kPa, nearly double that of Sn QSE (4.4 kPa), suggesting a notable improvement in mechanical robustness. This enhancement is attributed to a lower PDI index in Sn-FB QSE which forms a denser and more mechanically stable polymer network, implying its improvement in resisting sodium dendrite penetration.

The researchers also moved beyond coin-cell testing. Pressure-free pouch cells continued operating while being repeatedly folded and were able to power a smartphone. High-loading battery configurations and alternative cathode chemistries also showed promising results. In this work, a single-ion-conducting QSE with highly adaptable interphases was developed via a dual interlocked mediator engineering, incorporating Sn2+ salt as a in situ cationic polymerization initiator and DFOB– as a retarding agent to suppress the runaway polymerization in a PDOL-based system. This interlocking effect in Sn-FB QSE bulk not only yields a homogenous and mechanically robust network but also achieves = 0.94 by dissociating Na+-TFSI– and reducing Na–O interactions. Consequently, the other interlocking effect enables adaptable bilateral interphases. During cell operation, a hybrid NaSn alloy/inorganic-rich SEI was induced by Sn2+ together with other electrolyte species, which homogenizes the electric field and facilitates rapid Na+ transport and enables no-dendrite Na+ plating/stripping with low polarization for over 6000 h. Meanwhile, DFOB– oxidizes at the cathode to generate a thin yet robust CEI, mitigating electrolyte degradation and lowering interfacial Na+ diffusion resistance. Furthermore, a pressure-free pouch cell successfully operates for 19 cycles, and Sn-FB QSE is also compatible with high-mass-loading cathode NVP (5 mg cm–2) and NFM (17 mg cm–2), which underscores the potential of dual interlocked mediator engineering for practical ultrafast-charging and long-life SMBs. The team says the approach could be extended to lithium and potassium metal batteries while remaining compatible with existing battery manufacturing methods already existing.