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

The separator is one of four core components of every lithium-ion battery, sitting between the graphite anode and the oxide cathode. Its job is deceptively simple: physically keep the electrodes apart so electrons cannot short the cell, while its pores must admit liquid electrolyte and let lithium ions shuttle back and forth during charge and discharge. Polypropylene has long dominated this role because of its mechanical strength, chemical resistance, low cost and easy processability. Yet it is entirely petroleum-derived and non-biodegradable, meaning that as the battery fleet swells, so will a stream of durable plastic waste. The research team, led by Sajad Rasouli of the Research Institute of Petroleum Industry in Tehran with colleagues from Islamic Azad University, the Institute for Color Science and Technology and the University of Garmsar, set out to answer a question that has been largely neglected: could a biodegradable polymer actually beat polypropylene at its own game?

The answer hinged on two chemical interventions. First, the team chain-extended the PBAT using maleic anhydride activated by a dicumyl peroxide initiator. In this reaction, the anhydride ring opens and couples with the hydroxyl end groups of PBAT chains, forging covalent bridges between individual molecules. The result is a higher molecular weight polymer with reduced chain mobility, which translates into improved thermal and mechanical stability. Second, because PBAT is slightly polar and polypropylene is completely nonpolar, the two polymers would normally refuse to mix. The researchers solved this by blending in polypropylene-grafted-maleic anhydride as a compatibilizer: its polypropylene backbone merges with the pure polypropylene phase while its anhydride groups interact with PBAT macromolecules, knitting the two immiscible materials into a coherent whole.

Manufacturing relied on a four-step gravure printing route rather than conventional film blowing or electrospinning. Chain-extended PBAT was compounded with a polypropylene–polypropylene-grafted-maleic anhydride mixture in a single-screw extruder, with 30 percent by weight of dioctyl phthalate oil fed in as a liquid porogen and plasticizer. The molten compound was deposited onto a rotating gravure cylinder, compressed between impression cylinders into a uniform film of roughly 100 micrometers, then uniaxially stretched at 65 degrees Celsius to a final thickness of just 20 to 30 micrometers. The critical chemistry came afterward: the film was soaked in ethanol for two days, leaching out the dioctyl phthalate and leaving behind an interconnected network of nanoscale cavities. The team confirmed by Fourier-transform infrared spectroscopy that extraction efficiency reached about 99.3 percent, with the characteristic fingerprint peaks of the plasticizer vanishing completely after treatment.

The resulting microstructure is remarkable. In the pure chain-extended PBAT membrane, the extraction produced spherical, uniformly distributed cavities averaging just 68 nanometers in diameter with a standard deviation of 45.64 nanometers, a number density of 16.7 holes per square micrometer and a porosity of 42.9 percent. By comparison, most polyolefin-based separators reported in the literature exhibit oval cavities on the micrometer scale. The researchers attribute this morphology to the affinity between the ester-rich, aromatic dioctyl phthalate and the similarly structured PBAT, which allows the plasticizer to disperse homogeneously before removal. When the PBAT content was cut in half to 50 weight percent, the average cavity size swelled 204 percent to 208 nanometers, while pore density fell by 86 percent and porosity dropped to 34.2 percent. Below that ratio, at 25 weight percent PBAT, the compatibilizer could no longer prevent phase separation, yielding a heterogeneous, irregular pore structure unsuitable for ion transport.

Wettability measurements told a complementary story. Water contact angles of roughly 80 to 82 degrees for the high-PBAT membranes sit comfortably in the hydrophilic regime, meaning the pores will readily draw in the polar carbonate electrolyte. Reducing PBAT to 50 weight percent left the surface wettability essentially unchanged, and because its larger pores encouraged capillary penetration over time, the contact angle actually fell from 81.3 to 78.6 degrees during measurement. Bulk wettability, quantified through electrolyte uptake, told a similar tale: pure PBAT absorbed 128.7 percent of its weight in electrolyte, while the 50 percent PBAT formulation still absorbed 94.3 percent. Increasing polypropylene further degraded both uptake and surface wettability, pushing the contact angle toward the hydrophobic zone that battery engineers consider disastrous for ion transport. Balancing microstructure, wettability and mechanical reinforcement from the polypropylene fraction, the team identified the 50 weight percent chain-extended PBAT formulation, labeled BP50, as the optimum.

Electrochemical testing put the claims to the test in a CR2032 coin cell built with a graphite anode and a lithium nickel-cobalt-manganese oxide cathode. Electrochemical impedance spectroscopy yielded resistances of 2.84 ohms for the electrolyte and 36.82 ohms against lithium-ion penetration, with an estimated solid electrolyte interface thickness of 81.27 micrometers. The computed ionic conductivity of 2.86 millisiemens per centimeter dwarfs the 0.34 millisiemens per centimeter reported for polypropylene separators under comparable conditions, and the authors’ radar-chart comparison shows BP50 outperforming separators made from ultra-high molecular weight polyethylene, poly(vinylidene fluoride) variants, cellulose nanocrystals, collagen-chitosan hybrids, zeolite-coated polypropylene and other published polymer membranes. Linear sweep voltammetry confirmed reversible lithiation and delithiation with negligible side reactions up to 1.8 volts, delivering capacities of 69.10 and 63.52 milliampere-hours per gram during delithiation and lithiation respectively.

The team argues that PBAT’s biodegradability does not compromise safety inside the cell. Practical lithium-ion batteries normally operate below 45 degrees Celsius, and even aggressive high-rate use rarely pushes well-controlled cells past 60 degrees, whereas thermal or hydrolytic degradation of PBAT requires substantially higher temperatures along with moisture, oxygen and biological activity. The non-aqueous electrolyte also does not chemically corrode the polyester, and the chain-extension step further stabilizes the polymer by raising molecular weight and curbing chain mobility. Compared with electrospun separators that often suffer from weak mechanical strength and thermal shrinkage despite high porosity, the gravure-printed membranes combine safety, mechanical integrity and electrochemical stability in a single roll-to-roll-compatible process.

The broader implications are significant. A separator that can be printed at industrial speeds, using a blend in which half the polymer decomposes in soil within two months thanks to PBAT’s biodegradation rate of 2.16 per day, offers the battery industry a credible route away from persistent fossil plastics without sacrificing performance. Because the gravure printing route is already a mature printing-industry technology, scaling the process may require less reinvention than entirely novel fabrication schemes. If the printed BP50 membrane can be validated at commercial scale and over long cycle life, the humble separator, often an afterthought in battery design, could become a showcase for how sustainable materials science turns one of electrification’s biggest waste liabilities into an environmental advantage.

Beyond the headline numbers, the study offers a useful lesson in polymer blend physics. Immiscible polymer pairs typically form coarse, disconnected phases because the thermodynamic penalty of mixing their segments is high, and the interfacial tension drives the minor component into large droplets. A graft compatibilizer lowers this penalty by anchoring at the interface, reducing surface tension between the phases and stabilizing a fine, dispersed morphology. In this system, that interfacial engineering is what allows a nano-porous structure to survive the stretching and extraction steps: without well-dispersed PBAT domains, the leached porogen would leave behind irregular, micron-scale voids rather than the uniform 68-nanometer cavities that govern ion transport.

The choice of gravure printing also deserves attention from a manufacturing standpoint. Gravure coating is a high-throughput, roll-to-roll technique already ubiquitous in packaging and printed electronics, capable of depositing controlled film thicknesses at speeds far exceeding batch methods. Demonstrating that a functional battery separator can be patterned this way suggests a pathway in which separator fabrication could be integrated into existing printing infrastructure, potentially lowering capital costs and enabling thickness control at the micrometer scale that is difficult to achieve with conventional dry or wet stretching of polyolefin films.

The work also highlights how formulation trade-offs shape separator design. Porosity, pore size, mechanical strength and electrolyte uptake are coupled variables: raising the biodegradable fraction improved wettability and conductivity but relied on the polypropylene fraction for fatigue resistance and dimensional stability. The authors’ systematic sweep of blend ratios, tracking cavity size, pore density and absorption capacity across compositions, illustrates the kind of compositional mapping needed before any new separator chemistry can be considered commercially credible.

Finally, the environmental argument rests on quantified biodegradation rather than vague green claims. PBAT’s reported soil decomposition within two months, and its biodegradation rate exceeding that of polylactic acid, give the blend a defined end-of-life pathway, though real-world battery recycling streams, separator recovery and lifecycle emissions would need assessment before the sustainability benefit can be fully claimed.

Subject of Research: Development of a biodegradable chain-extended PBAT/polypropylene lithium-ion battery separator fabricated by gravure printing

Article Title: Novel high-performance lithium-ion battery separator made of chain-extended-poly(Butylene Adipate-co-terephthalate)/modified-polypropylene using gravure printing

Article References: Rasouli, S., Arshadi, M., Ataeefard, M., Ghamarpoor, R., & Salehi, M. M. (2026). Novel high-performance lithium-ion battery separator made of chain-extended-poly(Butylene Adipate-co-terephthalate)/modified-polypropylene using gravure printing. Journal of Materials Science: Polymers, 1(1), Article 20. https://doi.org/10.1007/s44493-026-00016-3

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00016-3

Keywords: lithium-ion battery, battery separator, PBAT, polypropylene, gravure printing, biodegradable polymer, ionic conductivity, maleic anhydride, chain extension, porosity, electrolyte wettability, energy storage