Effect of surface microstructure on electrochemical performance of garnet solid electrolytes

Accelerating the Development of LLZO in Solid-State Batteries Toward Commercialization: A Comprehensive Review

Solid-state batteries (SSBs) are under development as high-priority technologies for safe and energy-dense next-generation electrochemical energy storage systems operating over a wide temperature range. Solid-state electrolytes (SSEs) exhibit high thermal stability and, in some cases, the ability to prevent dendrite growth through a physical barrier, and compatibility with the "holy grail" metallic lithium. These unique advantages of SSEs have spurred significant research interests during the last decade. Garnet-type SSEs, that is, Li7La3Zr2O12 (LLZO), are intensively investigated due to their high Li-ion conductivity and exceptional chemical and electrochemical stability against lithium metal anodes. However, poor interfacial contact with cathode materials, undesirable lithium plating along grain boundaries, and moisture-induced chemical degradation greatly hinder the practical implementation of LLZO-based SSEs for SSBs. In this review, the recent advances in synthesis methods, modification strategies, corresponding mechanisms, and applications of garnet-based SSEs in SSBs are critically summarized. Furthermore, a comprehensive evaluation of the challenges and development trends of LLZO-based electrolytes in practical applications is presented to accelerate their development for high-performance SSBs.

Machine Learning on Microstructure-Property Relationship of Lithium-Ion Conducting Oxide Solid Electrolytes

Understanding the structure-property relationship of lithium-ion conducting solid oxide electrolytes is essential to accelerate their development and commercialization. However, the structural complexity of nonideal materials increases the difficulty of study. Here, we develop an algorithmic framework to understand the effect of microstructure on the properties by linking the microscopic morphology images to their ionic conductivities. We adopt garnet and perovskite polycrystalline oxides as examples and quantify the microscopic morphologies via extracting determined physical parameters from the images. It directly visualizes the effect of physical parameters on their corresponding ionic conductivities. As a result, we can determine the microstructural features of a Li-ion conductor with high ionic conductivity, which can guide the synthesis of highly conductive solid electrolytes. Our work provides a novel approach to understanding the microstructure-property relationship for solid-state ionic materials, showing the potential to extend to other structural/functional ceramics with various physical properties in other fields.

Solid Electrolyte Bimodal Grain Structures for Improved Cycling Performance

The application of solid-state electrolytes in Li batteries is hampered by the occurrence of Li-dendrite-caused short circuits. To avoid cell failure, the electrolytes can only be stressed with rather low current densities, severely restricting their performance. As grain size and pore distributions significantly affect dendrite growth in ceramic electrolytes such as Li7La3Zr2O12 and its variants; here, a "detour and buffer" strategy to bring the superiority of both coarse and fine grains into play, is proposed. To validate the mechanism, a coarse/fine bimodal grain microstructure is obtained by seeding unpulverized large particles in the green body. The rearrangement of coarse grains and fine pores is fine-tuned by changing the ratio of pulverized and unpulverized powders. The optimized bimodal microstructure, obtained when the two powders are equally mixed, allows, without extra interface decoration, cycling for over 2000 h as the current density is increased from 1.0 mA·cm-2, and gradually, up to 2.0 mA·cm-2. The "detour and buffer" effects are confirmed from postmortem analysis. The complex grain boundaries formed by fine grains discourage the direct infiltration of Li. Simultaneously, the coarse grains further increase the tortuosity of the Li path. This study sheds light on the microstructure optimization for the polycrystalline solid-state electrolytes.

Solid Interfaces for the Garnet Electrolytes

Solid-state electrolytes (SSEs) have attracted extensive interests due to the advantages in developing secondary batteries with high energy density and outstanding safety. Possessing high ionic conductivity and the lowest reduction potential among the state-of-the-art SSEs, the garnet type SSE is one of the most promising candidates to achieve high performance solid-state lithium batteries (SSLBs). However, the elastic modulus of the garnet electrolyte leads to deteriorated interfacial contacts, and the increasing in electronic conduction at either anode/garnet interface or grain boundary results in Li dendrite growth. Here, recent developments of the solid interfaces for the garnet electrolytes, including the strategies of Li dendrite suppression and interfacial chemical/electrochemical/mechanical stabilizations are presented. A new viewpoint of the double edges of interfacial lithiophobicity is proposed, and the rational design of the interphases, as well as effective stacking methods of the garnet-based SSLBs are summarized. Moreover, practical roles of the garnet electrolyte in SSLB industry are also discussed. This work delivers insights into the solid interfaces for the garnet electrolytes, which provides not only the promotion of the garnet-based SSLBs, but also a comprehensive understanding of the interfacial stabilization for the whole SSE family.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Insight into Dendrites Issue in All Solid-State Batteries with Inorganic Electrolyte: Mechanism, Detection and Suppression Strategies

All solid-state batteries (ASSBs) are regarded as one of the promising next-generation energy storage devices due to their expected high energy density and capacity. However, failures due to unrestricted growth of lithium dendrites (LDs) have been a critical problem. Moreover, the understanding of dendrite growth inside solid-state electrolytes is limited. Since the dendrite process is a multi-physical field coupled process, including electrical, chemical, and mechanical factors, no definitive conclusion can summarize the root cause of LDs growth in ASSBs till now. Herein, the existing works on mechanism, identification, and solution strategies of LD in ASSBs with inorganic electrolyte are reviewed in detail. The primary triggers are thought to originate mainly at the interface and within the electrolyte, involving mechanical imperfections, inhomogeneous ion transport, inhomogeneous electronic structure, and poor interfacial contact. Finally, some of the representative works and present an outlook are comprehensively summarized, providing a basis and guidance for further research to realize efficient ASSBs for practical applications.

Correlating the Microstructure and Current Density of the Li/Garnet Interface

Solid-state lithium batteries hold great promise for next-generation energy storage systems. However, the formation of lithium filaments within the solid electrolyte remains a critical challenge. In this study, we investigate the crucial role of morphology in determining the resistance of garnet-type electrolytes to lithium filaments. By proposing a new test method, namely, cyclic linear sweep voltammetry, we can effectively evaluate the electrolyte resistance against lithium filaments. Our findings reveal a strong correlation between the microscopic morphology of the solid electrolyte and its resistance to lithium filaments. Samples with reduced pores and multiple grain boundaries demonstrate remarkable performance, achieving a critical current density of up to 3.2 mA cm-2 and excellent long-term cycling stability. Kelvin probe force microscopy and finite element method simulation results shed light on the impact of grain boundaries and electrolyte pores on lithium-ion transport and filament propagation. To inhibit lithium penetration, minimizing pores and achieving a uniform morphology with small grains and plenty of grain boundaries are essential.

Reference Electrode Reveals Insights on Sodium Metal/Solid Electrolyte Interface Cycling and Voiding Behaviors at High Current Densities and Areal Capacities

Plating and stripping processes at solid metal electrode/solid electrolyte interfaces are of great significance for high-energy, solid-state batteries. Here, we introduce a Na metal reference electrode to a symmetric Na metal/sodium β″ alumina/Na metal cell and study both cycling and unidirectional protocols with a focus on high current density and areal capacity. For example, in a current ramp test at 5 mAh cm-2 we find a shift from stable to unstable interfacial polarization during stripping at ≳3 mA cm-2, and at 7.5 mA cm-2 we measure 100s of mV of voltage magnitude rise at the stripping electrode and 10s of mV of voltage changes at the plating electrode. In unidirectional testing (i.e., passing current in a single direction until cell failure), at 1.2 mA cm-2 we find only ∼40% of the initial Na foil could be transferred through the solid electrolyte and again observe 100s of mV (and larger) voltage magnitude rise at the stripping electrode and 10s of mV of voltage change at the plating electrode. This test also shows that the 100s of mV of interfacial polarization can be sustained for hours (at 1.2 mA cm-2) to tens of hours (in a test at 0.3 mA cm-2). Hence, across several test protocols we find a Na metal reference electrode provides quantitative insights on electrochemical interfacial behavior that are not revealed in two-electrode testing. We also built a two-dimensional model of our three-electrode symmetric cell to quantify the link between the measured interfacial potentials in our testing and changes in electrochemically active interfacial contact and find that 100s of mV of interfacial potential rise indicates loss of electrochemically active contact area of >80%. Our work provides a promising approach to clarify the coupled interfacial electrochemical and contact mechanics processes at solid metal electrode/solid electrolyte interfaces.

High Cathode Loading and Low-Temperature Operating Garnet-Based All-Solid-State Lithium Batteries - Material/Process/Architecture Optimization and Understanding of Cell Failure

All-solid-state lithium batteries (ASSLBs) are prepared using garnet-type solid electrolytes by quick liquid phase sintering (Q-LPS) without applying high pressure during the sintering. The cathode layers are quickly sintered with a heating rate of 50-100 K min-1 and a dwell time of 10 min. The battery performance is dramatically improved by simultaneously optimizing materials, processes, and architectures, and the initial discharge capacity of the cell with a LiCoO2 -loading of 8.1 mg reaches 1 mAh cm-2 and 130 mAh g-1 at 25 °C. The all-solid-state cell exhibits capacity at a reduced temperature (10 °C) or a relatively high rate (0.1 C) compared to the previous reports. The Q-LPS would be suitable for large-scale manufacturing of ASSLBs. The multiphysics analyses indicate that the internal stress reaches 1 GPa during charge/discharge, which would induce several mechanical failures of the cells: broken electron networks, broken ion networks, separation of interfaces, and delamination of layers. The experimental results also support these failures.

Low Interfacial Resistance and Superior Suppression to Li-Dendrite Penetration Facilitated by Air-Stable and Mechanically Robust Al/Mg-Co-Doped Li-La-Zirconate as Electrolyte for Li-Based Solid-State Cells

In the context of usage as a solid electrolyte (SE) for Li-based solid-state cells, the interfacial characteristics of Li-La-zirconate (LLZO) with the electrodes and the mechanical properties of LLZO influence the overall impedance and stability. In this regard, the newly developed air-stable Al/Mg-co-doped LLZO has been found to possess greater resistance to crack propagation (by ∼31%) and fracture stress (by ∼52%), along with elevated hardness and stiffness, as compared to simply Al-doped LLZO. Furthermore, as directly visualized via cross-section electron microscopy at the Li/LLZO interfaces, the air-stability, along with mechanical robustness of Al/Mg-co-doped LLZO, facilitates the complete absence of impurity phase and cracks at the Li/LLZO interface, unlike for the simply Al-doped LLZO. These result in a very low area specific resistance for the Li/"Al/Mg-co-doped LLZO" interface of ∼9 Ω cm2, which is ∼3 times lower than that at the Li/"Al-doped LLZO" interface and is also among the lowest reported to date for Li/LLZO interfaces, that too sans any surface/interfacial coating/engineering. Galvanostatic Li-plating/stripping cycles indicate that the critical current density toward initiating Li-dendrite nucleation/propagation is higher in the case of Al/Mg-co-doped LLZO SE, viz., ∼0.45 mA/cm2, than for the Al-doped counterpart (viz., ∼0.25 mA/cm2). Furthermore, Li-stripping/plating cycles @ 0.1 mA/cm2 have revealed outstanding stability of polarization voltage up to at least 100 cycles upon using Al/Mg-codoped LLZO as the SE, in contrast to the instability right from the 36th cycle onward with the Al-doped LLZO. This indicates superior suppression toward Li-dendrite nucleation/propagation by the Al/Mg-codoped LLZO, unlike by Al-doped LLZO, as also directly visualized via cross-section electron microscopy post-cycling. The air-stability induced a clean Li/LLZO interface (viz., good contact), which, together with the mechanical robustness of Al/Mg-codoped LLZO, resulted in the very low interfacial resistance and excellent suppression toward Li-dendrite nucleation/propagation, leading to high CCD and very stable long-term Li-stripping/plating. Overall, in addition to highlighting the notable advantages offered by the Al/Mg-co-doped LLZO solid electrolyte, the insights obtained as part of this work are expected to lead to the successful and facile development of high-performance solid-state Li-based cells.

Temperature and Pressure Effects on Unrecoverable Voids in Li Metal Solid-State Batteries

All-solid-state batteries (ASSB) can potentially achieve high gravimetric and volumetric energy densities (900 Wh/L) if paired with a lithium metal anode and solid electrolyte. However, there is a lack in critical understanding about how to operate lithium metal cells at high capacities and minimize unwanted degradation mechanisms such as dendrites and voids. Herein, we investigate how pressure and temperature influence the formation and annihilation of unrecoverable voids in lithium metal upon stripping. Stack pressure and temperature are effective means to initiate creep-induced void filling and decrease charge transfer resistances. Applying stack pressure enables lithium to deform and creep below the yield stress during stripping at high current densities. Lithium creep is not sufficient to prevent cell shorting during plating. Three-electrode experiments were employed to probe the kinetic and morphological limitations that occur at the anode-solid electrolyte during high-capacity stripping (5 mAh/cm2). The role of cathode-LLZO interface, which dictates cyclability and capacity retention in full cells, was also studied. This work elucidates the important role that temperature (external or in situ generated) has on reversible operation of solid-state batteries.

Effect of pulse-current-based protocols on the lithium dendrite formation and evolution in all-solid-state batteries

Understanding the cause of lithium dendrites formation and propagation is essential for developing practical all-solid-state batteries. Li dendrites are associated with mechanical stress accumulation and can cause cell failure at current densities below the threshold suggested by industry research (i.e., >5 mA/cm²). Here, we apply a MHz-pulse-current protocol to circumvent low-current cell failure for developing all-solid-state Li metal cells operating up to a current density of 6.5 mA/cm². Additionally, we propose a mechanistic analysis of the experimental results to prove that lithium activity near solid-state electrolyte defect tips is critical for reliable cell cycling. It is demonstrated that when lithium is geometrically constrained and local current plating rates exceed the exchange current density, the electrolyte region close to the defect releases the accumulated elastic energy favouring fracturing. As the build-up of this critical activity requires a certain period, applying current pulses of shorter duration can thus improve the cycling performance of all-solid-solid-state lithium batteries.

Understanding the evolution of lithium dendrites at Li6.25Al0.25La3Zr2O12 grain boundaries via operando microscopy techniques

The growth of lithium dendrites in inorganic solid electrolytes is an essential drawback that hinders the development of reliable all-solid-state lithium metal batteries. Generally, ex situ post mortem measurements of battery components show the presence of lithium dendrites at the grain boundaries of the solid electrolyte. However, the role of grain boundaries in the nucleation and dendritic growth of metallic lithium is not yet fully understood. Here, to shed light on these crucial aspects, we report the use of operando Kelvin probe force microscopy measurements to map locally time-dependent electric potential changes in the Li_6.25Al_0.25La₃Zr₂O₁₂ garnet-type solid electrolyte. We find that the Galvani potential drops at grain boundaries near the lithium metal electrode during plating as a response to the preferential accumulation of electrons. Time-resolved electrostatic force microscopy measurements and quantitative analyses of lithium metal formed at the grain boundaries under electron beam irradiation support this finding. Based on these results, we propose a mechanistic model to explain the preferential growth of lithium dendrites at grain boundaries and their penetration in inorganic solid electrolytes.

Processing and Properties of Garnet-Type Li7 La3 Zr2 O12 Ceramic Electrolytes

Garnet-type solid electrolyte Li₇ La₃ Zr₂ O₁₂ (LLZO) is widely considered as one of the most promising candidates for solid state batteries (SSBs) owing to its high ionic conductivity and good electrochemical stability. Since its discovery in 2007, great progress has been made in terms of crystal chemistry, chemical and electrochemical properties, and battery application. Nonetheless, reliable and controllable preparation of LLZO ceramics with desirable properties still remains as big challenges. Herein, this review summarizes various synthetic routes of LLZO ceramics and examines the influence of various key processing parameters on the chemical and electrochemical properties. Focusing on correlation of processing parameters and properties, this review aims to provide new insights on a reliable and controllable production of high-quality LLZO ceramic electrolytes for SSB application.

Polymorphism of garnet solid electrolytes and its implications for grain-level chemo-mechanics

Understanding and mitigating filament formation, short-circuit and solid electrolyte fracture is necessary for advanced all-solid-state batteries. Here, we employ a coupled far-field high-energy diffraction microscopy and tomography approach for assessing the chemo-mechanical behaviour for dense, polycrystalline garnet (Li₇La₃Zr₂O₁₂) solid electrolytes with grain-level resolution. In situ monitoring of grain-level stress responses reveals that the failure mechanism is stochastic and affected by local microstructural heterogeneity. Coupling high-energy X-ray diffraction and far-field high-energy diffraction microscopy measurements reveals the presence of phase heterogeneity that can alter local chemo-mechanics within the bulk solid electrolyte. These local regions are proposed to be regions with the presence of a cubic polymorph of LLZO, potentially arising from local dopant concentration variation. The coupled tomography and FF-HEDM experiments are combined with transport and mechanics modelling to illustrate the degradation of polycrystalline garnet solid electrolytes. The results showcase the pathways for processing high-performing solid-state batteries.

Nanoscale interface engineering of inorganic Solid-State electrolytes for High-Performance alkali metal batteries

All-solid-state metal batteries (ASSMBs) have been regarded as the ideal candidate for the next-generation high-energy storage system due to their ultrahigh specific capacity and the lowest redox potential. However, the uncontrollable chemical reactivity during cycling which directly determines the growth behaviour of metal dendrites, the low coulombic efficiency and the safety concerns severely limit their real-world applications.. Crystallographic optimization based on solid-state electrolytes (SSEs) provides an atomic-scale and fundamental solution for the inhibition of dendrite growth in metal anodes, which has attracted widespread attentions. From this perspective, we summarize the recent advance of the crystallographic optimization for various classes of solid-state electrolytes. We highlight the recent experimental findings of crystallographic optimization for a new generation of all-solid-state batteries, including lithium-ion batteries, sodium-ion batteries, magnesium-ion batteries, with the aim of providing a deeper understanding of the crystallographic reactions in ASSMBs. The challenges and prospects for the future design and engineering of crystallographic optimization of SSEs are discussed, providing ideas for further research into crystallographic optimization to improve the performance of rechargeable batteries.

A Nanoscale Design Approach for Enhancing the Li-Ion Conductivity of the Li10GeP2S12 Solid Electrolyte

The discovery of the lithium superionic conductor Li10GeP2S12 (LGPS) has led to significant research activity on solid electrolytes for high-performance solid-state batteries. Despite LGPS exhibiting a remarkably high room-temperature Li-ion conductivity, comparable to that of the liquid electrolytes used in current Li-ion batteries, nanoscale effects in this material have not been fully explored. Here, we predict that nanosizing of LGPS can be used to further enhance its Li-ion conductivity. By utilizing state-of-the-art nanoscale modeling techniques, our results reveal significant nanosizing effects with the Li-ion conductivity of LGPS increasing with decreasing particle volume. These features are due to a fundamental change from a primarily one-dimensional Li-ion conduction mechanism to a three-dimensional mechanism and major changes in the local structure. For the smallest nanometric particle size, the Li-ion conductivity at room temperature is three times higher than that of the bulk system. These findings reveal that nanosizing LGPS and related solid electrolytes could be an effective design approach to enhance their Li-ion conductivity.

Mechanical behaviour of inorganic solid-state batteries: can we model the ionic mobility in the electrolyte with Nernst-Einstein's relation?

Inorganic solid-state lithium-metal batteries could be the next-generation batteries owing to their non-flammability and higher specific energy density. Many research efforts have been devoted to improving the ionic conductivity of inorganic solid electrolytes. For a wide range of electrolytes including liquid and solid polymer electrolytes, an independent measurement or calculation of both electrolyte conductivity and diffusion coefficient is often time-consuming and challenging. As a result, Nernst-Einstein's relation has been used to relate the ionic conductivity to ionic diffusivity after the determination of either parameter. Although Nernst-Einstein's relation has been used for different electrolytes, we demonstrate in this perspective that this relation is not directly transferable to describe the ionic mobility for many inorganic solid electrolytes. The fundamental physics of Nernst-Einstein's relation shows that the relationship between the diffusion coefficient and electrolyte conductivity is derived for ionic mobility in a viscous or a gaseous medium. This postulation contradicts state-of-the-art experimental studies measuring the mechanical behaviour of inorganic solid electrolytes, which show that inorganic solid electrolytes are usually brittle rather than viscoelastic at ambient room temperature. The measurement of loss tangent is required to justify the use of Nernst-Einstein's relation. The outcome of such measurement has two implications. First, if the loss tangent of inorganic solid electrolytes is less than unity in the range of batteries operating temperatures, the impacts of using Nernst-Einstein's relation in modelling the ionic mobility should be quantified. Secondly, if the measured loss tangent is comparable to that of solid polymers and lithium metal, inorganic solid electrolytes may behave in a viscoelastic manner as opposed to the brittle behaviour usually suggested.

A Morphologically Stable Li/Electrolyte Interface for All-Solid-State Batteries Enabled by 3D-Micropatterned Garnet

Morphological degradation at the Li/solid-state electrolyte (SSE) interface is a prevalent issue causing performance fading of all-solid-state batteries (ASSBs). To maintain the interfacial integrity, most ASSBs are operated under low current density with considerable stack pressure, which significantly limits their widespread usage. Herein, a novel 3D-micropatterned SSE (3D-SSE) that can stabilize the morphology of the Li/SSE interface even under relatively high current density and limited stack pressure is reported. Under the pressure of 1.0 MPa, the Li symmetric cell using a garnet-type 3D-SSE fabricated by laser machining shows a high critical current density of 0.7 mA cm^-2 and stable cycling over 500 h under 0.5 mA cm^-2 . This excellent performance is attributed to the reduced local current density and amplified mechanical stress at the Li/3D-SSE interface. These two effects can benefit the flux balance between Li stripping and creep at the interface, thereby preventing interfacial degradation such as void formation and dendrite growth.

Evolving contact mechanics and microstructure formation dynamics of the lithium metal-Li7La3Zr2O12 interface

The dynamic behavior of the interface between the lithium metal electrode and a solid-state electrolyte plays a critical role in all-solid-state battery performance. The evolution of this interface throughout cycling involves multiscale mechanical and chemical heterogeneity at the micro- and nano-scale. These features are dependent on operating conditions such as current density and stack pressure. Here we report the coupling of operando acoustic transmission measurements with nuclear magnetic resonance spectroscopy and magnetic resonance imaging to correlate changes in interfacial mechanics (such as contact loss and crack formation) with the growth of lithium microstructures during cell cycling. Together, the techniques reveal the chemo-mechanical behavior that governs lithium metal and Li₇La₃Zr₂O₁₂ interfacial dynamics at various stack pressure regimes and with voltage polarization.

Evaluation of Ga0.2Li6.4Nd3Zr2O12 garnets: exploiting dopant instability to create a mixed conductive interface to reduce interfacial resistance for all solid state batteries

The next major leap in energy storage is thought to arise from a practical implementation of all solid-state batteries, which remain largely confined to the small scale due to issues in manufacturing and mechanical stability. Lithium batteries are amongst the most sought after, for the high expected energy density and improved safety characteristics, however the challenge of finding a suitable solid-state electrolyte remains. Lithium rich garnets are prime contenders as electrolytes, owing to their high ionic conductivity (>0.1 mS cm^-1), wide electrochemical window (0-6 V) and stability with Li metal. However, the high Young's modulus of these materials, poor wetting of Li metal and rapid formation of Li₂CO₃ passivating layers tends to give a detrimentally large resistance at the solid-solid interface, limiting their application in solid state batteries. Most studies have focused on La based systems, with very little work on other lanthanides. Here we report a study of the Nd based garnet Ga_0.2Li_6.4Nd₃Zr₂O₁₂, illustrating substantial differences in the interfacial behaviour. This garnet shows very low interfacial resistance attributed to dopant exsolution which, when combined with moderate heating (175 °C, 1 h) with Li metal, we suggest forms Ga-Li eutectics, which significantly reduces the resistance at the Li/garnet interface to as low as 67 Ω cm² (much lower than equivalent La based systems). The material also shows intrinsically high density (93%) and good conductivity (≥0.2 mS cm^-1) via conventional furnaces in air. It is suggested these garnets are particularly well suited to provide a mixed conductive interface (in combination with other garnets) which could enable future solid-state batteries.

Interfacial Defect of Lithium Metal in Solid-State Batteries

All-solid-state battery with Li metal anode is a promising rechargeable battery technology with high energy density and improved safety. Currently, the application of Li metal anode is plagued by the failure at the interfaces between lithium metal and solid electrolyte (SE). However, little is known about the defects at Li-SE interfaces and their effects on Li cycling, impeding further improvement of Li metal anodes. Herein, by performing large-scale atomistic modeling of Li metal interfaces with common SEs, we discover that lithium metal forms an interfacial defect layer of nanometer-thin disordered lithium at the Li-SE interfaces. This interfacial defect Li layer is highly detrimental, leading to interfacial failure such as pore formation and contact loss during Li stripping. By systematically studying and comparing incoherent, coherent, and semi-coherent Li-SE interfaces, we find that the interface with good lattice coherence has reduced Li defects at the interface and has suppressed interfacial failure during Li cycling. Our finding discovered the critical roles of atomistic lithium defects at interfaces for the interfacial failure of Li metal anode, and motivates future atomistic-level interfacial engineering for Li metal anode in solid-state batteries.

X-ray Nanoimaging of Crystal Defects in Single Grains of Solid-State Electrolyte Li7-3xAlxLa3Zr2O12

All-solid-state lithium batteries promise significant improvements in energy density and safety over traditional liquid electrolyte batteries. The Al-doped Li₇La₃Zr₂O₁₂ (LLZO) solid-state electrolyte shows excellent potential given its high ionic conductivity and good thermal, chemical, and electrochemical stability. Nevertheless, further improvements on electrochemical and mechanical properties of LLZO call for an in-depth understanding of its local microstructure. Here, we employ Bragg coherent diffractive imaging to investigate the atomic displacements inside single grains of LLZO with various Al-doping concentrations, resulting in cubic, tetragonal, and cubic-tetragonal mixed structures. We observe coexisting domains of different crystallographic orientations in the tetragonal structure. We further show that Al doping leads to crystal defects such as dislocations and phase boundaries in the mixed- and cubic-phase grain. This study addresses the effect of Al doping on the nanoscale structure within individual grains of LLZO, which is informative for the future development of solid-state batteries.

Surface Potential Regulation Realizing Stable Sodium/Na3 Zr2 Si2 PO12 Interface for Room-Temperature Sodium Metal Batteries

Inorganic Na₃ Zr₂ Si₂ PO₁₂ is prospective with a high ionic conductivity but suffers large interfacial resistance and stability issues against sodium metal, hindering its practical application in all-solid-state sodium batteries. A surface potential regulation strategy is adopted to address these issues. Na₃ Zr₂ Si₂ PO₁₂ (NZSP) ceramic with homogeneously-sintered surface is synthesized by a simple two-step sintering method to promote its uniform surface potential, which is favorable for mitigating the potential fluctuations at the interface against Na metal and enhancing interfacial compatibility. The Na/NZSP interface can be stabilized for over 4 months with a low interfacial resistance of 129 Ω cm² at 25 °C. The symmetrical Na/NZSP/Na cell exhibits ultra-stable sodium platting/stripping cycling for over 1000 cycles under 0.1 mA cm^-2 . Superior interfacial performance is well retained even under 0.2 mA cm^-2 at room temperature. The robust interface is further signified by its excellence under higher current densities of up to 0.85 mA cm^-2 at 60 °C. A 4 V all-solid-state Na₃ V_1.5 Cr_0.5 (PO₄ )₃ /NZSP/Na metal battery is demonstrated at ambient conditions, which exhibits superior rate capability and delivers a high reversible capacity of 103 mA h g^-1 under 100 mA g^-1 for over 400 cycles with a Coulombic efficiency of over 99%.

Water based synthesis of highly conductive GaxLi7-3xLa3Hf2O12 garnets with comparable critical current density to analogous GaxLi7-3xLa3Zr2O12 systems

Next generation lithium ion batteries are envisaged as those which feature an all solid-state architecture. This will enable the higher energy density storage required to meet the demands of modern society, especially for the growing electric vehicle market. Solid state batteries have, however, proved troublesome to implement commercially due to the lack of a suitable solid-state electrolyte, which needs to be highly conductive, have a low interfacial resistance and a suitably wide electrochemical stability window. Garnet materials are potential contenders for these batteries, demonstrating many of the desired properties, although there remain challenges to overcome. Here we report a facile synthesis of Li₇La₃Hf₂O₁₂ and Ga/Al_xLi_7-3xLa₃Hf₂O₁₂ garnets, with the synthesis of Ga_0.2Li_6.4La₃Hf₂O₁₂ requiring only dissolution of precursors in water and heating to 700 °C. Ga_0.2Li_6.4La₃Hf₂O₁₂ was shown to display a high room temperature conductivity (0.373 mS cm^-1 at 28 °C). Moreover, in Li|garnet|Li cells, we observed a comparable critical current density compared to Ga_0.2Lai_6.4La₃Zr₂O₁₂, despite a lower density and higher area specific resistance compared to literature values, suggesting Hf systems may be further engineered to deliver additional improvements for use in future solid state batteries.

A facile method for the synthesis of a sintering dense nano-grained Na3Zr2Si2PO12 Na+-ion solid-state electrolyte

We report dense Na₃Zr₂Si₂PO₁₂ with an average grain size of 546 ± 58 nm and prepared by a facile method.

Li1.5La1.5MO6 (M = W6+, Te6+) as a new series of lithium-rich double perovskites for all-solid-state lithium-ion batteries

Solid-state batteries are a proposed route to safely achieving high energy densities, yet this architecture faces challenges arising from interfacial issues between the electrode and solid electrolyte. Here we develop a novel family of double perovskites, Li_1.5La_1.5MO₆ (M = W⁶⁺, Te⁶⁺), where an uncommon lithium-ion distribution enables macroscopic ion diffusion and tailored design of the composition allows us to switch functionality to either a negative electrode or a solid electrolyte. Introduction of tungsten allows reversible lithium-ion intercalation below 1 V, enabling application as an anode (initial specific capacity >200 mAh g^-1 with remarkably low volume change of ∼0.2%). By contrast, substitution of tungsten with tellurium induces redox stability, directing the functionality of the perovskite towards a solid-state electrolyte with electrochemical stability up to 5 V and a low activation energy barrier (<0.2 eV) for microscopic lithium-ion diffusion. Characterisation across multiple length- and time-scales allows interrogation of the structure-property relationships in these materials and preliminary examination of a solid-state cell employing both compositions suggests lattice-matching avenues show promise for all-solid-state batteries.

Li/Garnet Interface Optimization: An Overview

Solid-state lithium batteries can improve the safety and energy density of the present liquid-electrolyte-based lithium-ion batteries. To achieve this goal, both solid electrolyte and lithium anode technology are the keys. Lithium garnet is a promising electrolyte to enable the next generation solid-state lithium batteries due to its high ionic conductivity, good chemical, and electrochemical stability, and easiness to scale up. It is relatively stable against Li metal but the poor contact area and the presence of resistive impurity or decomposition layers at the interface interfere with fast charge transfer, thereby, spiking the interfacial resistance, overpotential, local current density, and the propensity for dendrite growth. In this Review, we first summarize the recent understanding of the interfacial problems at the Li/garnet interface from both computational and experimental viewpoints while seizing the opportunity to shed light on the chemical/electrochemical stability of garnet against Li metal anode. Also, we highlight various interface optimization strategies that have been demonstrated to be effective in improving the interface performance. We conclude this Review with a few suggestions as guides for future work.

Interface Between Solid-State Electrolytes and Li-Metal Anodes: Issues, Materials, and Processing Routes

Li metal, which has a high theoretical capacity and negative electrochemical potential, is regarded as the "holy grail" in Li-ion batteries. However, the flammable nature of liquid electrolyte leads to safety issues. Hence, the cooperation of solid-state electrolyte and Li-metal anode is demanded. However, the short cycle life induced by interfacial issues is the main challenge faced by their cooperation. In this review, dendrite and interfacial side reactions are comprehensively analyzed as the main interfacial problems. Meanwhile, the "state-of-the-art" interphase materials are summarized. The challenges faced by each kind of material are underscored. Moreover, different processing routes to fabricate artificial interphase are also investigated from an engineering perspective. The processing routes suitable for mass production are also underscored.

Research Progresses of Garnet-Type Solid Electrolytes for Developing All-Solid-State Li Batteries

All-Solid-State Batteries (ASSBs) that use oxide-based solid electrolytes (SEs) have been considered as a promising energy-storage platform to meet an increasing demand for Li-ion batteries (LIBs) with improved energy density and superior safety. However, high interfacial resistance between particles in the composite electrode and between electrodes and the use of Li metal in the ASBS hinder their practical utilization. Here, we review recent research progress on oxide-based SEs for the ASSBs with respect to the use of Li metal. We especially focus on research progress on garnet-type solid electrolytes (Li₇La₃Zr₂O₁₂) because they have high ionic conductivity, good chemical stability with Li metal, and a wide electrochemical potential window. This review will also discuss Li dendritic behavior in the oxide-based SEs and its relationship with critical current density (CCD). We close with remarks on prospects of ASSB.

Multiscale Lithium-Battery Modeling from Materials to Cells

New experimental technology and theoretical approaches have advanced battery research across length scales ranging from the molecular to the macroscopic. Direct observations of nanoscale phenomena and atomistic simulations have enhanced the understanding of the fundamental electrochemical processes that occur in battery materials. This vast and ever-growing pool of microscopic data brings with it the challenge of isolating crucial performance-decisive physical parameters, an effort that often requires the consideration of intricate interactions across very different length scales and timescales. Effective physics-based battery modeling emphasizes the cross-scale perspective, with the aim of showing how nanoscale physicochemical phenomena affect device performance. This review surveys the methods researchers have used to bridge the gap between the nanoscale and the macroscale. We highlight the modeling of properties or phenomena that have direct and considerable impact on battery performance metrics, such as open-circuit voltage and charge/discharge overpotentials. Particular emphasis is given to thermodynamically rigorous multiphysics models that incorporate coupling between materials' mechanical and electrochemical states.

Effect of Liquid Electrolyte Soaking on the Interfacial Resistance of Li7La3Zr2O12 for All-Solid-State Lithium Batteries

The impact of liquid electrolyte soaking on the interfacial resistance between the garnet-structured Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte and metallic lithium has been studied. Lithium carbonate (Li₂CO₃) formed by inadvertent exposure of LLZO to ambient conditions is generally known to increase interfacial impedance and decrease lithium wettability. Soaking LLZO powders and pellets in the electrolyte containing lithium tetrafluoroborate (LiBF₄) shows a significantly reduced interfacial resistance and improved contact between lithium and LLZO. Raman spectroscopy, X-ray diffraction, and soft X-ray absorption spectroscopy reveal how Li₂CO₃ is continuously removed with increasing soaking time. On-line mass spectrometry and free energy calculations show how LiBF₄ reacts with surface carbonate to form carbon dioxide. Using a very simple and scalable process that does not involve heat-treatment and expensive coating techniques, we show that the Li-LLZO interfacial resistance can be reduced by an order of magnitude.

Surface-Dependent Stability of the Interface between Garnet Li7La3Zr2O12 and the Li Metal in the All-Solid-State Battery from First-Principles Calculations

The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte is of particular interest because of its good chemical stability under atmospheric condition, suitable for practical all-solid-state batteries (ASSBs). However, recent works observed electrochemical instability at the LLZO/Li interfaces. Herein, we have revealed the origin of the instability by performing a comprehensive first-principles investigation with a high-throughput interface structure search scheme, based on the density functional theory framework. Based on the constructed phase diagrams of low-index surfaces, we found that the coordinatively unsaturated (i.e. coordination number < 6) Zr sites exist widely on the low-energy LLZO surfaces. These undercoordinated Zr sites are reduced once the LLZO surface is in contact with the Li metal, leading to chemical instability of the LLZO/Li interface. Besides, the calculated formation and adhesion energies of interfaces suggest that the Li wettability on the LLZO surface is dependent on the termination structure. The employment of the approaches such as by controlling the synthesis atmosphere are needed for preventing the reduction of LLZO against the Li metal. The present analysis with comprehensive first-principles calculations provides a novel perspective for the rational optimization of the interface between LLZO electrolyte and Li metal anode in the ASSB.

New Concepts in Electrolytes

Over the past decades, Li-ion battery (LIB) has turned into one of the most important advances in the history of technology due to its extensive and in-depth impact on our life. Its omnipresence in all electric vehicles, consumer electronics and electric grids relies on the precisely tuned electrochemical dynamics and interactions among the electrolytes and the diversified anode and cathode chemistries therein. With consumers' demand for battery performance ever increasing, more and more stringent requirements are being imposed upon the established equilibria among these LIB components, and it became clear that the state-of-the-art electrolyte systems could no longer sustain the desired technological trajectory. Driven by such gap, researchers started to explore more unconventional electrolyte systems. From superconcentrated solvent-in-salt electrolytes to solid-state electrolytes, the current research realm of novel electrolyte systems has grown to unprecedented levels. In this review, we will avoid discussions on current state-of-the-art electrolytes but instead focus exclusively on unconventional electrolyte systems that represent new concepts.

Nonaqueous Polymer Combustion Synthesis of Cubic Li7La3Zr2O12 Nanopowders

Garnet-type lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZO) shows great promise as a solid electrolyte for future solid-state lithium batteries as it possesses a uniquely beneficial combination of high ionic conductivity, electrochemical stability against metallic lithium, and generally low reactivity in ambient conditions. Conventionally synthesized by using solid-state reactions, LLZO powders have also been prepared by using variations of sol-gel or combustion synthesis with sacrificial organic templates or polymers containing metal nitrate precursors. Herein, a novel nonaqueous polymer (NAP) method using metalorganic precursors and poly(vinylpyrrolidone) is demonstrated to easily form LLZO nanopowders. Compared to similar techniques using aqueous solutions with metal nitrates, the NAP method confers greater control over synthesis conditions. Undoped cubic phase LLZO is obtained after calcination at 700-800 °C between 0 and 4 h, and the NAP process is easily extended to Ta-doped LLZO. To elucidate the general formation mechanism of nanosized LLZO in the NAP combustion synthesis, scanning transmission electron microscopy is used to perform energy dispersive X-ray and electron energy loss spectral imaging. The results show that in situ formation of a carbonaceous foam during combustion physically segregates pockets of reagents and is responsible for maintaining the small particle size of the as-synthesized material during combustion and crystallization. The room temperature ionic conductivity of nanosized Ta-doped LLZO synthesized by using the NAP method was studied under various sintering conditions, with ionic conductivities between 0.24 and 0.67 mS cm^-1, activation energies between 0.34 and 0.42 eV, and relative densities in excess of 90% obtained by sintering at 1100 °C for between 6 and 15 h.

Microstrain and electrochemical performance of garnet solid electrolyte integrated in a hybrid battery cell

Garnet type solid electrolytes are promising candidates for replacing the flammable liquid electrolytes conventionally used in Li-ion batteries. Al-doped Li₇La₃Zr₂O₁₂ (LLZO) is synthesized using nebulized spray pyrolysis and field assisted sintering technology (FAST), a novel synthesis route ensuring the preparation of samples with a homogeneous elemental distribution and dense ceramic electrolytes. Ceramic preparation utilizing field assisted sintering, in particular the applied pressure, has significant influence on the material structure, i.e. microstrain, and thereby its electrochemical performance. The phenomenon of microstrain enhancement of electrochemical performance might open a new route towards improved garnet solid electrolytes. A detailed mechanism is proposed for the lattice distortion and resulting microstrain during sintering. The charge transfer resistance of Li-ions at the interface between LLZO and Li is characterized using AC impedance spectroscopy and is amongst the best reported values to date. Additionally, the solid electrolyte is integrated in a full hybrid cell, a practical approach combining all the advantages of the solid electrolyte, while maintaining good contact with the cathode material.

Influence of induced microstrain due to pressure application during field assisted sintering on the electrochemical performance of Al-doped LLZO.

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 LiNi_0.5Co_0.2Mn_0.3O₂ 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.

Interface in Solid-State Lithium Battery: Challenges, Progress, and Outlook

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes promise improved safety, higher energy density, longer cycle life, and lower cost than conventional Li-ion batteries. However, their practical application is hampered by the high resistance arising at the solid-solid electrode-electrolyte interface. Although the exact mechanism of this interface resistance has not been fully understood, various chemical, electrochemical, and chemo-mechanical processes govern the charge transfer phenomenon at the interface. This paper reports the interfacial behavior of the lithium and the cathode in oxide and sulfide inorganic solid-electrolytes and how that affects the overall battery performance. An overview of the recent reports dealing with high resistance at the anodic and cathodic interfaces is presented and the scientific and engineering aspects of the approaches adopted to solve the issue are summarized.