Garnet-type Solid-state Electrolytes Research Articles

A superior stable interlayer for dendrite-free solid-state lithium metal batteries

Garnet-type solid-state electrolyte (SSE) has emerged as a potential candidate for high-energy-density lithium (Li) metal batteries owing to its preferable Li-ion conductivity and non-flammability. However, the unexpected dendrite propagation restricts its practical application. Herein, we design a bifunctional Co3O4 layer on garnet-type Li6.5La3Zr1.5Ta0.5O12 (LLZTO) pellets by simply magnetron sputtering to suppress the Li dendrite growth. Small free energy change ΔG between Li and Co3O4 decreases the interfacial impedance (from 975 to 75 Ω∙cm−2). In-situ formed Co/Li2O layer effectively improves critical current density up to 4.8 mA∙cm−2 through homogenizing the electric-field distribution, blocking electron conduction, and providing mechanical protection for SSEs. As a result, the Li/Co3O4-SSE-Co3O4/Li symmetric cell shows extremely stable cycling performance (600 h at a current density of 1 mA∙cm−2 without short circuit), and Li/Co3O4-SSE/LFP full cell exhibits good electrochemical performance, comparable to that of equipped with liquid electrolyte. Specially, the cyclic voltammetry (CV) curves of the symmetric cell have been proposed to study the interface reaction. This work provides new thinking for designing bifunctional layer for high stable garnet-based solid-state lithium metal batteries.

A writable lithium metal ink

The recent revival of Li metal anodes (LMA) leads to a renewed interest in LMA as the ultimate choice for rechargeable lithium batteries towards high energy density. However, multiple challenges stand in the way of using LMA, of which high reactivity, dendrite growth, the difficulty of fabricating Li thin foils, and the flammability of organic liquid electrolytes are typical. Here, a writable Li metal ink (LMI) prepared by introducing biomass-derived carbon particles into molten Li is presented. Due to the significantly decreased surface tension, LMI is able to directly write on copper foils or other substrates that ultrathin Li foils with a remarkably small thickness ( can be achieved. The versatility of LMI is further demonstrated in addressing the interface issue between LMA and garnet-type solid-state electrolytes, where directly writing LMI on the garnet offers a perfect contact and enables an extremely low interfacial resistance of 6 Ω cm2, in sharp contrast to 939 Ω cm2 between the pure Li and the garnet. Due to the successful partnership with non-flammable solid-state electrolytes, ink-based technology may have a chance to bring us very close to the use of solid-state lithium metal batteries (SSLMBs) with high safety and high energy density.

Toward Moisture-Stable and Dendrite-Free Garnet-Type Solid-State Electrolytes

All-solid-state batteries using garnet-type solid-state electrolytes (SSEs) are promising candidates for safe, high energy density batteries due to their wide electrochemical stability window, high...

A High-Performance Carbonate-Free Lithium|Garnet Interface Enabled by a Trace Amount of Sodium.

Garnet-type solid-state electrolytes (SSEs) are promising for the realization of next-generation high-energy-density Li metal batteries. However, a critical issue associated with the garnet electrolytes is the poor physical contact between the Li anode and the garnet SSE and the resultant high interfacial resistance. Here, it is reported that the Li|garnet interface challenge can be addressed by using Li metal doped with 0.5 wt% Na (denoted as Li*) and melt-casting the Li* onto the garnet SSE surface. A mechanistic study, using Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) as a model SSE, reveals that Li2 CO3 resides within the grain boundaries of newly polished LLZTO pellet, which is difficult to remove and hinders the wetting process. The Li* melt can phase-transfer the Li2 CO3 from the LLZTO grain boundary to the Li*'s top surface, and therefore facilitates the wetting process. The obtained Li*|LLZTO demonstrates a low interfacial resistance, high rate capability, and long cycle life, and can find applications in future all-solid-state batteries (e.g., Li*|LLZTO|LiFePO4 ).

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.

Bi aliovalent substitution in Li7La3Zr2O12 garnets: Structural and ionic conductivity effects

We report on the synthesis of cubic-phase garnet-type solid-state electrolytes based on Bi-doped Li7La3Zr2O12 (LLZO). Bi aliovalent substitution of Zr in LLZO utilizing the Pechini processing method is employed to synthesize Li7−xLa3Zr2−xBixO12 compounds. A strong dependence of the ionic conductivity on Bi content is observed, and under our synthesis and sintering conditions, a >100-fold increase over the un-doped sample is observed for x=0.75. Cubic-phase Li6La3Zr1BiO12 compounds are generated upon annealing in air in the temperature range 650 °C–900 °C. In contrast, in the absence of Bi, the cubic garnet phase of Li7La3Zr2O12 is not formed below 700 °C and a transformation to the tetragonal phase is observed at ∼900 °C for this un-doped compound. The role of Bi in lowering the formation temperature of the garnet cubic phase and in the ionic conductivity improvements is investigated in this work. We ascribe the effect of Bi-doping on ionic conductivity increments to changes in Li+-site occupancy and lattice parameters and the reduction in the formation temperature for the cubic-phase formation to rate enhancements of the solid-state reaction. To identify the site occupancy of Bi in the garnet structure, we employ synchrotron extended x-ray absorption fine structure spectroscopy. Our results indicate that Bi additions occupy the Zr-type sites exclusively, to within the accuracy of the measurements.

In Situ Lithiophilic Layer from H+/Li+ Exchange on Garnet Surface for the Stable Lithium-Solid Electrolyte Interface.

Garnet-type solid-state electrolytes (SSEs) show a promising application in solid-state Li batteries. Poor interfacial contact with lithium causing large interfacial impedance and dendrite penetration is a problem. Inspired by unique H+/Li+ exchange of garnet electrolyte, we used an AgNO3 aqueous solution induced strategy to construct a lithiophilic layer in situ on the garnet surface without any specific apparatus. Experimental analysis reveals the uniform distribution of Ag nanoparticles and significantly enhanced affinity between the solid state electrolyte (SSE) and Li anode for the Li-Ag alloying. As expected, the interfacial area specific resistance (ASR) is greatly reduced to ∼4.5 Ω cm2, accompanying with long-cycling stability for ∼3500 h at 0.2 mA cm-2 and high critical current density of 0.75 mA cm-2. With modified SSEs, quasi-solid-state batteries with a LiFePO4 or LiNi0.5Co0.2Mn0.3O2 cathode operate well at room temperature and an all-solid-state LiFePO4/garnet/Li battery displays good cycling stability for over 200 cycles at 60 °C.

The Active Interface of Ta-Doped Li7La3Zr2O12 for Li Plating/Stripping Revealed by Acid Aqueous Etching

All-solid-state lithium batteries incorporating oxide-based solid electrolytes have attracted much attention as a promising battery system for enabling highly reversible Li metal anodes. However, the cycling stability of Li plating/stripping reactions at higher charging/discharging rates on garnet-type solid-state electrolytes must be improved to realize a practical Li metal anode for solid-state batteries. Here, we report that a short acid etching procedure performed in ambient air significantly activates the Ta-doped Li7La3Zr2O12 (LLZT) surface compared to polishing under inert gas atmosphere such as dry Ar. It has been believed that Li7La3Zr2O12 (LLZ) and related doped LLZ solid electrolyte surfaces need to be mechanically polished in dry Ar before the cell fabrication to remove Li2CO3 and LiOH that are present on the surface. However, a commonly used mechanical polishing procedure is found to form a thin electrochemically inactive layer on the LLZT surface, whereas a short acid etching procedure (e.g., HCl) removes the inactive layer, and the acid-etched LLZT exhibits excellent cycling stability.

A silicon anode for garnet-based all-solid-state batteries: Interfaces and nanomechanics

High-capacity electrode materials are indispensable for developing high energy density solid-state batteries. The lithium metal anode is attractive because of its high capacity and low electrochemical reduction potential, but its application is hampered by the dendrite issue. The silicon anode is a promising material having high capacity and invulnerability to undergoing dendrite formation, but is limited to the nanometer regime for the thickness of a Si anode. Herein, for the first time, we demonstrate a 1 μm thick solid-state silicon anode (10 times the typical thickness of Si anodes used in organic electrolyte) as an alternative to Li metal anode for solid-state batteries. This Si anode forms good contact with the garnet-type solid-state electrolyte and maintains structural integrity during the Li ion intercalation and extraction. The Si anode with the garnet electrolyte exhibits a high discharge capacity of 2685 mA h g−1 and an excellent initial Coulombic efficiency of 83.2%, higher than that of the Si anodes with an organic electrolyte (77.1%). Our mechanics modeling reveals that the strong nanomechanical constraints by the solid garnet electrolyte enables the substantial increase in the critical thickness of the Si anode from just nanometers to micrometers, toward high-capacity solid-state batteries.

Ultra-stable lithium plating/stripping in garnet-based lithium-metal batteries enabled by a SnO2 nanolayer

Owing to its high ionic conductivity (10−4 - 10−3 S cm−1), excellent stability toward lithium and a wide electrochemical window, the garnet-type solid-state electrolyte has been regarded as one of the most promising solutions to the viability of lithium metal-based batteries. However, the poor interfacial contact between the garnet and the lithium metal leads to an ultrahigh interfacial resistance (∼1000 Ω cm2) and uneven current distribution. In this work, the issue is effectively addressed by coating the garnet pellet with a SnO2 nanolayer. Attributed to the lithiation reaction between the lithium metal and SnO2, an ion-conducting interlayer is generated automatically between the garnet and lithium metal, which reduces their interfacial resistance from 1019 to 153 Ω cm2. A Li | SnO2-garnet | Li symmetric cell is then built and achieves a stable lithium plating/stripping for more than 900 h at 0.2 mA cm−2 without short circuit. Moreover, in a Li | SnO2-garnet | LiFePO4 full battery test, the SnO2 nanolayer enables the full battery to be stably operated for over 100 cycles at 0.3 C with a capacity retention of 98.6%, whereas the control group using bare garnet is unable to work at this high rate.

In-situ formed Li2CO3-free garnet/Li interface by rapid acid treatment for dendrite-free solid-state batteries

Garnet-type solid-state electrolytes (SSEs) are very promising due to their high ionic conductivities at room temperature and high stability against Li metal. However, the poor garnet/Li interfacial contact caused by Li2CO3 surface contaminant can lead to lithium dendrite growth and the performance decay of solid-state batteries (SSBs), which still hinders their practical application. Herein, a universal and simple method of rapid acid treatment is proposed to perfectly remove the surface Li2CO3 and retrieve a lithiophilic SSE surface. The SSE/Li interfacial resistance dramatically decreases from 940 Ω cm2 to 26 Ω cm2 at 30 °C. The acid treated garnet-SSE pellets exhibit an interfacial resistance comparable to the pellets with various surface coatings. In addition, the intrinsic garnet/Li interface remains stable during cycling, which enables the Li symmetric cells continuously cycle over 700 h under 0.2 mA cm−2 at 30 °C. And the LiFePO4/Li and LiCoO2/Li cells with acid treated garnet-SSE show excellent cycle and rate performances after eliminating the surface contaminant. These results indicate that rapid acid treatment not only guides a new understanding for an intrinsic garnet/Li interface but also is a simple and high-efficiency strategy to well address the interfacial issue for SSBs.

(Invited) Construction of Flexible Interfacial Layer for Solid-State Li Batteries

There is a growing interest in all-solid-state lithium batteries(ASLBs) for their safety and high energy density. However, the power density for ASLBs is still far away from satisfactory on the account of high interfacial resistance at electrode / solid electrolyte interface. For example, Garnet-type solid-state electrolytes (SSEs) are considered as a good choice for solid-state batteries, yet the interfacial issues with metallic Li limit their applications. In my group, we propose an ultra-simple and effective strategy to enhance the interfacial connection between garnet SSEs and Li metal just by drawing a graphite-based soft interface with a pencil[1]. Both experimental analysis and theoretical calculations confirm that the reaction between graphite-based interfacial layer and metallic lithium forms a lithiated connection interface with good lithium-ionic and electronic conductivity. Compared to the reported interfacial materials, the graphite material provides a soft interface with better ductility and compressibility. With the improvement by this soft interface, the impedance of symmetric Li cells significantly decreases from 1350 to 105Ωcm2 and the cell can cycle for over 1000hoursat a current density of 300 μA/cm2 at room temperature. Moreover, a solid-state battery with Li-metal anode, ternary cathode NCM523 and graphite-based interface modified garnet SSEs is fabricated and displays excellent rate capability and long cycling performance. In addition, LGPS-type materials show extremely high Li+ conductivity. But when in contact to Li metal, they can be reduced to produce Li2S, Li3P and Li-M alloy and form a mixed ionic and electronic conducting layer at the interface, which hinders the application of Li metal in such solid electrolyte system. We report on a simple approach for the modification of the interface between Li and Li10SnP2S12 sulfide solid electrolyte by utilizing ionic liquid[2]. Our study shows that the addition of 1.5M LiTFSI/Pyr13TFSI ionic liquid can not only promote intimate contact but can in-situ form a stable SEI layer at the interface between Li and solid electrolyte as well. A stable SEI layer instead of mixed conducting layer can prevent sulfide solid electrolyte from further decomposition and improve the cycle performance of batteries.

Lithium-Graphite Paste: An Interface Compatible Anode for Solid-State Batteries.

All-solid-state batteries (ASSBs) with ceramic-based solid-state electrolytes (SSEs) enable high safety that is inaccessible with conventional lithium-ion batteries. Lithium metal, the ultimate anode with the highest specific capacity, also becomes available with nonflammable SSEs in ASSBs, which offers promising energy density. The rapid development of ASSBs, however, is significantly hampered by the large interfacial resistance as a matched lithium/ceramic interface that is not easy to pursue. Here, a lithium-graphite (Li-C) composite anode is fabricated, which shows a dramatic modification in wettability with garnet SSE. An intimate Li-C/garnet interface is obtained by casting Li-C composite onto garnet-type SSE, delivering an interfacial resistance as low as 11 Ω cm2 . As a comparison, pure Li/garnet interface gives a large resistance of 381 Ω cm2 . Such improvement can be ascribed to the experiment-measured increased viscosity of Li-C composite and simulation-verified limited interfacial reaction. The Li-C/garnet/Li-C symmetric cell exhibits stable plating/striping performance with small voltage hysteresis and endures a critical current density up to 1.0 mA cm-2 . The full cell paired with LiFePO4 shows stable cycle performance, comparable to the cell with liquid electrolyte. The present work demonstrates a promising strategy to develop ceramic-compatible lithium metal-based anodes and hence low-impedance ASSBs.

A dopamine modified Li6.4La3Zr1.4Ta0.6O12/PEO solid-state electrolyte: enhanced thermal and electrochemical properties

A garnet-type solid-state electrolyte Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was modified using dopamine to improve the wettability of LLZTO with PEO, allowing 80 wt% LLZTO to be uniformly dispersed in 20 wt% PEO/LiTFSI polymer electrolyte.

An Electron/Ion Dual-Conductive Alloy Framework for High-Rate and High-Capacity Solid-State Lithium-Metal Batteries.

The solid-state Li battery is a promising energy-storage system that is both safe and features a high energy density. A main obstacle to its application is the poor interface contact between the solid electrodes and the ceramic electrolyte. Surface treatment methods have been proposed to improve the interface of the ceramic electrolytes, but they are generally limited to low-capacity or short-term cycling. Herein, an electron/ion dual-conductive solid framework is proposed by partially dealloying the Li-Mg alloy anode on a garnet-type solid-state electrolyte. The Li-Mg alloy framework serves as a solid electron/ion dual-conductive Li host during cell cycling, in which the Li metal can cycle as a Li-rich or Li-deficient alloy anode, free from interface deterioration or volume collapse. Thus, the capacity, current density, and cycle life of the solid Li anode are improved. The cycle capability of this solid anode is demonstrated by cycling for 500 h at 1 mA cm-2 , followed by another 500 h at 2 mA cm-2 without short-circuiting, realizing a record high cumulative capacity of 750 mA h cm-2 for garnet-type all-solid-state Li batteries. This alloy framework with electron/ion dual-conductive pathways creates the possibility to realize high-energy solid-state Li batteries with extended lifespans.

Interface Engineering for Garnet-Based Solid-State Lithium-Metal Batteries: Materials, Structures, and Characterization.

Lithium-metal batteries are considered one of the most promising energy-storage systems owing to their high energy density, but their practical applications have long been hindered by significant safety concerns and poor cycle stability. Solid-state electrolytes (SSEs) are expected to improve not only the safety but also the energy density of Li-metal batteries. The key challenge for solid-state Li-metal batteries lies in the low ionic conductivity of the SSEs and moreover the interface contact between the electrode and SSE. To achieve feasible solid-state Li-metal batteries, it is imperative that the ionic conductivity is improved, especially at the electrode-SSE interface. Herein, recent advances in interface engineering for solid-state Li-metal batteries are reported, mainly focusing on garnet-type SSEs. Various materials to modify the cathode-garnet and Li-garnet interfaces by intermediate layers, alloys, and polymer electrolytes are analyzed. Structural innovations for SSEs including composite electrolytes and multilayer SSE frameworks are reviewed, along with advanced characterization approaches to probe the interfaces, which will provide further insights for garnet-based solid-state batteries. Future challenges and the great promise of garnet-based Li-metal batteries are discussed to close.

Drawing a Soft Interface: An Effective Interfacial Modification Strategy for Garnet-Type Solid-State Li Batteries

Garnet-type solid-state electrolytes (SSEs) are considered to be a good choice for solid-state batteries, yet the interfacial issues with metallic Li limit their applications. Herein, we propose an ultrasimple and effective strategy to enhance the interfacial connection between garnet SSEs and Li metal just by drawing a graphite-based soft interface with a pencil. Both experimental analysis and theoretical calculations confirm that the reaction between the graphite-based interfacial layer and metallic lithium forms a lithiated connection interface with good lithium-ionic and electronic conductivity. Compared to the reported interfacial materials, the graphite provides a soft interface with better ductility and compressibility. With improvement by this soft interface, the impedance of symmetric Li cells significantly decreases and the cell cycle is stable for over 1000 h. Moreover, a solid-state battery with Li-metal anode, ternary NCM523 cathode, and treated-garnet SSEs is fabricated and displays excellen...

Surface Chemistry Mechanism of Ultra-Low Interfacial Resistance in the Solid-State Electrolyte Li7La3Zr2O12

The garnet-type solid-state electrolyte, based on the nominal formula Li7La3Zr2O12 (LLZO), is unique in that it is a fast Li-ion conductor, exhibits sufficient mechanical properties, and is also chemically and electrochemically stable against metallic Li. Despite these promising attributes, additional challenges must be overcome before solid-state batteries based on LLZO are viable. Demonstrating low Li–LLZO interfacial resistance (R Li–LLZO) is a critical milestone along the path to commercialization. While several studies have characterized R Li–LLZO, nearly all report values significantly higher than conventional LIBs employing liquid electrolytes. Thus, strategies to reduce R Li–LLZO to values comparable to, or lower than, LIBs are needed [1]. In this study, the impact of surface chemistry on the interfacial resistance between the LLZO solid-state electrolyte and a metallic Li electrode is revealed [1]. 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. [1] A. Sharafi, E. Kazyak, A. L. Davis, S. Yu, T Thompson, D. J. Siegel, N. P. Dasgupta, J. Sakamoto, Chem. Mater. 29, 7961(2017)

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.

Enhanced lithium ion transport in garnet-type solid state electrolytes

Al-substituted Li7La3Zr2O12 samples processed under argon show enhanced Li-ion transport and interfacial properties in symmetrical cells with lithium electrodes, compared to those prepared in air. In particular, the samples prepared under argon have higher ionic conductivities and lower interfacial impedances in symmetrical lithium cells, and show better DC cycling characteristics. The electronic conductivities are also somewhat higher. Pellets subjected to thermal treatment under the two types of atmospheres have different colors but exhibit similar microstructures. X-ray diffraction experiments suggest that there are slight structural differences between the two types of samples, but few dissimilarities were observed in elemental composition, distribution of ions, oxidation states, or bond lengths using laser-induced breakdown spectroscopy (LIBS), x-ray photoelectron spectroscopy (XPS), and extended x-ray absorption fine structure spectroscopy (EXAFS) to analyze the materials. Additionally, there was no evidence that La or Zr were reduced during the processing under Ar. Possible explanations for the improved electrochemical properties of the sample prepared under Ar compared to the one prepared in air include differences in grain boundary chemistries and conductivities and/or a small concentration of oxygen vacancies in the former.