The study, published in the journal Ionics by Shuonan Kong, Chunye Nong, Liping Pang, Wei Li, and Wei Wang, affiliated with Nanning Normal University and Guangxi University, tackles a problem that has long stood between paper-based separators and commercial viability. Cellulose paper, on paper so to speak, is an ideal candidate for separator duty. It exhibits outstanding electrolyte affinity, meaning it wets readily with the liquid carbonate electrolytes used in lithium-ion cells, and it possesses excellent thermal stability, retaining its structure at temperatures far beyond the melting points of polyethylene and polypropylene. These are not trivial advantages. When a polyolefin separator overheats, it can shrink or melt, allowing the electrodes to contact each other directly and triggering thermal runaway, the runaway exothermic reaction that underlies many battery fires. Paper separators, by contrast, hold their geometry under heat, promising intrinsically safer cells.

The catch lies in the microscopic architecture of paper itself. Bare paper substrates are constructed from interwoven cellulose fibers, and the network of fibers inevitably contains abundant large voids, pores that are far too big for a separator to be safe. In a functioning battery, lithium ions must migrate through the separator’s pore network, but the membrane must also block any pathway by which lithium dendrites, needle-like metallic structures that grow from the anode during repeated charging, could bridge the gap to the cathode. Large voids in a paper separator offer exactly such pathways. Worse, uneven pore distributions create localized regions of high current density that accelerate dendrite nucleation. If a dendrite penetrates the separator, the cell short-circuits internally, and the resulting heat can ignite the flammable electrolyte. This is why, despite their attractive bulk properties, unmodified paper separators have remained confined to laboratory curiosities rather than production lines.

The solution proposed by the Nanning team is elegantly simple: coat the paper with a second cellulose derivative, hydroxypropyl cellulose, applied via a roll coating technique. The substrate chosen was Xuan paper, a traditional Chinese paper famed for its fine fiber network and durability, best known to the world as the medium of classical calligraphy and ink painting. Roll coating is an industrially mature, scalable process, which matters enormously for any technology hoping to leave the laboratory. The researchers systematically varied the coating grammage, the mass of hydroxypropyl cellulose deposited per unit area, and tracked how this parameter transformed the separator’s microstructure and performance. The approach builds on a growing body of work on biomass-based separators, including prior studies of chitosan coatings, cellulose fiber acetylation, and nano-silica hybrid polymer electrolyte layers, but the choice of hydroxypropyl cellulose brings its own distinct advantages as a widely available, chemically tunable cellulose ether.

The mechanism by which the coating works is a story of pores being filled and bonds being forged. As the coating grammage increases, the hydroxypropyl cellulose gradually fills the pores between the cellulose fibers of the Xuan paper substrate, narrowing and regulating the pore size distribution that would otherwise invite dendrite penetration. Simultaneously, the coating builds extra inter-fiber bonds, crosslinking the mechanical integrity of the composite. The result is a notable increase in the mechanical strength of the separator, a critical property because separators must withstand the considerable stacking pressure of cell assembly and the mechanical stresses of repeated electrode expansion and contraction during cycling. A separator that tears or delaminates is a failed separator, no matter how well it conducts ions. By reinforcing the fiber network at the same time as it refines the pore structure, the hydroxypropyl cellulose coating addresses both of the fundamental weaknesses of bare paper in a single processing step.

Electrolyte handling is where the composite separators truly began to shine in comparison with the commercial benchmark. When the researchers compared their coated paper separators against Celgard, the polyolefin standard, the cellulose composites delivered superior performance in electrolyte absorption, electrolyte retention, and thermal resistance. Each of these properties feeds directly into battery performance and longevity. High electrolyte absorption ensures that ion-conducting pathways are fully wetted from the start of the cell’s life. Strong retention means the separator holds onto that liquid electrolyte over hundreds of cycles, resisting the gradual drying out that degrades ion transport and raises internal resistance in aging cells. And superior thermal resistance, as noted, translates directly into a wider safety margin against overheating events. The fact that all three properties were achieved simultaneously, rather than traded off against one another, underscores the coherence of the cellulose-on-cellulose design philosophy.

The electrochemical results provide the headline numbers. The team identified an optimum coating grammage of 5.2 grams per square meter, the formulation at which the composite separator presented its best cycling stability. Under a 1 C current density, meaning the cell discharges its nominal capacity in one hour, a lithium-ion cell built with this optimized separator achieved an initial discharge capacity of 130.9 milliampere-hours per gram. More impressively, the cell retained 98 percent of that capacity after 300 charge-discharge cycles. Capacity retention of this magnitude over hundreds of cycles indicates that the separator is successfully suppressing the parasitic side reactions, dendrite growth, and ion-transport degradation that normally erode cell capacity over time. The Celgard separator, tested under the same conditions, delivered lower capacity retention, meaning the coated paper composite outperformed the incumbent commercial technology on the metric that matters most to anyone who has watched a laptop battery fade: how long the cell keeps working.

Why should a cellulose separator cycle better than polyolefin? The technical explanation threads together the separator’s multiple roles. A separator is not merely a passive physical barrier; its surface chemistry and pore architecture shape the uniformity of lithium-ion flux at the electrode interface. The hydrophilic hydroxypropyl cellulose coating promotes homogeneous electrolyte distribution and consistent ion transport across the separator area, reducing the local current hotspots that seed dendrites. The refined, filled pore structure eliminates the oversized voids through which dendrites could most easily propagate. Meanwhile, the enhanced mechanical strength keeps the separator dimensionally stable under cycling stress, maintaining uniform electrode separation as the anode expands and contracts. Together, these effects preserve the cell’s internal geometry and electrochemical uniformity cycle after cycle, which is precisely what the 98 percent retention figure reflects.

The broader significance of this work lies in its sustainability and scalability credentials. The battery industry faces mounting pressure to reduce the environmental footprint of its supply chains, and separators are typically derived from petroleum-based polymers processed with energy-intensive techniques such as dry stretching or wet extrusion. A separator built from paper, coated with a biomass-derived cellulose derivative using roll coating, offers a green and scalable pathway, in the authors’ words, to high-performance separators. Xuan paper and hydroxypropyl cellulose are both renewable materials, and roll coating is compatible with existing industrial web-processing infrastructure, meaning the laboratory demonstration could plausibly be translated to continuous manufacturing without inventing new production equipment. The work also fits into a wider research movement, documented across recent literature on cellulose nanofibril membranes, fiber swelling strategies, and metal-organic framework coatings, that seeks to replace or upgrade synthetic separators with cellulose-based alternatives.

Challenges inevitably remain before calligraphy paper powers your phone. The study demonstrates cycling stability at 1 C over 300 cycles, a meaningful but still moderate test regime compared with the thousands of cycles and varied temperature conditions demanded of electric vehicle batteries. Questions of long-term chemical compatibility with electrolyte additives, behavior at extreme fast-charge rates, and cost at industrial scale will all require further investigation. Nevertheless, the demonstration that a simple, green coating can convert a fragile, void-riddled paper into a separator that beats Celgard on cycling retention is a striking result. It suggests that the future of battery safety and sustainability may owe as much to the ancient papermakers of China as to the polymer chemists of the modern era, and that sometimes the most advanced materials science begins with the humblest of starting materials.

Subject of Research: Hydroxypropyl cellulose-coated paper separators for high cycling stability lithium-ion batteries

Article Title: Hydroxypropyl cellulose-coated paper separators toward high cycling stability lithium-ion batteries

Article References: Kong, S., Nong, C., Pang, L., Li, W., & Wang, W. (2026). Hydroxypropyl cellulose-coated paper separators toward high cycling stability lithium-ion batteries. Ionics. https://doi.org/10.1007/s11581-026-07517-9

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07517-9

Keywords: lithium-ion batteries, battery separator, hydroxypropyl cellulose, Xuan paper, cellulose coating, roll coating, cycling stability, electrolyte retention, thermal stability, dendrite suppression, biomass materials, energy storage