Eutectic Solution Enables Powerful Click Reaction for In‐Situ Construction of Advanced Gel Electrolytes

Thiol‐ene click reaction is an intriguing strategy for preparing polymer electrolytes due to its high activity, atom economy and less side reaction. However, the explosive reaction rate and the use of non‐electrolytic amine catalyst hamper its application in in‐situ batteries. Herein, a nitrogen‐containing eutectic solution is designed as both the catalyst of the thiol‐ene reaction and the plasticizer to in‐situ synthesize the gel polymer electrolytes, realizing a mild in‐situ gelation process and the preparation of high‐performance gel electrolytes. The obtained gel polymer electrolytes exhibit a high ionic conductivity of 4 × 10−4 S cm−1 and lithium‐ion transference number ( tLi+ ) of 0.51 at 60 °C. The as‐assembled Li/LiFePO4 (LFP) cell delivers a high initial discharge capacity of 155.9 mAh g−1, and a favorable cycling stability with the capacity retention of 82% after 800 cycles at 1 C is also obtained. In addition, this eutectic solution significantly improves the rate performance of the LFP cell with high specific capacity of 141.5 and 126.8 mAh g−1 at 5 C and 10 C, respectively, and the cell can steadily work at various charge–discharge rate for 200 cycles. This powerful and efficient strategy may provide a novel way for in‐situ preparing gel polymer electrolytes with desirable comprehensive performances.


Introduction
Relative to liquid and solid-state electrolytes, gel polymer electrolytes (GPEs) have been identified as more promising media for advanced lithium metal batteries (LMBs) because of their compromised performance between safety and powerful ion-conductive ability. [1][2][3][4][5][6][7] However, the commonly reported gel electrolytes are prepared by ex-situ method, which cannot avoid the complicated preparation process accompanied by volatile solvent. [8,9] More importantly, the penetration difficulty and poor contact of the semi-solid electrolyte film with the electrode materials cause a high interfacial impedance, thus resulting in the performance fading or failure of the cell. [10] In recent years, in-situ polymerization methods with the precursor pre-injecting into the cells have become attractive for the fabrication of GPEs. The pre-injected liquid precursor fully infiltrates the electrode materials, and intimate junction between the electrolyte and electrode can be easily built after the in-situ polymerization process, which circumvent the tedious synthesis process and contact obstacles mentioned above. [11] Even so, in-situ prepared GPEs still cannot satisfy the demands for practical applications for several reasons. First, the heating or irradiation-responsive polymerizations are timeconsuming and energy-intensive. In addition, the mostly used liquid plasticizer in the in-situ GPEs are volatile and combustible organic carbonates, the safety issues need to be further avoided. Most importantly, the use of nonelectrolytic catalysts and initiators, and relative side reactions may cause adverse effects on the battery stability. [12] The click reaction is one of the most efficient strategies for preparing the polymer materials because of its high activity, moderate conditions and less side reactions, all of which are attractive for the construction of polymer electrolytes. [13][14][15][16][17][18][19] Thereinto, the Michael addition between the thiol-containing nucleophiles and the acrylates can introduce a S-C bond in the electrolyte matrix, which is beneficial for decreasing the electronegativity of the polymer chains and improving the ionic conductivity. [15] However, this reaction is not suitable for in-situ cell because of the ultrafast click rate under the triggering of amines catalyst, which reserves no time for the electrode wetting and cell assembling. Moreover, the residue of non-electrolytic amines also exerts a risk on the interfacial stability and subsequent battery performance. [13,16,17] In addition to polymerization methods, the inherent properties of liquid plasticizer are very crucial for the electrochemical performance of GPEs. As a novel ionic electrolyte, deep eutectic solvents (DESs) own a plenty of advantages, including easy preparation, available raw materials, flexible designability, high ionic conductivity and non-volatility, etc., both of which conform to the in-situ preparation principles of high-performance GPEs. [20][21][22][23][24][25] Herein, we design a eutectic solution based on the interactions between sulfolane (SL), 1,3-diamino-2-propanol and lithium salts for in-situ preparing GPEs, as shown in Figure 1. The combination of nitrogen-containing complex and lithium salt provides a high catalysis efficiency on the click reaction between poly(ethylene glycol) diacrylate (PEGDA) and pentaerythritol tetra(3-mercaptopropionate) (PETMP) or thiol-decorated polyhedral oligomeric silsesquioxane (POSS-SH), while the introduction of SL offers favorable electrochemical properties for the prepared gel electrolytes. In addition, the regulation of eutectic constitute via the extra addition of methyl acrylate (MA) ensures a mild gelation process for the in-situ assembling of lithium batteries. Each Thiol-ene click reaction is an intriguing strategy for preparing polymer electrolytes due to its high activity, atom economy and less side reaction. However, the explosive reaction rate and the use of non-electrolytic amine catalyst hamper its application in in-situ batteries. Herein, a nitrogencontaining eutectic solution is designed as both the catalyst of the thiol-ene reaction and the plasticizer to in-situ synthesize the gel polymer electrolytes, realizing a mild in-situ gelation process and the preparation of highperformance gel electrolytes. The obtained gel polymer electrolytes exhibit a high ionic conductivity of 4 × 10 −4 S cm −1 and lithium-ion transference number (t Li + ) of 0.51 at 60°C. The as-assembled Li/LiFePO 4 (LFP) cell delivers a high initial discharge capacity of 155.9 mAh g −1 , and a favorable cycling stability with the capacity retention of 82% after 800 cycles at 1 C is also obtained. In addition, this eutectic solution significantly improves the rate performance of the LFP cell with high specific capacity of 141.5 and 126.8 mAh g −1 at 5 C and 10 C, respectively, and the cell can steadily work at various charge-discharge rate for 200 cycles. This powerful and efficient strategy may provide a novel way for in-situ preparing gel polymer electrolytes with desirable comprehensive performances. component of the eutectic mixture interacts with each other and with the polymer network, decreasing the side reactions between the liquid components and the electrodes. The variation of DES components and their proportion relative to the polymer matrix permits a flexible performance optimization of the in-situ prepared GPEs. As a result, the GPEs demonstrate satisfied rate performance and long cycling lifespan (800 cycles at 1 C) when assembling with Li/LFP cells, and the interfacial stability between the in-situ formed GPEs and the lithium anodes delivers a long plating-stripping process of 1200 h (0.1 mA cm −2 , 60°C) in the lithium symmetric batteries.

Results and Discussion
1,3-Diamino-2-propanol (DAP) can coordinate with lithium salt via hydrogen bond interaction to form the deep eutectic solution, meanwhile the amine group serves as the catalyst of the click reaction between the thiol group and acrylate. In order to ensure a moderate gelation rate and subsequent electrochemical performance of the in-situ construction of polymer electrolytes, sulfone and methyl acrylate (MA) were introduced, and the reaction between DAP and MA led to the transformation from primary amine to secondary amine (DAAP) with slightly lower catalytic activity. In addition, trace amount of LiDFOB was added to enhance the performance of the obtained electrolyte because its preferential reduction than LiTFSI can form a LiF-rich solid electrolyte interphase (SEI) layer on the surface of lithium metal anode, [26] and the final eutectic solution is comprised of SL/LiTFSI/LiD-FOB with the molar ratio of 15:2:1. As shown in Figure 2a, the disappearance of the vinyl signal (6.0-6.5 ppm) in MA and the downfieldshift of methine group in DAP indicates the generation of DAAP species. The eutectic solution with different weight ratios was then mixed with the thiol-vinyl systems, PEGDA 600 and PETMP or PEGDA 600 and POSS-SH, to prepare the GPE, and the detailed crosslinking process is shown in Figure S1, Supporting Information. The obtained electrolytes are named as PS 1 GPE n and PS 2 GPE n , respectively, where n represents the weight fraction of the eutectic solution. Variation of DES content results in a difference in gelation time, as shown in Figure 2b, 60 wt% (A), 70 wt% (B) and 80 wt% (C) of DES were added in the reaction system, respectively, and the higher of the eutectic content, the slower of the gelation rate. A full solidification can also be obtained within 3 h even with a high content of eutectic solution (80 wt%). This mild gelation process provides sufficient time for the precursor to fully infiltrate the electrode materials, which is totally suitable for the in-situ assembling of lithium batteries. In order to ensure a full crosslinking, the gelation time was prolonged to 8 h at room temperature for all the in-situ GPE systems. The FT-IR spectra shown in Figure 2c and Figure S2, Supporting Information confirm the complete crosslinking reaction between PEGDA 600 and PETMP with no obvious residue of the vinyl and thiol groups. The thermal stability and the ionconducting ability of the obtained gel electrolytes were then evaluated. As shown in Figure 2d, both the decomposition temperatures of DES and various GPEs are around 150°C, which is high enough for battery application. The introduction of abundant eutectic solution in the polymer network provides a smooth atmosphere for ion migration without any obvious phase transformation in the DSC curves (Figure 2e), the ionic conductivity increases with the content increasing of the eutectic, and a high value of 1.7 × 10 −4 S cm −1 at 60°C was obtained in the PS 1 GPE 80 (Figure 2f). The value of the activation energy (E a ) decreased with the increase of eutectic ( Figure S3, Supporting Information), the reduction of E a value is not significant with relatively low content of eutectic from 60 wt% to 70 wt%, while an obvious change was observed in PS 1 GPE 80 , which owns high eutectic content. This phenomenon indicates that the migration of lithium ions in PS 1 GPE 60 and PS 1 GPE 70 depends on both the movement of the polymer segment and the complexation with SL molecule, and when the content of eutectic reaches 80 wt%, abundant SL could better promote the migration of Li + because of the stronger coordination ability between SL and Li + than the crosslinked EO chains with Li + . In addition, the lithium-ion transference number ( Figure S4, Supporting Information) and the oxidation-resistance of the obtained electrolytes were significantly enhanced relative to conventional PEO-based electrolytes thanks to the eutectic component, the t Li þ of PS 1 GPE 60 , PS 1 GPE 70 and PS 1 GPE 80 is 0.48, 0.42 and 0.51, respectively, and the redox window of DES and GPEs are both about 4.8 V (Figure 2g), which provides the possible application for high-voltage lithium battery.
In-situ Li/LFP and lithium symmetric cells were then assembled in the similar way mentioned above and tested. The rate performance of the Li/LFP cells based on pure DES and various in-situ formed GPEs was firstly compared, as shown in Figure 3a, for the GPE loaded with low content of eutectic (PS 1 GPE 60 ), the lowest discharge specific capacities at different rates were observed in the corresponding LFP cell, and with the increase of eutectic component, the values of the specific capacities increase. The in-situ assembled Li|PS 1 GPE 80 |LFP cell delivers excellent rate performance similar with the one assembled with the liquid eutectic electrolyte even at high current densities, which is mainly due to the SL component in the eutectic solution, [27,28] and the detailed specific capacities are 141.5 and 126.8 mAh g −1 at 5 C and 10 C, respectively. The voltage-capacity profiles of Li|PS 1 GPE 80 |LFP cell shown in Figure 3b exhibit slight expansion of the polarization from 0.5 C to 5 C, indicating that the interface between the in-situ formed PS 1 GPE 80 and the electrodes is extremely stable with low interfacial impedance, and this stability can enable a steady charge-discharge cycling totally for 200 cycles with a gradient increased rate (1 C, 2 C, 3 C, 5 C and 1 C) in the Li|PS 1 GPE 80 |LFP cell (Figure 3c). The long-term cycling properties of Li|DES|LFP, Li|PS 1 GPE 60 |LFP, Li|PS 1 GPE 70 |LFP and Li|PS 1 G-PE 80 |LFP cells were also explored, and the results are listed in Figure 3d. It can be seen that, even though the cell assembled with pure DES owns high initial discharge specific capacity, a rapid degradation was then observed with only 66.2% of capacity retention after 667 times cycling at 1 C. The construction of polymer skeleton via the Michael addition reaction significantly enhances the cycling stability of LFP cells, but the participant of polymer chains in the ion transport also results in a decrease of the initial discharge capacity in the Li|PS 1 GPE 60 |LFP cell. When the content of eutectic solution exceeds 60 wt%, both the initial discharge capacity and the cycling stability of the cells are favorable. The initial discharge specific capacities of the Li|PS 1 GPE 70 |LFP cell and Li|PS 1 GPE 80 |LFP cells are as high as 164.1 and 155.9 mAh g −1 , respectively, and the batteries can steadily work for a long time with highcapacity retentions of 82% (800 cycle) and 80.1% (700 cycle), respectively.
The long life-span of these cells confirms the advantages of the eutectic solution and the in-situ click reaction in the preparation of advanced polymer electrolytes once again. In addition, the chargedischarge curves of Li|PS 1 GPE 80 |LFP cell in different cycles are shown in Figure 3e, although the specific capacity decays with the increase of the working cycles, there is almost no voltage increase in the polarization platform, which indicates that the interfacial impedance between the electrolyte and the electrode does not increase during the cell cycling. It is worth noting that, except for the polymer skeleton and the in-situ polymerization method, the trace amount of LiDFOB in the eutectic solution also exert an important effect on the interfacial stability. [29,30] In comparation, the Li|PS 1 GPE 80 |LFP cell without LiDFOB was also tested, the result of its long cycling is shown in Figure S5, Supporting Information, and an obviously decreased cycling stability was observed with 88% of capacity retention after 400 cycles (0.5 C). In addition, the cycling and rate performances of the Li|PS 1 GPE 80 |LFP cell at room temperature were also evaluated, as shown in Figure S6, Supporting Information, the corresponding discharge capacities of Li|PS 1 GPE 80 |LFP cell at different current densities are lower than that at 60°C, which is consistent with the decreased migration ability of Li + at room temperature, but the cell can still work steadily at 0.5 C without an obvious capacity decay after 30 cycles. In order to further improve the energy density, the lithium batteries assembled with high-voltage cathodes (LCO, NCM622 and NCM811) were also tested ( Figure S7, Supporting Information), low specific capacities and rapid decay in Li/NCM622 and Li/NCM811 full cell indicate that the cathode electrolyte interphase (CEI) layer formed between the electrolyte and the cathodes is not stable enough, and the presence of abundant liquid substitute continuously penetrates into the active materials and cause side reactions, thus resulting in the collapse of cathode structure and a performance decay. Above problems might be solved by enhancing the intermolecular interaction between the plasticizer, salt and the polymer matrix, [31] or combining with ceramic elecrolyte to construct a sandwiched electrolyte structure. [32] Li/LCO full cell (0.2 C) at room temperature was also tested ( Figure S8, Supporting Information), and a relatively stable cycling performance with a capacity retention of 89.1% was obtained owing to the mitigated interfacial reaction at lower temperature.
In-situ lithium symmetric batteries were also prepared to test the lithium deposition properties during a repeated plating-stripping process. The critical current densities (CCD) of the lithium symmetric cells based on DES and in-situ formed PS 1 GPE 60 , PS 1 GPE 70 and PS 1 GPE 80 are shown in Figure 3f and Figure S9, Supporting Information. Relatively poor current-tolerance with low CCD of 0.45 mA cm −2 was observed in the DES and PS 1 GPE 60 assembled lithium symmetric cells, while significantly improved CCDs were obtained in the Li|PS 1 GPE 70 |LFP (0.9 mA cm −2 ) and Li|PS 1 GPE 80 |LFP cells (0.85 mA cm −2 ), demonstrating the synergistic effect of polymer matrix and the eutectic solution on inhibiting the dendrite-growth. In addition, the thiol reactant containing inorganic ingredient, POSS-SH, was also used for the construction of in-situ electrolytes in order to further improve the comprehensive performances ( Figure S10, Supporting Information). Transparent GPEs can also be obtained with different content of eutectic, and the use of POSS-SH did not exert a significant influence on the ionic conductivity, t Li þ and the cycling performance of corresponding LFP cell ( Figure S11, Supporting Information), but the critical current density of Li|PS 2 GPE 60 |Li and Li|PS 2 GPE 70 |Li cells increased to 1.09 mA cm −2 ( Figure S12, Supporting Information) and 1.1 mA cm −2 (Figure 3f), respectively, which can be attributed to the inorganic silicon element. [33,34] The Li|PS 2 GPE 70 |Li cell can steadily work under a relatively high current density (0.5 mA cm −2 ) for 120 h without a short-circuit, while a very short plating-stripping cycling was observed in the Li|PS 2 GPE 60 |Li cell ( Figure S13, Supporting Information), which further indicates the importance of the eutectic solution. Furthermore, the long-term lithium plating and stripping under the current density of 0.1 mA cm −2 was also evaluated. As shown in Figure 3g, the Li|PS 1 GPE 80 |Li cell short-circuited at 665 h, while the cycling time can be prolonged up to 1200 h without short circuit for Li|PS 2 GPE 80 |Li cell, although the polarization voltage is slightly higher than that in PS 1 GPE 80 -based system. In addition, the overpotential of Li|PS 2 GPE 80 |Li cell increased with the plating-stripping cycles, which is because the SEI layer formed between the electrolyte and the lithium anode is not robust enough, and the repeated collapse and reconstruction of the interphase lead to an increase of interfacial impedance. The inserted graphs of Li|PS 2 GPE 80 |Li cell indicate a smooth ion-migration process during the repeated lithium deposition.
In order to better explicit the interfacial information between the in-situ formed GPEs and the lithium metal anode, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to analyze the surface morphology and component of the lithium anode in Li|DES|LFP and Li|PS 1 GPE 80 |LFP cells after 700 times cycling at 1 C, and the Li|PS 2 GPE 70 |LFP cells after 400 times cycling at 0.5 C. As shown in Figure 4a, cracked solid electrolyte interface (SEI) was observed in the cycled lithium anode of Li|DES|LFP cell, while the lithium anodes in PS 1 GPE 80 and PS 2 GPE 80 systems were covered with a smooth SEI layer without obvious dendrite-growth, indicating that the interaction between the polymer skeleton and the eutectic solution can decrease the side reaction between the electrolyte and the electrode. Relative to the lithium anode in Li|PS 1 GPE 80 |LFP, the lithium surface is cleaner in the inorganic silicon-containing PS 2 GPE 80 system.  [35][36][37] respectively, which is consistent with the corresponding O 1s and F 1s spectra. [38][39][40] In comparation with DES-assembled lithium anode, the content of organic components in the GPE system is higher, which is mainly derived from the polymer matrix. Instead, stronger signals of C-F bonds (F 1s and C 1s), Li 2 CO 3 (C 1s and O 1s) and LiF (F 1s, 684.8 eV) in the DES-related system, which belong to the decomposition of lithium salt, were clearly observed. [41][42][43][44][45] These results indicate the uncoordinated lithium salt in pure DES can be further bound by the polymer network in the GPE system, thus decreasing the side reactions between the electrolyte and the electrode. The B 1s spectrum shown in Figure S8, Supporting Information also confirms a less decomposition of LiDFOB in the PS 1 GPE 80 -assembled lithium surface. [46,47] In addition, the S 2p spectrum also found the less decomposition of lithium salt (168.5 eV (S 2p 3/2 ) and 169.6 eV (S 2p 1/2 )) and the signal of S-C bond (163.4 eV (S 2p 3/2 ) and 164.5 eV (S 2p 1/2 )) belonging to the polymer matrix. [48,49] In a word, the interaction between the polymer network and the eutectic solution can not only endow the cell with excellent ion-migration ability, but can also provide a stable electrolyteelectrode interface.

Conclusion
In summary, we have designed a novel eutectic solution for catalyzing the thiol-Michael addition reaction and in-situ fabricating GPEs. The use of this liquid eutectic mixture not only enables a mild and efficient gelation process for in-situ cell assembling without any introduction of non-electrolytic components and heating treatment, but also improves the migration of lithium ion. More importantly, good interfacial contact and favorable interfacial stability was obtained with the help of in-situ assembling process and the interaction between the polymer network and the eutectic. As a result, the assembled in-situ lithium symmetric batteries demonstrate good current-tolerance with the critical current density of 1.1 mA cm −2 and a steady lithium plating-stripping behavior over 1200 h at the current density of 0.1 mA cm −2 . In addition, excellent rate and long-term cycling performances (800 cycles, 1 C) are also obtained in the in-situ assembled Li/LFP cells. This work provides a novel and efficient strategy for the in-situ preparation of highperformance polymer electrolytes for lithium batteries.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.