Lowering the Temperature of Solid Oxide Electrochemical Cells Using Triple-Doped Bismuth Oxides.

Despite the great potential of solid oxide electrochemical cells (SOCs) as highly efficient energy conversion devices, the undesirable high operating temperature limits their wider applicability. Herein, a novel approach to developing high-performance low-temperature SOCs (LT-SOCs) is presented through the use of an Er, Y, and Zr triple-doped bismuth oxide (EYZB). This study demonstrates that EYZB exhibits > 147 times higher ionic conductivity of 0.44 S cm-1 at 600 °C compared to commercial Y-stabilized zirconia electrolyte with excellent stability over 1000 h. By rationally incorporating EYZB in composite electrodes and bilayer electrolytes, the zirconia-based electrolyte LT-SOC achieves the unprecedentedly high performance of 3.45 and 2.02 W cm-2 in the fuel cell mode and 2.08 and 0.95 A cm-2 in the electrolysis cell mode at 700 °C and 600 °C, respectively. Further, a distinctive microstructural feature of EYZB that largely extends triple phase boundary at the interface is revealed through digital twinning. This work provides insights for developing high-performance LT-SOCs.

Toward solid-state Limetal-air batteries; an SOFC perspective of solid 3D architectures, heterogeneous interfaces, and oxygen exchange kinetics.

The full electrification of transportation will require batteries with both 3-5× higher energy densities and a lower cost than what is available in the market today. Energy densities of >1000 W h kg-1 will enable electrification of air transport and are among the very few technologies capable of achieving this energy density. Limetal-O2 or Limetal-air are theoretically able to achieve this energy density and are also capable of reducing the cost of batteries by replacing expensive supply chain constrained cathode materials with "free" air. However, the utilization of liquid electrolytes in the Limetal-O2/Limetal-air battery has presented many obstacles to the optimum performance of this battery including oxidation of the liquid electrolyte and the Limetal anode. In this paper a path towards the development of a Limetal-air battery using a cubic garnet Li7La3Zr2O12 (LLZ) solid-state ceramic electrolyte in a 3D architecture is described including initial cycling results of a Limetal-O2 battery using a recently developed mixed ionic and electronic (MIEC) LLZ in that 3D architecture. This 3D architecture with porous MIEC structures for the O2/air cathode is essentially the same as a solid oxide fuel cell (SOFC) indicating the importance of leveraging SOFC technology in the development of solid-state Limetal-O2/air batteries.

Extreme lithium-metal cycling enabled by a mixed ion- and electron-conducting garnet three-dimensional architecture.

The development of solid-state Li-metal batteries has been limited by the Li-metal plating and stripping rates and the tendency for dendrite shorts to form at commercially relevant current densities. To address this, we developed a single-phase mixed ion- and electron-conducting (MIEC) garnet with comparable Li-ion and electronic conductivities. We demonstrate that in a trilayer architecture with a porous MIEC framework supporting a thin, dense, garnet electrolyte, the critical current density can be increased to a previously unheard of 100 mA cm-2, with no dendrite-shorting. Additionally, we demonstrate that symmetric Li cells can be continuously cycled at a current density of 60 mA cm-2 with a maximum per-cycle Li plating and stripping capacity of 30 mAh cm-2, which is 6× the capacity of state-of-the-art cathodes. Moreover, a cumulative Li plating capacity of 18.5 Ah cm-2 was achieved with the MIEC/electrolyte/MIEC architecture, which if paired with a state-of-the-art cathode areal capacity of 5 mAh cm-2 would yield a projected 3,700 cycles, significantly surpassing requirements for commercial electric vehicle battery lifetimes.

3D Asymmetric Bilayer Garnet-Hybridized High-Energy-Density Lithium-Sulfur Batteries.

Lithium garnet Li7La3Zr2O12 (LLZO), with high ionic conductivity and chemical stability against a Li metal anode, is considered one of the most promising solid electrolytes for lithium-sulfur batteries. However, an infinite charge time resulting in low capacity has been observed in Li-S cells using Ta-doped LLZO (Ta-LLZO) as a solid electrolyte. It was observed that this cell failure is correlated with lanthanum segregation to the surface of Ta-LLZO that reacts with a sulfur cathode. We demonstrated this correlation by using lanthanum excess and lanthanum deficient Ta-LLZO as the solid electrolyte in Li-S cells. To resolve this challenge, we physically separated the sulfur cathode and LLZO using a poly(ethylene oxide) (PEO)-based buffer interlayer. With a thin bilayer of LLZO and the stabilized sulfur cathode/LLZO interface, the hybridized Li-S batteries achieved a high initial discharge capacity of 1307 mA h/g corresponding to an energy density of 639 W h/L and 134 W h/kg under a high current density of 0.2 mA/cm2 at room temperature without any indication of a polysulfide shuttle. By simply reducing the LLZO dense layer thickness to 10 µm as we have demonstrated before, a significantly higher energy density of 1308 W h/L and 257 W h/kg is achievable. X-ray diffraction and X-ray photoelectron spectroscopy indicate that the PEO-based interlayer, which physically separates the sulfur cathode and LLZO, is both chemically and electrochemically stable with LLZO. In addition, the PEO-based interlayer can adapt to the stress/strain associated with sulfur volume expansion during lithiation.

Amorphous-Carbon-Coated 3D Solid Electrolyte for an Electro-Chemomechanically Stable Lithium Metal Anode in Solid-State Batteries.

The use of solid-state electrolyte may be necessary to enable safe, high-energy-density Li metal anodes for next-generation energy storage systems. However, the inhomogeneous local current densities during long-term cycling result in instability and detachment of the Li anode from the electrolyte, which greatly hinders practical application. In this study, we report a new approach to maintain a stable Li metal | electrolyte interface by depositing an amorphous carbon nanocoating on garnet-type solid-state electrolyte. The carbon nanocoating provides both electron and ion conducting capability, which helps to homogenize the lithium metal stripping and plating processes. After coating, we find the Li metal/garnet interface displays stable cycling at 3 mA/cm2 for more than 500 h, demonstrating the interface's outstanding electro-chemomechanical stability. This work suggests amorphous carbon coatings may be a promising strategy for achieving stable Li metal | electrolyte interfaces and reliable Li metal batteries.

Garnet-Type Solid-State Electrolytes: Materials, Interfaces, and Batteries.

Solid-state batteries with desirable advantages, including high-energy density, wide temperature tolerance, and fewer safety-concerns, have been considered as a promising energy storage technology to replace organic liquid electrolyte-dominated Li-ion batteries. Solid-state electrolytes (SSEs) as the most critical component in solid-state batteries largely lead the future battery development. Among different types of solid-state electrolytes, garnet-type Li7La3Zr2O12 (LLZO) solid-state electrolytes have particularly high ionic conductivity (10-3 to 10-4 S/cm) and good chemical stability against Li metal, offering a great opportunity for solid-state Li-metal batteries. Since the discovery of garnet-type LLZO in 2007, there has been an increasing interest in the development of garnet-type solid-state electrolytes and all solid-state batteries. Garnet-type electrolyte has been considered one of the most promising and important solid-state electrolytes for batteries with potential benefits in energy density, electrochemical stability, high temperature stability, and safety. In this Review, we will survey recent development of garnet-type LLZO electrolytes with discussions of experimental studies and theoretical results in parallel, LLZO electrolyte synthesis strategies and modifications, stability of garnet solid electrolytes/electrodes, emerging nanostructure designs, degradation mechanisms and mitigations, and battery architectures and integrations. We will also provide a target-oriented research overview of garnet-type LLZO electrolyte and its application in various types of solid-state battery concepts (e.g., Li-ion, Li-S, and Li-air), and we will show opportunities and perspectives as guides for future development of solid electrolytes and solid-state batteries.

A Redox-Robust Ceramic Anode-Supported Low-Temperature Solid Oxide Fuel Cell.

A critical factor hampering the deployment of fuel-flexible, low-temperature solid oxide fuel cells (LT-SOFCs) is the long-term stability of the electrode in different gas environments. Specifically, for state-of-the-art Ni-cermet anodes, reduction/oxidation (redox) cycles during fuel-rich and fuel-starved conditions cause a huge volume change, eventually leading to cell failure. Here, we report a robust redox-stable SrFe0.2Co0.4Mo0.4O3 (SFCM)/Ce0.9Gd0.1O2 ceramic anode-supported LT-SOFC with high performance and remarkable redox stability. The anode-supported configuration tackles the high ohmic loss associated with conventional ceramic anodes, achieving a high open circuit voltage of ∼0.9 V and a peak power density of 500 mW/cm2 at 600 °C in hydrogen. In addition, ceramic anode-supported SOFCs are stable over tens of redox cycles under harsh operating conditions. Our study reveals that oxygen nonstoichiometry of SFCM compensates for the dimensional changes that occur during redox cycles. Our results demonstrate the potential of all ceramic cells for the next generation of LT-SOFCs.

Highly Performing Chromate-Based Ceramic Anodes (Y0.7Ca0.3Cr1- xCu xO3-δ) for Low-Temperature Solid Oxide Fuel Cells.

Exploitation of alternative anode materials for low-temperature solid oxide fuel cells (LT-SOFCs, 350-650 °C) is technologically important but remains a major challenge. Here we report a potential ceramic anode Y0.7Ca0.3Cr1- xCu xO3-δ ( x = 0, 0.05, 0.12, and 0.20) (YCC) exhibiting relatively high conductivity at low temperatures (≤650 °C) in both fuel and oxidant gas conditions. Additionally, the newly developed composition (YCC12) is structurally stable in reducing and oxidizing gas conditions, indicating its suitability for SOFC anodes. The I- V characteristics and performance of the ceramic anode infiltrated with Ni-(Ce0.9Gd0.1O2-δ)(GDC) were determined using GDC/(La0.6Sr0.4CoO3-δ)(LSC)-based cathode supported SOFCs. High peak power densities of ∼1.2 W/cm2 (2.2A/cm2), 1 W/cm2 (2.0A/cm2), and 0.6 W/cm2 (1.3 A/cm2) were obtained at 600, 550, and 500 °C, respectively, in H2/3% H2O as fuel and air as oxidant. SOFCs showed excellent stability with a low degradation rate of 0.015 V kh-1 under 0.2 A/cm2. YCC-based ceramic anodes are therefore critical for the advancement of LT-SOFC technology.

Nanointegrated, High-Performing Cobalt-Free Bismuth-Based Composite Cathode for Low-Temperature Solid Oxide Fuel Cells.

Cost-effective cathodes that actively catalyze the oxygen reduction reaction (ORR) are one of the major challenges for the technological advancement of low-temperature solid oxide fuel cells (LT-SOFCs). In particular, cobalt has been an essential element in electrocatalysts for efficiently catalyzing the ORR; nevertheless, the cost, safety, and stability issues of cobalt in cathode materials remain a severe drawback for SOFC development. Here, we demonstrated that by appropriate nanoengineering, we can overcome the inherent electrocatalytic advantages of cobalt-based cathodes to achieve comparable performance with a cobalt-free electrocatalyst on a bismuth-based fast oxygen ion-conducting scaffold that simultaneously enhances the performance and stability of LT-SOFCs. Consequently, the peak power density of the SOFCs reaches 1.2 W/cm2 at 600 °C, highest performance of a cobalt-free-based cathode that has been ever reported. In addition, by the surface-protecting effect of covered nanoelectrocatalysts, the evaporation of highly volatile bismuth is greatly suppressed, resulting in an 80% improvement in performance durability, the best among all reported bismuth-based fuel cells.

Three-Dimensional, Solid-State Mixed Electron-Ion Conductive Framework for Lithium Metal Anode.

Solid-state electrolytes (SSEs) have been widely considered as enabling materials for the practical application of lithium metal anodes. However, many problems inhibit the widespread application of solid state batteries, including the growth of lithium dendrites, high interfacial resistance, and the inability to operate at high current density. In this study, we report a three-dimensional (3D) mixed electron/ion conducting framework (3D-MCF) based on a porous-dense-porous trilayer garnet electrolyte structure created via tape casting to facilitate the use of a 3D solid state lithium metal anode. The 3D-MCF was achieved by a conformal coating of carbon nanotubes (CNTs) on the porous garnet structure, creating a composite mixed electron/ion conductor that acts as a 3D host for the lithium metal. The lithium metal was introduced into the 3D-MCF via slow electrochemical deposition, forming a 3D lithium metal anode. The slow lithiation leads to improved contact between the lithium metal anode and garnet electrolyte, resulting in a low resistance of 25 Ω cm2. Additionally, due to the continuous CNT coating and its seamless contact with the garnet we observed highly uniform lithium deposition behavior in the porous garnet structure. With the same local current density, the high surface area of the porous garnet framework leads to a higher overall areal current density for stable lithium deposition. An elevated current density of 1 mA/cm2 based on the geometric area of the cell was demonstrated for continuous lithium cycling in symmetric lithium cells. For battery operation of the trilayer structure, the lithium can be cycled between the 3D-MCF on one side and the cathode infused into the porous structure on the opposite side. The 3D-MCF created by the porous garnet structure and conformal CNT coating provides a promising direction toward new designs in solid-state lithium metal batteries.

Continuous plating/stripping behavior of solid-state lithium metal anode in a 3D ion-conductive framework.

The increasing demands for efficient and clean energy-storage systems have spurred the development of Li metal batteries, which possess attractively high energy densities. For practical application of Li metal batteries, it is vital to resolve the intrinsic problems of Li metal anodes, i.e., the formation of Li dendrites, interfacial instability, and huge volume changes during cycling. Utilization of solid-state electrolytes for Li metal anodes is a promising approach to address those issues. In this study, we use a 3D garnet-type ion-conductive framework as a host for the Li metal anode and study the plating and stripping behaviors of the Li metal anode within the solid ion-conductive host. We show that with a solid-state ion-conductive framework and a planar current collector at the bottom, Li is plated from the bottom and rises during deposition, away from the separator layer and free from electrolyte penetration and short circuit. Owing to the solid-state deposition property, Li grows smoothly in the pores of the garnet host without forming Li dendrites. The dendrite-free deposition and continuous rise/fall of Li metal during plating/stripping in the 3D ion-conductive host promise a safe and durable Li metal anode. The solid-state Li anode shows stable cycling at 0.5 mA cm-2 for 300 h with a small overpotential, showing a significant improvement compared with reported Li anodes with ceramic electrolytes. By fundamentally eliminating the dendrite issue, the solid Li metal anode shows a great potential to build safe and reliable Li metal batteries.

3D-Printing Electrolytes for Solid-State Batteries.

Solid-state batteries have many enticing advantages in terms of safety and stability, but the solid electrolytes upon which these batteries are based typically lead to high cell resistance. Both components of the resistance (interfacial, due to poor contact with electrolytes, and bulk, due to a thick electrolyte) are a result of the rudimentary manufacturing capabilities that exist for solid-state electrolytes. In general, solid electrolytes are studied as flat pellets with planar interfaces, which minimizes interfacial contact area. Here, multiple ink formulations are developed that enable 3D printing of unique solid electrolyte microstructures with varying properties. These inks are used to 3D-print a variety of patterns, which are then sintered to reveal thin, nonplanar, intricate architectures composed only of Li7 La3 Zr2 O12 solid electrolyte. Using these 3D-printing ink formulations to further study and optimize electrolyte structure could lead to solid-state batteries with dramatically lower full cell resistance and higher energy and power density. In addition, the reported ink compositions could be used as a model recipe for other solid electrolyte or ceramic inks, perhaps enabling 3D printing in related fields.

Improving microstructural quantification in FIB/SEM nanotomography.

FIB/SEM nanotomography (FIB-nt) is a powerful technique for the determination and quantification of the three-dimensional microstructure in subsurface features. Often times, the microstructure of a sample is the ultimate determiner of the overall performance of a system, and a detailed understanding of its properties is crucial in advancing the materials engineering of a resulting device. While the FIB-nt technique has developed significantly in the 15 years since its introduction, advanced nanotomographic analysis is still far from routine, and a number of challenges remain in data acquisition and post-processing. In this work, we present a number of techniques to improve the quality of the acquired data, together with easy-to-implement methods to obtain "advanced" microstructural quantifications. The techniques are applied to a solid oxide fuel cell cathode of interest to the electrochemistry community, but the methodologies are easily adaptable to a wide range of material systems. Finally, results from an analyzed sample are presented as a practical example of how these techniques can be implemented.

Rapid Thermal Annealing of Cathode-Garnet Interface toward High-Temperature Solid State Batteries.

High-temperature batteries require the battery components to be thermally stable and function properly at high temperatures. Conventional batteries have high-temperature safety issues such as thermal runaway, which are mainly attributed to the properties of liquid organic electrolytes such as low boiling points and high flammability. In this work, we demonstrate a truly all-solid-state high-temperature battery using a thermally stable garnet solid-state electrolyte, a lithium metal anode, and a V2O5 cathode, which can operate well at 100 °C. To address the high interfacial resistance between the solid electrolyte and cathode, a rapid thermal annealing method was developed to melt the cathode and form a continuous contact. The resulting interfacial resistance of the solid electrolyte and V2O5 cathode was significantly decreased from 2.5 × 104 to 71 Ω·cm2 at room temperature and from 170 to 31 Ω·cm2 at 100 °C. Additionally, the diffusion resistance in the V2O5 cathode significantly decreased as well. The demonstrated high-temperature solid-state full cell has an interfacial resistance of 45 Ω·cm2 and 97% Coulombic efficiency cycling at 100 °C. This work provides a strategy to develop high-temperature all-solid-state batteries using garnet solid electrolytes and successfully addresses the high contact resistance between the V2O5 cathode and garnet solid electrolyte without compromising battery safety or performance.

Garnet Solid Electrolyte Protected Li-Metal Batteries.

Garnet-type solid state electrolyte (SSE) is a promising candidate for high performance lithium (Li)-metal batteries due to its good stability and high ionic conductivity. One of the main challenges for garnet solid state batteries is the poor solid-solid contact between the garnet and electrodes, which results in high interfacial resistance, large polarizations, and low efficiencies in batteries. To address this challenge, in this work gel electrolyte is used as an interlayer between solid electrolyte and solid electrodes to improve their contact and reduce their interfacial resistance. The gel electrolyte has a soft structure, high ionic conductivity, and good wettability. Through construction of the garnet/gel interlayer/electrode structure, the interfacial resistance of the garnet significantly decreased from 6.5 × 104 to 248 Ω cm2 for the cathode and from 1.4 × 103 to 214 Ω cm2 for the Li-metal anode, successfully demonstrating a full cell with high capacity (140 mAh/g for LiFePO4 cathode) over 70 stable cycles in room temperature. This work provides a binary electrolyte consisting of gel electrolyte and solid electrolyte to address the interfacial challenge of solid electrolyte and electrodes and the demonstrated hybrid battery presents a promising future for battery development with high energy and good safety.

Chromium Poisoning Effects on Surface Exchange Kinetics of La0.6Sr0.4Co0.2Fe0.8O3-δ.

The presence of Cr has already been reported in literature to cause severe degradation to La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). However, fundamental understanding of Cr effects on the surface exchange kinetics is still lacking. For the first time, in situ gas phase isotopic oxygen exchange was utilized to quantitatively determine Cr effect on oxygen exchange kinetics of LSCF powder as a function of temperature and water vapor. Our investigations revealed that the formation of secondary phases such as SrCrO4, Cr2O3, Cr-Co-Fe-O, and La-Co-Fe-O can affect both the oxygen dissociation step and overall surface exchange. Specifically, Cr-containing secondary phases on the surface not only decrease the active sites for surface reactions but also alter the nearby stoichiometry of the LSCF matrix, thereby limiting surface oxygen transport. In addition, water molecules actively participate in the surface reactions and can further block the active sites.