vast improvements in energy density may be achieved with lithium metal anodes owing to their high gravimetric capacity (3869 mA h g −1 ) and low density (0.534 g cm −3 ). However, adoption of rechargeable lithium metal batteries has been unsuccessful thus far due to safety concerns associated with short circuits that occur when Li dendrites grow through the liquid electrolyte during the charging process. [4][5][6] Although several approaches have reduced dendrite formation, [7][8][9][10] to date the phenomenon has not been avoided under all relevant conditions. Nonflammable inorganic solid electrolytes, paired with Li metal anodes, could result in high energy density yet safe rechargeable lithium batteries. [11][12][13][14][15] As reviewed by Takada, [12] inorganic solid electrolytes have now been widely studied, [16][17][18][19][20] but are not yet commercialized. Monroe and Newman have suggested that dendrite growth during the plating process may be suppressed if the liquid electrolyte is replaced with a Li-ion conducting solid electrolyte of a sufficiently high shear modulus. [21,22] According to this criterion, numerous inorganic solid electrolytes should be able to suppress dendrite formation. However, multiple research groups have recently reported cases where ceramic solid electrolytes paired with a Li metal anode experience a short circuit Li deposition is observed and measured on a solid electrolyte in the vicinity of a metallic current collector. Four types of ion-conducting, inorganic solid electrolytes are tested: Amorphous 70/30 mol% Li 2 S-P 2 S 5 , polycrystalline β-Li 3 PS 4 , and polycrystalline and single-crystalline Li 6 La 3 ZrTaO 12 garnet. The nature of lithium plating depends on the proximity of the current collector to defects such as surface cracks and on the current density. Lithium plating penetrates/infiltrates at defects, but only above a critical current density. Eventually, infiltration results in a short circuit between the current collector and the Li-source (anode). These results do not depend on the electrolytes shear modulus and are thus not consistent with the Monroe-Newman model for "dendrites." The observations suggest that Li-plating in pre-existing flaws produces crack-tip stresses which drive crack propagation, and an electrochemomechanical model of plating-induced Li infiltration is proposed. Lithium short-circuits through solid electrolytes occurs through a fundamentally different process than through liquid electrolytes. The onset of Li infiltration depends on solid-state electrolyte surface morphology, in particular the defect size and density.

The solid lithium-ion electrolyte ''Li 7 La 3 Zr 2 O 12 '' (LLZO) with a garnet-type structure has been prepared in the cubic and tetragonal modification following conventional ceramic syntheses routes. Without aluminium doping tetragonal LLZO was obtained, which shows a two orders of magnitude lower room temperature conductivity than the cubic modification. Small concentrations of Al in the order of 1 wt% were sufficient to stabilize the cubic phase, which is known as a fast lithium-ion conductor. The structure and ion dynamics of Al-doped cubic LLZO were studied by impedance spectroscopy, dc conductivity measurements, 6 Li and 7 Li NMR, XRD, neutron powder diffraction, and TEM precession electron diffraction. From the results we conclude that aluminium is incorporated in the garnet lattice on the tetrahedral 24d Li site, thus stabilizing the cubic LLZO modification. Simulations based on diffraction data show that even at the low temperature of 4 K the Li ions are blurred over various crystallographic sites. This strong Li ion disorder in cubic Al-stabilized LLZO contributes to the high conductivity observed. The Li jump rates and the activation energy probed by NMR are in very good agreement with the transport parameters obtained from electrical conductivity measurements. The activation energy E a characterizing longrange ion transport in the Al-stabilized cubic LLZO amounts to 0.34 eV. Total electric conductivities determined by ac impedance and a four point dc technique also agree very well and range from 1 Â 10 À4 Scm À1 to 4 Â 10 À4 Scm À1 depending on the Al content of the samples. The room temperature conductivity of Al-free tetragonal LLZO is about two orders of magnitude lower (2 Â 10 À6 Scm À1 , E a = 0.49 eV activation energy). The electronic partial conductivity of cubic LLZO was measured using the Hebb-Wagner polarization technique. The electronic transference number t eÀ is of the order of 10 À7. Thus, cubic LLZO is an almost exclusive lithium ion conductor at ambient temperature.

Inspired by the ongoing debate about the ion dynamics in the lithium superionic conductor Li 10 GeP 2 S 12 (LGPS), we present neutron powder diffraction data in combination with analyses of differential bond valence and nuclear density maps to elucidate the underlying diffusion pathways in Li 10 GeP 2 S 12 . LGPS exhibits quasi-isotropic three-dimensional lithium diffusion pathways, which is a combination of one-dimensional diffusion channels crossing two diffusion planes. Furthermore, ultrasonic speeds of sound measurements are used to understand the lattice dynamics and obtain the Debye temperature of LGPS. Temperature dependent X-ray diffraction is performed in order to understand the local temperature-dependent behavior of the prevalent structural backbone, as well as the thermal stability of the material. At elevated temperatures, the superionic conducting Li 10 GeP 2 S 12 phase partially decomposes into Li 4 P 2 S 6 , explaining the deterioration of the ionic conductivity upon heating.

Solid electrolytes are considered a potentially enabling component in rechargeable batteries that use lithium metal as the negative electrode, and thereby can safely access higher energy density than available with today's lithium ion batteries. To do so, the solid electrolyte must be able to suppress morphological instabilities that lead to poor coulombic efficiency and, in the worst case, internal short circuits. In this work, lithium electrodeposition experiments were performed using single-crystal Li6La3ZrTaO12 garnet as solid electrolyte layers to investigate the factors that determine whether lithium penetration occurs through brittle inorganic solid electrolytes. In these single crystals, grain boundaries are excluded as possible paths for lithium metal propagation.However, Vickers microindentation was used to introduce sharp surface flaws of known size.Using operando optical microscopy, it was found that lithium metal penetration sometimes initiates at these controlled surface defects, and when multiple indents of varying size were present, propagates preferentially from the largest defect. However, a second class of flaws was found to be equally or more important. At the perimeter of surface current collectors, an enhanced electrodeposition current density causes lithium metal filled cracks to initiate and grow to penetration, even when the large Vickers defects are in close proximity. Modeling the electric field concentration for the experimental configurations, it was shown that a factor of 5 enhancement in field can readily occur within 10 micrometers of current collector discontinuities, which we interpret as the origin of electrochemomechanical stresses leading to failure. Such field amplification may determine the sites where supercritical surface defects dominate lithium metal propagation during electrodeposition, overriding the presence of larger defects elsewhere. Broader ContextAll-solid-state batteries can potentially store electricity at higher energy density and with greater safety than existing lithium-ion technology but require the use of lithium metal electrodes.Towards these goals, it is critical to understand possible failure modes when lithium metal electrodes are used with solid electrolytes, and especially the processes of metal dendrite formation and propagation. Here, we test the stability limits of lithium metal electrodeposition using high quality single crystals of LLZTO garnet solid electrolyte, at high current densities (5 to 10 mA/cm 2 ) equivalent to charging a battery at 1C-2C rates (1h to 0.5h charge times). We surprisingly observe that lithium metal filled cracks initiate at the edges of surface metal current collectors, rather than on millimeter-scale deliberately introduced surface cracks. At these current densities, lithium metal penetrates to short-circuit through ~2mm electrolyte thickness on the minute time scale. The results highlight a previously unrecognized failure mode for all-solidstate batteries and suggest that control of electric field distributions will be...

No abstract

The development of all-solid-state electrochemical energy storage systems, such as lithiumion batteries with solid electrolytes, requires stable, electronically insulating compounds with exceptionally high ionic conductivities. Considering oxides, garnet-type Li7La3Zr2O12 and derivatives, see Zr-exchanged Li6La3ZrTaO12 (LLZTO), have attracted great attention because of its high Li + ionic conductivity of up to 1 mS · cm −1 . Despite numerous studies focusing on conductivities of powder samples, only a few use time-domain NMR methods to probe Li ion diffusion parameters in single crystals. Here we report, for the first time, on temperature-variable 7 Li NMR relaxometry measurements using both laboratory and spin-lock techniques to probe Li jump rates in monocrystalline Li-bearing garnets. Timedomain NMR offers the possibility to study Li ion dynamics on both the short-range and long-range length scale. The techniques applied yield a fully consistent picture of correlated Li ion jump diffusion in LLZTO; the data perfectly mirror a modified BPP-type relaxation response being based on a Lorentzian-shaped relaxation function. The rates measured could be parameterized with a single set of diffusion parameters. Dynamic information about the elementary jump processes, such as jump rates and activation energies, were extracted from complete diffusion-induced rate peaks that are obtained when the relaxation rate is plotted vs inverse temperature. Results from NMR are completely in line with ion transport parameters derived from conductivity spectroscopy. Acknowledgement. We thank our colleagues at the University of Hannover and the TU Graz for valuable discussions. Financial support by the Deutsche Forschungsgemeinschaft

Fast Li-ion conducting garnets have shown excellent performance as chemically stable solid state Li electrolytes even at room temperature. However, because of phase formation and Li loss during preparation, reliably obtaining high Li-ion conductivities remains challenging. In this work, we show that an additional defect chemical species needs to be considered, namely, oxygen vacancies. We prove the existence of oxygen vacancies in all six investigated sample types: Ta-, Al-, and Ga-stabilized cubic Li 7 La 3 Zr 2 O 12 (LLZO) polycrystals and Ta-stabilized LLZO single crystals. Isotope exchange threedimensional analysis was used to characterize surface oxygen exchange (k*) and bulk oxygen diffusion (D*) enabled by the oxygen vacancies present in the LLZO variants. Remarkably high k* values of 10 −11 −10 −8 cm s −1 and D* values of 10 −15 −10 −11 cm 2 s −1 were found at 350 °C in air. In a further data analysis, the differences between the compositions are investigated, the concentration of oxygen vacancies is estimated, and the possible effects on the cation defect chemistry and phase formation of LLZO are discussed.

The ternary solid solution CeO 2 -ZrO 2 is known for its superior performance as an oxygen storage catalyst in exhaust gas catalysis (e.g. TWC), although the defect chemical background of these outstanding properties is not fully understood quantitatively. Here, a comprehensive experimental study is reported regarding defects and defect-related transport properties of cubic stabilized single crystalline (Ce x Zr 1Àx ) 0.8 Y 0.2 O 1.9Àd (0 r x r 1) solid solutions as a model system for CeO 2 -ZrO 2 . The constant fraction of yttria was chosen in order to fix a defined concentration of oxygen vacancies and to stabilize the cubic fluorite-type lattice for all Ce/Zr ratios. Measurements of the total electrical conductivity, the partial electronic conductivity, the ionic transference number and the non-stoichiometry (oxygen deficiency, oxygen storage capacity) were performed in the oxygen partial pressure range À25 o lg pO 2 /bar o 0 and for temperatures between 500 1C and 750 1C. The total conductivity at low pO 2 is dominated by electronic transport.A strong deviation from the widely accepted ideal solution based point defect model was observed.An extended point defect model was developed using defect activities rather than concentrations in order to describe the point defect reactions in CeO 2 -ZrO 2 -Y 2 O 3 properly. It served to obtain good quantitative agreement with the measured data. By a combination of values for non-stoichiometries and for electronic conductivities, the electron mobility could be calculated as a function of pO 2 , ranging between 10 À2 cm 2 V À1 s À1 and 10 À5 cm 2 V À1 s À1. Finally, the origin of the high oxygen storage capacity and superior catalytic promotion performance at a specific ratio of n(Ce)/n(Zr) E 1 was attributed to two main factors: (1) a strongly enhanced electronic conductivity in the high and medium pO 2 range qualifies the material to be a good mixed conductor, which is essential for a fast oxygen exchange and (2) the equilibrium constant for the reduction exhibits a maximum, which means that the reduction is thermodynamically most favoured just at this composition.