Two liquid metals mixed at room temperature form a non-Newtonian fluid anode that eliminates dendrites and slashes viscosity in potassium-ion batteries.
(Nanowerk Spotlight) Some materials refuse to commit to a single identity: solid or liquid, rigid or flowing. Apply force to oobleck, the cornstarch-and-water mixture familiar from school science fairs, and it resists like a solid. Leave it alone and it pours like a liquid. These are non-Newtonian fluids, materials whose viscosity shifts in response to applied stress.
That ability to toggle between resistance and flow has practical value well beyond the classroom. A battery electrode, for instance, must hold its shape inside a cell yet maintain intimate contact with the electrolyte and current collector. A material that stiffens under pressure but flows when coated onto a surface can satisfy both demands. This principle has driven researchers to build battery anodes from room-temperature liquid metals engineered into non-Newtonian states.
Liquid metal anodes solve one of the oldest problems in rechargeable batteries. During charging, solid metal anodes develop dendrites, needle-like growths that can pierce the separator and cause short circuits. A fluid surface cannot sustain such protrusions, making liquid electrodes inherently dendrite-free. Two candidate families have emerged: sodium-potassium (NaK) alloys, which offer strong electrochemical performance, and gallium-indium-tin (GaInSn) alloys, which provide good fluidity and low toxicity.
Each, however, carries liabilities. NaK alloys suffer from high surface tension that causes electrode leakage and structural collapse. GaInSn alloys show low electrochemical activity and corrode conventional copper and aluminum current collectors. The standard fix has been to blend liquid metals with solid carriers such as carbon black, creating composites with non-Newtonian flow behavior. But the solids trigger oxide formation, pushing viscosity so high that the paste becomes unworkable.
The reaction produces Ga₄Na, an intermetallic compound whose solid particles, suspended in the remaining liquid metal matrix, generate non-Newtonian flow behavior. Because the components bond chemically rather than mixing physically, the composite achieves atomic-level uniformity and avoids the oxide buildup that plagues solid-carrier systems.
Preparation process and characteristics of non-Newtonian fluid sodium potassium alloy. (a) Viscosity performance schematic diagram. (b) Synthesis process of sodium potassium alloy in non-Newtonian fluid state and infrared thermal imaging test. (c) Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) spectra of the non-Newtonian fluid. (d) The vibration self-healing property of the non-Newtonian alloy. (Image: Reproduced with permission from Wiley-VCH Verlag)
This marks the first reported formation of such a fluid electrode through a liquid-liquid reaction between two room-temperature liquid metals for potassium-ion battery anodes. The key question was whether the reaction yields Ga₄Na rather than other possible intermetallic phases such as Na₇Ga₁₃.
Several converging lines of evidence answered that question. Phase diagram analysis showed that the sodium content in the initial mixture sits close to the stoichiometry of Ga₄Na but falls well below the threshold required for Na₇Ga₁₃. The high proportion of potassium in the NaK alloy further dilutes sodium’s effective chemical activity, making sodium-rich phases even less accessible.
Density functional theory calculations reinforced this conclusion. Under ideal conditions, Na₇Ga₁₃ has a slightly more favorable formation energy. But correcting for the actual gallium concentration in the mixture shrank the energy gap between the two compounds to a negligible margin. X-ray diffraction patterns of the product matched standard Ga₄Na reference data, with no peaks corresponding to Na₇Ga₁₃.
The bonding evidence pointed in the same direction. After the reaction, sodium spectra revealed new states consistent with Na-Ga bond formation, and gallium spectra shifted toward higher binding energies, a signature of Ga₄Na. Potassium spectra showed no such changes, confirming that potassium remains free and does not participate in the intermetallic reaction.
The fluid’s rheological behavior depended on the ratio of Ga₄Na to NaK alloy. Below a mass ratio of 1:3, the mixture flowed like a conventional liquid with minimal viscosity variation. Between 1:3 and 1:4, viscosity curves displayed the characteristic oscillations of a non-Newtonian fluid. Above 1:4.5, excessive solid content caused local aggregation that disrupted this response.
These rheological properties translated directly into practical advantages during battery assembly. The NaK@GaInSn fluid registered viscosities roughly five times lower than a NaK@Super P composite made with carbon black. After battery disassembly, the separator in contact with NaK@GaInSn remained clean, while the Super P version was coated with residual material. The fluid also displayed vibration-responsive self-healing, rapidly returning to its original shape when notched and struck.
Electrochemical testing confirmed that the lower viscosity did not come at the cost of performance. In symmetric cell tests, the NaK@GaInSn electrode achieved an exchange current density more than twice that of NaK@Super P and nearly ten times that of bare potassium, indicating faster reaction kinetics. The NaK@GaInSn symmetric cell exceeded 2700 cycles with minimal polarization, while both comparison electrodes failed within 500 cycles.
Full cells paired the anode with a high-entropy Prussian blue cathode. At 0.2 C, the NaK@GaInSn configuration delivered a specific capacity of 106.4 mAh g⁻¹ with 93.7% capacity retention over more than 900 cycles. It outperformed both comparison systems in discharge capacity and coulombic efficiency across all tested current densities.
The work establishes a new construction principle for liquid metal electrodes. By replacing solid carriers with a second liquid metal that reacts to form structure-providing intermetallic compounds, the strategy sidesteps the viscosity penalty of oxide formation while preserving the dendrite-free and self-healing properties inherent to liquid electrodes.
Questions remain about stability under extreme conditions, solid electrolyte interphase formation on a fluid surface, and scalability to larger cell formats. But the core demonstration shows that liquid-liquid chemistry offers a viable route beyond the liquid-solid composites that have dominated this space.
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