A Li–Li4Ti5O12 Composite Anode for Reducing Interfacial Resistance of Solid‐State Batteries

The high energy density and stability of solid‐state lithium batteries (SSBs) have garnered attention. Garnet electrolytes are widely explored in SSBs due to their huge electrochemical potential window, high effective ionic conductivity, and reasonable production cost. However, the electrochemical stability of a metallic lithium anode and a garnet electrolyte pose obstacles to the widespread use of garnet‐based SSBs. To remedy these problems, Li4Ti5O12 (LTO) is added to the metallic lithium anode. With superior wettability on the garnet electrolyte compared to pure metallic Li, Li–LTO is a more desirable electrolyte. Increased wettability between the garnet electrolyte and Li–LTO composite is responsible for the larger absolute value of the interface formation energy found in the first principal density‐functional theory calculation. Since the interface resistance between the Li–LTO composite anodes (25 Ω cm2) and the Li metal (270 Ω cm2) is much lower, Li dendrite development is effectively suppressed. An all‐lithium battery with a Li–LTO anode and a LiFePO4 cathode shows excellent capacity retention of 95% after 450 cycles. This discovery may serve as inspiration for future efforts to create a metallic Li‐containing anode for lithium batteries and other functional LTO‐based composites.

DOI: 10.1002/sstr.202200374 The high energy density and stability of solid-state lithium batteries (SSBs) have garnered attention. Garnet electrolytes are widely explored in SSBs due to their huge electrochemical potential window, high effective ionic conductivity, and reasonable production cost. However, the electrochemical stability of a metallic lithium anode and a garnet electrolyte pose obstacles to the widespread use of garnet-based SSBs. To remedy these problems, Li 4 Ti 5 O 12 (LTO) is added to the metallic lithium anode. With superior wettability on the garnet electrolyte compared to pure metallic Li, Li-LTO is a more desirable electrolyte. Increased wettability between the garnet electrolyte and Li-LTO composite is responsible for the larger absolute value of the interface formation energy found in the first principal density-functional theory calculation. Since the interface resistance between the Li-LTO composite anodes (25 Ω cm 2 ) and the Li metal (270 Ω cm 2 ) is much lower, Li dendrite development is effectively suppressed. An all-lithium battery with a Li-LTO anode and a LiFePO 4 cathode shows excellent capacity retention of 95% after 450 cycles. This discovery may serve as inspiration for future efforts to create a metallic Li-containing anode for lithium batteries and other functional LTO-based composites.
of the artificial layer and to guarantee that it will provide longterm advantages for the homogenous Liþ flow, more complicated designs are necessary for finely tailoring the lithophilicity of the artificial layer to both the lithium side and the electrode side. Interfacial engineering has hit a new roadblock since no new innovations have been created that make use of novel coating processes or interfacial materials. As a result, it is vital to find a method that can build an efficient anode, preserve ionic conductivity, and stop the formation of lithium dendrites from the SSE.
Compared to artificially generating an interlayer between the anode and electrolyte, developing a composite anode is an alternative method for lowering interface resistance. [38,39] A metallic lithium containing composite anode that may give long-term improvement on the interface contact and relieve the formation of lithium dendrite is meaningful to solve the aforementioned concerns. In recent research, Li 0.3 La 0.5 TiO 3 (LLTO) was combined with lithium metal to make a composite anode for SSBs. [40,41] The perovskite-type oxide LLTO is typically characterized as a Li þ -conducting electrolyte, which implies that it is incapable of storing Liþ and may have a detrimental impact on the energy density of the anode. In previous studies on traditional LIBs with liquid electrolyte, Li 4 Ti 5 O 12 (LTO) was considered a stable anode material due to the minor lattice parameter modification during Li þ intercalation and deintercalation. LTO is used as an anode additive to build a Li-LTO composite anode to improve the overall performance of SSBs in this study. Similar to LLTO, LTO significantly improves the anode's wettability with garnet electrolyte, resulting in a better interface contact, and reduced interface resistance. Compared to the LLTO additive used in the prior study, LTO offers additional benefits, including the following: 1) composite anode with LTO additive presents lower interfacial resistance; 2) the LTO synthesis is less complicated and takes place at a lower temperature; and 3) the price is reduced since LTO does not include costly rare-earth elements (i.e., La). In addition, the LTO additive improves the uniformity of the Li þ flux during Li plating and stripping, hence enhancing the performance and stability of SSBs, particularly under high-current conditions.

Results and Discussion
After Li metal is melted at 250°C on the hot plate, the pure molten lithium will form a spherical shape ( Figure S4a, Supporting Information), indicating a poor surface with garnet electrolyte. To generate the Li-LTO composite, the LTO is then simply dispersed in molten Li metal at a temperature of 250°C with a weight of 5%. The mixture was stirred continuously until the Li-LTO compound became homogeneous ( Figure S4b, Supporting Information). Unlike the vigorous reaction between molten lithium and metal oxide material, reported in the previous study, [38] the LTO reacts gently with molten lithium in the glove box. After solidification, the XRD characterization of the metallic-containing samples was conducted with sample holders covered by Kapton films to avoid direct exposure of the sample to ambient air. Ex situ XRD profile of the Li, LTO, and Li-LTO composites is shown in Figure 1b. The peaks of the Li-LTO composite aligned with those of the metallic Li (PDF: 00-001-1131) and LTO (PDF: 01-082-1617), indicating no new phase was generated after the preparation of the Li-LTO composite. It was noted that minor peaks around 22°-32°(in line with those of Li 3 N PDF: 03-065-1896 and LiOH PDF: 01-085-0777) could be associated with unexpected surface contamination during XRD characterization, even though the sample holders for XRD were carefully covered with Kapton films. The preparation process of the Li-LTO composite was conducted in an argon-filled glove box, so these contaminations might not exist in the Li-LTO composite after the preparation process.
To further validate the reaction between LTO and Li metal occurred, the next step is to perform an XPS measurement, which will further characterize the Li-LTO composite. As demonstrated in Figure 1c, the oxidation state of Ti in LTO is predominantly in þ4 (80%) and þ3 (20%). After the processing, the oxidation state of Ti in the Li-LTO composite changed to þ4 (37%) and þ3 (63%). During processing, LTO interacts with lithium, changing the chemical states of Ti but not significantly changing the phase structure of LTO. The partial Ti reduction (from þ4 to þ3) may contribute to higher LTO electronic conductivity, which would be advantageous for better electronic transfers in the anode. It is known that the LTO itself can be used as the anode, which will not reduce the capacity by forming the Li-LTO composite anode.
The impact of integrating LTO into Li on the wettability of LLZTO was further validated by examining the contact angles of molten Li and Li-LTO composites on LLZTO. Because it had a poor wettability, the molten lithium had a considerable contact angle (about 125°), which demonstrated its lithiophobic character ( Figure 2a). Based on the findings of the previous study, traditional solid-state methods were used to produce cubic phase LLZTO. [40] The Li-LTO composite, in contrast, exhibited a rapid wetting process with a modest contact angle of 25° (  Figure 2b), which may have been related to the decreased surface tension. As can be seen in the cross-sectional scanning electron microscope (SEM) picture, the greater wettability of the material results in improved contact between the anode and the LLZTO electrolyte. Figure 2a demonstrates that the interface between LLZTO electrolyte and pure Li is riddled with voids and holes, whereas the interface between LLZTO electrolyte and Li-LTO exhibits a close contact and does not include any spaces or voids ( Figure 2b). EDS mapping demonstrates that there has been an improvement in the surface contact between the Li-LTO composite and the LLZTO, and it further verifies that the amount of LTO that is present in the Li-LTO composite is distributed evenly (Figure 2c). All these data indicate that the addition of LTO to Li metal may be able to successfully remedy the contact issue that exists between the Li metal and the LLZTO electrolyte. The distribution of the Ti element lends credence to the portrayal of LTO's dispersion in Li metal seen in Figure 2c. During the plating and stripping process, the cavities and holes on the Li and LLZTO interface might cause a considerable increase in interface resistance as well as an uneven distribution of Li þ flux (Figure 2d). A void-free contact between the solid electrolyte and the anode is provided by the Li-LTO composite. This helps to provide equal flow and smooth transfer of lithium ions.
To examine interfacial resistances, we produced two types of symmetric cells with pure Li anodes and Li-LTO composites.
As seen in Figure 3a, symmetric cells using Li-LTO composite anodes display significantly lower interfacial resistance than cells using pure Li metal electrodes, owing to the significantly improved interfacial contact. The pure Li symmetric cell has a high interfacial resistance of 270 Ω cm 2 , but the 5 wt% Li-LTO composite has a lower interfacial resistance of 25 Ω cm 2 compared to the prior work. [40] With the symmetric cells, overpotentials at various current densities and critical current densities (CCDs) were also examined ( Figure 3b). Compared to the Li symmetric cell, the Li-LTO symmetric cell has a reduced overpotential of 0.02 V at 0.1 mA cm À2 (0.06 V). Inadequate contact causes dendrite growth, resulting in a short circuit of a Li| LLZTO| Li symmetric cell at 0.1 mA cm À2 with growing current densities every 30 min (Figure 3b). Figure 3c demonstrates that a high CCD of 0.8 mA cm À2 may be attained with the Li-LTO| LLZTO| Li-LTO symmetric cell. Reduced interface resistance because of enhanced interfacial contact between the LLZTO electrolyte and Li-LTO composite anode is accountable for the decreased overpotential and increased CCD.
Long-term cycling was studied to determine the interface's stability. The symmetric cell with Li electrodes exhibited a lack of stability, as the overpotentials continued to rise with time. The symmetric cell experienced a short circuit in less than 6 h at 0.1 mA cm À2 (1 h per Li stripping and plating cycle, Figure 3d). In contrast, the symmetric cell with Li-LTO electrode demonstrated excellent cycle stability at 0.2 mA cm À2 for 450 h (450 cycles) (Figure 3e). Similar overpotentials (0.05 V) were measured at 300 and 306 h compared to the initial 6 h (Figure 3f ). In addition, the flat profiles of overpotential in individual cycles indicate a Li stripping/plating process. After Li stripping and plating cycling, the electrochemical impedance spectroscopy (EIS) of symmetric cells with a Li-LTO electrode revealed an interface resistance of 35 Ω cm 2 (Figure 3g), showing no noticeable change from its original state (25 Ω cm 2 ). After cycling, the symmetric cell was dismantled to see the interfacial contact in greater detail. Despite long-term cycling, the intimate contact between the Li-LTO electrode and LLZTO was maintained, as shown in Figure 3h. There was no evidence of Li dendritic development or penetration. Li þ flux deficiency and inhomogeneous current distribution may be the underlying causes of interface instability in a Li| LLZTO| Li symmetric cell. The unstable interface contact between the LLZTO electrolyte and the pure Li anode is the primary cause of cell short-circuiting and polarization. LTO enables a much enhanced and more stable Li þ distribution at the interface between the composite lithium anode and the garnet electrolyte. The first-principal density-functional theory (DFT) calculations were carried out to further analyze the causes of the enhanced wettability between the LLZTO electrolyte and the Li-LTO anode. E b = (E total ÀE Li ÀE LLZTO-LTO )/A, where E total is the energy of interface structures, E Li is the total energy of the Li surface (001) layer, and E LLZTO-LTO is the energy of the LLZTO with LTO interface in a vacuum, was used to compute the interface formation energy of Li metal on LLZTO with LTO. Li| LLZTO and Li-LTO| LLZTO interfaces were assembled using geometries and constructed using optimized structures that can reduce interfacial strain. Furthermore, the lowest surface energy Li (001), LLZTO (001), and LTO (001) slabs with a 20 Å vacuum layer were used to build the Li| LLZTO (Figure 4a), Li-LTO (Figure 4b), LTO|LLZTO (Figure 4c), and Li-LTO| LLZTO (Figure 4d) interface models. Li-LTO composites can improve the wettability at the garnet electrolyte interface, which is implied by the larger absolute value of interface formation energy.
To evaluate the practicability of composite anodes with typical lithium iron phosphate (LiFePO 4 , LFP) cathode materials, Li-LTO and pure Li metal were used to create two distinct types of complete batteries in the voltage range of 2.5-3.8 V for lengthy hours of cycling. Figure 5a depicts the encouraging performance of SSBs in terms of high capacity and safety. Li-LTO was used as the anode substance. It is demonstrated that the initial capacity of a full cell is 148 mAh g À1 at 1C, with 5% capacity loss after 450 cycles. The first charge-discharge curves for Li-LTO, garnet electrolyte, and LFP, as shown in Figure 5b, exhibit a low polarization of 0.23 V (first cycle) between charge and discharge at 1C. The slight fluctuation in charge-discharge is caused by the fluctuating temperature of the laboratory between day and night. In addition, the discharge potential stays steady over a cycle test, demonstrating the improved interfacial stability of LLZTO electrolyte and Li-LTO during Li þ migration. The LFP| LLZTO| Li-LTO full cell could be cycled at a constant 1C rate due to the rapid Li-ion transfer. In contrast, after 100 cycles, the capacity of a complete cell composed of pure Li metal degraded by around 40% (Figure S5, Supporting Information). Figure 5c depicts the rate performance test that was conducted to further establish the interface's stability. The charge steps go from 0.1C to 0.2C to 0.5C to 1C to 2C and then back to 0.1C. Literally, it demonstrates a very consistent capacity at varied rates and the ability to recover its previous capacity after completing the 2C rate. Figure 5d displays typical charge and discharge curves during cycling for a Li-LTO| LLZTO| Li-LTO full battery,  demonstrating constant plating/stripping behavior with a little overpotential. All the enhanced electrochemical experiments reveal a vastly improved interface between the LLZTO electrolyte and the Li-LTO anode, as well as a higher dendrite suppression efficiency. Figure 5e depicts the results of the complete cell EIS test, which indicated no discernible increase in resistance after long cycling. Long-term charge-discharge cycling increases the overall resistance, which may be attributed to the charge transfer and diffusion processes shown in the semicircle at low frequencies. After finishing the battery cycling, no lithium dendrites were observed at the Li-LTO and LLZTO contacts in Figure S6, Supporting Information.

Conclusion
To conclude, this study presented here resulted in the development of a composite anode that was made of metallic lithium and a tiny quantity of LTO additions. The addition of LTO has the potential to greatly reduce the surface tension of the molten lithium, which ultimately results in a surface contact between the anode and the solid electrolyte that is noticeably enhanced. The results of symmetric cells cycling, full battery performance, and DFT calculations demonstrate that the Li-LTO composite possesses decreased interface resistance as well as a robust ionic transport channel between the anode and the garnet electrolyte www.advancedsciencenews.com www.small-structures.com interface, which significantly limits dendrite formation. This is demonstrated by the fact that the Li-LTO composite was able to undergo complete battery cycling. Consequently, a better interface contact between the anode and electrolyte was created, which guaranteed a low interface resistance, low overpotentials, and stable lithium stripping and plating for 450 h without the formation of lithium dendrite. After 450 cycles at a rate of 1C, the entire battery that contains Li-LTO composite displays a remarkable cycling performance, with less than 5% specific capacity depreciation throughout the course of the test. The findings indicate that the fabrication of Li metal-based composites provides an alternative method for the fabrication of   high-performance SSBs that have low interfacial resistance and high capacity, and the recently developed technology for composite anodes will work well in large-scale manufacture.

Experimental Section
Material Synthesis: A solid-state reaction was used to prepare Li 6.4 La 3 Zr 1.5 Ta 0.6 O 12 (LLZTO), a garnet SSE. Typically, a high-energy ball mill (Fritsch-Pulverisette 6) was used to mix LiOH·H 2 O (Sigma), La 2 O 3 (Sigma), ZrO 2 (Sigma), and Ta 2 O 5 (Sigma) with isopropanol (IPA) for 6 h. To compensate the lithium loss while sintering at high temperature, 10% more LiOH·H 2 O was added. The combined powder was then precalcined for 12 h at 900°C. The calcined powder was ball-milled once more with IPA for another 6 h. The LLZTO powder was dried on a hot plate, then pressed into pellets with a diameter of 15 mm, which were sintered at 1150°C for 12 h. To eliminate any surface blemishes, the surface of the calcinated pellets was polished with sandpapers and the details were cleaned with IPA in an ultrasonic bath. Before being assembled into symmetric or full batteries, the polished pellets were then preserved in a glove box. Figures 1 and 2 show, respectively, the LLZTO pellet's shape and X-ray diffraction (XRD) profile.
The LTO was synthesized by a traditional solid-state method with LiOH·H 2 O (Sigma, 99.95%), and TiO 2 (anatase, Sigma) was ball-milled thoroughly with ethanol at the speed of 400 rpm for 4 h with the molar ratio. After the ball mill, the homogeneous slurry was obtained and dried at 100°C for 8 h to obtain the precursor powder. After that, the powder was calcined at 850°C for 12 h in a tube furnace (MTI, KSL-1100) under air to produce the LTO product. The SEM in Figure S3, Supporting Information, depicts the characteristics of LTO powder.
Cell Assembly: All assembly work was done within a glove box that is filled with argon. The Li-LTO compound was prepared by continuous stirring of a molten Li and LTO mixture at a weight ratio of 95:5 in a stainless steel crucible for 15 min at 250°C. To prepare symmetric cells, an LLZTO (garnet) pellet was coated with the Li-LTO composite on both sides. In the CR2032 coin cell, the Li-LTO| LLZTO| Li-LTO symmetric cells were sealed. For the full battery, (LFP) was used as the cathode. LFP, acetylene black, and polyvinylidene fluoride were combined in an 8:1:1 weight ratio with N-methyl pyrrolidone solution in a vacuum mixer to create the cathode slurry. The slurry was casted onto aluminum foil and then dried for 12 h at 120°C in a vacuum oven to thoroughly evaporate the solvent. It is required that the cathode foils need to be punched into 8 mm discs in diameter with 1-2 mg cm 2 active material loading. A tiny quantity (5 μL) of electrolyte comprising 1.0 mol L À1 LiPF 6 in diethyl carbonate and ethylene carbonate (volume ratio of 1:1) was added to enhance the contact between the LFP cathode and the LLZTO.
Characterizations: The crystal phases of the Li-LTO composite were examined using XRD (Bruker D8A). The Li-LTO composite anode samples were generated in an argon-filled glove box to prevent air contamination, and X-ray disc sample holders needed to be covered with Kapton tape to inhibit air inlet during XRD tests. The morphologies and element dispersions of the samples were investigated using a SEM (FEI Verios) and an energy-dispersive X-ray spectroscope (EDS). The composition and chemical condition of Li, LTO, and Li-LTO composites were investigated using X-ray photoelectron spectroscopy (XPS). The information was gathered using a monochromatic Al K (1486.6 eV) radiation source at 15 kV (10 mA) and a Kratos Axis Ultra XPS with a 165 mm hemisphere electron energy analyzer. The overpotential and voltage profiles for the symmetric cells and full batteries were tested by the battery test station (LANHE, Wuhan). For the purpose of obtaining EIS of the symmetric cells, a frequency response analyzer (Solartron 1260 A) was used. A potentiostat (Biologic VSP) was used to test the EIS of full batteries. [42] DFT Calculation: The Vienna Ab Initio Package [43,44] was used to perform all DFT computations within the generalized gradient approximation using the Perdew-Burke-Ernzerhof formulation. [45] Using a plane-wave basis set with a kinetic energy cutoff of 450 eV, the projected augmented wave potentials [46,47] were used to represent the ionic cores and take valence electrons into consideration. Using the Gaussian smearing technique and a width of 0.05 eV, partial occupancies of the Kohn-Sham orbitals were permitted. When the energy shift was less than 105 eV, the electronic energy was regarded as self-consistent. When the force variation was less than 0.05 eV Å À1 , a geometry optimization was deemed to have reached convergence. The dispersion interactions have been described using Grimme's DFT-D3 approach. The surface structures of the 2 Â 2 Â 1 Monkhorst-Pack K-point sampling were used for the Brillouin zone integral.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.