US researchers developed a meta-weakly solvating electrolyte that optimizes the solvation structure of sodium ions, enabling faster ion transport, reduced side reactions and improved interfacial stability in high-voltage sodium ion batteries. This design significantly extends cycle life and outperforms conventional and high-concentration localized electrolytes by promoting more uniform and stable electrode-electrolyte interfaces.
By ESS news
Researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) have developed a meta-weakly solvating electrolyte that reportedly can enable stable operation of high-voltage sodium-ion (Na-ion) batteries.
“The new electrolyte represents a novel strategy to regulate Na solvation structure, which can facilitate beneficial reactions and suppress undesirable reactions,” says the study’s lead author. An L. Phantold ESS news. “This results in less irreversible loss and degradation of cell materials under practical conditions.”
Most conventional battery electrolytes are designed to strongly solvate the metal ions, allowing ions to move through the liquid. However, this also tends to create a very stable “ion-solvent shell” that is difficult to disintegrate at the electrode surface. When that happens, the electrolyte molecules themselves often become involved in unwanted side reactions, forming unstable layers, consuming electrolyte and degrading the battery over time.
In contrast, the proposed electrolyte type is designed so that sodium ions are less tightly bound to solvent molecules and are instead guided to a more controlled, intermediate solvation structure. This changes the way the ions behave at the electrode interface and prevents overly stable ion-solvent shells that typically cause harmful side reactions and battery degradation.
To build the battery cell, the scientists used battery-grade sodium hexafluorophosphate (NaPF₆) and sodium bis(fluorosulfonyl)imide (NaFSI) salts, along with high-purity solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), triethyl phosphate (TEP), tris(2,2,2-trifluoroethyl) phosphate (TFP), and 1,1,2,2-tetrafluoroethyl. 2,2,3,3-tetrafluoropropyl ether (TTE).
Sodium-nickel-manganese-iron oxide (NFM424) cathodes were combined with hard carbon (HC) anodes, both fabricated by slurry casting on aluminum (Al) foil using binders such as polyvinylidene fluoride (PVDF), sodium carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR), together with conductive carbon (Super P carbon, C45). Electrodes were dried under vacuum before mounting.
Complete cells consisting of NFM424 cathodes and HC anodes were assembled in an argon-filled glove box using standard coin cell components and tested, with all electrochemical tests performed at 30 C using battery cyclers. Leakage current tests evaluated interfacial stability against reference aluminum (Al) and NFM424 electrodes. Electrolyte solvation structures were analyzed using nuclear magnetic resonance (NMR) spectroscopy.
Additionally, the scientists performed post-cycling analyzes on electrodes after 50 cycles, including scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS).
The tests showed that the proposed battery cell design achieves improved sodium mobility and outperforms conventional counterparts, which showed previous degradation and instability. In addition, leakage current tests confirmed that the battery cell using the meta-weakly solvating electrolyte achieves the best interfacial stability at high voltage, consistent with reduced free solvent reactivity and enhanced cathode-electrolyte interphase (CEI) formation.
It was also found that the cell retains 80% of its capacity after 500 cycles, compared to 100-300 cycles for the benchmark devices. Electrochemical impedance spectroscopy showed that this improvement arises from lower resistance to charge transfer, coupled with faster sodium desolvation and more efficient interfacial transport.
“These features effectively improve the electrochemical stability of the cell and reduce the degradation of active materials during long-term cycling,” Phan pointed out.
The new battery cell design was presented in “Meta-weak solvating electrolyte for high voltage sodium ion batteries”, published in Nanoenergy.
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