# Lithium salt-induced microphase separation in photopolymerized solid polymer electrolytes

Kingsley Ikenna Aniagbaoso, Monika Krol, Janne Ruokolainen, Antoine

Bousquet, Maud Save, Laurent Rubatat

► To cite this version:

Kingsley Ikenna Aniagbaoso, Monika Krol, Janne Ruokolainen, Antoine Bousquet, Maud Save, et al. Lithium salt-induced microphase separation in photopolymerized solid polymer electrolytes. Applied Materials Today, 2025, 47, pp.102946. 10.1016/j.apmt.2025.102946. hal-05368987

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# Lithium salt-induced microphase separation in photopolymerized solid polymer electrolytes

Kingsley Ikenna Aniagbaoso<sup>a,1, ID</sup>, Monika Król<sup>b,1, ID</sup>, Janne Ruokolainen<sup>b</sup>, Antoine Bousquet<sup>a, ID</sup>, Maud Save<sup>a</sup>, Laurent Rubatat<sup>a,* ID</sup>

## ARTICLE INFO

**Keywords:**

Solid polymer electrolytes

Block copolymers

Star architecture

Microphase separation

Photopolymerization

## ABSTRACT

This paper reports on lithium salt-induced microphase separation of linear and star block copolymers (BCPs) dispersed in monomer to produce solid polymer electrolytes (SPEs). The proposed one-step photopolymerization methodology uses pre-synthesized BCPs of Poly (styrene-co-benzyl methacrylate)-b-Poly [oligo(ethylene glycol) methyl ether acrylate] (P(S-co-BzMA)-b-POEGA) solubilized with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in oligo (ethylene glycol) methyl ether acrylate (OEGA). The resulting kinetically arrested glassy nano-domains of P(S-co-BzMA) are later fixed during the POEGA homopolymer matrix photopolymerization. A variety of morphologies can be produced by adjusting lithium salt content and BCP architectures, i.e., linear or star-like. Embedding the BCP nanostructures in the POEGA matrix acted as physical crosslinking that enhances the meCHANICAL strength by a factor of seven, compared to un-crosslinked POEGA, without sacrificing on ionic con ductivity. Remarkably, these innovative materials not only match the mechanical properties and conductivity of traditional cross-linked POEGA-based SPEs, but also open a valuable way towards recyclability.

## 1. Introduction

Polyethylene oxide (PEO) remains the most investigated polymer matrix for solid polymer electrolyte (SPE). It exhibits good stability against lithium metal and high solubility for a wide range of lithium salts [1]. Nevertheless, PEO-based SPEs exhibit rather low ionic conductivity compared to liquid electrolytes, and lack the mechanical strength required to impede lithium dendrite growth [2–4]. Lithium transport is promoted by PEO segmental motion within the amorphous regions [5]. Thus, various strategies have been employed to reduce PEO crystallinity by breaking up the long-range macromolecular symmetry, such as utilizing PEO with comb-like macromolecular architectures [6]. In that context block copolymers (BCPs) with PEO blocks [7,8], or comb-like PEO block [9–11], blended with lithium salts, have been explored as microphase separated SPEs to reach the best compromise between ionic conductivity and mechanical properties [12–14]. Interestingly, this approach has been extended to polymerization induced microphase separation (PIMS) to achieve co-continuous phases starting from liquid formulations that can be thermally-activated [15,16], or

photo-activated [17]. Nevertheless, to allow full solubility of the polymer in the PIMS formulation, solvents or ionic liquids are usually added, thus leading ultimately to quasi-SPEs. Recently, in situ PIMS was reported to produce quasi-SPEs directly on the positive and negative electrodes [18]. Similarly to PIMS, salt induced microphase separation of BCP and homopolymer blends was exploited to generate nano-structured SPEs [19–21].

In the present work, nano-structured SPEs are prepared by the combination of salt induced microphase separation of pre-synthesized BCP at nanometer scale and photopolymerization of the ion-conductive monomer. This mechanism differs from the typical PIMS process, where the polymerization starts from a reactive polymer soluble in the monomer with phase separation occurring continuously during the polymerization. The main interest of this BCP photopolymerization strategy lies in its ability to expedite the fabrication of nano-structured materials through a solvent-free process [22]. Thus the produced SPE is fully solvent-free which is not accessible with the regular PIMS process. In addition, this approach is compatible with an *in situ* preparation of the SPE onto the electrodes, which is highly desirable for achieving efficient

<footnote><sup>a</sup> Universite de Pau et des Pays de l'Adour, UPPA, CNRS, IPREM, Pau, France</footnote>

<footnote><sup>b</sup> Department of Applied Physics, School of Science, Aalto University, Espoo FIN-00076, Finland</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: laurent.rubatat@univ-pau.fr (L. Rubatat).</footnote>

<footnote n="1">¹ These authors contributed equally to this work.</footnote>
and conformal interfaces [18,23]. To do so, the ion conductive oligo (ethylene glycol) methyl ether acrylate (OEGA) monomer, used as solvent, was photopolymerized in presence of linear or star comb-like block copolymers (BCP) of poly (styrene-co-benzyl methacrylate)-b-Poly [oligo (ethylene glycol) methyl ether acrylate] (P(S-co-BzMA)-b-POEGA), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and a photoinitiator. The morphological evolution of the blends before and after polymerization together with the influence of varying the lithium salt content, were investigated. Ultimately, SPE mechanical and ion conducting properties are correlated to their morphologies.

## 2. Materials and methods

### 2.1. Materials

Benzyl Methacrylate (BzMA, Sigma-Aldrich, 96%) was passed through a silica gel column to remove monomethyl ether hydroquinone inhibitor. Styrene (S, Sigma-Aldrich, 99%†), Azobis(isobutyronitrile) (AIBN, Sigma-Aldrich, 98%), Oligo (ethylene glycol) methyl ether acrylate (OEGA, $DP_n = 8–9$, average $M_n = 480$ g mol⁻¹, Sigma-Aldrich), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Solvionic, 99.9% extra dry), were used as purchased. Pentaerythritol tetrakis[2-(dodecylthiocarbonothioylthio)-2-methylpropionate (4-arm-star DDMAT RAFT agent, Sigma-Aldrich, 97%), 4-cyano 4-[(dodecylsulfanylthiocarbonyl)-sulfanyl]-pentanoic acid (monofunctional RAFT agent, Sigma-Aldrich, 97%), Ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (TPO-L) photo-initiator (316 g mol⁻¹).

### 2.2. Photopolymerization

The photopolymerization of OEGA (with TPO-L) was initiated by irradiating the liquid resin, contained in PTFE molds, with a Thorlabs mounted LED (M365L2) ($\lambda_{max} = 365$ nm, $I_0 = 78$ mW cm⁻²) during 30 minutes.

### 2.3. Nuclear magnetic resonance (NMR)

NMR spectra were recorded using a Bruker 400 MHz spectrometer at 25 °C with 32 scans and a time delay $D1$ of 5 s. ¹H NMR measurements were performed at frequencies of 400 MHz. Deuterated chloroform (CDCl₃) and deuterated dichloromethane (CD₂Cl₂) were used as solvent respectively for crude and purified polymer. All products were dissolved with concentrations between 10 and 20 mg mL⁻¹.

### 2.4. Size exclusion chromatography (SEC)

Measurements were performed on a bank of 4 Shodex columns (KF801, KF8025, KF804 and KF806) each 300 mm × 8 mm at 30 °C with THF eluent controlled by a Malvern pump (Viscotek, VE1122) at a flow rate of 1.0 mL min⁻¹. The SEC apparatus is equipped with a refractive index (RI) detector (Viscotek VE3580), a Wyatt Heleos II Multi Angle Laser Light Scattering detector (MALLS, 18 angles, $\lambda_0 = 664.4$ nm) and a UV-visible detector (Viscotek-Malvern VE3210). Toluene was used as a flow marker. Sample concentrations were limited to the range of 1.5 to 4.5 mg mL⁻¹ and were filtered through polytetrafluorethylene (PTFE) membranes with a pore size of 0.45 µm prior to injection.

### 2.5. Fourier-Transform infrared spectroscopy (FTIR)

Real-time FT-IR spectroscopy was used to investigate the kinetic of photopolymerization of PPEGA in quartz capillary tube (2 mm thickness). The experiment was performed on a Thermo Scientific Nicolet iS50 FT-IR Spectrometer. After taking a background reading of the capillary tube, aliquot of polymerization resin was pipetted into the capillary. Polymerization was started by irradiating with a Thorlabs mounted LED (M365L2) ($\lambda_{max} = 365$ nm, current = 659 mA, $I_0 = 2.1$ mW

cm⁻²). Simultaneously, IR spectra were recorded for 400 s interval to determine the integral, $Int(6103 – 6245 cm⁻¹)$ of the vinylic peak at 6103 – 6245 cm⁻¹. Each measurement included an average of 2 scans with a resolution of 8 cm⁻¹. Spectra were analyzed using Omsec software.

### 2.6. Small angle X-ray scattering (SAXS)

SAXS measurements were collected on the BL11 NCD-SWEET beamline at ALBA synchrotron facility using 0.99 Å X-ray wavelength, a 2.9 m sample-to-detector distance, and standard data corrections. Scattering curves were plotted as a function of scattering vector $q = 4\pi \sin\theta / \lambda$, with measurements performed at room temperature. The experimental data was fitted with SasView application (detailed in the supporting information).

### 2.7. Transmission electron microscopy (TEM)

Samples were fixed directly to the pin without any previous heat treatment and microtomed at –80 °C on Leica EM UC7 Ultramicrotome, using a Diatome 25° diamond knife. The sections were collected on 300 mesh Copper grid coated with lacey carbon. Subsequently, such prepared samples were submerged in liquid nitrogen and transferred onto Petri dish placed in the styrofoam box filled with dry ice and exposed to $OsO_4$ vapour for approximately 2 h. After staining, grids were cryo-transferred to the microscope. CryoTEM measurements were performed on Schottky field-emission cryo-electron microscope JEOL JEM-3200FSC at 300 kV accelerating voltage, while keeping the temperature of the examined specimen at –187 °C. The micrographs were acquired in bright field mode using zero energy loss omega type filter with a slit width of 30 eV via Gatan Ultrascan 4000 CCD camera.

### 2.8. Differential scanning calorimetry (DSC)

DSC was performed using the Q100 from TA instruments under a N₂ atmosphere. For each sample, the temperature regime was initially stabilized at –90 °C for 5 min, heating and cooling was done within –80 and 120 °C with a rate of 5 °C min⁻¹. The second heating ramp was used for the determination of the glass transition temperature ($T_g$).

### 2.9. Thermogravimetry analysis (TGA)

TGA measurements were performed on a TGA 2 STAR® instrument from Mettler Toledo with a heating rate of 10 °C min⁻¹ under nitrogen flux.

### 2.10. Rheology

Rheology measurements were performed on Anton Paar modular compact rheometer MCR-302. The torque ranged from 0.5 nN m to 230 mN m. Samples were sandwiched between two 8 mm diameter parallel plate geometry at a constant shear stress of 25 Pa and a frequency of 5 Hz. The measurements were performed in temperature-controlled environment, between 25 and 100 °C.

### 2.11. Electrochemical impedance spectroscopy (EIS)

Free-standing SPE films were previously dried in a vacuum oven for 24 h and stored in a glove box. The films (8 mm diameter, thickness, L, from 90 to 470 µm) were sandwiched between two blocking gold electrodes plates in CESH-e (Enhanced Controlled Environment Sample Holder) cells (diameter 6.35 mm). EIS measurement using Biologic Potentiostat (equipped with EC-Lab software package) at frequencies from 1 Hz to 1 MHz with an oscillating voltage of 50 mV. Subsequently, these cells were preconditioned in a temperature chamber for 2 h at 90 °C. The measurements were carried out by gradually decreasing the temperature in 20 °C steps from 90 °C to –10 °C with each temperature
being maintained for 2 h to attain a thermal equilibrium. The ionic conductivity, $\sigma$ (S cm$^{-1}$), was determined by $\sigma = L/R.A$ where R is the measured resistance, L is the film thickness and A is the area of the electrode.

### 3. Results and discussion

The resin was formulated with OEGA monomer, LiTFSI salt, TPO-L as photoinitiator, and linear or star comb-like P(S-co-BzMA)-b-POEGA BCP. The synthesis of both BCPs, named L-BCP and S-BCP for respectively the linear and the star comb-like BCP, was achieved by reversible addition fragmentation chain transfer (RAFT) polymerization and have been discussed previously in reference 24. The RAFT polymerization of BzMA was controlled by copolymerization of styrene units, particularly needed for the tetrafunctional chain transfer agent [24]. The main macromolecular characteristics of the BCPs are given in Table 1. For reference, the details of the synthesis and characterization of BCPs are provided in the supporting information.

In terms of nomenclature, the SPE formulated with the linear or star comb-like BCP are called L-SPE-X% or S-SPE-X%, respectively. X being the weight percentage of the global soft phase, i.e., POEGA from the matrix and from the BCP. Moreover, when LiTFSI salt is added in the formulation, it is included in the soft phase and the [EO]/[Li] molar ratio is affixed to the SPE's name. As an example, L-SPE$_{30}$-92% refers to an electrolyte composed of the linear comb-like, L-BCP, a [EO]/[Li] ratio of 30 and a total soft phase of 92 wt% (including POEGA and LiTFSI).

Table 2 provides a detailed overview of the prepared formulations. A special care was taken to achieve homogenous formulations, by separately solubilizing the BCPs and LiTFSI in OEGA monomer, then blending the resulting mixtures in the desired proportions. The hard phase content refers to the P(S-co-BzMA) block of the BCP in the formulation. It is worth noting that, since the P(S-co-BzMA) block was poorly soluble in the LiTFSI/OEGA monomer mixture, the BCP amount that can be incorporated was limited.

#### 3.1. Liquid formulation characterization

The impact of lithium salt and BCP architecture on the liquid formulation morphology was investigated by SAXS. Fig. 1 displays the scattering patterns collected on the liquid L-SPE-82% and S-SPE-85% salt-free formulations, where no specific scattering features are observed. This is indicating that the BCP macromolecules are not microphase separated, but well solubilized in OEGA monomer. It is worth noting that contrast matching between the hard and soft phases is unlikely since SAXS data collected on the corresponding salt-free BCPs (cf. Fig. S5) revealed a peak for the S-BCP, thus indicating a microphase separation. On the other hand, no microphase separation was observed for L-BCP counterpart. The addition of LiTFSI salt in the formulations, S-SPE$_{30}$-92% and L-SPE$_{30}$-92% ([EO]/[Li]=30), resulted in more specific scattering patterns (cf. Fig. 1). Those can be associated to spherical form factors. Consequently, the addition of salt induced a transition from well

solubilized BCPs in monomer to self-assemble BCPs into spherical micelles (cf. Scheme 1). Indeed, it is known that LiTFSI is immiscible with P(S-co-BzMA) and increases the block segregation [10,24]. In parallel, the ethylene glycol units of OEGA monomer and POEGA block of the BCP easily coordinate lithium-ion, and solvates the salt to ultimately form the ion-conducting phase. Therefore, the formulation phase segregation results in formation of micelles with hard P(S-co-BzMA) core decorated with POEGA chains, dispersed in a [OEGA/LiTFSI] mixture. That salt content impact is consistent with the behavior of both neat BCPs (cf. Fig. S5). Indeed, it is only after doping the BCPs with LiTFSI ([EO]/[Li]=15), that pronounced structural peaks are observed, i.e., microphase separations.

Regarding the formulation microstructures, a simple two-phases system can be considered with P(S-co-BzMA) domains within a matrix POEGA block, OEGA monomer and LiTFSI. Indeed, the POEGA block corona cannot be distinguished from the OEGA matrix due to a rather similar electron density. Therefore, we assume that the scattering patterns primarily reflect the shape and dimension of the P(S-co-BzMA) domains. To further analyze the SAXS data, a modified Beaucage model was selected to account for the micellar shape including a large-scale aggregation contribution [25]. The detailed model is presented in the supporting information. Fig. 1a and b display the fitted curves (red curves), and the parameters are given in Table S3. For L-SPE$_{30}$-92% the micellar core radius, r, is determined as 16.8 nm with a dispersity, PDI, of 0.2. Regarding S-SPE$_{30}$-92%, r is 17.5 nm with a PDI of 0.18. Here a Percus-Yevick (PY) hard sphere structure factor was implemented to account for the slight excluded volume interactions [26–28]. The effective radius, $R_{eff}$ for L-SPE$_{30}$-92% and S-SPE$_{30}$-92% was determined as 28 and 23.9 nm respectively. $R_{eff}$ classically describes the closest approach distance of the hard spheres. This later parameter gives insight on the incompressible POEGA corona dimension. Regrettably, when LiTFSI concentration was increased to [EO]/[Li]=15, the formulations L-SPE$_{15}$-92% and S-SPE$_{15}$-92% become too viscous to be introduced in the SAXS capillaries. Thus, no data on their self-assembly characteristics prior to photopolymerization was collected. Nevertheless, it can be hypothesized that increasing salt content up to [EO]/[Li]=15 maintains the phase segregation, which is actually well supported by the elevated formulation viscosity.

#### 3.2. Film photopolymerization

The general reaction scheme describing the photopolymerization process is presented in the Scheme 1. FTIR spectroscopy indicated a high degree of OEGA monomer conversion, superior to 90% (cf. Fig. S6 and Eq. S11). That value was obtained by irradiating the formulation containing OEGA monomer, the photoinitiator, and LiTFSI with an intensity, $I_{365}$ nm, of 2.1 mW cm$^{-2}$ during 6.5 min. In the case of 90% OEGA conversion, the theoretical degree of polymerization of POEGA homopolymer ($DP = [OEGA]/[TPOL]$) would be about 297, i.e., 143 kg mol$^{-1}$, although attempts to verify the degree of polymerization experimentally by SEC and NMR were not successful due to the SPE's insolubility in THF

<table><caption>**Table 1**

Macromolecular characteristics of P(S-co-BzMA)-b-POEGA BCP.</caption><thead><tr><th></th><th><i>W</i><sub>BzMA</sub><sup>a</sup></th><th><i>W</i><sub>S</sub><sup>b</sup></th><th><i>W</i><sub>P(S-co-BzMA)</sub><sup>c</sup></th><th><i>W</i><sub>POEGA</sub><sup>d</sup></th><th><i>M</i><sub>n,Theo</sub><sup>e</sup><br>(kg mol<sup>-1</sup>)</th><th><i>M</i><sub>n,MALLS</sub><sup>f</sup><br>(kg mol<sup>-1</sup>)</th><th><i>D</i></th></tr></thead><tbody><tr><td>L-BCP</td><td>0.56</td><td>0.44</td><td>0.71</td><td>0.29</td><td>44</td><td>41</td><td>1.10</td></tr><tr><td>S-BCP</td><td>0.60</td><td>0.40</td><td>0.73</td><td>0.27</td><td>281</td><td>414</td><td>1.31</td></tr></tbody></table>

a Weight fraction of BzMA in the P(S-co-BzMA) block (cf. Eq S2) determined from $F_{BzMA}$, molar fraction measured by 1H NMR (cf. Eq. S1, Fig. S1 and Fig. S2).

b Weight fraction of styrene, S, in the P(S-co-BzMA) block.

c Overall weight fraction of P(S-co-BzMA) in P(S-co-BzMA)-b-POEGA BCP.

d Overall weight fraction of POEGA in the P(S-co-BzMA)-b-POEGA BCPs (cf. Eq S7).

e $M_n,\text{theo}$ is calculated from Eq. S4.

f $M_n$ values obtained from the SEC analysis with RI/MALLS detectors (cf. Fig. S3 and Fig. S4). See determination of $dn/dc$ values for each copolymer in supporting information.
<table><caption>**Table 2**
Summary of resin's compositions in terms of the volume percent of conducting phase.</caption><thead><tr><th></th><th>BCP <sup>a</sup></th><th>[OEGA]<br>[TPOL]</th><th>[EO]<br>[Li]</th><th>Hard Phase <sup>b</sup><br>(wt%)</th><th>Soft Phase <sup>c</sup><br>wt%</th><th>Φ<sub>cond</sub><sup>d</sup></th></tr></thead><tbody><tr><td>L-SPE-82%</td><td>L-BCP</td><td>330</td><td>-</td><td>18</td><td>82</td><td>-</td></tr><tr><td>L-SPE<sub>30</sub>-92%</td><td>L-BCP</td><td>330</td><td>30</td><td>8</td><td>92</td><td>0.91</td></tr><tr><td>L-SPE<sub>15</sub>-92%</td><td>L-BCP</td><td>330</td><td>15</td><td>8</td><td>92</td><td>0.91</td></tr><tr><td>S-SPE-85%</td><td>S-BCP</td><td>330</td><td>-</td><td>15</td><td>85</td><td>-</td></tr><tr><td>S-SPE<sub>30</sub>-92%</td><td>S-BCP</td><td>330</td><td>30</td><td>8</td><td>92</td><td>0.91</td></tr><tr><td>S-SPE<sub>15</sub>-92%</td><td>S-BCP</td><td>330</td><td>15</td><td>8</td><td>92</td><td>0.91</td></tr></tbody></table>

<sup>a</sup> The prefix *L* and *S* represent the BCP architecture, linear and star-like respectively.

<sup>b</sup> P(S-*co*-BzMA) block constitutes the hard phase of the electrolyte.

<sup>c</sup> POEGA or POEGA + LiTFSI constitute the soft phase of the electrolyte.

<sup>d</sup> Φ<sub>cond</sub> is the volume fraction of the conducting phase in the initial resin according to Eq. S8. Density of OEGA, P(S-*co*-BzMA), LiTFSI are taken as 1.05 g cm<sup>-3</sup>, 1.12 g cm<sup>-3</sup>, and 1.33 g cm<sup>-3</sup> respectively.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** SAXS patterns collected before photopolymerization on a) L-SPE-82% and L-SPE<sub>30</sub>-92% formulations, and (b) S-SPE-85% and S-SPE<sub>30</sub>-92% formulations. The red curves correspond to the fits using a modified Beaucage model [24].</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Reaction schemes describing the salt-induced microphase separation and the elaboration of the SPE by photopolymerization (exemplify with L-BCP).</figcaption></figure>

solvent. Finally, to ensure an optimal and fast conversion, a significantly higher irradiation intensity of 78 mW cm<sup>-2</sup> and a longer exposure time of 30 min were employed to produce the SPEs in custom-made PTFE molds.

### 3.3. Film structural characterization

Prior to characterization, the samples were vacuum-dried at 60 °C for 24 h and the TEM sections were stained by OsO<sub>4</sub>. TEM images and SAXS

curves of the salt-free L-SPE-82% and S-SPE-85% are shown in Fig. 2. L-SPE-82% SAXS pattern does not exhibit specific scattering feature. On the other hand, after photopolymerization, S-SPE-85% displays a structural peak at *q* = 0.15 nm<sup>-1</sup>, corresponding to a Bragg distance of 41.9 nm. In fact, the peak confirms that the SAXS contrast between the hard and the soft phases is sufficient even in the absence of LiTFSI. According to the TEM images, L-SPE-82% shows a macroscopic phase separation without defined pattern where the minority BCP phase is
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** SAXS profiles collected before and after photopolymerization on a) L-SPE-82% and b) S-SPE-85%. TEM images showing morphologies after photopolymerization of c) L-SPE-82% and d) S-SPE-85% samples.</figcaption></figure>

probably expulsed from the POEGA matrix in the form of micrometric oval domains. Unexpectedly, this particular sample reveals an inverted contrast compared to all other TEM images (discussed later). Indeed, $OsO_4$ usually stains preferentially the POEGA majority phase, leaving the P(S-co-BzMA) phase clear on the pictures. S-SPE-85% sample features a chain-like interconnected microstructure formed by BCP, in which the white P(S-co-BzMA) domains are dispersed in the dark grey POEGA phase (cf. Fig. S7 for larger scale images).

As previously discussed, inserting LiTFSI salt in the formulations impacts drastically the microphase separations. By comparing pre- and post-polymerization SAXS patterns of L-SPE30 and S-SPE30, insights into the microstructural evolution can be obtained. At [EO]/[Li]=30, the SAXS patterns of post-cured L-SPE30-92% (Fig. 3a) and S-SPE30-92% (Fig. 3b) revealed an upturns in the low *q*-range, i.e., from 0.03 to 0.08 nm⁻¹, attributed to P(S-co-BzMA) core aggregation. Those aggregates are clearly visible on the corresponding TEM micrographs (Fig. 3c and d, cf. Fig. S8 for larger scale images). The morphological data suggests that the initially formed spheroidal micelles evolved in *i*) agglomerates without sharp interfaces for the linear BCP and *ii*) agglomerates composed of discrete spheroids for the star BCP. Interestingly, short worm-like agglomerates of combined micelles can also be observed on S-SPE30-92% micrograph (Fig. 3d).

To further analyze the SAXS data, we theorize that kinetics of the frozen P(S-co-BzMA) core rearrangement are faster than completion of the OEGA polymerization. Ultimately, the P(S-co-BzMA) aggregates become kinetically trapped in the cured matrix. To reflect that mechanism, the modified Beaucage model [25] was used for data fitting, similarly to the pre-cured formulations (red curves in Fig. 3a and b). It is

worth noting that various two-phase models were evaluated to fit the data, however the most consistent one, over the entire data set, was the modified Beaucage model. The fitted curves were preferentially adjusted on the low-*q* regions (i.e., the higher intensity values). Thus, explaining the slight discrepancy between the fits and the higher *q*-data, more discernible on the log-log plots. Nevertheless, the main features, located in the low and intermediate *q*-regions, are well fitted. In result, the hard P(S-co-BzMA) core radius was estimated to be 16.2 nm with a PDI of 0.17 for L-SPE30-92% and 17.7 nm with a PDI of 0.16 for S-SPE30-92% (Table S3). Those values are consistent with the ones obtained for the corresponding pre-cured formulations, thus supporting that there was no alteration of the micellar structure itself. Nonetheless, the scattering patterns before and after curing differ significantly at low *q* due to the aggregation process occurring upon photopolymerization. Indeed, additional information regarding the fractal dimension of the system can be provided through the power law exponent, *D* (cf. Eq. S12 and Table S3), although determined in the rather limited low *q*-range. The *D* values range between 1.8 and 3, thus indicates presence of the mass fractals [25]. Also, the dimensions and therefore compactness of the mass fractals increases upon the photopolymerization for both L-SPE30-92% and S-SPE30-92%, supporting the hypothesis of micelle agglomeration.

The TEM images collected on the SPEs with an increased content of LiTFSI ([EO]/[Li] = 15) are provided in Fig. 4c and d. Additional, micrographs acquired at lower magnification are available on Fig. S9. L-SPE15-92% and S-SPE15-92% exhibit mainly worm-like agglomerates with a minor fraction of spheres or spheroids. A similar trend, from globular to worm-like micelles prompt by salt concentration increase,
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** SAXS profiles collected before and after photopolymerization on a) L-SPE₃₀-92% and b) S-SPE₃₀-92%. The red curves correspond to the fits using a modified Beaucage model [24]. TEM micrographs showing morphology after photopolymerization of c) L-SPE₃₀-92% and d) S-SPE₃₀-92% samples.</figcaption></figure>

was already reported for surfactant micelles in salt doped solvent [29, 30]. Here the BCP architecture influences significantly the dimension of the resulting self-assembly, with the worm-like structures formed by the L-BCP being longer, 200 to 250 nm, compared to those composed of the S-BCP, 70 to 100 nm. On the other hand, the diameters remain relatively close, with 16 and 17 nm for L-SPE₁₅-92% and S-SPE₁₅-92% respectively. Aggregation can be observed as well, although less severe, for L-SPE₁₅-92% sample. The corresponding SAXS data are shown in Fig. 4a and b. Slight upturns in the lower *q* region are present for both samples, thus supporting the nano-object aggregation. SAXS pattern of S-SPE₁₅-92% (Fig. 4b) shows a well pronounced structural peak at *q* = 0.13 nm⁻¹, corresponding to a Bragg distance of 48.3 nm between the scattering objects. This is the signature of large-scale domains exhibiting a significant concentration of scattering objects. On the other hand, the scattering pattern collected on sample L-SPE₁₅-92% (Fig. 4a) exhibits a rather weak structure factor, implying well-dispersed and distant scattering objects in the whole sample. For the consistency of the analysis, the data was fitted with the modified Beaucage model, incorporating a PY structure factor to account for the observed inter-particle interactions (red curves). In result, the core radius was estimated to be 16 nm with a PDI of 0.15 for L-SPE₁₅-92% and 16.6 nm with a PDI of 0.17 for S-SPE₁₅-92% (Table S3). It appears that increasing salt content rendered the hard P(S-co-BzMA) core slightly more compact and reduces the *D* power law exponent (Table S3). Indeed, the SPEs with [EO]/[Li]=30 were characterized by *D* values, and therefore mass-fractal dimension, of 3; whereas the SPEs with [EO]/[Li]=15 exhibit *D* values of 1.8. Thus, indicates different nature and lower compactness of the observed mass fractals prompted by the salt doping.

### 3.4. SPE characterizations

To allow comparison with the SPEs, POEGA samples with and without LiTFSI were prepared by initiating the photopolymerization of OEGA monomer under the same conditions as used for fabricating SPEs. The resulting films are POEGA-100% and POEGA₁₅-100%. A third film, with 1 mol% (relative to OEGA) of oligo ethylene glycol diacrylate (OEGDA) crosslinker, was prepared; denoted as POEGA₁₅-100% (crosslinked). DSC measurements collected on the pristine photopolymerized POEGA film are displayed in Fig. S10. They evidence clearly that crystallinity is lost with salt doping ([EO]/[Li]=15). The Tg of POEGA-100% is not clearly apparent; on the other hand, the Tg of POEGA₁₅-100% is evaluated at -50.3 °C (Table 3). Regarding the samples prepared from the S-BCP, the ones without salt exhibits a significant crystallinity, which also vanishes with salt doping (cf. Fig. S11). Moreover, the Tg increases slightly from -55 to -47.6 °C upon salt addition, for S-SPE₃₀-92% and S-SPE₁₅-92% respectively. In the SPEs, those specific Tg are associated to the POEGA matrix, exhibiting values that are comparable to the POEGA₁₅-100% one. A similar trend is observed for the SPE prepared from the L-BCP (cf. Fig. S12), with Tg of -56.8 and -40.7 °C, for L-SPE₃₀-92% and L-SPE₁₅-92% respectively. Thermogravimetric analysis was performed on SPEs with [EO]/[Li]=15 to assess their chemical stability (Fig. 5b). There was a slight loss in weight (about 4 wt%) after 100 °C. This is probably due to the moisture captured during sample manipulation. Overall, the SPEs exhibited thermal stabilities up to 300 °C thus demonstrating the capacity to withstand classical operating temperature conditions of lithium batteries, i.e., -20 to 60 °C [31].
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** SAXS profiles collected after photopolymerization on a) L-SPE<sub>15</sub>–92% and b) S-SPE<sub>15</sub>–92%. The red curves correspond to the fits using a modified Beaucage model [24]. TEM micrographs showing morphology after photopolymerization of c) L-SPE<sub>15</sub>–92% and d) S-SPE<sub>15</sub>–92% samples.</figcaption></figure>

<table><caption>**Table 3**
Main characteristics of the SPEs.</caption><thead><tr><th>Code</th><th>[EO]<br/>[Li<sup>+</sup>]</th><th>Soft Phase<br/>(wt%)</th><th>Φ<sub>cond</sub><sup>a</sup></th><th>G<sup>b</sup><br/>(kPa)</th><th>σ<sup>b</sup><br/>(S cm<sup>-1</sup>)</th><th>T<sub>g</sub><sup>c</sup><br/>(°C)</th><th>T<sub>0</sub><sup>d</sup><br/>(°C)</th></tr></thead><tbody><tr><td>L-SPE<sub>15</sub>–92%</td><td>15</td><td>92</td><td>0.91</td><td>52.9</td><td>1.30 × 10<sup>-4</sup></td><td>-40.7</td><td>-105</td></tr><tr><td>S-SPE<sub>15</sub>–92%</td><td>15</td><td>92</td><td>0.91</td><td>34.7</td><td>5.20 × 10<sup>-5</sup></td><td>-47.6</td><td>-129</td></tr><tr><td>POEGA<sub>15</sub>–100%</td><td>15</td><td>100</td><td>1</td><td>7.5</td><td>1.66 × 10<sup>-4</sup></td><td>-50.3</td><td>-97</td></tr><tr><td>POEGA<sub>15</sub>–100% (crosslinked)</td><td>15</td><td>100</td><td>1</td><td>31.7</td><td>n/a</td><td>n/a</td><td>n/a</td></tr></tbody></table>

<sup>a</sup> Volume fraction of the soft conducting phase, i.e., POEGA and LiTFSI, in the SPE.

<sup>b</sup> Measured at 30 °C.

<sup>c</sup> The T<sub>g</sub> values are extracted from Fig. S10, Fig. S11 and Fig. S12.

<sup>d</sup> The T<sub>0</sub> values are extracted from the VTF model (cf. the supporting information, Table S4).

To investigate the influence of morphology on the mechanical strength, rheology was performed on SPEs L-SPE<sub>15</sub>–92% and S-SPE<sub>15</sub>–92%. The resulting curves are displayed in Fig. 5a. In Table 3 are presented the storage moduli (G') of L-SPE<sub>15</sub>–92%, S-SPE<sub>15</sub>–92%, POEGA<sub>15</sub>–100%, and POEGA<sub>15</sub>–100% (crosslinked) as 52.9, 34.7, 7.5, and 31.7 kPa, respectively at 30 °C (Fig. 5a). These results demonstrate that embedding the linear BCP in the POEGA matrix increases the storage moduli by a factor of 7. In the case of the star BCP, the moduli increase by approximately 5-fold. Similarly, a comparable enhancement in the storage moduli (approximately 4-fold) is achieved by incorporating 1 mol% of OEGDA crosslinker, into POEGA<sub>15</sub>. These results suggest that BCP-derived morphologies in the SPEs act as physical-crosslinking points that strengthen the POEGA matrix. Additionally, the elongated domains of P(S-co-BzMA) observed in L-SPE<sub>15</sub>–92% sample improve further the mechanical strength. Another interesting

observation is the gradual decrease in modulus with temperature for L-SPE<sub>15</sub>–92% and S-SPE<sub>15</sub>–92%, indicating a transition of the SPE from a hard to a viscous material. Specifically, L-SPE<sub>15</sub>–92% exhibits a significant decline after reaching ca. 70 °C. This phenomenon is correlated with the T<sub>g</sub> of the P(S-co-BzMA) phase, which was previously reported to be approximately 76 °C [24]. Moreover, embedding BCP in POEGA instead of using a chemical cross-linker can be highlighted as a significant step forward in battery recycling.

The ionic conductivity of vacuum-dried samples was assessed by EIS. Fig. 5c illustrates a classical decline of the conductivities as the temperature decreases. This conductivity decrease can be attributed to the reduced segmental mobility of polymer chains at lower temperatures, which limits the ion transport across the SPEs [32,33]. The Vogel–Tammann–Fulcher equation (VTF, Eq. S17) [34] depicts accurately this temperature dependence as shown on Fig. 5c. The resulting
<figure><img src="image_8.png" /><figcaption>Fig. 5. a) Storage moduli (G') versus temperature, b) Thermogravimetric analysis and c) Temperature-dependent ionic conductivity of L-SPE15-92% (♦), S-SPE15-92% (□), POEGA15-100% (○) and POEGA15-100% (crosslinked) (○) SPEs. Short dash lines represent the VTF model fits of the conductivity data (cf. Eq. S17 and Table S4).</figcaption></figure>

fitting parameters are presented in Table S4. At a temperature of 30 °C, the ionic conductivity values for POEGA15-100%, L-SPE15-92%, and S-SPE15-92% are 0.17, 0.13, and 0.05 mS cm⁻¹, respectively (Table 3). L-SPE15-92% exhibits improved conductivities beyond 50 °C, comparable to POEGA15-100% (as seen Fig. 5c). This observation suggests that the open morphology with well-dispersed non-conductive nano-domains does not impede the ionic motion across the film. In contrast, S-SPE15-92% displays reduced conductivity, which may be attributed to the presence of macro-domains containing concentrated non-conductive nano-domains, as observed by TEM (Fig. 4c and d).

## 4. Conclusion

Nano-structured SPEs were fabricated by the salt induced micro-phase separation of pre-synthesized BCP, subsequently fixed by the photopolymerization. Two mechanisms underpinned the formation of such nanostructures. (i) The *ab initio* self-assembly of the BCPs into hard spheres upon the addition of lithium salt in the initial liquid resin. (ii) The kinetic arrest of the already formed hard spheres during the photopolymerization of OEGA monomer. This may include a subtle interplay between the photopolymerization of OEGA monomer and interactions of the self-assembled hard spheres, which depends strongly on the salt concentration. It has been demonstrated that the micro-structure ultimately influences the strength and conductivity of the SPEs. Indeed, the presence of kinetically arrested nanostructures in the POEGA matrix serves as physical crosslinkers, leading to a remarkable 7-fold enhancement in storage moduli in the case of L-SPE15-92% and a 5-fold improvement in the case of S-SPE15-92%, compared to a neat POEGA matrix. Also, the conductivity of L-SPE15-92% was approximately twice that of S-SPE15-92%, underscoring the substantial influence of BCP architecture. At this stage, further electrochemical characterizations and battery cycling tests are considered to better confirm the SPE's performances. The innovative nanostructured SPEs exhibit properties comparable to classical crosslinked POEGA, with the remarkable added benefit of recyclability. Additionally, this approach seamlessly integrates with battery manufacturing processes, thanks to the capability for *in situ* photopolymerization directly on the electrodes.

## Funding sources

Marie Skłodowska-Curie grant agreement No 860403.

## Notes

The authors declare no competing financial interest.

## CRediT authorship contribution statement

Kingsley Ikenna Aniagbaoso: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Monika Król: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Janne Ruokolainen: Writing – review & editing, Validation, Supervision. Antoine Bousquet: Writing – review & editing, Validation, Supervision. Maud Save: Writing – review & editing, Validation, Supervision. Laurent Rubatat: Writing – review & editing, Writing – original draft, Validation, Supervision, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
K. Aniagbaoso reports financial support was provided by European Commission. M. Krol reports financial support was provided by European Commission. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

The authors appreciate the provision of facilities and technical support by Aalto University at OtaNano-Nanomicroscopy Center (Aalto-NMC). The SAXS experiment was performed at BL11 NCD-SWEET beamline at ALBA Synchrotron with the collaboration of M. Malfois and ALBA staff. This project was carried out within the POLYSTORAGE European training network and have received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 860403.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.apmt.2025.102946.
# Data availability

Data will be made available on request.

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