Self‐Adaptive and Electric Field‐Driven Protective Layer with Anchored Lithium Deposition Enable Stable Lithium Metal Anode

Lithium metal battery has great development potential because of its lowest electrochemical potential and highest theoretical capacity. However, the uneven deposition of Li+ flux in the process of deposition and stripping induces the vigorous growth of lithium dendrites, which results in severely battery performance degradation and serious safety hazards. Here, the tetragonal BaTiO3 polarized by high voltage corona was used to build an artificial protective layer with uniform positive polarization direction, which enables uniform Li+ flux. In contrast to traditional strategies of using protective layer, which can guide the uniform deposition of lithium metal. The ferroelectric protective layer can accurately anchor the Li+ and achieve bottom deposition of lithium due to the automatic adjustment of the electric field. Simultaneously, the huge volume changes caused by Li+ migration change of the lithium metal anode during charging and discharging is functioned to excite the piezoelectric effect of the protective layer, and achieve seamless dynamic tuning of lithium deposition/stripping. This dynamic effect can accurately anchor and capture Li+. Finally, the layer‐modified Li anode enables reversible Li plating/stripping over 1500 h at 1 mA cm−2 and 50 °C in symmetric cells. In addition, the assembled Li‐S full cell exhibits over 300 cycles with N/P ≈ 1.35. This work provides a new perspective on the uniform Li+ flux at the Li‐anode interface of the artificial protective layer.


Introduction
[6] Therefore, the non-uniformity of Li + flux is the key factor that limits the large-scale application of LMBs.
[15] These strategies have been extensively investigated, in which the artificial protective layers can significantly modify the fragile properties of SEI, [16,17] reducing the ineffective consumption of Li and suppressing the formation of Li dendrites. [18]In the past years, diverse heterogeneous artificial protective layers including organic (b-PVDF, polyimide, protein-based membranes), [19] inorganic(SiO 2 , Cu 3 N, Li 2 S), [20,21] and hybrid system (Cu 3 N/SBR, ZrO 2 @PVDF-HFP) [22,23] have been constructed.However, current research on artificial protective layers can be summarized as "guided deposition," which can achieve uniform deposition in a short time, but it still faces huge challenges to achieve continuous uniform deposition in a long-term cycling.Therefore, there is an urgent need for an anchored deposition method that can accurately anchor and capture Li + , and achieve bottom-up deposition of metal lithium far from the separator.Ferroelectric materials are polarized spontaneously and can be reversibly switched by electric fields.BaTiO 3 is a typical ferroelectric and piezoelectric material with good ionic conductivity and can form an internal electric field in the presence of an external electric field and stress, [24] such an internal electric field can anchor and trap Li + , and allow the ideal deposition of lithium metal away from the separator to inhibit the growth of lithium dendrites.
In this work, we construct an artificial piezoelectric protective layer on Cu foil with tetragonal BaTiO 3 to form lithium anodes by Lithium metal battery has great development potential because of its lowest electrochemical potential and highest theoretical capacity.However, the uneven deposition of Li + flux in the process of deposition and stripping induces the vigorous growth of lithium dendrites, which results in severely battery performance degradation and serious safety hazards.Here, the tetragonal BaTiO 3 polarized by high voltage corona was used to build an artificial protective layer with uniform positive polarization direction, which enables uniform Li + flux.In contrast to traditional strategies of using protective layer, which can guide the uniform deposition of lithium metal.The ferroelectric protective layer can accurately anchor the Li + and achieve bottom deposition of lithium due to the automatic adjustment of the electric field.Simultaneously, the huge volume changes caused by Li + migration change of the lithium metal anode during charging and discharging is functioned to excite the piezoelectric effect of the protective layer, and achieve seamless dynamic tuning of lithium deposition/stripping.This dynamic effect can accurately anchor and capture Li + .Finally, the layermodified Li anode enables reversible Li plating/stripping over 1500 h at 1 mA cm À2 and 50 °C in symmetric cells.In addition, the assembled Li-S full cell exhibits over 300 cycles with N/P % 1.35.This work provides a new perspective on the uniform Li + flux at the Li-anode interface of the artificial protective layer.electrochemical deposition and investigate the effect of this mechanoelectric coupling strategy.Because the effect is not present at every position inside the BaTiO 3 , and its polarization direction is closely related to the direction of piezoelectric electric field. [25]In addition, polarization direction and intensity of BaTiO 3 can be adjusted by applied DC electric field due to its ferroelectricity. [26]It is expected that the polarization directions within the BaTiO 3 are unified and along the Z axis by high voltage corona polarization, and then the protective layer can obtain the electric field direction from the current collector side to separator.The volume effect caused by Li + migration during the plating and stripping of lithium metal can be used as the driving force for the migration of anchored Li + , which urge piezoelectric protective layer dynamically reverse its own polarization direction.This work not only explores the law of electric field distribution in the protective layer under different high voltage polarization directions, but also reveals the effect of different electric field distributions on the Li metal deposition process.As a result, the protective layer of BaTiO 3 under positive polarization named PBTO urge the Li + flux on the anode more uniform, and also accelerate deposition rate of Li + and realize ideal bottom-up deposition of metal lithium far from the separator.Due to its high mechanical strength, the artificial protective layer can also act as a physical barrier to inhibit lithium dendrites, further improving battery safety performance.Benefiting from the above versatilities, the symmetric battery assembled with the lithium anode protected by PBTO can cycle stably for 1500 h at 1 mA cm À2 , even when the test environment temperature up to 50 °C, the stability of the battery remains unchanged.In addition, the lithium-sulfur full battery assembled with a protective layer anode can stably cycle for 300 cycles at N/P % 1.35.

Results and Discussion
Due to high reactivity of lithium metal, the interfacial properties of lithium anode will directly determine the subsequent charging and discharging behavior. [27]And a layer with good ion permeability and electronic insulation could were designed with the anchoring effect of uniform Li + flow.As shown in Figure 1, the piezoelectric effect of BaTiO 3 is closely related to its own polarization direction.Usually, the polarization of BaTiO 3 has been neither strong nor uniform, which will lead to the polarization of each direction to cancel each other and weaken. [28,29]Therefore, it is important to build a completely uniform protective layer with the polarization direction pointing to the separator.After the protective layer obtained a uniform polarization direction through positive high voltage polarization, that is PBTO, the BaTiO 3 particles will gain omnidirectional force in all axial directions from Li + , eventually protective layer is further excited to produce the piezoelectric effect.Moreover, the defects in volume effect caused by Li + migration in LMBs will be converted into the advantage of driving Li + directed migration in electrolyte.When the metal lithium is plating, the protective layer is subjected to the compressive stress of the Z axis of the lithium metal, thus the polarization direction is reversed and far away from the separator.On the contrary, as the lithium metal is stripping, the protective layer will gain the Z-axis tensile stress.Simultaneously, the polarization direction of the protective layer points to the separator.Its internal electric field will drive the migration of Li + in each cycle of charge and discharge, and the ordered electric field direction can anchor lithium metal to achieve dense deposition.
The as-prepared BaTiO 3 maintains a good crystal structure and can be accurately matched to tetragonal BaTiO 3 (PDF #05-0626; Figure 2a).Figure 2b shows the Raman peaks near 305 and 715 cm À1 are recognized as the characteristic peaks of the ferroelectric tetragonal structure. [30]We further obtained energy-dispersive X-ray spectroscopy (EDS) mapping to examine the distribution of barium (Ba), titanium (Ti), and oxygen (O) within the BaTiO 3 .As shown in Figure 2c, the molar ratio of Ba and Ti was $ 1:1.Furthermore, the morphology of BaTiO 3 synthesized in this work were evaluated.The nanoscale BaTiO 3 of the protective layer shortens the ion transport distance.Scanning electron microscopy (SEM) of Figure 2d also proves that nanoparticles BaTiO 3 were successfully synthesized and Figure S2, Supporting Information shows its average particle size is about 200 nm.A highresolution TEM image (Figure 2e) reveals clear lattice fringes.Additionally, lattice fringes with d-spacing of 1.419 and 2.838 nm are also observed, which are attributed to the [202] and [101] lattice planes of tetragonal BaTiO 3 .Furthermore, energy-dispersive X-ray spectroscopy (EDS) measurements confirmed the presence of Ba, Ti, and O within nanoparticle BaTiO 3 were evenly distributed (Figure 2f).Further verification of the BaTiO 3 composition was achieved by X-ray photoelectron spectroscopy (XPS, Figure S3, Supporting Information).The XPS spectrum of Ba 3d in the BaTiO 3 powder in Figure 2g show that Ba 3d 5/2 and Ba 3d 3/2 peaks can be fitted by two peaks, in which located at approximately 777.9 and 793.1 eV corresponding to Ba 2+ in the ABO 3 perovskite structure and other two corresponding peaks (794.8 and 779.1 eV) demonstrate Ba 2+ in the nonperovskite structure. [31]From the high-resolution spectrum of Ti 2p (Figure 2h), the main valence states of Ti in the material are Ti 4+ and a small amount of Ti + , Ti 2+ .The fitting result of oxygen element (Figure 2i) also shows that the oxygen in the material is originally O 2À . [32]The above results show that the powder synthesized in this work is a high-purity ferroelectric tetragonal BaTiO 3 .
Owing to displacement of Ba 2+ and Ti 4+ for O 2À along the [110] axis of the tetragonal structure, the tetragonal BaTiO 3 equipped with the spontaneous polarization have demonstrated good ferroelectricity and piezoelectricity. [33]However, the spontaneous polarization intensity of BaTiO 3 is relatively low, and the piezoelectric effect of BaTiO 3 may not be effectively excited during the charging and discharging process.Therefore, the piezoelectric effect of the material can be excited as much as possible by the high-voltage corona polarization.As shown in Figure 3a, the tetragonal BaTiO 3 particles synthesized above were roll-coated on copper foil as a protective layer.The prepared current collector is further turned over and subjected to high-voltage polarization under an applied electric field, so as to current collector with different applied electric field directions.The contact angle difference (Figure S4, Supporting Information) between the polarized and unpolarized three current collectors and the electrolyte is very small, and they all have good wettability to the electrolyte, so the wettability between the current collector and the electrolyte is not affected after the  high voltage polarization.The topographic AFM images of current collector with coating layer are shown in Figure 3b,c.There was no significant difference between NBTO and PBTO particles.It is known that the KPFM mapping image of NBTO became darker after negative high voltage polarization treatment and the surface charge is positive, but the electric field strength is low.In comparison, the surface of the current collector PBTO treated with positive high voltage polarization possesses negative charge and high electric field intensity.The difference in surface charge confirms the afore-mentioned idea: the polarization direction of the current collector coating layer can be changed by adjusting the direction of the applied high voltage electric field.After removing the applied high voltage electric field, the difference in the retained electric field strength of the coating layer is caused by the fact that the charge distribution of each part of the layer are not completely consistent.
As shown in Figure 3d 1 ,e 1 , when the BTO-coated current collector is polarized by applying DC high voltages in different electric field directions, the surface and internal potential of the current collector show corresponding reverse changes, indicating that the polarization direction of the coating layer is consistent with the direction of the external voltage.Therefore, the surface electrical properties of PBTO and NBTO after polarization treatment are opposite, and these simulation results are also consistent with the above KPFM experimental results.Even the external high-voltage power is removed, the retained polarization direction of PBTO and NBTO is still consistent with the direction of the applied electric field (Figure 3d 2 ,e 2 ), but the electric field strength is weakened.The polarized current collector is placed inside the battery to simulate the current density distribution near the current collector during charging and discharging (Figure 3d 3 ,e 3 ).The results show that for the NBTO, the current density around the 3d bump is larger, which is caused by the tip effect of the Li metal deposition process, and there is a large difference in current density between it and the coating layer.Although PBTO also has a tip effect, the current density distribution near the coating layer is more uniform, which will be more conducive to the uniform deposition of Li + .For the Li + concentration distribution inside the battery, the Li + concentration of the forwardly polarized current collector PBTO is larger above its coating layer, while the Li + concentration near the NBTO is smaller, thus indicating that PBTO can promote Li + diffusion effect.At the same time, theoretical analysis results of BTO without high-voltage polarization treatment (Figure S5, Supporting Information) demonstrate that the current collector without polarization treatment does not have piezoelectric effect.To sum up, the above results show that the PBTO current collector can promote uniform deposition of Li metal and achieve bottom deposition, while the reverse-processed NBTO current collector inhibits the bottom deposition of Li metal.
As highlighted in Figure 4a, there is no change in the intensity of each peak of BTO in Li plating time.Additionally, the peak intensity at ~540 cm À1 changes irregularly (Figure 4b), while the intensity of the peak (~540 cm À1 ) was found to be enhanced during the PBTO plating process in Figure 4c.Notably, when the positive high voltage applied to the collector to polarize PBTO, the peak intensity of ~540 cm À1 gradually increases.Behavior of phonon vibration softening modes indicate that with the lithium deposition process, the Ti-O and Ti-O octahedron in PBTO are strengthened and polarity of coating layer is enhanced. [34,35]That is, the electric field driving force of lithium deposition becomes larger, which is consistent with the above experimental results.Comparing the surface morphology of lithium deposition, the results show that there is a small amount of lithium metal on the surface of BTO (Figure 4d), the surface of NBTO (Figure 4e) contains a large amount of lithium metal and the morphology is dense, while the surface of PBTO (Figure 4f) has almost no lithium metal, mainly BaTiO 3 particles.SEM images of Li deposition morphologies demonstrate the electrode thickness of BTO is 70.63 lm (Figure 4g) with the 0.2 mA cm À2 and 4 mA h cm À2 .Similarly, the metal lithium deposited in NBTO (Figure 4h) is loose and porous, and the thickness is thicker, while the lithium deposition of PBTO (Figure 4i) is denser and thinner, so PBTO realizes the bottom and uniform deposition of metal lithium.Figure S6, Supporting Information shows that when the cells with three kinds of current collectors are fully charged, the lithium of the anode will be fully stripped.
The lithium plating process was in situ traced by optical microscopy (OM) in Figure 5, in which the effect of polarization direction was investigated with respect to lithium plating process.As shown in Figure 5a, there is only a small amount of metal lithium between the unpolarized BTO coating layer and the Cu foil, and there is also a little of metal lithium on the opposite side of the Cu foil.When the coating layer is negative polarized NBTO, it can be found that the lithium metal enrichment between the Cu foil and BTO coating layer is clearly suppressed (Figure 5b), and lithium metal is almost nonexistent.For the NBTO current collector, when a reverse voltage is applied, the polarization direction of the coating layer is directed from the separator to the copper foil, and the polarization direction of the lithium metal deposition process is reversed, inhibiting the bottom deposition of lithium metal (Figure 1).On the contrary, almost all Li metals are distributed on the opposite side of the Cu foil.Interestingly, when the PBTO coating layer is used in Figure 5c, which indicates that there appears to be obvious lithium metal enrichment between the Cu foil and BTO Energy Environ.Mater.2024, 7, e12599 coating layer due to lithium-ion diffusion from the electrolyte to the current collector under the action of positive polarized electric field.In summary, the PBTO appears to have functional roles to promote lithium plating, and the direction of electric field in protective layer points to the Cu foil in the process.In contrast, the NBTO inhibits Li deposition.This phenomenon arises because the long distance between the positive and negative electrodes is present, which resulting in Li + are deposited on the outside of the Cu foil through both ends of the Cu foil and away from the protective layer.These results further suggest that the polarization direction inside the protective layer can be changed with the change of direction about high voltage electric field.Importantly, the direction of the electric field inside the protective layer can be automatically reversed during the charging and discharging of the battery.This result is also consistent with the previous theoretical simulation results.It can be seen from Figure 5d-f that as the deposition current increases, lithium metal is uniformly deposited between the Cu foil and the protective layer, and it also shows that the direction of the electric field inside the material still has the effect of promoting lithium deposition during high current discharge.However, owing to the limited space between the protective layer and the current collector, very high-capacity lithium metal cannot be accommodated.Therefore, under the condition of high current density and large capacity, PBTO can achieve uniform deposition of metal lithium at the bottom, and only number of lithium metal exceeding the carrying capacity will grow upward through the BTO layer, which is caused by the low specific surface area and poor lithiophilicity of commercial planar Cu foil.As results shown above, we confirmed that the direction of the internal electric field of the protective layer was changed in external highvoltage electric field, and the migration of Li + can be dramatically accelerated under internal electric field pointing in Cu foil.
The long cycle stability of electrodes prepared with different coating layer are shown in Figure 6a.PBTO/Li anode can cycle smoothly for 800 h, while NBTO finally ends with a short circuit after ~250 h of cycle.Meanwhile, for BTO/Li, the cycling stability of the battery is better than that of NBTO/Li, but its overpotential gradually increases when cycling to 400 h, which is attributed to the accumulation of dead Li and uneven Li plating/stripping process.Figure S7, Supporting Information demonstrate the symmetric battery assembled with the three kinds of polarized lithium anodes shows that the overpotential of the PBTO/Li anode is about 17 mV at initial 125 h.This shows that PBTO coating layer can improve the cycle stability of the battery due to the positive electric field.A related same phenomenon of three types of symmetrical cells is the quick decay of voltage hysteresis in the initial cycles, which can be explained by a stable SEI formation on the surface of the electrode.Figure 6b shows the electrochemical performance of symmetrical battery at a broad range of temperatures (25-50 °C) at 1 mA cm À2 and 1 mA h cm À2 , which establishes the fact that electrolyte volatilization caused by temperature increase has no effect on the cycling performance of PBTO/Li.To evaluate the reversibility and efficiency of lithium plating/ stripping behavior on different electrodes, half-cells were assembled, and their CE were examined.At a fixed area capacity of 1 mA h cm À2 and a current density of 1 mA cm À2 (Figure 6c), the PBTO/Li electrode achieved a high average CE of 99.2% after more than 100 cycles.The average CE of the NBTO/Li electrode is only 98.2%, which decreases rapidly after 10 cycles, clearly reflecting the polarization direction of ferroelectric materials is tuned to accelerate the reversible deposition of lithium metal.The different CE is not only related to the difference in the Li plating process between the different current collectors, but also related to the SEI composition in the interface structure.As shown in Figure S8, Supporting Information, the SEI components on different electrode surfaces are similar, but their contents are slightly different.Among them, the solvent and salt decompositions of the electrolyte on the surface of the PBTO electrode are relatively few, and the reaction products between the NBTO and the electrolyte are more.At the same time, Figure S9, Supporting Information proves that Ba and Ti elements only exist on the surface of the PBTO electrode, which further proves that the SEI formed on the surface of the PBTO electrode is thinner and promotes the bottom deposition of metallic lithium. [36]This is consistent with the previous analysis results.To investigate the practical application of Li-metal composite anodes in full cells, we tested the cycling performance of the full cells assembled with different current collectors and elemental sulfur as the cathode.As shown in Figure 6d, the full cell using PBTO/Li as the anode exhibits improved specific capacity at 0.5 C and maintains at 505 mA h g À1 after 300 cycles, proving the excellent Li + transport reversibility.Even if the charging and discharging conditions become severe, that is, when the N/P ratio is only 1.35, the cycle performance of the battery is still maintained under the limited lithium source.

Conclusion
In summary, we propose a strategy for the structural design of current collectors for Li metal anode utilizing high-voltage polarization to simultaneously alleviate the uneven Li deposition process and top growth problems.Verified by theoretical simulations and multiple insitu tracking experiments, the additional positive-direction high-voltage polarization treatment can maximize the release of the piezoelectricity of coating layer of the current collector along the specified polarization direction, which enables the faster migration of Li + within the polarized coating layer than untreated coating or reversely polarized coating, resulting in uniform lithium deposition along the bottom of the current collector.Impressively, the full cell with the PBTO matched with the sulfur cathode exhibits excellent cycling stability with an N/P value of only 1.35 for 300 cycles.Furthermore, the novel mechanism of anchoring Li metal deposition for piezoelectric coatings constructed in this work provides a promising solution to the challenges of uniform Li metal deposition and dynamic realization of metallic Li bottom deposition.In addition to dynamically realizing the reversal of the side polarization direction of the lithium metal negative electrode, this work has realized the transformation of force generated by the volume effect into the directional power to anchor the lithium metal deposition, which successfully releases the volume effect caused by the lithium deposition process or the lithium stripping process in a sustainable way.We believe that if the physical properties of the coating layer are further optimized and the way of large-scale application is explored, the full-cell performance can be further improved in the future.

Figure 1 .
Figure 1.Schematic diagram of BaTiO 3 as an ideal protective layer on Cu foil.

Figure 2 .
Figure 2. a) XRD and b) Raman spectra for BaTiO 3 nanoparticles.c) Energy-dispersive X-ray elemental distribution results and d) Scanning electron microscope images of BaTiO 3 .e) HRTEM image and f) EDS mappings of BaTiO 3 nanoparticles.g-i) XPS spectra of BaTiO 3 nanoparticles.

Figure 3 .
Figure 3. a) Schematic diagram of the preparation and high voltage polarization processes of current collector with protective layer of BaTiO 3 .b) KPFM topography and surface potential image obtained of BaTiO 3 under negative polarization and c) positive polarization conditions.d) Theoretical simulation results of NBTO and e) PBTO.Correspondingly, from left to right are the electric field distribution during the reverse polarization process, the electric field distribution remaining after polarization treatment, the current density distribution after polarization, and the Li + concentration distribution.

Figure 4 .
Figure 4.In situ Raman spectroscopy of BTO a), NBTO b) and PBTO c) current collectors during Li plating process.SEM images with top-view and crosssection of Li deposition morphologies of different current collectors: d, g) BTO, e, h) NBTO, and f, i) PBTO.

Figure 5 .
Figure 5.In situ optical microscopy visualization of Li electrodeposition on Li-Cu cells about a) BTO, b) NBTO, and c) PBTO, and in-situ deposition process of PBTO at different current densities and area capacities d-f).

Figure 6 .
Figure 6.a) Cycle performances of symmetric cell with protective layers with different polarization orientations.b) Cycling performance of PBTO in symmetric cell at different temperatures.c) Coulombic efficiency of cell under different polarization conditions.d) Electrochemical performances of the Li-S full cells with lithium anodes under different polarization directions.