Electrolytes For Solid-state Batteries Research Articles

Modeling study of stress generation of a single active material particle connected to solid electrolyte in solid-state batteries

One of the major problems with solid-state batteries (SSBs) is the mechanical degradation of the interfacial structures between active materials (AMs) and solid electrolytes (SEs). In this study, we established a single particle model to study stress generation of AM particles surrounded by SEs and their impact on the mechanical degradation of SSBs including Li7La3Zr2O12 (LLZO) or Li10GeP2S12 (LGPS). When the AM particle was constrained by the SE, the first principal stress of the AM particle was significantly higher than that of the AM particle in the liquid electrolyte. The changes in the direction and magnitude of the stress strongly depended on the locations of the AM/SE. The largest change in the stress was often observed in the interface of the SE and the trend is dependent on the Young's modulus ratio of AM to SE. Because the interface between the AM and SE experiences the most significant mechanical degradation, it is crucial to find the optimal combination of AMs and SEs based on the mechanical properties for the design of SSBs to reduce the probability of failure.Moreover, we investigated the effect of the material properties, porosity, and contact area between AMs and SEs on the lithium transport and stress evolution. To minimize the mechanical degradation of SSBs, it is necessary to increase the AM/SE contact ratio and to achieve homogeneous AM particle distribution that enables a higher local volume fraction of SE. The results of this study can provide valuable insights into the fracture behavior of SSBs and guide the electrode design to minimize the mechanical degradation.

Excellent Cathode Interfaces Achieved by Atomic/Molecualr Layer Depositions for Sulfide-Based All-Solid-State Batteries

All-solid-state Lithium batteries (ASSLBs) are attracting a large amount of attention due to the improved safety and high energy density compared with conventional liquid electrolyte-based Li-ion batteries (LIBs). Sulfide-based solid-state electrolytes (SSEs) exhibit ultrahigh ionic conductivity up to 10-2 S cm-1 at room temperature (RT). [1] However, the interface between sulfide SSEs and oxide cathode materials is considered unstable. The reason of the poor cathode interface stability is originated form the low oxidation potential of sulfide SSEs. This causes big chemical-potential gap between sulfides and oxide cathode materials, which would cause the formation of space-charge layer and increase the interfacial impedance. [1] Additionally, the low oxidation limit of sulfides leads to severe decomposition of SSEs at the cathode interface, thus affecting the interfacial Li+ transport. Designing coating materials for cathode particles is the most common strategy to alleviate the cathode interface problems. [2]Atomic/molecular layer deposition (ALD/MLD) is an advanced film fabricating technique, capable of tuning the interface property with atomic-level thickness control at relatively low temperature (< 400 oC). Compared with the conventional wet chemical method (e.g., sol-gel method), the ALD/MLD method can not only show the ability of developing Li-containing ternary oxides and organic coatings with uniform and conformal features, but also completely avoid the negative effects of using solvents and high-temperature treatments. [2-4] In our work, we employed ALD/MLD to elaborately design LiNbOx, LiZrOx, LiPOx, and PEDOT coating materials to solve the interface problem between sulfide SSEs and oxide cathode materials. [5-8] Ion-conducting and electron-insulating ALD coating layers can prevent the interfacial side reactions and improve the Li+ transport, while the semiconductor MLD layer can suppress the interfacial degradation. Therefore, excellent performance of sulfide-based ASSLBs was obtained. Ex/in-situ synchrotron X-ray characterizations (XANES, EXAFS, STXM) were used to analyze the local structure of ALD/MLD coatings and reveal the working mechanism of designed interfaces. The flexible design of functional cathode interface realized by ALD/MLD coatings paves the way to achieve advanced sulfide-based ASSLBs. Re ferences C. Yu, F. Zhao, X. Sun*, et al. Recent Development of lithium argyrodite solid-state electrolytes for solid-state batteries: synthesis, structure, stability and dynamics. Nano Energy 2021, 83, 105858.Y. Zhao, F. Zhao, X. Sun*, et al. Atomic/molecular layer deposition for energy storage and conversion. Chem. Soc. Rev., 2021, 50, 3889-3956.Y. Zhao, X. Sun*, et al. Addressing interfacial issues in liquid-based and solid-state batteries by atomic and molecular layer deposition, Joule 2018, 2, 2583-2604.Y. Zhao, X. Sun*. Molecular Layer Deposition for Energy Conversion and Storage, ACS Energy Lett. 2018, 3, 899-914.F. Zhao, X. Sun*, et al. Tuning bifunctional interface for advanced sulfide-based all-solid-state batteries. Energy Storage Mater. 2020, 33, 139-146.C. Wang, F. Zhao, X. Sun* et al. Manipulating Interfacial Nanostructure to Achieve High-Performance All-Solid-State Lithium-Ion Batteries, Small M e thods, 2019, 3, 1900261.S. Deng, X. Sun*, et al. Dual-functional interfaces for highly stable Ni-rich layered cathodes in sulfide all-solid-state batteries Energy Storage Mater. 2020, 27, 117-123.S. Deng, X. Sun*, et al. Eliminating the Detrimental Effects of Conductive Agents in Sulfide-Based Solid-State Batteries, ACS Energy Lett. 2020, 5, 1243-1251.

Progress and Opportunities in the Application of Cold Sintering to Solid-State, Electrochemical, and Ceramic Devices

The high temperature conventional sintering process has been practiced by humans for thousands of years, but only in recent decades has there been significant progress in modifying the sintering process to achieve lower temperatures or shorter dwell times.In this talk, we focus on the newest addition to the family of alternative sintering techniques; cold sintering. Cold sintering is characterized by the addition of a small amount of a transient sintering aid, such as an alkali hydroxide, to the parent ceramic powder. The mixture is then subjected to low temperatures (Tsinter < 400°C) and moderate uniaxial pressure (100 - 300 MPa) which results in a remarkable degree of densification for a wide range of ceramics. Broadly speaking, cold sintering is able to densify even the most refractory technical ceramics at around 30% of the melting temperature, whereas conventional wisdom asserts that the solid-state sintering process requires temperatures near 70% of the melting temperature.To illustrate the benefits of this dramatic reduction in sintering temperature, we will present recent examples of cold sintering applied to conductive ceramics for high temperature electrochemical devices. These examples include sodium ion electrolytes for solid-state batteries (e.g. Na3Zr2Si2PO12, β’’-Al2O3) and oxygen-ion electrolytes for solid oxide fuel cells (e.g. CeO2). For these electrolytes, we illustrate how the density and conductivity of the cold sintered electrolytes is comparable to the conventionally fired ceramics. In doing so, we also discuss the opportunities and open questions surrounding these ceramic materials which have been densified at low temperatures.These examples show how the elimination of elemental volatilization and grain size control can be easily achieved with lower temperature processing. We will conclude with a forward-looking assessment of some of the opportunities that low temperature sintering may enable, such as co-firing device layers and the inclusion of thermally fragile additives into the sintered ceramics. The low temperatures afforded by cold sintering presents new opportunities for the processing of many ceramic-dominated solid-state electrochemical devices.Figure Caption: The relative density is plotted as a function of sintering temperature for the prototypical refractory solid electrolyte, sodium-ion conducting β’’-Al2O3. Conventional sintering methods include hot pressing, spark plasma sintering, liquid phase sintering, and solid-state sintering. Cold sintering achieves high densities at exceedingly low temperatures for this refractory material. Figure 1

High-Performance Sulfide-based Solid-state Electrolytes and Interfaces for All-solid-state Li Batteries

All-solid-state Lithium metal batteries (ASSLMBs) cannot only maximize the energy density, but also show improved safety compared with the conventional liquid electrolyte-based Li-ion batteries. Solid-state electrolytes (SSEs), as the most important component in the ASSLMBs, can determine the electrochemical performance. High ionic conductivity is one of the most basic requirements for an excellent SSE. Sulfide-based SSEs exhibit ultrahigh ionic conductivity (> 10-2 S cm-1 at room temperature) that can be comparable to the liquid electrolytes, making this kind of SSEs popular and promising.1 However, air sensitivity and Li metal incompatibility are two major obstacles, which are together with the problematic cathode interface, hindering the development of sulfide SSEs for practical ASSLMBs.2, 3 In our work, we employed the strategy of element substitution to design F-substituted Li6PS5Cl, Sn-substituted Li6PS5I, and Sn-substituted Li3PS4 as high-performance Li-metal compatible sulfide SSEs.4-6 Using these element doping SSEs can derive functional interphases that contain LiF, LiI, and Li-Sn alloy, respectively. These interfacial contents were demonstrated as important to achieve stable Li plating/stripping. Importantly, we found that replacing P with Sn could significantly improve the air stability and ionic conductivity at the same time. The high reaction energy between the Sn-substituted PS4 tetrahedra and moisture was regarded as the main reason for the improved air stability, while the expanded unit cell and increased Li+ concentration led to superior ionic conductivity. Our modifications on various common-used sulfide SSEs with multi-function elements provide more choices of sulfide SSEs for practical ASSLMBs. At the cathode side,7-10 we used interface modification (coating strategy) to improve the interfacial Li+ transport and chemical inertness. Thus, excellent electrochemical performance was achieved. Re ferences N. Kamaya, R. Kanno*, et al. A lithium superionic conductor, Nat. Mater. 2011, 10, 682-686.C. Yu, F. Zhao, X. Sun*, et al. Recent Development of lithium argyrodite solid-state electrolytes for solid-state batteries: synthesis, structure, stability and dynamics. Nano Energy, 2021, 83, 105858.Y. Zhao, F. Zhao, X. Sun*, et al. Atomic/molecular layer deposition for energy storage and conversion. Chem. Soc. Rev., 2021, 50, 3889-3956.F. Zhao, X. Sun*, et al. Ultrastable anode interface achieved by fluorinating electrolytes for all-solid-state Li metal batteries, ACS Energy Lett. 2020, 5, 1035-1043F. Zhao, X. Sun*, et al. A Versatile Sn-Substituted Argyrodite Sulfide Electrolyte for All-Solid-State Li Metal Batteries, Adv. Energy Mater. 2020, 10, 1903422.F. Zhao, X. Sun*, et al. An Air-Stable and Li-Metal-Compatible Glass-Ceramic Electrolyte enabling High-Performance All-Solid-State Li-Metal Batteries. Adv. Mater. 2021, 33, 2006577.F. Zhao, X. Sun*, et al. Tuning bifunctional interface for advanced sulfide-based all-solid-state batteries. Energy Storage Mater. 2020, 33, 139-146.C. Wang, F. Zhao, X. Sun* et al. Manipulating Interfacial Nanostructure to Achieve High-Performance All-Solid-State Lithium-Ion Batteries, Small Methods, 2019, 3, 1900261.S. Deng, X. Sun*, et al. Dual-functional interfaces for highly stable Ni-rich layered cathodes in sulfide all-solid-state batteries Energy Storage Mater. 2020, 27, 117-123.S. Deng, X. Sun*, et al. Eliminating the Detrimental Effects of Conductive Agents in Sulfide-Based Solid-State Batteries, ACS Energy Lett. 2020, 5, 1243-1251.

Coral-like Li7La3Zr2O12-Filled PVDF-HFP/LiODFB Composite Electrolytes for Solid-State Batteries with Excellent Cycle Performance

Coral-like Li₇La₃Zr₂O₁₂-Filled PVDF-HFP/LiODFB Composite Electrolytes for Solid-State Batteries with Excellent Cycle Performance

Robust and high thermal-stable composite polymer electrolyte reinforced by PI nanofiber network

To address the flammable and chemical unstable problems of liquid electrolyte, the solid electrolyte is a promising candidate to replace liquid electrolyte for solid-state batteries. Herein, a composite polymer electrolyte (CPE) of 3D polyimide (PI)-nanofiber membrane-incorporated polyethylene oxide (PEO)/lithium bis (triflu-romethanesulphonyl) imid (LiTFSI) is reported. Three advantages of the PI nanofiber network in the CPE include providing a continuous, rapid transport channel of lithium ions to improve the Li-ion conductivity, improving the mechanical properties and stability, and effectively inhibiting the dendrite growth of Li metal. The PI/PEO/LiTFSI CPE delivers an ionic conductivity of 4.2 × 10−4 S cm−1 at 60 °C, a wider electrochemical window to 5.4 V, and an excellent thermal stability, which result in the excellent electrochemical performance of LiFePO4 full cells assembled with PI/PEO/LiTFSI CPE.

Lithium-Conducting Branched Polymers: New Paradigm of Solid-State Electrolytes for Batteries.

The past decades have witnessed rapid development of lithium-based batteries. Significant research efforts have been progressively diverted from electrodes to electrolytes, particularly polymer electrolytes (PEs), to tackle the safety concern and promote the energy storage capability of batteries. To further increase the ionic conductivity of PEs, various branched polymers (BPs) have been rationally designed and synthesized. Compared with linear polymers, branched architectures effectively increase polymer segmental mobility, restrain crystallization, and reduce chain entanglement, thereby rendering BPs with greatly enhanced lithium transport. In this Mini Review, a diversity of BPs for PEs is summarized by scrutinizing their unique topologies and properties. Subsequently, the design principles for enhancing the physical properties, mechanical properties, and electrochemical performance of BP-based PEs (BP-PEs) are provided in which the ionic conduction is particularly examined in light of the Li+ transport mechanism. Finally, the challenges and future prospects of BP-PEs in this rapidly evolving field are outlined.

Ultrathin polymer-in-ceramic and ceramic-in-polymer bilayer composite solid electrolyte membrane for high-voltage lithium metal batteries

Solid-state electrolyte for solid-state batteries is one of the current hotspots in the research field of high-safety and high energy density lithium metal batteries. This work reports a polymer-in-ceramic and ceramic-in-polymer bilayer ultrathin (∼11 μm) composite solid electrolyte (CSE) membrane by exploiting a spontaneous precipitation of Li7La3Zr2O12 in a polymer-lithium salt matrix. The polymer-in-ceramic layer is toward a high-voltage cathode, which can effectively inhibit oxidative decomposition of electrolytes. The ceramic-in-polymer layer toward the anode can provide good flexibility. Experimental results show that the CSE bilayer membrane has not only high ionic conductivity but also satisfactory Li+ transference number. It also exhibits excellent thermal and mechanical properties. High-voltage Li||LiCoO2 (4.5 V) cell employing this CSE membrane exhibits excellent cycling stability (151.18 mAh g−1 at 1 C after 100 cycles), and superior rate performance (124.31 mAh g−1 10 C). This work guides the improvement of both solid state electrolyte membrane and high-voltage solid-state lithium metal batteries.

Industry Chain and Technology Trends in China’s Solid-state Battery Industry

Solid-state battery is different from traditional lithium-ion battery, which is a kind of battery using solid electrode and solid electrolyte, and it has the advantages of high safety, long life, high charging and discharging efficiency, good high temperature resistance, simple assembly and processing, and easy to scale up. This paper first analyzes the industrial chain of solid-state batteries in China and the stakeholders in the process of industrial development, and finally draws a technology roadmap for the development of China’s solid-state battery industry based on industrial characteristics. In the future, along with energy transformation and national policy support, the focus should be on solid state electrolytes for solid state batteries, and gradually realize the development path of solid state battery electrolytes from liquid state of traditional lithium-ion batteries to solid-liquid hybrid, gel, solid state, and all-solid state.

Testing with Thin Lithium Anode and Practical Capacities for Fast Evaluation of Polymer Electrolytes for Solid-State Batteries

Lithium metal solid-state batteries can reach greater than 400 Wh/kg energy density compared to ~300 Wh/kg of conventional lithium-ion batteries. However, the amount of inactive material needs to be significantly reduced, and active material loading maximized. For example, the lithium metal anode needs to be <10 µm, the solid electrolyte separator layer needs to be <20 µm, and the cathode capacity >4 mAh/cm2. Several new inorganic and polymeric electrolytes have been discovered in the past decade. Nevertheless, the integration and testing of these novel candidate materials into functioning cells with the electrode loadings and dimensions required to meet the cell-level energy density goals have been limited. For example, typically, hundreds of microns thick lithium foils are used, which can hide any lithium losses at the anode due to reactions or physical isolation and lead to artificially long cycle life. The amount of lithium cycled is also typically too low (<1 mAh/cm2) compared to the desired 4 mAh/cm2 in both symmetric and full-cell tests. Such testing makes it difficult to gauge the progress of the field in reaching practical energy density goals. We argue that testing new materials solutions at full-cell level with practical electrode loading/dimensions can help understand their limitations quicker and progress faster towards reaching the cell-level energy density goal of >400 Wh/kg.In this work, using two polymer electrolytes (a “dry” and a gel-type), we present several examples that demonstrate the impact on solid-state battery performance as electrode thicknesses/loadings and cycling capacities are shifted towards practical values. First, using thermally-evaporated ultra-thin (1-10 µm) lithium metal anodes, we quantified the lithium losses during cycling at the lithium/polymer-electrolyte interface. Our results show severe lithium losses or ultra-low coulombic efficiency of a common gel-polymer electrolyte leading to complete consumption of the lithium reservoir within a few cycles. Second, we present examples that highlight the challenges of increasing cycling capacities towards practical values. These examples underscore the need to test novel materials solutions directly with such practical capacities, either in full-cell or symmetric-cell formats, to accelerate the materials development endeavors.

On the quality of tape-cast thin films of sulfide electrolytes for solid-state batteries

All-solid-state lithium batteries (ASSLBs) have the potential to increase energy density, improve safety, and allow for lower manufacturing costs compared to conventional, liquid-based Li-ion batteries. The thickness of solid electrolyte (SE) layer dictates the cell-level energy density and it is desirable to make the SE layer as thin as possible while maintaining uniformity and defect-free. Manufacturing a high-quality, thin sulfide SE layer at large-scale, however, is challenging. Previous studies have addressed the compatibility of materials used for manufacturing thin sulfide SE films, paving the way for further investigation of processing conditions and film quality. Here we report a strong correlation between the solid loading of dispersions and the quality of tape-casted thin sulfide SE films. We also demonstrate a method for quantifying the quality of thin SE films by observing both pin-hole defects and larger heterogeneous agglomerations of particles in the films. Our thin sulfide SE films containing ∼5 wt% binder are defect-free and show similar ionic conductivity compared to a cold-pressed, binder-free, thick SE pellet, resulting in an ∼11X reduction of area specific resistance. This work on the solid loading of the dispersion used in a scalable tape casting process provides insight for manufacturing high-quality, thin sulfide SE films and to increase the cell-level energy density of ASSLB.

Structure and Dynamics of LiPON and NaPON Oxynitride Phosphate Glasses by Solid-State NMR

Oxynitride phosphate glasses are a subfield of phosphate-based glasses, where part of the oxygen of the network has been replaced by nitrogen, in a process commonly called nitridation, of the phosphate melt. Because of this, the properties of the glasses are greatly enhanced, notably mechanical and chemical resistance, which has resulted in an important field with applications in glass-to-metal sealing or as electrolytes for solid-state batteries when processed as thin films. Whereas the properties of oxynitride phosphates have been thoroughly researched in view of their prospected applications, the elucidation of their structure remains uncomplete in some aspects, notably in the way that the network building groups are interconnected as well as from the point of view of the role of the modifiers onto the nitrided network structure. In the present work, 1D, 2D, REDOR, and INADEQUATE NMR experiments have been complemented with high-temperature NMR of lithium and sodium oxynitride phosphate glasses to study the structural arrangement of the building units and its evolution with temperature. The results have evidenced that the network organization adopted by the glasses distinctively depends on the nature of the modifier cation. However, the results exclude any significant phase separation or segregation of oxide and oxynitride regions, despite some clustering of nitride species may be occurring, mostly in the lithium-containing glasses.

Interfacial barrier free organic-inorganic hybrid electrolytes for solid state batteries

Organic-inorganic hybrid solid electrolytes (HSEs) are expected to overcome the inherent limitations of rigid fragile inorganic electrolytes for solid state batteries. Li-ion conductive filler such as garnet Li7La3Zr2O12 (LLZO) is proposed for the high performance of HSEs, unfortunately, which suffers from native surface layer resistance to Li-ion transport. Here we present highly conductive polyvinylidene fluoride (PVDF)-based HSEs incorporating LLZO fillers, whose resistive barriers are eliminated by dry etching. Our optimal composition of etched LLZO fillers (30 wt%) leads to ionic conductivity of 4.05 × 10-4 S cm-1, about two-fold improvement from non-etched counterpart. Li symmetric cells with etched fillers exhibit low interfacial resistance of 110 Ω cm2 and minimal overpotential of 90 mV. Moreover, high capacity of 79 mA h g-1 is highlighted at 4C, comparable or superior to liquid electrolyte or sulfide-based electrolyte devices. Interfacial environment in HSEs ideally modified for Li-ion transport is identified by 7Li NMR measurements.

Recent development of lithium argyrodite solid-state electrolytes for solid-state batteries: Synthesis, structure, stability and dynamics

Abstract Solid electrolyte plays a key role to enable good safety reliability and high performance of all-solid-state lithium batteries. Among the diverse solid electrolytes, argyrodites represent a relatively new and promising class of sulfide-based lithium-ion superconductors due to their high ionic conductivity at room temperature, low cost and good compatibility towards Li metal. However, lithium argyrodites remain challenging for practical applications despites of the continuous improvements from research. Herein, we review the recent research advancements and limitations of lithium argyrodites from material to battery level. Material synthesis based on solid and liquid routes are summarized. The chemical structures of lithium argyrodites, their lithium-ion transport mechanism and various optimization strategies for high ionic conductivity are discussed. The chemical and electrochemical stabilities of lithium argyrodites towards cathode and anode materials and corresponding improvement approaches are discussed in detail. The perspectives on future research directions of lithium argyrodites are provided.

Processing thin but robust electrolytes for solid-state batteries

The widespread adoption of high-energy-density solid-state batteries (SSBs) requires cost-effective processing and the integration of solid electrolytes of about the same thickness as the polymer-membrane separators found in conventional lithium-ion batteries. In this Review, we critically discuss the current status of research on SSB processing as well as recent cost calculations, and compare SSB oxide electrolyte material and processing options in terms of performance parameters for thick versus thin ceramics. We identify as critical for future SSB design the need to capture the thermal processing budget and the stability of the phase of interest for oxide solid electrolytes, namely lithium phosphorus oxynitride, sodium superionic conductors, perovskites and garnets, in addition to the classic plots of Arrhenius lithium transport and the electrochemical stability window. Transitioning to SSB oxide electrolyte films with thicknesses close to the range for lithium-ion battery separators could provide ample opportunities for low-temperature ceramic manufacture and potential cost reduction. High-performance solid-state electrolytes are key to enabling solid-state batteries that hold great promise for future energy storage. The authors survey the fabrication process of thin-film versus thick oxide-based solid-state electrolytes and discuss their material design and processing options.

Influence of Co2+ on the Structure, Conductivity, and Electrochemical Stability of Poly(Ethylene Oxide)-Based Solid Polymer Electrolytes: Energy Storage Devices

In the present work, using a simple solution cast technique, a series of poly(ethylene oxide) (PEO)-doped cobalt chloride (CoCl2) solid polymer electrolytes (SPEs) were successfully prepared. The effect of dopant on the morphology, structure, thermal, and electrochemical stability of the PEO films was systematically studied, and their ionic conductivity was examined. The Fourier transform infrared spectroscopy data provide evidence of the complex nature and existence of various microscopic interactions. The PEO ionic conductivity of 8.6 × 10−8 S cm−1 was found to increase to 3.5 × 10−3 S cm−1 upon the inclusion of 5 wt.% of CoCl2 at 303 K. An effort was made to understand the enhanced conductivity. Several studies were utilized to better understand the fundamental interplay of ion content and segmental motion using the Vogel–Tammann–Fulcher equation with typical investigations based on the fit of temperature-dependent conductivity data. The activation energy (Ea) decreased with increasing dopant concentration. The PCL5 transfer number (tion) was determined to be 0.93, evidence of the ionic nature of the doped electrolyte. Further, the purity and electrochemical stability of SPEs were studied using cyclic voltammetry and chronocoulometry. The thermal analysis showed reduced crystallinity and changes in glass transition and melting temperature at lower temperature, indicating enhanced amorphous content, thus confirming faster ion conduction. These SPEs with excellent electrical performance are promising candidates for electrolytes in solid-state batteries and other energy storage devices.

Cold Sintering for High-Temperature Electrochemical Applications

Although cold sintering remains an emerging processing technique, it is noteworthy to consider recent progress in materials that are of interest to electrochemical systems with elevated service temperatures. Here we discuss a broad number of materials ranging from alkali electrolytes for solid state batteries, to oxygen ion conductors for gas sensing. We also discuss the opportunity to co-fire mixed ionic conductors into multilayered devices. Given the low-temperature sintering temperatures with the cold sintering process (CSP), we will discuss some insights into operating ceramics at temperatures near or above their processing temperature.

Understanding the Structural Disorder Related to Ionic Conductivity Enhancement in Anti-Perovskite Ion Conductors

Amongst known families of solid electrolytes of potential interest for solid-state batteries, the anti-perovskites (APs) have recently attracted great attention for the high ionic conductivities observed in certain compositions, and, like perovskites, the compositional flexibility provided by the possibility for ion substitution onto multiple lattice sites. [1] For example, variations in the extent of disorder, that are accompanied by changes in the distribution of lithium migration energies.[2] In surveying transport in this family of ion conductors, we observed a trend wherein numerous compositions show upwards curvature on an Arrhenius plot, indicating a non-constant activation energy. This is unusual as most solids exhibiting non-Arrhenius behavior exhibit a negative deviation. The structure origins of such deviations are not well understood.Among the AP-structure SSEs, Li2OHCl and its variants are particularly interesting because of their relatively high conductivities at room temperature. Unlike Li3OCl, where six Li atoms occupy the vertices of the Li6O octahedral, in Li2OHX the vertices are occupied by four lithium atoms with two other sites vacant. H in Li2OHX sits at the oxygen atom in the center of the octahedron. The Li defects and the movement of H around the oxygen atom give rise to a change in the ionic conduction pathway as temperature varies (Figure 1). Figure 1. The ionic conduction pathway of Li and H in Li2OHCl at different temperatureHerein, we explain the non-Arrhenius conduction behavior in relation to defect and H movement, and our attempts to understand the structural origin(s) of this behavior through temperature-dependent X-ray and neutron scattering, calorimetry, and other methods. Implications for the design of solid electrolytes for solid state batteries will be discussed.This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Reference s : [1] Zhao, Y.; Daemen, L. L. Superionic conductivity in Lithium-rich anti-perovskites. J. Am. Chem. Soc. 2012, 134, 15042−15047.[2] Kwangnam Kim and Donald J. Siegel. "Correlating lattice distortions, ion migration barriers, and stability in solid electrolytes." Journal of Materials Chemistry A 7, no. 7 (2019): 3216-3227. Figure 1

Phase Behavior in Nasicon Electrolytes and Electrodes

An avenue to create safer batteries is to replace the liquid electrolyte with a non-flammable solid electrolyte.1–3 A good candidate is the Natrium superionic conductor (NaSiCON) Na1+xZr2SixP3-xO12 (0 x 3) which displays high bulk ionic conductivity and good relative stability towards other NaSiCON-based electrodes.2 Despite the sizeable share of research on the Na1+xZr2SixP3-xO12 material, the structural properties of NaSiCON are still poorly understood and as a result the optimisation of this class of materials often follows chemical intuition.4–9 Here, we analyse the phase behaviour of the NaSiCON electrolyte by constructing the Na1+xZr2SixP3-xO12 phase diagram as a function of temperature and composition (0 x 3) for the high-temperature rhombohedral phase. This phase is also common in several Na-based positive electrodes, such as Na3Ti2(PO4)3, Na3V2(PO4)3 and Na3Cr2(PO4)3. Using a multi-scale approach, based on density functional theory, the cluster expansion formalism and Monte Carlo simulations, we elucidate: i) the entire phase-diagram of NaSiCON as a function of temperature and Na content, identifying the regions providing the highest Na+-ion conductivity; ii) previously, unreported phase-separation behaviour in a specific region of the phase diagram, iii) the relationship between the population of Na-sites and the relative ratio of Si:P in the structure. We reveal that kinetic effects hinder the expected mechanism of phase separation in Na1+xZr2SixP3-xO12. We then extended these principles between the competition of thermodynamic and kinetic driving forces derived on Na1+xZr2SixP3-xO12 to popular mono-transition metal NaSiCON electrodes, which all tend to phase separate upon Na extraction/insertion. These results are important for the development of inexpensive Na-ion batteries.(1) Janek, J.; Zeier, W. G. A Solid Future for Battery Development. Nature Energy 2016, 1 (9), 16141. https://doi.org/10.1038/nenergy.2016.141.(2) Famprikis, T.; Canepa, P.; Dawson, J. A.; Islam, M. S.; Masquelier, C. Fundamentals of Inorganic Solid-State Electrolytes for Batteries. Nat. Mater. 2019, 18, 1278–1291. https://doi.org/10.1038/s41563-019-0431-3.(3) Canepa, P.; Dawson, J. A.; Sai Gautam, G.; Statham, J. M.; Parker, S. C.; Islam, M. S. Particle Morphology and Lithium Segregation to Surfaces of the Li7La3Zr2O12 Solid Electrolyte. Chemistry of Materials 2018, 30 (9), 3019–3027. https://doi.org/10.1021/acs.chemmater.8b00649.(4) Hong, H. Y.-P. Crystal Structures and Crystal Chemistry in the System Na1+xZr2SixP3−xO12. Materials Research Bulletin 1976, 11 (2), 173–182. https://doi.org/10.1016/0025-5408(76)90073-8.(5) Boilot, J. P.; Collin, G. Relation Structure-Fast Ion Conduction in the NASICON Solid Solution. J. Solid State Chem. 1988, 73, 160–171.(6) Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591. https://doi.org/10.1021/cr3001862.(7) Ma, Q.; Guin, M.; Naqash, S.; Tsai, C.-L.; Tietz, F.; Guillon, O. Scandium-Substituted Na3Zr2(SiO4)2(PO4) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors. Chemistry of Materials 2016, 28 (13), 4821–4828. https://doi.org/10.1021/acs.chemmater.6b02059.(8) Deng, Y.; Eames, C.; Nguyen, L. H. B.; Pecher, O.; Griffith, K. J.; Courty, M.; Fleutot, B.; Chotard, J.-N.; Grey, C. P.; Islam, M. S.; Masquelier, C. Crystal Structures, Local Atomic Environments, and Ion Diffusion Mechanisms of Scandium-Substituted Sodium Superionic Conductor (NASICON) Solid Electrolytes. Chem. Mater. 2018, 30 (8), 2618–2630. https://doi.org/10.1021/acs.chemmater.7b05237.(9) Zhang, Z.; Zou, Z.; Kaup, K.; Xiao, R.; Shi, S.; Avdeev, M.; Hu, Y.-S.; Wang, D.; He, B.; Li, H.; Huang, X.; Nazar, L. F.; Chen, L. Correlated Migration Invokes Higher Na+-Ion Conductivity in NaSICON-Type Solid Electrolytes. Advanced Energy Materials 2019, 9 (42), 1902373. https://doi.org/10.1002/aenm.201902373.

(Keynote) The Role of Interlayer Chemistry in Li-Metal Growth through a Garnet-Type Solid Electrolyte

Although garnet-type LLZO has been considered one of the most promising solid electrolyte for solid-state batteries (SSB), the instability of electrode/LLZO interface has been obstacle for the practical application of LLZO into the batteries.[1] Thus, a fundamental understanding of the lithium deposition behavior in the interface would aid in elucidating the underlying mechanisms of the short circuit failure due to unstable interface and designing the interface in SSBs. Herein, we successfully used an in operando microscopy technique to probe Li deposition through the LLZO electrolyte in an anode-free solid-state battery setup. More importantly, we carefully examined the interface with artificial interlayers, which revealed that Li plating is strongly dominated by the kinetics of alloying and precipitation through the metal interlayer. In addition, we confirmed that the interlayer also affects the sequential stripping process, influencing the electrochemical performance of the cell. Supported by these intriguing observations, we propose the dynamic roles of the interlayer during battery operation: as a buffer layer and a seed layer.The in-house cell and microscope system shown in Figure 1a were used for the in operando observations of Li deposition on LLZO. Li foil was attached to the bottom of the LLZO pellet and the top surface was pre-coated with the selected interlayer metal. We investigated electrochemical Li deposition behavior in the absence of an interlayer metal. Li metal begins to appear in island shapes under an applied galvanostatic current (0.1 mA cm-2) and continues to grow over time as shown in Figure 1b. It is worth noting that all of the small Li-metal islands first form at pre-existing defects on the pristine LLZO surface during initial lithiation and Li grows preferably at these islands during the subsequent plating. When the metal interlayer was introduced, the deposition behavior was significantly altered. With 30-nm-thick Au layer, we observed that Au interlayer changed color under electrochemical bias, which is indicative of the formation of a Li-Au alloy. These color changes are followed by the formation of island-type small precipitations. From this observation, we propose that Au interlayer plays the unexpected role of a “buffer layer”, which dynamically functions as a medium for Li redistribution by propagating alloying reactions.Given the proposed new role of the interlayer, we expected the Li-metal deposition behavior to critically depend on the thermodynamic and kinetic properties of the interlayer metal and its alloying nature with Li. Si and Ag were chosen in this study, considering the availability of various alloys and the appreciable Li diffusivities in their alloys. When Si layer is applied (Figure 2a), the color changed during the early stages as gold, but subsequent behavior was noticeably different from that of the Au; Li metal preferentially precipitates at only a few sites, and whisker-shaped Li metal rapidly grows at these sites. On the other hand, in Ag interlayer (Figure 2b), the alloying reaction occurs first, followed by the uniform formation of numerous small nuclei. Interestingly, the two reactions occur nearly simultaneously, implying that both the alloying reaction and precipitation in the alloy are so fast that the sequential processes are unable to be distinguished. Regarding on this, the metal interlayers can be regarded as seed matrix for lithium precipitation.Further, Ag interlayer is more reversible upon Li stripping than Au or Si. The better efficacy of the lithium deposition/stripping process with Ag interlayer was further validated by comparative electrochemical testing of two electrode cells constructed with ‘100-nm-thick metal layer│LLZO │Li’ configuration. The results show that the coulombic efficiencies of cells using the various interlayers are closely related to the Li deposition and stripping behavior. The cell with Ag layer, which induces the most-uniform and reversible Li deposition among the tested metals, exhibits the highest efficiency of 71%, while that with Si layer shows 28%. Thus, it indicates that the interlayer can significantly affect the electrochemical performance of anode-free SSBs that employ solid electrolytes by regulating lithiation and de-lithiation behavior at the interface.Considering the ease of interlayer deposition on the LLZO surface and its wide applicability, we expect that our findings will provide useful guidelines for securing optimal interfaces for SSBs.REFERENCES[1] Aguesse, F.; Manalastas, W.; Buannic, L.; Lopez del Amo, J. M.; Singh, G.; Llordés, A.; Kilner, J., ACS Applied Materials & Interfaces 9, 3808-3816. (2017) Figure 1