What Is a Composite Solid Electrolyte? Mechanisms, Materials and Conductivity.

 

In 1998, researchers at the University of Rome published a short paper in Nature describing a counterintuitive result.

Croce, Appetecchi, Persi and Scrosati dispersed nanoscale titania and alumina particles into a polymer electrolyte. The composite showed improved ionic conductivity, a higher lithium-ion transference number and stable interfacial behaviour against lithium, without the loss of mechanical integrity associated with liquid plasticizers.

The surprising part was that neither titania nor alumina conducts lithium ions. An electrically insulating additive had improved an ionic conductor.¹

Resolving this apparent paradox—and understanding what changes when the ceramic filler can conduct lithium—has occupied the field for more than two decades. This article summarises what published studies reveal about ion transport, ceramic loading, filler morphology, material selection and the limitations of composite solid electrolytes in solid-state battery research.

What is a composite solid electrolyte?

A composite solid electrolyte, or CSE, is a solid ionic conductor containing two or more distinct solid phases. It commonly consists of a lithium-salt-bearing polymer combined with an inorganic ceramic phase.

The purpose of combining the materials is to achieve a balance of ionic conductivity, mechanical strength, flexibility and electrode contact that neither component can provide alone.

Polymer electrolytes generally offer good interfacial contact, flexibility and relatively straightforward membrane processing. Their principal limitation is low room-temperature ionic conductivity, particularly when polymer crystallinity restricts segmental motion.

Ceramic electrolytes can provide higher ionic conductivity, thermal stability and mechanical rigidity. However, dense ceramics are brittle and may form poor physical contact with electrodes, resulting in high interfacial resistance.²

Composite solid electrolytes attempt to combine the complementary advantages of both material families.

Why can an insulating filler improve ionic conductivity?

Polyethylene oxide, or PEO, transports lithium ions mainly through its amorphous regions. Lithium coordination and transport are closely connected to the movement of polymer-chain segments. When PEO crystallises, chain mobility falls and ion transport becomes more difficult.

Dispersed ceramic nanoparticles can interfere with polymer crystallisation, increase the proportion of amorphous polymer and interact with the lithium salt. These interactions may promote salt dissociation and change the balance between mobile lithium ions and their counterions.

This explains why inert oxides such as Al₂O₃ and TiO₂ can improve conductivity and lithium-ion transference number even though the particles do not transport lithium through their bulk structure.¹

The magnitude of the effect depends strongly on particle size, surface chemistry, dispersion quality and the interaction between the particle surface, the polymer and the salt.

A lithium-conducting ceramic may provide these polymer-modification effects while also creating an additional pathway through the ceramic phase or along the polymer–ceramic interface.

Where do lithium ions travel inside a composite solid electrolyte?

In 2016, Zheng, Tang and Hu investigated this question using isotopic labelling and high-resolution solid-state nuclear magnetic resonance spectroscopy.

The researchers assembled symmetric cells using ⁶Li metal electrodes and a PEO–LLZO composite electrolyte. After cycling, they monitored the replacement of naturally abundant ⁷Li in the electrolyte by ⁶Li supplied by the electrodes.

The experiment provided direct evidence that lithium ions could preferentially travel through the LLZO ceramic phase, rather than solely through the PEO or the PEO–LLZO interface.³

In a related study, the ⁶Li content in the LLZO phase increased significantly after cycling, while enrichment in the polymer and interface regions was comparatively limited.⁴

The same research group later examined how the transport pathway changed across LLZO loadings from 5 to 50 wt% in PEO containing LiClO₄.⁵

LLZO content	Finding reported for the studied PEO–LiClO₄ system
Below approximately 20 wt%	Transport occurred mainly through the polymer-rich domain, while LLZO acted partly as an additional lithium-containing phase.
Above approximately 20 wt%	Additional lithium environments became detectable and were assigned to interfacial and altered LLZO regions.
Approaching 50 wt%	Transport shifted progressively towards a connected network of LLZO particles.

Important: These thresholds apply to the specific materials and processing conditions used in the study. Polymer chemistry, lithium salt, ceramic composition, particle size, surface treatment and membrane preparation can all shift the result.

Additives can also redirect the transport pathway. In a 50 wt% LLZO composite, adding tetraethylene glycol dimethyl ether increased ion mobility in the polymer and shifted the dominant conduction pathway away from the LLZO phase and back towards the PEO-rich region.⁴

However, there is still no universal agreement on which phase dominates lithium-ion transport in every composite system.

A 2025 impedance study of Ga-doped LLZO in PEO found that below approximately 85 wt% LLZO, transport was dominated by the PEO–LLZO interfaces, whereas above this threshold it occurred predominantly through the LLZO phase.⁶

Across the broader literature, the proposed pathways include the polymer bulk, ceramic bulk, ceramic surface, polymer–ceramic interface and interconnected combinations of these regions.²

What is the difference between ceramic-in-polymer and polymer-in-ceramic?

Chen, Li, Fan, Nan and Goodenough used the terms ceramic-in-polymer and polymer-in-ceramic to describe different PEO–LLZTO composite architectures.⁷

In a ceramic-in-polymer electrolyte, the polymer forms the continuous matrix and ceramic particles are dispersed within it.

In a polymer-in-ceramic architecture, the ceramic forms the dominant or interconnected structural framework, while the polymer occupies the pores, gaps and interfaces within that framework.

The distinction therefore depends on phase continuity and microstructure—not simply whether the ceramic exceeds 50 wt%.

Property	Polymer-in-ceramic system reported by Chen et al.7
Composition	80 wt% LLZTO and 20 wt% PEO-based polymer phase
Ionic conductivity	Above 10⁻⁴ S cm⁻¹ at 55 °C
Mechanical behaviour	Self-standing, flexible and mechanically robust
Lithium interface	Low plating and stripping impedance under the reported test conditions
Full-cell result	LiFePO₄|Li discharge capacity of 148.6 mAh g⁻¹ at 55 °C

Reported ceramic-network formation thresholds vary considerably. Reviews have compiled values around 50 wt% for LLZO, 63 wt% for LLTO, 63.3 wt% for LAGP and 70 wt% for LLZTO in selected three-dimensional composite architectures.⁸

These values should not be treated as universal design rules. Weight fraction does not directly represent ceramic volume fraction, and phase continuity is also affected by ceramic density, particle geometry, agglomeration and processing.

How do filler shape and particle quality affect conductivity?

Filler geometry can affect ion transport independently of ceramic chemistry and total loading.

In 2017, Liu, Lee and colleagues compared aligned and randomly dispersed ceramic nanowires in composite polymer electrolytes. The aligned-nanowire composite reached an ionic conductivity of 6.05 × 10⁻⁵ S cm⁻¹ at 30 °C—approximately an order of magnitude higher than the equivalent randomly dispersed system.⁹

The improvement was attributed to longer, more continuous pathways with fewer particle-to-particle junctions.

Other studies have reported similar advantages from interconnected ceramic structures. Representative examples include:

• A three-dimensional garnet nanofibre network reaching 2.5 × 10⁻⁴ S cm⁻¹ at room temperature.⁸
• A polymer electrolyte containing 15 wt% garnet nanosheets reaching 3.6 × 10⁻⁴ S cm⁻¹ at room temperature.⁸
• A PEO composite containing 40 wt% templated microporous LATP reaching 3.5 × 10⁻⁴ S cm⁻¹.¹⁰

These results do not mean that one-dimensional or three-dimensional fillers will always outperform nanoparticles. Alignment quality, ceramic surface condition, polymer infiltration and the resistance at each junction remain critical.

The physical quality of the ceramic particles also matters. Isaac, Devaux and Bouchet showed that the density of the inorganic electrolyte particles themselves can influence the conductivity of a composite electrolyte.¹¹

Which ceramic filler is suitable for a lithium-metal anode?

Material family	Representative behaviour and design considerations
Garnet LLZO, LLZTO	Cubic LLZO commonly shows room-temperature conductivity in the approximate range of 10⁻⁴–10⁻³ S cm⁻¹. It is considered one of the more chemically compatible oxide electrolytes for lithium-metal systems, although surface contamination, poor wetting, void formation and interfacial resistance remain practical challenges.12,13
NASICON LATP, LAGP	LATP and LAGP offer useful ionic conductivity but are reductively unstable against lithium metal. Titanium or germanium species can be reduced at low potential, causing interfacial degradation. Protective interlayers or polymer-rich interfaces are therefore normally required when these materials are used with lithium-metal anodes.10,12
Halide HLIC-1	Halide solid electrolytes can provide millisiemens-per-centimetre conductivity, strong oxidative stability and favourable mechanical processability. Many halide compositions are highly moisture sensitive and require controlled handling and storage.14,15
Sulfide Argyrodite, LGPS	Sulfide electrolytes can achieve some of the highest room-temperature conductivities among inorganic solid electrolytes and can often be densified under relatively low pressure. Their main challenges include moisture sensitivity, possible H₂S generation and chemical instability against some electrode materials.14
Inert oxide Al₂O₃, TiO₂	These materials do not conduct lithium through their bulk structure, but they may reduce polymer crystallinity, promote salt dissociation and improve conductivity or transference number when suitably dispersed.1

The most suitable filler therefore depends on more than bulk ionic conductivity. Researchers should also consider:

• Compatibility with the intended anode and cathode
• Operating temperature
• Required mechanical strength and flexibility
• Particle size and morphology
• Moisture sensitivity and handling environment
• Polymer and lithium-salt chemistry
• Whether the ceramic should form a dispersed phase or an interconnected framework

What do plasticizers change in a composite solid electrolyte?

Plasticizers can increase polymer-chain mobility, reduce crystallinity and improve salt dissociation. Their effect is not limited to increasing the measured conductivity.

As the isotope-tracing work on PEO–LLZO–TEGDME demonstrated, a plasticizer can change which phase carries most of the ionic current. Adding TEGDME to the composite increased mobility within the polymer-rich phase and shifted the dominant pathway away from the LLZO network.⁴

This illustrates an important design principle: two composites containing the same polymer and ceramic may behave differently if their solvent history, residual plasticizer content or drying conditions are different.

Plasticizers may also reduce mechanical modulus or introduce a residual liquid phase. A formulation described as a composite solid electrolyte may therefore behave more like a gel or quasi-solid electrolyte when a significant mobile-liquid fraction remains.

How are composite solid-electrolyte membranes prepared?

Many polymer–ceramic composite membranes are prepared through solution casting. The polymer, lithium salt and ceramic particles are dispersed in a solvent, cast into a film and dried under controlled conditions.

However, solution casting is not the only method. Other approaches include:

• Hot pressing
• Melt processing
• Electrospinning
• In-situ polymerisation
• Polymer infiltration into porous ceramic scaffolds
• Three-dimensional templating and ceramic-network formation

Processing affects ceramic dispersion, membrane porosity, residual solvent, polymer crystallinity and interfacial contact. These variables can change both the measured conductivity and the dominant transport pathway.

For solution-processed systems, many of the same dispersion and drying principles discussed in From Slurry to Structure and Understanding Electrode Fabrication Defects remain relevant. Thin composite membranes are often cast onto or supported by a porous separator or non-woven scaffold. Ceramic-coated separators represent a related separator-engineering strategy, although they should not be considered equivalent to a composite solid electrolyte unless the solid phases provide the principal ionic-conduction pathway.

How is ionic conductivity measured?

Ionic conductivity is commonly evaluated using electrochemical impedance spectroscopy.

A solid-electrolyte specimen with a known thickness and electrode-contact area is placed between ion-blocking electrodes, such as stainless steel, gold or platinum. The measured bulk resistance is converted into conductivity using:

σ = L ÷ (R × A)

where:

• σ is ionic conductivity
• L is the electrolyte thickness
• R is the measured resistance
• A is the electrode-contact area

Because solid-state cells are sensitive to stack pressure and contact area, both should be recorded alongside specimen thickness.

Electrochemical stability is evaluated separately, using a working-electrode and reference/counter-electrode configuration appropriate to the electrolyte and the intended voltage range. Conductivity and electrochemical-stability measurements should not be treated as the same experiment.

What limits composite solid electrolytes?

Composite solid electrolytes remain promising, but adding a ceramic does not automatically solve the limitations of either component.

Common challenges include:

• Polymer–ceramic interfacial resistance: Poorly matched interfaces can obstruct rather than accelerate lithium-ion transport.²
• Particle agglomeration: High ceramic loading can produce clusters, voids and discontinuous polymer regions.
• Poor ceramic connectivity: A high weight fraction does not guarantee a continuous lithium-conducting network.
• Mechanical trade-offs: Increasing ceramic content may improve stiffness but reduce flexibility and membrane integrity.
• Residual solvent or plasticizer: Incomplete drying can change conductivity, stability and classification as a fully solid electrolyte.
• Electrode contact: Good bulk conductivity does not guarantee a low-resistance interface with lithium or a composite cathode.
• Reproducibility: Small changes in particle surface condition, humidity, mixing and drying can produce significantly different results.

In short, the dominant conduction pathway changes with composition, morphology, additives and processing. Polymer–ceramic interfaces frequently impose one of the most important limits on performance, but polymer dynamics, salt dissociation, ceramic connectivity and electrode contact remain equally important design variables.

Common questions

Is a composite solid electrolyte the same as a hybrid or quasi-solid electrolyte?

Not necessarily. A composite solid electrolyte contains two or more solid phases. A gel or quasi-solid electrolyte contains a mobile liquid or plasticizer phase within a solid framework. Some materials described in the literature as composites also contain liquid additives and may therefore fall into both categories.

Which ceramic filler is most compatible with lithium metal?

Garnet-type LLZO and LLZTO are among the more chemically compatible oxide solid electrolytes for lithium-metal systems. However, practical interfaces can still suffer from poor wetting, surface contamination, void formation and high contact resistance. LATP and LAGP generally require a protective interlayer or polymer-rich interface when used against lithium metal.

Does the filler have to conduct lithium?

No. The original 1998 nanocomposite study used titania and alumina, neither of which conducts lithium through its bulk structure. The particles nevertheless improved conductivity and lithium-ion transference number by modifying the polymer and salt environment.¹

What ionic conductivity has been reported for composite electrolytes?

Representative published values include:

• 6.05 × 10⁻⁵ S cm⁻¹ at 30 °C for an aligned ceramic-nanowire composite.⁹
• 2.5 × 10⁻⁴ S cm⁻¹ at room temperature for a three-dimensional garnet-nanofibre network.⁸
• 3.5 × 10⁻⁴ S cm⁻¹ for a PEO composite containing 40 wt% microporous LATP.¹⁰
• Above 10⁻⁴ S cm⁻¹ at 55 °C for the polymer-in-ceramic PEO–LLZTO system reported by Chen et al.⁷

These values should only be compared when temperature, salt concentration, sample thickness, electrode configuration and test method are also considered.

At what loading does the ceramic network percolate?

There is no universal threshold. Reported values vary with ceramic chemistry, density, particle shape, surface treatment and processing. Selected studies and reviews report transitions from polymer-dominated to ceramic-connected transport over a broad range—from approximately 50 wt% to more than 80 wt% ceramic.^6,8

Does the same design logic apply to sodium batteries?

Yes. Similar composite strategies are used in sodium systems, with sodium salts, sodium-conducting ceramics and polymers selected for compatibility with sodium electrodes. The specific transport mechanisms and interfacial reactions differ from lithium systems.

For a broader comparison, see Sodium-Ion vs Lithium-Ion Batteries and explore our sodium-ion research materials.

Selecting materials for a composite-electrolyte study

Composite-electrolyte research is rarely about identifying one universally superior material. The result depends on how the phases are combined, which phase is continuous and what happens at the interfaces between them.

Beyond Battery supplies research-grade solid-electrolyte powders, including:

• LLZO
• LLZTO
• LATP
• LAGP
• Halide HLIC-1

The full range, together with separators, current collectors, cathode sheets and cell hardware, is in our solid-state battery materials collection. Different particle sizes, dopant compositions and pack quantities may be available. Technical documentation, including TDS, SDS and CoA, is available on request.

When contacting our team, share your polymer, lithium salt, operating temperature, intended electrode chemistry and preferred preparation method. We can help identify suitable filler options and supporting materials for your formulation.

Contact the Beyond Battery technical team about your composite-electrolyte project.

References

1. Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nanocomposite polymer electrolytes for lithium batteries. Nature 1998, 394, 456–458. DOI: 10.1038/28818.
2. Horowitz, Y.; Lifshitz, M.; Greenbaum, A.; Feldman, Y.; Greenbaum, S.; Sokolov, A. P.; Golodnitsky, D. Review—Polymer/Ceramic Interface Barriers: The Fundamental Challenge for Advancing Composite Solid Electrolytes for Li-Ion Batteries. J. Electrochem. Soc. 2020, 167 (16), 160514. DOI: 10.1149/1945-7111/abcd12.
3. Zheng, J.; Tang, M.; Hu, Y.-Y. Lithium ion pathway within Li₇La₃Zr₂O₁₂–polyethylene oxide composite electrolytes. Angew. Chem. Int. Ed. 2016, 55, 12538–12542. DOI: 10.1002/anie.201607539.
4. Zheng, J.; Dang, H.; Feng, X.; Chien, P.-H.; Hu, Y.-Y. Li-ion transport in a representative ceramic–polymer–plasticizer composite electrolyte: Li₇La₃Zr₂O₁₂–polyethylene oxide–tetraethylene glycol dimethyl ether. J. Mater. Chem. A 2017. DOI: 10.1039/C7TA05832B.
5. Zheng, J.; Hu, Y.-Y. New insights into the compositional dependence of Li-ion transport in polymer–ceramic composite electrolytes. ACS Appl. Mater. Interfaces 2018, 10, 4113–4120. DOI: 10.1021/acsami.7b17301.
6. Seo, J.; Nasir, M.; Park, H. J. Lithium-ion conduction pathways in LLZO–PEO composite solid electrolytes. ACS Appl. Energy Mater. 2025, 8, 1518–1525. DOI: 10.1021/acsaem.4c02489.
7. Chen, L.; Li, Y.; Li, S.-P.; Fan, L.-Z.; Nan, C.-W.; Goodenough, J. B. PEO/garnet composite electrolytes for solid-state lithium batteries: from “ceramic-in-polymer” to “polymer-in-ceramic”. Nano Energy 2018, 46, 176–184. DOI: 10.1016/j.nanoen.2017.12.037.
8. Zhang, X.; Cheng, S.; Fu, C.; et al. Advancements and Challenges in Organic–Inorganic Composite Solid Electrolytes for All-Solid-State Lithium Batteries. Nano-Micro Letters 2025, 17, 2. DOI: 10.1007/s40820-024-01498-y.
9. Liu, W.; Lee, S. W.; Lin, D.; Shi, F.; Wang, S.; Sendek, A. D.; Cui, Y. Enhancing ionic conductivity in composite polymer electrolytes with well-aligned ceramic nanowires. Nature Energy 2017, 2, 17035. DOI: 10.1038/nenergy.2017.35.
10. Small components play a big role—fillers in composite solid-state electrolytes for lithium metal batteries. Energy Materials 2024. DOI: 10.20517/energymater.2024.57.
11. Isaac, J. A.; Devaux, D.; Bouchet, R. Dense inorganic electrolyte particles as a lever to promote composite electrolyte conductivity. Nature Materials 2022, 21, 1412–1418. DOI: 10.1038/s41563-022-01343-w.
12. Quasi-solid-state electrolytes—strategy towards stabilising Li|inorganic solid electrolyte interfaces in solid-state lithium-metal batteries. Energy Materials 2023. DOI: 10.20517/energymater.2023.03.
13. Wang et al. Accelerating the development of LLZO in solid-state batteries toward commercialization: a comprehensive review. Small 2024. DOI: 10.1002/smll.202402035.
14. Bouguern, M. D. et al. Comparative Advances in Sulfide and Halide Electrolytes for Commercialization of All-Solid-State Lithium Batteries. Advanced Materials 2026. DOI: 10.1002/adma.202513255.
15. Ren, T.; Chen, X.; Zhang, H.; Zhang, H.; Wang, W. Humidity stability of halide solid-state electrolytes. J. Mater. Chem. A 2025, 13, 38609–38632. DOI: 10.1039/D5TA06216K.

Compiled by the Beyond Battery technical team in Singapore from the published literature cited above. This article is provided for research and educational purposes. All referenced products are supplied for laboratory research use only.