Quasi-solid polymer electrolytes occupy a strategic position in battery design. Conventional liquid electrolytes conduct lithium ions quickly and wet electrode surfaces efficiently, but they leak, evaporate and burn. Fully solid-state electrolytes eliminate these hazards, yet they struggle with brittle mechanical behaviour, poor contact with electrodes and stubbornly low ionic conductivity at room temperature. Quasi-solid systems blend a polymer matrix with a small quantity of liquid or plasticising component, capturing much of the ionic mobility of a liquid while retaining the mechanical integrity and processability of a solid. The concept is not new; polymer electrolytes have been studied since the early 1980s, and nanocomposite approaches date back to landmark work in the late 1990s showing that ceramic nanoparticles could dramatically improve polymer electrolyte performance.

The central difficulty, as the review by Alexander Pavlovskii and colleagues makes clear, is that no single electrolyte material excels at everything. A practical electrolyte must conduct lithium ions rapidly, block electrons, withstand high voltages, resist dendrite penetration, remain mechanically robust and, crucially, form stable interfaces with both the anode and the cathode. These requirements frequently pull in opposite directions. Polymers such as poly(ethylene oxide), or PEO, are flexible and easy to process but conduct ions poorly at room temperature and oxidise above roughly four volts. Fluorinated polymers such as poly(vinylidene fluoride-co-hexafluoropropylene) offer better electrochemical stability but depend heavily on the liquid trapped in their pores. Gel polymer electrolytes based on polyacrylonitrile, polycarbonates and cellulose each bring their own trade-offs between conductivity, stability and sustainability.

This is where inorganic fillers enter the picture. The review organises the field’s filler strategies into several families: inert oxide ceramics such as silica, alumina, titania and zirconia; lithium-ion-conducting ceramics such as garnet-structured LLZO and NASICON-type LATP; and more exotic porous frameworks including metal-organic frameworks and covalent organic frameworks. Inert oxides do not conduct lithium ions themselves, yet decades of evidence show they can substantially raise conductivity and improve cycling. The classical explanation invokes Lewis acid-base interactions: surface hydroxyl and metal centres on the ceramic compete with lithium ions for coordination with the polymer chains and the salt anions, loosening the polymer structure, promoting segmental motion and freeing more charge carriers to move. Computational studies have recently added nuance, suggesting that silica nanoparticles can also create favourable percolating pathways for lithium transport along polymer-filler interfaces.

Lithium-ion-conducting ceramics go a step further by acting as genuine fast-ion conduits embedded in the polymer. Garnet-type Li7La3Zr2O12 and its tantalum-doped variants boast bulk conductivities that rival liquids, and when distributed as particles, fibres or three-dimensional frameworks within a polymer matrix, they provide continuous highways for lithium ions. Recent work highlighted in the review shows that the garnet surface chemistry matters enormously: lithium-rich surfaces can trap anions and raise the transference number, meaning a larger fraction of the total current is carried by lithium ions rather than counterions, while fluorinated surface treatments such as fluoroethylene carbonate modification improve wettability against lithium metal and suppress parasitic reactions. Three-dimensional garnet frameworks infiltrated with in-situ polymerised monomers represent one of the most sophisticated versions of this strategy, combining ceramic-level conductivity with intimate electrode contact.

Metal-organic frameworks and covalent organic frameworks are the newcomers. These crystalline, porous materials offer tunable pore chemistry at the atomic scale. Cationic covalent organic framework nanosheets, first reported as fast lithium-ion conductors in 2018, provide ordered one-dimensional channels through which ions migrate, and functionalised variants such as lanthanum-decorated ZIF-8 have been shown to enhance quasi-solid electrolyte performance. Because their pore surfaces can be designed to anchor anions, expose lithium-binding sites or catalyse beneficial decomposition reactions, these frameworks act less like passive fillers and more like programmable interfacial machinery. The review treats them as a frontier direction, noting that cost and scalability remain open questions.

Perhaps the most consequential theme of the review is interfacial chemistry, specifically the formation of the solid electrolyte interphase on the anode and the cathode electrolyte interphase on the cathode. These nanometre-thin layers, formed by electrolyte decomposition during the first charge cycles, determine whether a battery lives or dies. A good interphase conducts lithium ions, blocks electrons and stops growing; a bad one consumes active lithium, impedes ion flow and allows dendrites to nucleate. The authors emphasise that inorganic fillers reshape both layers. Ceramic surfaces can nucleate inorganic-rich interphases composed of lithium fluoride, lithium oxide and lithium carbonate species, which are mechanically strong and electrochemically stable. Fillers can also locally concentrate lithium ions at the electrode surface, homogenising the flux and delaying dendrite initiation, a mechanism implicated in the dendrite-free cycling achieved by several sandwich-structured and multilayer quasi-solid designs.

On the cathode side, the challenges are different but equally severe. High-voltage nickel-rich layered oxides release reactive oxygen and dissolve transition metals, which migrate to the anode and poison it. Conventional PEO-based electrolytes oxidise at these potentials, and recent theoretical work has mapped the degradation pathways and predicted cathode electrolyte interphase components. The review describes how tailored fillers and layered electrolyte architectures, including quasi-double-layer designs with adjustable interphases, can buffer the cathode surface, scavenge harmful species and extend the practical voltage window. Strong Lewis-acid coordinated polymer systems have even demonstrated compatibility with 4.8-volt cathodes in all-solid-state cells delivering over 580 watt-hours per kilogram, a figure that rivals or exceeds the best liquid-electrolyte cells.

The authors are careful to frame these advances against the remaining obstacles. Scalable fabrication is the elephant in the laboratory: many of the highest-performing composite electrolytes rely on intricate in-situ polymerisation, electrospinning or framework synthesis that would be difficult to translate to gigafactory roll-to-roll processing. Advanced operando characterisation is needed to watch interphases form and evolve in real cells rather than inferring them from post-mortem analysis. The review also points to data-driven electrolyte optimisation, in which machine learning navigates the vast compositional space of polymers, salts, solvents and fillers far faster than trial-and-error experimentation. Sustainability concerns appear as well, with recent demonstrations of PFAS-free quasi-solid electrolytes and fluorine-free formulations that form lithium-oxide-rich interphases signalling a push toward greener chemistry.

What emerges from the survey is a coherent design philosophy rather than a single breakthrough material. The performance of a quasi-solid polymer electrolyte is not dictated by any one component but by the engineered interplay between polymer chemistry, filler identity, filler surface treatment and the resulting electrode-electrolyte interphases. Rational filler selection, the authors conclude, is as important as the polymer matrix itself, and interface engineering is the lever that converts good materials into practical batteries. With the International Energy Agency projecting continued rapid growth in electric vehicle adoption and safety standards such as IEC 62660-3 tightening requirements for automotive cells, the incentive to commercialise non-flammable, high-performance electrolytes has never been stronger. Quasi-solid polymer electrolytes, armed with intelligently chosen inorganic fillers, may be the architecture that finally delivers it.

Subject of Research: Roles of inorganic fillers in quasi-solid polymer electrolytes for lithium-ion batteries

Article Title: Interfacial chemistry and hybrid phase engineering in quasi-solid polymer electrolytes: roles of inorganic fillers in advanced lithium-ion batteries

Article References: Pavlovskii, A. A., Pushnitsa, K., Kosenko, A., Novikov, P., & Popovich, A. A. (2026). Interfacial chemistry and hybrid phase engineering in quasi-solid polymer electrolytes: roles of inorganic fillers in advanced lithium-ion batteries. Ionics. https://doi.org/10.1007/s11581-026-07488-x

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07488-x

Keywords: quasi-solid polymer electrolytes, lithium-ion batteries, inorganic fillers, solid electrolyte interphase, cathode electrolyte interphase, garnet ceramics, metal-organic frameworks, covalent organic frameworks, interface engineering, hybrid materials, solid-state batteries, ionic conductivity

At the heart of the review is a deceptively simple observation: the overall efficiency and degradation of a battery are determined by the contributions of its individual components, and the most consequential of those components are often the thinnest. When a lithium-ion cell is charged for the first time, the electrolyte decomposes on the surface of the negative electrode, forming a nanometer-scale film known as the solid electrolyte interphase, or SEI. A counterpart film, the cathode electrolyte interphase or CEI, forms on the positive electrode. These films are paradoxical creatures: they are a necessary evil, passivating the electrode and preventing further electrolyte breakdown, but they also add resistance that restricts the movement of lithium ions during electrochemical reactions. As the review emphasizes, the formation of the SEI, the CEI, and lithium dendrites creates major resistance at the electrode materials, and this interfacial drag is a central cause of capacity fade, impedance growth, and eventual battery failure.

The SEI has been studied since the early days of rechargeable lithium technology, but its chemistry remains fiendishly complex. It is a heterogeneous mosaic of inorganic compounds such as lithium fluoride, lithium oxide, and lithium carbonate, embedded in organic species derived from solvent decomposition. The review highlights recent work showing that the functional groups present at the surface of silicon anodes, one of the most promising high-capacity negative electrode materials, directly shape which SEI components form and how stable the film turns out to be. Researchers are increasingly designing electrolyte formulations specifically to regulate the SEI on lithium metal anodes, tailoring the balance between organic-rich and inorganic-rich interphase compositions to achieve both fast ion transport and long-term mechanical resilience. Quantitative frameworks for anti-corrosive passivation design, aimed at extending the calendar life of lithium metal batteries, represent the newest frontier in this effort.

On the positive side of the cell, the review surveys the evolution of cathode chemistry from the layered oxides that dominate today’s market to the high-voltage spinels and lithium-rich compositions that promise greater energy density. Layered lithium nickel manganese cobalt oxides remain the industry standard, but increasing the nickel content raises capacity at the cost of structural instability, surface reactivity, and transition metal dissolution. High-voltage spinel cathodes such as lithium manganese nickel oxide offer operating voltages approaching five volts, but they suffer from electrolyte oxidation and oxygen loss at the electrode surface. The review discusses surface modification, doping, electrolyte optimization, and the deliberate introduction of oxygen deficiencies as complementary strategies to stabilize these demanding materials. Recent work on cobalt-free lithium nickel oxide electrodes for sulfide-based all-solid-state batteries shows how far the push toward sustainable, high-voltage cathodes has progressed.

Lithium-rich layered oxides deserve special attention because they deliver extraordinary capacities by activating anionic redox, meaning that oxygen in the crystal lattice participates in charge storage alongside the transition metals. The catch is voltage fade: with repeated cycling, the operating voltage drifts downward, eroding energy density. Recent research has traced this phenomenon to trapped molecular oxygen within the cathode structure, a finding that finally gives designers a concrete target. The review also covers structural stabilization of lithium-rich cathodes through composite electrolytes that induce a rigid yet adaptive interphase at the cathode surface, protecting the material while preserving its exceptional capacity. Surface modification of these lithium-rich cathodes, the review notes, is one of the most active areas in current battery research.

The most tantalizing prize in the field remains the lithium metal anode. Replacing graphite with pure lithium could dramatically increase energy density, because lithium offers the highest theoretical specific capacity of any anode material and the lowest electrochemical potential. But lithium metal is notoriously unruly. During charging, lithium ions plate onto the anode surface, and if the deposition is not uniform, needle-like structures called dendrites grow across the cell. These can pierce separators, cause internal short circuits, and in the worst cases trigger fires. Even short of catastrophic failure, dendrites create dead lithium that is electrically disconnected from the electrode, permanently draining the cell’s capacity. The kinetics of the lithium electrodeposition process, including the role of exchange current density in determining whether lithium deposits smoothly or erupts into dendrites, are now understood in far greater detail than a decade ago.

Against this backdrop, the review devotes substantial space to artificial interphase engineering, one of the most promising strategies for making lithium metal anodes practical. Instead of relying on the spontaneously formed, often fragile native SEI, researchers build engineered protective layers on the lithium surface before the cell ever operates. These artificial interphases can be designed to conduct lithium ions efficiently while blocking electrons and mechanically suppressing dendrite initiation. Three-dimensional host structures that guide bottom-up lithium deposition represent a related approach, physically confining where lithium is allowed to plate. Recent demonstrations of vertically oriented one-dimensional titania nanoflake architectures show how carefully designed scaffolds can stabilize lithium deposition. Even more striking, recent experiments have achieved high plating currents without dendrite formation at the interface between a lithium anode and a solid electrolyte, challenging long-held assumptions about the limits of fast charging in lithium metal systems.

Another emerging strategy highlighted in the review is the use of multicomponent lithium metal alloys as negative electrodes. Rather than plating pure lithium, these electrodes exploit solid-solution alloys, in which lithium is stored within a matrix of other metals. Recent work on multicomponent solid-solution alloy anodes and on lithium-aluminum alloy electrodes for all-solid-state batteries demonstrates that alloying can smooth the electrochemical potential landscape, moderate volume changes, and suppress dendrite growth. Computational tools are accelerating this search: neuroevolution potentials, a machine learning technique, have recently been used to screen ion transport in lithium-rich alloys and predict which compositions will conduct lithium fastest, compressing years of trial-and-error experimentation into computational screening.

The electrolyte itself is undergoing a quiet revolution. The review discusses electrolyte diluents engineered with large electrostatic potential differences to enable fast charging and slow discharging in lithium metal batteries, as well as anion-enrichment interfacial strategies that enable high-voltage, anode-free configurations. All-solid-state batteries, in which a solid electrolyte replaces the flammable liquid, appear throughout the review as both a solution and a new set of challenges. Solid electrolytes promise improved safety and the possibility of pairing lithium metal anodes with high-voltage cathodes, but they introduce their own interfacial problems, including contact loss and space-charge layers. Adaptive interphase designs that work under pressure-free conditions are among the recent breakthroughs bringing all-solid-state cells closer to commercialization.

What emerges from the Ionics review is a picture of a field converging on a single insight: the interface is everything. Whether the challenge is dendrite formation on lithium metal, oxygen loss from high-voltage cathodes, or transition metal crossover poisoning the anode, the decisive battles are fought in layers of material only nanometers thick. The stabilization strategies surveyed, from artificial interphases and alloy anodes to surface-modified cathodes and rationally designed electrolytes, all aim to make those invisible walls stronger, thinner, and more ion-conductive. If the pace of recent progress continues, the review suggests, the next generation of batteries may deliver the combination of high energy density, long cycling life, and safety that has eluded the industry for decades, and it will do so by mastering chemistry at a scale far smaller than the eye can see.

Subject of Research: Recent advances in electrode materials, interfacial chemistry, and stabilization strategies for lithium-ion and lithium-metal batteries

Article Title: Recent advances in lithium-ion and lithium-metal batteries: Electrode materials, interfacial chemistry, and stabilization strategies for enhanced performance

Article References: Singh, V., Sharma, Y., Kumar, A., Singh, D., Anand, V., & Heera, P. (2026). Recent advances in lithium-ion and lithium-metal batteries: Electrode materials, interfacial chemistry, and stabilization strategies for enhanced performance. Ionics. https://doi.org/10.1007/s11581-026-07547-3

Image Credits: AI Generated

DOI: 10.1007/s11581-026-07547-3

Keywords: lithium-ion batteries, lithium-metal anodes, solid electrolyte interphase, cathode electrolyte interphase, dendrite suppression, artificial interphase engineering, high-voltage cathodes, lithium-rich layered oxides, multicomponent lithium alloys, all-solid-state batteries, electrolyte design, battery degradation