The potential to enable unprecedented performance, durability, and safety has created the impetus to develop bulk-scale all-solid-state batteries employing metallic Li as the negative electrode. ...Owing to its low density, low electronegativity and high specific capacity, Li metal is the most attractive negative electrode. However, failure caused by the formation of dendrites has limited the widespread use of rechargeable batteries using metallic Li negative electrodes coupled with liquid electrolytes. One approach to mitigate the formation of dendrites involves the use of a solid electrolyte to physically stabilize the Li–electrolyte interface while allowing the facile transport of Li-ions. Though in principle this approach should work, it has been observed that at high Li deposition rates Li metal can propagate through relatively hard ceramic electrolytes and Li dendrite formation causing short circuit has been reported. Why this occurs is poorly understood, emphasizing the need to close the knowledge gap and facilitate the development of advanced batteries employing solid electrolytes. Here, through precise microstructural control, striking electron microscopy, and high-resolution surface spectroscopy, we directly observed for the first time the propagation of Li metal through a promising polycrystalline solid electrolyte based on the garnet mineral structure (Li6.25Al0.25La3Zr2O12). Moreover, we observed that Li preferentially deposits along grain boundaries (intergranularly). These results offer insight into the electrochemical-mechanical phenomena that govern the stability of the metallic Li–polycrystalline solid electrolyte interface and are essential to the maturation of solid-state batteries.
Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in ...potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, Li7La3Zr2O12, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the Li7La3Zr2O12 grain boundaries were found to have a reduced bandgap, around 1–3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li+ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.Solid electrolytes are promising for enabling the use of Li metal anodes but Li infiltration along grain boundaries can lead to battery failure. Li infiltration in a model solid oxide electrolyte is now found to be strongly associated with local electronic band structure.
The stability and kinetics of the Li–Li7La3Zr2O12 (LLZO) interface were characterized as a function of temperature and current density. Polycrystalline LLZO was densified using a rapid hot-pressing ...technique achieving 97 ± 1% relative density, and <10% grain boundary resistance; effectively consisting of an ensemble of single LLZO crystals. It was determined that by heating to 175 °C, the room temperature Li-LLZO interface resistance decreases dramatically from 5822 (as-assembled) to 514 Ω cm2; a > 10-fold decrease. In characterizing the maximum sustainable current density (or critical current density – CCD) of the Li-LLZO interface, several signs of degradation were observed. In DC cycling tests, significant deviation from Ohmic behavior was observed. In post-cycling tests, regions of metallic Li were observed; propagating parallel to the ionic current. For the cells cycled at 30, 70, 100, 130 and 160 °C, the CCD was determined to be 50, 200, 800, 3500, and 20000 μA cm−2, respectively. The relationships and phenomena observed in this work can be used to better understand the Li-LLZO interface stability, enabling the use of batteries employing Li metal anodes.
•The Li-LLZO interface kinetics and stability are affected by temperature.•The Li-LLZO interface resistance decreases by >10-fold upon heating to 175 °C.•Reducing the charge-transfer resistance increased the maximum current density.•Exceeding the maximum current density resulted in metallic Li propagation.
The impact of surface chemistry on the interfacial resistance between the Li7La3Zr2O12 (LLZO) solid-state electrolyte and a metallic Li electrode is revealed. Control of surface chemistry allows the ...interfacial resistance to be reduced to 2 Ω cm2, lower than that of liquid electrolytes, without the need for interlayer coatings. A mechanistic understanding of the origins of ultra-low resistance is provided by quantitatively evaluating the linkages between interfacial chemistry, Li wettability, and electrochemical phenomena. A combination of Li contact angle measurements, X-ray photoelectron spectroscopy (XPS), first-principles calculations, and impedance spectroscopy demonstrates that the presence of common LLZO surface contaminants, Li2CO3 and LiOH, result in poor wettability by Li and high interfacial resistance. On the basis of this mechanism, a simple procedure for removing these surface layers is demonstrated, which results in a dramatic increase in Li wetting and the elimination of nearly all interfacial resistance. The low interfacial resistance is maintained over one-hundred cycles and suggests a straightforward pathway to achieving high energy and power density solid-state batteries.
Despite their different chemistries, novel energy-storage systems, e.g., Li–air, Li–S, all-solid-state Li batteries, etc., face one critical challenge of forming a conductive and stable interface ...between Li metal and a solid electrolyte. An accurate understanding of the formation mechanism and the exact structure and chemistry of the rarely existing benign interfaces, such as the Li–cubic-Li7–3x Al x La3Zr2O12 (c-LLZO) interface, is crucial for enabling the use of Li metal anodes. Due to spatial confinement and structural and chemical complications, current investigations are largely limited to theoretical calculations. Here, through an in situ formation of Li–c-LLZO interfaces inside an aberration-corrected scanning transmission electron microscope, we successfully reveal the interfacial chemical and structural progression. Upon contact with Li metal, the LLZO surface is reduced, which is accompanied by the simultaneous implantation of Li+, resulting in a tetragonal-like LLZO interphase that stabilizes at an extremely small thickness of around five unit cells. This interphase effectively prevented further interfacial reactions without compromising the ionic conductivity. Although the cubic-to-tetragonal transition is typically undesired during LLZO synthesis, the similar structural change was found to be the likely key to the observed benign interface. These insights provide a new perspective for designing Li–solid electrolyte interfaces that can enable the use of Li metal anodes in next-generation batteries.
Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte that could enable solid-state-batteries (SSB) employing metallic Li anodes. For a SSB to be viable, the stability and charge transfer ...kinetics at the Li-LLZO interface should foster facile plating and stripping of Li. Contrary to these goals, recent studies have reported high Li-LLZO interfacial resistance which was attributed to a contamination layer that forms upon exposure of LLZO to air. This study clarifies the mechanisms and consequences associated with air exposure of LLZO; additionally, strategies to minimize these effects are described. First-principles calculations reveal that LLZO readily reacts with humid air; the most favorable reaction pathway involves protonation of LLZO and formation of Li2CO3. X-ray photoelectron spectroscopy, scanning electron microscopy, Raman spectroscopy, and transmission electron microscopy were used to characterize the surface and subsurface chemistry of LLZO as a function of relative humidity and exposure time. Additionally, electrochemical impedance spectroscopy was used to measure the Li-LLZO interfacial resistance as a function of surface contamination. These data indicate that air exposure-induced contamination impacts the interfacial resistance significantly, when exposure time exceeds 24 h. The results of this study provide valuable insight into the sensitivity of LLZO to air and how the effects of air contamination can be reversed.
Replacing state-of-the-art graphite with metallic Li anodes could dramatically increase the energy density of Li-ion technology. However, efforts to achieve uniform Li plating and stripping in ...conventional liquid electrolytes have had limited success. An alternative approach is to use a solid electrolyte to stabilize the Li interface during cycling. One of the most promising solid electrolytes is Li7La3Zr2O12, which has high ionic conductivity at room temperature, high shear modulus and chemical and electrochemical stability against Li. Despite these properties, Li filament propagation has been observed through LLZO at current densities below what is practical. By combining recent achievements in reducing interface resistance and optimizing microstructure, we demonstrate Li cycling at current densities competitive with Li-ion. Li|LLZO|Li cells are capable of cycling at up to 0.9 ± 0.7 mA cm−2, 3.8 ± 0.9 mA cm−2, and 6.0 ± 0.7 mA cm-2 at room temperature, 40 and 60 °C, respectively. Extended stability is shown in Li plating/stripping tests that passed 3 mAh cm−2 charge per cycle for a cumulative capacity of 702 mAh cm−2 using a 1 mA cm−2 current density. These results demonstrate that solid-state batteries using metallic Li anodes can approach charge/discharge rates and cycling stability comparable to SOA Li-ion.
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•Critical current densities as high as 6.0 mA cm−2 were demonstrated at 60 °C.•702 mAh cm−2 total Li was plated at 60 °C with 3.0 mAh cm−2 per half cycle.•Li7La3Zr2O12 is stable against Li during extended cycling.•Ohmic behavior, EIS, and visual inspection verify short-free electrolyte.
Solid-state electrolytes (SSEs) have attracted substantial attention for next-generation Li-metal batteries, but Li-filament propagation at high current densities remains a significant challenge. ...This study probes the coupled electrochemical-morphological-mechanical evolution of Li-metal-Li7La3Zr2O12 interfaces. Quantitative analysis of synchronized electrochemistry with operando video microscopy reveals new insights into the nature of Li propagation in SSEs. Several different filament morphologies are identified, demonstrating that a singular mechanism is insufficient to describe the complexity of Li propagation pathways. The dynamic evolution of the structures is characterized, which demonstrates the relationships between current density and propagation velocity, as well as reversibility of plated Li before short-circuit occurs. Under deep discharge, void formation and dewetting are directly observed, which are directly related to evolving overpotentials during stripping. Finally, similar Li penetration behavior is observed in glassy Li3PS4, demonstrating the relevance of the new insights to SSEs more generally.
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•Multiple morphologies of Li penetration are possible in ceramic solid electrolytes•The Li within these structures can be reversibly cycled•The most common Li filaments propagate by a mechanical crack-opening mechanism•The dynamic evolution of filament morphology can be correlated to voltage signatures
A key challenge for commercialization of solid-state batteries with Li-metal anodes is the propagation of Li filaments at high current densities. This work utilizes operando video microscopy to study the dynamic evolution of Li penetration within ceramic solid electrolytes. Four morphology types are identified and studied under a range of battery-relevant conditions. Morphology evolution is linked to electrochemical signatures by synchronizing video microscopy with the voltage response of the cells, providing an avenue for on-board diagnostics. A key observation is the reversible plating and stripping of Li filaments before short-circuiting occurs, indicating that if Li propagation is identified through voltage analysis, catastrophic failure modes could be avoided. These findings represent an important step toward understanding and overcoming the challenge of Li penetration and enabling high-rate-capability Li-metal solid-state batteries for applications such as electric vehicles.
The dynamic evolution of Li penetration within ceramic solid electrolytes with Li-metal anodes is investigated with operando video microscopy. Four unique morphology types are identified and studied under a range of battery-relevant conditions in both in-plane and through-plane cell geometries. This work informs future studies to identify the mechanistic origins of Li penetration in ceramic solid electrolytes and how to suppress it to enable high-rate-capability Li-metal solid-state batteries for applications such as electric vehicles.
Herein, we report on the characterization of a Li–S hybrid cell containing a garnet solid electrolyte (Li7La3Zr2O12, LLZO) and conventional liquid electrolyte. While the liquid electrolyte provided ...ionically conductive pathways throughout the porous cathode, the LLZO acted as a physical barrier to protect the Li metal anode and prevent polysulfide shuttling during battery operation. This hybrid cell exhibited an initial capacity of 1000 mAh/g(S) and high Coulombic efficiency (>99%). The interface between the liquid electrolyte and LLZO was studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). These results indicate that a spontaneous interfacial reaction layer formed between the LLZO and liquid electrolyte. XPS depth profiling experiments indicate that this layer consisted of Li-enriched phases near the surface (e.g., Li2CO3) and intermediate Li–La–Zr oxides in subsurface regions. The reaction layer extended well beyond the LLZO surface, and bulk pristine LLZO was not observed even at the deepest sputtering depths used in this study (∼90 nm). Overall, these results highlight that developing stable electrode/electrolyte interfaces is critical for solid-state batteries and their hybrids.
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