A protein foamed binder forms stable ion channels in dense lithium sulfur cathodes, delivering higher volumetric energy and faster ion transport without sacrificing mechanical stability.
(Nanowerk Spotlight) As electric transportation grows and renewable energy use expands, the limitations of current lithium-ion batteries have become clear. Their energy density, meaning the amount of energy stored in a given mass or volume, restricts how far vehicles can travel and how compact storage systems can be built.
These constraints have drawn attention to lithium sulfur chemistry, which in theory can store several times more energy by weight than conventional lithium-ion materials. Sulfur is abundant and inexpensive but turning that potential into practical batteries has proved difficult.
Sulfur electrodes depend on open pores and liquid electrolyte to let lithium ions move freely. Those same pores take up valuable space and reduce the amount of energy that fits into each cubic centimeter. When manufacturers compress the electrodes to increase density, the porous network collapses, slowing ion transport and shortening battery life. This trade-off between density and ion movement has blocked progress.
A study published in Small Structures (“Noncollapsible, Meringue-Structured Cathodes for a Volumetrically Efficient Li–S Battery”) addresses this problem by rethinking the binder, the material that holds the cathode together. By converting this adhesive into a structural framework, the researchers created a protein foamed binder that forms straight, stable ion channels through the electrode.
The method draws from the texture of a meringue, where tiny air pockets are trapped in a solid matrix. During fabrication, air bubbles are captured in the binder and become microscopic channels once the film dries. These channels remain open even after compression, maintaining ion transport in a dense structure.
This marks a significant advance in volumetrically efficient lithium sulfur battery performance, combining compactness with rapid ion movement and long cycling stability. Earlier sulfur cathodes rarely achieved this outcome without compromising other key properties.
“Our work focuses on improving energy per liter, not by adding heavy materials, but by redesigning the internal framework that connects sulfur particles to the current collector,” Petar Jovanović, a Research Fellow in the Department of Mechanical & Aerospace Engineering at Monash University and the paper’s first author, tells Nanowerk.
Protein-foamed binder (PFB) templates produced by vigorous whipping of carboxymethyl cellulose and denatured bovine serum albumin (BSA). (A) Photograph of PFB with a schematic showing adsorption of the protein molecules at the air–water interface physically cross-linking with the by carboxymethylcellulose (CMC) chains by various types of physical interactions and SEM micrograph of the dried PFB showing the preservation of porosity after drying. (B) FTIR spectra showing the physical interaction between CMC and BSA. (C) SAXS curve of PFB with a sketch of the nanoscale structure in the inset. (Image: Reproduced from DOI:10.1002/sstr.202500537, CC BY) (click on image to enlarge)
The binder combines bovine serum albumin, a common protein, with carboxymethylcellulose, a cellulose-based polymer already used in water processed electrodes. Heating unfolds the protein, so its surface stabilizes air bubbles. When the mixture is whipped, coated, and dried, the bubbles leave behind vertical channels about five to ten micrometers wide.
“The protein and polymer form numerous physical cross links, which create a strong and adhesive network,” explains Jovanović. “This prevents the channel structure from collapsing during calendering while allowing electrolyte to wet the pores. The recipe uses water and inexpensive materials, keeping it compatible with existing production.”
The team performed mechanical tests confirming the structural benefit. With the protein binder, electrode density increases from 0.51 to 1.62 g cm⁻³ after calendering. Porosity decreases from 74 % to 21 %, yet tortuosity rises only from 2.02 to 2.67.
In contrast, electrodes made with a standard binder crack under pressure and show tortuosity that almost doubles. Adhesion tests show that the new binder holds the coating to the current collector about three times more strongly, and indentation tests reveal higher hardness and elasticity. These properties help the electrode survive compression and the large volume changes that occur when sulfur reacts to form lithium sulfide.
Electrochemical tests confirm the dense, channel-templated cathodes maintain rapid ion transport. At 0.2C they deliver 1,283 mAh g⁻¹ and 625 mAh g⁻¹ at 4C, recovering 96 % of capacity on return to 0.2C, while controls drop to 298 mAh g⁻¹ and recover only 87 %.
“These data indicate that straight channels maintain ion flow even in dense films,” Jovanović points out.
Measured volumetrically, the compact cathodes reach 1,550 mAh cm⁻³ and 3,285 Wh L⁻¹, maintaining 1,149 and 992 mAh cm⁻³ at 4 and 12 mg cm⁻² sulfur loadings, remaining dense yet fast and sustaining capacity even at 4C.
Long-term cycling supports the mechanical design. At 2C, cells with the templated binder retain 94 % capacity after 500 cycles and maintain Coulombic efficiency above 99 %. Even at 16 mg cm⁻² sulfur loading and 1.44 g cm⁻³ density, the cells deliver 16.6 mAh cm⁻² at 0.05C and 13.6 mAh cm⁻² at 0.1C for 50 cycles.
Microscopy after testing shows that the straight channels remain visible and intact. The binder’s strength and adhesion appear to prevent cracking and separation inside the cathode.
The study compares this strategy with earlier attempts that raised density but lost power or required intricate fabrication. It also contrasts the method with approaches that used Teflon fiber networks or heavy metal oxide scaffolds. By focusing on the binder, the new design keeps processing simple, uses renewable materials, and still achieves high density and strong rate capability.
The key insight is that controlling tortuosity matters more than total pore volume. Short, direct ion paths are the factor that preserves performance when electrodes are compacted.
Beyond lithium sulfur systems, the same idea could extend to other high energy chemistries that need dense but accessible electrodes. The protein and cellulose derived materials form a tough network without fluorinated binders or toxic solvents, aligning with industry efforts toward safer and greener processing. The channel templating step uses only whipping and drying, both of which fit within standard manufacturing.
The team notes that full cell optimization still requires balancing the electrolyte to sulfur ratio and matching capacities of both electrodes, but the structural bottleneck in the cathode is effectively addressed.
“Our work demonstrates a practical, manufacturing ready path toward higher volumetric energy in lithium sulfur batteries,” Jovanović concludes. “By turning the binder into a structural tool rather than a passive glue, the method links simple processing with clear performance gains. The protein foamed network supports dense packing, maintains fast ion movement, and resists fatigue during cycling.”
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