Review—Polymer Electrolytes for Rechargeable Batteries: From Nanocomposite to Nanohybrid

Numerous studies have been performed incorporating inactive inorganic fillers in PEs. TiO₂, Al₂O₃, ZnO_x, ZrO₂ and SiO₂ are some of the most widely employed inorganic fillers (Table I).^28,34–36 Nevertheless, the effect of these inorganic fillers on the electrochemical properties is still debated. At first, it was claimed that the addition of fillers could enhance the mechanical properties of the PE and at the same time decrease of the crystallinity of PEO, improving the ionic conductivity at temperatures below the melting transition.³⁴ However, several studies demonstrated that the overall effect depends on several factors, such as type of filler and size of the particles.^37–39 Fig. 2a compares the ionic conductivity of different type of CPEs; the highest values are obtained with liquid electrolytes and the lowest one for filler-free polymer electrolytes. The addition of nanofillers increases the ionic conductivity of PE by two orders of magnitude at room temperature.⁴⁰ Additionally, it can be appreciated that the nature of the filler has also an effect on the ionic conductivity (σ_free < σ_Al2O3 < σ_TiO2), suggesting that the chemical nature of inorganic particles and their surface groups play important role on the ionic transport properties of the resulting PEs.

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Dissanayake et al.,⁴¹ studied the effect of the particle size on the ionic conductivity of a PE comprised of PEO and lithium triflate (LiCF₃SO₃, LiTf, Fig. 2b). The conductivity was greatly improved with the smallest particles having the highest surface area. By decreasing the particle size from 10 μm to 10–20 nm, ionic conductivity was increased by one order of magnitude at 70 ^oC, reinforcing the importance of "nano-scaled" mixing between organic and inorganic phases in dictating the ionic transport behavior of CPEs.

To evaluate the role of the surface chemistry, Croce and co-workers⁴² studied the influence of nanoparticle surfaces on electrochemical properties by tuning nanoparticle surface chemistry (acidic, neutral and basic surfaces). Based on the results of the study, coordination between polymer, salt, and fillers was hypothesized. This mechanism is explained by Lewis acid-base interactions. In the case of acidic surfaces, –OH groups compete in the coordination with the basic oxygen atoms of the polymer and anions through hydrogen bonding, providing additional lithium transport pathways. Alternatively, the surface protons contribute to the solvation of the anion, increasing dissociation. With neutral particles, due to the lower concentration of –OH groups, the number of possible interactions is weaker, leading to a lower favorable effect. Finally, where the surface provides a basic character, the particles will not interact neither with the polymer nor the anion, resulting in a negative effect on the ionic conductivity. This Lewis acid-base approach was supported by the experimental data of PEO₂₀LiTf with the addition of 10 wt% Al₂O₃ with different surfaces, where acidic surfaces offered the highest ionic conductivity values among all tested fillers.⁴²

Besides, the particle concentration is also important. A high content of inorganic nanofillers enhances the mechanical properties but reduces the ionic conductivity of the electrolyte.⁴⁴ As it can be observed in Fig. 2c, at high concentration of inorganic nanofillers (>4 wt% SiO₂), the storage modulus increases, whereas the ionic conductivity is negatively affected. Favorably, due to the enhanced mechanical stiffness of the membrane, dendrites formation is also hindered.⁴⁵ Figure 2d shows the overall effect of nanoparticles size on the dendrite growth: large particles are able to retard the dendrite formation (2) compared to particle-free electrolyte (1), while, smaller particles directly prevent dendrites formation (3).

As discussed in previous section, the addition of passive fillers into a PE may have a beneficial effect by means of improved mechanical strength and increased polymer chain mobility; however, these inactive nanoparticles cannot directly contribute to the ionic conductivity of the electrolyte.

In this regard, the combination of SPEs with Li-ion conductive inorganic electrolytes has attracted much attention in an attempt to surmount the limitation of each individual component. Among all the existing inorganic electrolytes, NASICON and garnet are the ones that have recently stimulated the most research interest for the development of nanocomposites. NASICON-type electrolytes, such as Li_1+xAl_xTi_2−x(PO₄)₃ (LATP)⁴⁶ and Li_1+xAl_xGe_2−x(PO₄)₃ (LAGP)⁴⁷ have been used as active fillers in composites due to their wide electrochemical window, high ionic conductivity and moisture stability, while garnet-type ones with chemical structures derived from Li₇La₃Zr₂O₁₂ (LLZO)^48,49 have been chosen because of their outstanding compatibility with lithium metal anode.

Despite this increasing number of works focused on the replacement of inactive fillers by Li-ion conductive inorganic ones, there is a controversy regarding the impact that this may have in the overall performance of the CPE. Consequently, it is easy to find literature reporting an increase of the ionic conductivity of the CPE compared to neat PE,^48,50 as well as, some others showing negligible or even detrimental effect^49,51 after ceramic filler introduction. Nevertheless, it is generally agreed that the main decisive factors are the amount, the chemical composition and the size of the filler.

Overall, the ionic conductivity and lithium diffusion path understanding in composite electrolytes containing active fillers remains the major issue to be addressed. Depending on the particles size, shape and concentration, several scenarios can be sketched. As an example, Fig. 3a depicts the main lithium conduction pathways proposed for four CPEs, distinguishing between spherical shape nanoparticles (1 and 2) and nanowires (3 and 4). Moreover, depending on the composition, another classification can be done, i.e., between polymer-rich electrolytes (Fig. 3a-1), on which PE is the main component, and ceramic-rich electrolytes (Fig. 3a-2) where inorganic particles are predominant.⁵² The former CPE offers three Li-motion pathways: through both inorganic particle and PE (orange dashed line), across neat PE (grey dashed line) and along PE/nanoparticle interfaces (black dashed line). When exceeding the percolation threshold, as is the case with ceramic-rich CPEs, an alternative Li-diffusion path is created through the inorganic particles (yellow dashed line).

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Since the first report on the introduction of a nano-sized active filler, γ-LiAlO₂, in a PEO-based PE by Scrosati et al.,⁵³ many works have been undertaken taking this polymer as reference matrix. Figure 3b compares the ionic conductivity reported for some representative PEO-based CPEs. While the improvement in the ionic conductivity is widely attributed to the increase of PEO amorphicity produced by the inorganic particle introduction, Kieffer et al.⁴⁶ concluded that there should be other factors affecting the ionic conductivity when adding active fillers. After comparing the performance of PEO/LiClO₄ with LATP to the one with inactive fillers they found similar glass transition and crystallinity degree regardless the inorganic particle type; however, a higher ionic conductivity was obtained for the CPE with active fillers. In view of these results and taking into account that LATP content was below the percolation threshold (4 vol% content), the mechanism proposed involved the balancing of the chemical potential of lithium between both phases and possibly a Li⁺ exchange, since both active filler and PEO/LiClO₄ share lithium as a common component.

To further understand the transport mechanism of ionic species in CPEs, solid-state nuclear magnetic resonance (ssNMR) is emerging as one of the most promising techniques. A combination of isotope labeling and high-resolution Li ssNMR was employed to track the Li-ion pathway in a PEO-LLZO CPEs.⁵⁴ The monitoring of the replacement of ⁷Li from the CPE by ⁶Li from the Li metal electrodes during plating and stripping experiment showed a preference of Li-ion to go through the ceramic pathway rather than the neat polymer or the PEO-LLZO interface. More recently, ssNMR studies related to a CPE based on PEO and lithium bis(trifluoromethanesulfonyl)imide {Li[N(SO₂CF₃)₂], LiTFSI} with 10 vol% of Li_6.55Ga_0.15La₃Zr₂O₁₂ microparticles proved that Li exchange could take place at the soft-hard interface between Li_6.55Ga_0.15La₃Zr₂O₁₂ and PEO.⁵⁵ Even if this exchange process was characterized by slow kinetics, these results suggest that increasing the compatibility and interaction between both phases by further surface modification of the filler or changes in the polymer functional groups may allow this process to happen in larger length scale.

In great contrast to previous studies where high active filler content was shown to have a detrimental effect on the ionic conductivity,^51,49 PEO-LAGP based CPE with high ionic conductivities and high loadings above 50 wt% have been reported.^47,56 Even so, in the case of extremely high filler content, the addition of an intermediate thin upper layer based on PEO between the Li metal surface and the CPE was required for improving the contact and preventing the reaction of LAGP.⁵⁶

Beyond PEO, polycarbonates have attracted much attention in the design of CPEs. Poly(propylene carbonate) (PPC)/LiTFSI/Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) electrolyte provided a promising ionic conductivity at 20 ^oC of 5.2 × 10⁻⁴ s cm⁻¹ with a lithium transference number of 0.75.⁵⁷ The high transference number was ascribed to the interactions of the nano-sized LLZTO with both TFSI⁻ and PPC polymer chains, favoring the salt dissociation and inhibiting anion diffusion. Besides, the introduction of garnet in a poly(ethylene carbonate) (PEC) -based CPE could improve the interface with lithium metal avoiding side reactions with the electrolyte.⁵⁸

Finally, the effect of the particle shape in the ionic conductivity of the CPE is evaluated and compared in Fig. 3c. It has been proposed that the replacement of nanoparticles by nanowire fillers may provide a continuous Li-ion transport pathway through the highly conducting ceramic phase.^59–61 Namely, the replacement of Li_0.33La_0.557TiO₃ (LLTO) nanoparticles by LLTO randomly dispersed nanowires (Fig. 3a-3) in a poly(acrylonitrile) (PAN)/LiClO₄ PE, provided an increase of the ionic conductivity from 1.0 × 10⁻⁶ s cm⁻¹ to 5.4 × 10⁻⁶ s cm⁻¹ at room temperature.⁶¹ This performance was further enhanced by the alignment of the nanowires in the conduction direction (Fig. 3a-4), allowing the improvement of ionic conductivity by another order of magnitude, highlighting the importance of obtaining a CPE free of crossing junctions as happens in the case of nanoparticles or randomly dispersed nanowires.

All in all, even if different mechanisms have been proposed for the ionic conduction in CPE, the exact role of the active filler inside the PE remains unclear. The scant data available related to the study of the Li-ion exchange between the different CPE components, evidence the need of new strategies to boost this interaction. Moreover, considering that when decreasing the particle size to the nano-scale the surface/volume ratio drastically increases, customizing the nanoparticle surface may be crucial for promoting the Li-ion conduction at the phase boundary.

With respect to CPEs, one of the greatest attractiveness of hybrid inorganic-organic materials relies on the possibility of tailoring to a great detail the properties of the designed material.

The first applications of hybrid inorganic-organic electrolytes in lithium batteries date back to the second half of the eighties—early nineties,^32,63 following the development of sol-gel derived hybrid inorganic-organic materials⁶⁴ and of hybrid inorganic-organic protonic conductors.³³ These systems (Ormolytes, lithium-doped Ormosyls, Ormocers, 3D-HION, etc.)¹³ were prepared by sol-gel reaction of silanes in the presence of oligo- or polyethers. In some configurations, silicon atoms or other metal centers function as cross-linkers between polyether chains.^65–69 Later on, this approach was developed to obtain cross-linked hybrid systems with poly(ethylene glycol) (PEG) chains anchored to inorganic clusters (Fig. 4a).^70–74

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Sol-gel technique was also used to grow in situ the inorganic particles.^75–77 In situ growth of SiO₂ nano-particles directly into the PEO matrix provides a hybrid material with peculiar properties, with respect to traditional composites prepared by simple physical mixing. SiO₂ particles are more uniformly distributed in the polyether matrix and the polyether chains are either mechanically wrapped or chemically anchored to the particle surface. This results in much stronger PEO-SiO₂ and in a higher conductivity than in traditional composites (Fig. 4b).⁷⁷

If in situ growth results in uniform particles' dispersion, the size, shape and functionalization of the inorganic clusters can be controlled at best by using preformed hybrid fillers, such as polysilsesquioxanes (POSS). This strategy allows the introduction of oligosiloxanes with specific organic functionalities. POSS bearing PEG chains hinder the crystallization of the polymer host, thus increasing the conductivity of the resulting hybrid (Fig. 4c).⁷⁸ Additionally, polymerizable functional groups can be used as crosslinker, resulting in networked polymer electrolytes with relatively high shear modulus and conductivity.^79,80 Alternatively, anionic-functionalized POSS, neutralized with lithium ions, can be used as polyanionic salt.^81,82 The functionalized POSS clusters act as charge carriers sources and, if no binary salt is added, the resulting electrolyte is a single-ion conductor. Interestingly, NMR studies indicate that, with respect to polymeric single-ion conductors, the rigid cage-structure of polyanionic POSS hinders the formation of ionic aggregates, thus favoring lithium mobility.⁸³

Beside particle size, shape and surface chemistry, it might be also interesting to control the porosity of the filler. Great control of the pore size can be achieved, for example, by using Metal Organic Frameworks (MOFs). MOFs have been used as neat lithium-ion conductors (by soaking with carbonates),⁸⁴ as scaffold for ionic liquids,⁸⁵ for carbonate-based electrolytes,⁸⁶ and as fillers in composite polymer electrolytes.^87,88 In this case, comparable results to other composite electrolytes have been achieved, such as room temperature conductivities between 10⁻⁵ and 10⁻⁴ s cm⁻¹ and cycling at 60–70 °C.⁸⁹ In other cases, MOFs structure was specifically modified to increase the lithium transference number. As example, MOFs with adjusted pore size were used as selective ions channels (Fig. 4d),⁹⁰ whereas cationic MOFs were utilized as anions traps.⁹¹

Altogether, the most common strategy to prepare hybrid inorganic-organic electrolytes is by grafting organic functionalities on the surface of inorganic particles, which are then dispersed in the polymer-salt complex. The resulting composites with hybrid fillers are called nano-hybrid electrolytes. Fumed silica, either pristine or hydrophobic, is one of the many fillers employed in composite electrolytes, especially due to its ability for forming network structures with liquids.^92,93 Capping of silanols with other functionalities has been later performed to enhance both mechanical and transport properties. Fan et al.⁹⁴ were the first to use SiO₂ functionalized with oligoethylene oxide chains, although with no beneficial effect on the conductivity. Archer and coworkers⁹⁵ tethered longer PEG chains on SiO₂ and TiO₂. By mixing these functionalized particles with lithium salt, hybrid electrolytes were obtained which showed an interesting mix of transport and mechanical properties. Specifically, these materials showed superionic properties, i.e., decoupled transport and mechanical properties. Interestingly, no significant deleterious effect due to the inorganic core chemistry was observed. The ionic conductivity of these materials can be further enhanced by adding small amounts of solvent, without significant deterioration of the rheological properties.⁹⁶

Interesting effects were observed also by grafting ionic functionalities. Particles with surface cationic functionalities were first deployed by Archer and coworkers,⁹⁷ this time mainly for application in combination with ionic liquids. The addition of imidazolium-tethered silica particles results in either the delay of the ionic liquid crystallization or the increase of the ionic conductivity and of the transference number, due to the hindering of the salt crystallization and slowdown of the anion mobility.⁹⁸ Similarly, Sato et al.⁹⁹ prepared a solid electrolyte with high conductivity by adding small amounts of ionic-liquid to cation-functionalized SiO₂ particles (Fig. 4e). Cationic functionalized particles were also used in combination with carbonate-based liquid electrolytes, resulting in enhanced transport properties,¹⁰⁰ retarded dendrites formation,¹⁰¹ and higher cyclability of Li ∣∣ LTO cells.¹⁰²

Finally, inorganic particles can be functionalized with anionic groups. The first electrolytes of this type used silica functionalized with alkyl-sulfonate groups,^103,104 which show conductivity between 10⁻⁶ s cm⁻¹ and 10⁻⁴ s cm⁻¹, depending on the host matrix.

The type of anionic group, functionalization density, chain length and flexibility are critical to obtain higher dissociation and conductivity. Higher conductivities can be achieved by increasing the loading of ionic groups on the surface of the particle,¹⁰⁵ by using anions with higher dissociability (e.g., trifluoroborate groups,¹⁰⁶ trifluoromethanesulfonamide¹⁰⁷) by increasing the chain flexibility next to particle surface,¹⁰⁸ and by grafting both ionic groups and cation acceptor chains (Fig. 4f).^107,109

Despite all the efforts, the conductivity of single-ion conducting CPE is in general too low for room temperature operation. The conductivity is enhanced by adding a lithium salt, but this is compensated by a decrease of the lithium transference number.¹⁰³ Practical electrolytes may be though obtained by finding a compromise between these two instances. A good example is the use of sulfonate-functionalized SiO₂ as additive in carbonate-based gel-polymer electrolytes, which results in an increase of both ionic conductivity and transference number.¹¹⁰

As a last remark, it was recently reported that conductivity could be greatly enhanced by using nanometric Al₂O₃, instead of SiO₂,¹⁰⁷ suggesting that the chemistry and morphology of the inorganic core may play a role in determining the transport properties of nanohybrid electrolytes.

CPEs are generally prepared by physical mixing of a polymer/salt complex with inorganic particles either in dry state (e.g., ball-milling) or in common solvent. CPEs are very attractive in terms of synthesis due to the simplicity of the preparation procedures and the low cost of the individual components. No chemical modifications are involved and no expensive precursors are required. In comparison, HPEs, in which organic and inorganic phases are connected by strong covalent or ionic bonds, are prepared in two ways: a) a bottom-up approach, where HPEs are obtained directly by reaction of cross-linkable precursors, typically by sol-gel technique; b) a nano-hybrid approach, where hybrid inorganic-organic fillers are firstly prepared and then introduced in a polymer or liquid matrix. HPEs synthesized from bottom-up approaches are generally characterized by high conductivity (up to ca. 5 × 10⁻⁴ s cm⁻¹ at room temperature, in the absence of plasticizer), low or no tendency to crystallize, and high thermo-mechanical stability due to the presence of a 3D network. On the downside, mechanical and transport properties are strongly coupled, which means that the mechanical modulus of the most conductive materials is relatively low. In contrast, the hybrid-composite approach is very versatile, as it allows obtaining materials with a wide range of properties, by using a large variety of host matrices, functional groups, and by choosing inorganic particles with specific size and chemistry. Furthermore, the synthetic process is relatively easy and controllable, as it involves only the functionalization of the particles surface. As an example, it is relatively easy to prepare single-ion conductors, e.g., by grafting the inorganic particles surface with anionic groups. This is probably one of the easiest and most effective ways to immobilize anions in a polymer electrolyte, resulting in single-ion conducting electrolytes with conductivities approaching those of binary electrolytes.¹⁰⁷ However, both ways involve certain synthetic steps for building chemical linkage between organic and inorganic materials, making HPEs of lesser interest compared to CPEs in terms of upscaling for practical applications. Hence, efficient and scalable synthetic methods for easily grafting organic moieties to inorganic substrates should be carefully explored in the search of robust HPEs.

In the case of CPEs, the ionic conductivity is rarely above 10⁻⁴ s cm⁻¹ at room temperature, unless high amounts of plasticizers are added, which then results in a significant deterioration of the mechanical properties. Furthermore, classical CPEs are intrinsically binary conductors, although in some cases an increased transference number is achieved. A notable exception is represented by systems with ionomer-type polymer host, which, however, are characterized by low ionic conductivity. Indeed, one of the hardest obstacles is represented by the mutual dependence of ionic conductivity, transference number and modulus, which prevents increasing at the same time all three properties. The same limitations are valid also for HPEs: it is possible to selectively increase some of the performance parameters, but it is difficult, if not impossible, to maximize all at a time.

Figure 5 shows the multifunctional graphs of ionic conductivity vs elastic modulus and apparent transference number for several polyether-based polymer and hybrid electrolytes. In the case of the elastic modulus, the strong coupling with the conductivity is clear. Conductivities higher than 10⁻⁴ s cm⁻¹ are reached only with moduli lower than 10 MPa, by plasticized electrolytes or by mechanically-weak HPEs. For the transference number, only the non-plasticized systems are reported, for sake of clarity. In this case the coupling is less obvious, but a clear trend is observed at least for HPEs, with the conductivity decreasing of almost three orders of magnitudes when the transference number is increased between 0.3 and 1. Both CPEs and HPEs reach higher conductivities and transference numbers than PEO/LiX systems. Nonetheless, the transference numbers for the most conductive systems are below 0.5. Truly single-ion conduction is achieved only by HPEs, at a cost of decreasing the conductivity.

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The comparison shows that the HPE and CPE approaches led to clear enhancement of selected transport parameters. Possibly, a combination of the two approaches may lead to a further enhancement of both conductivity and transference number.

In the quest towards high energy, solid-state batteries, the attention has gradually shifted towards inorganic electrolytes, which are characterized by high conductivity and transference numbers equal to one. However, with respect to PEs, inorganic electrolytes have a clear disadvantage in terms of flexibility, interfacial properties (in particular regarding the ability of establishing and maintaining a good physical contact with the electrodes)¹¹ and processability.⁴⁹ CPEs with lithium-conducting fillers are considered as a possible compromise solution between inorganic and polymer electrolytes, as they could couple the excellent transport properties of inorganic electrolytes with the flexibility, interfacial properties and processability of polymer electrolytes. Indeed, some studies on CPEs with lithium-conducting fillers report conductivities over 10⁻⁴ s cm⁻¹ and increased transference number with respect to reference filler-free PEs.^57,115,116 Nonetheless, the ionic conductivity and transference number are still lower than in inorganic electrolytes. Sluggish ion transport in the polymer host matrix and across the polymer/ceramic interphase limits the overall conductivity, whereas binary transport in the organic phase results in a decreased transference number.

The conduction mechanism in systems were both inorganic and polymer phases are lithium-conducting is complex, and these systems have not been studied as in depth as classical CPEs, Fig. 6. The charge transport mechanism, and in particular between the two phases, has not been fully elucidated. Consequently, it is difficult to develop a strategy to optimize the transport properties. Ideally, the highest lithium conductivity is expected for ceramic-rich CPEs with bulk, mono-crystalline inorganic particles, lithium percolation pathways through the highly conducting ceramic phase and no grain boundary throughout the migration path. However, in a practical composite separator, lithium ions will have to cross multiple resistive particle/particle and particle/polymer interfaces. Thus, it is likely that ceramic-rich electrolytes will be at disadvantage in respect to the polymer-rich counterparts due to the blocking of the percolation pathways through the polymer phase.¹¹⁷ Hence, the interface between inorganic and organic phases appears playing a decisive role in determining the transport properties. Engineering of the surface of the inorganic particles may be a key element to enhance the interface charge transfer, providing that the surface modification is ionically conductive and has a good compatibility with the surrounding polymer matrix, e.g., coating the surface of garnet electrolyte with dopamine¹¹⁸ or lithium fluoride.¹¹⁹ Another possibility could be the direct covalent coupling between inorganic particles and polymer matrices. In the last case, or if surface modification with organic molecules is performed, the resulting material could be classified as a nanohybrid electrolyte. In other words, the nanohybrid approach, which has been already successfully proved with inactive inorganic nanoparticles, might be a promising choice also when implemented to lithium-ion conducting fillers.

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With regards to the cell performance, both classes of electrolyte materials have been traditionally tested at light cycling conditions, i.e. at medium-high temperatures (≥50 °C), low C-rates (<1 C), with low-voltage cathode materials (e.g., LiFePO₄), and with low active material loading (areal capacities lower than 1 mAh cm⁻²). At least in recent years a few reports have appeared, either with CPEs or with HPEs, showing cycling at room temperatures.^80,120–125 Independently of the type of electrolyte used, it appears that room temperature cycling is possible, at least at low C-rates, in systems with conductivity >10⁻⁴ s cm⁻¹, or with ultra-thin electrolyte layers.¹²⁶

At the same time, it is becoming customary to test solid state electrolytes also with higher-energy cathodes, such as lithium nickel manganese cobalt oxide (NMC), lithium nickel cobalt aluminum oxide (NCA), LiFe_xMn_1−xPO₄ (LFMP), or LiNi_0.5Mn_1.5O₄ (LNMO).^{124,127–131} In this regard, it must be noted that for PEO-based PEs the electrochemical stability is reportedly limited up to 4 V. Thus, the use of high-voltage cathode materials may require switching to other type of more stable host matrices, e.g., possibly polycarbonates or polyesters. In this case, CPEs may be advantaged with respect to HPEs, as the latter are usually based on polyether moieties. Another possible strategy is to use layered electrolyte systems, in which a polyether-based electrolyte is in contact with the anode, but not with the cathode.^132,133 Finally, another possibility could be to develop hybrid inorganic-organic electrolyte based on non-polyether moieties.

With respect to the lithium metal anode, good plating/stripping performance has been demonstrated with CPEs,^52,115 probably due to good mechanical properties of these electrolytes. Although there are few reports on the plating/stripping properties of HPEs,^91,134 it is expected that single-ion conducting systems to have a good resistance against dendrites.¹³⁵ In this case, the combination of the two approaches might be a promising way to enhance the stability with lithium metal, by means of combined enhancement of mechanical properties and transference number.

In conclusion, there has been, in recent years, an evident improvement of the cell performance with SPEs, in particular regarding the operation temperature and cathode chemistry/cell voltage. This improvement was observed with different types of materials (HPEs, CPEs, gel-polymer electrolytes, polycarbonate-based electrolytes, etc.), although CPEs with active fillers have shown particularly promising results. Nonetheless, further improvements are needed in the test conditions, in order to enhance the representativity of the results. In practical terms, this means reducing the amount of lithium at the anode, using thinner electrolyte layers, and especially increasing the areal cathode capacity (>1 mAh cm⁻²).