# Lithium Nitrate as a SEI-Stabilizing Additive in Single-Ion Conducting Polymer Electrolytes for Lithium–Metal Batteries

Yunfan Shao, Tony Bertaux, Kai Shi, Maider Zarrabeitia, Fannie Alloin,
Dominic Bresser, Cristina Iojoiu

► To cite this version:

Yunfan Shao, Tony Bertaux, Kai Shi, Maider Zarrabeitia, Fannie Alloin, et al.. Lithium Nitrate as a SEI-Stabilizing Additive in Single-Ion Conducting Polymer Electrolytes for Lithium-Metal Batteries. ACS Applied Materials & Interfaces, 2026, 18 (4), pp.6912-6925. 10.1021/acsami.5c21796 . hal-05526085

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# Lithium Nitrate as SEI-Stabilizing Additive in Single-Ion Conducting Polymer Electrolytes for Lithium-Metal Batteries

Yunfan Shao¹, Tony Bertaux¹, Kai Shi²,³, Maider Zarrabeitia²,³, Fannie Alloin¹,⁵, Dominic Bresser²,³,⁴, Cristina Iojoiu¹,⁵,*

¹ Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France

² Helmholtz Institute Ulm (HIU), 89081 Ulm, Germany

³ Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany

⁴ Ulm University (UUlm), 89069 Ulm, Germany

⁵ Réseau sur le Stockage Electrochimique de l'Energie (RS2E), CNRS, FR3459, 80039 Amiens Cedex,
France

Corresponding author e-mail: [Cristina.Iojoiu@grenoble-inp.fr](mailto:Cristina.Iojoiu@grenoble-inp.fr)

Keywords: lithium metal, single ion conducting polymer electrolyte, solid electrolyte interphase, LiNO₃ additive, all solid batteries

## Abstract

Single-ion conducting polymer electrolytes (SIPEs) are among the most promising candidates to realize lithium-metal batteries, which commonly suffer from the risk of lithium dendrite growth and continuous electrolyte decomposition at the lithium-electrolyte interface. Herein, we report the introduction of lithium nitrate (LiNO₃) as an additive into a SIPE based on a poly(ethylene oxide) backbone and grafted perfluorosulfonate anions. By comparing SIPEs with and without LiNO₃ concerning, e.g., the charge transport, electrochemical stability, reversibility of the lithium stripping/plating process, electrode morphology and surface composition, it appears that the introduction of LiNO₃ yields a thinner, but more robust solid electrolyte interphase, which greatly benefits the eventual performance, Coulombic efficiency, and reversibility of the lithium stripping and plating. The advantageous effect of LiNO₃ is finally confirmed in Li|SIPE|LFP solid-state battery cells providing a substantially longer cycle life.
# 1. Introduction

In pursuing even higher energy densities than provided presently by lithium-ion batteries, the potential use of lithium metal at the negative electrode has attracted great interest in recent years owing to its high specific capacity of 3860 mAh g⁻¹ and low redox potential of -3.04 (V vs. SHE).¹,² However, after decades of studies, lithium metal electrodes are still only used in very few commercial batteries, as lithium metal suffers a series of issues such as the risk of dendrite growth,³,⁴ the formation of an inhomogeneous and continuously growing solid electrolyte interphase (SEI),⁵,⁶ the formation and accumulation of “dead” Li,⁷ and frequently a rather poor Coulombic efficiency (CE).⁸

Solid polymer electrolytes are considered a promising electrolyte class to mitigate the aforementioned challenges.⁹,¹⁰ Among them, single-ion conducting polymer electrolytes (SIPEs) have shown excellent performance and have proven effective in suppressing dendritic lithium deposition, benefitting from their high lithium transference number ($T_{Li+}$), commonly approaching unity, thanks to the immobilization of the anionic functional groups to the polymer backbone by covalent tethering.¹¹,¹² Accordingly, the lithium cation becomes the only “free” ion in the electrolyte, which hinders the formation of a concentration gradient in the electrolyte, thus, inhibiting lithium dendrite formation, as predicted by the models of Chazalviel¹³ and Tikekar¹⁴.

However, lithium metal still reacts with (presumably all) polymer electrolytes to some extent, resulting in the formation of an SEI.¹⁵,¹⁶ As for liquid electrolytes, the nature of the SEI has a crucial impact on the battery performance. It is commonly considered important to form a thin, stable and highly cation conducting SEI to effectively and efficiently passivate the surface of the lithium metal electrode and prevent further degradation of both the electrolyte and the lithium metal electrode. Contrarily, a thick and inhomogeneous SEI is commonly expected to significantly increase the interfacial resistance

and to reduce the reversibility of the lithium plating/stripping process. Additionally, the nature of the SEI and the $Li^+$ diffusivity within/across the SEI is critical for the eventual performance of the SIPE in the battery cell, as it is frequently the bottleneck for the overall charge transport, especially at elevated current densities.¹⁷

Hence, the control of the SEI formation on lithium metal electrodes in combination with both liquid and solid electrolytes has received a lot of attention. Common strategies include coating the lithium surface with a protective layer,¹⁸⁻²⁰ adding functional additives,²¹ and using lithium alloys.²² The use of additives in liquid electrolytes is a very easy and cost-efficient approach to tune the SEI and improve its stability. Additives can be organic molecules such as vinylene carbonate (VC),²³ fluoroethylene carbonate (FEC),²⁴ or lithium salts such as lithium nitrate ($LiNO_3$),²⁵⁻²⁷ lithium bis(oxalato)borate ($LiBOB$),²⁸⁻³⁰ $LiF^{31}$ or even ammonium salts.³² Additionally, lithium nitrate ($LiNO_3$) has been applied widely in Li-S batteries as an anti-shuttle agent by efficiently passivating the lithium electrode in ether-based electrolytes³³ and $Li-O_2$ batteries.³⁴,³⁵ Recent studies also unrevealed its effect in eliminating water hazard³⁶ and stabilizing lithium metal electrodes.³⁷,³⁸ However, the use of additives has been limited so far to liquid electrolytes basically, as they need to be dissolved in the solvating medium.

Although most studies on SIPEs claim the suppression of lithium dendrite formation, only rather few provide a comprehensive investigation of the reactions occurring at the interface with the lithium-metal electrode.³⁹,⁴⁰ Indeed, for polymer electrolytes, the surface monitoring and analysis is much more challenging, as it requires the careful separation of the two phases without causing any damage to the lithium-metal surface and the interphase.⁴¹

Herein, we examined the interfacial reaction between lithium metal and a newly developed SIPE as well as the impact of incorporating $LiNO_3$ as functional additive to enhance our understanding of the SEI in such cases and its
influence on the electrochemical processes. The results show that the incorporation of LiNO₃ into the SIPE leads to a reduced electrolyte decomposition by yielding a thinner and more effective SEI and, thus, an improved reversibility and efficiency of the lithium stripping/plating process.

## 2. Experimental Section

### 2.1 Materials

Lithium nitrate was purchased from Alfa Aesar. Photoinitiator 2-Hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) was from Ciba. All the chemicals were used without purification.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Structure of I₁₀₀₀p-SO₃-db SIPE.</figcaption></figure>

Carbon coated Al foil was purchased from MSE supplies, thickness of 16 µm (with 1 µm of carbon coating). LFP powder was purchased from MTI corp. Binder PVDF (Solef® 6020) was form Solvay S.A. Conductive carbon black powder (Super C 65) was from TIMCAL.

### 2.2 Electrolyte Preparation

I₁₀₀₀p-SO₃-db SIPE was synthesized followed the protocol in our previous work.⁴² The molecular structure of I₁₀₀₀p-SO₃-db is shown in Scheme 1.

The SIPE was synthesized by a two-step polycondensation reaction described in our previous work,⁴² which yielded an ionic linear

polymer bearing C=C double bond, allowing to form a crosslinked membrane. The linear SIPE was mixed with photoinitiator and LiNO₃ (0, 0.5 and 1.0 wt.%) and cured by UV irradiation, yielding a crosslinked self-standing film. The additive of 0.5 and 1.0 wt.% LiNO₃ in SIPE correspond to ratios between Li sulfonate from SIPE and LiNO₃ of ~10:1 and 5:1, respectively.

The I₁₀₀₀p-SO₃-db SIPE was first dissolved in DI water with 4 wt.% of photoinitiator, 6 wt.% of nanocrystalline cellulose and 0, 0.5, 1 wt.% LiNO₃ and stirred until complete dissolution. The water then removed by freeze drying to yield a homogenous mixture. The mixture was laminated into a thin film with thickness of ~60 µm. The thin film was placed under a mercury arc lamp and crosslinked by UV irradiation for 2 minutes. The crosslinked SIPE membrane was then dried at 80 °C under vacuum for 20 h and stored in an Ar-filled glovebox.

### 2.3 Ionic Conductivity and Lithium Transference Number

Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) of stainless steel (SS)|SIPE|SS cells. The cells were placed in a Vötsch chamber with controlled temperature sequence from 20 to 90 °C with a 10 °C interval. For each temperature, the cell was measured with EIS after 1 h of isothermal process. The impedance spectra were conducted from 7 MHz to 0.1 Hz with a bias potential of 20 mV by a Biologic VMP300. The ionic conductivity is calculated with the following equation:

$$ \sigma = \frac{l}{R_b \cdot A} $$

Where l is the distance between two electrodes, and A is the contact area of electrolyte with electrode, Rb is the bulk resistance of the electrolyte. To confirm the geometric parameters of the membranes, the samples was disassembled
after all the EIS tests and the surface area and thickness was measured again.

The conductivity-temperature relation was fitted with the following VTF equation2,3:

$$ \sigma = \sigma_0 \cdot T^{\frac{1}{2}} \cdot e^{-\frac{E_a}{K_b \cdot (T-T_0)}} $$

where, $\sigma_0$ is the pre-exponential factor which related to the number of charge carriers, $T_0$ is the polymer ideal transition temperature with zero configurational entropy, $E_a$ is pseudo-activation energy and $K_b$ is the Boltzmann constant.

Lithium transference number $T^+$ was measured by Bruce-Vincent method.43 Symmetrical Li|SIPE|Li cells were placed in isothermal chamber at 80 °C and rested for 12 h before measurement. The cell was first conducted with an EIS measurement form 7 MHz to 0.1 Hz with a bias voltage of 20 mV, and then rest for 1 h. Then the cells were conducted with chronoamperometry (CA) with 20 mV for 10 h, and again rest for 1h after polarization. An EIS measurement was taken after the polarization with the same parameters.

The $T^+$ can be calculated by:

$$ T_{Li}^{+} = \frac{I_{ss}(\Delta V - I_{0}R_{0})}{I_{0}(\Delta V - I_{ss}R_{ss})} $$

where $I_{ss}$ is the steady-state current, $I_0$ is initial state current, $R_0$ and $R_{ss}$ were the interfacial resistance of the initial and steady-state.

The $T^+$ was measured twice with pristine Li|SIPE|Li cells and the cells after 80 hour of lithium stripping/plating process to stabilize the Li|SIPE interphase. The Li plating/stripping process consist of 5 cycles (4 h plating and 4 h stripping) of plating/stripping at 10 µA cm⁻² and then 5 cycles at 100 µA cm⁻². The testing protocol is shown in Figure S1.

## 2.4 Electrochemical Characterizations

The electrochemical tests are conducted on a Biologic VMP300. The data was processed with Biologic EC-lab software.

Electrochemical impedance spectra were carried out with a bias potential of 20 mV with a frequency range of 7 MHz to 0.1 Hz. For each frequency, three measurements were performed, and the final spectra represent the average.

The CV of the electrolyte was conducted with coin cell with Li|SIPE|SS configuration. Before the measurements, cells were placed in an isothermal chamber at 80 °C and rest for 12 h. The cells were cycled with a scan rate of 0.2 mV s⁻¹ between 4.0 and -0.5 V (vs. Li⁺/Li) with a Biologic VMP 300 potentiostat. The CVs start at OCV of the cell and scan towards oxidation up to 4.0 V and then reduced to -0.5 V. The amount of lithium plating/stripping during CV scanning was calculated by integration of the corresponding peaks with Biologic EC lab software.

The critical current density was determined by a current scan test on symmetric Li|SIPE|Li cells from 0 to 2 mA with a scan rate of 0.1 µA s⁻¹ at 80 °C.

Lithium stripping/plating experiments were conducted on Li|SIPE|Li and Li|SIPE|Cu cells at 80 °C. In the symmetric lithium stripping/plating experiments, chronopotentiometry was applied with current density of 0.1 mA cm⁻² for 4 h of each charge and discharge. Two EIS was record in every stripping/plating cycle. In the asymmetric lithium stripping/plating experiments, plating on Cu was last for 4 h in each cycle, and followed by stripping until the cell voltage reached 1 V. In modified asymmetric lithium stripping/plating experiments, a precondition cycle of plating (2 mAh cm⁻²) and stripping until 1 V was applied. Followed by another 2 mAh cm⁻² of Li was plated on Cu as lithium reservoir. Subsequently, charge/discharge cycling of 0.1 mAh cm⁻² per cycle was repeated until the cell voltage exceeding 1 V. An EIS was recorded after each stripping/plating.

## 2.5 Raman Spectrometry

Raman spectra was carried out with a Renishaw Virsa Raman spectrometer, equipped with 785 nm laser source and x50 LF objective lens. The
laser power was 10 mW. The sample was prepared in an Ar-filled glove box, and seal with glass window to avoid contact with ambient atmosphere during the measurement.

## 2.6 SEM

SEM images of lithium surface were collected with a ZEISS Gemini SEM. The samples were prepared in Ar-filled glove box and transfer into SEM with airlock protection. Images were collected by SE2 detector with EHT = 5 kV. SEM images was processed by ImageJ software.

## 2.7 XPS

XPS was performed with a SPECS XPS consisting of monochromatic Al Kα radiation (1486.6 eV) and Phoibos 150 spectrometer from Surface Concept. The spectrometer was equipped with a micro channel plate and delay line detector. The cycled Li|SIPE|Li cells were disassembled in an Ar-filled glovebox. The Li electrode and SIPE were transferred to the XPS in an air-tight container. The spectra were collected with an X-ray source power of 200 W (15kV), 30 eV pass energy and 0.1 eV energy steps in a transmission fixed mode.

XPS data was fitted with XPSPEAK4.1 software (written by Raymund W.M. Kwok, The Chinese University of Hong Kong). All the XPS data was calibrated by the hydrocarbon peak at 285 eV before fitting.⁴⁴ The photoelectron energy referenced the NIST X-ray Photoelectron Spectroscopy Database (https://srdata.nist.gov/xps/).

## 2.8 Electrode Preparation and Cell Assembly

The LFP electrodes were prepared with a formulation of LFP:Super C65:PVDF:I₁₀₀₀-SO₃-db = 65:10:5:20. All the ingredients were mixed with mortar and pestle with NMP to form a homogeneous slurry and cast on carbon coated Al foil. The electrode was dried at 80 °C overnight and cut into discs with a diameter of 12 mm. The

electrodes were calendared to reduce the porosity and dried again under vacuum. The active material mass loading was about 1.9 - 2.0 mg cm⁻².

Inert electrodes including stainless-steel and Cu was cleaned with ethanol before use.

All the cells with SIPE in this work, including CV cells, symmetrical Li||Li cells, Li|SIPE|Cu cells, and LMB cells were assembled in CR2023-type coin cells in an Ar-filled glovebox.

## 2.9 Data Processing

The data was processed with Microsoft Office suite and open-source python package pandas (https://pandas.pydata.org/). All the data was plotted with matplotlib, which is an open-source 2D graphics package used in python for data visualization.

# 3. Results and Discussion

## 3.1 Charge Transport

The ionic conductivity and lithium transference number of the crosslinked neat SIPE (Scheme 2) and in presence of 0.5 and 1.0 wt.% LiNO₃ was determined to understand the impact of the addition of free ionic species. As expected, the conductivity-temperature dependencies follow a VTF-type behavior.⁴⁵ As shown in Figure 1a, the ionic conductivity of the LiNO₃-comprising SIPEs exhibited almost the same as the bare SIPE at elevated temperatures (≥ 60 °C), e.g., 4×10⁻⁵ S cm⁻¹ at 90 °C. At lower temperatures, the LiNO₃-comprising SIPEs showed a slightly lower ionic conductivity than the neat SIPE with 7.5×10⁻⁷, 7.0×10⁻⁷ and 6.3×10⁻⁷ S cm⁻¹ for the 0, 0.5, 1.0 wt.% LiNO₃-comprising SIPEs at 20 °C, respectively, due probably to the interactions of LiNO₃ and polymer chains. The interactions between additional LiNO₃ and SIPE chains were investigated using differential scanning calorimetry (DSC) and Raman spectroscopy. In DSC thermogram (Figure S2), a glass transition
temperature was observed at -38 °C indicating that polymer chain mobility remains unchanged upon the addition of these small amounts of LiNO₃. To further elucidate the dissociation of LiNO₃ in the SIPE medium, Raman spectra were recorded for pure LiNO₃ salt and SIPE samples containing different concentrations of LiNO₃ (0.5, 1.0, 5.0 and 10.0 wt.%) as shown in Figure S3. The spectra between 750-1100 cm⁻¹ (Figure S4) were normalized using the aromatic rings band at 995 cm⁻¹ ⁴⁶ which characteristic of the bare SIPE polymer. The non-degenerate stretching of the NO₃⁻ anion, observed between 1060-1020 cm⁻¹ (overlapped with the SO₃⁻ anion from SIPE), increase in intensity with the concentration of LiNO₃ salt. However, at lower concentrations (0.5 and 1 wt.%), the Raman spectra remained almost unchanged, suggesting negligible interactions between LiNO₃ and SIPE. Additionally, the interaction between the polymer chains and Li⁺ ions was monitored in the 880-770 cm⁻¹ region⁴⁷,⁴⁸ corresponding to ether-coordinated Li⁺. A clear increase in the peak at 860 cm⁻¹ was observed becoming particularly visible for the 5.0 and 10.0 wt.% samples. This reflects an increase in interaction with salt concentration and, consequently, salt dissolution.⁴⁹ A distinct bump at 1070 cm⁻¹ also appeared with increasing salt concentration, especially at 10 wt.%, indicating the formation of LiNO₃ aggregates.⁵⁰ Overall, both DSC and Raman spectroscopy measurements confirm that the small amounts of 0.5, 1.0 wt.% LiNO₃ added are effectively dissolved and dissociated within the SIPE matrix but due to very small doesn't affect significantly the SIPE chain and ion mobilities as well as ion concentration.

Given the slightly higher ionic conductivity for the lower LiNO₃ content, we focused first on the neat SIPE and the SIPE containing 0.5 wt.% LiNO₃ for the basic electrochemical characterization.

The Li⁺ transference number was determined using the Bruce-Vincent method which yielded a very high Li⁺ transference number of 0.98 for the 0.5 wt.% LiNO₃-comprising SIPE in the pristine

state and essentially the same value with 0.96 after 80 h of lithium plating/stripping, indicating the formation of a stable Li|SIPE interface as reflected by no obvious change during the polarization steps (see Figure S5-6). The remarkable number of 0.96, which is very close to the bare SIPE (>0.99), indicates that also when introducing LiNO₃ as additive, the electrolyte could maintain the single-ion conductivity, presumably due to the low dissociation and/or high transference number of LiNO₃.⁵¹

## 3.2 Electrochemical Stability

As apparent from the determination of the transference number, the interfacial resistance of the cells was evolving over several days, suggesting the formation/modification of the (spontaneously formed) passivation layer on the lithium electrode, i.e., the SEI. To further investigate this evolution, an aging test was conducted by placing the Li|SIPE|Li cells in an isothermal chamber at 80 °C and open circuit conditions. Electrochemical impedance spectroscopy (EIS) was conducted during the aging process, which is shown in Figure S7-8. Two semicircles were observed which correspond to the bulk resistance of the electrolyte at higher frequencies and the interfacial resistance at lower frequencies. By fitting the EIS data with an equivalent circuit, as presented in Figure S9, the interfacial resistance has been plotted vs. the square root of time, as depicted in Figure 1b. For both samples, linear trends were found, which could be associated with a growth of the SEI, limiting the charge transport. The SEI resistance of the LiNO₃-comprising SIPE increased and after ~64 h showed a stabilizing trend, which can be explained by the formation of a rather stable SEI. While the SIPE without the addition of LiNO₃ showed a similar trend with a linear evolution with a greater slope, stabilizing after 144 h. Accordingly, the presence of LiNO₃ permits a faster stabilization of the interface, i.e.,
the faster formation of a more stable SEI,
resulting in a lower interfacial resistance.

Subsequently, cyclic voltammetry (CV)
experiments with an inert working electrode
(stainless steel), were conducted to examine the
electrochemical stability, especially upon
oxidation, as well as the lithium plating/stripping
at low potentials.

As shown in Figure 1c and 1d, for both SIPEs, i.e.,
with and without LiNO₃, no obvious oxidation
before 4.0 V was observed. At low potential,
lithium plating/stripping was observed. Contrary
to the oxidation process, a continuous evolution
of the intensity of the plating/stripping was
observed for both electrolytes. For the first cycle,
both electrolytes showed a relatively low amount
of lithium plated at potentials below -0.5 V (vs.
Li⁺/Li), indicating a large polarization of the Li⁺
reduction process. However, during cycling, the
two electrolytes behaved differently. For the neat
SIPE, the amount of plated Li decreased after the
third cycle and the plating/stripping coulombic
efficiency (CE) stabilized at only 20%. The weak
reduction process could be associated with the
passivation of the stainless-steel electrode
inducing a strong polarization of the process.
Regarding the poor CE, one explanation could be
the spontaneous oxidation of the fresh lithium
formed by plating. However, as the amount of
deposited lithium was rather low, its consumption
by chemical reaction had a significant impact on
the lithium plating/stripping CE.

Concerning the 0.5% LiNO₃-comprising SIPE,
from the third cycle, both the lithium plating
kinetics and CE started to improve up to some
stabilization from the 5th cycle. A comparison of
the amount of reversible lithium and CE for both
cells are shown in Figure 1e,f. A CE higher than
80% can be reached with the 0.5 wt.% LiNO₃-
comprising SIPE, confirming that the LiNO₃
additive has a significant effect on the Li|SIPE
interface. The LiNO₃ enables the formation a
stabilized passivation layer that mitigates further
reaction between the electrolyte and the lithium-
metal electrode. Additionally, it appears that the

passivation layer is characterized by a lower
resistance, thus, allowing the lithium cations
migrate through the passivation layer more easily,
as reflected by the higher CE and amount of
lithium deposited.

The critical current density of the SIPEs
without/with LiNO₃ was determined using
Li|SIPE|Li cells at 80 °C (Figure S10). The
potential of the cells and the corresponding
derivation (dJ/dE) are presented as function of
current density applied. The critical current
density could be determined as the point where a
nonlinear relationship between potential and
current density was observed, i.e., deviation
started to increase. This phenomenon of extra
polarization indicated the depletion of the charge
carriers in the system. For both SIPEs, i.e.,
without and with LiNO₃, critical current densities
larger than 1.25 mA cm⁻² were observed, further
corroborating that the presence of LiNO₃ did not
greatly affect the charge transport in the SIPE.

**3.3 Lithium Stripping and Plating**

Lithium stripping/plating tests with symmetric
Li|SIPE|Li cells are shown in Figure S11. The
cells were subjected to chronopotentiometric
cycling (0.1 mA cm⁻², 4 h per dis-/charge) for over
400 h. All three SIPEs showed stable
plating/stripping with low overpotential and no
sign of degradation, polarization or short circuit,
indicating a dendrite-free lithium deposition. The
stable cycling and rectangular shapes of the
voltage profiles could be ascribed to the single-
ion conducting behavior of the electrolytes.
However, the plating/stripping with these
symmetric cells, employing rather thick lithium
electrodes (~800 µm) masks (to some extent) the
reactivity of the SIPE in contact with lithium at
the interface. Therefore, we performed
plating/stripping tests with limited lithium
resources with the aim to have more information
on the CE of the lithium plating/stripping process.
Therefore, Li|SIPE|Cu cells were used. First, a
fixed amount of lithium was plated on the Cu foil,
which is represented by the charge passed in the
external circuit Qp. Since initially no lithium was
on the Cu electrode, all the lithium plated formed a limited Li reservoir. Then a depletive stripping process followed to remove Li from the surface of Cu with a cut-off voltage of 1.0 V, which indicated that all removable Li was stripped. The method is demonstrated in Figure 2a. The amount of lithium removed from the Cu surface was represented by the amount of charge passing through the external circuit $Q_s$. The CE of the lithium could be simply determined by the equation: $CE = Q_s/Q_p = t_s/t_p$, where $Q_s$, $Q_p$ are the absolute values of charge of the stripping and plating, respectively, and $t_s$ and $t_p$ are the times of the corresponding processes (as the absolute value of the current is constant).

The plating/stripping tests were conducted with both the 0.5 wt.% LiNO₃-containing and the neat SIPE for 20 cycles (Figure 2b,c). Each cycle consisted of 0.1 mA cm⁻² plating for 4 h (i.e., 0.4 mAh cm⁻² or theoretically 1.9 µm thickness of lithium), and 0.1 mA cm⁻² stripping until the cell voltage exceeded 1.0 V. During the first cycle, for both electrolytes, a rather high polarization (0.2-0.3 V) was observed, indicating the difficulty of the Li nucleation on the surface of the pristine Cu foil, accompanied by a low CE. This could be related to the different reactions at the surface of Cu, including the SEI formation and reduction of surface oxides (e.g., Cu₂O + 2Li⁺ + 2e⁻ → 2Cu⁰ + Li₂O), inducing the surface chemistry modification.⁵² The CE evaluation of both cells is shown in Figure 2e. For the SIPE comprising 0.5 wt.% LiNO₃, a quick improvement of the CE from 51% to 79% is observed up to the 4th cycle, followed by a slower improvement up to 94%. In comparison, the SIPE without the additive exhibited only a gradual improvement up to 80% after 20 cycles.

The polarization behavior during cycling in the Li|Cu cells is the result from two competing processes: the surface chemistry modification which may become more favorable for metallic lithium nucleation, thereby improving the interfacial properties and reducing polarization, and the continuous growth of the SEI, which increases polarization. In the sample without

LiNO₃, the very low CE in the initial cycles lead to the reactivity between fresh deposit lithium and electrolyte. This reactivity induced the formation of a SEI and a substantial amount of dead lithium metal (protected by the SEI) accumulating on the copper electrode surface. These surface properties modification resulted in an apparent decrease in polarization. In contrast, the sample with 0.5 wt.% LiNO₃ achieved a much higher CE after only three cycles. Consequently, the formation of a stable SEI, slightly increasing over time, could become the dominant process. This leads to a slightly gradual increase in polarization during subsequent cycles. Although the polarization trends differed between the two samples, the absolute polarization values for the LiNO₃-containing SIPE remained consistently lower than those for the bare SIPE throughout the experiment. After 20 cycles, the polarization values were approximately 90 mV and 140 mV for the LiNO₃-containing SIPE and the bare SIPE, respectively. The higher CE values and lower polarization indicate that the presence of LiNO₃ permits the formation of a more stable, but less resistive SEI layer during the Li plating/stripping tests.

To avoid the impact of the severe side reactions when always stripping the complete amount of lithium on the Cu foil, a modified method based on the work of Adams et al.⁵³ was applied (Figure 3) for the determination of the CE. Accordingly, using again the Li|SIPE|Cu cell setup, a Li plating and stripping cycle with a large capacity of 2 mAh cm⁻² (theoretically about 9.6 µm of lithium metal) was applied first to “pre-condition” the surface of the Cu electrode. After this preconditioning step, another 2 mAh cm⁻² of Li was plated on the Cu electrode, serving as limited lithium reservoir ($Q_R$). Subsequently, plating/stripping of a small amount of lithium ($Q_s$ and $Q_p$, 0.1 mAh cm⁻², 1/20 compared to the reservoir) was conducted until the Li reservoir was completely consumed, as indicated by the stripping voltage exceeding its upper limit of 1.0 V. The experimental procedure is depicted in Figure 3a. The average CE could be calculated
using the equation: $CE_{avg} = (\Sigma Q_s)/(\Sigma Q_p + Q_r)$.

Compared to the previous plating/stripping on the Cu electrode to determine the CE, this method is considered more accurate, since the initial loss of Li is compensated by the preconditioning step.

The voltage profiles of the Li|SIPE|Cu cells using the neat SIPE, as well as the 0.5 wt.%, and 1 wt.% LiNO₃-comprising SIPEs are shown in Figure 3b-d. Lithium plating/stripping of 0.1 mAh cm⁻² was conducted for 146 cycles (corresponding to ΣQₚ = ΣQₛ = 14.6 mAh cm⁻²), until the stripping voltage exceeded 1.0 V, indicating that the Li reservoir was depleted. Thus, an average CE of 88% was determined for the neat SIPE. For the 0.5 and 1 wt.% LiNO₃-comprising SIPEs, very similar results were obtained with a CE of 88% and 87%, respectively, which is consistent with reported values in PEO/LiTFSI-based polymer electrolytes and multiblock single-ion polymer electrolyte.⁵⁴,⁵⁵

EIS spectra were recorded during the plating/stripping process. The EIS spectra obtained before and after the preconditioning step are shown in Figures S12-13, which showed the transition from a blocking Li|SIPE|Cu cell to a symmetric Li|SIPE|Li cell with the formation of a passivation layer. The EIS measurements during the subsequent cycling with an areal capacity of 0.1 mAh cm⁻² are shown in Figure 4a-c. The cell with the neat SIPE showed an interfacial resistance of 1100 Ω for the first cycle, which slightly decreases during the next 50 cycles with a minimum value of 850 Ω and then started to increase towards the 146th cycle with an interfacial resistance of 1740 Ω. The interfacial resistance of the Li|SIPE|Li|Cu cells was always higher than in the corresponding symmetric Li|SIPE|Li cell (200-400 Ω), indicating that it is more difficult to plate Li on a Cu electrode, yielding a presumably more inhomogeneous lithium deposition. Notably, the shape of the second semicircle was getting more depressed, while the Li reservoir was depleting. The depressed semicircle could be ascribed to the

weakening symmetry of the two interfaces (Li|SIPE|Li and Li|SIPE|Li|Cu), by the heterogeneity of the passive layers formed on both interfaces.

The EIS spectra for the cells comprising the 0.5 and 1 wt.% LiNO₃-comprising SIPE during the plating/stripping cycling exhibited a much more stable behavior than in the case of the neat SIPE. As shown in Figure 4b-c, the LiNO₃-comprising SIPEs presented a stable total resistance of less than 1600 Ω for ~150 cycles, while the neat SIPE presents a higher total impedance of 2500 Ω. The much smaller resistance also led to a much lower polarization during the plating/stripping tests. The bulk resistance of the electrolytes showed an increasing trend as for all the samples tested before, whereas the interfacial resistance was more constant (Figure 4d). We suppose that the bulk resistance increased due to the loss of contact between the SIPE and the electrode, owing to the morphological changes (increased roughness) of the lithium electrode. These assumptions were further examined by an *ex-situ* analysis of the cells.

### 3.4 Understanding the Effect of LiNO₃

After cycling, the different Li|SIPE|Cu cells were disassembled in an Ar-filled glove box. The images of the disassembled cells are shown in Figure S14. The color of the SIPE membranes became brownish after long-term cycling. The lithium metal kept its silver-white smooth surface. A thin layer of gray matter was observed on both sides of the SIPEs. SEM images indicate that this is presumably mossy lithium and/or lithium-containing species resulting from the reaction with the electrolyte, generated during cycling. These mossy species were peeled off from the electrodes easily and remained on the surface of the SIPE membrane, leading to shining surfaces of the Li and Cu electrodes.

The mossy species on the SIPE membranes showed different morphologies, which could explain their different electrochemical behavior
during the lithium plating/stripping tests. An edge
of these mossy species remaining on the SIPE is
displayed in Figure S15. A clear boundary can be
observed between the mossy lithium species and
the SIPE membrane.

The cross-sectional SEM micrographs of the Cu-SIPE recovered from the Li|SIPE|Cu cells after plating/stripping for 350 h (Figure 5a) showed that the mossy lithium species between the Cu electrode and the SIPE membrane had a thickness of 7.0±1.2 µm. No dendritic lithium that intrudes into the SIPE was observed, which agrees with the conclusion of the lithium plating/stripping tests. Compared to the 9.6 µm deposited lithium as a reservoir, the thickness of the “dead” lithium species remaining on the Cu electrode was significant. The contact between the current collector and the mossy lithium seems to be poor, and a clear decohesion is observed (Figure 5a).

In Figure 5b, the mossy lithium on the SIPE
without additive showed a rough surface with
high porosity. While with the LiNO₃-comprising
SIPEs, the lithium deposition was denser with a
smooth surface. Similar effect has also been
overserved very recently in a LiNO₃-containing
liquid electrolyte⁵⁶ and gel polymer electrolyte.⁵⁷
The dense lithium deposition could lead to less
volume expansion, which could achieve better
contact between the electrode and the electrolyte.

Due to the long-term average CEs being similar,
i.e., ~86% in all three configurations, it suggests
that the impact of adding LiNO₃ on the long-term
lithium loss due to parasitic reactions was less
relevant. In fact, this somehow agrees with the
observation that similar CEs were observed for
various polymer electrolytes (e.g. PEO/LiTFSI)
with different methods.⁵⁴,⁵⁵ Combined with the
observation of the rather loosely attached mossy
lithium species, as observed by SEM, it appears
that a significant part of the long-term coulombic
inefficiency may be ascribed to the formation of
“dead lithium”, i.e., lithium that has irreversibly
reacted and/or metallic lithium that lost the
electronic contact to the current collector owing
to the pronounced volume changes and changes

in morphology. Nonetheless, in the presence of
LiNO₃, denser lithium deposits were obtained,
along with a lower interfacial resistance, which is
certainly still beneficial.

To investigate the chemical nature of the SEI and
species formed at the interface, XPS was
conducted on pristine and cycled lithium metal
electrodes as well as the SIPE membranes. For
this purpose, symmetric cells comprising the
SIPEs with/without 0.5 wt.% LiNO₃ were
assembled and cycled for 48 h (current density:
0.1 mA cm⁻², 0.4 mAh cm⁻² per half cycle) at
80 °C and then disassembled in an Ar-filled glove
box. The collected and analyzed C 1s, O 1s and F
1s spectra of cycled SIPEs are shown in Figure 5c.
The spectra recorded for the pristine SIPEs and
for the lithium metal electrodes (before and after
cycling) are shown in Figures S16-20.

For the pristine SIPEs without (Figure S16) and
with LiNO₃ (Figure S17), all the chemical bonds
from the SIPE were observed, such as
hydrocarbons (C-C), ethers (C-O-C) and
fluorinated species (-CF₂) at 285.0, 286.4 and
290-291 eV, respectively, in the C 1s region.⁴⁴,⁵⁸,⁵⁹
Interestingly, the SIPE with LiNO₃ as additive,
showed a larger concentration of C-O-C bonds on
the surface, which might indicate that the SIPE
orientation changes, or some reactions occur
during the incorporation of the additive into the
SIPE. In addition, the surface of the SIPEs is
covered by two other species, i.e., carbonyl (C=O)
and ester (O-C=O) groups at 287.5 and 288.5 eV,
respectively, which might be due to the reactivity
of our SIPE with the ambient atmosphere.
However, it might not be discarded that the C=O
peak corresponds to C-O bonds close to the
aromatic ring (see Scheme 2) that induce a peak
shift towards higher binding energies.⁵⁹ The
presence of ether groups is confirmed from the O
1s spectrum at 532.5 eV, as well as some O-C=O
bonds at 534 eV.⁶⁰ The perfluorinated side chain
(CF₂-O) was also observed in the F 1s spectra at
688.1 eV (C-F) and at 536 eV, corresponding to
CF₂-O in O 1s.⁶¹ The presence of LiF in the bare
SIPE (not observed for the LiNO₃-comprising
SIPE) may be due to the dehydrofluorination
process of the SIPE during membrane preparation, comparable to the polyvinylidene difluoride (PVdF) binder during electrode preparation.⁶² The sulfonate group is confirmed by both the 168.2 eV signal in the S 2p spectrum and the peak at 534.3 eV in the O1s spectrum.

The spectra recorded for the SIPE surfaces after cycling were significantly different. For the SIPE without the additive (Figure 5c top), the changes after cycling were mainly observed in the F 1s, O 1s, and C 1s spectra. The F 1s spectrum shows a weak signal, suggesting that the thickness of the SEI composed of SIPE degradation products is around 10 nm, as the C-F₂ species are barely detected and the XPS detection limit is close to 10 nm. Regarding the chemistry of the formed SEI onto SIPE, the O 1s and C 1s region provided relevant information. In the C 1s and O 1s spectra, a large contribution of species appearing at high binding energies are observed (please note that the formed degradation products exhibit similar chemical bonds as the SIPE, therefore the peaks corresponding to the degradation products might be also partially related to the SIPE). These species might be related to carbon/oxygen species, such as C=O, O-C=O, and/or O-(C=O)-O from the decomposition of the SIPE. As suggested by Aurbach et al.,⁶³ the cleavage of ether bonds might occur upon the reduction of ether-based electrolytes, generating alkoxides and radicals which led to a series of side reactions. Recent studies pointed out that more complicated organic molecules could be generated during the reduction of ethers, including ethylene, branched ethers, or vinyl ethers.⁶⁴,⁶⁵ Based on literature data, it may be assumed that the new peaks were due to the cleavage of ether bonds, forming degradation products on the polymer surface, which contains multiple organic degradation products, which could be ethers, vinyl ether and carbonyl compounds (e.g., aldehydes, ketones and carboxylates).⁶⁶,⁶⁷

For the cycled SIPE with LiNO₃ (Figure 5c bottom), the presence of polymer degradation products at high binding energies in the O 1s and C 1s spectrum were also detected, but at much

lower concentration than for the cycled SIPE without the additive. The lower intensity of these peaks suggests a decreased degradation of the SIPE in the presence of LiNO₃. In addition, the F 1s spectrum clearly displays the (-CF₂)-related peak from bulk SIPE, which is in good agreement with the decreased degradation products observed in the O 1s and C 1s spectrum, indicating that the SEI was thinner than in the case of the neat SIPE (< 10 nm).

The chemical composition of the SEI on the lithium metal surface was also analyzed. Both cycled lithium metals using SIPEs without/with LiNO₃ additive display a clear LiF peak, together with a -CF₂ peak at higher binding energies in the F 1s spectra, which could be related to the degradation of the SIPE as also observed on the pristine/cycled SIPEs, as well as some remaining SIPE. However, in the presence of LiNO₃, the LiF:CF₂ ratio varied, showing a more pronounced LiF formation than for the one without the additive, which is in agreement with what has been observed on the surface of the cycled SIPE with LiNO₃. In both C 1s spectra, the cycled lithium metal surfaces show species from the native layer, such as Li₂CO₃, compounds from the remaining SIPE (C-O-C and CF₂) and the formation of degradation products, which could be ethylene, branched ethers, and CH₃CHOLi. It is worth mentioning that, as occurred for the SIPE, the degradation products are having similar chemical bonds as the pristine SIPE and the native passivation layer on the lithium metal electrodes. Therefore, it is difficult to distinguish each specific compound in the degradation products. It should also be noted that the O 1s spectrum fitting is rather difficult due to the very close binding energies of carbonates, hydroxides and oxides. However, considering the broader O 1s peak observed on the cycled lithium metal samples, it might indicate the formation of additional peaks/species which could be assigned to Li₂O, which is also frequently present in the native passivation layer of pristine lithium metal.⁶⁸ Moreover, the O 1s spectra exhibit CSO₂-O moieties, which are confirmed by the S 2p spectra (Figures S18 and S19) at 168.2 eV. In addition,
the S 2p region indicates the presence of a series of degradation products at 158-166 eV which could be assigned to polysulfides and Li₂S, suggesting once more the degradation of the SIPE.¹⁵ Finally, the N 1s spectrum obtained for the cycled lithium metal surface using the SIPE with LiNO₃ (Figure S19) confirmed the reduction of the LiNO₃ additive, forming Li₃N.⁶⁹

For both lithium metal surfaces, i.e., without and with LiNO₃, the findings indicate that the SEI is similar, with an inorganic-rich layer composed of, e.g., Li₂CO₃, Li₂O, LiF, and Li₂S, with a C-O-C organic contribution close to the lithium-metal electrode and an organic-rich composition (mainly carbonyl-based products, which could be due to the cleavage of ether bonds) close to the SIPE. The main difference concerning the chemical composition of the SEI on the lithium side for the SIPE with LiNO₃ is the fact that it contains one more inorganic species, Li₃N, and a larger concentration of LiF. Moreover, the presence of LiNO₃ appears to result in a thinner SEI, while it is thicker in the absence of LiNO₃. The SEI chemistry compositions are illustrated in Figure 5d. In sum, XPS confirms that adding LiNO₃ could effectively alleviate the degradation at the Li|SIPE interface and lead to a more robust inorganic-rich SEI, which is commonly favorable for the eventual performance.

### 3.5 Evaluation in Li|SIPE|LFP Cells

To confirm the stabilizing effect of adding LiNO₃, LMB cells were assembled and cycled with LFP positive electrodes at 80 °C. The specific capacity and CE and their evolution upon cycling are shown in Figure 6a. The corresponding charge/discharge profiles are shown in Figure 6b-c. The first three cycles were conducted at a rather low C rate of C/20 (1C = 170 mA g⁻¹) as formation cycles, and the following cycles were conducted at C/2. The first cycles exhibited essentially the same specific capacity of 142 mAh g⁻¹, which provides a good basis to compare the performance of the two different cells. The cells comprising SIPEs without and with 0.5 wt.% LiNO₃ showed a capacity retention of 42% and 70% with an

average CE of 99.79% and 99.98% over 250 cycles, respectively. The cells employing the LiNO₃-comprising SIPE showed a greater cycling stability and reversibility. The capacity loss was calculated for each cycle, as shown in Figure S20. The cells comprising the LiNO₃-containing SIPE exhibited stable cycling during 250 cycles with a low average capacity loss of 0.16% per cycle. This was much lower than the cell with the neat SIPE, which showed an average capacity decay of 0.36% per cycle. This superior performance presumably originates from the favorable interphase at the SIPE | Li interface and the larger Li deposits; with the latter also restricting to some extent the volume changes and, thus, the contact loss upon cycling.

### Conclusion

The addition of 0.5 wt.% LiNO₃ to an ether-type single-ion conducting polymer electrolyte yields a stabilized SEI at the SIPE | Li interface, leading to a reduced interfacial resistance and cell polarization as well as an improved Li stripping/plating reversibility for the initial cycles, while the overall charge transport appears to be little affected. Upon long-term stripping and plating, however, the coulombic efficiency becomes essentially the same – independent from the presence of LiNO₃, indicating that other factors such as the loss of contact owing to the substantial volume changes and changes of the electrode morphology become dominant. Nonetheless, the advantageous modification of the eventually thinner SEI, characterized by a greater contribution of LiF and the additional presence of Li₃N, enables a substantially enhanced performance of Li|SIPE|LFP cells, with a greatly improved capacity retention and coulombic efficiency. To ensure practical applicability, future work will prioritize rate-dependent and high-rate stability assessments.

### Author contributions
C.I., D.B., F.A. and Y.S. conceived and designed the study. Y.S. and T.B. performed the material synthesis, characterization, and battery testing. K.S. contributed to the SEM experiments. M.Z. contributed to the XPS experiments. The manuscript was written through contributions of all authors. All the authors discussed the results and commented on the manuscript.

## Conflict of interest

There are no conflicts to declare.

## Data availability

Data for this article, including XPS and electrochemical measurements are available at Open Science Framework at DOI.org/10.17605/OSF.IO/2ZMA6.

## Acknowledgement

The authors would like to acknowledge the financial support from the Agence Nationale de la Recherche (ANR) and the German Federal Ministry of Education and Research (BMBF) within the MOLIBE project (ANR-19-ENER-0007 and 03SF0583A). Moreover, we would like to thank Dr. Claire Villevieille for the helpful discussion.

## Supporting Information

**Figure S1.** Scheme of the measurement of the Li⁺ transference number.

**Figure S2.** DSC thermogram of the SIPEs.

**Figure S3.** Raman spectra of LiNO₃ salt and the SIPEs without additive and with different LiNO₃ concentrations.

**Figure S4.** Raman spectra of the SIPEs without additive and with different LiNO₃ concentrations between 750 to 1100 cm⁻¹.

**Figure S5.** Determination of lithium transference number for the pristine SIPE comprising 0.5% LiNO₃, before the stabilization of the electrolyte/electrode interface.

**Figure S6.** Determination of the lithium transference number for the SIPE comprising 0.5% LiNO₃ after 80 h of lithium plating/stripping.

**Figure S7.** EIS spectra of the Li|SIPE|Li cell comprising the bare SIPE upon aging at 80 °C.

**Figure S8.** EIS spectra of the Li|SIPE|Li cell comprising the 0.5% LiNO₃-comprising SIPE upon aging was performed at 80 °C.

**Figure S9.** Equivalent circuit model for the Li|SIPE|Li cell system.

**Figure S10.** Determination of critical current density of the I₁₀₀₀p-SO₃-cr SIPE. (a) Current scan of a Li|SIPE|Li cell at 80 °C, from 0 to 2 mA cm⁻¹, with a rate of 0.1 µA s⁻¹. (b) derivation of potential.

**Figure S11.** Lithium plating/stripping test Li|SIPE|Li cells: (A) bare SIPE, (B) and (C)SIPE with 0.5% and 1.0% of LiNO₃ additive. The potential of the LiNO₃-containing samples was normalized by the electrolyte bulk resistance to eliminate the difference in sample thickness.

**Figure S12.** EIS spectra during the preconditioning of a Li|SIPE|Cu cell with the bare SIPE.

**Figure S13.** EIS spectra during the preconditioning of the Li|SIPE|Cu cells comprising the (a) 0.5 and (b) 1.0% LiNO₃-comprising SIPEs, inset is the magnified spectra at high frequencies.

**Figure S13.** EIS spectra during the preconditioning of the Li|SIPE|Cu cells comprising the (a) 0.5 and (b)
1.0% LiNO₃-comprising SIPEs, inset is the magnified spectra at high frequencies.

**Figure S14.** Images of the disassembled Li|SIPE|Cu cells. The diameter of the stainless-steel discs was 16 mm. (A) Cell comprising the bare SIPE. (B) Cell comprising the 1% LiNO₃-comprising SIPE (lithium electrode). (C) cell comprising the 1% LiNO₃-comprising SIPE (copper electrode).

**Figure S15.** Edge of electrochemically inactive/isolated lithium on the SIPE surface, the sample was recovered from a Li|SIPE|Cu cell comprising the bare SIPE after 350 h of cycling.

**Figure S16.** XPS spectra of pristine SIPE without additive.

**Figure S17.** XPS spectra of pristine SIPE with 0.5% LiNO₃ additive

**Figure S18.** XPS spectra of cycled Li from symmetric cell with SIPE without additive.

**Figure S19.** XPS spectra of Li from symmetric cells with SIPEs with LiNO₃ additive.

**Figure S20.** Capacity decay for each cycle of the Li|SIPE|LFP cells.
<figure><img src="image_3.png" /><figcaption>Scheme 2. Molecular structure of the crosslinked SIPE.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 1. (a) Ionic conductivity of SIPEs. (b) Interfacial resistance evolution of the Li|SIPE|Li cells vs. the square root of time during one week. (c,d) Cyclic voltammograms of (c) the neat and (d) the 0.5 wt.% LiNO₃ containing SIPE. (e) Amount of lithium reversibly stripped after plating and (f) Coulombic efficiency as a function of cycle number for the CV experiments.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 2. (a) Scheme of the lithium plating/stripping process on a Cu working electrode. (b,c) Lithium plating/stripping conducted with Li|SIPE|Cu cells at 80 °C with (b) the neat SIPE and (c) the 0.5 wt.% LiNO₃-containing SIPE following the approach illustrated in (a). (d) Charge/discharge profiles of the 5th stripping/plating cycle. (e) Evolution of the Coulombic efficiencies upon lithium stripping and plating.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 3. (a) Scheme of the revised lithium plating/stripping protocol employing Li|SIPE|Cu cells for the determination of the long-term Coulombic efficiency. (b-d) Lithium plating/stripping conducted with Li|SIPE|Cu cells employing (b) the neat SIPE as well as (c) the 0.5 wt.% and (d) 1.0 wt.% LiNO₃-comprising SIPE.</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

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

<figure><img src="image_10.png" /><figcaption>Figure 4. (a-c) Impedance spectra recorded for Li|SIPE|Cu cells during the lithium plating/stripping experiments presented in Figure 3b-d employing (a) the neat SIPE as well as (b) the 0.5 wt.% and (c) the 1 wt.% LiNO₃-comprising SIPE. (d) Evolution of the interfacial resistance during the lithium plating/stripping tests.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure 5. (a) Cross-sectional SEM micrograph of a Li|SIPE|Cu cell without LiNO₃ after cycling for 350 h. (b) SEM micrograph of the SIPE membranes recovered from the different Li|SIPE|Cu cells after the lithium plating/stripping test shown in Figure 3b-d. (c) XPS spectra of the neat SIPE and the LiNO₃-comprising SIPE after lithium plating/stripping in Li|SIPE|Li cells. (d) Illustrative schemes of the SEI composition with and without LiNO₃ as additive.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 6. (a) Li|SIPE|LFP cells employing the neat and 0.5 wt.% LiNO₃ comprising SIPE cycled at C/2 and 80 °C. (b-c) Charge/discharge profiles of cells employing the (b) neat and (c) 0.5 wt.% LiNO₃ comprising SIPE.</figcaption></figure>
References

(1) Varzi, A.; Raccichini, R.; Passerini, S.; Scrosati, B. Challenges and Prospects of the Role of Solid Electrolytes in the Revitalization of Lithium Metal Batteries. *J. Mater. Chem. A* **2016**, *4* (44), 17251–17259. https://doi.org/10.1039/C6TA07384K.

(2) Lin, D.; Liu, Y.; Cui, Y. Reviving the Lithium Metal Anode for High-Energy Batteries. *Nat. Nanotechnol.* **2017**, *12* (3), 194–206. https://doi.org/10.1038/nnano.2017.16.

(3) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. *Chem. Rev.* **2017**, *117* (15), 10403–10473. https://doi.org/10.1021/acs.chemrev.7b00115.

(4) Harry, K. J.; Hallinan, D. T.; Parkinson, D. Y.; MacDowell, A. A.; Balsara, N. P. Detection of Subsurface Structures underneath Dendrites Formed on Cycled Lithium Metal Electrodes. *Nat. Mater.* **2014**, *13* (1), 69–73. https://doi.org/10.1038/nmat3793.

(5) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Wei, F.; Zhang, J.-G.; Zhang, Q. A Review of Solid Electrolyte Interphases on Lithium Metal Anode. *Adv. Sci.* **2016**, *3* (3), 1500213. https://doi.org/10.1002/advs.201500213.

(6) An, S. J.; Li, J.; Daniel, C.; Mohanty, D.; Nagpure, S.; Wood, D. L. The State of Understanding of the Lithium-Ion-Battery Graphite Solid Electrolyte Interphase (SEI) and Its Relationship to Formation Cycling. *Carbon* **2016**, *105*, 52–76. https://doi.org/10.1016/j.carbon.2016.04.008.

(7) Chen, K.-H.; Wood, K. N.; Kazyak, E.; LePage, W. S.; Davis, A. L.; Sanchez, A. J.; Dasgupta, N. P. Dead Lithium: Mass Transport Effects on Voltage, Capacity, and Failure of Lithium Metal Anodes. *J. Mater. Chem. A* **2017**, *5* (23), 11671–11681. https://doi.org/10.1039/C7TA00371D.

(8) Ota, H.; Sakata, Y.; Wang, X.; Sasahara, J.; Yasukawa, E. Characterization of Lithium Electrode in Lithium Imides/Ethylene Carbonate and Cyclic Ether Electrolytes: *II. Surface Chemistry*. *J. Electrochem. Soc.* **2004**, *151* (3), A437. https://doi.org/10.1149/1.1644137.

(9) Li, J.; Cai, Y.; Wu, H.; Yu, Z.; Yan, X.; Zhang, Q.; Gao, T. Z.; Liu, K.; Jia, X.; Bao, Z. Polymers in Lithium-Ion and Lithium Metal Batteries. *Adv. Energy Mater.* **2021**, *11* (15), 2003239. https://doi.org/10.gh3b3j.

(10) He, X.; Bresser, D.; Passerini, S.; Baakes, F.; Krewer, U.; Lopez, J.; Mallia, C. T.; Shao-Horn, Y.; Cekic-Laskovic, I.; Wiemers-Meyer, S.; Soto, F. A.; Ponce, V.; Seminario, J. M.; Balbuena, P. B.; Jia, H.; Xu, W.; Xu, Y.; Wang, C.; Horstmann, B.; Amine, R.; Su, C.-C.; Shi, J.; Amine, K.; Winter, M.; Latz, A.; Kostecki, R. The Passivity of Lithium Electrodes in Liquid Electrolytes for Secondary Batteries. *Nat. Rev. Mater.* **2021**, *6* (11), 1036–1052. https://doi.org/10.1038/s41578-021-00345-5.

(11) Diederichsen, K. M.; McShane, E. J.; McCloskey, B. D. Promising Routes to a High Li + Transference Number Electrolyte for Lithium Ion Batteries. *ACS Energy Lett.* **2017**, *2* (11), 2563–2575. https://doi.org/10.1021/acsenergylett.7b00792.

(12) Stolz, L.; Hochstädt, S.; Röser, S.; Hansen, M. R.; Winter, M.; Kasnatscheew, J. Single-Ion *versus* Dual-Ion Conducting Electrolytes: The Relevance of Concentration Polarization in Solid-State Batteries. *ACS Appl. Mater. Interfaces* **2022**, *14* (9), 11559–11566. https://doi.org/10.1021/acsami.2c00084.

(13) Chazalviel, J.-N. Electrochemical Aspects of the Generation of Ramified Metallic Electrodeposits. *Phys. Rev. A* **1990**, *42* (12), 7355–7367. https://doi.org/10.1103/PhysRevA.42.7355.

(14) Tikekar, M. D.; Archer, L. A.; Koch, D. L. Stability Analysis of Electrodeposition across a Structured Electrolyte with Immobilized Anions. *J. Electrochem. Soc.* **2014**, *161* (6), A847–A855. https://doi.org/10.1149/2.085405jes.

(15) Xu, C.; Sun, B.; Gustafsson, T.; Edström, K.; Brandell, D.; Hahlin, M. Interface Layer Formation in Solid Polymer Electrolyte Lithium Batteries: An XPS Study. *J Mater Chem A* **2014**, *2* (20), 7256–7264. https://doi.org/10.1039/C4TA00214H.
(16) Andersson, E. K. W.; Wu, L.-T.; Bertoli, L.; Weng, Y.-C.; Friesen, D.; Elbouazzaoui, K.; Bloch, S.; Ovsyannikov, R.; Giangrisostomi, E.; Brandell, D.; Mindemark, J.; Jiang, J.-C.; Hahlin, M. Initial SEI Formation in LiBOB-, LiDFOB- and LiBF₄-Containing PEO Electrolytes. *J. Mater. Chem. A* **2024**, 12 (15), 9184–9199. https://doi.org/10.1039/D3TA07175H.

(17) Borzutzki, K.; Nair, J. R.; Winter, M.; Brunklaus, G. Does Cell Polarization Matter in Single-Ion Conducting Electrolytes? *ACS Appl. Mater. Interfaces* **2022**, 14 (4), 5211–5222. https://doi.org/10.1021/acsami.1c19097.

(18) Jin, C.; Wu, Z.; Li, G.; Luo, Z.; Li, N.-W. Phosphazene-Based Flame-Retardant Artificial Interphase Layer for Lithium Metal Batteries. *Acta Phys.-Chim. Sin.* **2025**, 41 (8), 100094. https://doi.org/10.1016/j.actphy.2025.100094.

(19) Zheng, G.; Lee, S. W.; Liang, Z.; Lee, H.-W.; Yan, K.; Yao, H.; Wang, H.; Li, W.; Chu, S.; Cui, Y. Interconnected Hollow Carbon Nanospheres for Stable Lithium Metal Anodes. *Nat. Nanotechnol.* **2014**, 9 (8), 618–623. https://doi.org/10/gf7qt7.

(20) Zeng, J.; Liu, Q.; Jia, D.; Liu, R.; Liu, S.; Zheng, B.; Zhu, Y.; Fu, R.; Wu, D. A Polymer Brush-Based Robust and Flexible Single-Ion Conducting Artificial SEI Film for Fast Charging Lithium Metal Batteries. *Energy Storage Mater.* **2021**, 41, 697–702. https://doi.org/10/gk8bns.

(21) Qiao, Y.; Yang, H.; Chang, Z.; Deng, H.; Li, X.; Zhou, H. A High-Energy-Density and Long-Life Initial-Anode-Free Lithium Battery Enabled by a Li2O Sacrificial Agent. *Nat. Energy* **2021**, 6 (6), 653–662. https://doi.org/10.1038/s41560-021-00839-0.

(22) Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Li-Alloy Based Anode Materials for Li Secondary Batteries. *Chem. Soc. Rev.* **2010**, 39 (8), 3115. https://doi.org/10.1039/b919877f.

(23) Xu, Y.; Wu, H.; He, Y.; Chen, Q.; Zhang, J.-G.; Xu, W.; Wang, C. Atomic to Nanoscale Origin of Vinylene Carbonate Enhanced Cycling Stability of Lithium Metal Anode Revealed by Cryo-Transmission Electron Microscopy. *Nano Lett.* **2020**, 20 (1), 418–425. https://doi.org/10.1021/acs.nanolett.9b04111.

(24) Stępień, D.; Wolff, B.; Diemant, T.; Kim, G.-T.; Hausen, F.; Bresser, D.; Passerini, S. Insights into the Lithium Nucleation and Plating/Stripping Behavior in Ionic Liquid-Based Electrolytes. *ACS Appl. Mater. Interfaces* **2023**, 15 (21), 25462–25472. https://doi.org/10.1021/acsami.3c01722.

(25) Phiri, I.; Kim, J.; Oh, D.-H.; Ravi, M.; Bae, H.-S.; Hong, J.; Kim, S.; Jeong, Y.-C.; Lee, Y. M.; Lee, Y.-G.; Ryou, M.-H. Synergistic Effect of a Dual-Salt Liquid Electrolyte with a LiNO₃ Functional Additive toward Stabilizing Thin-Film Li Metal Electrodes for Li Secondary Batteries. *ACS Appl. Mater. Interfaces* **2021**, 13 (27), 31605–31613. https://doi.org/10.1021/acsami.1c04972.

(26) Mou, P.; Zhang, L.; Lu, C.; Luo, Y.; Yuan, W.; Li, L.; Chen, Y.; Shen, Z.; Xu, J.; Shu, J. Mask-Based Separator with Sustained-Release LiNO₃ as Dendrite Growth Barrier for Potassium Metal Battery. *Adv. Energy Mater.* **2023**, 13 (22), 2300734. https://doi.org/10.1002/aenm.202300734.

(27) Dutta, A.; Matsushita, K.; Kubo, Y. Impact of Glyme Ether Chain Length on the Interphasial Stability of Lithium-Electrode in High-Capacity Lithium-Metal Battery. *Adv. Sci.* **2024**, 11 (32), 2404245. https://doi.org/10.1002/advs.202404245.

(28) Li, S.; Chen, Y.-M.; Liang, W.; Shao, Y.; Liu, K.; Nikolov, Z.; Zhu, Y. A Superionic Conductive, Electrochemically Stable Dual-Salt Polymer Electrolyte. *Joule* **2018**, 2 (9), 1838–1856. https://doi.org/10/gg8rr8.

(29) Liang, W.; Shao, Y.; Chen, Y.-M.; Zhu, Y. A 4 V Cathode Compatible, Superionic Conductive Solid Polymer Electrolyte for Solid Lithium Metal Batteries with Long Cycle Life. *ACS Appl. Energy Mater.* **2018**, 1 (11), 6064–6071. https://doi.org/10/gg8rsb.

(30) Wu, F.; Mullaliu, A.; Diemant, T.; Stepien, D.; Parac-Vogt, T. N.; Kim, J.; Bresser, D.; Kim, G.; Passerini, S. Beneficial Impact of Lithium Bis(Oxalato)Borate as Electrolyte Additive for High-voltage Nickel-rich Lithium-battery Cathodes. *InfoMat* **2023**, 5 (8), e12462. https://doi.org/10.1002/inf2.12462.
(31) Lu, Y.; Tu, Z.; Archer, L. A. Stable Lithium Electrodeposition in Liquid and Nanoporous Solid Electrolytes. *Nat. Mater.* **2014**, *13* (10), 961–969. https://doi.org/10.1038/nmat4041.

(32) Di Cillo, D.; Bargnesi, L.; Lacarbonara, G.; Arbizzani, C. Ammonium and Tetraalkylammonium Salts as Additives for Li Metal Electrodes. *Batteries* **2023**, *9* (2), 142. https://doi.org/10.3390/batteries9020142.

(33) Rosenman, A.; Elazari, R.; Salitra, G.; Markevich, E.; Aurbach, D.; Garsuch, A. The Effect of Interactions and Reduction Products of LiNO₃, the Anti-Shuttle Agent, in Li-S Battery Systems. *J. Electrochem. Soc.* **2015**, *162* (3), A470–A473. https://doi.org/10.1149/2.0861503jes.

(34) Hayashi, Y.; Sohmiya, M.; Otsuka, H.; Ito, K.; Kubo, Y.; Horiba, T.; Saito, M. Synergy between LiNO₃ and Tetraglyme-Sulfone Dual-Solvent Electrolyte Solutions in Li-O₂ Batteries. *J. Electrochem. Soc.* **2020**, *167* (16), 160531. https://doi.org/10.1149/1945-7111/abd04a.

(35) Iliksu, M.; Khetan, A.; Yang, S.; Simon, U.; Pitsch, H.; Sauer, D. U. Elucidation and Comparison of the Effect of LiTFSI and LiNO₃ Salts on Discharge Chemistry in Nonaqueous Li-O₂ Batteries. *ACS Appl. Mater. Interfaces* **2017**, *9* (22), 19319–19325. https://doi.org/10.1021/acsami.7b03592.

(36) Zhang, X.; Wang, Y.; Ouyang, Z.; Wang, S.; Zhao, X.; Xu, Q.; Yuan, B.; Yue, X.; Liang, Z.; Geng, S.; Tang, S.; Sun, H. Dual-Functional Lithium Nitrate Mediator Eliminating Water Hazard for Practical Lithium Metal Batteries. *Adv. Energy Mater.* **2024**, *14* (6), 2303048. https://doi.org/10.1002/aenm.202303048.

(37) Zhang, W.; Lu, Y.; Feng, Q.; Wang, H.; Cheng, G.; Liu, H.; Cao, Q.; Luo, Z.; Zhou, P.; Xia, Y.; Hou, W.; Zhao, K.; Du, C.; Liu, K. Multifunctional Electrolyte Additive for High Power Lithium Metal Batteries at Ultra-Low Temperatures. *Nat. Commun.* **2025**, *16* (1), 3344. https://doi.org/10.1038/s41467-025-58627-3.

(38) Gu, S.; Zhang, Y.; Li, M.; Lin, Q.; Xu, G.; Zhang, N. Internal Electron-Donation Allocation Design for Intrinsic Solubilization of Lithium Nitrate in Ester Electrolyte for Stable Lithium Metal Batteries. *Angew. Chem. Int. Ed.* **2025**, *64* (1), e202410020. https://doi.org/10.1002/anie.202410020.

(39) Zhou, W.; Wang, S.; Li, Y.; Xin, S.; Manthiram, A.; Goodenough, J. B. Plating a Dendrite-Free Lithium Anode with a Polymer/Ceramic/ Polymer Sandwich Electrolyte. *J Am Chem Soc* **2016**, *4*.

(40) Nguyen, H.-D.; Kim, G.-T.; Shi, J.; Paillard, E.; Judeinstein, P.; Lyonnard, S.; Bresser, D.; Iojoiu, C. Nanostructured Multi-Block Copolymer Single-Ion Conductors for Safer High-Performance Lithium Batteries. *Energy Environ. Sci.* **2018**, *11* (11), 3298–3309. https://doi.org/10/gfv2z4.

(41) Zor, C.; Turrell, S. J.; Uyanik, M. S.; Afyon, S. Lithium Plating and Stripping: Toward Anode-Free Solid-State Batteries. *Adv. Energy Sustain. Res.* **2023**, 2300001. https://doi.org/10.1002/aesr.202300001.

(42) Ngo, H. P. K.; Shao, Y.; Bertaux, T.; Nguyen, T. K. L.; Solier, J.; Planes, E.; Judeinstein, P.; Alloin, F.; Sanchez, J.-Y.; Iojoiu, C. Single-Ion Conducting Polymer Electrolyte with Excellent Interfacial Stability toward the Lithium Metal. *ACS Appl. Energy Mater.* **2025**, *8* (5), 2819–2827. https://doi.org/10.1021/acsaem.4c02859.

(43) Bruce, P. G.; Vincent, C. A. Steady State Current Flow in Solid Binary Electrolyte Cells. *J. Electroanal. Chem. Interfacial Electrochem.* **1987**, *225* (1–2), 1–17. https://doi.org/10.1016/0022-0728(87)80001-3.

(44) Biesinger, M. C. Accessing the Robustness of Adventitious Carbon for Charge Referencing (Correction) Purposes in XPS Analysis: Insights from a Multi-User Facility Data Review. *Appl. Surf. Sci.* **2022**, *597*, 153681.

(45) Diederichsen, K. M.; Buss, H. G.; McCloskey, B. D. The Compensation Effect in the Vogel–Tammann–Fulcher (VTF) Equation for Polymer-Based Electrolytes. *Macromolecules* **2017**, *50* (10), 3831–3840. https://doi.org/10.1021/acs.macromol.7b00423.
(46) Kim, H.; Kim, J. R.; Paeng, K.; Sohn, D. Characterization of Porous Microspheres Prepared via γ-Ray Irradiation. *Colloid Polym. Sci.* **2018**, *296* (12), 2015–2023. https://doi.org/10.1007/s00396-018-4417-5.

(47) Caruso, T.; Capuleoni, S.; Cazzanelli, E.; Agostino, R. G.; Villano, P.; Passerini, S. Characterization of PEO-Lithium Triflate Polymer Electrolytes: Conductivity, DSC and Raman Investigations. *Ionics* **2002**, *8* (1–2), 36–43. https://doi.org/10.1007/BF02377751.

(48) Alloin, F.; Hirankumar, G.; Pagnier, T. Temperature-Dependent Raman Spectroscopy of Lithium Triflate-PEO Complexes: Phase Equilibrium and Component Interactions. *J. Phys. Chem. B* **2009**, *113* (52), 16465–16471. https://doi.org/10.1021/jp904356a.

(49) Edman, L. Ion Association and Ion Solvation Effects at the Crystalline–Amorphous Phase Transition in PEO–LiTFSI. *J. Phys. Chem. B* **2000**, *104* (31), 7254–7258. https://doi.org/10.1021/jp000082d.

(50) Rosas, J.; Virya, A.; Lian, K. Roles of Hydrogen Bonds in Ionic Conductivity in the LiNO₃–Poly(Vinyl Alcohol) Electrolyte: A Real-Time Raman Spectroscopic Study. *J. Phys. Chem. C* **2024**, *128* (3), 1139–1146. https://doi.org/10.1021/acs.jpcc.3c06174.

(51) Hayashi, Y.; Yamada, S.; Ishikawa, T.; Takamuki, Y.; Sohmiya, M.; Otsuska, H.; Ito, K.; Kubo, Y.; Saito, M. Enhancement of Bifunctional Effect for LiNO3/Glyme Electrolyte by Using Dual Solvent System for Li-O2 Batteries. *J. Electrochem. Soc.* **2020**, *167* (2), 020542. https://doi.org/10.1149/1945-7111/ab6975.

(52) Kirillov, E.; Rodygin, K.; Ananikov, V. Recent Advances in Applications of Vinyl Ether Monomers for Precise Synthesis of Custom-Tailored Polymers. *Eur. Polym. J.* **2020**, *136*, 109872. https://doi.org/10.ghrm89.

(53) Adams, B. D.; Zheng, J.; Ren, X.; Xu, W.; Zhang, J. Accurate Determination of Coulombic Efficiency for Lithium Metal Anodes and Lithium Metal Batteries. *Adv. Energy Mater.* **2018**, *8* (7), 1702097. https://doi.org/10.1002/aenm.201702097.

(54) Sahore, R.; Mayer, A.; Steinle, D.; Counihan, M. J.; Chen, X. C.; Tepavcevic, S.; Bresser, D.; Westover, A. S. Electroanalytical Exploration of Li Loss at the Solid Electrolyte-Anode Interface in Anode-Free Batteries with Polymer Electrolytes. *J. Electrochem. Soc.* **2024**, *171* (5), 050505. https://doi.org/10.1149/1945-7111/ad430f.

(55) Müller, P.; Szczuka, C.; Tsai, C.-L.; Schöner, S.; Windmüller, A.; Yu, S.; Steinle, D.; Tempel, H.; Bresser, D.; Kungl, H.; Eichel, R.-A. Capacity Degradation of Zero-Excess All-Solid-State Li Metal Batteries Using a Poly(Ethylene Oxide) Based Solid Electrolyte. *ACS Appl. Mater. Interfaces* **2024**, *16* (25), 32209–32219. https://doi.org/10.1021/acsami.4c03387.

(56) Yi, R.; Xu, K.; Zhao, W.; Ren, Z.; Dong, Q.; Shao, H.; Shen, Y.; Chen, L. Coordination Topology Design in Anion-Rich Solvated Electrolytes for High-Voltage Lithium Metal Batteries. *Angew. Chem. Int. Ed.* **2025**, e202423439. https://doi.org/10.1002/anie.202423439.

(57) Yim, T.; Kim, K.; Hassig, M. Q.; Nedsaengtip, J.; Zhang, T.; Jamkar, S.; Li, C. Y.; Kalra, V. Lithium-Nitrate-Containing Gel Polymer Electrolyte for Carbonate-Based Anode-Free Lithium Metal Batteries. *ACS Appl. Mater. Interfaces* **2025**, *17* (26), 37851–37862. https://doi.org/10.1021/acsami.5c03252.

(58) Beamson, G.; Briggs, D. *High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database*; Wiley: Chichester, 1992.

(59) Briggs, D.; Beamson, G. Primary and Secondary Oxygen-Induced C1s Binding Energy Shifts in x-Ray Photoelectron Spectroscopy of Polymers. *Anal. Chem.* **1992**, *64*, 1729–1736. https://doi.org/10.1021/ac00039a018.

(60) Aurbach, D.; Weissman, I.; Schechter, A.; Cohen, H. X-Ray Photoelectron Spectroscopy Studies of Lithium Surfaces Prepared in Several Important Electrolyte Solutions. A Comparison with Previous Studies by Fourier Transform Infrared Spectroscopy. *Langmuir* **1996**, *12* (16), 3991–4007. https://doi.org/10.1021/la960076k.
(61) Friedman, A. K.; Shi, W.; Losovyj, Y.; Siedle, A. R.; Baker, L. A. Mapping Microscale Chemical Heterogeneity in Nafion Membranes with X-Ray Photoelectron Spectroscopy. *J. Electrochem. Soc.* **2018**, *165* (11), H733–H741. https://doi.org/10.1149/2.0771811jes.

(62) Sharma, J.; Totee, C.; Kulshrestha, V.; Ameduri, B. Spectroscopic Evidence and Mechanistic Insights on Dehydrofluorination of PVDF in Alkaline Medium. *Eur. Polym. J.* **2023**, *201*, 112580. https://doi.org/10.1016/j.eurpolymj.2023.112580.

(63) Aurbach, D.; Zaban, A.; Ein-Eli, Y.; Weissman, I.; Chusid, O.; Markovsky, B.; Levi, M.; Levi, E.; Schechter, A.; Granot, E. Recent Studies on the Correlation between Surface Chemistry, Morphology, Three-Dimensional Structures and Performance of Li and Li-C Intercalation Anodes in Several Important Electrolyte Systems. *J. Power Sources* **1997**, *68* (1), 91–98. https://doi.org/10.1016/S0378-7753(97)02575-5.

(64) Liu, Q.; Cresce, A.; Schroeder, M.; Xu, K.; Mu, D.; Wu, B.; Shi, L.; Wu, F. Insight on Lithium Metal Anode Interphasial Chemistry: Reduction Mechanism of Cyclic Ether Solvent and SEI Film Formation. *Energy Storage Mater.* **2019**, *17*, 366–373. https://doi.org/10.1016/j.ensm.2018.09.024.

(65) Ushakova, E. E.; Frolov, A.; Reveguk, A. A.; Usachov, D. Yu.; Itkis, D. M.; Yashina, L. V. Solid Electrolyte Interface Formation between Lithium and PEO-Based Electrolyte. *Appl. Surf. Sci.* **2022**, *589*, 153014. https://doi.org/10.1016/j.apsusc.2022.153014.

(66) Siggel, M. R. F.; Thomas, T. D. Linear Correlation of Oxygen Core-Ionization Energies of Alcohols and Acids with Those of the Corresponding Methyl and Ethyl Ethers and Esters. *J. Electron Spectrosc. Relat. Phenom.* **1989**, *48* (1), 101–116. https://doi.org/10.1016/0368-2048(89)80008-8.

(67) Smith, S. R.; Thomas, T. D. Acidities and Basicities of Carboxylic Acids. Correlations between Core-Ionization Energies, Proton Affinities, and Gas-Phase Acidities. *J. Am. Chem. Soc.* **1978**, *100* (17), 5459–5466. https://doi.org/10.1021/ja00485a033.

(68) Chen, Z.; Kim, G.-T.; Kim, J.-K.; Zarrabeitia, M.; Kuenzel, M.; Liang, H.-P.; Geiger, D.; Kaiser, U.; Passerini, S. Highly Stable Quasi-Solid-State Lithium Metal Batteries: Reinforced Li1.3Al0.3Ti1.7(PO4)3/Li Interface by a Protection Interlayer. *Adv. Energy Mater.* **2021**, *11* (30), 2101339. https://doi.org/10.1002/aenm.202101339.

(69) Xiong, S.; Xie, K.; Diao, Y.; Hong, X. Properties of Surface Film on Lithium Anode with LiNO3 as Lithium Salt in Electrolyte Solution for Lithium–Sulfur Batteries. *Electrochimica Acta* **2012**, *83*, 78–76.
Lithium Nitrate as SEI-Stabilizing Additive in Single-Ion Conducting Polymer Electrolytes for
Lithium-Metal Batteries

Yunfan Shao<fnref n="1" />, Tony Bertaux<fnref n="1" />, Kai Shi<fnref n="2" />,<fnref n="3" />, Maider Zarrabeitia<fnref n="2" />,<fnref n="3" />, Fannie Alloin<fnref n="1" />,<fnref n="5" />, Dominic Bresser<fnref n="2" />,<fnref n="3" />,<fnref n="4" />, Cristina Iojoiu<fnref n="1" />,<fnref n="5" />,*

Corresponding author e-mail: [Cristina.Iojoiu@grenoble-inp.fr](mailto:Cristina.Iojoiu@grenoble-inp.fr)

Keywords: lithium metal, single ion conducting polymer electrolyte, solid electrolyte interphase, LiNO3 additive, all solid batteries

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

<footnote n="2">² Helmholtz Institute Ulm (HIU), 89081 Ulm, Germany</footnote>

<footnote n="3">³ Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany</footnote>

<footnote n="4">⁴ Ulm University (UUlm), 89069 Ulm, Germany</footnote>

<footnote n="5">5 Réseau sur le Stockage Electrochimique de l'Energie (RS2E), CNRS, FR3459, 80039 Amiens Cedex,
France</footnote>
<figure><img src="image_13.png" /><figcaption>**Figure S1.** Scheme of the measurement of the $Li^+$ transference number.</figcaption></figure>

## 2. Result and Discussion

<figure><img src="image_14.png" /><figcaption>**Figure S2.** DSC thermogram of the SIPEs.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure S3.** Raman spectra of LiNO₃ salt and the SIPEs without additive and with different LiNO₃ concentrations.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Figure S4.** Raman spectra of the SIPEs without additive and with different LiNO₃ concentrations between 750 to 1100 cm⁻¹.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure S5.** Determination of lithium transference number for the pristine SIPE comprising 0.5% LiNO₃, before the stabilization of the electrolyte/electrode interface.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Figure S6.** Determination of the lithium transference number for the SIPE comprising 0.5% LiNO₃ after 80 h of lithium plating/stripping.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure S7.** EIS spectra of the Li|SIPE|Li cell comprising the bare SIPE upon aging at 80 °C.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure S8.** EIS spectra of the Li|SIPE|Li cell comprising the 0.5% LiNO₃-comprising SIPE upon aging was performed at 80 °C.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S9.** Equivalent circuit model for the Li|SIPE|Li cell system.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>**Figure S10.** Determination of critical current density of the I₁₀₀₀p-SO₃-cr SIPE. (a) Current scan of a Li|SIPE|Li cell at 80 °C, from 0 to 2 mA cm⁻¹, with a rate of 0.1 µA s⁻¹. (b) derivation of potential.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure S11.** Lithium plating/stripping test Li|SIPE|Li cells: (A) bare SIPE, (B) and (C)SIPE with 0.5% and 1.0% of LiNO₃ additive. The potential of the LiNO₃-containing samples was normalized by the electrolyte bulk resistance to eliminate the difference in sample thickness.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure S12.** EIS spectra during the preconditioning of a Li|SIPE|Cu cell with the bare SIPE.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Figure S13.** EIS spectra during the preconditioning of the Li|SIPE|Cu cells comprising the (a) 0.5 and (b) 1.0% LiNO₃-comprising SIPEs, inset is the magnified spectra at high frequencies.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Figure S14.** Images of the disassembled Li|SIPE|Cu cells. The diameter of the stainless-steel discs was 16 mm. (A) Cell comprising the bare SIPE. (B) Cell comprising the 1% LiNO₃-comprising SIPE (lithium electrode). (C) cell comprising the 1% LiNO₃-comprising SIPE (copper electrode).</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Figure S15.** Edge of electrochemically inactive/isolated lithium on the SIPE surface, the sample was recovered from a Li|SIPE|Cu cell comprising the bare SIPE after 350 h of cycling.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S16.** XPS spectra of pristine SIPE without additive.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Figure S17.** XPS spectra of pristine SIPE with 0.5% LiNO₃ additive.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure S18.** XPS spectra of cycled Li from symmetric cell with SIPE without additive.</figcaption></figure>

<figure><img src="image_31.png" /></figure>
<figure><img src="image_32.png" /><figcaption>**Figure S21.** Capacity decay for each cycle of the Li|SIPE|LFP cells.</figcaption></figure>
References

(1) Ngo, H. P. K.; Shao, Y.; Bertaux, T.; Nguyen, T. K. L.; Solier, J.; Planes, E.; Judeinstein, P.; Alloin, F.; Sanchez, J.-Y.; Iojoiu, C. Single-Ion Conducting Polymer Electrolyte with Excellent Interfacial Stability toward the Lithium Metal. *ACS Appl. Energy Mater.* **2025**, 8 (5), 2819–2827.
https://doi.org/10.1021/acsaem.4c02859.

(2) Tammann, G.; Hesse, W. Die Abhängigkeit Der Viscosität von Der Temperatur Bie Unterkühlten Flüssigkeiten. *Z. Für Anorg. Allg. Chem.* **1926**, 156 (1), 245–257.
https://doi.org/10.1002/zaac.19261560121.

(3) Fulcher, G. S. ANALYSIS OF RECENT MEASUREMENTS OF THE VISCOSITY OF GLASSES. *J. Am. Ceram. Soc.* **1925**, 8 (6), 339–355. https://doi.org/10.1111/j.1151-2916.1925.tb16731.x.

(4) Bruce, P. G.; Vincent, C. A. Steady State Current Flow in Solid Binary Electrolyte Cells. *J. Electroanal. Chem. Interfacial Electrochem.* **1987**, 225 (1–2), 1–17. https://doi.org/10.1016/0022-0728(87)80001-3.

(5) Evans, J.; Vincent, C. A.; Bruce, P. G. Electrochemical Measurement of Transference Numbers in Polymer Electrolytes. *Polymer* **1987**, 28 (13), 2324–2328. https://doi.org/10/bfmccj.

(6) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. NIH Image to ImageJ: 25 Years of Image Analysis. *Nat. Methods* **2012**, 9 (7), 671–675. https://doi.org/10.1038/nmeth.2089.

(7) Biesinger, M. C. Accessing the Robustness of Adventitious Carbon for Charge Referencing (Correction) Purposes in XPS Analysis: Insights from a Multi-User Facility Data Review. *Appl. Surf. Sci.* **2022**, 597, 153681.

(8) Hunter, J. D. Matplotlib: A 2D Graphics Environment. *Comput. Sci. Eng.* **2007**, 9 (3), 90–95.
https://doi.org/10.1109/MCSE.2007.55.