Li 6−x Fe 1−x Al x Cl 8 Solid Electrolytes for Cost‐Effective All‐Solid‐State LiFePO 4 Batteries

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

Currently, electrolytes used in lithium-ion batteries are flammable and leaky, causing safety problems when used under high temperatures or extreme conditions. Solid electrolytes can fundamentally avoid the occurrence of such issues, at the same time, it can significantly increase the energy density of solid state lithium batteries. Solid electrolytes are classified into inorganic solid electrolytes and solid polymer electrolytes. Although inorganic solid electrolytes have the high mechanical strength and room temperature ionic conductivity, it has poor flexibility and complicated preparation process. In contrast, solid polymer electrolyte has good elasticity and good interface contact, which can overcome many disadvantages of the inorganic solid electrolyte. Among them, polyethylene oxide (PEO) is first proposed as a polymer electrolyte. Due to its excellent mechanical properties, electrochemical stability and thermal stability, it has been a hot topic for polymer solid electrolytes for decades. However, its low room temperature ionic conductivity limits practical application. Given the problems existing in PEO-based polymer electrolytes, several methods for improving ionic conductivity have been proposed. The primary methods include plasticization, the organic-inorganic composite, and polymer blending. Comparing these methods, it can be seen that adding plasticizers to the polymer can effectively increase the conductivity of the electrolyte, but at the same time it will cause a decrease in the mechanical properties, and the safety issues limit its application in lithium-ion batteries. Inorganic fillers can balance the problems of ionic conductivity and mechanical properties, but the construction of a good filler-polymer dispersion system is complicated in preparing such composite electrolytes, design a new type of multi-dimensional the ion transport network also face tremendous challenges. Undoubtedly, in the future research, inorganic fillers will be the dominant means in the modification of composite electrolytes. Compared with the former two modification methods, polymer blending is easy to operate and friendly to industry, but low ionic conductivity at room temperature still exist. To give full play to the advantages of polymer blending, using it as an auxiliary means to improve the performance of composite electrolytes will be the primary development direction of such modification methods. PEO is the most widely studied polymer electrolyte. Even though a large number of modification methods have emerged in its development history, the ionic conductivity at room temperature is still limited to a relatively low level. To meet the needs of the application in lithium-ion batteries, a high operating temperature (50−70℃) is still needed, the biggest problem remains exist. To solve the problems in polymer solid electrolytes, the research directions in the future may mainly focus on: Expanding the modification method of PEO to other polymer electrolyte host materials, broadening the scope of research, and finding the available polymer electrolytes at room temperature. Concerned about the preparation of ultra-thin polymer electrolytes, reducing the ion transmission path to increase the transmission efficiency of lithium ions between positive and negative electrodes. Effectively combines inorganic solid electrolytes and solid polymer electrolytes to give full play to their advantages. In addition, exploring the effect of inorganic filler morphology on the conductivity of the composite electrolyte will also deserve attention. Many one-dimensional, two-dimensional materials can build ion transport networks so that lithium ions can travel rapidly along the path. With the deepening of research, it is expected that PEO-based polymer electrolytes may replace liquid electrolytes and become an essential cornerstone for building the next generation of safe and reliable lithium-ion batteries.

<p indent="0mm">In order to deal with the increasingly severe global energy crisis as well as serious environmental pollution, it is urgent to develop new energy resources. Although lithium-ion batteries have only been successfully introduced into the market since 1990s, there is still an exponentially growing demand for lithium-ion batteries all over the world. As a considerable novel energy source, lithium-ion batteries have significant potential, which is widely used in electric vehicles, energy-storage systems, metallurgy, ceramic production, glass manufacturing and aviation fuel industries. Among them, the solid-state lithium-ion batteries (SSLIBs) represent a promising evolution in battery technology, offering superior safety, energy density, and longevity compared to conventional liquid electrolyte batteries. Traditional lithium-ion batteries, which use liquid electrolytes, suffer from safety issues such as leakage, flammability and thermal runaway. SSLIBs replace these liquid electrolytes with solid electrolytes, mitigating these risks and enabling higher energy densities due to the ability to accommodate lithium metal anodes. The SSLIB consists of a positive electrode, a negative electrode and a solid electrolyte. Compared with liquid electrolytes, solid electrolytes avoid the direct contact between positive electrodes and negative electrodes, fundamentally eliminating the risk of lithium dendrites piercing the separator, so as to enhance the safety of lithium batteries greatly. Furthermore, the SSLIBs are expected to be a hot spot in future lithium battery research due to the advantages of high energy density, wide operating voltage range and long cycle life. Advancements in SSLIBs technology are crucial for the next generation of electric vehicles, portable electronics, and large-scale energy storage systems, pushing the boundaries of battery performance, safety and sustainability. Solid electrolytes are regarded as the key component of SSLIBs, serving dual functions as both separators and electrolytes. The currently developed solid electrolytes can be divided into two categories: Polymer solid electrolytes and inorganic solid electrolytes, in which the oxide solid electrolytes, as the earliest developed inorganic solid electrolytes, play an indispensable role in the development of solid-state battery industry. The typical oxide solid electrolytes, including sodium super ionic conductor (NASICON), garnet (LLZO), inorganic perovskite (LLTO), and LiPON films, are reviewed in this paper. NASICON, for example, is known for its high ionic conductivity and structural stability. Garnet-type electrolytes, such as LLZO, possess high ionic conductivity and chemical stability with lithium. Inorganic perovskite (LLTO) provides a good balance between ionic conductivity and mechanical strength. LiPON films, widely used in thin-film batteries, have excellent electrochemical stability and compatibility with lithium metal. In addition, the crystal structures and ion-transport mechanism, the solutions to improve conductivity and measures to improve the compatibility between electrodes and electrolytes are introduced, along with a comparation of oxide solid electrolytes. Finally, several suggestions for future development directions are proposed. These include enhancing the ionic conductivity of solid electrolytes, improving the interface compatibility between the electrolytes and electrodes, and developing cost-effective and scalable manufacturing processes. Additionally, further research is needed to understand the degradation mechanisms and long-term stability of solid-state batteries. These recommendations aim to provide a more comprehensive reference for ongoing and future research on oxide solid electrolytes, supporting the broader goal of advancing solid-state lithium battery technology.

Electrolytes are key components in electrochemical storage systems, which provide an ion-transport mechanism between the cathode and anode of a cell. As battery technologies are in continuous development, there has been growing demand for more efficient, reliable and environmentally friendly materials. Solid-state lithium ion batteries (SSLIBs) are considered as next-generation energy storage systems and solid electrolytes (SEs) are the key components for these systems. Compared to liquid electrolytes, SEs are thermally stable (safer), less toxic and provide a more compact (lighter) battery design. However, the main issue is the ionic conductivity, especially at low temperatures. So far, there are two popular types of SEs: (1) inorganic solid electrolytes (InSEs) and (2) polymer electrolytes (PEs). Among InSEs, sulfide-based SEs are providing very high ionic conductivities (up to 10−2 S/cm) and they can easily compete with liquid electrolytes (LEs). On the other hand, they are much more expensive than LEs. PEs can be produced at less cost than InSEs but their conductivities are still not sufficient for higher performances. This paper reviews the most efficient SEs and compares them in terms of their performances and costs. The challenges associated with the current state-of-the-art electrolytes and their cost-reduction potentials are described.

With the rapid development of energy storage technology, solid‐state lithium batteries with high energy density, power density, and safety are considered as the ideal choice for the next generation of energy storage devices. Solid electrolytes have attracted considerable attention as key components of solid‐state batteries. Compared with inorganic solid electrolytes, solid polymer electrolytes have better flexibility, machinability, and more importantly, better contact with the electrode, and low interfacial impedance. However, its low ionic conductivity, narrow electrochemical stability window (ESW), and poor mechanical properties at room temperature limit its development and practical applications. In recent years, many studies have focused on improving the ionic conductivity of polymer electrolytes; however, few systematic studies and reviews have been conducted on their ESWs. A polymer electrolyte with wide electrochemical window will aid battery operation at a high voltage, which can effectively improve their energy density. Moreover, their stability toward lithium metal anode is also important. Therefore, this review summarizes the recent progress of solid polymer electrolytes on the ESW, discusses the factors affecting ESW of polymer electrolytes, and analyzes a strategy to broaden the window from the perspective of molecular interaction, polymer structural design, and interfacial tuning. The development trends of polymer electrolytes with wide electrochemical windows are also presented.image

Recently, demands on energy storage systems of high capacity and efficiency are continuously increased as the growth in commercialization of new renewable energy generation, such as wind and solar power, electric vehicle and portable electronic devices. Li-ion battery(LIB) has been introduced in diverse electronic products as one of the promising energy storage devices because of good energy density and long cycle life to answer these demands. For these reasons, study on LIB actively progressed to improve the performance of LIB. However, there are critical obstacles to make advance in energy density, such as increase in voltage and capacity, of conventionally used LIB. The commercialized LIB system use liquid electrolytes as for Li-ion migration between two electrodes. This liquid electrolyte deteriorate the stability of LIB, bring about overheating and explosion as temperature changes originated mainly from increase in operating voltage. In addition, leakage of organic based liquid electrolyte from the inside of LIB could have noxious effect on health. Therefore, alternatives on liquid electrolyte have been highly required to resolve these limitations. As increase in efforts to develop new electrolyte types, Inorganic solid electrolytes have received attentions to substitute it for liquid electrolytes. Inorganic solid electrolytes is one of the promising alternatives of liquid electrolyte because of their excellent thermal stability and good Li ion conductivity. Among inorganic electrolytes, Li10GeP2S12 (LGPS) indicates the great Li ion conductivity to be comparable with Li ion conductivity of liquid conductivity. Despite of this superior Li ion conductivity, the understanding of mechanism for Li ion diffusion in LGPS has not been clearly understood. This study explains correlation between Li ion diffusion and local environments of LGPS to understand mechanism of Li ion migration by first principles calculation. LGPS has enough Li ion concentrations and two types of migration paths. Li ion could migrate in frameworks of Ge-S and P-S(fig.1a). These frameworks secure c-axis and ab-plane paths for Li ion diffusions. There are also two types of methods for first principles calculation of Nudged Elastic Band(NEB) method of Li ion diffusion. One is defect migration (single ion migration) and the other is multi ion concentration migration(fig.1). Firstly, defect migration was analyzed by NEB. We setup three local environments for understanding the correlation in c-axis direction migration. The three situations are only one Li ion without any other Li ions in LGPS(fig.1b), one Li ion in the migration path around Li ions existent in the other non-migration paths(fig.1c) and only one vacancy with existent of Li ions in the migration path(fig.1b). Although the first and second cases of Li ion migration indicates similar tendency and energy barrier for Li ion migration, the third case shows that energy barrier in same migration path is more higher than the energy barrier of the other cases. This means that the factor influencing the Li ion diffusion is not the local Li ions in non-migration path but the near Li ions in the migration path. In addition to single ion diffusion, multi ion concentration migration is examined and this type has no vacancy and assumes that the all Li ions in the path diffuse all together. The energy barrier is lower than the energy barrier in all of single ion migration cases(fig.1f). This indicates that simultaneous migration of all Li ions in migration path is energetically beneficial. The ab-plane migration has two migration paths and the energy barriers are relatively higher than the energy barrier of c-axis migration path(fig.1e). This means that Li ion migration is mainly progressed in c-axis rather than ab-plane regardless of local environments in LGPS. Therefore, this understanding of mechanism for Li ion diffusion in LGPS would be effective guide line for developing the superior inorganic solid electrolyte based on LGPS. Figure 1

Inorganic solid electrolytes are one of the key materials instead of organic liquid electrolytes for next-generation all-solid-state batteries (ASSBs). The advantages of ASSBs are long cycle life and high safety due to the non-flammable of the solid electrolyte. The ASSBs can be stacked layer by layer, reducing the volume in battery packaging and resulting in increased energy density. These ASSBs can be applied to wearable or healthcare or smart home devices in the form of multilayer ceramic battery (MLCB), in addition to energy storage devices and electric vehicles. However, the MLCBs are difficult to fabricate due to the high sintering temperature and high interfacial resistance of the solid electrolyte. The Li-based solid electrolytes of NASICON-type have received much attention as prospective solid electrolytes due to their high ionic conductivity and high stability against moisture. Especially, lithium aluminum germanium phosphate (LAGP) is potential solid electrolyte material in the NASICON family due to superior stability. LAGP glass electrolyte was selected to enable sintering at low temperatures and reduce resistance at the interface for the application to MLCB.In this study, a LAGP-based solid electrolyte ceramic sheet was prepared using glass/powder composite at 750 °C. The obtained LAGP-based solid electrolyte exhibited a NASICON-type hexagonal structure and a total ionic conductivity of 1.45 × 10-4 S/cm at 25 ˚C. A Coin cell with the LAGP composite electrolyte, LiCoO2 (LCO) as cathode, and Li metal as anode was assembled. The coin cell was charged and discharged by using a battery cycler and the charge capacity was approximately 130 mAh∙g-1. Additionally, we prepared a LiMnPO4 glass-based cathode (LMP) using conventional melt quenching techniques and then stacked it layer by layer with the LAGP electrolyte and co-sintered at 650°C. After co-sintering the LAGP/LMP cell, it was confirmed that the interface between the LAGP composite electrolyte and the LMP cathode was well attached, and its ionic conductivity maintained a value similar to that of the LAGP composite electrolyte. Subsequently, an electrochemical analysis of the LAGP/LMP cell was performed.

All-solid-state batteries (ASSBs) using inorganic solid electrolytes are expected to achieve high safety and high energy density as alternatives to conventional lithium-ion batteries using liquid electrolytes. However, in many cases, rate capability of ASSBs is poor compared with that of the conventional liquid system. This is because the contact interface between solid active materials and solid electrolyte is insufficient, resulting in high resistance for electrochemical properties. The solid electrode-electrolyte interface in ASSBs is related to the morphological structure such as porosity and mixed state. It has been confirmed that pressurization reduces the grain boundaries of solid electrolytes and improves conductivity1). From this improvement, it is generally known that applying high pressure when assembling ASSBs improves the contact area of the solid-solid interface, including electrode and electrolyte interface. However, the effect of pressure on particle-particle contact and battery performance is not understood enough. In this study, we investigated the effect of pressure-induced three-dimensional structure on the electrochemical properties of ASSBs by using X-ray computed tomography (CT). ASSBs were prepared at various stack pressures using a composite cathode consisting of oxide active materials LiNi1/3Co1/3Mn1/3O2 (NCM), sulfide solid electrolyte Li10GeP2S12 (LGPS), and acetylene black. Electrochemical properties were investigated in ASSBs of NCM composite cathode|LGPS solid-electrolyte|In-Li anode. The charge–discharge measurements were performed using various stack pressure, which indicates that the capacity increases as the stack pressure increases. To analyze the factors behind this result in terms of the internal structure of the ASSBs, we performed X-ray CT with synchrotron radiation at 20 keV using a homemade operando measurement cell that electrode diameter is 1 mm. This measurement system allows us to analyze the three-dimensional structure of composite cathode during monitoring applied pressure, electrochemical measurements, and X-ray CT measurements. From the X-ray CT data, we succeeded in quantifying the changes in the porosity of the composite cathode and the contact state between the active material and electrolytes by changing the pressure. Quantitative analysis revealed that the porosity and contact state correlate with the charge–discharge properties. Additionally, CT analysis revealed that the voids are preferentially presented near active materials and these voids spread the horizontal direction (not pressurization direction), resulting in different porosities of composite cathode and solid electrolyte layer. Since it was confirmed that voids remained in the vicinity of the active material even when the pressurization was continued, controlling these voids decreases the voids of the entire cathode layer. Pressure dependencies on AC impedance during X-ray CT measurements showed that the apparent conductivity of ASSBs increases linearly as the confinement stack pressure increases. This result is caused by suggesting that the decrease in porosity and tortuosity in cathode layer, which is detected by CT analysis. Additionally, date of tortuosity shows over-pressurization may bypass Lithium conduction path. The improved internal structure such as contact state due to pressurization enhances the apparent conductivity and charged-discharge characteristics of ASSBs. With the latest synchrotron radiation measurement technology and image processing technology, high-precision analysis has become possible.1) Jean-Marie Doux, Yangyuchen Yang, Darren H. S. Tan, Han Nguyen, Erik A. Wu, a Xuefeng Wang, Abhik Banerjeea and Ying Shirley Meng, J. Mater. Chem. A, 8, 5049, (2020).

The safety issues concerning conventional Li+ ion batteries (LIBs) are attributed to the use of flammable organic liquid electrolyte (LE), which could result in fire and explosion. To address the safety issues and increase the durability, All-solid-state Li+ ion batteries (ASLBs) using inorganic solid electrolyte (SE) are considered as an ideal alternative for developing safe LIBs. Their wide range of operating temperature, from room temperature to over 90°C, is also an important advantage with regards to energy storage systems.SE can be classified into two types: sulfide-based SEs and oxide-based SEs. First, in the case of sulfide-based SEs, thio-LISICON, and Li-argyrodite are present and these have high ionic conductivity that are comparable to LE . Furthermore, a sulfide-based SE with a polymer electrolyte has been extensively investigated as well. However, one of the major disadvantages of a sulfide-based SE is that it is chemically unstable in air as it reacts with moisture in the air to produce H2S gas, which is a hazardous.On the contrary, oxide- based SEs including garnet, NASICON type may be attractive because they not only have high ionic conductivity but also, are chemically stable in air. Given these aspects, many researchers are devoted to the combination of oxide-based SEs and polymer electrolytes by adding oxide-based SE powders in a polymer matrix, as polymer electrolytes exhibit more suitable mechanical properties than those of ceramics, but show low ionic conductivity at room temperature.The combination of oxide-based SEs and polymers have paved new ways to create better electrolytes which incorporate both the high ionic conductivity of the oxide-based SE and the stable mechanical properties of polymers. In the case of composite electrolytes, many researchers anticipated that the higher the oxide-based SE (herein, ceramic) content in a composite electrolyte, the higher the Li+ ion conduction contribution from the ceramic. To investigate the effect of Li+ ion conduction from ceramic powders, analytical research studies according to the ceramic content have been conducted. Unexpectedly, even if the ratio of ceramic powder is increased, the ceramic’s contribution to Li+ ion conductivity could not be confirmed as there is no path for the Li+ ions to percolate through the ceramic due to high interfacial resistance in the ceramic powders. Furthermore, this ceramic powder acts as a resistor due to high interfacial resistances between the ceramic powder and polymer electrolyte. In this work, scalable and flexible composite electrolyte film (SFCEF) was fabricated based on a fiber-shaped ceramic and polymer support. Ceramic fibers of Li1.3Ti1.7Al0.3(PO4)3 (LATP) were prepared by sintering the precursor-coated sacrificial template and then infiltrated with a polyethylene oxide (PEO) polymer to obtain the FSCEF. The LATP fibers induced continuous Li+ ion channels, allowing the FSCEF to show an ionic conductivity exceeding 10−4 S cm−1 at 60 °C. The synergistic action of the ceramic frameworks and supportive PEO resulted in enhanced mechanical flexibility. Furthermore, the possibility of using a FSCEF in all-solid-state batteries was confirmed by conducting electrochemical performance tests on a Li/FSCEF/LCO (LiCoO2) cell. We expect that the herein reported findings will contribute to the synthesis of thin and flexible solid-state electrolyte films with manufacturing scalability for promising high-voltage all-solid-state batteries.

Significant scientific and economic challenges are presented by developing battery systems with increased energy storage capacity. Geopolitical issues might pose supply chain risks for lithium (Li)-ion batteries, which are frequently used in portable electronics. As an alternative, sodium-ion batteries (NIBs), have drawn attention from researchers all over the world due to the perspective of being lower cost systems[1]. In comparison to lithium-ion batteries, solid-state sodium-ion batteries (SSSBs) promise as a safe, scalable, and environmentally friendly alternative. For the production of robust and adaptable SSSBs, the development of highly conductive, electrochemically stable, and chemically stable solid electrolytes (SEs) is essential [2]. With greater durability, adaptability, and safety, solid-state batteries with inorganic solid electrolytes are being heavily investigated as safer and perhaps more energy-dense alternatives to liquid electrolyte cells.Sulfides solid electrolytes have been extensively investigated due to their mechanical softness and high ionic conductivity. However, the electrochemical oxidation stability of those materials are as low as 2 V (vs. Na+/Na)[3]. Alternatively, the excellent ion conductivities and stability at high potentials have made halide compounds, intriguing candidates for solid electrolytes in batteries. These compounds offer fast ion transport, good stability at highly oxidative potentials, and are easily processed. A wide range of chemistries, such as A3MX6 (A = Li, Na; M = Sc, In, Y, Yb, Er, Hf, Ho, Dy, Al, Zr; X = F, Cl, Br, I), have been synthesized with tunable properties and the characterizations of those materials shows relevant electrochemical proprieties [2].In our work, we investigate different Na-based chloride compounds by Density Functional Theory (DFT). We expand beyond the reported compositions to consider possible cation-substituted compounds. Here, we report on the structures, ionic conduction pathways and ionic conductivity (computed by molecular dynamics methods) expected for such compounds. In the next months, we will also evaluate the compounds´ chemical and electrochemical stability. Our work moves us one step closer towards the development of stable and highly conductive solid electrolytes for Na-based solid-state batteries.References :[1] J. Mater. Chem. A, 2021,9, 281-292[2]J. Mater. Chem. A, 2022,10, 21565-21578[3] ACS Energy Lett. 2022, 7, 10, 3293–3301

Inorganic solid electrolytes with fast ionic conductivity are critical components of fuel/electrolysis cells, all-solid-state batteries, membrane and separation reactors, sensors and other electrochemical devices. For example, solid oxygen ion (O2-) conductors are used in solid oxide fuel/electrolysis cells (SOFCs/SOECs) for high-efficiency conversion between chemical and electrical energy and chemical production from CO2. One major challenge in engineering such materials is the diminished grain boundary (GB) conductivity in polycrystalline oxide ceramics. This phenomenon, which was initially accredited to highly resistive silica-based glassy impurity phases, is attributed to the formation of intrinsic space charge layers (SCLs) directly adjacent to GB cores. The positive potential at the GB core (due to large concentrations of oxygen vacancies) results in a depletion of oxygen vacancies in the space charge layer as well as the segregation of charge-compensating acceptor solutes from grain interiors which ultimately reduces GB conductivity. Several strategies have been explored to improve GB conductivity by understanding and mitigating the SCL effects. While the standard Poisson–Boltzmann approach models the near-grain boundary defect behavior in dilute solid solutions and recent models by Mebane et al [1] and Virkant et al [2] incorporate the effects of defect-defect interactions (in concentrated solid solutions) and chemomechanical stress, respectively, there is yet to be a model developed that can predict individual GB conductivities in concentrated solid solutions based on direct experimental measurements of near-GB defect chemistry.Here, we present an experimental-computational framework that predicts which GBs in a polycrystalline electrolyte likely facilitate ionic conductivity. We developed a thermodynamic phase-field modeling framework and applied it to a model oxygen electrolyte Gd0.25Ce0.75O2-δ, wherein the GB-to-GB variability in defect concentrations (Gd3+ solutes, electrons localized at Ce3+, and oxygen vacancies) was measured microscopically using scanning transmission electron microscopy electron energy-loss spectroscopy (STEM EELS). These data were used to predict the GB-to-GB variability in cross-GB ionic conduction using a unique model, which prioritizes reproducing microscopically observed GB defect concentrations and considers defect-defect interactions in highly concentrated solid solutions, making the framework applicable to other technologically relevant solid electrolytes. Across the GBs studied, we revealed a non-monotonic relationship between depletion and GB conductivity, with the highest conductivity predicted for intermediate depletion amounts. This work lays the foundation for future experimental-computational research and advanced data-driven design of solid electrolytes needed for functional and energy storage/conversion devices.[1] D. S. Mebane, R. A. D. Souza, Energy & Environmental Science, 2015.[2] K. S. N. Vikrant, R. Edwin García, Energy & Environmental Science, 2018.[3] H. Vahidi, W.J. Bowman (Submitted). Acknowledgments HV and WJB acknowledge primary support from the UCI new faculty startup funding. This research was partially supported by the National Science Foundation Materials Research Science and Engineering Center program through the UC Irvine Center for Complex and Active Materials (DMR-2011967). We acknowledge the use of facilities and instrumentation at the UC Irvine Materials Research Institute (IMRI) supported in part by the same NSF MRSEC (DMR-2011967). AM, DM, and AC acknowledge funding from the NSF [WB1] [hv2] and the WVU MAE research department for the use of the Thorny Flat HPC Cluster for the numerical modeling work and computational resources.

All-solid-state lithium batteries (ASSBs) are considered like the next generation of electrochemical energy storage devices because of their safety and their high energy density up to 400Wh/kg. The all-solid-state battery is composed of a positive electrode (e.g., NMC (Li(Ni1∕3Mn1∕3Co1∕3)O2)), a negative one (Li), and a solid electrolyte, which is the critical component, composed of inorganic compound and/or solid polymer electrolyte (SPE) [1]. On the one hand, SPE have the advantage of an easy shaping with the technique already used in the manufacture of batteries and their flexibility allows obtaining a good interface with the electrode materials [2]. On the other hand, inorganic compounds generally have high ionic conductivity (10-3S/cm), they are stable electrochemically vs Li/Li+ and their transport number is close to 1 to promote the diffusion/migration of Li+ ions only. In order to combine the advantages of both types of materials, hybrid materials have been designed [3].First, a new adaptable polymer matrix based on PEO derivatives has been developed. A solvent-free synthesis has been designed from liquid and commercial precursors Poly(ethylene glycol) methyl methacrylate (PEGM) and Poly(ethylene glycol) dimethacrylate (PEGDM). These latter were copolymerized with Lithium 3-[(trifluoromethane)sulfonamidosulfonyl] propyl methacrylate (MTFSLi). The anion TFSI- is thus grafted to the PEO network, allowing exclusively the diffusion of the Li+ ion. This polymer matrix can be thus classified as single-ion conductor. In addition, the proportions between PEGM and PEGDM, acting as crosslinker, can be easily tuned. The PEGDM proportion controls the cross-linking density which modifies not only the glass transition temperature (Tg), but also the storage modulus (E’) and the free volume, and consequently the ionic conductivity of the polymer matrix which is the main property required for SPE. Finally, a ratio of PEGM:PEGDM (80:20) allows obtaining homogeneous SPE, self-standing and soft at room temperature, amorphous with a Tg of -41°C. The EO/Li ratio was then modulated in single-ion networks with the MTFSLi proportion, and an optimum in ionic conductivity of 2.10-7 S/cm @25°C has been measured for EO/Li ratio of about 20. The transport number measured by the Bruce and Vincent method [4] was t+=1,0 for the single ion conductor, demonstrating the effective grafting of the counter anion. This value has been validated by measuring the diffusion coefficients of 7Li and 19F in NMR. To our knowledge, this is the first time a real unity transport number is reported in the literature for a polymer electrolyte. Moreover, the electrochemical stability of this single ion system extends from 0 to 6V vs Li+/Li. These experiments suggest the interest of using the single-ion polymer in the formulation of composite positive electrode with high potential cathode materials, due to its stability in potential. Li metal is considered the ultimate anode for Li-batteries. The long-term electrochemical stability has been evaluated in symmetric Li/polymer electrolyte/Li cell. The cell is cycled at a constant current density of 0.2 mA/cm2. After 200 h of cycling the voltage reaches solely 2V. Accordingly, the grafting of the anion to the network limits an overpotential in the cell, allowing a more stable system over time.Second, the garnet Li6.4La3Zr2Al0.2O12 (LLZO) of the oxide family was associated to the previous polymer matrix. This garnet presents a high conductivity (10-4 S/cm @ 25°C), a good chemical stability not only against lithium metal but also against air and humidity, which is not the case of sulphides homologues. A 3D-fibrous network obtained by the electrospinning method was carried out with a solution containing the inorganic precursors, a polymer and an appropriate solvent [5]. Then this membrane is calcined at 850°C to obtain the 3D fibrous network of LLZO. The resulting self-standing porous inorganic material (~80% of porosity) exhibits a conductivity of the order of 10-6 S/cm @25°C. It was then impregnated with the polymer matrix precursors by capillarity and following by a polymerization. A self-standing membrane as thin as 80 µm containing 9%vol of LLZO was obtained. The conductivity of this hybrid membrane is comparable to that of the polymer (10-7S/cm @ 25°C). The resulting hybrid is mainly polymeric where the ionic conduction was realized by the polymeric matrix and the inorganic acts as a mechanical reinforcement. Preliminary results on high content of LLZO hybrid will also be presented, where a synergy of the both components is observed.[1] Janek. Nature Energy. 2016. Vol 1(9)p.16141. [2] Yue. Energy Storage Materials. 2016. Vol 5p.139-164. [3] Zheng. Chem Soc Rev. 2020. Vol 49p.8790-8839. [4] Bruce. Solid State Ion. 1988. Vol 28–30p.918–922. [5] A. La Monaca. Electrochemistry Communications. 2019. Vol 104p.106483. Figure 1

All-solid-state lithium-ion batteries are increasing in interest as the next-generation battery due to their safety, high energy density, and long lifetime in achieving a decarbonized society. However, all-solid-state lithium-ion batteries, using inorganic solid electrolytes instead of liquid electrolytes, face challenges in charging/discharging rates and capacity due to their much lower ion conductivity[1]. Therefore, it is essential to explore novel solid electrolytes with high ionic conductivity.Solid electrolytes with particularly high ion conductivity are called superionic conductors, such as Li10GeP2S12 (LGPS)[2]and Li1.3Al0.3Ti1.7(PO4)3 (LATP)[3]. Theoretical studies suggest that their fast ionic conduction is governed by a non-trivial conduction mechanism, simultaneous multiple-ion hopping, which is driven by Coulomb interactions of lithium ions instead of classical single-ion hopping[4]. Multiple-ion hopping, namely concerted migration, results in a lower energy barrier for ion migration than single-ion hopping. Thus, triggering concerted migration would be crucial to achieving high ionic conductivity.To induce concerted migration, chemical substitution may be effective, as seen in LATP. LATP is a superionic conductor with a NASICON-type structure in which the Ti4+ site of the parent compound LiTi2(PO4)3 is partly substituted with Al3+. Though the parent compound has a low ionic conductivity of 3.13×10–6 S/cm, the ionic conductivity in LATP of 5.1×10–3 S/cm increases by approximately three orders of magnitude[5]. A theoretical study suggests the conduction mechanism of LATP is governed by concerted migration, meaning a chemical substitution leads to concerted migration, resulting in such a significant enhancement.To find ionic conductors with high ionic conductivity, we focused on LiTa2PO8 [6]for the following reasons: First, its ionic conductivity (1.6×10–3 S/cm) is comparable to that of LATP without chemical substitutions. Second, theoretical calculations predict that a substantially weaker Li-Li interaction is expected in LiTa2PO8 [7] than in LATP[4], implying that the conduction mechanism may be classical. Given the increase in ionic conductivity in LATP, high ionic conductivity might be achieved when concerted migration is induced/enhanced in LiTa2PO8. Though the substitutions of Ta5+/P5+ sites with tetravalent ions of typical elements such as Te4+ [8]and Si4+[9] have been studied, no drastic increase in ionic conductivity has been achieved, and the highest ionic conductivity is 1.6×10–3 S/cm. In this study, we performed chemical substitutions of Ta5+ with transition metal ions, Ti4+ and W6+.Polycrystalline powder of Li1±xTa2–xMxPO8 (M = Ti4+, W6+) (x = 0 ~ 0.2) was prepared by solid-state reaction. The lattice parameters determined by XRD patterns decreased as the substitution amount x increased, indicating successful substitutions of Ti4+ and W6+ since Ti4+ and W6+ have smaller ionic radii than Ta5+. The x dependence of the lattice constants indicates the solubility limit is xTi = 0.08 for the case of Ti4+, while the linear decrease of the lattice constants in the case of W6+ indicates the solubility limit is above xW = 0.20. Elemental analysis via SEM/EDX also supports the above results. The ionic conductivity of xTi = 0.08 and xW = 0.20 was measured by the AC impedance method in the range of 1.0×102~5.0×107 Hz. The results showed that Ti4+ substitution increases ion conductivity, while W6+ substitution decreases compared with non-doped LiTa2PO8, meaning the increase in the number of Li ions is effective in enhancing the ionic conductivity in LiTa2PO8. However, unlike LATP, the increase in ionic conductivity is at most twice as large.

Organic ionic plastic crystals (OIPCs) have attracted attention as novel organic solid electrolyte materials, but their insufficient mechanical strength and ionic conductivity have prevented their application. In this study, a lithium salt, lithium bis(fluorosulfonyl)amide (LiFSA), and an inorganic solid electrolyte, Li7La3Zr2O12 (LLZO), were added to an OIPC, N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C2epyr][FSA]). The fabricated organic-inorganic hybrid solid electrolytes were evaluated thermally, mechanically, and electrochemically to reveal which factors affect the properties of the electrolytes. All samples showed excellent thermal stability regardless of LiFSA or LLZO concentration, and they were found to be highly plastic and ion-conductive solids at a wide range of temperatures. It was also revealed that the addition of LLZO raised the nanoindentation stiffness (HIT) of the [C2epyr][FSA]/LiFSA composites. The ionic conductivity of the hybrid electrolytes was higher than that of the pristine OIPC, reaching a value of 2.1 × 10-4 S cm-1 at 25 °C upon addition of appropriate amounts of LiFSA and LLZO. Overall, samples with higher LiFSA concentration and moderate LLZO concentration exhibited higher ionic conductivity. Cyclic voltammetry results showed that the [C2epyr][FSA]/LiFSA/LLZO composites were lithium-ion conductors. These findings indicate that by optimizing the concentrations of lithium salt and LLZO, it would be possible to realize their applications as solid electrolytes.

Enhanced lithium dendrite suppression ability through SiO2 substitution in superionic halogen-rich argyrodites and their application in all-solid-state lithium batteries