Highly ion conductive cross-linked ionogels for all-quasi-solid-state lithium-metal batteries

Lithium-metal batteries (LMBs) play significant roles in our modern life because of their significant intrinsic merits, such as their high capacity, low density, very low redox potential of the lithium metal, etc.^1–7 Among the different components of LMBs, the electrolytes are more crucial for the development of advanced LMBs with high specific capacity and energy density.^7–10 Generally, organic liquid electrolytes are used in traditional LMBs. These liquid electrolytes create serious problems such as leakage, flammability, explosiveness, etc. Moreover, the growth of lithium dendrites on lithium metal electrodes produces dead lithium and builds an unstable solid electrolyte interface (SEI). In addition, a short circuit may happen, affecting directly the cycle life and safety of the LMBs.^11,12 Recently, solid-state ionogels (IGs) are considered more promising and new alternative candidates than liquid electrolytes to address these issues. IGs are composed of an inorganic host network and an ionic liquid electrolyte (ILE) containing ionic liquids (ILs) and lithium salts.^13–18 In IGs, the major part is ILE, and IGs display the main properties of ILE except outflow.^17,19–21 Basically, ionic liquids are molten salts at ambient conditions and typically consist of an asymmetric organic cation such as imidazolium and an inorganic anion.^22,23 Recently, ILs have been considered promising, novel, and safe electrolytes for LMBs due to their suitable basic properties, such as high ionic conductivity, a wide electrochemical potential stability window, etc.^24,25 Generally, poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-HFP)], poly(methyl methacrylate) (PMMA), and poly(ethylene oxide) (PEO) are used as common host polymer matrices to prepare IGs.²⁶ The cross-linked IGs synthesized via UV-photo or thermal polymerization technique have received great attention due to their flexibility, leakage proof, high thermal stability, high ionic conductivity at room temperature, wide potential stability window, high lithium–ion transference number (>0.5), blocking of lithium dendrite formation, etc.^2,27–31 Recently, studies of ion transport mechanisms in ionogels have also received great attention for fundamental research. Generally, ion transport in polymer electrolytes occurs via ionic hopping and segmental motion of polymer chains in the amorphous region of the polymer matrix.³² Impedance or dielectric spectroscopy has been widely used to study the controlling parameters (ionic conductivity, segmental motion, etc.) for ion transport.³³ 

In this work, we have prepared cross-linked ionogel films based on poly(ethylene glycol) diacrylate (PEGDA) and ILE via thermal polymerization for the fabrication of LMBs. The synthesized ionogels showed superior electrolyte parameters such as flexibility, thermal stability, ionic conductivity, a potential stability window, a lithium–ion transference number, and the stability of the SEI. The temperature dependence of the ionic conductivity obtained from the complex impedance plot followed the VTF relationship. We have also studied the performance of the LMBs, fabricated with this ionogel as an electrolyte at different current rates and rate capabilities.

All chemicals used in this work were purchased from Sigma-Aldrich. The chemicals poly(ethylene glycol) diacrylate (PEGDA, average M_n ∼ 575), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄, ≥99.0%), lithium tetrafluoroborate (LiBF₄, 98.0%), and 2,2′-Azobis(2-methylpropionitrile) (AIBN, 98.0%) were used without further purification. The LiBF₄ salt has often been studied instead of the widely used LiPF₆ in lithium cells because of its improved thermal and hydrolytic stability. The LiBF₄ has demonstrated several other important advantages over the LiPF₆: improved performance at sub-zero temperatures, improved passivation of the Al current collector, and improved performance at the positive electrode.³³ In addition, EMIMBF₄ ionic liquid was chosen due to its high thermal and electrochemical stability, high ionic conductivity, low viscosity, and chemical and electrochemical compatibility. The ionogels (IGs) were synthesized by adding the required amount of ionic liquid electrolyte (ILE) and PEGDA monomer via thermal polymerization. At first, ILE containing 10, 30, and 50 mol. % of LiBF₄ lithium salt in EMIMBF₄ ionic liquid was prepared with magnetic stirring for 2 h. 30 mol. % of LiBF₄ lithium salt was chosen because 30 mol. % of LiBF₄ was optimized to provide the highest ionic conductivity at room temperature of the prepared ionogels for battery applications, which will be discussed later. Finally, an appropriate amount of ILE and PEGDA was mixed in the presence of 1 wt. % AIBN as a thermal initiator with continuous magnetic stirring for another 2 h. The homogeneous mixed solution was injected into a Teflon separated glass plate cell, which was then heated at 70 °C for 12 h. The self-standing uniform film was stored in an Ar-filled glove box for further use. Two different compositions of IGs were prepared. The compositions of IGs are the following: Ionogel-1 (IG-1): PEGDA: ILE (50:50 w/w. %) and Ionogel-2 (IG-2): PEGDA: ILE (20:80 w/w. %). A photograph of the solid, self-standing, and flexible IG-2 film with 30 mol. % salt concentration film is shown in Fig. 1, along with the schematic preparation method of IGs by thermal curing.

The Fourier transform infrared spectroscopy of all IGs was studied in a Perkin Elmer Spectrometer (model Spectrum 100) at room temperature with an attenuated total reflectance (ATR) cell in the wave number region 4000–500 cm⁻¹ at a resolution of 1 cm⁻¹. The TGA experiments with IGs were performed using the Q600 TA instrument to study the thermal degradation from 30 to 600 °C at a heating rate of 10 °C min⁻¹ under N₂ flowing at 50 cm³/min.

The surface morphology of the gold-coated ionogel was studied using a field emission scanning electron microscope (JEOL, model JSM-6700F).

The ionic conductivity of IGs at different temperatures was measured by complex impedance spectroscopy using an LCR meter (HIOKI, model IM 3536 LCR) in the frequency range of 4 Hz to 8 MHz in a cryostat with temperature stability of ±0.10 K in vacuum (0.01 mbar). The IGs with a thickness of 0.27 mm were sandwiched between two stainless steel (SS) blocking electrodes. The impedance measurement of the 30 mol. % of LiBF₄ in the EMIMBF₄ ionic liquid electrolyte was carried out by dipping two stainless steel (SS) electrodes into the electrolyte.

The measurement of the Li⁺ transference number of the IG-2 and 30 mol. % of LiBF₄ in EMIMBF₄ ILE were carried out using a symmetrical Li∣IG-2∣Li cell setup. The IG-2 was sandwiched between two Li metal electrodes of diameter 16 mm, where the Li metal worked as a counter and working electrode. To measure the Li⁺ transport number of the ILE, Li disks were used for sandwiching a Whatman fiberglass layer impregnated with the 30 mol. % of LiBF₄ in EMIMBF₄ ILE (about 50 µL). The CR2032 coin cell (MTI Corp.) was assembled under an argon gas atmosphere in a glove box (MBRAUN, LABstar ECO, Germany). The Li transference number was measured in a multichannel electrochemical analyzer (Bio-Logic, model VMP3) using electrochemical impedance spectroscopy and chronoamperometry with an external DC potential step of ΔV = 10 mV. The frequency range of the measured impedance spectra was 100 mHz–1 MHz.

To estimate the electrochemical potential stability window (EPSW) of the IGs, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were performed with an electrochemical analyzer (Bio-Logic, VMP3). The iongels were sandwiched between electrodes Li/Cu for the cathodic stability window (CSW) and Li/SS for the anodic stability window (ASW) .

Lithium symmetrical cells (Li∣IG-2∣Li and Li∣IG-2∣Cu) were used to study the interfacial stability between the IG-2 and the lithium metal. These cells were assembled using a CR2032 coin cell assembly in an Ar-filled glove box with H₂O and O₂ < 0.5 ppm. The cycling test of the symmetrical cells was performed using the same electrochemical analyzer at room temperature at current densities of 0.05, 0.1, and 0.2 mA·cm⁻² for 1 h for both stripping and plating cycles for each current density.

The cathode for the LMB was prepared by coating Al foil (MTI Corp.) with N-methyl-2 pyrrolidone (NMP) based homogeneous slurry containing LiFePO₄ powder (Sigma-Aldrich, >97%), acetylene carbon black as a conducting material, and PVdF as a binder in the ratio of 80:10:10 (w/w). Then the coated Al foil was dried at 100 °C overnight under a vacuum. The electrodes were cut into circular discs with a diameter of about 10 mm. The mass loading of LiFePO₄ was 2.54 mg/electrode. The circular lithium metal foil (Alfa Aesar) electrodes of diameter ∼14 mm used as a reference electrode were punched into the rolled piece of lithium metal in a glove box. The Li-metal battery (Li|IG-2|LiFePO₄) was assembled under an argon atmosphere in a glove box to study the electrochemical performance. The cycling test of the fabricated half-cell was carried out in a multichannel electrochemical analyzer (Bio-Logic, model VMP3) at room temperature. The cell was tested at several current densities (C/n), where C ∼ theoretical capacity (170 mA h g⁻¹) of LiFePO₄ and n ∼ 1, 2, 4, 6, 8, 10, 15, and 20 in a potential range of 2.0–4.5 V.

Fourier-transform infrared (FTIR) spectroscopy has been used to illustrate the consequences of the thermal polymerization of PEGDA monomer. Figure 2(a) shows the FTIR spectra of IG-2 before and after thermal polymerization. The peaks near about 780–820 and 1580–1660 cm⁻¹, which correspond to the C=C twisting vibration and the C=C stretching vibration of the acrylate groups, respectively (guided by the dotted lines), confirm the polymerization. Figures 2(b) and 2(c) exhibit an enlarged view of the corresponding regions of the FTIR spectra. It is observed in these figures that the characteristic peaks of the monomer disappeared after thermal polymerization, indicating complete cross-linking of the monomer and the formation of cross-linked networks. Figure 2(d) shows the thermogravimetric analysis (TGA) thermograms to test the thermal stability of the IGs. The TGA traces of IGs show a non-linear relation of weight loss with temperature corresponding to non-uniform cross-linked monomers, and TGA curves indicate the different stages of weight loss.²⁸ The first step at 100 °C corresponded to the decomposition of absorbed surface moisture inside the IGs film. The second step at around 190 °C was attributed to the decomposition of polymer chains. The third step and major weight loss at around 350 °C were attributed to the decomposition of the polymer backbone chain, and the last flat step (above 450 °C) was due to leftover inorganic residue.³⁴ It is observed that the IGs show negligible thermal degradation up to 280 °C, which confirms the safety of the IGs for application in LMBs. Figure S1 shows the FESEM image of the IG-2 representing a plane surface morphology.

To check the anodic potential stability window of IG-2 with 30 mol. % salt concentration, the linear sweep voltammetry (LSV) of the symmetric Li∣IG-2∣SS cell was performed in the voltage range of 2.5–6.0 V (Li/Li⁺) with a scan rate of 0.1 mV s⁻¹ at 30 °C. The result is shown in Fig. 4(a). It is observed that the anodic breakdown occurs at about 5.15 V vs Li/Li⁺. The cyclic voltammetry (CV) of Li∣IG-2∣Cu cell was carried out to test the cathodic voltage stability window of IG-2 in the range of −0.5 to 3.0 V vs Li/Li⁺. The reversible lithium deposition and dissolution were found in the copper electrode in the lower voltage scale at around 0 V. Therefore, the IG-2 (30 mol. %) shows a wide electrochemical stability window in the range of 0–5.15 V.

We have performed a lithium stripping/plating cycling experiment to test the dynamical stability of the solid electrolyte interface for the long-term stability of LMBs. Figure 4(b) shows the time dependent voltage curves (plating stands for positive voltage and stripping stands for negative voltage) of the symmetric Li∣IG-2∣Li cell with different current densities of 0.05, 0.1, and 0.2 mA cm⁻² at 30 °C and insets present an enlarged view of the respective current densities. It is observed that the polarization voltage increases (±44 mV for the first cycle) for the first 50 cycles and then stabilized for the next 122 cycles at ±79 mV at 0.05 mA cm⁻² current density. The polarization voltage of the cell is about ±183 and ±256 mV for current densities of 0.1 and 0.2 mA cm⁻², respectively. It indicates that polarization voltage increases with the increase in current density, but the stable polarization voltage stage is still observed after 325 cycles. The low polarization voltages of the symmetric cell signify stable performance during the plating/stripping of the lithium metal electrode. The observation of a slightly high polarization voltage may be due to the thickness of the ionogels. Actually, thicker IGs constitute a physical barrier to hinder lithium–ion conduction.³⁹ We have also measured the lithium stripping/plating of Li/ILE/Li cells for a comparative study. The time dependent voltage profile of the cell is presented in Fig. S4. The comparative ILE cell exhibits a slight rise in voltage after 120 h, which indicates that the fabricated cell with ionogel is better at inhibiting dendrite growth. Another critical parameter is Coulombic efficiency (CE) for investigating the reversibility of the metallic Li plated into the host. The CE is defined as the ratio of the lithium stripping capacity (areal charging capacity) to the lithium plating capacity (areal discharging capacity) for each cycle. Figure S5 shows the Coulombic efficiency of the Li||Li cell for different cycles at current densities of 0.05 and 0.1 mA cm⁻², and the inset of Fig. S5 represents the voltage-areal capacity curves of the Li||Li cell at different current densities for different cycle numbers. We have also investigated the CE performance of the Li||Cu cell at a current density of 0.1 mA cm⁻², which is shown in Fig. S6. The value of Coulombic efficiency is 63.4% in the first cycle due to irreversible lithium consumed to form the SEI layer and gradually increases with fluctuation with an increase in cycles. The CE of the Li||Cu cell is very stable for the 20–50 cycling range and then drops gradually. This type of Coulombic efficiency of the Cu is due to the dendrite growth and uneven lithium nucleation that induce an unstable solid electrolyte interface layer.⁴⁰ Furthermore, the time evolution of the electrochemical impedance spectra shown in Fig. 4(c) has been investigated to obtain insight into this process. The spectra were fitted to an equivalent circuit model,¹³ which is shown in the inset of Fig. 4(c). The electrochemical impedance spectra in Fig. 4(c) indicate an Ohmic resistance at high frequencies corresponding to the ionic conductivity of the IGs (R_e) and a semi-circle at low frequencies corresponding to the surface resistance of the Li electrode (R_s). The time dependence of R_s and R_e obtained from the equivalent circuit model is shown in Fig. 4(d). It is observed in Fig. 4(d) that the value of R_s increases significantly due to the formation of the solid electrolyte interface (SEI) over the first 50 cycles. It is worth mentioning that the value of R_e is much lower than R_s and did not change during the lithium plating/stripping process, indicating that the electrolyte is not affected by this process. The stable value of R_s is observed up to the 204th cycle, and then the value increased slightly. Therefore, IG-2 has good compatibility with lithium metal.

Finally, the optimized ionogel (IG-2) having the highest ionic conductivity was used as an electrolyte in the solid-state Li-metal battery (LMB) in the configuration Li∣IG-2∣LiFePO₄ (CR2032 coin cell) to reveal its application and electrochemical performance at room temperature. Figure 5(a) shows the charge–discharge voltage profile (voltage vs specific capacity) in the voltage range of 2.0–4.5 V (vs Li/Li⁺) at different current rates at 30 °C. The LMB delivers specific discharge capacities of 156, 133, 111, 98, 86, 76, 60, and 48 mA h g⁻¹ at current rates of C/20, C/15, C/10, C/8, C/6, C/4, C/2, and 1C, respectively, as shown in the inset of Fig. 5(a). The LMB exhibits 3.5–3.7 V (vs Li/Li⁺) charging voltage plateaus and 3.1–3.3 V (vs Li/Li⁺) discharging voltage plateaus for the current rate of C/20. The voltage gap ΔV (difference between charging and discharging voltage plateaus) slightly increases with the increase of the current rates due to slow Li⁺-ion transport into the LiFePO₄ electrode.⁴¹ The charge–discharge specific capacities gradually decrease with the increase of the current rates due to the restriction of Li⁺-ion migration in IG-2.^13, Figure 5(b) shows the CV profile of the LMB between 2.0 and 4.5 V (vs Li⁺/Li) at different scan rates such as 0.1, 0.3, and 0.6 mV s⁻¹ at 30 °C. It is observed that sharp electrochemical redox peaks are observed, and the small potential gap between the cathodic and anodic peaks indicates smaller polarization in the lithium metal battery. The cycling performance of the fabricated LMB up to 50 cycles at a current rate of C/20 at 30 °C is shown in Fig. 5(c). The LMB delivers low discharge capacity for the first few cycles due to the formation of a stable solid electrode–electrolyte interface and then reaches a maximum capacity. Thereafter, a slow decrease in the discharge specific capacity of the battery is observed with the increase in the cycle number. The discharge specific capacity delivered by the fabricated battery is 129 mA h g⁻¹ at the 50th cycle, resulting in a capacity retention of ∼83% at the 50th cycle. It is noted that 24% capacity fading occurred after the 49th cycle due to the reaction of the IL cation (EMIM⁺) with lithium metal resulting in an increase in interfacial impedance and the formation of F⁻ based reactive species. Other reasons may be the evaporation of ILE and the formation of the passive layer on the electrode–electrolyte interface.^11,42,43 Actually, an unstable solid electrode–electrolyte interface is a serious issue for the cycling life of LMBs. Different electrolyte additives are introduced to form robust solid electrode–electrolyte inter-phases for suppressing Li dendrite growth.^44–46 The fabricated battery shows good specific capacity, 98% Coulombic efficiency, and good capacity retention. The electrochemical impedance plots of the LMB at different cycles are depicted in Fig. 5(d). All the electrochemical impedance spectra show semicircles in the high-to-medium frequency region that can be interpreted as interfacial phenomena, such as passivation layer formation on the electrode surface.⁴⁷ The diameter of the semicircle, i.e., inter-phase resistance, slightly increases with long-term cycling performance.

Finally, a comparison of the physicochemical properties of the cross-linked IGs and the electrochemical results of the IGs based battery of this present work with those of the most significant results reported in the literature is shown in Table I.^{27,28,48–55} It is observed in Table I that the IG-2 exhibits the highest ionic conductivity at room temperature, a comparatively high lithium transference number, and a good electrochemical potential stability window. It is also observed in Table I that the LMBs fabricated with IG-2 displays a comparatively high discharge specific capacity at room temperature.

Highly Li–ion conductive and stable cross-linked flexible ionogels have been synthesized via thermal polymerization of poly(ethylene glycol) diacrylate (PEGDA) in the presence of an ionic liquid electrolyte (ILE) based on 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) ionic liquid and lithium tetrafluoroborate (LiBF₄) salt. The temperature dependence of the ionic conductivity of the ionogels follows the VTF relation indicating the ion transport mechanism in the ionogels is associated with the segmental dynamics of the polymer chains. The IG-2 with a 30 mol. % salt concentration shows very high ionic conductivity (12.59 mS cm⁻¹) and a high lithium–ion transference number (∼0.56) at 30 °C. The IG-2 also has the ability to build a robust and conductive solid electrolyte interface on the lithium electrode surface. The ionogel shows excellent performance as a self separator in the all-quasi-solid-state lithium-metal battery (Li∣IG-2∣LiFePO₄) at 30 °C. The battery delivered a high discharge specific capacity of 156 mA h g⁻¹ at the current rate of C/20 at 30 °C with 83% capacity retention at the 50th cycle.

The supplementary material contains an SEM image of ionogel, ionic conductivity, lithium–ion transference number, and lithium stripping/plating results of ionic liquid electrolyte (ILE). The supplementary material also contains areal capacity with Coulombic efficiency of the Li||Li and Li||Cu cells fabricated with ionogel.

This work was supported by the Raja Ramanna Fellowship program of the Department of Atomic Energy (DAE), Government of India.

The authors have no conflicts to disclose.

Pulak Pal: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Validation (equal); Visualization (equal); Writing – original draft (equal). Aswini Ghosh: Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.