Among the contenders in the new generation energy storage arena, all-solid-state batteries (ASSBs) have emerged as particularly promising, owing to their potential to exhibit high safety, high energy density and long cycle life. The relatively low conductivity of most solid electrolytes and the often sluggish charge transfer kinetics at the interface between solid electrolyte and electrode layers are considered to be amongst the major challenges facing ASSBs. This review presents an overview of the state of the art in solid lithium and sodium ion conductors, with an emphasis on inorganic materials. The correlations between the composition, structure and conductivity of these solid electrolytes are illustrated and strategies to boost ion conductivity are proposed. In particular, the high grain boundary resistance of solid oxide electrolytes is identified as a challenge. Critical issues of solid electrolytes beyond ion conductivity are also discussed with respect to their potential problems for practical applications. The chemical and electrochemical stabilities of solid electrolytes are discussed, as are chemo-mechanical effects which have been overlooked to some extent. Furthermore, strategies to improve the practical performance of ASSBs, including optimizing the interface between solid electrolytes and electrode materials to improve stability and lower charge transfer resistance are also suggested.

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New horizons for inorganic solid state ion conductors

Li deposition is observed and measured on a solid electrolyte in the vicinity of a metallic current collector. Four types of ion-conducting, inorganic solid electrolytes are tested: Amorphous 70/30 mol% Li2S-P2S5, polycrystalline β-Li3PS4, and polycrystalline and single-crystalline Li6La3ZrTaO12 garnet. The nature of lithium plating depends on the proximity of the current collector to defects such as surface cracks and on the current density. Lithium plating penetrates/infiltrates at defects, but only above a critical current density. Eventually, infiltration results in a short circuit between the current collector and the Li-source (anode). These results do not depend on the electrolytes shear modulus and are thus not consistent with the Monroe–Newman model for “dendrites.” The observations suggest that Li-plating in pre-existing flaws produces crack-tip stresses which drive crack propagation, and an electrochemomechanical model of plating-induced Li infiltration is proposed. Lithium short-circuits through solid electrolytes occurs through a fundamentally different process than through liquid electrolytes. The onset of Li infiltration depends on solid-state electrolyte surface morphology, in particular the defect size and density.

Mechanism of Lithium Metal Penetration through Inorganic Solid Electrolytes

All-solid-state batteries (ASSBs) have attracted enormous attention as one of the critical future technologies for safe and high energy batteries. With the emergence of several highly conductive solid electrolytes in recent years, the bottleneck is no longer Li-ion diffusion within the electrolyte. Instead, many ASSBs are limited by their low Coulombic efficiency, poor power performance, and short cycling life due to the high resistance at the interfaces within ASSBs. Because of the diverse chemical/physical/mechanical properties of various solid components in ASSBs as well as the nature of solid-solid contact, many types of interfaces are present in ASSBs. These include loose physical contact, grain boundaries, and chemical and electrochemical reactions to name a few. All of these contribute to increasing resistance at the interface. Here, we present the distinctive features of the typical interfaces and interphases in ASSBs and summarize the recent work on identifying, probing, understanding, and engineering them. We highlight the complicated, but important, characteristics of interphases, namely the composition, distribution, and electronic and ionic properties of the cathode-electrolyte and electrolyte-anode interfaces; understanding these properties is the key to designing a stable interface. In addition, conformal coatings to prevent side reactions and their selection criteria are reviewed. We emphasize the significant role of the mechanical behavior of the interfaces as well as the mechanical properties of all ASSB components, especially when the soft Li metal anode is used under constant stack pressure. Finally, we provide full-scale (energy, spatial, and temporal) characterization methods to explore, diagnose, and understand the dynamic and buried interfaces and interphases. Thorough and in-depth understanding on the complex interfaces and interphases is essential to make a practical high-energy ASSB.

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Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes.

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.

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

Solid state electrolyte systems boasting Li+ conductivity of >10 mS cm−1 at room temperature have opened the potential for developing a solid state battery with power and energy densities that are competitive with conventional liquid electrolyte systems. The primary focus of this review is twofold. First, differences in Li penetration resistance in solid state systems are discussed, and kinetic limitations of the solid state interface are highlighted. Second, technological challenges associated with processing such systems in relevant form factors are elucidated, and architectures needed for cell level devices in the context of product development are reviewed. Specific research vectors that provide high value to advancing solid state batteries are outlined and discussed.

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Review—Practical Challenges Hindering the Development of Solid State Li Ion Batteries

Li7La3Zr2O12 (LLZO) garnet is attracting interest as a promising Li-ion solid electrolyte. LLZO exists in a tetragonal and cubic polymorph where the cubic phase exhibits ∼2 orders of magnitude higher Li-ion conduction. It has been suggested that a critical Li vacancy concentration (0.4–0.5 atoms per formula unit) is required to stabilize the cubic polymorph of Li7La3Zr2O12. This has been confirmed experimentally for Al3+ doping on the Li+ site. Substitution of M5+ (M = Ta, Nb) for Zr4+ is an alternative means to create Li vacancies and should have the same critical Li vacancy concentration, nevertheless, subcritically doped compositions (0.25 moles of Li vacancies per formula unit) have been reported as cubic. Adventitious Al, from alumina crucibles, was likely present in these studies that could have acted as a second dopant to introduce vacancies. In this work, Al-free subcritically doped (Li6.75La3Zr1.75Ta0.25O12) and critically doped (Li6.5La3Zr1.5Ta0.5O12) compositions are investigated. X-ray diffraction indicates that both compositions are cubic. However, upon further materials characterization, including SEM analysis, Raman spectroscopy, Electrochemical Impedance Spectroscopy, and neutron diffraction it is evident that the subcritically doped composition is a mixture of cubic and tetragonal phases. The results of this study confirm that 0.4–0.5 Li vacancies per formula unit are required to stabilize the cubic polymorph of LLZO.

Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O12 (LLZO)

The effect of hot-pressing temperature on the microstructure and Li-ion transport of Al-doped, cubic Li7La3Zr2O12 (LLZO) was investigated. At fixed pressure (62 MPa), the relative density was 86%, 97%, and 99% when hot-pressing at 900°C, 1000°C, and 1100°C, respectively. Electrochemical impedance spectroscopy showed that the percent grain-boundary resistance decreased with increasing hot-pressing temperature. Hot pressing at 1100°C resulted in a total conductivity of 0.37 mS/cm at room temperature where the grain boundaries contributed to 8% of the total resistance; one of the lowest grain-boundary resistances reported. We believe hot pressing is an appealing technique to minimize grain-boundary resistance and enable correlations between LLZO composition and bulk ionic conductivity.

Isabel N. David

Papers

Tetragonal vs. cubic phase stability in Al – free Ta doped Li7La3Zr2O12 (LLZO)

Microstructure and Li‐Ion Conductivity of Hot‐Pressed Cubic Li7La3Zr2O12

A preliminary investigation of fracture toughness of Li7La3Zr2O12 and its comparison to other solid Li-ionconductors