Ionic Porous Organic Polymers Synthesized Using Solvothermal and Mechanochemical Methods as Solid Electrolytes for Lithium Metal Batteries

Lithium metal batteries (LMBs) have considerable potential to become the next generation of energy storage devices because of their high energy density. However, when used with liquid electrolytes, dendrite growth and thermal runaway can cause serious safety problems. Solid-state electrolytes with high Li + conductivity, durability, and stability can effectively solve these problems. However, existing solid electrolytes do not meet the desirable criteria of having low interfacial contact, high electrochemical stability, high conductivity, and low manufacturing cost. Herein, we demonstrate that Li + -coordinated silicate and sulfonate porous organic polymers (denoted as Li + @Si-S-POPs) synthesized using solvothermal (ST) and mechanochemical (MC) methods can be used as solid electrolytes for LMBs. Bene�ting from the incorporated ionic groups, permanent porosity, and low interfacial resistance, products synthesized using ST and MC methods exhibited high Li + conductivity values of 1.1 × 10 − 4 and 1.5 × 10 − 4 S cm − 1 at room temperature and Li + transference numbers of 0.94 and 0.96, respectively. Such excellent conduction behaviors facilitate high Li plating/stripping stability on Li metal electrodes, demonstrating the potential of Li + @Si-S-POPs for use in solid-state LMBs.


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Given their high volumetric and gravimetric energy density, lithium (Li) batteries containing Li metal anodes have the potential to become the next generation of batteries. 1However, when used with liquid electrolytes, Li metal anodes are prone to capacity fading and thermal runaway because they are highly susceptible to side reactions.Li 0 passivation and dendrite growth are other critical problems that can cause battery failure.To solve these problems, a stable and e cient solid electrolyte interphase has been developed. 2,3 owever, all-solid-state Li metal batteries (LMBs) with solid-state electrolytes can effectively address safety concerns and ensure stable cycling performance. 4,5 olymers such as polyethylene and polycarbonate were used in rst-generation solid electrolytes.However, they were limited by low Li + conductivity (σLi + ) and transference numbers (tLi + ). 6Recently, glass ceramics, including oxides and sul des, have been developed and widely studied due to their high σLi + and tLi + , even outperforming liquid electrolytes. 7However, the hardness of these ceramic materials leads to high contact resistance and their instability when exposed to open air, causing long-term cycling problems.
Porous crystalline polymers, such as covalent organic frameworks, have been used as an alternative because of their high electrochemical performance and stability.However, they also suffer from the issue of contact resistance. 8Given that successful solid electrolytes should have high σLi + and tLi + , low interfacial contact, low manufacturing cost, high electrochemical stability, and high mechanical strength to suppress dendrite growth, porous organic polymers (POPs) are suitable materials for Li + conduction.
POPs are composed of lightweight atoms, such as C, H, O, and N, linked by strong covalent bonds, which endow them with amorphous networks, permanent porosity, and chemical stability. 9POPs containing anionic groups can conduct Li + rapidly, selectively, and reliably.Moreover, their mechanically soft amorphous nature reduces interfacial resistance, and their solvothermal or mechanochemical synthesis is cost-effective.
Recently, Li + -conducting solid electrolytes based on POPs have received increasing attention [10][11][12][13][14] because their permanent pores favor the selective uptake of liquid electrolytes and act as channels that facilitate the rapid transport of Li + .Apart from utlizing the porosity of POPs by impregnating them with liquid solvents and electrolytes, 8, 15 other strategies have been developed to reduce the contributions of Li + salts.These include grafting the pore walls of POPs with moieties found in polyelectrolytes, such as polyethylene glycol, 16 and imbuing intrinsic charge into POPs through synthesis with ionic groups, postsynthetic modi cation, and ion exchange, thereby creating ionic POPs (iPOPs). 13Compared with neutral POPs, iPOPs exhibit better ionic conductivity and single-ion conduction behavior. 8With ionic moieties lining the pore channels of POP backbones, Li + have directional ion conducting pathways, promoting rapid and homogeneous transport while eliminating the need for freely mobile anions and requiring only trace amounts of organic solvents.Such a strategy enhances the mobility of Li + , leading to an increase in their cationic transference number (t Li+ ) to over 0.8.This results in improved cycling stability and practical viability in batteries as well as a reduction in dependence on liquid components in the battery. 13iPOPs are promising as ion conductors without the need for the addition of Li salts, including freely mobile anions that can reduce t Li+ .However, only a few studies have described the use of iPOPs, even POPs, for Li-based batteries.For instance, Angulakshmi et al. successfully synthesized a triazene-p-phenylenediamine-based POP with a conductivity of 1.3 10 − 4 S cm − 1 at room temperature and a t Li + of approximately 0.5. 17Zhou et al. used POPs as llers to prepare polyethylene oxide-based composite solid-state electrolytes with a conductivity of 3.2 10 − 5 S cm − 1 and a t Li+ value of 0.18. 18rthermore, Ye et al. synthesized POPs with a conductivity of 2.7 10 − 5 S cm − 1 . 19OPs are typically synthesized using energy-intensive, high-temperature, and small-scale (usually less than 200 mg) solvothermal (ST) methods spanning several days.These methods can hinder upscaling and commercial adaptation because of their time ine ciency, high cost, and adverse environmental effects.Thus, a rapid, green, and cost-effective method for the synthesis of iPOPs is necessary.
Mechanochemical (MC) synthesis has received considerable interest as a simple, economical, and environmentally friendly alternative to ST synthesis for constructing network materials and polymers. 20th MC methods, iPOPs can be synthesized at larger scales, without the use of solvents, and at ambient temperatures, thus signi cantly reducing the adverse environmental effects of chemical synthesis, energy consumption, and production cost.Purely MC routes have been used for synthesizing covalent organic frameworks (COFs) for the construction of triformylphloroglucinol-based COFs. 21MC-assisted routes involving the use of heat at some stage in the synthesis of imine-linked COFs have been explored to create energy storage materials.For example, two studies have focused on the mechanochemical construction of H + -conducting COF-based materials. 22,23 ensch et al. reported a "beat and heat" method for polyimide-linked COFs. 24A study demonstrated H + conductivity at subzero temperatures by using a mechanochemically synthesized COF-based composite membrane. 25However, despite considerable progress in this eld, the use of mechanochemistry for the synthesis of porous polymer-based Li conductors has been notably overlooked.To date, no study has mechanochemically synthesized pristine porous polymers for single-ion conductive solid electrolytes in Li batteries.
Herein, we demonstrate that Li + -coordinated silicate and sulfonate POPs (denoted as Li + @Si-S-POPs) are viable solid electrolytes for LMBs.Li + @Si-S-POP is an anionic POP based on Si-S-POPs and contains abundant anionic sulfonates and hexacoordinate silicates that facilitate single Li + conduction.We determined that both ST and MC methods were equally effective in synthesizing single Li + conductors; however, both of these methods have their pros and cons.The ST method involves typical Schiff-base condensation reactions, and the MC method uses grinding in a mortar and pestle to construct iminelinked polymers (Fig. 1).Li + @Si-S-POPs prepared using ST and MC methods exhibited a high σLi + of 1.1 10 − 4 and 1.5 10 − 4 S cm − 1 at room temperature, respectively, and single-ion conductive t Li+ values of 0.94 and 0.96, respectively.We added 20 wt% of propylene carbonate to iPOP samples (see Fig. 3f and Table S1 for relevant details).Such ion conduction characteristics endow Li metal electrodes and even full cells with high Li plating/stripping stability, indicating the competitiveness of Si-S-POP materials and the effectiveness of both ST and MC techniques.To synthesize Li + @Si-S-POPs, tri-benzaldehyde-silicate with 2Et 3 NH + , labeled as 1, was reacted with 2-amino-5-[E-2-(4-amino-2sulfophenyl)ethenyl]benzenesulfonic acid to produce polymerized products, which were subsequently subjected to ion exchange to incorporate Li + as counter ions.When the ST technique was used, the color of the monomer solution changed to light red immediately upon the addition of catalysts (Fig. 1b).However, to ensure the formation of long and extended polymeric networks, we continued the reaction for 3 days.When the MC technique was used, the progress of the reaction was effectively monitored through colorimetric analysis.Monomers were initially light yellow.Continuous grinding for 10, 20, and 40 min caused their color to change to dark yellow, orange, and dark orange, respectively, indicating the formation of extended conjugated networks (Fig. 1c).After the completion of reactions, we washed the products with various organic solvents to remove oligomeric products (see SI for details).This MC method is time saving and easy to operate.Moreover, the MC method conforms to the concept of green chemistry because it is conducted at room temeprature and ambient conditions without the use of solvents.However, the ST method ensures a higher degree of reaction progress than the MC method, as indicated by the color of the resulting products.
To con rm the formation of POP networks through both ST and MC techniques, we examined the progress of the reaction by using Fourier-transform infrared spectroscopy (Fig. 2a).The Si-precursor monomer 1 and amino-benzenesulfonic acid exhibited the characteric peaks of aldehydes and amines at 1,660 and 1,619 cm − 1 , respectively.With the progression of the MC reaction, we observed an absorption peak at 1,660 cm − 1 and a neighboring peak at 1,600 cm − 1 , which were attributed to a decrease in aldehydes and an increase in imines, respectively.Accompanied by a visible color change of the powder from beige yellow to orange and red, the presence of a peak at 1,600 cm − 1 indicated the formation of C = N imine linkages and the dimishing peak at 1,660 cm − 1 indicated the consumption of aldehyde monomers and aldehyde-terminated oligomers.The products obtained using both ST and MC methods did not have crystalline two-dimensional (2D) characteristic peaks of COFs at less than 10° because monomer 1 was not symmetric, indicating the amorphous nature of the network (Fig. 2b).The products obtained using the MC method had many sharp peaks originating from 2D stacked morphologies at the meso to macro scale.The presence of numerous peaks within the range of 10° to 50° indicated that the products have many different morphologies and may contain a broad distribution of crosslinked polymer lengths.Given that the aldehyde monomer is not symmetric, it is reasonable to assume that even ST products are similar to MC products.Thus, the resulting products cannot be crystalline or periodic.
To con rm architectural rigidity and complete structure formation, we obtained nitrogen gas sorption isotherms to determine the porosity of samples from both methods (Fig. 2c).Both Si-S-POP (MC) and Si-S-POP (ST) exhibited reversible type I isotherms.However, Si-S-POP-Li (MC) had a considerably lower Brunauer-Emmett-Teller (BET) surface area (40.0 m 2 g − 1 ) than did Si-S-POP-Li (ST; 229.2 m 2 g − 1 ) due to exfoliation during MC synthesis, which resulted in thin layered structures with less pores for N 2 adsorption.The reduction in the BET surface area of Li + @Si-S-POP-Li (ST; 14.2 m 2 g − 1 ) and Li + @Si-S-POP-Li (MC; 5.2 m 2 g − 1 ) indicated that Li + were successfully coordinated to silicates and sulfonates.The thermal stability of the two base polymers was examined through thermogravimetric analysis (TGA; Fig. 2d).TGA curves indicated that Si-S-POP (ST) was stable up to 400°C under a nitrogen atmosphere and that the thermal stability pro le of Si-S-POP-Li (MC) was similar to that of Si-S-POP-Li (ST).However, a higher percentage of mass loss was noted from 180 to 380°C.This can be attributed to the lower degree of stacking resulting from the self-exfoliating nature of mechanochemical synthesis, which makes the particles less aggregated than POPs obtained through the ST method.Overall, our TGA ndings align with those of previous studies on mechanochemically synthesized COFs. 22,23,26 Sanning electron microscopy images revealed the morphology of both samples (Fig. 2e, f).For Si-S-POPs (ST), a rectangular grain-like nanotube morphology was observed; this observation is in agreement with that of a previous study. 27However, the particles synthesized using the MC method were markedly atter and wider, resembling plates, and were composed of stacked laminated sheets, a visual feature attributable to exfoliation and shearing that occur due to grinding during the MC synthesis process (Fig. S1).
To examine the σLi + of materials, we prepared self-standing pellets of Li + @Si-S-POPs (ST) and Li + @Si-S-POPs (MC) using a cold-pressing method.Approximately 70 mg of the dried sample was pressed in a 15mm stainless-steel die between two stainless steel electrodes in a coin cell con guration.The obtained pellet showed a dense morphology, with a thickness of approximately 0.2 mm.Subsequently, the ionic conductivity of the pellet containing 20 wt.% of propylene carbonate was evaluated at varied temperatures using electrochemical impedance spectroscopy.Li + @Si-S-POPs (ST) and Li + @Si-S-POPs (MC) had σLi + of 1.1 10 − 4 and 1.5 10 − 4 S cm − 1 at room temperature, respectively, indicating a marked increase in ionic conductivity compared with that of its contemporary POP-based electrolyte counterparts (Fig. 3a, b).This nding can be attributed to the abundance of lithiophilic groups in the pore channels of POPs.Potentiostatic polarization was utilized to determine the conduction behavior of Li + only.Li + @Si-S-POPs (ST) and Li + @Si-S-POPs (MC) exhibited single-ion conductive t Li+ values of 0.94 and × × 0.96, respectively (Fig. 3c, d).These high t Li+ values re ect the advantage of abundant installed silicates and sulfonates on the backbones of POPs, indicating that σLi + is predominantly attributable to Li + .The Arrhenius plot of samples revealed a proportional increase in logarithmic σLi + against a rise in temperature, yielding an activation energy value of 0.129 and 0.133 eV for ST and MC samples, respectively.Although t Li+ values were considerably high, σLi + values were one order of magnitude lower than the recorded values of other porous polymers, such as COFs. 28However, our Li + @iPOPs still outperformed all other POPs reported (Fig. 3f).To the best of our knowledge, this is the rst report of iPOPs for LMBs.
Before incorporating pellet-type iPOP solid electrolytes in Li metal battery cells, we tested the electrochemical stability of both ST and MC samples through linear sweep voltammetry.Both samples exhibited high electrochemical stability, with negligible increases in current up to 4.2 and 4.1 V for ST and MC samples, respectively (Fig. 4a).However, the stability window can be expanded to increase its competitiveness with current state-of-the-art solid electrolytes.To investigate the applicability of iPOPs as solid-state electrolytes, coin cells were assembled to obtain Li|Li + @Si-S-POPs|Li symmetric cell con guration (Fig. 4b).Subsequently, galvanostatic Li plating and stripping on Li metal electrodes was conducted at a current density of 10 µA cm − 2 for 2 hours in each cycle.Over 200 hours of cycling, the symmetric cell demonstrated considerably stable plating/stripping behavior, as evidenced by a low increase in overpotential, stabilizing at 0.01 V versus Li/Li + and the absence of any irreversible uctuation.

Conclusion
In this study, we demonstrated the synthesis of Li + coordinated iPOPs (Li + @Si-S-POPs) by using ST and MC methods.These methods yielded products containing abundant anionic sulfonates and hexacoordinate silicates.We found that both ST and MC methods were equally effective in synthesizing Li + conductors for LMBs.With ionic groups lining the pore channels of POP backbones, Li + have directional ion conducting pathways, promoting rapid and homogeneous transport.This unique structure enabled Li + @Si-S-POPs (ST) and Li + @Si-S-POPs (MC) to exhibit excellent ion conduction behaviors, with σLi + of 1.1 10 − 4 and 1.5 10 − 4 S cm − 1 and tLi + of 0.94 and 0.96 at room temperature, respectively.This high conduction behavior promoted the stable cycling of Li plating/stripping on Li metal electrodes.
Overall, this is the rst study to demonstrate the viability of iPOPs prepared using both ST and MC methods as single Li      S-POPs (MC), Li + @Si-S-POPs (ST), and Li + @Si-S-POPs (MC).e, f, SEM images revealing the rod-like and rectangular morphologies of Si-S-POPs (ST) and plate-like and aked morphologies of Si-S-POPs (MC), respectively.
Electrochemical characterization of Li + @Si-S-POPs.a, b, Nyquist plots of Li + @Si-S-POP (ST) and Li + @Si-S-POP (MC), ranging from room temperature to 60 °C, respectively.c, d, Electrochemical impedance spectra before and after polarization at 10 mV and the corresponding polarization curves for Li + @Si-S-POP (ST) and Li + @Si-S-POP (MC), respectively.e, Arrhenius plots of Li + @Si-S-POP (ST) and Li + @Si-S-POP (MC) revealing low activation energies, which correspond to a hopping-type mechanism.29][30][31] Detailed data are summarized in Table S1.
Electrochemical stability and symmetric cell data.Linear sweep voltammetry curves of stainless steel|Li using Li + @Si-S-POP as solid electrolytes at 1 mV s −1 .b, Galvanostatic Li plating/stripping pro les of Li|Li + @Si-S-POPs|Li symmetric cells at 0.019 mA cm −2 .Li was plated/stripped per cycle at room temperature.

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Table S1
is not available with this version.