A Single‐Layer Piezoelectric Composite Separator for Durable Operation of Li Metal Anode at High Rates

Piezoelectric ceramic and polymeric separators have been proposed to effectively regulate Li deposition and suppress dendrite growth, but such separators still fail to satisfactorily support durable operation of lithium metal batteries owing to the fragile ceramic layer or low‐piezoelectricity polymer as employed. Herein, by combining PVDF‐HFP and ferroelectric BaTiO3, we develop a homogeneous, single‐layer composite separator with strong piezoelectric effects to inhibit dendrite growth while maintaining high mechanical strength. As squeezed by local protrusion, the polarized PVDF‐HFP/BaTiO3 composite separator generates a local voltage to suppress the local‐intensified electric field and further deconcentrate regional lithium‐ion flux to retard lithium deposition on the protrusion, hence enabling a smoother and more compact lithium deposition morphology than the unpoled composite separator and the pure PVDF‐HFP separator, especially at high rates. Remarkably, the homogeneous incorporation of BaTiO3 highly improves the piezoelectric performances of the separator with residual polarization of 0.086 μC cm−2 after polarization treatment, four times that of the pure PVDF‐HFP separator, and simultaneously increases the transference number of lithium‐ion from 0.45 to 0.57. Beneficial from the prominent piezoelectric mechanism, the polarized PVDF‐HFP/BaTiO3 composite separator enables stable cyclic performances of Li||LiFePO4 cells for 400 cycles at 2 C (1 C = 170 mA g−1) with a capacity retention above 99%, and for 600 cycles at 5 C with a capacity retention over 85%.


Introduction
The successful implementation of lithium-ion batteries (LIBs) in smartphones and electric vehicles has dramatically reshaped our consumption habits and transportation patterns in a more convenient and eco-friendly way. [1]owever, the capacity growth of LIBs is constrained by the inherent theoretical restriction of conventional intercalation materials, hardly satisfying the demands for next-generation energy storage devices. [2]From the perspective of anodes, due to the ultra-high theoretical gravimetric capacity (3860 mA h g −1 ) and the lowest standard electrochemical potential (−3.04 V, vs standard hydrogen electrode), [3] lithium (Li) metal is considered the most desired anode material.However, the growth of dendrites, the intrinsic electrodeposition morphology of Li, upright on the Li metal anode toward the separator during the Li deposition process not only attenuates the reversible utilization of Li metal anode but also precipitates the internal short circuit (separator penetration), which poses serious safety risks and hinders the mass production of Li metal batteries (LMBs). [4,5]Therefore, regulating the Li electroplating process to bring about dendrite-free and compact electrodeposition morphology is critical for practical application of LMBs.
Various physical factors govern the plating/stripping behavior of Li metal, such as current density, stress, ion flux, and electric field. [6]elective nucleation/growth can be triggered as a result of local unevenness of these physical quantities around the nucleation substrates, leading to dendrite growth.Numerous researchers have now manipulated the physical fields described above to tackle the harsh deposition behavior of Li-ions.Three-dimensional (3D) Li metal anodes or electronic conductivity skeletons are designed to mitigate the deposition inhomogeneity via scaling down the local current density, [7,8] but the reduction in volumetric energy density induced by this protocol is not negligible.The artificial solid electrolyte interface (SEI) layers, whether inorganic, organic, or their mixture always serve as ion flux regulators, [9][10][11] but the additional thickness of these artificial layers takes up extra volume inside the batteries, and the polyolefin separators used in these studies have poor thermal stability.Although a stiff solid electrolyte has been proposed to mechanically suppress the growth of Li dendrite, [12] its poor interface contact with Li metal induces a large interface resistance and uneven Li-ion deposition.Additionally, owing to the existence of local fragility of micro-defects and microcracking, even the very hardened ceramic material is unable to impede the penetration of dendrites. [13,14]The electric field and ion flux always promote each other in the deposition process.On the surface of the plating substrate, any tiny protrusions can generate unevenness of electric fields accumulated around the protrusions, where the ion flux is always enhanced compared with that on a flat surface, causing the dendrite to grow longer. [15]As a critical component of batteries, separators play a pivotal role in isolating the positive and negative electrodes, providing ion transport channels, and ensuring the safe operation of the batteries. [16]Meanwhile, the separator that directly contacts the Li metal anode has a giant potential for regulating Li-ion deposition and inhibiting dendrite formation. [17]Many functional separators with evenly distributed pore structures and regular pore sizes have been developed to achieve stable plating/stripping of Li metal. [18,19]In addition, a straininduced piezoelectric β-PVDF separator is studied to prohibit dendrite growth, but its electrochemical performance in LMBs needs further exploration, [20,21] and the shrinkage of the porous PVDF separator due to the heat produced during high-current cycle also requires profitable remedies.Moreover, modified separators with functional groups absorbing Li-ion at molecular level enable the formation of a uniform ion flux even at large current densities. [22,23]Along these lines, it is critical to develop functional separators that possess electric fields leveling and ion flux regulating capabilities to boost the performance of Li metal anode.
Here, we design a homogeneous piezoelectric composite separator to retard the accumulation of local electric fields at the dendrite tip and achieve long-term operation of the Li metal anode at high rates.PVDF-HFP, for its easy mixing characteristics with inorganic filler, is chosen as the base material of the composite separator.BaTiO 3 (BTO) is selected as the inorganic component due to its excellent piezoelectricity, electric insulation, electrochemical inertness, and low cost. [24]The properties of 25-μm Celgard 2500 separator, PVDF-HFP separator, PVDF-HFP/BTO composite separator (BT10), and poled PVDF-HFP/ BTO composite separator (BT10P) are characterized in parallel conditions.Notably, with the addition of BaTiO 3 filler, the composite separator owns the residual polarization of 0.086 μC cm −2 after the polarization process at 1000 V, over fourfold of the pure PVDF-HFP separator, and meanwhile, the transference number of Li-ion is increased from 0.45 to 0.57.We demonstrate that the BT10P separator effectively homogenizes Li deposition and yields a dendrite-free anode when squeezed by the local facial protrusions or the dendrites by generating a local counter voltage that moderates the local-intensified electric field and inhibits Li deposition in the hot zone.Attributed to the piezoelectric mechanism, Li||LiFePO 4 cells containing the BT10P separator exhibit stable cyclability for 400 cycles at 2 C (1 C = 170 mA g −1 ) with over 99% capacity retention and 600 cycles at 5 C with more than 85% capacity retention.

Design Conception and Properties of Piezoelectric Separator
Electric fields are unavoidably enhanced near the protuberant tips during cell operation owing to the microscopic roughness of electrode surface. [15]As a result, Li-ions concentrate because of the strong electric fields and preferentially deposit on the tip area, which further enhances the electric field.Notably, this selective deposition behavior undergoes self-enhancement, resulting in continuous dendrite growth until penetrating the separator.On the contrary, when the piezoelectric BT10P separator is subjected to the pressure of dendrites, the residual polarization induced by the dipole of the BTO domain generates a voltage against the electric field direction around the dendrite tip, preventing the selfamplification of dendrite growth.Therefore, the deposition slows down on the protruded spot, leaving a dendrite-free anode (Figure 1a).
Giving the PVDF-HFP/BTO composite separator macroscopic piezoelectricity requires a polarization treatment, though the ferroelectric BTO particles have spontaneous polarization property.According to Figure S1a, Supporting Information, the dipole direction of the BTO particles incorporated into the PVDF-HFP is randomly oriented, in which the polarization charge of each BTO particle compensates for each other, and the separator exhibits electrical neutrality without macroscopic piezoelectricity.When an external electric field is applied to the separator, the dipole direction of the BTO particles is forced to realign to the same direction (Figure S1b, Supporting Information) and maintain it after opening the circuit (Figure S1c, Supporting Information), causing the composite separator to display macroscopic piezoelectricity.
Figure S2, Supporting Information illustrates the digital images and thicknesses of different separators.All the separators are cut into discs with a 16 μm diameter, and the thicknesses of the PVDF-HFP, BT10, and BT10P separator are well-controlled to have the same 25 μm thickness as the commercial Celgard 2500 separator.The crystal structure of BTO particles is characterized by X-ray diffraction (XRD), as shown in Figure S3a, Supporting Information.The characteristic peaks at 2θ = 22.3°, 31.5°,38.9°, 44.9°, 45.4°, 50.6°, 51.0°, 51.1°, 56.0°, and 56.3°correspond to the (100), (011), (111), (002), (020), (012), (021), (120), (112), and (121) lattice planes of BTO (PDF #05-0626), respectively, which confirm that the BTO powder holding a perovskite structure with spontaneous ferroelectric effect belongs to the tetragonal P4mm space group. [25]From the scanning electron microscope (SEM) images (Figure S4, Supporting Information), the particle size of the BTO is about 300 nm. Figure 1b depicts the XRD result of the PVDF-HFP, BT10, and BT10P separators.For the pure PVDF-HFP separator, characteristic peaks corresponding to the (020), (110), (021), and (200) crystal planes proves that the PVDF-HFP belongs to α-phase. [26]The XRD pattern of the BT10P composite separator shows the same characteristic peaks of both PVDF-HFP and BTO as the BT10 separator, verifying that the structures of BTO and PVDF-HFP (Figure S5, Supporting Information) remain stable during the preparation process and the polarization treatment.The Fourier transform infrared (FT-IR) spectra of the BTO powder, PVDF-HFP separator, and composite separator are exhibited in Figure S3b, Supporting Information and Figure 1c, respectively.BTO particles demonstrate a characteristic band around 568 cm −1 that adheres to the Ti-O vibration. [27]he peaks of FT-IR spectrum in Figure 1c at 487, 534, 614, and 974 cm −1 are attributed to α-phase PVDF-HFP. [28]The peak at 876 cm −1 is the symmetric stretching vibration of -CF 2 .The two peaks at 1172 and 1400 cm −1 are -CF 3 and C-F stretching vibration, respectively. [29,30]There is no discernible peak shift when comparing the FT-IR spectra of the BT10 and the BT10P composite separators with that of the pure PVDF-HFP separator, indicating that the addition of BTO powder to PVDF-HFP and the polarization treatment scarcely change the bonding status of PVDF-HFP.
Energy Environ.Mater.2024, 7, e12510 SEM images of different separators are illustrated in Figure 1d-g.Celgard 2500 separator (Figure 1d and Figure S6a, Supporting Information) has a tensile morphology with a pore size of ~60 nm (Figure S7a, Supporting Information).The distribution of pore diameter of the PVDF-HFP separator ranges from 0.5 to 0.75 μm (Figure S7b, Supporting Information) and the SEM images also depict an uneven pore structure (Figure 1e and Figure S6b, Supporting Information).In contrast, the BT10 and BT10P separators (Figure 1f,g and Figure S6c,d S8b, Supporting Information), and that the composite separator is singlelayered without any coating when viewed in cross-section (Figure S8c, Supporting Information).The electrical displacement of the PVDF-HFP separator and the BT10 separator is analyzed using a ferroelectric tester.The measured polarization vs electric field (P-E) loops under various external electric fields are shown in Figure 1h,i.The slim hysteresis loops of the PVDF-HFP separator show a nearly linear relationship between polarization and electric field, and the residual polarization turns out to be weak, indicating a feeble piezoelectric effect.Regarding the BT10 separator, as the external electric field increases, the polarization gradually rises and the P-E loop turns to be more hysteretic, exhibiting typical piezoelectric features.Moreover, after the polarization treatment, the d33 value of the BT10P separator can reach 60 pC N −1 , confirming the clear polarization of the BT10P separator.To compare the mechanical robustness, the tensile strengths of all the separators are tested, and the results are shown in Figure 1j.Due to the uniaxial stretching process of the Celgard 2500 separator, it has the maximum tensile strength (83 MPa) along the non-stretching axis and the minimum of 9.6 MPa onward the stretching axis.The tensile strengths of the PVDF-HFP separator, BT10 separator, and BT10P separator are very close around 28 MPa.The close tensile results for the PVDF-HFP, BT10, and BT10P separators exclude the effect of mechanical strength differences on the inhibition of dendrite growth.
The heat distribution and the thermal stability of the separator at different temperatures are studied using a forward-looking infrared radiometer (FLIR).As illustrated in Figure S9, Supporting Information, the PVDF-HFP/BTO composite separator sustains good stability throughout the heating process, with rapid heat transfer and more uniform heat distribution in comparison with the Celgard 2500 separator and the PVDF-HFP separator.Wettability of the separators is assessed via contact angle measurement, as shown in Figure S10, Supporting Information.Both the PVDF-HFP separator and the PVDF-HFP/BTO composite separator with polar groups that provide better wettability with the electrolyte own smaller contact angles than that of the Celgard 2500 separator.The electrolyte uptakes of the separators (Figure S11, Supporting Information) follow the same trend as the wettability performance.

Influence of Dipole Direction of the Piezoelectric Separator on Li Deposition Morphology
To find out whether the polarization direction impacts the Li deposition morphology, Li||Li symmetric cells are assembled using the BT10P separator and Li foil.Only one deposition process is carried out at a current density of 1 mA cm −2 for 2 h.After disassembly, the deposition morphology of Li on the Li foil is observed by SEM.The schematic diagram with the polarization direction of the BT10P separator pointing to the deposited electrode is shown in Figure 2a, which has the densest deposition morphology (Figure 2b,c) compared with the other cases.When the polarization direction is reserved, a needle-like dendrite morphology and a loose deposited layer are obtained (Figure 2d-f).Therefore, a much more uniform Li deposition layer can be acquired when the polarization direction of the piezoelectric separator is oriented to the deposited electrode.In order to rule out the possibility that the differences between the top and bottom surfaces of the separator affect the Li deposition morphology, the BT10 separator is applied to assemble the Li‖Li cell.Loose deposition layers and dendrites are significantly observed on both the upper and lower surfaces of the BT10 separator in contact with the deposited electrodes (Figure 2g-l), which can exclude the influence of the differences between the upper and lower surfaces of the separator.The results above directly demonstrate that the pole direction of the piezoelectric separator has a crucial influence on the deposition morphology and that only when the polarization direction is directed toward the substrate can the voltage against the electric field direction generated by the BT10P separator remarkably suppress dendrite growth.

Electrochemical Performance of Piezoelectric Separator
The electrochemical oxidation window of the separators is tested through linear scanning voltammetry (LSV) with the SS‖Li cells.As depicted in Figure S12, Supporting Information, the current of the cell with the BT10P separator starts to rise at 4.5 V, the same as the other three separators, which is suitable for the operating voltage of LiFePO 4 (LFP) cells.
The ionic conductivity and Li + transference number (t Li þ ) of various separators are depicted in Figure 3a and the corresponded electrochemical impedance spectra (EIS) and chronoamperometry (CA) curves are shown in Figures S13 and S14, Supporting Information, respectively.The ionic conductivities of the Celgard 2500 separator and the PVDF-HFP separator are 1.148 and 0.834 mS cm −1 at room temperature, respectively.And the ionic conductivities of the composite separators are similar to that of the PVDF-HFP separator, indicating that the addition of BaTiO 3 particles and the polarization treatment have little effect on the ionic conductivity of the separator.The Li + transference number is calculated by the symmetric Li||Li cells with various separators (Equation S2, Supporting Information).The values for the BT10 and BT10P separators are 0.54 and 0.57, respectively, superior to those of the Celgard 2500 separator (0.4) and PVDF-HFP separator (0.45), owing to the local Lewis acid environment created by the ceramic BaTiO 3 filler fixing the lithium salt anion and then increasing the t Li þ . [31]i||Li symmetric cells equipped with different separators holding 50 μL 1.0 M LiPF 6 -EC/DMC/DEC (1:1:1 in volume) electrolyte are performed to quantify the long-term stability of the Li metal anode.As shown in Figure S15, Supporting Information, the initial nucleation overpotential curves show that the BT10P separator-based cell delivers the minimum voltage dip, illustrating the lowest nucleation barrier of the BT10P separator.Due to the piezoelectric effect, the BT10P symmetric cell exhibits an extended lifespan of 200 h both at 2 mA cm −2 with an area capacity of 1 mAh cm −2 and at 1 mA cm −2 with a higher area capacity of 5 mAh cm −2 (Figure S16a,b, Supporting Information).The EIS spectra for the different cycles (Figure S16c-f, Supporting Information) further demonstrate that the piezoelectric BT10P separator provides a stable interface that suppresses the accumulation of dead Li and keeps the impedance of the cell relatively stable after 125 cycles.Li|| Cu cells containing different separators are assembled to measure the coulombic efficiency (CE) performance in the presence of 1.0 M LiPF 6 -EC/DMC/DEC (1:1:1 in volume) electrolyte.By comparing the initial nucleation overpotential on the Cu substrate (Figure S17, Supporting Information), the Li||Cu cell with the BT10P separator delivers the lowest one, in line with the results of the Li||Li symmetric cell.As shown in Figure 3b, the cells with the Celgard 2500 separator, PVDF-HFP separator, and BT10 separator show clear fluctuations and CE attenuations after 40 cycles at the current density of 1 mA cm −2 with the capacity of 1 mAh cm −2 , which reflects the irreversible plating on the Li metal surface.On the contrary, the cell with the BT10P separator exhibits stable CEs over 120 cycles at the same condition and the average CE value is over 93%.
The EIS of Li||LFP cells with different separators tested at open-circuit potential is shown in Figure 3c.A semicircular arc and a tilted straight line in the high-and low-frequency zone can be observed.In the highfrequency zone, the intersection of the semicircular arc and the real axis corresponds to the ohmic resistance (R s ), representing the ohmic resistance of the electrolyte, separator, and other cell components.The diameter of the semicircular arc represents the charge transfer resistance (R ct ) of the working electrode, as related to the electrochemical reaction kinetics on the working electrode surface.The R ct of the cell containing the PVDF-HFP separator is apparently reduced compared with the Celgard 2500 separator-based cell, which is caused by the increased pore size of the PVDF-HFP separator facilitating the interfacial reaction of the electrode.The R ct of the cells equipped with the composite separator is further reduced, which can be attributed to the improved t Li þ promoting the charge exchange in the electrolyte/electrode interface. [32]he effects of different separators on the electrochemical reactions in Li||LFP cells are analyzed by cyclic voltammetry (CV) (Figure 3d) in the voltage window of 2.5-4.2V.The oxidation/reduction peaks at 3.6 and 3.3 V correspond to the redox reactions of Fe 2+ and Fe 3+ during the charge/discharge process of the LFP cathode, accompanied by the plating/stripping of Li + on the Li metal anode.In contrast, the position of the redox peaks of the BT10P separator-based cell almost coincides with that of the other cells, indicating that the electric field generated by the BT10P separator cannot cause the impedance for Li + transfer.No distinct changes in the positions of the redox peaks are observed in the second and third CV cycles (Figure S18, Supporting Information), indicating significant electrochemical stability of the prepared separators.
Figure 3e shows the initial charging and discharging profiles of Li|| LFP full cells using different separators at 0.1 C between 2.5 and 4.2 V.All the full cells deliver a discharge specific capacity of around 160 mAh g −1 , verifying that the BTO filler and the polarization treatment barely affect the capacity performances of the Li||LFP cells.Additionally, Figure 3f depicts the rate characteristics of the Li||LFP cells, and throughout the test, the charging and discharging rates are the same.The BT10P separator-based cell has discharge specific capacities of 166.40, 164.45, 159.42, 151.03, 141.97, and 108.39 mAh g −1 , respectively, from 0.1 C to 5 C.These values are marginally higher than those of cells based on other separators.
Figure 3g-j shows the cyclic performances of Li||LFP cells at various C-rates.At modest current densities (0.5 C and 1 C), the cyclic performances of the cells with different separators are essentially identical.As the current density becomes larger, the piezoelectric property of the BT10P separator improves the cell's cycling stability: at 2 C, after 400 cycles, the Li||LFP cell delivers 131.76 mAh g −1 with a capacity retention of above 99%, while the performance degradations of the cells with other separators occur at about 270 cycles; at 5 C, the BT10P cell lasts 600 cycles with a prominent average Coulombic efficiency of 99.5% and capacity retention of 86.3%, whereas the cells based on other separators present very limited continuance of approximately 200 cycles.Based on Sand's time model, [33] the dendrites have an easier time growing as the current density rises.Combining Figure 3g-j, the cycling performances of the cells at different C-rates are in line with this model.For the Celgard 2500 separator, PVDF-HFP separator, and BT10 separator, the decline in cyclic performance can be detected with increasing C-rate, which proves that the accumulation of Li deposition layer (dead Li layer) intensifies with increasing current density, leading to a gradual decay of capacity during the cycling tests.For the BT10P separator, its ability to inhibit dendrites is not exploited due to the uniform Li deposition at low Crates.At large C-rates, the increased extrusion strength of the Li dendrites or deposited layers on the BT10P separator makes its piezoelectricity expresses, and thus suppresses the growth of dendrites and prolongs the lifespan of the batteries.The charge/discharge profiles of the different cycles at 5 C are compared, as shown in Figure S19, Supporting Information.The composite separators notably narrow the gap between the charge/discharge platform, because the improvement of t Li þ is beneficial to faster charging/discharging. [32] Additionally, the potential gaps between the charging and discharging platforms of the cell based on the Celgard 2500 separator, PVDF-HFP separator, and BT10 separator gradually expand with the cycle number, proving that the accumulation of dead Li aggravates the electrochemical polarization and the fading of capacity.Impressively, the charge/discharge platforms of the cell with the BT10P separator almost overlap during various cycles, further indicating that the BT10P separator with piezoelectric property enables uniform Li deposition and alleviates the accumulation of dead Li.Also, the structure of the BT10P separator remains the same after the 200th cycle at 5 C (Figure S20, Supporting Information).In addition, cells using a highloading cathode with 12 mg cm −2 are assembled to reach high energy density, and the cycling performance at 2 C (4 mA cm −2 ) is shown in Figure S21, Supporting Information.At such a high-current density, the BT10P cell still displays durable cycling performance with a specific capacity of 85 mAh g −1 for 50 cycles, while the reference cells rapidly decline in capacity.Moreover, reducing the thickness of the separator is considered a valuable way to improve the energy density of batteries, and the thin BT10P separator with a thickness of 15 μm also exhibits a prolonged lifespan at 5 C than that of the 15μm BT10 separator (Figure S22, Supporting Information).The cycling performance of Li||LiNi 0.8 Co 0.1 Mn 0.1 O 2 cells equipped with different separators is further evaluated under the current density of 5 C (1 C = 180 mA g −1 ), shown in Figure S23, Supporting Information.The BT10P separator-based cell again delivers marked cycling Energy Environ.Mater.2024, 7, e12510 stability, with little capacity loss over 400 cycles, validating the practical application potential of the BT10P separator.

Information on the Surface of the Post-Cycled Li Metal Anode
SEM images provide direct proof that the piezoelectric BT10P separator suppresses dead Li accumulation.The morphologies of the Li metal anode of Li||LFP cells equipped with different separators are investigated after the 200th cycle at 5 C, as shown in Figure 4. Loose deposition layers with needle-like dendrites can be apparently observed on the Li metal anodes dissembled from the cells with the Celgard 2500 separator, the PVDF-HFP separator, and the BT10 separator.In sharp comparison, the anode in the BT10P cell exhibits a very compact and smooth deposition morphology.The cross-section images (Figure 4c,f,  i,l) reveal that the Li metal anode with the BT10P separator owns the thinnest deposition layer of about 40 μm, while the other separators have a deposition layer of more than 75 μm, nearly twice that of the BT10P separator, and the excessively thick deposited layer leads to the expansion of electrochemical polarization, which is consistent with the result of the cycling performance at 5 C. Additionally, the morphologies of the Li metal electrodes in the Li||Li symmetric cells with different separators operating at a current density of 1 mA cm −2 with a constant plating/stripping capacity of 5 mAh cm −2 for 100 h are also observed.As depicted in Figure S24, Supporting Information, it can be observed that the Li||Li symmetric cell equipped with the BT10P separator exhibits the smoothest plating morphology and no Li dendrites after a long cycle with a higher plating capacity (Figure S24d, Supporting Information).While the Li deposition morphologies of the other three types of separators are porous and covered with Li dendrites (Figure S24a-c, Supporting Information).These Li deposition morphology test results further validate the ability of the BT10P separator to inhibit dendrite growth.The elemental information of the SEI layer on cycled Li metal anode is studied by X-ray photoelectron spectroscopies (XPS), and the results are depicted in Figure S25, Supporting Information.Four peaks are identified in the C 1s spectrum as ROCO 2 Li, C=O, C-O, and C-C/C-H at 289.9, 288.4,286.5, and 284.8 eV, respectively. [34,35]For the F 1s spectra, two peaks corresponding to Li x PO y F z (686.7 eV) and LiF (684.7 eV) are detected, both from the decomposition of LiPF 6 . [36]It is noteworthy that neither the C 1s nor the F 1s spectra reveal any discernible changes, and that the peak areas of the identical components in various samples are remarkably similar.In addition, Ti and Ba are not detected in the Ti 2p and Ba 3d plots of all the post-cycled anodes.These results prove that the addition of BaTiO 3 into the separator does not change the composition of the SEI layer.

Ex/In Situ Observation of Li Plating Morphology and Finite Elements Analysis
The ex situ SEM images of the Li deposition process on Cu foil are shown in Figure 5a.Time points for Li deposition are set to be 5, 30, and 60 min, and the current density is fixed at 1 mA cm −2 .Needlelike dendrites randomly distributed on the Cu substrate are observed after 5-min plating using the Celgard 2500 separator.After continuous deposition for 30 min, lengthened and twisted Li whiskers with different sizes become more apparent.As the capacity increases, massive broken Li chunks appear, which is the dead Li that leads to lower CEs.The PVDF-HFP and BT10 separator-based Li||Cu cells show similar  Energy Environ.Mater.2024, 7, e12510 morphological evolution.Worm-shaped Li irregularly nucleate on the substrate in the initial 5 min and grow longer and coarser as the plating time increases, forming a loose plating morphology.Such a random dispersion of Li nuclei could trigger a locally enhanced electric field and intensified ion flux, leading to a nonuniform plating morphology.Notably, the piezoelectric BT10P separator induces an entirely different plating evolution, although the Li nuclei distribution is not very homogeneous.The SEM images show a tendency that Li favors to deposit around the protrusions but not on their tips.Further, the protrusions gradually join each other as the time extended rather than being isolated and loose as that of the PVDF-HFP and BT10 separator, and finally form a uniform and dense deposition layer.The morphology evolution confirms that the piezoelectricity of the BT10P separator guides the deposition of Li-ions around the protrusions and leads to a plating layer without dendrites.
The morphological evolution processes of Li deposition using the BT10 and BT10P separators are also observed by in situ optical microscopy at a current density of 10 mA cm −2 for 600 s, capturing pictures every 200 s.As shown in Figure 5b,c, the white BT10 and BT10P separator and the fresh Li-plated layer with metallographic luster can be apparently observed during the deposition process.Narrow, needle-like Li dendrites begin to grow between the BT10 separator and the substrate electrode after 200 s, and gradually increase with plating time.The force of the growing dendrite oriented upright lifts the separator rapidly and leaves a void-rich, loose deposition morphology at the end of the plating process.In comparison, the BT10P separator exhibits superior inhibition of dendrite growth at the high-current density.The pressure-induced counter potential from the BT10P separator guides the reduction of Li + at the void space rather than on the dendrite's tip, leaving a thinner and denser deposited layer than that of the BT10 separator.
Finite elements simulation can shed light on how the piezoelectric mechanism of the BT10P separator works.In the simulation models, the deposited substrate with a protuberance is covered by equivalent separators with/without piezoelectric characteristics whose pole direction faces the substrate, representing BT10P and BT10 separators, respectively.Electric field and Li-ion diffusion behavior simulations around the protuberance are conducted.As shown in Figure 5d, the deformed piezoelectric separator with a counter voltage significantly brings down the potential difference around the protuberance, which can be inferred that the accumulation of electric field at this place is diminished.However, in Figure 5e, the color of potential is bluer around the deformed zone of the non-piezoelectric separator, indicating the aggregation of the electric field at the protuberance.As can be expected, the diffusion behaviors of Li-ions are different between the two situations.For the piezoelectric separator (Figure 5d), Li-ion diffusion flux (black arrow) circumvents the protuberance and tends to deposit in the regions lower than the protrusion rather than on the protrusion tip, facilitating the formation of a flat deposition layer.For the non-piezoelectric separator (Figure 5e), the Li + accumulates mainly at the protrusion, exhibiting a locally enhanced ion flux, promoting the growth of dendrites.
The simulated concentration field of Li-ion, as shown in Figure 5f,g, illustrates the same outcomes that the piezoelectric separator (Figure 5f) decentralizes the ion flux around the protuberance while the nonpiezoelectric one (Figure 5g) gathers it due to the enhanced electric field resulted from the "tip effect.In addition, Figure 5h calculates the average Li-ion concentration in the direction of separator thickness, and impressively, the two separators have opposite concentration variation along the horizontal direction.The piezoelectric separator fundamentally changes the ion concentration distribution that a valley (~0.1 mol L −1 ) around the protrusion and higher values (~0.6 mol L −1 ) far away from the protrusion are exhibited, signifying that the piezoelectric separator reliably regulates Li-ion flux to form an even deposition layer.Notably, when the plating layer becomes flat, namely, when the piezoelectric separator is not squeezed by any local protuberance, the uneven distribution of the excited local voltage disappears, and the distribution of Li-ion concentration becomes uniform on the separator surface, as shown in Figure S26, Supporting Information.Therefore, it can be concluded that the piezoelectricity gives the separator the ability to regulate the electric field, which makes it serve as an electric field buffer.When it is pressed, it prevents the electric field from accumulating at the top of the dendrites and further readjusts the Li-ion flux, thus depositing a uniform Li-plated layer.

Conclusion
In summary, we design a homogeneous PVDF-HFP/BTO composite separator with simultaneously enhanced thermal stability and upgraded piezoelectric performances that successfully inhibit dendrite growth and enable durable operation of Li metal anode at high rates.The t Li þ of the composite separator is also remarkably promoted by incorporating BTO filler, which is favorable for reducing the overpotential at high C-rates.By characterizing the Li deposition morphology, the piezoelectric polarized PVDF-HFP/BTO composite separator can remarkably suppress the dendrite growth and decrease the thickness of the deposited layer.Finite elements simulations prove that when the piezoelectric separator is deformed by the dendrite, the generated counter potential can relieve the over-concentrated electric field around the dendrites' tip and further homogenize the Li-ion flux.Due to the piezoelectric mechanism, the Li||LiFePO 4 cells with polarized PVDF-HFP/BTO composite separator exhibits significantly improved cycling performance with over 99% capacity retention after 400 cycles at 2 C and over 85% capacity retention after 600 cycles at 5 C.This work provides an effective composite separator strategy to stabilize the high-rate Li metal anode for an extended lifetime.
, Supporting Information) have similar morphology and exhibit a more uniform porous structure than the PVDF-HFP separator, as confirmed by the pore size distribution concentrated at 0.5 μm shown in Figure S7c,d, Supporting Information.The element distribution images of the composite separator (Figure S8a, Supporting Information) exhibit that the BTO particles are homogeneously distributed, embedded in the pore structure of the separator or covered by PVDF-HFP (Figure

Figure 2 .
Figure 2. Effects of polarization direction of BT10P separator on the Li deposition morphology.a-c) Polarization direction of BT10P separator is directed toward the deposited electrode.d-f) Polarization direction of BT10P separator is directed backwards the deposited electrode.g-i) Upper surface of BT10 separator faces the deposited electrode.j-l) Lower surface of BT10 separator faces the deposited electrode.

Figure 4 .
Figure 4. Top-view and cross-section SEM images of Li metal anode dissembled from Li||LFP cells after 200 cycles at 5 C. a-c) Cell with Celgard 2500 separator.d-f) Cell with PVDF-HFP separator.g-i) Cell with BT10 separator.j-l) Cell with BT10P separator.

Figure 5 .
Figure 5. Ex/in situ observation of Li deposition morphology and finite elements analysis.a) Ex situ SEM images of the Li deposition process on Cu foil using the Celgard 2500, PVDF-HFP, BT10, and BT10P separators at current density of 1 mA cm −2 .b, c) In situ optical microscope images of Li deposition morphology using the BT10 and BT10P separators at current density of 10 mA cm −2 .The white dash line represents the height of the substrate.Scale bars are 50 μm.d, e) Electric field and Li-ion diffusion behavior simulations for separators with/without piezoelectricity.f, g) Concentration distribution simulations for separators with/without piezoelectricity.h) Average Li-ion concentration in the direction of separator thickness as a function of horizontal position.