La2O3 Filler's Stabilization of Residual Solvent in Polymer Electrolyte for Advanced Solid‐State Lithium‐Metal Batteries

Polymer solid electrolytes (SEs) with high safety and flexibility are ideal for advanced lithium‐metal solid‐state batteries (SSBs). Among various polymer SEs, polyvinylidene fluoride‐co‐hexafluoropropylene (PVDF‐HFP) polymer SEs have gained increased attention for their high dielectric constants, high ionic conductivity, and excellent flexibility. However, severe side reactions at the interface caused by the decomposition of residual DMF solvent significantly reduce the cycle life of PVDF‐HFP‐based SSBs. Herein, La2O3 nanoparticles are used as new inorganic fillers to form a PVDF‐HFP/LiFSI/La2O3‐40% composite polymer electrolyte (PVDF‐HFP/La2O3 CPE). Benefiting from the interaction between La2O3 and N,N‐dimethylformamide (DMF) solvent molecules, the cell cycling stability is greatly improved. In addition, the PVDF‐HFP/LiFSI solid electrolyte (PVDF‐HFP SE) containing 40 wt% La2O3 has the highest ionic conductivity of 1.33 × 10−3 S cm−1 at 25 °C. It also exhibits a higher lithium‐ion transference number of 0.52 and lower polarization. The PVDF‐HFP/La2O3 CPE here ensures high ionic conductivity and stable interface chemistry in SSB, demonstrating a promising application potential.

DOI: 10.1002/smsc.202300017 Polymer solid electrolytes (SEs) with high safety and flexibility are ideal for advanced lithium-metal solid-state batteries (SSBs). Among various polymer SEs, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) polymer SEs have gained increased attention for their high dielectric constants, high ionic conductivity, and excellent flexibility. However, severe side reactions at the interface caused by the decomposition of residual DMF solvent significantly reduce the cycle life of PVDF-HFP-based SSBs. Herein, La 2 O 3 nanoparticles are used as new inorganic fillers to form a PVDF-HFP/LiFSI/La 2 O 3 -40% composite polymer electrolyte (PVDF-HFP/La 2 O 3 CPE). Benefiting from the interaction between La 2 O 3 and N,N-dimethylformamide (DMF) solvent molecules, the cell cycling stability is greatly improved. In addition, the PVDF-HFP/LiFSI solid electrolyte (PVDF-HFP SE) containing 40 wt% La 2 O 3 has the highest ionic conductivity of 1.33 Â 10 À3 S cm À1 at 25°C. It also exhibits a higher lithium-ion transference number of 0.52 and lower polarization. The PVDF-HFP/La 2 O 3 CPE here ensures high ionic conductivity and stable interface chemistry in SSB, demonstrating a promising application potential.
conductivity, [26] enhancing the mechanical properties, [27] and improving the chemical stability at the cathode/anode interface. [28] The specific modification strategies include polymer blending, [29] grafting, [30] crosslinking, [31] and adding inorganic fillers. Among them, it is an effective strategy to add inorganic fillers to prepare composite SEs. The fillers are mainly divided into conductive active fillers (Li 7 La 3 Zr 2 O 12 -based SE, [32] Li 0.29 La 0.57 TiO 3 , [33] Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 , [34] Li 10 GeP 2 S 12 , [35] etc.) and nonconductive inert fillers (ZrO 2 , [36] Al 2 O 3 , [37] TiO 2 , [38] etc.). Adding fillers can enhance the ionic conductivity of polymeric SEs. The addition of filler reduces the orderliness of the polymer to form more amorphous regions, thus enhancing the ionic conductivity. [39] In addition, the existence of channels for ion transport on the surface of the filler particles accelerates ion transport. [40] Simultaneously, adding fillers can also significantly enhance the mechanical strength, broaden the electrochemical voltage window of polymeric SEs, and improve the interfacial stability between electrolytes and electrodes.
In polymer SE systems, PVDF-HFP polymer electrolyte is attracting great attention for their high dielectric constants, good electrochemical stability, and thermal stability. [41] Note that during the preparation of PVDF-HFP polymer electrolyte even after sufficient thermal treatment, there is still very tiny residual DMF solvent within the polymer matrix, which is not dissociative but will contribute good ionic conductivity to the electrolyte. [42] Nevertheless, owing to the interfacial side reactions and other interfacial issues caused by the decomposition of DMF at high voltages, the residual DMF solvent will drastically reduce the cycling stability of the cell. [43] To address this issue, great efforts have focused on understanding the role of DMF in polymers, regulating DMF residues, and developing novel solvents. [43b,44] In addition, adding inorganic SEs, for example, Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 (LATP) [45] and Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 (LLZTO), [46] seems a good strategy to improve interfacial stability since interfacial side reactions of fillers with residual DMF solvents can be suppressed. However, these aforementioned inorganic electrolytes are complicated to synthesize and very expensive. Therefore, it is of great interest to explore inexpensive inorganic fillers for achieving stable PVDF-HFP polymer. Zhang et al. [46] proposed that La atoms in LLZTO would form complexes with N atoms and C═O group of DMF solvent molecules. Thus La-based compounds may have potential for stabilizing DMF solvents. However, there are no relevant studies on the effect of La-based oxides on PVDF-HFP-based SEs.
In this work, we discover that La 2 O 3 nanoparticles can be incorporated into the polymer electrolyte as a useful inorganic filler, and a stable CPE composed of PVDF-HFP matrix with La 2 O 3 nanoparticles and LiFSI is developed. Surprisingly, La 2 O 3 nanoparticles have a good adsorption effect on DMF molecules, which can effectively inhibit its decomposition at high voltages, thus resulting in enhanced electrochemical stability of the CPE. Moreover, with the addition of La 2 O 3 nanoparticles, the obtained PVDF-HFP/La 2 O 3 CPE exhibits better mechanical properties and thermal stability. More importantly, the SSB assembled based on this CPE achieves excellent battery performance. Figure 1a displays the X-ray diffraction (XRD) patterns of prepared electrolyte membranes. The diffraction peaks include all the characteristic peaks of the PVDF-HFP matrix and La 2 O 3 nanoparticles without other impurity peaks, indicating that the inorganic filler and polymer matrix was successfully compounded and no new phase was formed. In addition, the comparison in the XRD shows that the intensity of the diffraction peaks of La 2 O 3 decreases after being embedded in the PVDF-HFP SE, which means a reduction in the crystallinity degree of La 2 O 3 . More importantly, the crystallinity of PVDF-HFP SE decreases with the addition of La 2 O 3 , which indicates that adding La 2 O 3 filler and LiFSI can increase the amorphous area of the PVDF-HFP matrix. The scanning electron microscopy (SEM) image of PVDF-HFP/La 2 O 3 CPE is presented in Figure 1b. As shown, the membrane surface is relatively dense and smooth, suggesting that La 2 O 3 particles are uniformly distributed within the PVDF-HFP matrix, which can facilitate internal Li þ transport. As further evidenced by the energy-dispersive spectral (EDS Figure 1c-f ) mapping images of the PVDF-HFP/La 2 O 3 CPE, it is obvious that the corresponding elements are distributed evenly. The thickness of this CPE is about 100 mm presented from its cross-sectional image in Figure S1, Supporting Information. The Fourier-transform infrared (FTIR) spectra of obtained membranes are given in Figure  g-h. The characteristic peaks at 1659 and 1380 cm À1 represent C═O vibration and -CH 3 in DMF molecule. [45] The peaks observed at 1170 and 1070 cm À1 are ascribed to -CF 2 absorption peak in PVDF-HFP matrix, and the peaks at 835 and 876 cm À1 are assigned to the amorphous phase of PVDF-HFP. [47,48] The bands at 658 and 673 cm À1 belong to uncoordinated O═C-N (free DMF) and coordinated [Li(DMF) x ] þ (bound DMF), respectively. [46] It should be noted that the band at 673 cm À1 in obtained membranes demonstrates that the residual DMF solvent exists in the electrolyte as bound DMF.

Results and Discussion
Ionic conductivity is a vital property of SEs. To investigate the effect of La 2 O 3 nanoparticles addition on the ionic conductivity of PVDF-HFP SE, the ionic conductivity of obtained CPEs with different La 2 O 3 contents (0%-60%) was measured, and the results are shown in Figure 2a. With increasing the proportion of La 2 O 3 , the CPE containing 40% La 2 O 3 exhibits the highest roomtemperature ionic conductivity of 1.33 Â 10 À3 S cm À1 , which is higher than that of PVDF-HFP SE without La 2 O 3 nanoparticles (9.74 Â 10 À4 S cm À1 ). This could benefit from the reduced crystallinity of the PVDF-HFP matrix with the addition of La 2 O 3 , which can be further confirmed by differential scanning calorimetry (DSC). [49] However, excessive La 2 O 3 filler will hinder Li þ conduction and present a decrease in the ionic conductivity of PVDF-HFP/La 2 O 3 CPEs, which is attributed to the aggregation of nanoparticles. The DSC curves of PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE were examined ( Figure S2, Supporting Information). The endothermic peak of PVDF-HFP SE appears at 169°C, which represents the melting temperature (T m ) of PVDF-HFP. When adding La 2 O 3 , the peaks decrease significantly to 157°C, which reveals the reduced crystallization stability of PVDF-HFP. [47] The impedance spectra of PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE at different temperatures are presented in Figure S3, Supporting Information. The Arrhenius plots of PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE are shown in Figure 2b, from which the corresponding activation energy is calculated to be 0.227 and 0.191 eV, respectively. The PVDF-HFP/ La 2 O 3 CPE shows relatively lower activation energy, indicating the facilitation of Li þ migration. Figure 3d and S4, Supporting Information, show the direct current polarization and the AC impedance curves before and after polarization of the PVDF-HFP/La 2 O 3 CPE and PVDF-HFP SE. The calculated lithium transference number (t Li þ ) of PVDF-HFP/La 2 O 3 CPE reaches 0.54, much higher than that of PVDF-HFP SE (0.36). The higher t Li þ of the PVDF-HFP/La 2 O 3 CPE indicates that the addition of La 2 O 3 filler can effectively immobilize the movement of FSI À and promote the effective migration of Li þ . [50] The linear scanning voltammetry (LSV) curve shows that with the introduction of La 2 O 3 nanoparticles, the decomposition of DMF at 3.8 V in PVDF-HFP SE was greatly suppressed and the electrochemical stability window was extended from 4.4 to 4.5 V (Figure 2c), indicating its better compatibility with highvoltage lithium batteries. [48] The cyclic voltammetry (CV) measurements of the assembled LiFePO 4 (LFP)||PVDF-HFP/La 2 O 3 CPE||Li ( Figure 2e) and LFP||PVDF-HFP SE||Li (Figure 2f ) were performed at room temperature from 2.5 to 4.0 V. The CV curves of LFP||PVDF-HFP/La 2 O 3 CPE||Li are basically overlapped, indicating the superior electrochemical stability of the composite electrolyte membrane. [51] Good mechanical strength is an important property of SEs. The tensile strengths of obtained PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE were investigated and are presented in Figure S5, Supporting Information. Compared with PVDF-HFP SE, the tensile strength of PVDF-HFP/La 2 O 3 CPE is remarkably increased. Good thermal stability is critical for SEs. Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of pure PVDF-HFP, PVDF-HFP SE, and PVDF-HFP/La 2 O 3 CPE, respectively. As presented in TGA results ( Figure S6, Supporting Information), the slight weight loss of the membranes before 140°C is ascribed to the captured moisture, and the evaporation of residual DMF solvent occurs at 140-200°C. Therefore, the DMF solvent residues of electrolyte membranes are less than 10%. The apparent weight loss at 200-310°C is derived from the thermal degradation of LiFSI. The complete thermal decomposition of PVDF-HFP/ La 2 O 3 CPE occurs at 310°C. [47] Nevertheless, the thermal stability of PVDF-HFP/La 2 O 3 CPE is still considerable for applications in lithium batteries.
To elucidate the interaction between La 2 O 3 and DMF molecules, FTIR and X-ray photoelectron spectroscopy (XPS) of PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE were performed www.advancedsciencenews.com www.small-science-journal.com ( Figure 3). The peaks at 1659 and 1380 cm À1 are assigned to C═O and -CH 3 of DMF solvent (Figure 3a). [45] Clearly, the C═O and -CH 3 peaks of DMF are weakened and shifted to 1662 and 1383 cm À1 with the addition of La 2 O 3 , respectively, which is attributed to the strong interactions of La 2 O 3 with DMF. [45] Furthermore, FTIR spectra of La 2 O 3 and pure DMF solvent mixtures were employed to demonstrate the interaction between La 2 O 3 and DMF molecules (Figure 3b,d). With the increase of La 2 O 3 , the peak intensity of free DMF decreases and the peak position is significantly shifted, which means that La 2 O 3 interacts with the DMF molecule. [46] The XPS measurements of PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE further demonstrate the interaction between La 2 O 3 and DMF molecule. The N1s spectrum peaks appear at 398.86 and 400.69 eV, corresponding to S─N─S bonds in LiFSI [52] and N─C═O in DMF, [42] respectively (Figure 3c) Figure S7, Supporting Information). The adsorption energy of DMF on La 2 O 3 is À0.637 eV, much higher than that on PVDF-HFP (À0.129 eV), revealing that La 2 O 3 has strong adsorption with residual DMF solvent in CPE.
In addition, the XPS spectra of LFP cathodes with PVDF-HFP SE and PVDF-HFP/La 2 O 3 CPE after 30 cycles were also characterized to describe the chemical environment at the LFP cathode interface. The Li-F (685.1 eV) appears in the F 1s (Figure 4a) derived from the decomposition of LiFSI. [53] The spectrum peak at 688.0 eV is assigned to C-F attributed to the decomposition of PVDF-HFP. [54] The C═O (533 eV) and C─O (531 eV) of Li 2 CO 3 presented in O1s are attributed to the decomposition of PVDF-HFP and DMF (Figure 4c). [55] The N─C═O (400.01 eV) [42] and Li x NO y (401.5 eV) [48] detected in N1s spectra are ascribed to DMF solvent and the hysteretic decomposition of DMF, respectively (Figure 4b). Compared with PVDF-HFP SE, the significantly decreased peak intensities of F1s, O1s, and N1s spectra using PVDF-HFP/La 2 O 3 CPE indicate that the side reactions of the cathode interface are greatly inhibited. [45] Figure 4d,e shows the SEM images of fresh lithium metal, and the cycled lithium-metal anode after 30 cycles of LFP||PVDF-HFP SE||Li, and LFP||PVDF-HFP/La 2 O 3 CPE||Li, respectively. The lithium-metal anode using PVDF-HFP SE exhibits extremely rough surfaces and irregular breakage, indicating severe lithium-related  Figure 5a-c. The LFP||PVDF-HFP SE||Li cell is damaged after 100 cycles and its discharge capacity decreases from 138 to 97 mAh g À1 with average coulombic efficiency of 83%. Benefiting from the addition of La 2 O 3 , the LFP||PVDF-HFP/La 2 O 3 CPE||Li cell delivers significantly improved electrochemical stability. The LFP||Li cell exhibits a high reversible capacity of 145 mAh g À1 at the first cycle. A high coulomb efficiency of 99.5% and superior cycling stability with 94.5% capacity retention rate can be obtained after 180 cycles. In addition, we also tested the cycling performance of full cells assembled with CPEs containing 30% and 50% La 2 O 3 , respectively ( Figure S8 and S9, Supporting Information). The results demonstrate that the cell assembled with the CPE containing 40% La 2 O 3 has better cycling stability. As presented in Figure 5d-f and S10, Supporting Information, the PVDF-HFP/La 2 O 3 CPE-based battery also shows a much better rate capability. It exhibits a capacity of 164, 162, 160, 156, 151, and 148 mA h g À1 at 0.1, 0.2, 0.3, 0.5, 0.8, and 1C, respectively, The voltage profiles of Li||PVDF-HFP SE||Li and Li||PVDF-HFP/ La 2 O 3 CPE||Li symmetrical cells are shown in Figure 5g. As shown clearly, the Li||PVDF-HFP SE||Li symmetric cell displays a short circuit after 200 h, while Li||PVDF-HFP/La 2 O 3 CPE||Li symmetric cell exhibits a much longer cycle time and smaller polarization voltage, owing to its better interfacial stability for lithium anode. Critical current density (CCD) is commonly used to evaluate the ability of SEs to suppress lithium dendrites. [56] The CCD of the Li||PVDF-HFP SE||Li cell is increased from 1.1 to 2.2 mA cm À2 after the addition of La 2 O 3 (Figure 5h), indicating that the PVDF-HFP/La 2 O 3 CPE could better inhibit the growth of lithium dendrites.
To further evaluate the flexibility and practical application potential of the PVDF-HFP/La 2 O 3 CPE used in lithium-metal batteries, the LFP||PVDF-HFP/La 2 O 3 CPE||Li pouch cells were assembled. The cycle performance of the above cell is evaluated at 0.1C. The battery delivers a high discharge capacity of 150 mAh g À1 and a high capacity retention of 96.7% after 170 cycles (Figure 6a,b). The solid-state pouch cell can successfully brighten the light-emitting diodes (LEDs) even in the bent cases, as shown in Figure 6c-f, implying the practical application potential of LFP||PVDF-HFP/La 2 O 3 CPE||Li SSBs.

Conclusion
To address interfacial instability caused by the residual DMF solvent in PVDF-HFP polymer, we developed a new La 2 O 3 filler into the PVDF-HFP matrix to form PVDF-HFP/La 2 O 3 CPE. Thanks to the reduced crystallinity of PVDF-HFP, the PVDF-HFP SE containing 40 wt% La 2 O 3 nanoparticles exhibited an enhanced ionic conductivity of 1.33 Â 10 À3 S cm À1 at 25°C. In addition, this CPE exhibited higher lithium-ion transference number and better mechanical strength. More importantly, the La 2 O 3 nanoparticles had a strong adsorption effect on DMF solvent molecules, which could effectively inhibit their decomposition on the positive or negative interfaces. The assembled cells achieved superior cycling performance and rate capability. All the above results indicate that PVDF-HFP/La 2 O 3 CPE has great application potential for advanced lithium-metal SSBs.

Experimental Section
Solid Electrolyte Synthesis: La 2 O 3 was purchased from Shuitian Reagent and was sintered at 900°C for 12 h before use ( Figure S11, Supporting Information). Lithium bis(fluoro sulfonyl) imide (LiFSI) was obtained from Duoduo Chemical Technology Company. We adopted the film-casting method to prepare composite SE membranes. Typically, PVDF-HFP (Mw = 400 000, Sigma-Aldrich) and LiFSI (0.75:1 weight ratio) were first dissolved in N,N-dimethylformamide (DMF, 99.8%, Aladdin) solvents. After magnetically stirring for 12 h, an appropriate quantity of La 2 O 3 nanoparticles (mass ratio of PVDF-HFP: La 2 O 3 = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were added to the solution. The mixture was then stirred and ultrasonically processed to obtain a homogeneous electrolyte slurry. Then, the electrolyte slurry was cast onto glass plates by a doctor blade. Finally, the membranes were obtained by further drying at 60°C for 24 h under vacuum and stored in an argon-filled glovebox for use. The thickness of the film was 100 μm. The obtained CPEs were denoted as PVDF-HFP/LiFSI and PVDF-HFP/LiFSI/La 2 O 3 , respectively.
Material Characterization: XRD patterns were collected on an Empyrean-2 diffractometer to characterize the crystalline structure of La 2 O 3 filler and obtained CPEs. The morphology of the obtained CPE membranes and cycled lithium metals were observed by a JEOL/JSM-7610FPlus SEM. FTIR spectral measurements were conducted on a Thermo Scientific Nicolet iS50. TGA using a Netzsch STA 449F3 instrument with a rate of 10°C min À1 from 25 to 600°C under nitrogen (N 2 ) atmosphere to evaluate the thermal stability of CPEs. DSC measurements were performed on a TA Q2000 instrument with a heating rate of 10°C min À1 from 25 to 200°C under air atmosphere. The stress-strain curve tests were conducted by a universal testing machine (CMT6103). XPS (XPS PHI 5000 VersaProbe) was performed to analyze obtained electrolyte membranes and LFP cathode surface chemistry. www.advancedsciencenews.com www.small-science-journal.com Electrochemical Measurements: Electrochemical impedance spectroscopy (EIS) of symmetric SS (stainless steel) ||CPE||SS CR2016 coin cell was measured to evaluate the ionic conductivity σ (S cm À1 ) of electrolyte membranes at a Multi Autolab/M204 with a frequency of 1 Â 10 À6 Hz to 0.1 Hz. The ionic conductivity was calculated according to the formula where L (cm) represents the thickness of the CPE Membrane, R (Ω) is the bulk resistance, and S (cm 2 ) is the effective test area. The activation energy E a of obtained electrolyte membranes was calculated by the classical Arrhenius equation where A is the pre-exponential factor, R is the thermodynamic constant, and T is the absolute temperature. The Li þ transference number of CPEs was assembled in a Li||CPE||Li coin cell and evaluated by combined DC polarization/AC impedance, and t Li þ was calculated based on the following equation where I 0 and I s are the initial and steady-state currents values and R 0 and R s are the interfacial resistances before and after polarization. ΔV is the polarization potential with 10 mV used in this work. LSV was measured to determine the electrochemical stability window of CPEs by Li||CPE||SS coin cells in the test voltage range of 0 to 6 V with a scan rate of 1 mV s À1. The CV measurements were measured by a Multi Autolab/M204 in LPF||CPEs||Li full cells at a scanning rate of 0.05 mV s À1 .
Battery Testing: The LFP cathode was synthesized by mixing LFP active material, super P, and polyvinylidene fluoride (PVDF, Sigma-Aldrich) binder with the mass ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP, Aladdin) solvent. Then the slurry was cast on an Al foil current collector and dried at 80°C for 12 h under vacuum to remove the residual NMP. The average mass loading of LFP active material was about 2.0 mg cm À2 . Specifically, the mass loading of the cathode active material in the pouch cell was about 1.72 mg cm À2 , and the mass loading of the cathode active material in the coin cell assembled with PVDF-HFP/LiFSI SE and PVDF-HFP/La 2 O 3 CPE was 2.65 and 2.21 mg cm À2 , respectively. The chargedischarge tests of cells were conducted on the CT-4008T Neware battery testing system at 25°C between 2.5 and 4 V.
Density Functional Theory (DFT) Calculation: To probe the interactions between DMF and La 2 O 3 in CPEs, the adsorption energies were investigated based on DFT calculations performed by the DMol 3 code. The generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) functional and double numerical plus polarization (DNP) basis was used to describe the exchange-correlation potential. The optimal geometric convergence criteria of energy iteration, force, and atomic displacement were 1.0 Â 10 À5 Ha, 2 Â 10 À3 Ha Å, and 5 Â 10 À3 Å, respectively. The Monkhorst-Pack scheme with 2 Â 2 Â 1 k-point was used for structural optimization. To reduce the interactions between neighboring layers, the vacuum thickness was set to be more than 15 Å. The formula of adsorption energy is defined as follows.
where E total , E DMF , and E La2O3 denote the energies of DMF-La 2 O 3 , DMF, and pristine La 2 O 3 (0 1 1), respectively. Statistical Analysis: The ionic conductivity data of obtained electrolyte membranes were averaged over at least three replicate measurements. Values were expressed as mean AE SD (n = 3), and the values were determined by "Statistics on Rows" using OriginPro 8. The Arrhenius plots were determined via "Fitting Line" using OriginPro 8. The mass loading of LFP active material was the average of three replicates measurements. The better rate capability data were obtained by three independent tests under the same conditions. The values were expressed as mean AE SD (n = 4) as presented in Figure S10, Supporting Information, and the values were determined by "Statistics on Rows" using OriginPro 8.