# Single-Ion Conducting Polymer Electrolyte with Excellent Interfacial Stability toward the Lithium Metal

Houang Phong Khanh Ngo, Yunfan Shao, Tony Bertaux, Thi Khanh Ly Nguyen, Justine Solier, Emilie Planes, Patrick Judeinstein, Fannie Alloin, Jean-Yves Sanchez, Cristina Iojoiu

► To cite this version:

Houang Phong Khanh Ngo, Yunfan Shao, Tony Bertaux, Thi Khanh Ly Nguyen, Justine Solier, et al. Single-Ion Conducting Polymer Electrolyte with Excellent Interfacial Stability toward the Lithium Metal. ACS Applied Energy Materials, 2025, 8 (5), pp.2819-2827. 10.1021/acsaem.4c02859. hal-05345120

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Distributed under a Creative Commons Attribution 4.0 International License
Single-ion Conducting Polymer Electrolyte with
Excellent Interfacial Stability Towards the Lithium
Metal

Houang Phong Khanh Ngo<fnref n="1" />,<fnref n="2" />, Yunfan Shao<fnref n="1" />,<fnref n="2" />, Tony Bertaux<fnref n="1" />, Thi Khanh Ly Nguyen<fnref n="1" />, Justine Solier<fnref n="1" />, Emilie Planes<fnref n="1" />, Patrick Judeinstein<fnref n="2" />,<fnref n="3" />, Fannie Alloin<fnref n="1" />,<fnref n="4" />, Jean-Yves Sanchez<fnref n="1" /> and Cristina

Iojoiu<fnref n="1" />, <fnref n="4" />*

<footnote n="1">¹ Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France</footnote>

<footnote n="2">² Université Paris-Saclay, CNRS, CEA, LLB, 91191 Gif-sur-Yvette, France</footnote>

<footnote n="3">³ Université Paris-Saclay, CNRS, LPS, 91405 Orsay, France</footnote>

<footnote n="4">⁴ Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS, FR3459, 80039 Amiens Cedex, France</footnote>
**Abstract**

To improve the safety and energy density of lithium metal batteries, polymer electrolytes address the challenge by replacing flammable liquid electrolytes. We herein design an amorphous solid-state single-ion conducting polymer electrolyte (SIPE) with a strict alternating sequence of perfluorosulfonate lithium salt and polyethylene glycol (PEG) with a very homogeneous dispersion of lithium salt in the electrolyte. Amorphous SIPE tailored with a moderate crosslink degree and reinforced with nanocrystals cellulose (NCC) exhibits a high Li⁺ conductivity exceeding 10⁻⁵ S cm⁻¹ at 60 °C with a lithium transference number close to unity and excellent stability with Li metal. A long-term lithium stripping/plating was achieved in symmetric Li|SIPE|Li cells for over 80 days without dendrite growth. The Li|SIPE|LiFePO₄ cells provide an excellent rate capability performance with a high specific capacity of 163 and 132 mAh g⁻¹ at C/20 and C respectively. The long-term cycling tests exhibited good performance and an average coulombic efficiency of 99.9% for over 160 cycles. These excellent results confirmed the potential of the rational-designed single-ion conducting polymers as a safe and efficient solid-state electrolyte for next-generation high-performance energy storage.

**Key words:** polymer; electrolyte; single-ion; ionomer; lithium metal; battery
# 1.Introduction

With a never-satisfied demand for higher energy density, Li-metal batteries (LMBs) are considered a promising candidate to substitute Li-ion batteries with graphite negative electrodes.¹⁻³ Lithium metal, with a high specific capacity of 3860 mAh g⁻¹ and low redox potential of -3.04 V vs. SHE, is deemed as a perfect negative electrode for high specific energy batteries.⁴ However, the safety of LMB is facing challenges due to the lithium dendrite formation, which originates from concentration gradient polarization in electrolytes, unstable solid electrolyte interphase (SEI), inhomogeneous charge distribution on the electrode surface, etc.⁵⁻⁷ The conventional polymer electrolytes based on a blend of lithium salts and a host polymer, are much safer than usual liquid electrolytes.⁸,⁹ The reference host polymer is the commercially available poly(oxyethylene) (PEO).¹⁰,¹¹ The high Lewis basicity of the ether repeating unit, i.e. the high donor number (DN) of PEO, allows dissolving and solvating lithium salts with low anion basicity such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium triflate (LiTf), etc.¹²⁻¹⁴. In a typical PEO-based electrolyte, the Li⁺ conduction mechanism occurs mostly in the amorphous phase and depends strongly on the chain segmental motion.¹⁵ The DN of PEO in combination with its low acceptor number (AN) leads to a low cationic transference number ranging from 0.1 to 0.2 for the blend with LiTFSI.¹⁶ The low Tₚ⁺ strongly contributes to dendrite formation and growth, especially when high current density is applied. At this point, full anion depletion occurs at the lithium/electrolyte interface and leads to significant cell polarization.¹⁷ Single-ion conducting polymer electrolytes (SIPEs) would theoretically offer a solution to this issue, by the virtue of charge neutrality, preventing the establishment of concentration gradients¹⁸ thereby overcoming the limitations of the conventional blend-type SPEs.¹⁹⁻²² In a pioneering work, Bannister et al.²³ blended polyanions based on perfluorocarboxylate anions with PEO. Due to the limited basicity of these anions and poor dissociation of Li cation, very low conductivities were obtained. Then,
perfluorosulfonate anions, with lower basicity than perfluorocarboxylate were turned into SIPE by anionic ring opening copolymerization of an oxirane-based ionic monomer with ethylene oxide (EO).²⁴ Due to reactivity ratios that favor the EO homopolymerization, the content of ionic monomer incorporation is too low to provide sufficient conductivities. Afterward, other PEO-based SIPEs were developed and studied.¹¹,²⁵⁻²⁷

It has been already mentioned that the conduction mechanism in such systems heavily relied on the polymer segmental motion²⁸, therefore one significant challenge is connected with the antagonist relationship between ionic conductivity and mechanical strength. Even though the conductivity has been greatly improved for some SIPE, especially for brushed and stars copolymers, the mechanical properties are very poor and need to be integrated in a rigid polymer matrix such as poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP).²⁹,³⁰ SIPE based on di-/tri- block copolymers have been reported³¹,³² such as P(STFSI-Li)-PEO-b-P(STFSI-Li), (poly(styrene trifluoromethane sulfonylimide of lithium) P(STFSI-Li), with a $T_{Li}^+ > 0.85$ and ionic conductivity of $1.3 \times 10^{-5}$ S cm⁻¹ at 60 °C with nanophase separated morphology with lamellar structure with PEO-rich and glassy TFSI-rich domains. However, it is important to note in these materials the anion is anchored on the rigid block and fairly good conductivity is achieved only when the PEO phase is melted. Due to the affinity of PEO with the lithium salt anchored to the rigid block, the phase separated microstructure is lost and unfortunately this also results in a significant reduction of the mechanical strength. Other solvating polymers, such as PVDF, polycarbonates and polysiloxane³³,³⁴,²⁸,²⁹ have been explored as well, however in solvent-free configuration nearly only the PEO-based SIPE chains could provide decent conductivities.

Another critical issue concerning the Li metal anode and dendrite formation is related to the microscopic ionic distribution near the electrode. This distribution significantly influences the morphology of the deposited material.³⁵ A uniform distribution of Li salts can prevent local $Li^+$
concentration gradients and charge localization on the Li electrode surface, resulting in consistent electrochemical Li deposition. Previous studies on SIPEs obtained through co-crosslinking, ring-opening copolymerization, chemical modification and, obviously, block copolymerization have shown that the ionic functions are not evenly distributed along the polymer chains.<sup>31,36,37</sup>

Herein, we designed a novel SIPE utilizing polyethylene glycol as backbone and lithium perfluorosulfonate as side chains, in a strict alternating sequence *via* a polycondensation reaction to reach ionic homogeneity. By introducing a low amount of cross-linkable double bond and nanocrystal cellulose (NCC) filler, solid-state 3D network structures were achieved with high chain mobility and good mechanical properties. The resulting solvent-free SIPE exhibited excellent ionic conductivity of 3.5×10<sup>-5</sup> S cm<sup>-1</sup>, with a nearly perfect T<sub>Li</sub><sup>+</sup> of 0.97 at 80 °C. Long-term lithium plating/stripping experiments with this SIPE indicated stable cycling for over 80 days without any short-circuit, confirming a stable, homogeneous and dendrite-free lithium deposition, which demonstrated the strong potential of these SIPE materials in realizing safe and stable Li-metal batteries.

## 2. Results and discussion

### 2.1 Electrolyte preparation

A novel linear SIPE, named I<sub>x</sub>p-SO<sub>3</sub>-db, was designed with a Li solvating polyether as backbone (PEG) and Li-perfluorosulfonate and vinyl double bond as side chains. The synthesis involved a two-step polycondensation reaction (Scheme 1a). This method enables the synthesis of SIPEs with highly controlled structures, notably ensuring uniform dispersion of the perfluorosulfonate anion along the Li<sup>+</sup> solvating polymer backbone. The distance between two lithium perfluorosulfonate anions is fixed by the molar mass of the PEG used for the synthesis. Three precursors I<sub>x</sub>p-SO<sub>3</sub>, obtained from PEG with molecular weights of 400, 600, and 1000 g mol<sup>-1</sup> (corresponding to EO/Li
ratios of 9, 14, and 23, respectively) were synthesized with an overall molecular weight of 5 kg mol⁻¹. To achieve well-defined oligomers with hydroxyl-terminated chains and controlled molecular weights (Mn), a calculated stoichiometric imbalance between two monomers was employed by using the Carothers equation.³⁸

The precursor Iₓp-SO₃ was then reacted with 3-chloro-2-chloromethyl-1-propene, introducing isobutenyl moieties to obtain unsaturated ionic polycondensate Iₓp-SO₃-db. The chemical structure and purity of Iₓp-SO₃-db were confirmed by ¹H and ¹⁹F NMR (Figure S1) and Raman (Figure S2). The average molecular weights (Mw) of Iₓp-SO₃-db, measured by size exclusion chromatography coupled multi-angle light laser scattering (SEC-MALLS), ranged from 24 to 35 kg mol⁻¹ with polydispersity index (PDI) ranging between 1.4 to 1.7 (Table S1). Cation exchange of Iₓp-SO₃-db-Na into Iₓp-SO₃-db-Li was achieved with a yield exceeding 96%, measured by atomic adsorption spectroscopy (Scheme 1b, Table S1). The Raman spectra of I₁₀₀₀p-SO₃-db and I₁₀₀₀p-SO₃-cr membranes are presented in the Figure S2, showing a decrease in the intensity of the peaks corresponding to vinyl double bonds. However, the I₁₀₀₀p-SO₃-cr exhibited elastic properties, making them difficult to manipulate in a glove box.

To further improve the mechanical strength of the crosslinked membranes, nanocrystalline cellulose (NCC) was added to Iₓp-SO₃-db. In previous works,³⁹⁻⁴¹ we demonstrated that the addition of 6 wt.% of NCC, with a diameter of 2 -20 nm and L/d ratios of 10-100, significantly enhances mechanical strength with negligible effect on conductivity. Solubility tests were conducted on the membranes by immersing them in acetonitrile to evaluate their soluble part. The membranes remained completely insoluble, indicating that the UV irradiation conditions used were effective and well-suited for crosslinking. Crosslinked membranes, obtained by film casting and UV irradiation, resulted in soft, self-standing membranes (Figure S3).

## 2.2 Thermal analysis
All the Iₓp-SO₃-db and Iₓp-SO₃-cr samples exhibited excellent thermal stability, with decomposition temperatures exceeding 330°C under nitrogen (Figure 1a). The DSC thermograms (Figure 1b) show the absence of melting peaks, confirming that all Iₓp-SO₃-db and Iₓp-SO₃-cr are fully amorphous. Crosslinking the membrane led to a slight increase in the glass transition temperature (T_g), with a 10 °C rise compared to the I₄₀₀p-SO₃ ionomer. However, a much more significant increase in T_g, over 40 °C, was observed when the EO/Li ratio decreased from 23 to 9 (Figure S4). Specifically, the T_g values for I₁₀₀₀p-SO₃-cr, I₆₀₀p-SO₃-cr, and I₄₀₀p-SO₃-cr are -40, -22, and 0 °C, respectively. It is well known that adding salt to PEO induces a T_g increase due to interactions between the PEO chains and Li cation. However, for Iₓp-SO₃-cr, the T_g values are much higher than those typically reported for PEO-lithium salts blends.⁴² For example, in an amorphous blend of cross-linked PEO and an aromatic salt with a structure similar to ionic moiety of Iₓp-SO₃-cr (perfluorosulfonate grafted to the aromatic ring),⁴³ the T_g increased by only 10°C when the EO/Li varied from 28 to 10. This significant increase in T_g for Iₓp-SO₃-cr can be attributed to two main factors: i) the presence of rigid aromatic rings in the polymer backbone, which significantly restricts chain mobility of structural units directly bonded and ii) the anchoring of the anion to the polymer backbone in the Iₓp-SO₃-cr prevents lithium salt aggregation, thereby enhancing the interaction between Li cations and PEG chains, resulting in a higher T_g.

## 2.3 Morphological study

The dispersion of nanofillers was assessed using SEM, on cross-sectional images of cryo-fractured I₁₀₀₀p-SO₃-cr membranes containing 6 wt.% NCC (Figure S5). As seen on SEM images, a good contrast was achieved with the secondary electron detector. A relatively dark, co-continuous phase and light nodules can be seen. Given the material observed, it appears that the first phase can identified as the polymer phase and the other as the NCCs. It seems that the latter present a homogeneous dispersion how shown in Figure S5c. Moreover, the observed particle sizes ranged
from 4 to 14 nm, consistent with the average size of the NCCs used. Notably, no evidence of
nanofiller agglomeration was detected. The uniform dispersion of NCC within this ionomeric
membrane was significantly better compared to nanocomposites based on PEO and lithium
salts.⁴⁴,⁴⁵ We ascribed this improvement of the dispersion, (free from aggregates) to the amorphous
nature of the ionomer, in contrast to the semi-crystalline structure of PEO electrolytes. In semi-
crystalline membranes, NCCs can be expelled into inter-spherulitic regions during crystallization,
leading to the formation of aggregates. This phenomenon was not observed in the SIPE membrane,
which likely accounts for the better nanofiller dispersion.

**2.4 Ionic conductivity**

The conductivity of I₁₀₀₀p-SO₃-db, I₁₀₀₀p-SO₃-cr without and with NCC are presented in the Figure S6. It can be observed that the crosslinking of I₁₀₀₀p-SO₃-db induces a decrease in conductivity with a factor less than 2.5. This decrease could be explained by the T_g increase as well as the lower contribution of anion to the overall conductivity, which is shown in anion self-diffusion coefficient measured by PFG-NMR (Table S2). In the crosslinked samples, both the anion (D_F) and cation (D_Li) diffusion coefficients were decreased, due to the increase in viscosity. However, this decrease is much higher for D_F with a factor of 29, while there was only 2 for D_Li. The calculated T_Li⁺ were 0.74 and 0.97 for I₁₀₀₀p-SO₃-db and I₁₀₀₀p-SO₃-cr respectively. The addition of NCC decreased slightly the conductivity values. This decrease is mostly related to the adding of non-conducting phase, because the T_g of I₁₀₀₀p-SO₃-cr is not affected by NCC, and remained roughly the same, i.e. -40°C. This hypothesis is also confirmed by the rather same diffusion coefficients obtained for the samples with and without NCC (Table S2). However, the membranes obtained by crosslinking of I₁₀₀₀p-SO₃-db with NCC show good mechanical resistance, therefore for the following parts only the membranes with NCC were studied. The ionic conductivity of different Iₓp-SO₃-cr with NCC is presented in Figure 1c. A significant decrease in
conductivity is observed with the increase of EO/Li. The conductivity at 90 °C was 3.5×10⁻⁵, 1.9×10⁻⁵ and 4.5×10⁻⁶ S cm⁻¹ for I₁₀₀₀p-SO₃-cr, I₆₀₀p-SO₃-cr and I₄₀₀p-SO₃-cr, respectively. The higher conductivity obtained with I₁₀₀₀p-SO₃-cr can be linked to its lower T_g value. All the curves exhibited a VTF-type behavior, suggesting the conductivity of the Iₓp-SO₃-cr is strongly linked to polymer segmental motion, i.e., the chain flexibility.⁴⁶ To minimize the impact of chain mobility on the conductivity, and highlight the ionic concentration effect, the conductivity as a function of 1000/(T-T₀) is represented in Figure 1d (T₀ is referred to as the Vogel temperature, indicating an ideal glass transition temperature, typical around 50 °C below T_g)⁴⁷ and it can be observed that at high temperature the conductivities of the different ionomers are on a similar master curve, suggesting similar activation energies between the three different SIPEs, despite different Li concentration. From these results, it can be concluded that the mobility of the chains is the main parameter that affects the conductivity. The gap observed at low temperatures could be due to the empirical determination of T₀. The linear evolution of the ionic conductivity vs 1000/(T-T₀) is in line with the VTF behavior.

Based on these conductivity results, the following parts of the electrochemical study will be
focused only on I₁₀₀₀ p-SO₃-cr with 6% of NCC.

**2.5 Lithium transference number**

Both electrochemical methods and PFG-NMR were used to determine the Li transference
number. A cationic transference number near unity was obtained by both Bruce-Vincent and
Sorensen methods,⁴⁸,⁴⁹ (Figure 1e and Figure S7). Furthermore, the self-diffusion coefficients of
¹⁹F and ⁷Li nuclei for I₁₀₀₀p-SO₃-cr, measured by PFG-NMR technique permit to determine a T₉ᵢ⁺
equal to 0.98 (Table S2). To sum up, the cross-linked ionomer I₁₀₀₀p-SO₃-cr is a true SIPE as it
exhibits nearly unity cationic transference number determined by three different techniques. This
result is fully consistent with the behavior of the structure of a crosslinked anionic network.
## 2.6 Electrochemical stability

Electrochemical stability of I₁₀₀₀p-SO₃-cr SIPE was examined by linear sweep (LSV) and cyclic voltammetry (CV) tests, which were conducted on a Li|I₁₀₀₀p-SO₃-cr|stainless-steel (SS) cell at 80 °C. The LSV experiment is presented in Figure S8 and shows a strong oxidation starting at ~5.2 V vs. Li⁺/Li. However, a small oxidative current could be observed at ~4 V vs. Li⁺/Li in accordance with the beginning of the ether functions oxidation.⁵⁰ A typical CV trace is shown in Figure 1f. With the anodic sweep up to 4.0 V vs. Li⁺/Li, a flat line without any oxidation peak was observed, suggesting good stability of the SIPE. At low potential, lithium plating and stripping on the SS electrode were observed, with a coulombic efficiency equal to 40%. Minor reduction peaks were also noticed at ~1.5 V vs. Li⁺/Li, which could relate to the oxides on the SS surface.⁵¹

The limiting current density of the I₁₀₀₀p-SO₃-cr membrane was examined by a current scan test with a Li|I₁₀₀₀p-SO₃-cr|Li cell. The potential of the cell is plotted as a function of current density in Figure S9a. With increasing current density, the potential increased following a linear relationship at the beginning and then sharply increased indicating the Li⁺ depletion close to the electrode. The derivative curve (Figure S9b) enables us to determine a liming current density of the SIPE equals to ~1.2 mA cm⁻². This is one of the best values that reported for solvent-free SIPEs.

## 2.7 Lithium stripping/plating

Lithium stripping/plating experiments were carried out with symmetric Li|I₁₀₀₀p-SO₃-cr|Li cells. Step-increased current densities were applied from 10, 80, 120, 150, to 200 µA cm⁻² for four hours per stripping/plating process across the cell at 80 °C, which corresponds to capacities of 0.04, 0.32, 0.48, 0.60, and 0.80 mAh cm⁻², respectively. The potential profiles of the Li stripping/plating are shown in Figure 2a. Long-term cycling over 80 days with stable overpotentials indicates a homogenous lithium deposition without dendrite growth. The overpotential of the cells remained
constant over 20 days with a current density of 120 µA cm⁻², suggesting excellent stability of I₁₀₀₀p-SO₃-cr with lithium metal. With higher current density (≥ 150 µA cm⁻²), a mild increase of overpotential was observed at the beginning of the step and then stabilized, which might be ascribed to the larger amount of stripped/plated lithium (for the 200 µA cm⁻², c.a. 4 µm of lithium was dissolved/deposited each half cycle) which induced evolutions on both the electrode surface morphology and SEI properties (rigidity and thickness).⁵² However, the potential profiles showed a rectangular shape with a flat plateau even at an elevated current density of 200 µA cm⁻² (Figure 2b), which is a distinctive sign of single-ion conducting, i.e., the lack of concentration gradient and polarization.⁵³ In addition, the linear evolution of the potential difference between Li stripping and plating vs. the current density (Figure 2c) and the EIS spectra recorded during the cycling (Figure 2d) presents a mild evolution of both interfacial and bulk resistances (Figure 2e). This is due to the SEI formation and morphological/chemical evolution on surface of Li electrodes during the long cycling, confirmed the stability of the SIPE/Li interface. Additionally, ex-situ SEM images were collected from Li|I₁₀₀₀p-SO₃-cr|Li cells after 200 h of stripping/plating at 100 µA cm⁻² (Figure S10), in which a clear boundary between the SIPE membrane and lithium could be observed. Neither dendritic nor mossy lithium could be observed in the SEM images, indicating that a dense and dendrite-free lithium plating was achieved. These excellent results indicate the I₁₀₀₀p-SO₃-cr SIPE shows remarkable stability against lithium metal.

## 2.8 Evaluation in LMB cells

The SIPE was finally evaluated in solid-state LMB cells with LFP as positive electrode. The linear I₁₀₀₀p-SO₃-db was added in the composite electrode to form ion pathways inside the electrode, which is essential for the electrode performance. The electrode was calendared to reduce its porosity and tortuosity before cell assembly. The images of electrodes (mass loading ~1.5 mg cm⁻²) before and after calendaring are shown in Figure S11.
The cell was first tested with varied C rates at 80 °C (Figure 3a-b), following a sequence of C/20, C/10, C/5, C/2 and C (C= 170 mA g⁻¹) which achieved specific capacities of 163, 160, 156, 143, and 132 mAh g⁻¹. After the cycles at C, the C-rate was reduced to C/10 and the cell exhibited a capacity of 157 mAh g⁻¹, suggesting the capacity change with increased current density is fully reversible. The good rate capability of the solid-state LMB cell indicates a good percolation of ionic pathway in the composite electrode and ideal contact between electrode and crosslinked SIPE membrane, which allows the cell to cycle at high current density with moderate capacity losses. Notably, the dis-/charge profiles showed remarkably long, flat plateau shapes with tiny polarizations. As compared to a cell with the non-calendared electrode (Figure S12), the cell with a calendared composite LFP electrode showed much better rate capability which underlined the importance of the porosity minimization in solid-state battery. The calendaring process could effectively reduce the porosity and tortuosity of the electrode leading to an improvement of the ionic and electronic transport through the electrode. In an additional LMB cell (Figure S13), even at a higher current density of 2 C, a capacity of more than 110 mAh g⁻¹ (c.a. 65% of the theoretical capacity of LFP) could be reached with a lower cut-off voltage, to compensate the cell polarization.

The long-term cycling test of LMB at a relatively high current density is presented in Figure 3 c-d. After 3 formation cycles, the cells cycled at C/2 with an initial capacity of 148 mAh g⁻¹. After 160 cycles, the capacity was decreased to 129 mAh g⁻¹, corresponding to a capacity retention of 87%, showing a good cycle stability of the cell. Moreover, the cell exhibited an impressive average coulombic efficiency of 99.9% over 160 cycles, indicating an excellent reversibility of the dis-/charge. Additionally, solid-state Li|I₁₀₀₀p-SO₃-cr|LFP cell with higher active material mass loading of ~6 mg cm⁻² were prepared and tested at current density of C/10 (Figure S14). The cells with LFP electrodes with mass loading of ~6 showed a satisfying initial specific capacity of 147 mAh g⁻¹ with an average Coulombic efficiency of 99.9% and no capacity decay over 29 cycles and remained
cycling. The cell with higher LFP loading demonstrated the potential of operating solid-state Li-metal batteries with the I₁₀₀₀p-SO₃-cr SIPE in more realistic conditions. With the cycling tests, the I₁₀₀₀p-SO₃-cr SIPE showed excellent performance in cycle stability which is comparable and even higher than other SIPES reported earlier.⁵⁴⁻⁵⁷

### 3. Conclusions

In this work, we designed novel SIPEs with a PEG-based backbone and a low basicity perfluorosulfonate anion. SIPEs with three different lithium concentrations were achieved by tuning the length of the PEG sequence on the polymer backbone. Flexible self-standing membranes reinforced with NCC were obtained. The I₁₀₀₀p-SO₃-cr showed the best ionic conductivity of 3.5×10⁻⁵ S cm⁻¹ at 90 °C, due to a high chain mobility. A T₉₀ close to one was measured by both electrochemical and PFG-NMR methods. The symmetrical Li|I₁₀₀₀p-SO₃-cr|Li cell exhibited impressive stability with cycling for more than 80 days without any sign of dendrite formation, demonstrating the excellent compatibility between SIPE and lithium metal electrodes. The LMB cells with LFP-based positive electrodes presented long-term stability. Therefore, the rationally designed I₁₀₀₀p-SO₃-cr SIPE can be a promising candidate for dendrite-free solid-state lithium metal batteries with high performance and safety.

### Supporting information

The experimental section, additional data and comments are included in the Supporting information. The authors have cited additional references within the Supporting information.⁵⁸⁻⁶¹

### Acknowledgments
The authors would like to acknowledge the financial support of the Agence Nationale de la Recherche and Bundesministerium für Bildung und Forschung (ANR-19-ENER-0007 and 03SF0583A) and French Research Ministry for funding the PhD of H.P.K Ngo. The authors would like to acknowledge ERAS Labo for providing the p-SO₃ material. We would like to thank Dr. Francine Roussel-Dherbey and Mr. Théodore Barret for their help on SEM images. This research is in the field of the Centre of Excellence of Multifunctional Architecture Materials CEMAM (grant ANR-10-LABX-44-01).

**Corresponding author:**

cristina.iojoiu@grenoble-inp.fr

**Author contributions**

C.I. and J-Y.S. conceived and designed the material. H.N., T.B., J.S performed the synthesis and preparation of the SIPE. H.N., T.N., J.S., E.P. and P.J. performed the characterizations of the material. Y.S. and H.N. performed the electrochemical characterizations and battery tests under CI and F.A. supervision. C.I. and Y.S. analyzed the data and prepared the manuscript. All authors discussed the results and commented on the manuscript. ‡These authors contributed equally.

**Data availability**

Data for this article, including NMR, IR spectra, thermal analysis, and electrochemical measurements are available at Open Science Framework at DOI: 10.17605/OSF.IO/A67PE.
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Synthesis route of the SIPEs: (a) polycondensation reactions; (b) Ion exchange; and (c) UV crosslinking. The crosslinkage is highlighted in red color.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** (a) TGA thermograms of SIPEs; (b) DSC traces of linear and crosslinked SIPEs with varied EO/Li ratios; (c) Ionic conductivity of the crosslinked SIPEs membranes; (d) Ionic conductivities of SIPEs with different EO/Li ratios as a function of 1000/(T-T₀); (e) Chronoamperometry experiments conducted on Li|I₁₀₀₀p-SO₃-cr|Li cells with a bias voltage of 20</figcaption></figure>
mV. The inset is corresponding EIS spectra before and after chronoamperometry experiments; (f)
CV trace of Li|I<sub>1000p</sub>-SO<sub>3</sub>-cr|SS cells at 80 °C, swept between -0.5 to 4.0 V vs. Li<sup>+</sup>/Li with a sweep
rate of 0.2 mV s<sup>-1</sup>. (all the crosslinked membranes contain 6% of NCC)
<figure><img src="image_6.png" /><figcaption>**Figure 2.** (a) Li stripping/plating experiment involving varying current densities conducted on Li|I₁₀₀₀p-SO₃-cr|Li cells at 80 °C. (b) Magnification of Li stripping/plating cycles in the range from 70 to 72 days with a current density of 200 µA cm⁻². (c) The potential during the Li stripping/plating</figcaption></figure>
experiments as a function of the applied current density. (d) EIS spectra of the Li|I₁₀₀₀p-SO₃-cr|Li cells recorded during the Li stripping/plating experiments, and (e) corresponding electrolyte bulk and interfacial resistance as a function of applied current density.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** (a) C rate test conducted on Li|I₁₀₀₀p-SO₃-cr|LFP cells at 80 °C, (b) selected dis-/charge profiles. (c) Long-term cycling test conducted on Li|I₁₀₀₀p-SO₃-cr|LFP cells at 80 °C at a current density of C/2 and (d) selected dis-/charge profiles of 1st, 2nd, 4th, 50th, 100th, 200th, and 300th cycles</figcaption></figure>
from high capacity to low capacity, respectively. The LFP mass loading in the electrodes is ~1.5
mg cm⁻².
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Supporting Information

Single-ion Conducting Polymer Electrolyte with
Excellent Interfacial Stability Towards the Lithium
Metal

Houang Phong Khanh Ngo<fnref n="1" />‚, Yunfan Shao<fnref n="1" />‚, Tony Bertaux<fnref n="1" />, Thi Khanh Ly Nguyen<fnref n="1" />, Justine Solier<fnref n="1" />, Emilie Planes<fnref n="1" />, Patrick Judeinstein<fnref n="2" />‚<fnref n="3" />, Fannie Alloin<fnref n="1" />‚<fnref n="4" />, Jean-Yves Sanchez<fnref n="1" />, Cristina Iojoiu<fnref n="1" />, 4*

<footnote n="1">¹ Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LLEPMI, 38000 Grenoble, France</footnote>

<footnote n="2">² Université Paris-Saclay, CNRS, CEA, LLB, 91191 Gif-sur-Yvette, France</footnote>

<footnote n="3">³ Université Paris-Saclay, CNRS, LPS, 91405 Orsay, France</footnote>

<footnote n="4">4 Réseau sur le Stockage Electrochimique de l'Energie (RS2E), CNRS, FR3459, 80039 Amiens Cedex,
France</footnote>

<footnote>* Corresponding authors:
cristina.iojoiu@grenoble-inp.fr</footnote>
# Experimental

## Materials

All the solvents were used as received. The DGME were dried by distillation over calcium hydride and stored with molecular sieves (3 Å) in a glovebox before use. All of the reagents used for polymer synthesis were dried under vacuum at 80 °C for 48 h and stored in a glovebox. The monomer sodium-5-(3',5'-difluorophenyl)-3-oxoperfluoropentane sulfonate (p-SO₃) was provided by ERAS Labo. The ion-exchange resin Amberlite IR120 H-form with the exchange capacity of 1.8 meq mL⁻¹ by wet volume was purchased from Sigma Aldrich. Nanocrystalline cellulose (NCC) with a diameter of 2 to 20 nm and factor form L/d of 10-100 are obtained from FP Innovation.

## Synthesis of single-ion conducting polymers

The ionic polymer with double-bonds e.g., Iₓp-SO₃-db was synthetized in one pot two steps reactions: (i) synthesis of an ionic precursor, Iₓp-SO₃, with a molar mass of 5000 g mol⁻¹ terminated in OH and (ii) polycondensation between the Iₓp-SO₃ synthetized in the first step and 3-chloro-2-chloromethyl-1-propene. Three Iₓp-SO₃-db with an EO (ethylene oxide)/Li ratio of 9, 14 to 23 were synthetized respectively by varying the length of used PEG (from 400 to 1000 g mol⁻¹). The molecular mass of repeat Iₓp-SO₃ was controlled by the ratio between the PEGs and p-SO₃, the amount of each was determined following the Carothers equation. In a typical procedure, for the synthesis of I₁₀₀₀p-SO₃-db, 4 g (4 mmol) of PEG was dissolved in 15 mL of DGME and 20 mL of toluene into a tri-neck round bottom flask equipped with a Dean-Stark apparatus. The mixture was heated at 150 °C and the solution was dried by azeotropic distillation during 2 h, after the toluene was removed by distillation and the bath was cooled down at 60 °C. Then 1.5 equivalents of NaH (0.3 g, 12.5 mmol) was added in the reaction. After 2 h of stirring at 60°C, the bath was heated-up to 120 °C and the corresponding amount of monomer p-SO₃
(1.35 g, 3.12 mmol) dissolved in approximate 10 mL DGME was added. The reaction was followed by ¹⁹F NMR spectroscopy i.e., the evolution of the aromatic fluorine peak at -108 ppm. After 2 hours, the reaction was complete and 0.114 mL (0.123 g, 1 mmol) of 3-chloro-2-chloromethyl-1-propene was added at 70 °C. The reaction was stirred overnight (about 12 h) at 70°C.

The product was recovered by precipitating the reaction mixture in n-pentane and rinsed several times with fresh n-pentane to remove the excess of solvents from the reaction.

The resulting polymer was dried under vacuum and then was dissolved in deionized water and a dilute acetic acid solution (~ 0.1 mol L⁻¹) was added in order to neutralize the NaOH formed after the hydrolysis of NaH excess. The solution with a pH close to 7 was transferred in a filtration milipore cell and filtrated under pressure (~ 2 bar) through a Millipore ultrafiltration membrane with 3 kDa pores to remove residual inorganic salts and low molecular weight polymer. The water was removed by freeze-drying. The obtained ionic polymer was dried at 80 °C for 48 h at 10⁻³ bar. The yield after purification was about 80 %. The NMR of final product is presented in Figure S1.

## Ion exchange from Na to Li

The Amberlite H-form was exchanged into Li-form and introduced in a column. Then the solution of an ionomer of 5% in water: acetonitrile (1:1 in volume) was slowly added on the top of column containing a large excess of cation (more than 10 times than the ionomer). After the cation exchange on the column the Iₓp-SO₃-db was recovered by evaporating the solvent using rotavapor. Almost 100 % of ionomer's quantity was obtained after the ion exchange process. The neat materials were dried carefully under vacuum at 80 °C for 48 h.

## Preparation of crosslinked membranes
0.2 g Iₓp-SO₃-db and 8 mg (4 wt.%) photoinitiator Irgacure® 2959, (1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one), was dissolved in 5 mL of deionized water. For the membrane with NCC 6 wt.% nanocrystalline cellulose (NCC, 0.3 ml of 4% in water) was added to reinforce the membrane mechanical properties. The homogeneous solutions (with and without NCC) were degassed under reduced vacuum and was poured into a PTFE petri dish (ø=3.5 cm). Solvent was evaporated in an oven at 60 °C. These films were crosslinked with a Heraeus F300 UV system for twice irradiation of 30 s, in a quartz covered cell filled with Argon. Cross-linked membranes with thickness around 60 µm were dried under vacuum at 80 °C for 48 h and stored in a glovebox (Figure S2).

## Raman Spectroscopy

The Raman spectra were collected by a Reni-Shaw Invia confocal Raman spectrometer equipped with a CCD photographic sensor with resolution of 1 cm⁻¹. Raman spectra are accumulated with a 785 nm diode laser excitation, at a power of 14.8 mW cm². Data processed with Renishaw's Wire 3.3 software.

## Atomic absorption spectroscopy (AAS)

The ion exchange efficiency was determined using a Perkin Elmer PinAAcle 900F AAS, and the data were treated with the Syngistix software. The lamps for absorption corresponding to different elements such as Li, Na, K, and Mg were used to detect the presence of these ions in the sample.

## Size exclusion chromatography coupled multi-angle laser light scattering (SEC-MALLS)

The SEC measurements were performed with the WATERS 515 HPLC, with a differential refractometer SOPARES RI2000 and a light scattering detector WYATTDAWN EOS at 690 nm. The data were treated
with ASTRA 6 software. NaNO₃ (0.1 M) in DMF was used as solvent at a flow rate of 1 mL.min⁻¹ through a
10 AGILENT 2xPLgel-Mixed-D column.

Membrane solubility tests

A definite mass of dried cross-linked membrane was immersed in volume of deuterated acetonitrile (AcN-D₃). After 24 h, the liquid phase contained the soluble ionomer was extracted and added in an NMR tube. An insert containing a reference solution of 0.5 M tetrabutyltetrafluoroborate in AcN-D3 was introduced in the NMR tube to quantify the amount of ionomer dissolved by the ratio of integrals between the SIPE characteristic peaks and the reference solution. For all the three samples any soluble SIPE was detected.

Scanning electron microscopy (SEM) analysis

To obtain a net fracture surface, membranes were frozen in liquid nitrogen, then fractured. These latter have been observed in cross-section. Before observation, they have been coated with carbon. SEM observations were performed on a ZEISS Ultra 500 using accelerating voltage of 3 kV. Obtained image was post-treated and analyzed using ImageJ software.¹ In details, for NCC dispersion, images were analyzed using a binary method in grayscale in order to quantify domain sizes using “Analyze particles” from ImageJ.

The SEM analysis of the Li|SIPE interface was carried with a Li|SIPE|Li cell which has been cycled with Li stripping/plating experiments of 200 h (current density =100 µA cm⁻²). The cell was disassembled and the sample was prepared inside an Ar-filled glovebox, The sample transferred to SEM with protection of an airlock chamber.
Thermal analysis

The differential scanning calorimetry (DSC) was carried out with a DSC 1 STARe instrument of METTLER
TOLEDO. The dry samples were sealed in aluminum crucibles in an Ar-filled glovebox. The samples were
submitted to a ramp in temperature from -150 °C to 100 °C, at a scan rate of 10 °C min⁻¹, under N₂ flux of
50 mL min⁻¹.

The thermogravimetric analysis (TGA) measurement was realized under N₂ flux of 50 mL min⁻¹ using TGA
1 STAR instrument of METTLER TOLEDO. The analysis was performed in temperature range 20-800 °C with
a scan rate 10 °C.min⁻¹.

Electrochemical characterizations

The electrochemical characterizations were carried out with a Biologic VMP3 potentiostat, and the data
was processed by Biologic EC-lab software. All the measurements were performed at least 3 times, for
the conductivity the average is represented (the difference between conductivity values were less than
10%), for the other measurements we selected the results for which at least two of them are very close
generally a results with good reproducibility were obtained for all the measurements).

Ionic conductivity

The ionic conductivity was measured by Electrochemical Impedance Spectroscopy (EIS) method. The cross-linked SIPE membrane (ø= 9 mm) was placed between two stainless-steel electrodes in a Swagelok cell. The cell was submitted to a two-cycle heating/cooling process, stepwise from 20 °C to 90 °C, with a temperature ramp of 5 °C h⁻¹ and an electrochemical impedance spectrum was measured every hour after the temperature stabilization.
Ionic conductivity measured was calculated by the following equation:

$$ \sigma = \frac{e}{R \times S} $$

where e is the thickness of the electrolyte membrane measured after the experiment, S is the electrode surface area, and R is the bulk resistance of the electrolyte.

## Cyclic voltammetry

The cyclic voltammetry (CV) measurement was performed using a Biologic VMP3 potentiostat. Cross-linked membranes were sandwiched between lithium metal and stainless-steel electrodes. CV was performed from OCV in a range of -0.5 V to 4.0 V vs. Li⁺/Li, at a scan rate of 0.2 mV s⁻¹, in oven at 80 °C.

## Lithium transference number ($T_{Li}^{+}$)

Transference number of Li⁺ was determined using three different methods, i.e., low-frequency impedance, Bruce-Vincent method and self-diffusion coefficients measured by Pulsed-field gradient nuclear magnetic resonance (PFG NMR).

For impedance method, the symmetric Li|SIPE|Li coin cells were conducted with an EIS measurement with a frequency range of 1 MHz to 1 mHz. The impedance was fitted with equivalent circuit in Scheme S1 using Biologic EC-lab software. According to MacDonald,² the transference number of a binary system is calculated with following equation:

$$ T_{Li^{+}} = \frac{R_{el}}{R_{el} + R_{diff}} $$
where $T_{Li}^{+}$ is the Li transference number, $R_{el}$ and $R_{diff}$ are the electrolyte resistance and the lithium diffusion resistance, respectively.

With the Bruce-Vincent method,³,⁴ the symmetric Li|SIPE|Li coin cells were conducted with an EIS measurement, a chronoamperometry polarization with 20 mV of bias voltage for 10 h and followed by another EIS measurement. Lithium transference number was determined by the following equation:

$$T_{Li}^{+} = \frac{I_{s}(\Delta V - I_{i}R_{i})}{I_{i}(\Delta V - I_{s}R_{s})}$$

Where, $I_{i}$, $R_{i}$ and $I_{s}$, $R_{s}$ represent the response of currents and interfacial resistances before and after polarization, respectively, and $\Delta V$ is the bias voltage.

<figure><img src="image_8.png" /><figcaption>Scheme S1. Equivalent circuit for symmetric Li|SIPE|Li cells.</figcaption></figure>

## Pulsed-field gradient nuclear magnetic resonance (PFG NMR)

The self-diffusion coefficients were measured by PFG-NMR using a Bruker Avance III HD spectrometer equipped with a diffusion probe of 5 mm and a temperature regulation unit (20 - 80°C). The frequencies are 400.13, 376.50, and 155.51 MHz for ¹H, ¹⁹F and ⁷Li, respectively. Stimulated echo sequence was used. The maximum magnitude of the pulsed field gradient was 900 G.cm⁻¹, the diffusion delay $\Delta$ was adjusted between 50 and 100 ms, and the gradient pulse length $\delta$ was set between 1ms and 5ms depending on the diffusion coefficient and the nature of the mobile species. The self-diffusion coefficients of Li and F
containing species, i.e., Li ion and polymer backbone respectively, were determined from the classical Stejskal-Tanner equation:⁵

$$ln \frac{I}{I_0} = -DG^2\gamma^2\delta^2(\Delta - \frac{\delta}{3})$$

where G is the magnitude of the two gradient pulses applied, γ is the gyromagnetic ratio of the nucleus under study and I and I₀ are the integrated intensities of the signal obtained respectively with and without gradient pulses. Here, 16 equally spaced gradient steps were applied to each sample. Data acquisition and treatment were performed with Bruker Topspin software. The T₉₀⁺ was calculated with the following equation:

$$T_{Li^+} = \frac{D_{Li}}{D_{Li} + D_F}$$

where D₉₀⁺ and D₇₀ are self-diffusion coefficients of Li and F containing species.

## Lithium plating/stripping

Chronopotentiometry (CP) technique was used to study the plating/stripping processes of Li⁺ in a symmetric Li|SIPE|Li cell. In each plating/stripping step, a constant current density was imposed for 4 h, following by 45 minutes of rest. After 10 pre-conditioning cycles (at 10 µA cm⁻²), the current density was gradually increased from 50 to 200 µA cm⁻².

## Battery assembling and testing

Electrodes with active material loading varied between 1.5 to 6 mg cm⁻² were achieved for different tests. LiFePO₄ (LFP)-based electrode were prepared with a mass ratio of LFP: carbon black:l₁₀₀₀p-SO₃⁻
db:PVDF = 65:10:20:5. The electrodes were cast on carbon-coated Al foils and then calendared by a rolling press at ambient temperature between protection of two layers of Al foil. The electrode was calendared multiple times until the minimum thickness of electrode is achieved. The thickness of electrode changed significantly after calendaring from 17 to 5 µm and from 50 to 38 µm for the electrode with mass loading respectively, 1.5 and 6 mg cm⁻². The cells were performed with CR2032-type coin cell with a sandwich structure of Li|SIPE|LFP. The battery cycling was carried out with galvanostatic charge/discharge ranged between 2.5 to 3.7 V vs. Li⁺/Li with a Biologic VMP3 potentiostat.

<table><caption>**Table S1.** SEC-MALLS and AAS results</caption><thead><tr><th rowspan="2">Electrolyte</th><th rowspan="2">EO/Li ratio</th><th colspan="3">SEC-MALLS</th><th colspan="3">AAS</th></tr><tr><th>M<sub>n</sub> kg.mol<sup>-1</sup></th><th>M<sub>w</sub> kg.mol<sup>-1</sup></th><th>DPI</th><th><sup>7</sup>Li %</th><th><sup>23</sup>Na %</th><th><sup>39</sup>K %</th></tr></thead><tbody><tr><td>I<sub>1000</sub> p-SO<sub>3</sub>-db</td><td>23</td><td>18</td><td>24</td><td>1.3</td><td>97.1</td><td>2.9</td><td>-</td></tr><tr><td>I<sub>600</sub> p-SO<sub>3</sub>-db</td><td>14</td><td>21</td><td>35</td><td>1.7</td><td>97.4</td><td>1.2</td><td>1.4</td></tr><tr><td>I<sub>400</sub> p-SO<sub>3</sub>-db</td><td>9</td><td>19</td><td>24</td><td>1.3</td><td>96.1</td><td>1.5</td><td>2.3</td></tr></tbody></table>

<table><caption>**Table S2.** Diffusion coefficients of <sup>7</sup>Li and <sup>19</sup>F nuclei, and the T<sub>Li</sub><sup>+</sup> of I<sub>1000</sub> p-SO<sub>3</sub> at 80 °C measured by PFG-NMR</caption><thead><tr><th>Electrolyte</th><th>D<sub>F</sub> (*10<sup>-12</sup> m<sup>2</sup>.s<sup>-1</sup>)</th><th>D<sub>Li</sub> (*10<sup>-12</sup> m<sup>2</sup>.s<sup>-1</sup>)</th><th>T<sub>Li<sup>+</sup></sub></th></tr></thead><tbody><tr><td>I<sub>1000</sub> p-SO<sub>3</sub>-cr</td><td>1.4</td><td>4.1</td><td>0.74</td></tr><tr><td>I<sub>1000</sub> p-SO<sub>3</sub>-cr<br>(without NCC)</td><td>0.048</td><td>2.0</td><td>0.97</td></tr></tbody></table>
<table><tr><td>I<sub>1000</sub> p-SO<sub>3</sub>-cr<br>(with NCC)</td><td>0.045</td><td>2.0</td><td>0.98</td></tr></table>

<figure><img src="image_9.png" /><figcaption>**Figure S1.** ¹H and ¹⁹F-NMR spectra of I₁₀₀₀p-SO₃-db in acetone-d₆ with corresponding site assignments and peak integration</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure S2.** Raman spectra of I₁₀₀₀p-SO₃-db and crosslinked I₁₀₀₀-SO₃-cr SIPEs.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure S3.** Images of (a) I₁₀₀₀p-SO₃-db and (b) I₁₀₀₀p-SO₃-cr.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure S4.** Glass transition temperature of SIPEs as a function of Li concentration.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure S5.** (a-b) Scanning electron micrographs of the cryo-fractured surface of $I_{1000}p-SO_3 + 6$ wt.% cellulose whiskers nanocomposite film with corresponding Image J treated images, respectively. (c) Particles size distribution of NCC.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure S6.** Ionic conductivity of linear SIPE and crosslinked SIPEs with/without NCC as a function of 1000/T.</figcaption></figure>

<figure><img src="image_15.png" /></figure>
**Figure S7.** EIS spectrum of symmetrical Li|I₁₀₀₀p-SO₃-cr|Li cell at 80 °C, with scanning frequencies between 1 MHz and 1 mHz.

<figure><img src="image_16.png" /></figure>

**Figure S8.** Linear sweep voltammogram of Li|I₁₀₀₀p-SO₃-cr|SS cells at 80 °C, with a scanning rate of 0.2 mV s⁻¹.
<figure><img src="image_17.png" /><figcaption>**Figure S9.** Determination of critical current density of the I₁₀₀₀p-SO₃-cr SIPE. (a): Current scan of a Li|I₁₀₀₀p-SO₃-cr|Li cell at 80 °C, from 0 to 2 mA, with a rate of 0.1 µA s⁻¹. (b): 1st order derivative of potential.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure S10.** Cross-sectional image of Li|SIPE|Li cells after Li stripping/plating experiments of 200 h (current density =100 µA cm⁻²): (a) Li|SIPE|Li sandwich structure, with a SE2 detector; (b) magnified Li|SIPE interface, with an InLens detector.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure S11.** Cross-sectional images of pristine and calendared LFP composite electrodes. The images are collected with SEM equipped with a backscattered electron detector. The thickness of the electrode decreases from 17 µm to 5 µm after calendaring.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S12.** C-rate test conducted on Li|I₁₀₀₀p-SO₃-cr|LFP cells at 80 °C with non-calendared composite LFP electrodes: (a) specific capacity; (b) Discharge profiles of different C-rates.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Figure S13.** C-rate test conducted on a Li|I₁₀₀₀p-SO₃-cr|LFP cell at 80 °C up to 2C: (a) specific capacity; (b) Discharge profiles of different C-rates.</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure S14.** Galvanostatic cycling test conducted on Li|I₁₀₀₀p-SO₃-cr|LFP cells with high mass loading LFP electrodes (~6 mg cm⁻²) at 80 °C and C/10: (a) specific capacity; (b) Charge/discharge profiles of selected cycles.</figcaption></figure>
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