# Functional Composite Separators with Cation-Trapping Abilities

Jason Richard, Navid Solati, Arvinder Singh, Valentin Meunier, Yoko Toda,

Alexis Grimaud, Arnaud Perez, Christel Laberty-Robert

► To cite this version:

Jason Richard, Navid Solati, Arvinder Singh, Valentin Meunier, Yoko Toda, et al., Functional Composite Separators with Cation-Trapping Abilities. ACS Applied Energy Materials, 2024, 7 (10), pp.4335-4346. 10.1021/acsaem.4c00094. hal-04754174

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Submitted on 25 Oct 2024

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Functional composite separators with
cations trapping abilities

Jason Richard,ª Navid Solati,ª Arvinder Singh,ª Valentin Meunier,ᵇ,ᶜ Yoko Toda,ª
Alexis Grimaud,ᵇ,ᶜ,ᵈ Arnaud Perezª,ᶜ, Christel Laberty-Robert a *

**Abstract**

Separators in Li-ion batteries are crucial components that have a great impact on battery safety and lifespan. The design of multifunctional separators is desired to tackle problems such as thermal runaway or electrode degradation. However, addressing both of these challenges is hardly achievable. Here, we propose a versatile process to form hybrid separators with tunable architecture and surface chemistry. Combining electrospinning with sol-gel chemistry, homogeneous nanofibers made of poly(vinylidene fluoride-co-hexafluoropropylene) and up to 37 wt.% of silica were obtained, showing high mechanical properties and outstanding thermal stability. Multilayer separators were designed to achieve shutdown properties: outer layers of hybrid fibers maintained the dimensional stability of the separator while an inner layer of polymer fibers melted to raise the ionic resistance. Notably, in the battery cell configuration, we demonstrate the advantages of incorporating hybrid fibers, which help maintain the dimensional stability of the separator. The hybrid nanofibers were also functionalized with amine or lithium sulfonate groups to trap hydrofluoric acid (HF) or metallic cations. XPS analyses revealed the protonation of amine functions on separators fibers after cycling in LiNiO₂ || graphite coin cells, which evidenced the scavenging of acidic species such as HF. In addition, we observed a correlation between the amount of Ni cations deposited on the graphite after cycling and the capacity loss attributed to the graphite electrode. The functional separator incorporating terpyridine molecules allowed for a reduced nickel crossover from LiNiO₂ to graphite and a better capacity retention compared to the other separators.

<footnote>ª Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC
Paris 06, 4 Place Jussieu, 75005 Paris, France</footnote>

<footnote>ᵇ Chimie du Solide et de l'Energie (CSE), Collège de France, UMR 8260, 75231 Paris Cedex
05, France</footnote>

<footnote>ᶜ RS2E, Réseau Français sur le Stockage Electrochimique de l'Energie, CNRS 3459, 80039 Cedex 1 Amiens, France</footnote>

<footnote>d Department of Chemistry, Boston College, Merkert Chemistry Center, 2609 Beacon St.,
Chestnut Hill, MA, 02467 USA</footnote>
**Keywords**

Li-ion batteries, functional separators, nanofibers, thermal runaway, cation crossover, sol-gel, $LiNiO_2$

**Graphical abstract**

Hybrid separator

<figure><img src="image_2.png" /></figure>

SHUTDOWN ARCHITECTURE

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

TRAPPING FUNCTIONS

<figure><img src="image_4.png" /></figure>

**Introduction**

Safe and efficient batteries are needed to support the energy transition toward low carbon energy sources. Among all mature electrochemical energy storage technologies, lithium ion batteries (LIB) currently offer the highest specific energy and energy density at stack level (314 Wh.kg⁻¹ and 773 Wh.l⁻¹ respectively).¹ Improving their safety and lifespan remains a topical issue to further increase their efficiency and spread the technology. Indeed, the properties of the separator have a large influence on the long-term stability of the cells because of its critical role as electrical insulator between the electrodes.²⁻⁴ Designing safe and robust separators thus appears as an effective strategy to improve LIB performances and durability without decreasing the specific energy or modifying the electrodes or the electrolyte.

Porous separators prevent the direct contact between the electrodes while ensuring the transport of lithium ions through the liquid electrolyte. Currently, polyolefin porous film are the state-of-the-art separators because of their affordability and their high electrochemical and mechanical stability. However, they have low wettability and low thermal stability, which limits the charge rate and cannot efficiently prevent thermal runaway events.⁵
Electrospun poly(vinylidene fluoride) (PVDF) or poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) fibers have risen as an interesting alternative due to their excellent wettability with traditional carbonate-based electrolytes.⁶,⁷ However, these separators also suffer from low thermal stability and poor mechanical properties. Reinforcing PVDF fibers with ceramic particles is an efficient method to improve the separators robustness.⁸ In these composite materials, the inorganic components may be added as particles⁹,¹⁰ or inorganic layers.¹¹ Among the different metal oxides, SiO₂ allowed for the best enhancement of the thermal stability of the separators.¹² Besides their composition, the morphology of the inorganic entities is also of importance: the smaller the inorganic particles, the better the mechanical properties,¹³ and 1D nanowires or nanotubes showed even better enhancement of performances than particles.¹⁴,¹⁵ This can be explained by a larger interface between polymer and silica domains, further improving the mechanical properties. In addition, the coating of PVDF-HFP fibers with SiO₂ nanoparticles improves the wettability and conductivity of the separators, as recently illustrated by Shi et al.¹⁶ Yet, it should be noted that these approaches are based on mixing or adding the inorganic particles or coating to the separators, strategies that may lead to the leaching of particles into the electrolyte.⁸,¹⁷

Furthermore, designing functional separators able to trap impurities in the liquid electrolyte allows for a lower degradation rate of the battery and an increased lifespan.⁵,¹⁸ Adopted strategies can be classified in different categories: i/ trapping of transition metal cations by ion-exchange¹⁹ or by complexation,²⁰ ii/ scavenging of HF with Lewis base functions such as amines,²¹,²² iii/ stabilization of the LiPF₆ salt,²³ iv/ design of multifunctional separators combining several of these effects.²⁴,²⁵ Noteworthy, Aurbach *et col.* investigated the ability of polyamine-based separators to trap HF and transition metal cations dissolved from the cathode such as Mn²⁺ and Mn³⁺.²⁶ They quantified the capacity retention and the amount of Mn deposited on the anode after cycling full cells with polyolefin or functional separators. With poly(divinylbenzene-4-vinylpyridine) as functional separator for instance, they systematically observed higher capacity retention and lower final Mn amounts in anodes for all temperatures, cycling conditions and cathode materials they tested. This effect was mainly attributed to the HF scavenging ability rather than the Mn trapping capability.

The design of multifunctional separators having both inorganic reinforcements and chemical functions to trap impurities thus appears as a promising way to achieve both safer and more sustainable LIB. While functional composite separators have been investigated for Li-S batteries,²⁷⁻³⁰ they remain scarcely explored for LIB. Woo et al. deposited an Al₂O₃ layer on commercial polyethylene (PE) separators by atomic layer deposition and grafted sodium carboxylate functions.¹⁹ The capacity retention was improved compared to the PE and to the PE-Al₂O₃ separators, which was attributed to the exchange of Mn²⁺ with Na⁺. However, the thermal and mechanical properties were not reported. Zuo et al. functionalized a commercial PE separator by dip-coating in a solution containing PVDF, ethyl cellulose, and amino-functionalized SiO₂ particles (SiO₂-NH₂).³¹ The amine functions on the silica particles improved the dispersion of the particles on the separator, which further enhanced the mechanical
robustness and the thermal stability. Despite this enhancement, the shrinkage still reached 20% after 30 min at 150°C. The capacity fading and the impedance increase were also reduced, which was suggested to come from the SiO₂-NH₂ scavenging properties.

In 2019, we introduced electrospun hybrid PVDF-HFP/SiO₂ fiber-based separators for Na-ion batteries.³² Compared to a pure PVDF-HFP electrospun separator, the hybrid separator showed high porosity, wettability and electrolyte uptake, as well as good thermal stability. These properties allowed for an excellent power rate capability. However, we did not investigate the effect of the architecture and chemical functionalization of the separators on their properties and on the cell performances.

Here we propose a versatile strategy to combine the robustness of composite separators and the impurities trapping ability of functional separators. These separators consist of electrospun nanofibers made of PVDF-HFP and *in situ* generated domains of amorphous silica. The silica precursors are condensed in acidic conditions, generating intermixed polymer and ceramic domains within the nanofibers rather than macro-segregated phases. This original organization enhances mechanical properties and greatly improves the thermal stability, with low shrinkages up to 200°C. Shutdown abilities were then targeted with a multilayer design: while hybrid layers maintain the dimensional stability, the polymer layers melt around 100°C, reducing the porosity and drastically increasing the ionic resistance. Fully charged cells assembled with this multilayer separator were also able to maintain the OCV up to at least 120°C, while the other cells underwent short-circuits. In addition, amines or lithium sulfate trapping functions were grafted directly on the silica domains of the hybrid nanofibers to scavenge HF and/or metallic cations. The crossover of Ni species from the cathode to the anode and the capacity retention were quantified after cycling in LiNiO₂ (LNO) || 1M LiPF₆, EC/EMC 30/70 || graphite coin cells for the different separators. Moreover, XPS studies revealed the protonation of –NH₂ functions onto the separators after cycling, which evidences the ability of these functional separators to scavenge acidic species such as HF.

# Experimental section

**Chemicals.** Tetraethyl orthosilicate (TEOS) (99%), N,N-dimethylformamide (DMF), lithium chloride (LiCl), (3-Aminopropyl)triethoxysilane (APTES, 99%), hydrochloric acid (HCl, 37wt.%) and 4'-chloro-2,2':6',2"-terpyridine (99%) were purchased from Sigma Aldrich and used as received. 2-4(chlorosulfonylphenyl)ethyltrichlorosilane (CSPTC, 50% in methylene chloride) and N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane (AAPDMS, 97%) were purchased from ABCR and used as received. Poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) (Solef 21216®) was provided by Solvay and used as received. Toluene was purchased from VWR and water traces were removed with molecular sieve 3Å.

**Synthesis of Hybrid Electrospun Separator.** For the electrospinning of the Hybrid Electrospun Separators (HES), 10 wt.% of PVDF-HFP was first dissolved in DMF before addition of TEOS with a PDVF-HFP:TEOS mass ratio of 1:2. An aqueous solution of H₂O/HCl (pH=1) was
then added to the mixture under vigorous magnetic stirring with a H₂O:TEOS mol ratio of 4:1 to hydrolyze the TEOS. The solution was kept at 60°C for 20 minutes and was then cooled down to room temperature while maintaining the magnetic stirring overnight. For the electrospinning of the pure polymer-based separator (PES), the 10 wt.% PVDF-HFP in DMF solution was directly used as the precursor.

For the electrospinning of the Hybrid Electrospun Separators with the terpyridine encapsulated (HES@terpy), the 4-chloro-terpyridine was first dissolved in DMF with TEOS with a terpyridine/TEOS mol ratio of 1:10. An aqueous solution of H₂O/HCl (pH=1) was then added to the mixture under vigorous magnetic stirring with a H₂O:TEOS molar ratio of 4:1 to hydrolyze the TEOS. Finally, PVDF-HFP was added to the solution with a PDVF-HFP:TEOS:DMF mass ratio of 1:2:9 and the mixture was magnetically stirred overnight to obtain a homogeneous solution.

For the electrospinning of the Hybrid Electrospun Separators functionalized with lithium sulfonate functions HES-SO₃Li, 10 wt.% of PVDF-HFP was first dissolved in DMF before addition of TEOS and CSPTC with a PDVF-HFP:TEOS:CSPTC mass ratio of 10:6:39. No water was added. The solution was kept at 70°C for 3 hours and was then cooled down to room temperature while maintaining the magnetic stirring overnight.

**Electrospinning.** A Nanospinner from Inovenso was used as the electrospinning setup for membrane synthesis. For a typical separator synthesis, 2 mL of the precursor solution was electrospun at 20 kV with a feed rate of 1.0 mL.h⁻¹ on a layer of non-adhesive paper attached to a rotating drum. The nozzle-collector distance was 15 cm and the relative humidity was maintained at 15%. After the electrospinning, membranes were removed from the paper and dried overnight at 70 °C. The membranes obtained were white, flexible and resistant, with a homogeneous aspect and a thickness of around 40 to 60 µm.

For multilayered membranes, two syringes were filled with either a pure polymer precursor solution, either a hybrid precursor solution. Typically, for HES@PES, 1 ml of the pure polymer precursor solution was first electrospun on a layer of non-adhesive paper, immediately followed by the electrospinning of 1 ml of the hybrid precursor solution and finally by 1 ml of the pure polymer precursor solution.

**Hot-pressing.** Multilayer heterostructure separators PES@HES and HES@PES were hot-pressed to reduce their porosity and improve their shutdown behavior. The separators were placed between PTFE sheets and a laminar press was used. The pressing time was set to 10 minutes, the temperature to 80°C and the pressure to 10 MPa.

**Post functionalization.** To obtain amino functionalized hybrid separator (HES-NH₂), a HES membrane was first dried overnight under vacuum to remove the remaining traces of water. Then, the membrane was introduced in a 3 neck round bottom flask with dry toluene and heated to reflux under nitrogen. APTES was then introduced into the solution via a syringe with a HES:APTES mass ratio of 2:1 and the reflux was maintained for 6 hours. The membrane
was then separated and rinsed with ethanol before being dried overnight at 70°C. A change in membrane color was observed, from white to pale orange.

For the diamino functionalized hybrid separator (HES-NH-NH₂), the protocol was the same as for HES-NH₂ but AAPDMS was used instead of APTES.

**Characterization of HES. Scanning Electron Microscopy (SEM)** images were acquired on a Hitachi S-3400N operating at 6 kV, after deposition of a gold layer of 20 nm on the samples. The fiber diameter was measured using ImageJ software on at least 30 fibers. **Thermal shrinkage** measurements were made on membrane samples of 400 mm² placed between two layers of heat-resistant paper and let 30 min in an oven at different temperature. Dimensions were measured using a caliper with a precision of ± 0.1 mm. **Differential Scanning Calorimetry (DSC)** were performed on a TA Instruments DSC Q20 under nitrogen (50 ml.min⁻¹) at a heat rate of 5°C.min⁻¹ using aluminum pan with pierced covers. **Dynamic mechanical analyses (DMA)** were performed with a Discovery DMA850 from TA Instruments. Separators' samples were placed in a calibrated film tension clamp and stress strain tests were performed at 30°C with a constant strain rate.

**The porosity** of the different separators was measured using the following protocol: at least 20 mg of membrane was first weighted with a precision of 0.01 mg and immersed in n-dodecane. The excess of solvent was then eliminated by pressing the membrane between two layers of absorbing paper and the membrane was finally weighted again. Dodecane was chosen because of its excellent wettability with all the separators, its low viscosity and low volatility. Porosity was then calculated using the following equation:

$$P = \frac{\rho_{HES} (m_2 - m_1)}{\rho_{HES} (m_2 - m_1) + \rho_{dod} m_1}$$

With P the porosity of the separator, m₁ the weight of the separator before immersion and m₂ the weight after immersion in dodecane, ρdod the density of dodecane (0.75 g.cm⁻³) and ρHES the density of the fibers in the HES, which was estimated to 1.95 g.cm⁻³ (ρPVDF-HFP = 1.78 g.cm⁻³ and ρSiO₂ = 2.2 g.cm⁻³). Each measurement was repeated 4 times and the mean values and standard deviation were reported. For a commercial Celgard2400 separator with a porosity of 41% according to the characteristics given by the company, the method gave a value of 40.9 ± 0.3%.

**Infrared spectra** were acquired in attenuated total reflection (IR-ATR) mode on a Spectrum400 from Perkin Elmer in the range 550-4000 cm⁻¹. **X-ray photoelectron spectroscopy (XPS)** were performed on a Scienta Omicron Argus X-ray photoelectron spectrometer equipped with an Al Kα monochromatic radiation source (hν = 1486.6 eV) operating at 280 W power. Peak deconvolutions were performed with CasaXPS software. **Elemental analyses** were performed on a Elementar Vario Micro Cube apparatus to determine the carbon C wt.% content in HES. The amounts of PVDF-HFP (PVDF-HFP wt.%) and SiO₂ (SiO₂ wt.%) in HES were then calculated from the following equations:
$$PVDF-HFP \text{ wt.\%} = C \text{ wt.\%} / C_{PVDF-HFP}$$

$$SiO_2 \text{ wt.\%} = 100\% - PVDF-HFP \text{ wt.\%}$$

With C wt.% and $C_{PVDF-HFP}$ standing for the carbon content measured in HES and the PVDF-HFP powder respectively (C wt.% = 22.911 ± 0.033 wt.% and $C_{PVDF-HFP}$ = 36.058 ± 0.001 wt.%).

**Ionic conductivity** of the HES separators was measured by electrochemical impedance spectroscopy (EIS) in a symmetrical cell at 30°C, using a SI1260 Impedance analyzer from Solartron Instruments. Aluminum disks were used as blocking electrodes and 20 µL of LiPF₆ 1M in EC:EMC 30:70 wt.% (LP57) as electrolyte. Fig. S3 and S5 show the Nyquist plots of the separators. The electrolyte bulk resistance ($R_b$) was estimated from the intercept of the semicircle with the real axis. The ionic conductivity $\sigma$ was then calculated from the $R_b$ values by employing the formula $\sigma = \ell/R_b A$, where $\ell$ is the thickness of the dry separator and A the contact area between the separator and the electrode.

**Electrochemical performances** of the separators were measured in LiNiO₂||1M LiPF₆, EC/EMC 30/70||graphite coin cells. To ensure that the porosity of the electrodes is properly filled with electrolyte, the total volume of the electrolyte in the cell is 50 µL, evenly divided on both sides of the separator. LNO loading was 2.96 mAh.cm⁻². Graphite loading on negative electrode was chosen so that the full cells are cathode-limited. All electrodes and separators were dried in a Buchi glass oven at 70 °C under vacuum for 24 h prior to being transferred into an Ar filled glovebox. All cells were then assembled in the glovebox using 2032-type coin cells.

All cells were galvanostatically cycled using a VMP3 or MPG2 potentiostat (Biologic, France) at room temperature. The upper cut-off voltage was fixed to 4.5V and the lower cut-off voltage was fixed to 2V. The accelerated cycling protocol was: 6h at OCV, one cycle at C/20 to form the SEI layer, ten cycles at C/2 with an upper cut-off voltage at 4.5V hold until a current density limit of C/20 is reached, one cycle at C/20 and one final charge at C/20 (Fig. S10). The final charge allows the maximum quantity of dissolved nickel ions to be reduced at the graphite electrode, thereby rendering subsequent ICP quantification more meaningful and pertinent. C-rate is defined from the theoretical capacity of LiNiO₂ material ($C_{th}$ = 275 mAh.g⁻¹). For the shutdown effect measurement, after the final charge at C/20, the temperature is increased to 60°C. The cell is cycled at this temperature at C/10 followed by charging with the same current. Then, the evolution of the cell potential in the charged state (4.0 V) was monitored while heating the cell up to 145 °C.

**Inductively coupled plasma mass spectrometry (ICP-MS)** analyses were carried out with a Nexion 2000 (Perkin Elmer) to determine the nickel content in the graphite electrodes after cycling. The protocol used is identical to that reported by Meunier et al.³³ Briefly, cells were de-crimped in an argon-filled glove box after cycling to recover graphite electrodes. The graphite electrodes were subjected to a back-and-forth washing process using dimethylmethyl carbonate (DMC) to effectively remove the adsorbed nickel that are deposited as a salt from the electrolyte from the electrode surfaces. Graphite electrodes were
then dissolved in 2 wt.% nitric acid (Suprapur, 65% HNO3, Merck, Ni impurities ≤ 5.0 ppb and Li impurities below 0.5 ppb), sonicated and let at rest overnight. Solutions were then filtered with polypropylene 0.2 µm syringe filters (VWR) and analyzed. Nickel amount was quantified after the calibration line was obtained with five standard points from multi-element standard solution (certified reference materials in 2% HNO3, CPA Chem, Bulgaria). Error bars on data plots were obtained from duplicata of a same condition.

Slippage analyses were performed following the methodology initially reported by Bloom et al.³⁴,³⁵ and adapted by Meunier et al. for graphite/LiNiO₂ cells.³³ The derivative voltage analysis were performed on discharge curve of C/20 cycles using VAWQuum v1.2 freeware provided by the Department of Physics and Atmospheric Science, Dalhousie University, Canada.³⁶ The reference curves employed for the fitting, i.e. LiNiO₂ vs Li⁺/Li and graphite vs Li⁺/Li, were recorded at C/20. The recorded data points were reduced to 100 points using linear interpolation and the discharge endpoints have been set to 0 mAh to meet software requirements. The fitting model is shown as follows:

$$ \frac{dV^{\text{model}}}{dQ_{\text{full cell}}} = \left( mass_{\text{pos.}} * \frac{dV}{dQ_{\text{pos. vs Li}}} + S_{\text{pos.}} \right) - \left( mass_{\text{neg.}} * \frac{dV}{dQ_{\text{neg. vs. Li}}} + S_{\text{neg.}} \right), $$

with mass<sub>pos.</sub> and mass<sub>neg.</sub> (g) representing the masses of the active material for the positive and negative electrodes, respectively. S<sub>pos.</sub> and S<sub>neg.</sub> (mAh) are the relative capacity slippages of the positive and negative electrodes, respectively. The measured capacity curves of full cells were fitted with the model by adjusting the masses and capacity slippages according to the reference curves.
Results and discussion

**Hybrid separators.** Polymer or hybrid polymer/silica separators were synthesized by electrospinning, as illustrated in Scheme 1. The pure polymer electrospun separators (called **PES** hereafter) were produced directly from a solution of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). To obtain hybrid PVDF-HFP/SiO₂ electrospun separators (called **HES** hereafter), tetraethyl orthosilicate (TEOS) was added as a silica precursor to the initial PVDF-HFP solution and hydrolyzed with acidic water. Then, the electrospinning process allowed the simultaneous condensation of silica and precipitation of PVDF-HFP through solvent evaporation. Flexibles homogeneous white membranes were obtained in both cases. For HES, an extremely high content of silica of 37 wt.% was measured by elemental analysis (see experimental section), which corresponds to the theoretical ratio expected for a total conversion of TEOS into SiO₂. SEM images (Fig. 1a) show highly porous membranes (77 vol.% for PES and 82 vol.% for HES, measured by solvent uptake) made of nanofibers with mean diameters of 140 nm and 160 nm for PES and HES respectively (Table 1). In addition, the average fiber diameter can be controlled by modifying the polymer concentration in the solution (Fig. S1). Interestingly, the smooth and homogeneous morphology of the fibers confirms that silica in HES does not form discrete particles onto the fibers but rather forms fractal domains within them.³⁷,³⁸ This feature comes from the *in situ* generation of the silica in acidic sol-gel conditions. A major advantage is that the silica can be incorporated in large amounts while being intimately mixed with the polymer at the nanoscale, which may prevent the leaching of silica particles in the electrolyte without the need for any binder. In addition, as the fiber contains both polymer and SiO₂, the separator's thickness is mainly controlled by the volume of electrospun solution; the value is about 40-50 µm for both separators.

<figure><img src="image_5.png" /><figcaption>Scheme 1: schematic representation of the method to obtain pure polymer electrospun separators (PES) and hybrid electrospun separators (HES).</figcaption></figure>

The hybridization of PVDF-HFP with silica drastically enhanced the thermal stability and the flame resistance of the separator. The evolution of the shrinkage with the temperature radically differs from PES to HES (Fig. 1b and 1c). For PES, the separator began to shrink at
120°C and reached a shrinkage of 91% at 160°C. For HES, almost no shrinkage was observed up to 160°C, and it only reached 12% at 200°C. SEM images of PES and HES after 30 min at 160°C confirm the macroscopic observations: while fibers completely melted for PES, the microstructure of HES did not change (Fig. 1b). This improvement of PVDF-HFP nanofibers' thermal stability matches the best value recently reported by Shi *et al.* (see Table S1 for a comparison of representative composite separators).¹⁶ This enhanced thermal stability most probably come from the high content of silica and the organization of the silica domains. Differential Scanning Calorimetry (DSC) analyses were performed to gain more insight into the separators thermal properties. Surprisingly, the range of melting temperature was found similar for PES and HES (120-160°C, Fig. S2). Thus, the lower shrinkage observed for HES cannot be accounted for an increased melting temperature of the PVDF-HFP in the hybrid fibers. Similar observations for PVDF-HFP/SiO₂ composite membranes were reported in the literature, with reduced shrinkage but similar melting temperature.¹⁴,³⁹ This may come from a protective effect of the inorganic domains that prevent the fiber from merging despite the melting of the polymer. In addition, HES showed higher flame resistance than PES. PES melted and burned as soon as the flame approached, whereas HES burned much more slowly (Fig. 1d). After 25 s of flame contact, a fragile residual framework is obtained, which is most likely the residual silica network.

<table><caption>Table 1: properties of polymer (PES) and hybrid (HES) separators</caption><thead><tr><th></th><th>Porosity <sup>a</sup></th><th>Mean fibers<br>diameter /<br>nm</th><th>Young<br>Modulus<br>/ Mpa</th><th>Tensile<br>strenght /<br>Mpa</th><th>Cristallinity of<br>PVDF-HFP <sup>b</sup></th></tr></thead><tbody><tr><td>PES</td><td>77.0% ± 1.2%</td><td>140</td><td>21 ± 3</td><td>7.5 ± 1.9</td><td>25 %</td></tr><tr><td>HES</td><td>81.6% ± 0.4%</td><td>160</td><td>99 ± 3</td><td>5.5 ± 0.8</td><td>9 %</td></tr></tbody></table>

<footnote><sup>a</sup>measured by solvent uptake (dodecane); <sup>b</sup>calculated from the fusion enthalpy measured by DSC (with ΔH<sub>f</sub> = 104.7 J.g<sup>-1</sup> for 100% crystalline PVDF-HFP)</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 1: Thermal and mechanical properties of PVDF-HFP electrospun separators (PES) and hybrid electrospun separators (HES). a) SEM images of the separators at room temperature. b) Selected images of the separators at different temperatures and c) corresponding evolution of the shrinkage after 30 min at different temperatures. d) Flame test images of PES and HES. e) Typical stress-strain curves of PES and HES at 30°C (elongation rate = 2%.s⁻¹).</figcaption></figure>

Incorporating silica within the fibers also greatly increased the stiffness of the separators. Stress-strain tests were performed on PES and HES (Fig. 1e). HES presents a much higher Young's modulus than PES due to the presence of silica: 99 MPa and 21 MPa respectively (Table 1). This represents an increase of 370%, largely surpassing previously reported values (Table S1). Interestingly, Raghavan *et al.* also synthesized hybrid electrospun separators made of PVDF-HFP and *in situ* generated silica, but the increase in the Young modulus was only +70% with 9 wt.% of silica.⁴⁰ It may arise from the alkaline sol-gel conditions they used, favoring the formation of discrete silica particles, whereas the acidic conditions in this study tend to from extended silica networks.⁴¹ To compare both approaches, we designed a new hybrid separator that incorporates 9 wt.% of silica generated *in situ* under acidic conditions. The two strategies resulted in separators with similar Young's Modulus: 15.7 and 18 MPa for alkali and acidic conditions, respectively (Table S2). Furthermore, decreasing the silica content of HES from 37 to 9 wt.% markedly decreased the thermal stability of the separators (Fig. S3). Therefore, significant enhancements in the thermal and mechanical properties of HES are only achieved when high amounts of silica are incorporated into the fibers. The acidic *in situ* strategy allows for the homogeneous implementation of such high contents. However, HES presents slightly lower elongation at break and ultimate tensile strength than PES, which may come from the lower content of polymer in the hybrid separator and its lower crystallinity (9% for HES vs 25% for PES, Table 1).

**Heterostructure separators.** In order to benefit from both excellent thermal stability and high stiffness of HES, and superior elongation at break and tensile strength of PES, we designed multilayers separators. The versatility of the electrospinning process allowed us to prepare two different 3-layers architectures, as depicted on Fig. 2. **PES@HES** has a central
layer of polymer fibers and two outer layers of hybrid fibers, while **HES@PES** has a central layer of hybrid fibers sandwiched between two layers of polymer fibers.

The thermal shrinkage values for PES@HES and HES@PES are in between those of pure PES and HES (Fig. S4a). At 200°C for instance, PES@HES and HES@PES shrink by 50% and 61% respectively. Although lower than for PES, these values remain rather high. One possible explanation is the very high initial porosity of the separators. The porosity of the multilayer separators was thus reduced by hot-pressing. For both architectures, the evolution of shrinkage and porosity with temperature was compared before and after pressing (Fig. S4b and S4c). For all separators, shrinkage values increase and the porosity decreases in the 140-180°C range, corresponding to the melting of the pure polymer layers. As expected, pressing separators reduced their maximal shrinkage, down to 23% and 49% for PES@HES and HES@PES respectively. Interestingly, the dimensions of the pressed separators then reach a plateau and do not change up to 200°C. This may arise from the high thermal stability of the hybrid layers limiting any further evolution.

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<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

Fig. 2: Schemes of the multilayers separators with alternating layers of pure PVDF-HFP fibers (PES) or hybrid PVDF-HFP and SiO₂ fibers (HES): a) scheme of PES@HES, b) scheme of HES@PES. Evolution with temperature of shrinkage, porosity and resistance for the pressed multilayers separators: c) PES@HES, d) HES@PES. For PES@HES, the increase of resistance from 9 Ω to 168 Ω. between 140°C and 180°C is equivalent to a reduction in conductivity from 4.2 10⁻⁴ S.cm⁻¹ to 2.2 10⁻⁵ S.cm⁻¹. The thickness of the separators is about 40-50 µm and the surface area is 1.33 cm².
The evolution of the shrinkage, porosity and ionic resistance for pressed PES@HES and HES@PES were then investigated (Fig. 2c and 2d). The resistance was measured by electrochemical impedance spectroscopy (EIS), by extrapolation of the capacitive tail from the Nyquist plot (Fig. S5). Between 30°C and 140°C, the evolution of shrinkage, porosity and resistance are negligible for both separators. Then, between 140°C and 200°C, the shrinkage values for pressed PES@HES slightly increase up to 23% while the resistance drastically increases from 9 Ω to 168 Ω. This is equivalent to a reduction in the separator conductivity from 4.2 10⁻⁴ S.cm⁻¹ to 2.2 10⁻⁵ S.cm⁻¹, taking into account a constant thickness and surface area of 50 µm and 1.33 cm², respectively. Overall, PES@HES exhibit a better thermal stability at high temperature compared to HES@PES. The less pronounced increase in resistance (from 10 Ω to 38 Ω) for HES@PES can be a sign of its poor shutdown effect. This may come from the difference in architecture, the location of heat-resistant hybrid layers outside being more efficient than inside. These results show that the properties of separators can be tuned by a proper structural engineering.

Even though a separator inside a battery pack experiences high pressures at mildly high temperatures, it can be misleading to predict its behavior based on the results obtained from a hot-pressed sample. Therefore, a shutdown effect experiment was performed on PES@HES, HES@PES, as well as PES and HES (Fig. S7). In order to facilitate the analysis procedure of the obtained results, the time values at the last step of the experiments are converted to their corresponding temperatures and the figures are redrawn and shown in Fig. 3. The cell which was constructed using PES (shown in Fig. 3a) revealed a linear increase of current starting at around 86 °C. This behavior can be due to the gradual closure of the porosity of the separator. The completed closure of all porosity was achieved around 106 °C, where abrupt current fluctuations can be seen. On the other hand, HES showed a different trend (Fig. 3b). No significant current variations were detected until 115 °C. The current value at this temperature started to increase and the cell experienced current fluctuations immediately at 117 °C. The absence of a gradual shutdown process for HES confirms the effect of the fractal domains of silica, which prevents the structural deformation and hence postponing the shutdown temperature to around 117 °C. As expected from our previous experiments, the shutdown effect performance of HES@PES and PES@HES (Fig. 3c and Fig. 3d, respectively) had characteristic behavior of both PES and HES. While both samples showed gradual current increase at similar temperatures to PES, their main current fluctuations (i.e. complete shutdown) happened at around 120 °C, which is comparable to the shutdown temperature of HES. Another important feature of these results is that no current fluctuation is observed at characteristic shutdown temperature of PES (around 105 °C), confirming that the presence of HES layer(s) in the heterostructures promotes the structural stability of adjacent PES layer(s) too.
<figure><img src="image_11.png" /><figcaption>Fig. 3: The evolution of the current for the cells assembled using a) PES, b) HES, c) HES@PES, and d) PES@HES separators with respect to temperature.</figcaption></figure>

**Trapping functions.** As HES exhibit interesting thermal behavior, HES separators were then functionalized with a variety of chemical moieties to trap HF and dissolved transition metal cations to avoid their deposition on the negative electrode. Different approaches were explored using commercial precursors (Fig. 4). Lithium sulfonate $-SO_3Li$ functions were introduced via the co-condensation of TEOS and a sulfonated silane before electrospinning. The acidic protons were then exchanged with lithium to obtain $HES-SO_3Li$. Terpyridine molecules were added to the PVDF/TEOS solution before electrospinning to directly obtain $HES@terpy$. In this case, terpyridine molecules are simply encapsulated in the hybrid fibers without any covalent bond. Post-grafting strategies were employed to functionalize the hybrid HES fibers with either amine or diamine moieties to obtain $HES-NH_2$ and $HES-NH-NH_2$ respectively. For these amino-functionalized separators, the post-grafting method was chosen instead of co-condensation due to the fast condensation rate of aminosiloxane molecules.
<figure><img src="image_12.png" /><figcaption>Fig. 4: Schemes of the synthesis route of the functional hybrid separators and their respective SEM images: a) HES-SO₃Li, b) HES@terpy, c) HES-NH₂, d) HES-NH-NH₂.</figcaption></figure>

The morphologies of the hybrid separators can be observed on SEM images (Fig. 4). All separators present fibers with a high porosity of 75%-85% (Table S3), except HES-SO₃Li, which has a slightly lower porosity of 65%, possibly due to the larger fibers (670 nm for HES-SO₃Li, 170-250 nm for the other functionalized separators, Table S3). The fibers are smooth and no large aggregates were observed, suggesting for a homogeneous functionalization. Only HES-NH₂ presents tiny aggregates mainly localized at fibers interconnections, which could be attributed to the amino-silica domains. These aggregates were not observed for HES-NH-NH₂ because the precursor bears only two hydrolysable alkoxy functions, preventing the formation of a three dimensional network.

The success of the chemical functionalization was confirmed by infrared spectroscopy (FTIR), elemental analysis and X-ray photoelectron spectroscopy (XPS). The signal of the functional molecules could be detected by FTIR in the final separators for HES-SO₃Li, HES@terpy and HES-NH₂ (Fig. S8) confirming the introduction of the functions. The quantity of functions was then determined by measuring the C, N and S content of the materials with CHNS elemental analysis. The lowest functionalization rate was 0.48 mmol<sub>terpy</sub>/g<sub>mat</sub> for HES@terpy and the highest was 1.86 mmol<sub>SO₃H</sub>·g<sub>mat</sub><sup>-1</sup> for HES-SO₃Li (Table 2), which compares well with the ionic exchange capacity of conventional cation exchange membranes.<sup>42</sup> HES-NH₂ and HES-NH-NH₂ have a similar density of amino functions of 0.66 and 0.84 mmol·g<sub>mat</sub><sup>-1</sup> respectively. Besides, information about the localization of the functions can be deduced from the comparison of XPS elemental analysis with CHNS results. While CHNS analysis gives the total S/C or N/C ratio in the separator, XPS analysis determines the S/C or N/C ratio at the
surface of the fiber. For HES@terpy, the surface N/C ratio is 24 times lower than the total N/C ratio, meaning that most of the terpyridine molecules are located inside the fibers (Table 2). On the contrary, the surface N/C ratio is higher than the total N/C ratio for HES-NH₂ and HES-NH-NH₂, which confirms that amine functions are preferentially localized at the surface of the fibers, as expected from a post-grafting strategy.

<table><caption>Table 2: density of functions in the functionalized hybrid separators</caption><thead><tr><th></th><th>Functions introduced</th><th>Total functions measured <sup>a</sup></th><th>Total S/C or N/C atom. ratio <sup>b</sup></th><th>Surface S/C or N/C atom. ratio <sup>c</sup></th></tr></thead><tbody><tr><td>HES-SO<sub>3</sub>Li</td><td>2.09 mmol<sub>SO<sub>3</sub>H</sub>/g<sub>mat</sub></td><td>1.86 mmol<sub>SO<sub>3</sub>H</sub>/g</td><td>0.068</td><td>0.019</td></tr><tr><td>HES-NH<sub>2</sub></td><td>2.61 mmol<sub>NH<sub>2</sub></sub>/g<sub>mat</sub></td><td>0.66 mmol<sub>NH<sub>2</sub></sub>/g</td><td>0.022</td><td>0.035</td></tr><tr><td>HES-NH-NH<sub>2</sub></td><td>excess</td><td>0.84 mmol<sub>NH</sub>/g</td><td>0.036</td><td>0.043</td></tr><tr><td>HES terpy</td><td>0.44 mmol<sub>terpy</sub>/g<sub>mat</sub></td><td>0.48 mmol<sub>terpy</sub>/g</td><td>0.048</td><td>0.002</td></tr></tbody></table>

The ionic conductivity of the separators with electrolyte (1M LiPF₆ in EC/EMC) was evaluated by electrochemical impedance spectroscopy (EIS) to determine the influence of the chemical functionalization (Table S4). Because of the very low values of impedance, only the capacitive tail due to the ion-blocking electrodes are visible on Nyquist plots (Fig. S9). All the electrospun separators show high ionic conductivities ranking from 0.9 to 1.8 mS.cm⁻¹, which are higher than for the commercial microporous polyolefin separator Celgard 2400 (0.7 mS.cm⁻¹). This may be attributed to the much higher porosity of the electrospun separators (65% - 85%, Table S3) compared to the Celgard 2400 (41%). Interestingly, among the electrospun separators, all hybrid electrospun separators show higher ionic conductivities than the pure polymer electrospun separator (PES). This comes from an improved wettability due to the presence of silica domains, as previously reported in the literature.¹²,⁴³ Contact angle measurements were done to investigate the differences in wettability, but a ~0° angle was measured for all the electrospun separators, most probably due to their fiber-based morphology with high porosity and low tortuosity rather than their chemical composition.

The membranes were tested as separators in LiNiO₂ || 1M LiPF₆, EC/EMC 30/70 || graphite coin cells. The cells were cycled using an accelerated cycling protocol with a high cut-off voltage of 4.5V (description of protocol in experimental section and in Fig. S9). First, the N 1s spectra of the HES-NH₂ and HES-NH-NH₂ were recorded before and after cycling to analyze the chemical evolution of amine functions and their capacity to trap HF (Fig. 5). For both HES-NH₂ and HES-NH-NH₂, there is a clear increase in the ratio of NH₃⁺ vs NH₂ functions after cycling: from 49% to 61% for HES-NH₂ and from 11% to 18% for HES-NH-NH₂ (Table 3). This evolution must likely come from the trapping of HF species generated in the electrolyte. The amount of captured protons was estimated to 79 and 59 µmol.g⁻¹ for HES-NH₂ and HES-NH-NH₂

<footnote><sup>a</sup> density of functions measured by CHNS elemental analysis
<sup>b</sup> Total atomic ratio of S/C for HES-SO₃Li and N/C for the other separators, calculated from the mass ratio obtained by CHNS elemental analysis
<sup>c</sup> Surface atomic ratio of S/C for HES-SO₃Li and N/C for the other separators, measured by XPS.</footnote>
respectively considering the quantity of amino functions. For HES@terpy, a similar qualitative evolution could be observed, but the low signal of N 1s before cycling did not allow for a quantitative analysis (data not shown). To the best of our knowledge, this is the first direct evidence of the *in situ* scavenging ability of a functional separator towards acidic species. In addition, the S 2p spectra of the ion-exchange HES-SO₃Li separator were compared before and after the cycling tests (Fig. 5, right column). Both spectra show a double peak due to the spin-orbit splitting, with a shift of the S 2p₃/₂ peak from 168.4 eV before cycling to 169.7 eV after cycling. This shift may come from the exchange of Li⁺ with Ni²⁺ cations on the sulfonate functions, suggesting the trapping of dissolved Ni²⁺ cations on the functional separator. Taking into account that all the –SO₃⁻ functions incorporated in the separator are accessible to the electrolyte and that two –SO₃⁻ functions are needed to trap one Ni²⁺ cation, we estimated that up to 0.93 mmol·g⁻¹ of Ni²⁺ may be scavenged by the separator.

<figure><img src="image_13.png" /><figcaption>Fig. 5: XPS N 1s spectra of HES-NH₂ (left) and HES-NH-NH₂ (middle) amino-functionalized hybrid separators before and after cycling, with deconvolution of –NH₂ and –NH₃⁺ peaks. XPS S 2p spectra of HES-SO₃Li before and after cycling (right). LiNiO₂ || 1M LiPF₆, EC/EMC 30/70 || graphite coin cells.</figcaption></figure>

<table><caption>Table 3: Ratio of amine -NH₂ and ammonium -NH₃⁺ functions measured by XPS on amino-functionalized hybrid separators, before and after cycling.</caption><thead><tr><th rowspan="2"></th><th colspan="2">HES-NH<sub>2</sub></th><th colspan="2">HES-NH-NH<sub>2</sub></th></tr><tr><th>% NH<sub>2</sub></th><th>% NH<sub>3</sub><sup>+</sup></th><th>% NH<sub>2</sub></th><th>% NH<sub>3</sub><sup>+</sup></th></tr></thead><tbody><tr><td>Before cycling</td><td>51%</td><td>49%</td><td>89%</td><td>11%</td></tr><tr><td>After cycling</td><td>39%</td><td>61%</td><td>82%</td><td>18%</td></tr></tbody></table>

The graphite contribution to the capacity loss was measured by the slippage method proposed by Bloom *et al.*³⁴,³⁵ and further developed by Dahn *et al.*³⁶ (see experimental
section). The amount of Ni in the graphite after cycling was then measured by inductively
coupled plasma mass spectrometry (ICP-MS) and plotted against the capacity loss linked to
the negative electrode in Fig. 6. The specific contribution of the negative electrode was
determined by the analysis of the cell capacity slippages; this capacity loss is attributed to the
depletion of lithium inventory because of the solid electrolyte interface (SEI) growth and
destabilization. Except for HES-NH₂, a linear correlation is observed between the amount of
Ni in graphite and the capacity loss related to the negative electrode. This result supports the
theory that the crossover of Ni cations and its deposition on the graphite causes a depletion
of lithium ions and thus decreases the capacity. Leung ascribed such effect to catalytic
reactions occurring at the metallic centers deposited in the solid electrolyte interface in the
case of Mn crossover.⁴⁴ Unexpectedly, cells cycled with HES-NH₂ and HES-NH-NH₂ have both
higher capacity losses and Ni amounts in graphite than the other cells. This result contrasts
with reported works in which polyamine-based separators such as poly(divinylbenzene-co-4-
vinylpyridine)²¹,²² or polyvinylpyrrolidone²⁴,²⁵ showed better capacity retention. Likewise,
Woo et al. did not observe a lower capacity retention for an amino-functionalized hybrid
separator compared to the bare separator.¹⁹ This discrepancy may come from the initial
presence of already protonated –NH₃⁺ functions in HES-NH₂ and HES-NH-NH₂ evidenced by
XPS, which could favor the degradation of LiPF₆ and the formation of HF. It would explain the
high capacity loss of cells made with HES-NH₂, in which half of the amine functions are
protonated before the cell assembly (Table 3). For HES-SO₃Li, the amount of Ni measured in
the graphite after cycling (692 ppm, 4.1 µg.cm⁻²) was lower than that for HES (863 ppm, 5.1
µg.cm⁻²), which can be attributed to the trapping of Ni²⁺ on sulfonate functions, as suggested
by the XPS results. At last, only HES@terpy shows both lower capacity loss and final amount
of Ni in the graphite than the pure polymer separator PES. This confirms the ability of
terpyridine to trap metallic cations, as demonstrated by Meier et al. in a solution containing
PF₆⁻ as counter anions.⁴⁵ Interestingly, this study showed the relationship between the stability
of the complex formed and the nature of the central metal ion, with Ni complexes being more
stables than Cu and Mn complexes. Additionally, the better capacity retention is consistent
with previous reports in which pyridine-based molecules have showed positive effects on
capacity retention when grafted on separators²⁴ or simply incorporated in the polymer,²³ as
for HES@terpy. Similarly, Banerjee et al.²⁰ reported a higher trapping capacity of separators
mixed with EDTA than polyamine based separators. It indicates that the chemical moities are
accessible even when incorporated within the fiber; thus, the nature and scavenging ability of
the functions may be more important than their location.
<figure><img src="image_14.png" /><figcaption>Fig. 6: Capacity loss measured by the slippage method of LiNiO₂ || 1M LiPF₆, EC/EMC 30/70 || graphite coin cells versus the amount of Ni in graphite measured by ICP after cycling.</figcaption></figure>

Combining the results obtained by derivative voltage analysis (DVA) and Coulombic efficiency (CE), the following conclusions can be drawn. While CE values greater than 99% are obtained after 3 cycles for HES@NH-NH₂ or HES@NH₂, voltage slippage of the positive electrode associated with a degradation at the negative electrode is observed even after cycle 10, indicating that despite a good CE, a parasitic reaction is still modifying the balancing of the cell. The opposite trend is obtained for HES@SO₃Li with no slippage observed after 10 cycles, but a CE of only 90% recorded during the initial cycles, indicating copious reactions at the beginning of cycling. Instead, HES@terpy shows during the initial cycle a limited slippage associated with a CE that reaches quickly 99%, indicating that long-term cycling can be obtained with a cutoff voltage of 4.5V for this separator.

Conclusions

We demonstrated herein a simple strategy to design hybrid functional separator with high thermal stability, shutdown effect, and HF scavenging or cations trapping functions. The combination of sol-gel chemistry with the electrospinning process allowed for the synthesis of hybrid nanofibers made of 37 wt.% amorphous silica having excellent thermal stability. The versatility of the process was exploited to design multilayer separators exhibiting both shutdown effect and low shrinkage: while polymer layers melt and close the porosity, hybrid
layers ensure the dimensional stability. The shutdown experiments in real cells show the interest of having a hybrid separator in the separator design. Next, various amine or sulfonate based functions were successfully introduced in the hybrid fibers by encapsulation or covalent linking to the silica domains. The functions were localized within the fibers or at their surface depending on the functionalization strategy. The separators were then used in LNO|graphite cells which were cycled in accelerated stress tests. XPS analysis on *post mortem* amine functionalized separators showed an increase in the NH₃⁺/NH₂ ratio, proving the ability of these separators to trap acidic species such as HF in the liquid electrolyte. However, the final amount of Ni in graphite and the capacity loss were higher for these separators probably because of the initial presence of NH₃⁺ functions, which may have a detrimental effect; thus, careful attention should be paid to the initial protonation state of amino functionalized separators when designing such materials. Finally, the capacity loss measured was correlated with the amount of Ni measured in the graphite after cycling. The hybrid separator with encapsulated terpyridine molecules showed the lowest capacity loss and amount of Ni.

Clearly, the integration of functions to hybrid separator by electrospinning constitutes an interesting tool to enhance the battery safety and to suppress performance degradation related to all sorts of acid-related parasitic reactions. This could be beneficial for LIB performances containing NMC cathode and could be extended to broader classes of cell chemistries including Li-S batteries, with functional separator inhibiting the shuttle effect of polysulfide species based on electrostatic repulsion.

## Associated content

The Supporting Information describes the detailed characterization of the electrospun separators including: the thermal stability shrinkage and porosity of hybrid and multilayered separators, the Nyquist plots for all EIS analyses, the textural properties of the functional separators and their infrared spectra, the description of the accelerated cycling protocol.

## Author information

### Corresponding author

**Christel Laberty-Robert** - Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC Paris 06, 4 Place Jussieu, 75005 Paris, France.
https://orcid.org/0000-0003-3230-3164

### Authors

**Jason Richard** - Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC Paris 06, 4 Place Jussieu, 75005 Paris, France. https://orcid.org/0000-0002-8107-7688
**Navid Solati** - Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC Paris 06, 4 Place Jussieu, 75005 Paris, France. https://orcid.org/0000-0003-4981-7911

**Arvinder Singh** - Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC Paris 06, 4 Place Jussieu, 75005 Paris, France. https://orcid.org/0000-0001-9904-2013

**Valentin Meunier** - Chimie du Solide et de l'Energie (CSE), Collège de France, UMR 8260, 75231 Paris Cedex 05, France

**Yoko Toda** - Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC Paris 06, 4 Place Jussieu, 75005 Paris, France

**Alexis Grimaud** - Chimie du Solide et de l'Energie (CSE), Collège de France, UMR 8260, 75231 Paris Cedex 05, France. https://orcid.org/0000-0002-9966-205X

**Arnaud Perez** - Laboratoire Chimie de la Matière Condensée de Paris LCMCP, Sorbonne Université, UPMC Paris 06, 4 Place Jussieu, 75005 Paris, France. https://orcid.org/0000-0003-1659-554X

## Notes

The authors declare no competing financial interest.

## Acknowledgments

This work was supported by College de France. V. Meunier and A. Grimaud thank the European Union's horizon 2020 research and innovation program under grant agreement 57189, BIG-MAP for its support. The authors warmly thank Antoine MICHE from LRS for XPS analysis and discussion, Laurence ROZES for discussion about thermal and mechanical properties of hybrid materials and François BAUDIN for CHNS analyses. The authors thank Jean-Marie TARASCON for his inspiring discussions on batteries. The authors acknowledge the French National Research Agency for its support through the Labex STORE-EX project (ANR-10LABX-76-01).

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