English

• Lithium metal batteries (LMBs) have attracted much attention due to its high theoretical specific capacity (3860 mAh/g) [1, 2]. Unfortunately, safety hazards generating from the uncontrollable interfacial reactions between liquid electrolytes (LEs) and high energy electrodes still hinder the practical application of LMBs [3].

  Considering this situation, replacing LEs with gel polymer electrolytes (GPEs) will efficaciously improve the cycling performance and safety of LMBs. GPEs generally consist of polymer network and small organic molecules, which present good physical properties to resist volume deformation of electrodes [4, 5]. Even so, traditional strategies applied to prepare GPEs are attributable to ex-situ preparation methods, resulting in the low lithium-ion transference number and poor interfacial contact [6-8]. Accordingly, researchers have proposed the in-situ polymerization of GPEs to ameliorate these problems. This methodology can effectively bridge an integrated interface between the electrode particles and GPEs, thus deliberately enhancing the interfacial conformability [9]. During a routine procedure, a precursor solution including initiators, monomers, and LEs are primarily injected into the cell package. After effectively wetting the active materials, a certain external condition is took to initiate polymerization inside the battery, thus crafting steady conformal interfaces [10].

  Although the advantages of in-situ polymerization are obvious, most previous in-situ polymerization components are initiators and non-electrolytic monomers, which is not easily implementable under moderate external conditions [11]. Therefore, a facile preparation method for in-situ GPEs must be developed to meet the practical requirements. Among all the candidate monomers, 1,3-dioxolane (DOL) is able to form poly(1,3-dioxolane) (PDOL) via ionic ring-opening polymerization, which can be initiated by most of the Lewis acids [12]. It is believed that the electrochemical reduction products generated from PDOL forms passivate interphases on Li anode to prevent continuous degradation of GPEs, enabling highly reversible charge-discharge performance [13]. Nevertheless, the non-crosslinked structure of obtained PDOL is unable to supply sufficient mechanical properties to inhibit the volume deformation of electrodes [14]. Actually, this problem can be ameliorated by adding inorganic nano fillers into the GPEs.

  Silicon dioxide (SiO₂) nanofibers have been proved to enhance the thermal stability, mechanical strength, and electrochemical performances of GPEs [15, 16]. With the incorporation of SiO₂ nanoparticles, the pathways are created to transport Li⁺ through constructing the interactions between SiO₂ surface and anions [17]. However, this transportation path is inconsecutive, which means that a connected Li⁺ transfer network cannot be built, owing to the isolation distribution characteristics of nanoparticles [18]. Thus, the effect of enhancing Li⁺ mobility is reduced. Considering this situation, because of the highly efficient in synthesizing the nanofibers with high porosity and favourable flexibility, electrospinning technique can be applied to transform SiO₂ into a unique 3D nanofiber frame [19]. Such 3D reinforced architecture via incorporating GPEs into a SiO₂ spinning membrane will take effect as an enhancement strategy for interfacial Li⁺ transport channel. Even so, less reports focus on the combination between 3D SiO₂ nanofiber frame and in-situ SNCGPEs.

  In this study, DOL is infiltrated into the SiO₂ porous membrane to fabricate PDOL via the process of in-situ ionic ring-opening polymerization (Scheme 1). The oxygen atoms of PDOL together with Si-O of SiO₂ construct a more efficient channel for Li⁺ migration. Consequently, the t_Li⁺ and σ at 30 ℃ of PDOL-SiO₂ are separately 0.80 and 1.68 × 10⁻⁴ S/cm. PDOL-SiO₂ manifests the electrochemical decomposition potentials of 4.90 V. At 0.5 mA/cm², Li|PDOL-SiO₂|Li cell shows a steady cycling performance for nearly 1400 h. LFP|PDOL-SiO₂|Li battery can cycle steadily at 0.5 C at 30 ℃, keeping a capacity retention rate of 89% after 200 cycles. Even at 2 C, the capacity retention rate can maintain at 78% after 300 cycles. It is obvious that SiO₂ spinning membrane makes PDOL perform a better property in LMBs than that of PE membrane. This contribution provides a innovative strategy for accelerating Li⁺ transportation via designing PDOL molecular chains throughout the SiO₂ membrane, which is crucial for high-energy-density LMBs.

  Scheme 1

  

  Scheme 1.  Fabrication process of in-situ polymer electrolytes (PDOL-SiO₂).

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  For preparing SiO₂ nanofiber membrane, the stable and homogenous tetraethyl orthosilicate (TEOS) and poly(vinyl alcohol) (PVA) solutions (TEOS: PVA = 8:1) were fabricated using a conventional electrospinning parameters (Voltage: 20 kV; air gap distance: 15 cm; inner diameter of spinneret: 0.4 mm; flow rate of solution: 1 mL/h). The optimal concentration of the PVA/TEOS solution was 11% w/w. In addition, the experiment was conducted consistently at 25 ℃ in the air. Finally, SiO₂ membrane were calcinated for 8 h at 800 ℃ in air to take away the residual PVA. In the process of in situ preparation of PDOL-SiO₂, about 2 mol/L LiTFSI and 0.3 mol/L LiDFOB were added into a mixed solution of DOL reagent with 30 wt% SN and completely dissolved at 30 ℃ with continuous stirring for 1 h. About 200 µL of the precursor solution was injected into a SiO₂ membrane and then assembled into the cells. The assembled cells were aged for 120 min to make sure that the electrodes were well-wetted with the precursor solution. Subsequently, heating at 60 ℃ for 12 h would make the DOL completely polymerize into PDOL. For comparison, polyethylene (PE) membrane was applied to combine with the precursor solution in the same way. The other information including chemical materials, preparation of LiFePO₄ (LFP) cathodes, materials characterization, and electrochemical measurements were provided in supporting information.

  The morphologies of SiO₂ membrane and PDOL-SiO₂ were observed by SEM. Obviously, SiO₂ membrane presents a porous structure composed of nanowires with a diameter of about 200 nm (Figs. S1a and b in Supporting information), which will assist in adsorbing precursor solutions and maximizing the t_Li⁺ of PDOL-SiO₂ after ionic ring-opening polymerization [20]. In Figs. S1c and d (Supporting information), after the in-situ ionic ring-opening polymerization, SiO₂ membrane manifests a wrinkly surface, meaning that the entire SiO₂ support are full of PDOL. As expected, thermal polymerization reaction make PDOL-SiO₂ form a high compatibility interface to act as Li⁺-conductive path [21]. Figs. S2a–e (Supporting information) illustrate the digital photographs of flexural strength of SiO₂ membrane, clearly demonstrating that the SiO₂ membrane has good bending resistance, which is beneficial to support the PDOL. In Figs. S2g (Supporting information), it is clearly that liquid precursor solution translate into a transparent gel when compared with that in Figs. S2f (Supporting information), indicating that DOL monomers polymerize into the large molecules.

  FTIR was took to verify the chemical groups evolution of PDOL-SiO₂ (Fig. 1a). For DOL, the peaks located at 1029 cm⁻¹ and 1081 cm⁻¹ are attributed to the C-O-C structure, while peak at 915 cm⁻¹ is ascribed to the C-H out of plane vibration [22]. Compared with DOL, the PDOL-SiO₂ displays an obvious displacement in the C-O-C vibration and the disappearance of the C-H out-of-plane vibration, suggesting that the DOL monomers are bound together by ionic ring-opening polymerization [23]. This also confirms the macroscopic phenomenon observed in Figs. S1 (Supporting information). Moreover, the peaks belong to -CF₃ and -SO₂ structures are observed in the range of 1000~1400 cm⁻¹ in spectral line of PDOL-SiO₂, which mean the successful addition of lithium salt. For SN, the peak located at 2254 cm⁻¹ is attributable to nitrile group, while the added PDOL decreases the intensity of this peak owing to the interaction between PDOL and SN [24].

  Figure 1

  

  Figure 1.  (a) Fourier infrared spectrometer (FTIR) spectra of materials at 4000~400 cm⁻¹. X-ray photoelectron spectroscopy (XPS) spectra on (b) C 1s, (c) Li 1s and (d) O 1s of PDOL-SiO₂.

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  XRD patterns of SiO₂, PE, PDOL-SiO₂, and PDOL-PE are also showed in Figs. S3a and b (Supporting information). PE shows two sharp peaks from 20° to 25°, which validate that PE membrane presents the crystal structure [25]. In the case of PDOL-PE, the above peaks can still be observed, which indicates the maintainence of crystal structure with the introduction of PDOL. Although the appropriate crystallinity will be beneficial to the stability of mechanical properties, the crystal phase is not only inconducive to the Li⁺ transportation, but also increased the impedance of the electrolyte [26]. On the contrary, only a broad peak at about 21° is seen in both the pattern of PDOL-SiO₂ and SiO₂, confirming the amorphous phase of electrospun SiO₂ fibrous membrane and PDOL-SiO₂. The amorphous phase is able to capture more polymeric pecursor solution and form a closer relationship between supporting membrane and PDOL, which is crucial for transporting Li⁺. Thus, the amorphous characteristics will make PDOL-SiO₂ manifest a better Li⁺ transportation than that of PDOL-PE.

  TGA measurement was used to examine the decomposition process of PDOL-SiO₂ (Fig. S3c in Supporting information). For SiO₂, a negligible weight loss in the range of 30~800 ℃ is showed in the curves, indicating that this membrane displays a excellent thermostable performance. The polymerization leads to a improvement of the stability up to 100 ℃ for PDOL, while PDOL-SiO₂ makes a gradual weight loss in the range of the same temperature, which may be due to the elimination of moisture [27, 28]. Both of them exhibit the weight loss start from 400 ℃, being attributed to the decomposition of LiTFSI [29]. Because of the addition of SN, the additional mass drop in the range of 150~300 ℃ is also observed in TGA curves PDOL and PDOL-SiO₂ [30]. Specially, compared to PDOL, the weight before 450 ℃ of PDOL-SiO₂ reduces more slowly, which mean that the application of SiO₂ membrane is beneficial for improving the temperature resistance of PDOL.

  DSC measurement was employed to examine the physical properties of PDOL-SiO₂. For comparison, PE membrane is also applied as a support for PDOL. Fig. S3d (Supporting information) displays the DSC curves of PDOL-SiO₂ and PDOL-PE. The T_g of PDOL-PE is at around -8 ℃ and no endothermic peak can be observed, which demonstrated that PDOL-PE exits in amorphous state due to the absence of further transitions above T_g [31]. In contrast, no thermodynamic transition behavior can be observed in curves of PDOL-SiO₂, indicating that PDOL-SiO₂ remains the amorphous state within the test temperature range. Thus, PDOL-SiO₂ possesses the improved segmental motion in the networks of SiO₂ with a wider range of adaptive temperatures, which may bring the higher t_Li⁺ [32, 33].

  To investigate the chemical change in the materials, XPS was applied to analyze the PDOL-SiO₂ and SiO₂ membrane (Figs. 1b–d). In the C 1s and O 1s spectra of PDOL-SiO₂, the C-C (284.84 eV) and C-O (286.71 eV and 532.14 eV) peaks are assigned to the molecular chains of PDOL. In addition, C=O (288.12 eV and 533.80 eV) and MeCOO- (289.55 eV) peaks belonged to ester structures of LiDFOB also appear, while the observation of C-F peak (292.94 eV) is ascribed to the addition of LiTFSI and LiDFOB. For Li 1s, peak at 56.10 eV can be assigned to the Li⁺ of LiTFSI and LiDFOB. Thus, FTIR spectroscopy together with XPS analysis confirm the successful fabrication of PDOL-SiO₂. Generally, the binding energy (BE) of peak shift positively mean the electrons transfer among different atoms [34]. Thus, when compared with Si-O peak (533.01 eV) of SiO₂ in Fig. S3e (Supporting information), that peak of PDOL-SiO₂ in Fig. 1d shift positively to 533.08 eV. Such positive shift proves the coordination between Li⁺ and oxygen atoms of SiO₂, indicating that the SiO₂ membrane is also involved in the transportation of Li⁺.

  Fig. S4 (Supporting information) and Fig. 2a show the EIS and σ of the PDOL-SiO₂ and PDOL-PE in the temperature range from 0 to 90 ℃. The σ at 30 ℃ of PDOL-SiO₂ is 1.68 × 10⁻⁴ S/cm, higher than that of PDOL-PE (1.43 × 10⁻⁵ S/cm). A higher σ is indicative of the faster ionic migration, which will improve the charge-discharge performance of batteries at room temperature. Furthermore, as an significant parameter, the t_Li⁺ is employed to evaluate the Li⁺ transport efficiency of lithium batteries (LMBs) (Fig. 2b). t_Li⁺ for PDOL-SiO₂ is 0.80 at 30 ℃, which is larger than that of GPEs in recent reports (Fig. S5 in Supporting information). The large t_Li⁺ will reduce concentration polarization and shows a positive effect on the charge-discharge capacity of batteries, as the low t_Li⁺ will cause concentration gradients in electrolytes and finally leads to a premature failure of batteries [35]. The high σ and t_Li⁺ together confirm the vital function of SiO₂ membrane in accelerating the Li⁺ transportation as analyzed above.

  Figure 2

  

  Figure 2.  (a) Temperature-dependent ionic conductivity for PDOL-SiO₂ and PDOL-PE. (b) Chronoamperometry profiles and AC impedance spectra of lithium symmetric cell with PDOL-SiO₂ before and after polarization. (c) LSV curves of PDOL-SiO₂ at 3.0~6.0 V. (d) CV curves of LFP|PDOL-SiO₂|Li battery at 2.4~4.5 V.

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  LSV was applied to obtain the appropriate electrochemical window of the electrolyte for the following cycling investigation (Fig. 2c). It shows that the decomposition potentials of PDOL-SiO₂ at 30 ℃ is about 4.9 V, which is much higher than that of PEO (3.8 V). The high oxidative resistance of inorganic ceramic material mainly accounts for this result [36].

  To evaluate the cathode compatibility of PDOL-SiO₂, LMBs of LFP|PDOL-SiO₂|Li were measured at 30 ℃ by different methods. CV curves of LFP|PDOL-SiO₂|Li are shown in Fig. 2d. The oxidation-reduction process of Fe²⁺/Fe³⁺ of LFP|PDOL-SiO₂|Li displays a couple of redox peaks in the CV curves, which is related to extracting/embedding process of Li⁺ [37]. All samples exhibit conspicuous overlaps and small potential differences due to the favourable ionic conductivity, which represents the enhancement of electrochemical reversibility [38].

  The cycling performance of batteries using PDOL-SiO₂ or PDOL-PE as electrolyte were investigated at 30 ℃. The LFP|PDOL-PE|Li battery can only keep a capacity retention rate of 56% after 60 cycles at 0.5 C (Fig. S6a in Supporting information). More seriously, when under a constant current of 1 C, the capacity retention rate will decrease to 50% after 100 cycles (Fig. S6b in Supporting information). By contrast, LFP|PDOL-SiO₂|Li presents a capacity retention rate of 89% after 200 cycles at 0.5 C, while that is 89% after 100 cycles under a constant current of 1 C, which corresponds to discharge capacities of 120.11 and 114.89 mAh/g, respectively (Figs. 3a and b, Figs. S7a and b in Supporting information).

  Figure 3

  

  Figure 3.  Cycling performance of LFP|PDOL-SiO₂|Li batteries at (a) 0.5 C, (b) 1 C, (c) various current densities and (d) 2 C at 30 ℃.

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  The rate performances of LFP|PDOL-SiO₂|Li and LFP|PDOL-PE|Li were also investigated at 30 ℃ from 2.4 V to 4.2 V (Fig. 3c and Fig. S6c in Supporting information). The discharge specific capacity of LFP|PDOL-SiO₂|Li at the currents of 0.5 C, 1 C and 2 C are 137.4, 115.7 and 93 mAh/g, respectively (Fig. S7c in Supporting information), which are higher than those of LFP|PDOL-PE|Li. LFP|PDOL-SiO₂|Li presents a better discharge specific capacity than that of LFP|PDOL-PE|Li either at low or at high rates, which is attributed to the faster Li⁺ transportation resulting from the construction of fast ion channel between SiO₂ and PDOL. Based on this, LFP|PDOL-SiO₂|Li cycling at 2 C are also investigated at 30 ℃. PDOL-SiO₂ makes LFP operate stably for 300 cycles under a constant current of 2 C with the discharge capacity and capacity retention rate of 88 mAh/g and 78%, respectively (Fig. 3d and Fig. S7d in Supporting information).

  To understand the cycling behaviors of LFP|PDOL-SiO₂|Li and LFP|PDOL-PE|Li, the EIS of the above batteries recorded before cycling was compared with that at 100^th cycle (Fig. S8 in Supporting information). The R₀ at high frequency represents ohmic resistance of lithium battery, while R₁ is the interface impedance between lithium anode and GPEs [39]. Moreover, the R₂ of LFP-GPEs interface impedance at intermediate-frequency and the Li⁺ diffusion in LFP cathodes presented by a low-frequency tail are also observed in EIS curves before cycling and after 100 cycles [39]. With the assist of Zview software, the variation of fitted resistance of LFP|PDOL-SiO₂|Li and LFP|PDOL-PE|Li before cycling and after 100 cycles are showed in Fig. S9 (Supporting information). After cycling at 30 ℃, the R₀ of LFP|PDOL-SiO₂|Li and LFP|PDOL-PE|Li increase from 8.04 and 5.58 Ω to 26.87 and 27.03 Ω, respectively, indicating that the lithium metal changes structurally during charge-discharge process. In addition, R₁ and R₂ of LFP|PDOL-SiO₂|Li change from 2.06 and 128.40 Ω to 231.20 and 50.32 Ω, respectively, while those of LFP|PDOL-PE|Li change from 138.00 and 207.70 Ω to 39.88 and 774.60 Ω, respectively, which is attributed to the interface variation during cycling [35]. Obviously, LFP|PDOL-SiO₂|Li performs the smaller increase of R₂ than that of LFP|PDOL-PE|Li, indicating that application of SiO₂ porous membrane makes PDOL more stable with LFP in the charge-discharge process than that of PE. However, the value of R₁ of LFP|PDOL-SiO₂|Li increases, while that of LFP|PDOL-PE|Li decrease, indicating that compatibility of PDOL-PE with lithium anode is better than that of PDOL-SiO₂, which needs the further research in the future.

  The lithium plating and stripping processes of Li|PDOL-SiO₂|Li battery were evaluated at 30 ℃ (Fig. S10 in Supporting information). Li metal anode of the Li|PDOL-SiO₂|Li cell can cycle stably for nearly 1000 h with an overpotential of less than 50 mV at a current density of 0.5 mA/cm², which indicatives that Li|PDOL-SiO₂|Li cell has a small resistance and a decreased polarization of the SEI film. The high t_Li⁺ conduces to facilitate a uniform deposition and stripping of Li⁺ at the electrode/electrolyte interface, which may explain the above phenomenon [40]. However, compared with the recent reference, the stable cycling time of this system in lithium symmetric battery is shorter [41]. The difference is mainly caused by the different polymer applied in the batteries with different molecular chain structures. Introduction of some rigid structure, such as the phenyl structure in OP-10, may offer the electrolyte better resistance to lithium dendrite growth when compared with the pure ether structure.

  In summary, DOL is successfully infiltrated into the SiO₂ porous membrane to fabricate PDOL via the in-situ ionic ring-opening polymerization process. PDOL-SiO₂ presents a better thermal stability than that of LEs. The oxygen atoms of PDOL together with Si-O of SiO₂ construct a more efficient Li⁺ migration channel. On account of this, the t_Li⁺ and σ at 30 ℃ of PDOL-SiO₂ are enhanced to 0.80 and 1.68 × 10⁻⁴ S/cm, respectively. PDOL-SiO₂ manifests the electrochemical decomposition potentials of 4.90 V. At 0.5 mA/cm², Li|PDOL-SiO₂|Li cell shows a much more steady cycling performance for nearly 1400 h. Noticeably, LFP|PDOL-SiO₂|Li battery cycled at 0.5 C at 30 ℃ shows a capacity retention rate of 89% after 200 cycles. Even at 2 C, the capacity retention rate can maintain at 78% after 300 cycles. SiO₂ membrane based PDOL performs a better property in LMBs than that of PE membrane. This study provides an innovative strategy for accelerating Li⁺ transportation via designing PDOL molecular chains throughout the SiO₂ membrane, which is crucial for high performance LMBs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work is supported by the Department of Science and Technology of Zhuhai City (No. ZH22017001200059PWC) and the Department of Science and Technology of Guangdong Province, China (No. 2019A050510043).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.093.

  
  
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  Scheme 1  Fabrication process of in-situ polymer electrolytes (PDOL-SiO₂).

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  Figure 1  (a) Fourier infrared spectrometer (FTIR) spectra of materials at 4000~400 cm⁻¹. X-ray photoelectron spectroscopy (XPS) spectra on (b) C 1s, (c) Li 1s and (d) O 1s of PDOL-SiO₂.

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  Figure 2  (a) Temperature-dependent ionic conductivity for PDOL-SiO₂ and PDOL-PE. (b) Chronoamperometry profiles and AC impedance spectra of lithium symmetric cell with PDOL-SiO₂ before and after polarization. (c) LSV curves of PDOL-SiO₂ at 3.0~6.0 V. (d) CV curves of LFP|PDOL-SiO₂|Li battery at 2.4~4.5 V.

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  Figure 3  Cycling performance of LFP|PDOL-SiO₂|Li batteries at (a) 0.5 C, (b) 1 C, (c) various current densities and (d) 2 C at 30 ℃.

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