Blocking lithium dendrite growth in solid-state batteries with an ultrathin amorphous Li-La-Zr-O solid electrolyte

Lithium dendrites have become a roadblock in the realization of solid-state batteries with lithium metal as high-capacity anode. The presence of surface and bulk defects in crystalline electrolytes such as the garnet Li7La3Zr2O12 (LLZO) facilitates the growth of these hazardous lithium filaments. Here we explore the amorphous phase of LLZO as a lithium dendrite shield owing to its grain-boundary-free microstructure, stability against lithium metal, and high electronic insulation. By tuning the lithium stoichiometry, the ionic conductivity can be increased by 4 orders of magnitude while retaining a negligible electronic conductivity. In symmetric cells, plating-stripping shows no signs of lithium penetration up to 3.2 mA cm−2. The dense conformal nature of the films enables microbatteries with an electrolyte thickness of only 70 nm, which can be cycled at 10C for over 500 cycles. The application of this material as a coating on crystalline LLZO lowers the interface resistance and increases the critical current density. The effectiveness of the amorphous Li-La-Zr-O as dendrite blocking layer can accelerate the development of better solid-state batteries. The formation and growth of dendrites in solid-state lithium metal batteries is a common cause of failure. Here, thin-film amorphous Li-La-Zr-O shows high resistance to lithium penetration, making it promising for thin-film solid-state batteries and as a coating for bulk ceramic electrolyte.

Lithium garnet Li 7 La 3 Zr 2 O 12 (LLZO) electrolyte is a potential candidate for the development of solid-state batteries with lithium metal as high-capacity anode. But ceramic LLZO in the form of pellets or polycrystalline films can still suffer from lithium dendrite penetration because of surface and bulk inhomogeneities and grain boundaries with non-negligible electronic conductivity. In contrast, the amorphous phase of LLZO (aLLZO) possesses a grain-boundary-free microstructure with moderate ionic conductivity (10 -7 S cm -1 ) and high electronic insulation (10 -14 S cm -1 ), which in the form of thin coatings can offer resistance to lithium dendrite growth. We explore the electrochemical properties and applications of aLLZO ultrathin films prepared by sputtering deposition. The defect-free and conformal nature of the films enables microbatteries with an electrolyte thickness as low as 70 nm, which withstand charge-discharge at 0.2 mA cm -2 for over 500 cycles.
In Li/aLLZO/Li symmetric cells, plating-stripping at current densities up to 3.2 mA cm -2 shows no signs of lithium penetration. Finally, we show that the application of aLLZO as a coating on LLZO ceramic pellets significantly impedes the formation of Li dendrites.
File list (2) download file view on ChemRxiv manuscript_aLLZO.pdf (4.81 MiB) download file view on ChemRxiv supplementary_information_aLLZO.pdf (4.35 MiB) (aLLZO) possesses a grain-boundary-free microstructure with moderate ionic conductivity (10 −7 S cm −1 ) and high electronic insulation (10 −14 S cm −1 ), which in the form of thin coatings can offer resistance to lithium dendrite growth. We explore the electrochemical properties and applications of aLLZO ultrathin films prepared by sputtering deposition. The defect-free and conformal nature of the films enables microbatteries with an electrolyte thickness as low as 70 nm, which withstand charge-discharge at 0.2 mA cm −2 for over 500 cycles. In Li/aLLZO/Li symmetric cells, plating-stripping at current densities up to 3.2 mA cm −2 shows no signs of lithium penetration. Finally, we show that the application of aLLZO as a coating on LLZO ceramic pellets significantly impedes the formation of Li dendrites.
Increasing power requirements in portable devices and the electrification of the automotive industry are pushing the demand for batteries with higher energy capacities and power rates. Solid-state batteries with Li metal as anode are foreseen as the next generation of energy storage devices, given the 10-fold higher capacity of Li metal with respect to traditional graphite anodes. 1,2 Solid Li-ion conductive electrolytes, that potentially can enable such batteries, have been the subject of considerable interest in recent years. Among different families of solid electrolytes, the lithium garnet Li 7 La 3 Zr 2 O 12 (LLZO) can be regarded as one of the main contestants, owing to its high ionic conductivity up to 10 −3 S cm −1 and a wide electrochemical stability window, against metallic lithium and high potential cathodes. 3,4 This ceramic electrolyte suffers however from a major drawback: Its polycrystalline nature makes it prone to the growth of lithium metal dendrites which can short circuit the battery. [5][6][7] Although some debate is still ongoing in the research community, the main consensus points towards surface inhomogeneties, bulk defects and non-negligible electronic conductivities along the grain boundaries as the reason for lithium metal nucleation and dendrite growth through the solid electrolyte. [8][9][10] Very recently, Kim et al. showed that a laser annealing treatment on a bulk LLZO pellet leads to the formation of an amorphized surface and this increases significantly the critical current density and lifetime of the electrolyte. 11 They claim that this in-situ formed amorphous surface blocks electron injection and hinders the formation of Li dendrites. However, the laser annealing procedure employed in that work does not allow to control and optimize the properties of the amorphous surface layer. In contrast, ex-situ coating processes such as physical vapor deposition (PVD) guarantee a well-controlled composition and better homogeneity. Kalita et al. first showed the feasibility of preparing amorphous Li-La-Zr-O thin films by RF magnetron sputtering and characterized the ionic conductivity of this material as a function of the sputtering power, reporting values in the order of 10 −7 S cm −1 . 12 Garbayo et al. further investigated the amorphism in this type of electrolyte thin films prepared by pulsed-laser deposition (PLD) and revealed a relation between the ionic conductivity, the Li concentration and the formation of glassy states with regards to the processing temperature. 13 These both studies highlighted the potential of amorphous phase of LLZO, but did not go beyond characterizing stand-alone films nor demonstrated any application either in a thin-film battery configuration or as coating in a bulk solid-state battery. This leaves open the question whether the amorphous phase of LLZO could prevent the formation of Li dendrites, as it has been shown for other glassy electrolyte materials like Li 3 PO 4 and its oxynitride analogous (LiPON). 14,15 In this work we present a thorough investigation of the chemical structure and electrochemical properties of amorphous Ga-doped Li-La-Zr-O (aLLZO) films prepared by magnetron sputtering. By tuning the excess lithium in the films, the ionic conductivity can be increased over three orders of magnitude, up to 10 −7 S cm −1 , while retaining a negligible electronic conductivity of 10 −14 S cm −1 . In this way it is possible to prepare ultrathin conformal and grain-boundary free films that can act as injection barrier for electrons while allowing Li ions to be transported across. The stability of the ultrathin films against metallic lithium is investigated by plating-stripping lithium in half and symmetric cells, showing no interfacial degradation and resistance to short circuits at currents up to 3.2 mA cm −2 . Finally, the applicability of this material is demonstrated in the form of ultrathin solid electrolyte for thin-film solid-state batteries, showing a functional battery with an unprecedented electrolyte thickness of 70 nm, as well as surface coating to block formation of Li dendrites in bulk ceramic LLZO. aLLZO thin films were deposited at room temperature by magnetron sputtering from a stoichiometric Li 6.25 Ga 0.25 La 3 Zr 2 O 12 target. To control the lithiation level of the amorphous film, a Li 2 O target was simultaneously co-sputtered (as shown in Figure 1.a), allowing to tune the mass fraction of LLZO and Li 2 O by controlling the sputtering power on each target. We previously used this method to compensate lithium losses in the preparation of crystallized LLZO thin films,. 16,17 Besides compensating lithium losses, it is an effective way to tune the Li-ion concentration in the electrolyte film and modify its electrochemical properties.
To characterize the morphology, composition, ionic and electronic properties, films were prepared on Pt-coated Si and on insulating MgO single crystal substrates. As seen in the cross-section secondary electron microscopy (SEM) image in Figure 1.b, the as-deposited films, with a thickness of about 70 nm, are homogeneous and conformally cover the substrate.
The 3D tomogram in Figure 1.c shows the elemental structure of the film reconstructed from a gas-assisted focused ion beam time-of-flight secondary ion mass spectroscopy (FIB-ToF-SIMS) measurement. 18 The measurement reveals a uniform distribution of the matrix elements of the aLLZO film over the Pt-coated Si substrate. Detailed plots of each element's signal and a depth profile can be found in Figures S1 and S2.
In Figure 1.d the XRD diffractogram of the aLLZO film exhibits no sharp reflections but a broad hump, in contrast to the well-defined reflections in the LLZO film crystallized at 700 • C (cLLZO) that match well the reference pattern of cubic Ga-doped LLZO (ICSD 430603). 19 This confirms the amorphous nature of the as-deposited films, with no long-range order in the crystal structure, and is consistent with the studies of Kalita et al. 12 and Garbayo et al. 13 To further investigate the short-range order and chemical structure of the films, we   The ionic conductivity of the films was measured by through-plane impedance spectroscopy using silicon as substrate with Pt as back electrode and patterned Au top electrodes, as illustrated in the inset of Figure 2.a. A Nyquist plot of the impedance response of a selected sample is shown in Figure 2.a. The characteristic semicircle arising from the electrolyte response can be modeled with a Randles equivalent circuit, 22 yielding the electrolyte resistance from which the ionic conductivity can be derived. The tail at lower frequencies is due to the blocking nature of the electrodes and the resulting accumulation of charges at the interfaces. The ionic conductivity σ depends on both the density of mobile ions and their mobility and can be described with an Arrhenius relationship: where E a is the activation energy for ion migration, k B the Boltzmann's constant, T the temperature, and µ 0 a pre-exponential factor. 23 To determine whether the change in ion conductivity is due to a change in the density or mobility of mobile ions, we performed temperature dependent impedance spectroscopy and current transient measurements (see SI for details). We find that increasing the amount of Li 2 O up to 33% of the total mass the increases density of mobile ions and decreases the activation energy, which increases mobility ( Figure 2.c-e). Consequently, we assign the increased ionic conductivity to an increased amount of Li + ions, which reduces the electrostatic interaction between the disordered Zr-O and La-O chains and the mobile Li + ions, resulting in a decrease in activation energy. This is consistent with findings on other amorphous solid electrolytes such as lithium thiophosphate (LPS) and lithium phosphorus oxynitride (LiPON). 24 Above a critical mass fraction we assume that the added Li + ion forms aggregates which reduce the ion conductivity, as suggested by the obtained mobile Li + ion density, which does not increase further after the critical mass fraction of 0.66 LLZO:Li 2 O is reached.   which is equivalent to about 110 mA h g −1 assuming an ideal density of the cathode film. At 10C, the microbattery shows a capacity of 39 µA h cm −2 µm −1 , a 70% of the capacity at 1C.
To demonstrate the robustness of the ultrahin aLLZO electrolyte, the thin-film battery was cycled with a current density of 0.2 mA cm −2 (10C) for 500 cycles, as shown in Figure 4.c.
After 500 cycles (about 100 hours operation), the capacity degrades to about 60% of the initial capacity while retaining a stable Couloumbic efficiency of about 97.6%. The capacity fading is likely due to interfacial degradation of the electrolyte on the cathode side, but the  The interface resistance is almost identical to the non-coated side although the bulk resistance of the aLLZO-coated side is higher ( Figure S8).
Lithium plating-stripping was performed by applying currents in forward and reverse direction between both Li metal contacts. The current and voltage curves as a function of time is presented in Figure 5.c.The current density was increased step-wise from 50 µA cm −2     incorporated within a focused ion beam/scanning electron microscope (FIB/SEM) dualbeam instrument from Tescan (Brno, Czech Republic) was used. This technique has been recently reported to provide a significant enhancement of generating secondary ions (up to two orders of magnitude, depending on a material) [S5,S7], resulting in higher signalto-noise ratio and, therefore increased spatial resolution. Furthermore, a simultaneous coinjection of fluorine gas during a TOF-SIMS measurement shows potential for separating mass interference [S6] (which under standard vacuum conditions constitutes one of the main drawbacks of this technique). Finally, the initial studies seem to indicate that fluorine can have capability of altering the polarity of generated secondary ions during FIB sputtering. This feature, allowing the complete sample's chemical structure to be provided from a single volume (i.e. without any supplementary gas, two separate measurements have to be conducted to show the distribution of positively and negatively ionizing elements; since TOF-SIMS is a destructive technique, the data has to be acquired from different volumes then), was particularly important in the case of the sample investigated in this work as it enabled to detect elements like Li (which ionizes positively) and Pt and Au (which dominantly ionizes negatively) in the same measurement. The sample surface was bombarded with a continuous monoisotopic 69 Ga + primary ion beam, which was used as both, a sputtering S-5 and analysis beam. The 20 kV beam energy was applied to ensure high lateral resolution whilst maintaining sufficient depth resolution. The 4D data set, i.e. a 3D (x, y and z ) array with an associated mass spectrum for each data point, was recorded at approx. 110±1 pA ion current and 32 µs dwell time from a 10 µm×10 µm area with 512×512 pixels and 2x2 binning. Figure S1.a shows a secondary electron image of the sputtered crater, with sharp edges and a smooth bottom that indicate no FIB-induced roughness. A XeF 2 precursor was used as a source of fluorine. TOF-SIMS Explorer 1.12.2.0 from TOFWERK (Thun, Switzerland) was used for data collection and analysis. Mass spectra were mass calibrated using the secondary ion signals of the main sample elements ( 7 Li + , 24 Mg + , 90 Zr + , 195 Pt + and 197 Au + ), the substrate ( 28 Si + ) and the primary ion beam ( 69 Ga + ). 3D elemental tomography plots were created using the the Mayavi's mlab module for Python. Standard ToF-SIMS depth profiling was performed with a ToF.SIMS 5 system from IONTOF. For sputtering, a Cs + -ion gun was employed with an acceleration voltage of 2 kV on an area of 300 × 300 µm 2 . The primary ion source used for analyzing was Bi + ions with S-6 an acceleration voltage of 25 kV. The negative-charged ions extracted from a 100 × 100 µm 2 area within the sputtering crater were used for the analysis. A floodgun was used to avoid surface charging. Figure S2 shows the ToF-SIMS depth profile of extracted negative ions of the oxide compounds composing the aLLZO film (ZrO -, LaO -, and LiO -) and the metal electrodes (Auand Pt -). In this case, since the measurement was performed in the negative mode without a GIS system, the oxides of Li, La and Zr had to be analyzed instead of the individual isotopes because these elements tend to ionize positively.

X-ray diffractometry (XRD)
XRD diffractograms were acquired in a Bruker D8 Discover XRD system in a grazingincidence mode with Cu K α1 radiation at an incident angle ω = 2°and measuring in the range 2θ = 10°-60°. S-7

Raman spectroscopy
Raman spectroscopic characterization was performed on a WITec Alpha 300R microscope (300 mm focal length, 600 g/mm grating) equipped with a thermoelectrically cooled EMCCD-Detector. Acquisition parameters for excitation at λ = 532 nm were optimized for both amorphous and crystallized samples individually to avoid laser-induced changes (40 mW, 25 s integration time and 20 mW, 1 s integration time, respectively. Objective NA = 0.55).
Spectra displayed in Figure 1.e are averages of 25(400) spectra acquired over an area of 100 µm (200 µm) for the amorphous (crystallized) sample. Signatures of cosmic rays and a linear background were removed and scaling is indicated in the panel.

Fourier-transform infrared (FT-IR) spectroscopy
FTIR measurements were conducted with a BRUKER single reflection attenuated total reflection (ATR) unit (diamond ATR crystal) incorporated in a BRUKER Tensor 27 spectrometer in the wavenumber range of 340-4000 cm −1 .

Impedance spectroscopy and current transient measurements
Electrical characterization of the aLLZO films was performed through plane with Pt as back electrode and Au as top electrode using a Paios measurement system (Fluxim AG). Electrochemical impedance spectrocopy (EIS) spectra were recorded between 100 mHz and 10 MHz with an a.c amplitude below 70 mV. Measurements were performed in an Ar filled glovebox at temperatures ranging from room temperature to 200 • C. The temperature was regulated using a Linkam LTSE-420-P heating stage integrated with the measurement system. The temperature on the sample's surface was logged using a PT100 temperature sensor.
The ionic conductivity was extracted from the EIS measurements by fitting the imaginary part of the modulus. Figure S3 Figure S4.a-b for two different LLZO:Li 2 O mass fractions. After subtracting the background current, the extracted charge is calculated by integration over the measured current transient (Figure S4.c). The ion density n is then calculated from the extracted charge ∆C with ∆C = qnd, where q is the elementary charge and d is the thickness of the aLLZO layer. Figure S4.d shows the obtained mobile ion S-9 densities at different voltage biases. At biases higher than 1.20 V the curve begins to flatten, indicating that most of the mobile ions present in the aLLZO layer are being measured.
Here, we use a bias of 2 V for the quantification of the density of mobile ions. However, since larger biases degrade the device, larger densities cannot be excluded. To quantify the electronic conductivity, we measure the steady-state current 1 hour after applying both a positive and a negative bias of 1.2 V. This is shown in Figure S5.a-b as an example, with the obtained electronic conductivities summarized in Figure S5.c. In order to cancel out the effect of the internal electric field due to the different metals used as contacts and possible interfacial effects, we take the mean over both values. The mean values are then used to quantify the activation energies and electronic conductivities, as shown in Figure S4

Galvanostatic and potentiostatic cycling of Pt/Li half-cells and symmetric Li/Li cells
To determine the electrochemical stability of the aLLZO electrolyte, the films were measured in galvanostatic and potentiostatic mode in a Pt/Li half-cell. To determine the stability at high potentials, a cyclic voltammetry (CV) measurement was performed between 1 V and 5 V vs. Li/Li + at a rate of 2 mV s −1 (see Figure S6).

Galvanostatic cycling of thin-film batteries
Charge-discharge curves of the full thin-film battery with aLLZO as electrolyte were recorded in galvanostatic mode within a potential range of 3 V to 4.25 V vs. Li/Li + . The cells were tested with charge-discharge current densities ranging from 22 µA cm −2 (1C) to 0.22 mA cm −2 (10C). Long-term cycling was performed at 0.22 mA cm −2 for 500 cycles (about 100 h). The oscillations in the discharge capacities are due to temperature variations inside the glovebox between day and night.

In-plane Li plating-stripping with in-operando microscopy
In-plane Li plating-stripping was performed on the aLLZO-coated and uncoated sides of the ceramic LLZO pellet using the Li metal contacts with a spacing of 0.50 mm. Prior to the plating-stripping process, EIS was measured to determine the interface resistance. Plattingstripping of Li was performed at current densities ranging from 50 µA cm −2 to 3.20 mA cm −2 for a total charge transfer of 25 µA h cm −2 , with 15 min rest steps in between. During the Li platting-stripping process, a Dino-Lite AM73115MZT digital microscope was used to monitor the surface of the LLZO pellet.
S-13 Figure S8: EIS measured between two Li contacts with a lateral separation of 0.50 mm on the uncoated and aLLZO-coated sides of the LLZO ceramic pellet.