Engineering Ferroelectric Interlayer between Li1.3Al0.3Ti1.7(PO4)3 and Lithium Metal for Stable Solid‐State Batteries Operating at Room Temperature

The poor contact and side reactions between Li1.3Al0.3Ti1.7(PO4)3 (LATP) and lithium (Li) anode cause uneven Li plating and high interfacial impendence, which greatly hinder the practical application of LATP in high‐energy density solid‐state Li metal batteries. In this work, a multifunctional ferroelectric BaTiO3 (BTO)/poly(vinylidene fluoride‐co‐trifluoroethylene‐co‐chlorotrifluoroethylene) (P[VDF‐TrFE‐CTFE]) composite interlayer (B‐TERB) is constructed between LATP and Li metal anode, which not only suppresses the Li dendrite growth, but also improves the interfacial stability and maintains the intimate interfacial contact to significantly decrease the interfacial resistance by two orders of magnitude. The B‐TERB interlayer generates a uniform electric field to induce a uniform and lateral Li deposition, and therefore avoids the side reactions between Li metal and LATP achieving excellent interface stability. As a result, the Li/LATP@B‐TERB/Li symmetrical batteries can stably cycle for 1800 h at 0.2 mA cm−2 and 1000 h at 0.5 mA cm−2. The solid‐state LiFePO4/LATP@B‐TERB/Li full batteries also exhibit excellent cycle performance for 250 cycles at 0.5 C and room temperature. This work proposes a novel strategy to design multifunctional ferroelectric interlayer between ceramic electrolytes and Li metal to enable stable room‐temperature cycling performance.


Introduction
Solid-state electrolytes (SSEs) such as organic polymer, inorganic ceramic and organic-inorganic composite SSEs have been reported for high-energy density solid-state Li metal batteries (SLMBs). [1]Among them, the solid polymer electrolytes (SPEs) possess many advantages including relative stability towards lithium metal, the facile processability and good contact with electrodes. [2][5][6][7][8] Ceramic electrolytes are increasingly attractive due to their high ionic conductivity, wide electrochemical window and excellent mechanical properties. [9]A variety of ceramic electrolytes have been developed, including perovskite-type, [10] halide-type, [11] garnettype, [12] sulfide-type [13,14] and sodium superionic conductor NASICON-type. [15,16]Therein, the NASICON-type Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) is exceedingly promising for SLMBs due to its high RT ionic conductivity, low cost and especially excellent air stability. [17,18]nfortunately, the LATP exhibits large interfacial resistance with electrodes owing to its rigid intrinsic properties and particularly unstable interface against Li metal originating from the reduction reaction of Ti 4+ . [19]In addition, the formation of Li dendrites would aggravate the side reactions because of the broken solid electrolyte interface (SEI) layer (Figure 1a).[22][23] Various strategies for tackling the interfacial issues between LATP and Li metal anode have been proposed previously.Al 2 O 3 -coated LATP was prepared via atomic layer deposition (ALD) method, [24] while the ionically insulating coating layer blocks the ion diffusion and deteriorates the performance of solidstate batteries.The metal-based interlayers that can alloy with Li metal at a high temperature (~200 °C) are electronically conductive, [25] which cannot totally avoid the side reactions between Li metal and LATP.It is widely acknowledged that SPEs exhibit flexible nature, adjustable Li salt concentration and good interfacial compatibility with Li metal anode.Based on the above advantages, it is believed that the SPEs are suitable for the interlayer between the LATP electrolytes and Li metal anode. [26]As the most common polymer electrolyte, polyethylene oxide (PEO)-based polymer electrolyte was as a matter of course utilized as interlayer, [27][28][29][30] unfortunately, The poor contact and side reactions between Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) and lithium (Li) anode cause uneven Li plating and high interfacial impendence, which greatly hinder the practical application of LATP in high-energy density solid-state Li metal batteries.In this work, a multifunctional ferroelectric BaTiO 3 (BTO)/poly(vinylidene fluoride-co-trifluoroethylene-cochlorotrifluoroethylene) (P[VDF-TrFE-CTFE]) composite interlayer (B-TERB) is constructed between LATP and Li metal anode, which not only suppresses the Li dendrite growth, but also improves the interfacial stability and maintains the intimate interfacial contact to significantly decrease the interfacial resistance by two orders of magnitude.The B-TERB interlayer generates a uniform electric field to induce a uniform and lateral Li deposition, and therefore avoids the side reactions between Li metal and LATP achieving excellent interface stability.As a result, the Li/LATP@B-TERB/Li symmetrical batteries can stably cycle for 1800 h at 0.2 mA cm −2 and 1000 h at 0.5 mA cm −2 .The solid-state LiFePO 4 /LATP@B-TERB/Li full batteries also exhibit excellent cycle performance for 250 cycles at 0.5 C and room temperature.This work proposes a novel strategy to design multifunctional ferroelectric interlayer between ceramic electrolytes and Li metal to enable stable room-temperature cycling performance.
whose low RT ionic conductivity can only allow the operation of the batteries at 60 °C or even higher temperature, [29,30] offset the advantages of LATP at RT.Moreover, the PEO-based interlayers still face the issue of Li dendrite growth owning to the deficient Li-ion flux and poor mechanical strength. [28,30]erein, we developed a multifunctional interlayer by introducing the strong ferroelectric material BaTiO 3 (BTO) into poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) (P[VDF-TrFE-CTFE]) (denoted as B-TERB) as a highly ionic conductive interfacial layer.This interlayer not only can induce the homogeneous and lateral Li-ion deposition, but also reduce the Li/LATP interfacial impendence and suppress side reactions.The ferroelectric B-TERB interlayer exhibits a spontaneous dipole moment, which can be reversed in the electric field of battery to generate an inverse polarized electric field.The inverse polarized electric field can neutralize the locally concentrated electric field to relieve the "tip effect", enabling the uniform Li ions distribution and achieving horizontal Li deposition along its surface (Figure 1b,c).The "tip effect" can be explained as the inevitable existence of protuberances on the surface of the lithium metal anode.The electric field is unevenly distributed near the high-curvature position during lithium plating.Intrinsically, Li ions will move to protuberances owing to the local intensive electric field near the high-curvature position.Therefore, the irregular Li ions movement induced by uniform electric field distribution results in the formation of "lithium tips". [31]The elimination of Li tips combining with the uniform Li ion transport of B-TERB interlayer effectively suppresses the growth of Li dendrites to avoid their side reactions with LATP, which ensures the excellent solid-state battery performance during prolonged cycles.The Li/LATP@B-TERB/Li symmetrical batteries can stably cycle for 1800 h at 0.2 mA cm −2 and 1000 h at 0.5 mA cm −2 under RT.The LiFePO 4 (LFP)/LATP@B-TERB/Li full batteries at RT also exhibit excellent cycle performance for 250 cycles at 0.5 C.This work provides a universal interface design strategy for advanced all-solid-state batteries equipped with ceramic electrolytes.

Results and Discussion
The LATP pellet was fabricated by a coldpress process followed by sintering technique (see Appendix S1, Supporting Information), whose optical image is shown in Figure S1a, Supporting Information.The scanning electron microscope (SEM) image of pristine LATP pellet exhibits dense primary crystals and obvious grain boundaries (Figure S1b, Supporting Information).The X-ray diffraction (XRD) pattern of LATP pellet matches well with the standard pattern of LiTi 2 (PO 4 ) 3 (PDF#35-0754) (Figure S1c, Supporting Information). [32]The purephase structure leads to a high ionic conductivity of 3.67 × 10 −4 S cm −1 at 25 °C (Figure S1d, Supporting Information).Two parallel interlayers were designed to clarify the mechanism of alleviating Li dendrites by ferroelectric effect.One is P(VDF-TrFE-CTFE) (denoted as TER) and the other is BTO and P(VDF-TrFE-CTFE) composite (denoted as B-TERB).As shown in Figure 2a, the XRD pattern of BTO presents two split peaks at around 45 o (2θ), which are corresponding to (002) and ( 200) planes of ferroelectric tetragonal (t-BTO) phase, respectively. [33]The transmission electron microscope (TEM) image in Figure S2a demonstrates that the BTO has clear lattice fringes with a lattice spacing of 0.230 nm and 0.283 nm that could be ascribed to the (111) and (101) planes of tetragonal phase.In t-BTO with tetragonal phase, a spontaneous polarization could be produced via movement of titanium atoms along the c-axis, which can be switched in the applied electric field, therefore leading to the strong ferroelectric property (Figure S2b, Supporting Information). [34]iezoelectric force microscopy (PFM) was used to estimate the ferroelectric properties of TER and B-TERB interlayer, which allows imaging and manipulation of ferroelectric domains. [35]The surface potential of the interlayer was measured via a conductive probe applied with bias voltage.The mapping of the piezo-responsive amplitudes of TER interlayer and B-TERB interlayer shows that the TER interlayer exhibits polarization response voltage of 0-1.9 V, while the B-TERB interlayer exhibits a much higher response voltage of 0-3.2 V (Figure 2b,c).Distribution histogram of piezo-responsive amplitudes in Figure 2d shows that the peak piezo-responsive voltage of B-TERB interlayer with a peak height of 4.2% is ~2.9V, while that of TER interlayer with peak height 10.0% is ~1.7 V.The mean value of piezo-responsive amplitudes of TER and B-TERB are 0.244 mV and 0.557 mV, respectively (Figure S3a, Supporting Information).A higher and more uniform polarization response voltage illustrates that the B-TERB interlayer possesses a stronger ferroelectric than TER interlayer. [36]The domain boundaries of B-TERB are more distinct than that of TER in the mapping of piezo-responsive phases (Figure S3b,c, Supporting Information).Hysteresis loops curves of piezo-responsive amplitudes and phases at voltage bias show that the phase of TER interlayer was from −90 to 60 degrees (Figure 2e), while that of B-TERB interlayer in  and stability, [37] which could be attributed to the intrinsic strong piezoelectric property of BTO.
To further clarify the influence of strong ferroelectric interlayer on the Li-ion deposition behavior, the finite element simulations were conducted on the distributions of electric field and Li-ion concentration at the Li metal/interlayer interface. [38]Polarization response voltage of varying interlayers could affect the electric field distribution on Li anode surface.Non-ferroelectric-coated Li metal and the TER-coated Li exhibits concentrated electric field lines, while the B-TERB-coated Li induces an extremely uniform electric field distribution (Figure S4, Supporting Information).Consequently, the homogeneous electric field distributions results in much more uniform Li-ion concentration distributions in B-TERB-coated Li interface than that in Li and TER-coated Li (Figure 2g-i).The uniform electric field and Li-ion concentration distributions by strong ferroelectric effect is beneficial to the Li deposition behavior at the Li/LATP interface.
The interlayers were constructed between LATP and Li metal, whose thicknesses are controlled to around 2 μm (Figure 3a-c).The surface morphologies of LATP@TER and LATP@B-TERB present that the TER and B-TERB interlayers are uniformly covered on the crystalline grains of LATP (Figure S5a,b, Supporting Information).In addition, the surface elemental maps of C, F and Ba suggests that the TER and B-TERB interlayers are evenly distributed on the LATP surface (Figure S6, Supporting Information).The cross-sectional SEM images present that the pristine LATP/Li interface exhibits poor contact with obvious gap (Figure S7a, Supporting Information), while tight contact is observed after constructing a TER and B-TERB interlayers (Figure 3d).Therefore, the Li/LATP@TER/Li and Li/LATP@B-TERB/Li batteries present a much lower interfacial resistance of 411.1 Ω and 456.1 Ω than that of the Li/LATP/Li batteries (56937.8Ω, Figure 3e).
The cycling stability of Li/Li symmetrical batteries was evaluated to examine the availability of the ferroelectric-based interlayer at RT. [39] The Li/LATP/Li batteries without interlayer not only exhibit high polarization voltage of 2-3 V, but also present serious side reactions due to direct contact between LATP and Li metal, which become open-circuited only after 2 h (Figure S7b, Supporting Information).The B-TERB interlayer was firstly optimized by exploring the suitable concentration of BTO in TER.The B-TERB interlayer consisting of 10 wt% BTO (10%B-TERB) and 20 wt % BTO (20%B-TERB) were prepared to assemble Li/Li symmetric batteries.As shown in Figure S8, Supporting Information, the Li/Li symmetric batteries with 10%B-TERB achieved a better cycling stability than that with 20%B-TERB.Therefore, the 10%B-TERB interlayer was adopted for further measurements and characterizations in the following discussion (denoted as B-TERB).As shown in Figure 3f, the Li/LATP@B-TERB/Li batteries could stably cycle for 1800 h at 0.2 mA cm −2 and RT, while the polarization of Li/LATP@TER/Li batteries began to sharply increase after cycling only 500 h and came to open-circuited after cycling 700 h, which could be attributed to the formation of severe Li dendrites and side reaction between LATP and Li dendrites. [28]Similarly, the Li/LATP@B-TERB/Li batteries could stably cycle 200 h at 0.2 mA cm −2 followed by 1000 h at 0.5 mA cm −2 and RT, while the Li/LATP@TER/Li batteries present an increased polarization just after 200 h cycle at 0.5 mA cm −2 under the same test condition (Figure 3g).In addition, the Li/LATP@B-TERB/Li batteries exhibited a much better performance than Li/LATP@TER/Li batteries at 0.1 mA cm −2 and RT (Figure S9a, Supporting Information).These improved performances of Li/ LATP@B-TERB/Li batteries could be ascribed to the uniform Li deposition induced by the strong ferroelectric interlayer of B-TERB. [40]The Li/LATP@B-TERB/Li batteries present much better cycling stability at higher current density than other reported batteries using LATP with interlayers (Table S1, Supporting Information).28][29][30] The interfacial resistance of Li/LATP@TER/Li and Li/LATP@B-TERB/Li batteries before and after cycling was examined to further reveal the suppression of side reactions by B-TERB interlayer.The initial resistance of Li/LATP@TER/Li and Li/LATP@B-TERB/Li batteries after 90 h cycling increases from 411.1 Ω to 493.4 Ω, and from 456.1 Ω to 524.0 Ω, respectively, (Figure S9b, Supporting Information).However, the resistance of Li/LATP@TER/Li batteries after 1100 h cycling at 0.5 mA cm −2 is 6796.5 Ω, almost five times higher than that of Li/LATP@B-TERB/Li (1392.1 Ω) after the same cycling (Figure 3h).Similarly, the resistance of Li/LATP@TER/Li batteries after 1300 h cycling at 0.1 mA cm −2 is as high as 23893.3Ω, beyond six times higher than 3758.7 Ω of Li/LATP@B-TERB/Li after 2100 h cycling.Since the side reaction products between LATP and Li metal exhibit much lower ionic conductivity than pristine LATP, [41]  which deteriorate the interfacial contact and increase the interfacial resistance.The different resistance increase of the cycled Lisymmetrical batteries reveals that the severe reduction reaction between LATP and Li dendrites occurred in Li/LATP@TER/Li batteries during the prolonged cycle, while it was well suppressed in Li/ LATP@B-TERB/Li batteries.The B-TERB interlayer can neutralize locally concentrated deposition electric field and promote the uniform distribution of Li-ion, therefore alleviate the tip effect to achieve the Li uniform deposition at the Li metal anode surface, which leads to the dendrite-free and side-reaction-suppressed LATP-based SLMBs. [33]ue to the polarizing power of the high ferroelectric material, an effective polarization electric field is established which opposes the applied field in the battery and scales with the dielectric constant.As a consequence, the electrical field lines are drawn towards the high dielectric material (dictated by Gauss Law), which leads to a lowering of the divergence of the electrical field in the vicinity of the Li metal deposition to alleviate the tip effect. [42]These results suggest that construction of a strong ferroelectric interlayer can achieve the stable and highly efficient ion transport in LATP/Li interface.Generally, the polymer-ceramic composite layer exhibits higher mechanical strength than the single polymer layer, [43] which was also considered as an effective method to suppress the Li dendrite growth. [44]In order to strengthen the effect of ferroelectric property in this work, we combine MgO with TER (10% MgO) as interlayer (TERM) to construct a polymer-ceramic layer.As shown in Figure S10, Supporting Information, the Li symmetrical batteries with TERM interlayer exhibits a worse performance than that with B-TERB at various galvanostatic current densities.Therefore, the strong ferroelectric property of composite interlayer is significant to the uniform Li-ion distribution and deposition behavior between ceramic electrolyte pellets and Li metal anode.
Furthermore, Li-Cu batteries were assembled and a fixed areal capacity of Li (1 mAh cm −2 ) was plated on Cu electrode to explore the Li deposition behaviors with different interlayers.Different Li deposition behaviors occurred and granular dendrites appeared on the Cu foil in the batteries with TER interlayer (Figure S11a, Supporting Information and Figure 4a), which easily pierce through the interlayer to contact LATP.In sharp contrast, planar and tabular Li growth was observed on the Cu foil of the batteries using B-TERB interlayer (Figure 4b).This indicates that designing the layer with strong ferroelectric effect is an efficient strategy to suppress the Li dendrites under plating processes, which endows the ferroelectric interlayer a great application potential for LATP-based SLMBs.
The cycled LATP with TER interlayer presents needle-like surface morphology after ~700 h cycle, which signifies the seriously reduced-LATP, and its structure has been destroyed (Figure 4c).Whereas, the cycled LATP with B-TERB interlayer almost present the same morphology as pristine LATP (Figure 4d and Figure S11b Supporting Information).The X-ray photoelectron spectroscopy (XPS) of the LATP pellet before and after cycling was characterized to identify the side reaction between LATP and Li metal.The Ti 2p of pristine LATP SSEs presents the typical peaks of Ti 4+ 2p 3/2 (458.0 eV) and Ti 4+ 2p 1/2 (463.8eV) (Figure S11c, Supporting Information), [45] while the Ti 2p of cycled LATP with TER interlayer becomes weaker, which suggests that a large amount of Ti 4+ in LATP has been reduced by Li metal (Figure 4e).In contrast, the Ti 2p of cycled LATP with B-TERB interlayer still presents the state of Ti 4+ 2p 3/2 (458.3 eV) and Ti 4+ 2p 1/2 (464.0 eV) (Figure 4f), indicating that the strong ferroelectric B-TERB interlayer is quite effective on suppressing the side reaction against Li metal due to Li dendrite-free derived from the regulated Li deposition at the interface between LATP and Li metal.In addition, the cycled LATP pellet with B-TERB interlayer exhibits the same XRD patterns as the pristine LATP pellet (Figure S11d, Supporting Information), while another set of diffraction patterns are found in the XRD of the cycled LATP with TER interlayer (marked with black arrows) (Figure 4g-i), which can be attributed to Li-rich phase Li 3 Al 0.3 Ti 1.7 (PO 4 ) 3 formation for the lithium intercalation reaction of LATP. [41]Therefore, the strong ferroelectric interlayer can effectively prevent Li dendrite growth and side interfacial reaction between LATP and Li metal.
Evolution of Li deposition on Li foil with different interlayers via finite element simulations clarifies the forming mechanism of planar and tabular Li growth with strong ferroelectric B-TERB interlayer.As shown in Figure S12, Supporting Information, the growth direction of Li and TER-coated Li is almost perpendicular to electrolyte owing to the concentrated electric field, which easily causes severe dendrites propagation on Li and TER-coated Li (Figure 5a,b).In contrast, the uniform electric field and Li ions concentration distribution in B-TERB-coated Li induce Li horizontal growth and uniform Li deposition on B-TERB-coated Li (Figure 5c).The evolution of Li plating further verifies that the strong ferroelectric interlayer plays a significant role in uniform Li deposition behavior.The electrochemical performances of full cells with different interlayers were tested at RT.The EIS exhibits that the resistance of LFP/LATP@TER/ Li and LFP/LATP@B-TERB/Li batteries are 369.4Ω and 420.8 Ω, respectively (Figure S13, Supporting Information).A little larger resistance of LFP/LATP@B-TERB/Li batteries is owing to the stronger ferroelectric response from B-TERB interlayer.The RT rate capacities of LFP/LATP@B-TERB/Li batteries at 0.1 C, 0.2 C, 0.5 C and 1 C are 160.3,154.1, 141.8 and 115.2 mAh g −1 , respectively.After rate cycling, its capacity can still restore to 157.9 mAh g −1 at 0.1 C and the capacity retention rate is as high as 91.3% after 140 cycles.In contrast, the capacity of LFP/LATP@TER/Li batteries only deliver 87.7 mAh g −1 at 0.5 C and 18.9 mAh g −1 at 1 C, which return to 157.8 mAh g −1 at 0.1 C but presents sharp capacity fade just after 35 cycles (Figure 5d).
In addition, the LFP/LATP@B-TERB/Li batteries present excellent RT cycling performance with a higher capacity retention rate of 78.6% after 250 cycles at 0.5 C and still delivered 105.9 mAh g −1 at 250th cycle, which was much higher than 0.9 mAh g −1 of LFP/LATP@TER/Li batteries (Figure 5e).After 250 cycles at 0.5 C, the resistance of LFP/LATP@TER/Li batteries increased to 1650179.6 Ω owing to severe side reaction, while the resistance of LFP/LATP@B-TERB/Li was only 1018.4 Ω (Figure 5h).There was almost no capacity decay in the different cycles of LFP/LATP@B-TERB/Li at 0.5 C (Figure 5f), indicating that the side reaction was well suppressed due to uniform Li deposition induced by the strong ferroelectric effect.The severe Li dendrites and side reaction in LFP/ LATP@TER/Li batteries caused continuous capacity decay in the different cycles (Figure 5g).
When combined with high-voltage Ni 0.8 Co 0.1 Al 0.1 O 2 (NCA) cathode, the full batteries with B-TERB interlayer also exhibit obviously better cycling performance and higher capacity retention after 310 cycles at 0.1 C and RT (Figure 5i), and the coulombic efficiency (CE) is as high as 100.0%.The NCA full batteries with different interlayers present similar capacity at the initial 5th cycle (Figure S14a, Supporting Information).Whereas, the NCA/LATP@B-TERB/Li batteries could still deliver a capacity of 106.7 mAh g −1 after 310 cycles for 8.6 months, but the solid-state NCA/LATP@TER/Li batteries could only deliver a capacity of 77.9 mAh g −1 (Figure S14b, Supporting Information).The strong ferroelectric B-TERB interlayer contributes to uniform electric field and ion concentration distributions, which induces the homogeneous Li deposition to suppress the Li dendrite growth.In addition, the B-TERB interlayer significantly inhibits the side reactions between LATP pellet and Li metal to achieve excellent performance of solid-state LATPbased LMBs.

Conclusion
We design a strong ferroelectric polymer-based electrolyte layer (B-TERB) constructed between LATP pellet and Li metal, which effectively induces the uniform Li deposition and enables the SLMBs stably operating at room temperature.The strong ferroelectric B-TERB interlayer generates an inverse polarized electric field, which achieves uniform electric field and ion concentration distributions at the interface with Li metal anode to alleviate the "tip effect" and promotes the planar Li deposition.In addition, the B-TERB interlayer improves the contact between LATP pellet and Li metal to tremendously reduce the interfacial resistances of SLMBs and suppress side reactions between LATP and Li metal, which ensures the fast interfacial ion conduction of SLMBs at RT during long cycling.As a result, the Li/LATP@B-TERB/Li batteries can stably cycle for 1800 h at 0.2 mA cm −2 and 1000 h at 0.5 mA cm −2 .Furthermore, the solid-state LFP/LATP@B-TERB/Li batteries achieve excellent cycling performance and a CE of approximately 100% at 0.5 C after 250 cycles.Even combined with high-voltage cathode, the solid-state NCA/LATP@B-TERB/Li batteries can still cycle for more than 300 cycles at 0.1 C.This multifunctional ferroelectric interlayer provides a universal strategy for suppressing Li dendrite growth and side reactions of ceramic electrolytes with highly active Li metal anode.

Experimental Section
Preparation of the Dense LATP Ceramic Pellets: The LATP ceramic powder was pressed into pellets in a 15 mm mold through cold pressing under a pressure of 4 MPa, then the ceramic pellets were sintered at 900 °C for 6 h.During sintering, the ceramic was crystallized and shrunk, the LATP pellets with diameter of ~12 mm and thickness of ~500 μm were obtained.Finally, the sintered LATP ceramic pellets were polished to ~300 μm by sandpaper.Preparation of Various Interlayers: P(VDF-TrFE-CTFE) (average M w ~450 000 Piezotech, Arkema) polymer was dissolved in the DME solvent by stirring for 12 h, then obtained the precursor solution for TER interlayer.BaTiO 3 (Aladdin) powder and LiTFSI (Canrd) were added in the solution stirring for 12 h forming the precursor solution (10 wt.% and 20 wt.% BTO in solution) for B-TERB interlayer.The precursor solution was stirred in an argon-filled glove box.The interlayer was prepared by dripping a drop of the precursor solution (approximately 4-5 μL) on the surface of LATP ceramic pellets and evaporating the solvent at 100 °C for 30 min.After evaporating, the LATP with polymer-based electrolyte layer was placed in an argon-filled glove box for further drying.Assembly of the Batteries and Electrochemical Measurements: The solid-state Li symmetric batteries, LFP and NCA full batteries were all assembled in an argon-filled glove box.The Li foil with thickness of 90 μm was punched into small discs of 8 and 10 μm.These Li discs were used to assemble Li/LATP@TER/Li, Li/LATP@B-TERB/Li, LFP/LATP@TER/Li, LFP/LATP@B-TERB/Li, NCA/ LATP@TER/Li and NCA/LATP@B-TERB/Li batteries.To guarantee the integrality of LATP pellets in batteries, the common gaskets in 2032 coin cells were substituted by conductive sponge.The LiFePO 4 cathode was composed of 80 wt% LiFePO 4 , 10 wt% PVDF binder, and 10 wt% super P, which dissolved in N-1-methyl-2-pyrolidone (NMP) to obtain a homogeneous slurry, and then was casted on aluminum foil and dried at 80 °C for 20 h.NCA cathode was prepared by the similar method.In order to keep intimate contact, the TER interlayer precursor solution was dripped at cathode/LATP interface uniformly.The Li symmetrical batteries were tested under different current densities with an hour of charge and discharge time.The LFP and NCA full batteries were cycled in a galvanostatic mode, the former with a voltage range of 2.4-4.2V, and the latter with a voltage range of 2.7-4.3V.All electrochemical measurements of batteries were conducted by Land Test System at room temperature.Material Characterization: The EIS of batteries was measured by an electrochemical workstation, with a frequency range of 7 MHz to 0.1 Hz and a disturbing voltage amplitude of 10-20 mV.Crystal structures of LATP and BTO were examined by XRD (Rigaku D/max 2500/PC diffract meter, Riga Corp., Shibamata, Japan), using Cu Kα radiation with 2θ from 5°to 90°and a scanning speed of 10°min −1 .SEM (HITACH S4800) equipped with Energy Dispersive Spectrometer (EDS) was utilized to observe surface and cross-section morphologies of the LATP pellets and interlayer.XPS (PHI5802) was operated to analyze the valence states of the LATP pellets before and after cycling with Li metal.The piezoelectricity was characterized by AFM (Bruker, DIMENSION ICON) using a Scm-tip in the PFM module with Peak force tapping mode.
Figure 2f was from −80 to 80 degrees.The more distinct domain, enlarged polarized window and more obvious butterfly curve of B-TERB interlayer further indicate whose intensified polarization intensity

Figure 1 .
Figure 1.Illustration of Li metal plating between LATP and Li metal a) without interlayer, with b) TER and c) B-TERB interlayer.

Figure 2 .
Figure 2. a) XRD patterns of BTO; Characterization of piezoelectricity via piezoelectric force microscope (PFM).Mapping of piezo-responsive amplitudes of b) TER interlayer and c) B-TERB interlayer; d) Distribution histogram of piezo-responsive amplitudes; Hysteresis loops curves of piezoresponsive amplitudes and phases of e) TER interlayer and f) B-TERB interlayer; Li-ion concentration gradient lines distribution simulations (gray arrow lines) on Li foil with varying interlayers: g) Li, h) Li@TER and i) Li@B-TERB.