In Situ High‐performance Gel Polymer Electrolyte with Dual‐reactive Cross‐linking for Lithium Metal Batteries

Lithium metal batteries have been considered as one of the most promising next‐generation power‐support devices due to their high specific energy and output voltage. However, the uncontrollable side‐reaction and lithium dendrite growth lead to the limited serving life and hinder the practical application of lithium metal batteries. Here, a tri‐monomer copolymerized gel polymer electrolyte (TGPE) with a cross‐linked reticulation structure was prepared by introducing a cross‐linker (polyurethane group) into the acrylate‐based in situ polymerization system. The soft segment of polyurethane in TGPE enables the far migration of lithium ions, and the ‐NH forms hydrogen bonds in the hard segment to build a stable cross‐linked framework. This system hinders anion migration and leads to a high Li+ migration number ( tLi+  = 0.65), which achieves uniform lithium deposition and effectively inhibits lithium dendrite growth. As a result, the assembled symmetric cell shows robust reversibility over 5500 h at a current density of 1 mA cm−2. The LFP¦¦TGPE¦¦Li cell has a capacity retention of 89.8% after cycling 800 times at a rate of 1C. In summary, in situ polymerization of TGPE electrolytes is expected to be a candidate material for high‐energy‐density lithium metal batteries.


Introduction
[3] However, the poor compatibility of Li metal with commercial carbonate-based electrolytes poses a challenge in practical applications of lithium metal batteries (LMBs). [4]7][8] Furthermore, the uneven Li plating/stripping can easily trigger the growth of Li dendrite, what is worse, the internal short circuit, consequently induces serious safety issues. [9]Therefore, solving the uncontrolled growth of lithium dendrites is now a major challenge for lithium metal battery applications. [10]So far, many methods have been proposed to solve the above problems to obtain high-performance LMBs, including adding additives to the electrolyte, [11] optimization of the anode, [12] and constructing stable artificial SEI films [13][14][15] to prevent the continuous reaction between the lithium cathode and the electrolyte and inhibit the growth of lithium dendrites.
Tremendous efforts have been devoted to designing the highly compatible electrolyte system to stabilize the Li metal anode.Replacing the commonly used separator/liquid electrolytes route with the solid/gel state electrolyte system can fundamentally solve the safety problem of lithium metal batteries. [16]Solid polymer electrolyte (SPE) is a composite of polymer and lithium salt, which has a high mobility number. [17]However, the SPE commonly shows low ionic conductivity at room temperature (<10 −4 S cm −1 ) and relatively high interfacial resistance due to its high crystallinity and poor interfacial contact with the electrode. [18]The emerging strategies such as the incorporation of active ceramic fillers are recently proposed to improve the ionic conductivity of SPEs, the optimized content of active fillers helps to reduce the crystallinity of the polymer matrix and to form continuous Li + conductive channels at the interface with the polymer. [19]However, limited by the inherent insufficiency of the inorganic material [20] and its agglomeration and inhomogeneous dispersion in the polymer matrix, it causes new difficulties for the engineering design of SPEs. [21]el polymer electrolyte (GPE) consists of organic electrolyte and polymer matrix, which exhibits better ionic conductivity than SPE. [22]he polymer matrix in GPE provides enough mechanical strength and toughness to resist the volume expansion during the lithium plating/ stripping and the short circuit and thermal runaway caused by lithium dendrite piercing the separator. [23]In addition, the GPE ensures the physical integrity of the electrolyte, and its liquid phase enables favorable interfacial wettability. [17]Currently, the most of GPEs are prepared as thin films, and the frequently used polymer substrates are polyethylene oxide (PEO), [24,25] polyvinylidene fluoride (PVDF), [26,27] and polymethyl methacrylate (PMMA). [28,29]However, the thin films do not fully exploit the advantages of GPE.Additional electrolyte needs to be introduced in order to completely wet the electrode, this facilitates the performance of the cathode material. [30]n situ polymerization of GPE is achieved by injecting a mixture of LE, polymer and initiator (generally AIBN [31] and BPO [32] ) into the cell as a precursor solution, and initiating the polymerization by UV light, [33] heat, [34] and radiation. [35]In addition, thermal initiation is used preferentially in most studies because the thermal energy is more easily transferred to the interior of the cell.The commonly used polymer monomers today include acrylates, [36] carbonates, [37] and cyclic ethers. [38]In situ polymerization of GPE ensures homogeneous contact between the solid and liquid phases at the molecular level and enhances their interactions through the additional cohesion capability of GPE. [17,39]Different polymer monomers can modulate the mechanical strength of the electrolyte, and GPE can be multi-functionalized by introducing groups and adding inorganic ceramics, functional additives, etc. [40,41] Therefore, a reasonably designed polymer framework can significantly improve the electrochemical performances of LMBs.
In this work, a tri-monomer copolymerized gel polymer electrolyte (TGPE) with a highly cross-linked structure was prepared by in situ polymerization in LE with poly (ethylene glycol) diacrylate (PEGDA), pentaerythritol triacrylate (PETA) and poly(propylene glycol), tolylene 2,4-disocyanate terminated (PPO).The soft segment mainly consists of polyether and polyester, which imparts proper softness, facilitates Li + transport, and improves Li + migration behavior.Furthermore, the hard segment such as the benzene ring ensures the overall mechanical strength of GPE. [42]In addition, -NH forms hydrogen bonds in the hard segments, which can build a cross-linked mesh structure and ensure mechanical strength.TGPE exhibits a high migration number (0.65) and excellent compatibility with lithium metal anode, which delivers highly reversible Li plating/stripping ability over 5500 h at a current density of 1 mA cm −2 .Furthermore, the TGPE is adapted to different cathode materials.The LFP¦¦TGPE¦¦Li cell achieves a capacity of 126.6 mAh g −1 with the excellent capacity retention of 89.8% for 800 cycles at a rate of 1C.More impressively, TGPE achieves capacity retention of 79.3% for 400 cycles at a rate of 1C with the LiNi 0.8- Co 0.1 Mn 0.1 O 2 cathode.

Results and Discussion
The synthesis process is shown in Figure 1 and The polymerization mechanism is shown in Figure S1, Supporting Information. [42,43]The precursor solution is injected into the cell with the heat treatment at 60 °C, and the AIBN decomposes, generating free radicals that trigger the co-polymerization of PEGDA, PETA, and PPO.The isocyanate in PPO and the hydroxyl group in PETA polymerize under the catalysis of dibutyltin laurate (DBTDL), forming a cross-linked reticulation structure (Figure 2a).The -NH in TGPE forms intermolecular hydrogen bonds with the C=O (Figure S2, Supporting Information).The polymer chain of the other three polymers shows a disordered arrangement (Figure 2b-d).Specifically, the straight chains of PEGDA became longer after polymerization (Figure 2c).
As shown in Figure S3, Supporting Information, the synthesized TGPE showed a translucent gel state from a macroscopic scale, and the success of the polymerization reaction was further verified by Fourier transform infrared spectroscopy (FTIR) (Figure 2e).Upon polymerization, the absorption peak from the stretching vibration of C=C (1630 cm −1 ) almost disappeared in the polymer matrix, and the absorption peaks of -NCO (2270 cm −1 ) in PPO and -OH (3525 cm −1 ) in PETA disappeared in the polymer matrix, and the absorption peak associated with N-H (2971 cm −1 ) appeared in the polymer matrix, indicating that the polymerization was completed. [44,45]In addition, Figure 2f,g provides the 1 H NMR spectra and 13 C NMR spectra of the three monomers and TGPE.The peaks at 5.85, 6.09, and 6.28 ppm in PETA and 5.96, 6.19, and 6.31 ppm in PEGDA represent the chemical shifts of hydrogen atoms on C=C. [46]The peaks at 4.26 ppm in PETA represent the chemical shift of the hydrogen atom in the hydroxyl group and the peak at 124.1 ppm in PPO denotes the chemical shift of the carbon atom in the -NCO group.These peaks disappeared in TGPE, and the chemical shift peak of the hydrogen atom on the C adjacent to polyurethane (4.65 ppm) and the chemical shift peak of C=O on polyurethane (155.3 ppm) appeared in TGPE, [47] confirming the success of the co-polymerization.The conversion rates of different monomers were studied by NMR hydrogen spectroscopy.The monomer conversions can be estimated from the ratio of the integrated area of the monomer in the post-polymerization gel to the monomer CH 2 = in the precursor solution. [48]The conversion rates of PEGDA only and PETA only were 86.5% and 93.9%, respectively.Since the chemical shift peaks of the PEGDA and PETA double bonds overlapped, the conversion of PEGDA and PETA polymerization was estimated by the sum of the integrated areas of the two monomers, which was 91% (Figure S4, Supporting Information).When TGPE was synthesized, the conversion of PEGDA and PETA polymerization was 92.1%, and the conversion of PETA and PPO polymerization was also estimated by the ratio of the integrated area of -OH in the post-polymerization gel to that in the precursor solution, which was 96.6%.
We further investigated the fire resistance of the liquid electrolyte and the obtained gel polymer electrolytes (Figure S5, Video S1, Supporting Information).The liquid electrolyte tends to catch fire and continuously burn after ignition.Remarkably, TGPE does not continue to burn after leaving the fire source, which proves that TGPE can effectively immobilize the solvent, and prevent the leakage and volatility of the liquid electrolyte.Figure 3a shows the thermogravimetric analysis (TGA), the weight loss of TGPE occurs at above 100 °C, and this mass loss mainly comes from the volatilization of the solvent, and the final mass retention is 33%.Since PETA is a trifunctional group cross-linked to a higher degree and cross-linked density, the solvent is wrapped in the reticulated polymer to form a continuous and stable polymer electrolyte.This results in the GPE retaining part of the solvent at a high temperature.It is easier to lock the electrolyte molecules in the system when the crosslinking density increases, so there is less loss during the subsequent heating process.
The ionic conductivity and mobility number of the electrolyte have a decisive influence on the performance of the battery.In order to investigate the effect of different components in the electrolyte on the ionic conductivity, the ionic conductivities of different GPEs and LE at the temperature ranging from 30 to 80 °C were tested.The Arrhenius plots of ionic conductivity of each GPE with LE are shown in Figure 3b.The LE shows the largest R2 error, indicating that the ionic conductivity of LE is easily influenced by the temperature.The ionic conductivity of TGPE at room temperature is 8.676 × 10 −4 S cm −1 , which is slightly lower than that of liquid electrolyte (1.871 × 10 −3 S cm −1 ), owing to the polymer content of GPE being more than 7%. [17]The ionic conductivity of TGPE is higher than the other three gel electrolytes because the Energy Environ.Mater.2024, 7, e12497 introduction of the PPO chain segment enables the fast migration of lithium ions. [42]The ionic conductivity of each GPE is in accordance with the Arrhenius equation: where A is the exponential prefactor, E a is the activation energy, k is the Boltzmann constant, and T is the Kelvin temperature.The fitting results are shown in Table S1, Supporting Information, the A value of TGPE is 0.28, which is lower than that of the liquid electrolyte (0.8), and the A of the electrolyte is related to the activation entropy (ΔS) of the liquid in Eyring theory; the smaller values in TGPE than in LE indicate that ion motion is less random (i.e., more correlated) than in LE. [49] In addition, the A of the electrolyte is also related to the carrier concentration, with more dissociated ion pairs in LE. [50] the E a value of 0.14 eV in TGPE is very close to that of the liquid electrolyte (0.16 eV), indicating that the activation energy potential of ion transport in TGPE is similar to that of LE.In summary, although the addition of the polymer reduces the carrier concentration in the system, it does not limit the ion movement and the mobility of dissociated ions remains high.The steady-state current method was used to measure the t Li þ , which is a key factor in evaluating the lithium-ion migration in the electrolyte.The polarization curve of the symmetric cell and the impedance plots before and after the steady-state are shown in Figure 3c.The equivalent circuit model of the impedance of the Li¦¦Li symmetric cell is shown in Figure S6, Supporting Information.R b is the bulk resistance, which is related to the electronic conductivity of the electrode material.The semicircle in the mid-frequency region is the interfacial impedance (R s ), which represents the resistance of Li + passing through the interface.The low-frequency region is the charge transfer impedance (R ct ), which represents the desolvation resistance of Li + before crossing the interface. [48]PE has a high t Li þ (0.65) (Specific parameters in Table S2, Supporting Information), while the t Li þ of liquid electrolyte is only 0.4 (Figure S7, Supporting Information).On one hand, the polymer framework can limit the self-diffusion of anions and electrolyte molecules.On the other hand, the organized chain structure of the polymer can provide fast lithium ion channels.The t Li þ of P(PEGDA) is only 0.39, which is due to the low degree of cross-linking of the chain segments.The polymerization of DGPE and P (PETA) is a haphazard molecular chain, which can also restrict the movement of the anion and make the t Li þ increase.
To investigate the high voltage stability of the gel electrolyte, the electrochemical window of the Stainless Steel (SS)¦¦Li cell was tested by linear scanning voltammetry (LSV) in the voltage range of 3.0-6.0V with a scan rate of 1 mv s −1 .As shown in Figure 3d, the low oxidation current of TGPE persists until 4.5 V, which is higher than the irreversible oxidation voltage of LE (4.2 V).Other GPEs also show similar oxidation voltages to the TGPE.This is due to the excellent chemical stability of polymer and the immobilization of the anion, which suppresses the anodic current and thus increases the electrochemical window.The interfacial kinetics can be investigated by the nucleation overpotential of lithium deposition on the copper foil surface.As shown in Figure 3e.The nucleation overpotential of the TGPE-based half-cell is 71 mV, which is lower than that of the liquid electrolyte (125 mV) because of the high Li + mobility number of the TGPE.Among other gel electrolytes, the P(PETA)-based half-cell has the highest nucleation overpotential (307 mV), which corresponds to its worst coulombic efficiency cyclability, while the DGPE (234 mV) and P(PEGDA) (266 mV) based half-cells have similar nucleation overpotentials.The above results were analyzed in conjunction with the impedance test of the Li¦¦Li symmetric cell, the R ct of the TGPE-based symmetric cell is only 137.9 Ω (Figure S8, Table S3, Supporting Information), representing a low diffusion potential barrier of Li + from the electrolyte to the substrate surface.These lead to its low overpotential and nucleation barrier, which increase its electrochemical reversibility. [51]n order to further the intercalation and de-intercalation behavior of Li + in these electrolyte systems, the galvanostatic intermittent titration technique (GITT) measurements were carried out at a current density of 0.1C in the voltage range of 2.5-4.0V.The GITT curves of TGPE and Li + diffusion coefficients are shown in Figure 3f.The charge/discharge curve of TGPE has satisfactory symmetry and the voltage changes during the current pulse and open circuit are small, which indicates the favorable electrochemical reversibility and the superior Li + intercalation/de-intercalation processes. [52]The charge/discharge curves of the liquid electrolyte and the rest of the gel electrolytes are shown in Figure S9, Supporting Information.We can observe that although the LE, DGPE, P(PEGDA), and P(PETA) show symmetrical charge/discharge curves, the voltage variation during the current pulse and the open-circuit processes is larger than that of TGPE.These results were further confirmed by the theoretical calculations, the Li + diffusion coefficient of TGPE is 1.95 ×10 −10 cm 2 s −1 , which is better than that The average Coulomb efficiency of the lithium plating/stripping process was measured in lithium-copper cells using different electrolytes.The average Coulomb efficiency of the cell using TGPE was 97.70% (Figure 4a), which was higher than that of DGPE (89.56%),P (PEGDA) (94.03%),P(PETA) (91.64%), and LE (91.74%) (Figure S10, Supporting Information).In addition, the morphology of lithium deposited on Cu substrate was observed by scanning electron microscopy (SEM).In the Li¦¦DGPE¦¦Cu cell, the lithium deposited layer is a loose structure and has more cracks with a thickness of 38.7 μm (Figure S11a, Supporting Information).the lithium deposited layer becomes tighter but also has cracks in the Li¦¦P(PEGDA)¦¦Cu and Li¦¦P (PETA)¦¦Cu cells, with a reduced thickness of 18.97 μm (Figure S11b, Supporting Information) and 29.03 μm (Figure S11c, Supporting Information).There is mossy dead lithium production in the Li¦¦LE¦¦¦Cu cell with a thickness of 26.57μm (Figure S11d, Supporting Information).The lithium plated in the Li¦¦TGPE¦¦Cu cell has a dense particle morphology (Figure 4b) and the plating thickness is only 14.18 μm (Figure 4c).This dense lithium deposition can effectively reduce the side reactions between the lithium metal and the electrolyte, resulting in a high Coulomb efficiency as well as good cycling stability.
To further investigate the stability of the electrolyte systems, the lithium metal anode and the Li¦¦Li symmetric cells with different electrolytes were assembled and cycled at a current density of 1 mA cm −2 .As shown in Figure 4d, the Li¦¦LE¦¦Li symmetric cell exhibited a minimum overpotential of 50 mV, but the overpotential significantly increases to an extremely high value due to the serious parasitic reaction at the interface, indicating that the lithium metal-electrolyte interface was unstable.This is mainly due to the continuous thickening of SEI and the continuous growth of lithium dendrites. [53]Li¦¦P(PEGDA)¦¦Li symmetric cells show a gradual decrease in overpotential indicating that the P(PEGDA)-based electrolyte and separator were penetrated by lithium dendrites, and the growth of fine dendrites eventually pierced the separator, leading to short circuit, and cell failure.In addition, the drop in overpotential at the beginning of the Li¦¦DGPE¦¦Li and Li¦¦P (PETA)¦¦Li cycle is related to the hydroxyl group in the polymer, which is unstable in contact with the lithium metal, accelerating the accumulation of dendrites and failing with increasing polarization voltage in subsequent cycles.The overpotential of the TGPE-based cell is 100 mV, which maintains a stable voltage distribution and a flat voltage plateau over 5500 h.This indicates the TGPE can effectively optimize the lithium plating/stripping behaviors and suppress the growth of lithium dendrite.In addition, the charge/discharge curves of Li¦¦TGPE¦¦Li symmetric cells with different current densities and area capacities have been studied.At an area capacity of 3 mAh cm −2 and a current density of 1 mA cm −2 (Figure 4e), the Li¦¦TGPE¦¦Li symmetric cell was stably cycled for 1800 h and the overpotential decreased from an initial 110-90 mV.At an area capacity of 1 mAh cm −2 and a current density of 3 mA cm −2 (Figure 4f), the symmetric cell cycled steadily for 1700 h, although with a large overpotential (180 mV).At an area capacity of 5 mAh cm −2 and a current density of 5 mA cm −2 (Figure 4g), the Li¦¦TGPE¦¦¦ Li symmetric cell was still able to cycle stably for 900 h with an increased overpotential of 220 mV.In summary, although polyethers and polyesters in the polymer chain segments contribute to lithium ion migration, disordered polymer chains have no significant effect on lithium ion deposition.Especially, when there are residual reactive groups in the polymer chains, serious side reactions can occur and reduce the stability of the electrolyte to lithium metal.The intermolecular hydrogen bonds formed by polyurethanes in TGPE allow them to maintain an orderly chain structure and build a continuous ion diffusion channel.In addition, the restriction of anions by the stable framework of TGPE avoids the formation of a space charge region at the lithium-anode interface, reduces the side reactions caused by anions, forms a stable SEI during cycling, and achieves excellent longcycle performance of the symmetric cells.
The electrochemical performances of each GPE and liquid electrolyte were further investigated by assembling the Li¦¦LFP cell.First, the impedance of the cell before cycling was analyzed (Figure S12, Supporting Information).The Li¦¦TGPE¦¦LFP cell shows the lowest interfacial impedance, which is benefited by its excellent ion transportability, cohesion, and adhesion ability. [17]As shown in Figure 5a, the discharge-specific capacities of TGPE cells were 160, 153, 140, 126, 108, and 66 mAh g −1 at rate of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C with the voltage of 2.5-4.0V, respectively.When the rate returns to 0.2C, the discharge-specific capacity recovers to about 153 mAh g −1 , indicating the excellent rate performance of TGPE cells.It was comparable to that of the liquid electrolyte, especially the discharge-specific capacity of 66 mAh g −1 remains at a rate of 5C.Furthermore, the high Li + mobility number of TGPE enables the fast Li + kinetics and exhibits low and stable overpotential at a high rate, showing a satisfactory redox platform between 3.15 and 3.82 V (Figure 5b).By contrast, the other electrolytes show relatively poor rate performances, the DGPE-based cell has a discharge-specific capacity of only 30 mAh g −1 at 5C.The P (PEGDA)-based and P(PETA)-based cells show an increase in dischargespecific capacity when the current density return to 0.2C.The discharge capacity of P(PEGDA)-based cell changes from 141.5 to 148.6 mAh g −1 , and the discharge capacity of P(PETA)-based cell changes from 122.9 to 138.8 mAh g −1 .This means the cells need longer activation time.TGPE cells have an initial-specific capacity of 128 mAh g −1 , and the discharge-specific capacity remains 115 mAh g −1 after 800 cycles.The corresponding capacity retention rate is calculated to be 89.8%,which is obviously superior to the LFP¦¦LE¦¦Li cell (74.4%).Other GPEs showed large capacity decay during the cycle (Figure 5c), which was due to the poor Li + kinetics and irreversible consumption of active LM and electrolyte.Recent works on gel electrolytes are listed in Table S4, Supporting Information, and our work shows the best cycling stability.Figure S13, Supporting Information shows the charge/discharge curves of TGPE at different cycles.It can be seen that the redox platform is ranging from 3.3 to 3.5 V, which does not change with the increase in cycle times, further demonstrating the electrochemical stability of TGPE.
We further investigated the morphology of the lithium metal after cycling 100 times.The scanning electron microscopy (SEM) images are shown in Figure 5d.In the Li¦¦TGPE¦¦LFP cell, a uniform lithium deposition layer was found at the lithium surface, indicating that the intermolecular hydrogen bonding in TGPE ensures both a stable polymer framework and an orderly chain structure, which allows the soft segments to induce uniform deposition of lithium ions on the lithium metal surface.It also ensures sufficient mechanical strength of the TGPE, which enables it to effectively suppress lithium dendrites and mitigate the volume expansion of lithium metal during cycling.Furthermore, the absence of obvious dead lithium indicates that the excellent mechanical properties of TGPE can effectively inhibit the growth of lithium dendrites.In the liquid electrolyte cell, a thick and uneven lithium deposition layer with obvious lithium dendrite was observed, and a more severe volume expansion of LM is also shown in Figure S14a, Supporting Information.In the other GPE cells, there is the same thick lithium deposition layer with obvious cracks and lithium dendrites on the anode surface (Figure S14b, Supporting Information).The growth of lithium dendrite leads to the rupture of the polymer (Figure S14c,d, Supporting Information), indicating that the polymer without the introduction of polyurethane groups is not able to induce uniform Li + deposition and does not have sufficient mechanical strength to suppress the growth of dendrite.
X-ray photoelectron spectroscopy (XPS) was performed on the lithium metal surface after 100 cycling of Li¦¦TGPE¦¦LFP cells.The XPS of C 1s, F 1s, and O 1s after etching for 0, 60, and 120 s is shown in Fig- ure 5e.Typically, the C 1s spectrum can be divided into four peaks corresponding to C-C (284.8 eV), C-O (286.5 eV), C=O (287.6 eV), CF 2 (289.8eV), [48] and the surface composition was identified as ROCO 2 Li, Li 2 O, Li x PO y F z , Li 2 CO 3 , and LiF in combination with O 1s and F 1s.Among them, ROCO 2 Li and Li 2 CO 3 are compounds produced by the decomposition of carbonate solvents, [54] Li x PO y F z and Li 2 O are the decomposition products of LiPF 6 salt, and LiF is a compound produced by the decomposition of fluorinated solvent and lithium salt.LiF has high mechanical strength, which can further enhance the strength of SEI and inhibit the growth of lithium dendrites. [55]In addition, the peak intensity of ROCO 2 Li and LiF, the main components of SEI, changed lightly with increasing the etching time, indicating the formation of stable and homogeneous SEI.It helps suppress the parasitic reaction between lithium metal and electrolyte, thus enlonging the cycle life of the battery.
In addition, TGPE was coupled with the high-voltage LiNi 0.8- Co 0.1 Mn 0.1 O 2 (NCM811) cathode to demonstrate its versatility.As shown in Figure 5f, the discharge-specific capacities of Li¦¦TG-PE¦¦NCM811 cells are 200, 194, 182, 170, 153, and 108 mAh g −1 at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C with the voltage of 3.6-4.25V (Figure 5g), respectively.The initial-specific capacity of TGPE at a rate of 1C is 170.3 mAh g −1 , which is close to the liquid electrolyte (173.4 mAh g −1 ) but higher than other GPE cells (Figure 5h).The dischargespecific capacity remained at 133.6 mAh g −1 after 400cycles, with a capacity retention rate of 79.3%, which was better than that of liquid electrolyte (75.26%) and other GPE cells.The P(PETA)-based cell performs the worst exhibiting a discharge-specific capacity of only 88 mAh g −1 after 400 cycles, with a capacity retention rate of only 55.4%.
To demonstrate the commercial potential of the TGPE, Li¦¦TGPE¦¦LFP pouch cells (Figure 6a) were assembled for charge and discharge tests at a current density of 0.4 mA cm −2 , and their specific parameters are shown in Figure S15, Supporting Information.Typically, the mass loading of the LiFePO 4 cathode was 15 mg cm −2 and the injection coefficient was 3.6 Ah g −1 .As shown in Figure 6b, the initial discharge capacity of the Li¦¦TGPE¦¦LFP pouch cell is 80 mAh and the discharge capacity was maintained at 90 mAh.The pouch cell delivers a serve cycle life of 180 cycles and a coulomb efficiency of over 99%.In contrast, the Li¦¦LE¦¦LFP pouch cell failed after only 22 cycles (Figure S16, Supporting Information).In addition, to further demonstrate the commercial potential of TGPE, TGPE was applied to Graphite¦¦LFP pouch cell.The Graphite¦¦TGPE¦¦LFP pouch cell delivers a serve cycle life of over 800 cycles and a capacity retention rate of 85% (Figure S17a, Supporting Information).The specific parameters are shown in Figure S17b, Supporting Information.As shown in Figure 6c, the TGPE-based pouch battery can still light up the LED after folding and shearing (Video S2, Supporting Information).After shearing, the flame test shows that the LED continues to work under the flame-treating (Video S3, Supporting Information), indicating that the TGPE battery has a high safety level.Connecting two pouch batteries in series can also provide high output voltage for wireless charging of cell phones (Figure 6d), indicating the significant practicality of TGPE batteries under various conditions.

Conclusion
In summary, a cross-linked reticulation structure of GPE containing polyurethane groups was prepared by in situ polymerization, in which the -NH in the polyurethane hard segment forms hydrogen bonds to build a stable cross-linked structure that hinders the migration of anions, while the polyurethane soft segment contributes to the migration of lithium ions, thus achieving a high mobility number of 0.65.TGPE has an ionic conductivity of 8.676 × 10 −4 S cm −1 and high safety properties and an electrochemical window of 4.5 V, allowing it to be matched with high voltage cathodes.Benefiting from the fluorinated solvent as well as the polymer cross-linked structure forming a stable SEI, an electrodeelectrolyte interface with good compatibility was obtained, and uniform lithium deposition was achieved.The symmetric cell using TGPE achieved a stable cycle of over 5500 h at a current density of 1 mA cm −2 , and the LFP¦¦TGPE¦¦Li cell exhibited excellent cycle performance with a capacity retention of 89.8% at 1C for 800 cycles.Thus, this work provides a new strategy for the development and application of high energy density, long cycle, and high safety LMBs.Preparation of gel polymer electrolyte: Preparation of TGPE by in situ polymerization.PEGDA, PETA, and PPO were dissolved in the liquid electrolyte (1.0 M LiPF 6 dissolved in EMC/FEC, 9:1 by volume) in the molar ratio of 6:2:1, 5% dibutyltin dilaurate as the catalyst, and 1% AIBN as initiator.The TGPE was obtained by heat-initiated polymerization at 60 °C for 10 h.As a comparison, a di-monomer copolymerized gel polymer electrolyte (DGPE) was prepared by mixing PEGDA and PETA in a molar ratio of 3:1 and by self-polymerization of PEGDA and PETA separately, the preparation steps are similar to the above.All the above steps were carried out in an argon-filled glove box with moisture and oxygen content <0.01 ppm.

Experimental Section
Preparation of LiFePO 4 and NCM cathode: LiFePO 4 powder, carbon black and PVDF were dissolved into NMP in the mass ratio of 8:1:1, and the mixed slurry was coated onto the Al foil, and then the NMP was removed by vacuum drying at 120 °C for 24 h.The mixture-coated Al foil was cut into 12 mm diameter discs, and the mass loading of the LiFePO 4 cathode was 3.5 mg cm −2 .NCM cathodes can be prepared by replacing the LiFePO 4 powder with NCM powder using the above method, the mass loading of the NCM cathode was 4 mg cm −2 .
Characterization: The chemical structures and interactions between functional groups of the samples were examined by Fourier infrared spectroscopy (FTIR, Thermo Scientific Nicolet 6700). 1 H NMR and 13 C NMR spectra were performed with Bruker Avance III 400 MHz and DMSO was used as a deuterated reagent.The surface morphology of the samples was characterized by scanning electron microscopy.Thermogravimetric (TG) curves related to the small molecule content and thermodynamic processes of the samples were measured on a NETZSCH STA2500 in an N 2 atmosphere at a heating rate of 10 °C min −1 over the temperature range of 30-800 °C.The surface morphology of lithium metal Energy Environ.Mater.2024, 7, e12497 was characterized by scanning electron microscopy (SEM, TESCAN MIRA LMS).In-depth X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific K-Alpha spectrometer.
Electrochemical measurements: The ionic conductivity of the samples was characterized by AC impedance spectroscopy between 30 and 80 °C.AC impedance maps were acquired on an electrochemical workstation (Multi Autolab M204) from 0.1 MHz to 100 Hz with an amplitude of 10 mV.The cells were assembled from prepared gel polymer electrolytes, stainless steel sheets, and polypropylene separators (Celgard 2500).The ionic conductivity was calculated by the following equation: where σ is the ionic conductivity, R b is the bulk resistance, L is the thickness of the polypropylene separator, and S is the contact area between the stainless steel sheet and the polypropylene separator.Symmetric Li¦¦TGPE¦¦Li cells were assembled and current-time curves were recorded when a polarization voltage of 10 mV was applied to them.The AC impedance of the lithium/electrolyte/lithium battery was measured before and after polarization at frequencies ranging from 0.1 MHz to 0.01 Hz.The lithium-ion migration number (t Li þ ) was calculated by the following equation: where ΔV is the applied polarization voltage; I 0 and I s are the initial and steady currents, respectively; R 0 and R ss are the interface resistances before and after polarization, respectively.SS¦¦electrolyte¦¦Li cells were assembled and the electrochemical window stability of the samples was tested by the Linear sweep voltammetry (LSV) method.The voltage range was 1-6 V and the scan rate was 1 mV s −1 .
The test method for the average Coulomb efficiency of Li¦¦Cu cells was reported by Zhang et al. [56] The Cu substrate was pretreated with one lithium deposition/stripping cycle with a capacity of 5 mAh cm −2 at a current density of 0.5 mA cm −2 , followed by deposition of a lithium bank (Q T ) at a capacity of 5 mAh cm −2 .Afterward, the cell is discharged-charged at a capacity of 1 mAh cm −2 (Q C ) for n cycles, followed by a final stripping of the remaining Li reservoir at a current density of 0.5 mA cm −2 to 1 V.The final stripping charge (Q S ), corresponding to the quantity of Li remaining after cycling, is measured.The average CE over n cycles can be calculated as: The half cell is assembled with LFP or NCM as cathode, lithium foil as the anode, polypropylene (Celgard 2500) as separator and gel electrolyte precursor as the electrolyte.In situ polymerization occurred at 60 °C for 10 h after assembly.All charge/discharge tests of the batteries were using the LANHE CT2001A battery testing system.The voltage range of the LFP half-cell was 2.5-4.0V, and the NCM half-cell was 2.8-4.3V.
The Li¦¦LFP cells were subjected to GITT experiments using an Autolab 302 system with a voltage range of 2.5-4.0V.Each polarization process consisted of a current pulse of 0.1 mA for 10 min followed by an open-circuit resting period of 10 min to relax to the quasi-equilibrium potential.In addition, the chemical diffusion coefficient D Li þ (cm 2 s −1 ) was calculated according to the following equation:

Figure 3 .
Figure 3. a) Thermogravimetric analysis (TGA) curves of P(PETA), P (PEGDA), DGPE, and TGPE.b) Arrhenius curves of LE, P(PETA), P(PEGDA), DGPE, and TGPE.c) Chronoamperometry profiles curves of Li¦¦TGPE¦¦Li cells at a polarization voltage of 10 mV.The inset shows the corresponding EISs of the symmetric cell before and after polarization.d) LSV curves of LE, P (PETA), P(PEGDA), DGPE, and TGPE with lithium foil as the reference electrode and stainless steel as the working electrode at a scan rate of 1 mV s −1 .e) Discharge curves of Li¦¦Cu half-cells with different electrolytes at Li nucleation overpotential.f) The GITT curves of TGPE and chemical diffusion coefficient curves of Li + (D Li þ ) at a potential range of 2.5-4.0V with a current of 0.1C.

Figure 4 .
Figure 4. a) CE avg tests of Li plating/stripping in Li¦¦TGPE¦¦Cu.SEM images b) top and c) cross section of the Li deposition obtained by plating on Cu substrate in Li¦¦Cu cells using TGPE.d) Polarization voltage curves of Li¦¦Li symmetric cells with different electrolytes at a current density of 1 mA cm −2 and a capacity of 1 mAh cm −1 .Polarization voltage curves of Li¦¦Li symmetric cells with TGPE at e) a current density of 1 mA cm −2 and a capacity of 3 mAh cm −1 , f) a current density of 3 mA cm −2 and a capacity of 1 mAh cm −1 , and g) a current density of 5 mA cm −2 and a capacity of 5 mAh cm −1 .

Figure 5 .
Figure 5. a) Rate performance of Li¦¦LFP cells with different electrolytes.b) Charge and discharge curves of Li¦¦TGPE¦¦LFP at different rates.c) Cycling performance of Li¦¦LFP cells with different electrolytes at 1C. d) SEM images of LM cross section (top) and surface (bottom) of Li¦¦TGPE¦¦LFP after 100 cycles at 1C. e) The high-resolution XPS spectra of the C 1s, F 1s, and O1s for LM after etching 0, 60, and 120 s. f) Rate performance of Li¦¦NCM811 cells with different electrolytes.g) Charge and discharge curves of Li¦¦TGPE¦¦NCM811 at different multipliers.h) Cycling performance of Li¦¦NCM811 cells with different electrolytes at 1C.