A Sodium Polysulfide Battery with Liquid/Solid Electrolyte: Improving Sulfur Utilization Using P2S5 as Additive and Tetramethylurea as Catholyte Solvent

<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.

The demand for power sources for electric vehicles and portable devices has increased rapidly. All-solid-state lithium-ion batteries are promising attention due to their safety and energy density. Especially, the development of solid electrolytes is necessary to realize all-solid-state batteries. Solid electrolytes with high ionic conductivity like sulfides, oxides, and chlorides are used for long-distance conduction in the entire all-solid-state batteries. In addition, those with low ionic conductivity like lithium salts are also used as coating materials and sintering aids. Our group reported active materials and electrolytes using Li2SO4 [1, 2].One of the features required for solid electrolytes is wide electrochemical windows. Although various sulfide solid electrolytes have been studied due to their high ionic conductivity and ductility, they have narrow electrochemical windows [3], leading to oxidative and reductive decomposition during cell cycling. Thus, it is important to consider the electrochemical windows of solid electrolytes.Conventionally, the electrochemical windows of solid electrolytes have been investigated using electrochemical techniques like cycling voltammetry and linear sweep voltammetry (LSV). The working electrode is composed of the objective solid electrolytes and conductive carbon. However, for some materials with low ionic conductivity, it is challenging to ensure ionic conduction at long distances in the composite working electrode. Therefore, few reports have shown that the redox potential of a low ionic-conductive material including Li2SO4 is experimentally determined by electrochemical tests, although the electrochemical stability of lithium ion conductors was investigated using first-principles calculation methods [3, 4].In this study, we used LSV measurements using all-solid-state cells to investigate the redox potential of Li2SO4. The composite working electrode comprised Li2SO4, vapor-grown carbon fiber (VGCF), and oxide solid electrolyte. Li2SO4 and VGCF were ball-milled and the obtained Li2SO4-VGCF and oxide electrolyte were mixed in an agate mortar and pestle to prepare composite working electrodes. As oxide electrolytes, 90Li3BO3·10Li2SO4 glass-ceramic (LBSO) [2] was used in cells for an anodic scan, and 67Li2O·33LiI glass (LOI) [5] was used in cells for a cathodic scan because they have better oxidation or reduction tolerance than sulfide electrolytes. In addition, it was found that the decomposition of sulfide electrolytes affected the LSV curves when sulfide electrolytes were used in a composite working electrode or as a separator layer that directly contacted with the composite working electrode. Therefore, the cell configuration was Li-In/Li3PS4/LBSO (or LOI)/Li2SO4-VGCF-LBSO (or LOI). To confirm the decomposition of Li2SO4 after the LSV measurements, X-ray photoelectron spectroscopy (XPS) measurements were performed on the cells, and the redox mechanism of Li2SO4 was discussed.In the anodic scan, a current peak derived from the oxidation of Li2SO4 was observed at approximately 4.3 V vs. Li-In (4.9 V vs. Li+/Li), which was close to the standard electrode potential of oxidation from SO4 2– to S2O8 2– [6]. In addition, in the cathodic scan, a current peak derived from the reduction of Li2SO4 was observed at approximately 1.5 V vs. Li-In (2.1 V vs. Li+/Li), which almost corresponds to the potential of reduction from SO4 2– to SO3 2– [6]. Furthermore, the XPS analyses confirmed that Li2SO4 was finally reduced to Li2S and Li2O.In summary, the LSV measurements were performed with the all-solid-state cell using oxide electrolytes instead of sulfide electrolytes, which eliminated the overlap of a current caused by the decomposition of sulfide electrolytes. The oxidative and reductive decomposition voltages of Li2SO4 were close to each standard electrode potential. We also found a way to experimentally determine the electrochemical potential of any materials by modifying the cell design. This study will help spread the importance of potential windows and the realization of all-solid-state lithium-ion batteries. Reference s : [1] K. Nagao et al., Sci. Adv. 6 (2020) eaax7236.[2] M. Tatsumisago et al., JCS-Japan, 125 (2017) 433.[3] Y. Zhu et al., ACS Appl. Mater. Interfaces, 7 (2015) 23685.z[4] W. D. Richards, et al., Chem. Mater., 28 (2016) 266.[5] Y. Fujita et al., Electrochemistry, 89 (2021) 334.[6] A. J. Bard et al., Standard Potentials in Aqueous Solution, Routledge (2017).

Polymer electrolytes based on ionic liquids provide a safe solution for future solid-state high-energy-density batteries. In this work, we report a novel class of solid polymer electrolytes for fast lithium-ion conduction based on phosphonium poly(ionic liquid)s (poly(IL)s). First, a new family of poly(diallyldimethylphosphonium) poly(IL)s was synthesized by cyclopolymerization of diallyldimethylphosphonium iodide following anion exchange with different sulfonamide-based anions. The characterization of the poly(IL)s confirmed the formation of a stable 5-member ring phosphonium cationic polymer backbone. Then, solid polymer electrolytes (SPEs) were prepared by doping the phosphonium sulfonamide poly(IL)s with different contents of lithium fluorosulfonamide salt (LiFSI). The nature of the ionic interactions and lithium transport of SPEs with different anion systems was deeply investigated by FTIR, DSC, ionic conductivity, solid-state 7Li NMR, and molecular modeling simulations. Polymer-in-salt phosphonium SPEs showed excellent properties, with high ionic conductivities up to 1.5 × 10-3 S cm-1 at 80 °C, high lithium transference numbers up to 0.7, and a wide electrochemical stability window superior to state-of-the-art SPEs. Finally, we showed that phosphonium poly(IL) electrolytes allowed successful Li+ plating/stripping as a solid electrolyte in lithium metal symmetrical cells, showing constant overvoltage limited to 0.06 V working at 0.1 mA cm-2 current density at 60 °C, and stable cycling without signs of short-circuiting during 200 cycles at 40 °C.

Enhanced Li-O2 battery performance, using graphene-like nori-derived carbon as the cathode and adding LiI in the electrolyte as a promoter

Solid-state sodium ion batteries are frequently referred to as the most promising technology for next-generation energy storage applications. However, developing a suitable solid electrolyte with high ionic conductivity, excellent electrolyte–electrode interfaces, and a wide electrochemical stability window, remains a major challenge. Although solid-polymer electrolytes have attracted great interest due to their low cost, low density and very good processability, they generally have significantly lower ionic conductivity and poor mechanical strength. Here, we report on the development of a low-cost composite solid polymer electrolyte comprised of poly(ethylene oxide), poly(vinylpyrrolidone) and sodium hexafluorophosphate, mixed with indium arsenide nanowires. We show that the addition of 1.0% by weight of indium arsenide nanowires increases the sodium ion conductivity in the polymer to 1.50 × 10−4 Scm−1 at 40 °C. In order to explain this remarkable characteristic, we propose a new transport model in which sodium ions hop between close-spaced defect sites present on the surface of the nanowires, forming an effective complex conductive percolation network. Our work represents a significant advance in the development of novel solid polymer electrolytes with embedded engineered ultrafast 1D percolation networks for near-future generations of low-cost, high-performance batteries with excellent energy storage capabilities.

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) incorporating lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) offer excellent potential for solid-state lithium metal batteries (SSLMBs) due to their wide electrochemical stability window and high lithium salt solubility. LiTFSI supplies Li+ ions and also acts as a plasticizer, promoting the formation of the amorphous phase, which is vital for ionic conduction. Reduced ionic conductivity can result from very high crystallinity, spherulitic morphology, and secondary crystallization disrupting the ion pathways. The [EO:Li] molar ratio critically governs crystallization behavior and ionic transport; a ratio of 10:1, close to the stable amorphous eutectic composition (9:1), promotes amorphous phase formation and high ionic conductivity while limiting excessive crystallization. However, high salt content at this ratio leads to membrane stickiness, low mechanical strength, and handling difficulties. Here, we present a parafilm-assisted peeling technique that enables the fabrication of freestanding PEO/LiTFSI membranes while preserving their structural integrity. SEM analysis shows that conventional Teflon-mold peeling yields cracked, stretched surfaces, whereas parafilm-assisted films are smooth and defect-free. FTIR confirms Li+–PEO coordination with no parafilm–electrolyte interaction. Both methods deliver similar electrochemical performance: ∼4.30 V stability window, ionic conductivity 3.96 × 10−3 S cm−1 at 50 ° C (E a ∼ 0.898 eV), and Li+ transference number ∼0.14–0.15. Symmetric Li|SPE|Li cells cycle stably for 300 h at 50 ° C , while LiFePO4 full cells retain ∼161 mAh g−1 and >98% Coulombic efficiency over 30 cycles, confirming parafilm-assisted peeling as an effective fabrication method. This approach presents an effective means to address processing difficulties, thereby extending the viability of PEO based SPEs in next-generation energy storage devices.

The development and interest in environmentally friendly cars nowadays present new challenges to the lithium-ion batteries. With the safety problems caused by the liquid electrolyte in conventional batteries, the development of new electrolytes is becoming increasingly urgent. Based on the existing literature, this paper discusses the following: (1) the advantages and disadvantages of solid-state electrolytes; (2) Requirements for solid electrolytes; (3) Classification and characteristics of solid electrolytes; (4) Properties of different types of electrolytes; (5) Future application and research direction. Performance and possible applications of different solid-state electrolytes are presented in this paper. Specifically, it is found that inorganic solid electrolytes are the best in ion transference number, making them suitable for quick charging batteries. Polymer-based solid electrolytes perform better at electrochemical window, so they can be used to enhance the energy density of batteries. Polyethylene glycol (PEG) based composite solid electrolytes and sulfide solid electrolytes have the highest ionic conductivity, so they are optimum materials for high-rate performance batteries.

Solid-state lithium-metal (Li0) batteries are gaining traction for electric vehicle applications because they replace flammable liquid electrolytes with a safer, solid-form electrolyte that also offers higher energy density and better resistance against Li dendrite formation. Solid polymer electrolytes (SPEs) are highly promising candidates because of their tuneable mechanical properties and easy manufacturability; however, their electrochemical instability against lithium-metal (Li0), mediocre conductivity and poorly understood Li0/SPE interphases have prevented extensive application in real batteries. In particular, the origin of the low Coulombic efficiency (CE) associated with SPEs remains elusive, as the debate continues as to whether it originates from unfavoured interfacial reactions or lithium dendritic growth and dead lithium formation. In this work, we use state-of-the-art cryo-EM imaging and spectroscopic techniques to characterize the structure and chemistry of the interface between Li0 and a polyacrylate-based SPE. Contradicting the conventional knowledge, we find that no protective interphase forms, owing to the sustained reactions between deposited Li dendrites and polyacrylic backbones and succinonitrile plasticizer. Due to the reaction-induced volume change, large amounts of cracks form inside the Li dendrites with a stress-corrosion-cracking behaviour, indicating that Li0 cannot be passivated in this SPE system. On the basis of this observation, we then introduce additive engineering, leveraging from knowledge of liquid electrolytes, and demonstrate that the Li0 surface can be effectively protected against corrosion using fluoroethylene carbonate, leading to densely packed Li0 domes with conformal and stable solid-electrolyte interphase films. Owing to the high room-temperature ionic conductivity of 1.01 mS cm-1, the high transference number of 0.57 and the stabilized lithium-electrolyte interface, this improved SPE delivers an excellent lithium plating/stripping CE of 99% and 1,800 hours of stable cycling in Li||Li symmetric cells (0.2 mA cm-2, 1 mAh cm-2). This improved cathodic stability, along with the high anodic stability, enables a record high cycle life of >2,000 cycles for Li||LiFePO4 and >400 cycles for Li||LiCoO2 full cells.

Enabling all-solid-state Li-ion batteries requires solid electrolytes with high Li ionic conductivity and good electrochemical stability. Following recent experimental reports of Li3 YCl6 and Li3 YBr6 as promising new solid electrolytes, we used first principles computation to investigate the Li-ion diffusion, electrochemical stability, and interface stability of chloride and bromide materials and elucidated the origin of their high ionic conductivities and good electrochemical stabilities. Chloride and bromide chemistries intrinsically exhibit low migration energy barriers, wide electrochemical windows, and are not constrained to previous design principles for sulfide and oxide Li-ion conductors, allowing for much greater freedom in structure, chemistry, composition, and Li sublattice for developing fast Li-ion conductors. Our study highlights chloride and bromide chemistries as a promising new research direction for solid electrolytes with high ionic conductivity and good stability.

The electrochemical stability window of solid electrolyte is overestimated by the conventional experimental method using a Li/electrolyte/inert metal semi-blocking electrode because of the slow kinetics of the decomposition reactions due to the small contact area between LGPS and inert metal. Since the battery is cycled in the overestimated stability window, the decomposition of the solid electrolyte at the interfaces occurs but has been ignored as a cause for high interfacial resistances in previous studies, limiting the performance improvement of the bulk-type solid-state battery despite the decades of research efforts. Thus, there is an urgent need to identify the intrinsic stability window of the solid electrolyte. We propose a new experimental method to measure the intrinsic electrochemical stability window of solid electrolytes using a Li/electrolyte/electrolyte-carbon cell. The Li/electrolyte/electrolyte-C cell provides improved kinetics from large and continuous physical contacts between solid electrolyte and carbon to facilitate the thermodynamically favorable decomposition reactions of the solid electrolyte. Moreover, the use of the Li/electrolyte/electrolyte-C cell mimics the cell configuration in the bulk-type solid-state battery and represents the real microstructural architectures in the solid-state electrode composite, where carbon and solid electrolyte are mixed with the active material. The intrinsic stability window of the most promising solid electrolytes, Li10GeP2S12 (LGPS) and cubic Li-garnet Li7La3Zr2O12(LLZO), were examined using the novel experimental method. The results indicate that both of these solid electrolyte materials have much narrower electrochemical window than what was previously claimed (0 – 5 V), as shown in Figure 1. The cathodic and anodic decomposition products for both electrolytes were also characterized by XPS test. The measured stability windows as well as the decomposition products agree well with our first-principles calculations, indicating that our proposed cell design could obtain the intrinsic electrochemical stability window of solid electrolyte. Our work offers a new research orientation towards the minimization of the interfacial resistance of all-solid-state batteries. At the cycling voltages beyond the stability window of the solid electrolyte, the decomposition products of the solid electrolyte would form as an interphase at the electrolyte/electrode and electrolyte/carbon interfaces. Therefore, the performance of the bulk-type solid-state battery is greatly influenced depending on the properties of the decomposition interphases, such as ionic conductivity, electronic conductivity, and electrochemical reversibility. The influences of the decomposition of different solid electrolytes on the interfacial resistance will be discussed in detail. Specific recommendations for the engineering of sulfides and oxides solid electrolyte materials will be provided to obtain a higher performance all-solid-state batteries. Our most recent experimental results about the interface engineering of LGPS-based all-solid-state battery with consideration of the decomposition of LGPS will also be presented. On the other hand, the reversible decomposition of the solid electrolyte at two different voltages allows us to design an all-solid-state lithium ion battery using a single material. Based on this concept, a single-LGPS battery (Figure 2) was prepared using LGPS-C as cathode and anode, and pure LGPS as solid electrolyte, while the carbon mixed in the electrodes can be considered as an extension of current collector. The single-LGPS battery exhibited a remarkably low interfacial resistance due to the improvement of interfacial contact, the modification of the interfacial interactions, and the suppression of the strain/stress at the interface, providing a promising direction to address the most challenging interfacial problem in all-solid-state batteries. Additional implications of the single-material battery concept will also be presented.

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.

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

The ability to utilize Li metal as the anode in Li-ion or Li batteries has been a long-sought target since the development of modern battery technology because of its high capacity compared to graphite-based anodes (3860 mAh/gLi vs. 372 mAh/ggraphite), thus offering the prospect of significantly higher cell energy densities. However, Li faces several drastic challenges that have impeded its successful use as an anode so far. These include:1 (1) Large volume expansion upon Li stripping and plating, leading to rapid mechanical degradation; (2) Poor Coulombic (round-trip) efficiencies during cycling, which leads to loss of active Li (formation of so-called “dead Li”); and (3) Formation of dendritic structures upon Li deposition, which grow catastrophically and ultimately short-circuit the cell. Researchers have attempted to address these issues by developing solid electrolyte materials of sufficient mechanical strength to physically block dendrites, but such composites generally have high ionic resistances2 and poor performance at high powers3 due to the minimum thicknesses required for fabricating and processing without breakage. Recently, attention has shifted towards the introduction of “designer” material layers on Li through pre-formed SEI structures.4-6 Such a motif can be accomplished, for example, by introducing clean Li surfaces to donor molecules or additives that contain desirable constituents, such as fluoride ligands, that will presumably be incorporated upon decomposition at the Li surface. However, one major challenge is a lack of fundamental understanding of how such molecules decompose when in contact with a Li surface, either under dry conditions or in the presence of electrolyte. Consequently, there remains little understanding regarding how to intentionally build SEI layers with targeted compositions, morphological and electronic properties, and how these properties translate into electrochemical metrics such as Coulombic efficiency and cycle life. In this work, we present our recent studies of the decomposition reactions of oxide- and fluoride-containing gases on Li surfaces for artificial SEI formation. We first demonstrate controllable growth (10 – 100’s of nm thickness) of targeted ionic materials such as Li2O and LiF which is achieved through judicious selection of the reactant gas and reaction conditions. In gases with multiple potential SEI-forming constituents, we find that reaction kinetics and the decomposition pathway determine which elements are ultimately imparted to an SEI, thereby introducing an ability to rationally tune the SEI by tuning the gas structure and chemistry. We report X-ray photoelectron (XPS) with depth-profiling and X-ray diffraction (XRD) analysis of films to describe their chemical composition and crystallinity. We also employ several methodologies to characterize the electronic and electrochemical behavior of such by-design films, including electrochemical impedance, Li-Li cycling, Li-Cu plating/stripping measurements, and constant-current discharge to failure to measure dendrite propagation times, and compare their behavior to bare Li. Special emphasis will be placed on the mechanistic origin of electrochemical properties, which are derived from Li+ transport through film grain boundaries, and on the failure modes of such films during cycling. Tikekar, M. D.; Choudhury, S.; Tu, Z. Y.; Archer, L. A., Design principles for electrolytes and interfaces for stable lithium-metal batteries. Nat Energy 2016, 1, 1-7.Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A., A lithium superionic conductor. Nat Mater 2011, 10 (9), 682-686.Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H. H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.; Shao-Horn, Y., Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chem Rev 2016, 116 (1), 140-162.Li, Y.; Sun, Y.; Pei, A.; Chen, K.; Vailionis, A.; Li, Y.; Zheng, G.; Sun, J.; Cui, Y., Robust Pinhole-free Li3N Solid Electrolyte Grown from Molten Lithium. 2018, 4 (1), 97-104.Lin, D. C.; Liu, Y. Y.; Chen, W.; Zhou, G. M.; Liu, K.; Dunn, B.; Cui, Y., Conformal Lithium Fluoride Protection Layer on Three-Dimensional Lithium by Nonhazardous Gaseous Reagent Freon. Nano Lett 2017, 17 (6), 3731-3737.Lu, Y. Y.; Tu, Z. Y.; Archer, L. A., Stable lithium electrodeposition in liquid and nanoporous solid electrolytes. Nat Mater 2014, 13 (10), 961-969.

Lithium-ion batteries have high capacity and energy density, but there is a problem in terms of safety because an organic electrolyte solution is flammable. Therefore, development of large-scale lithium-ion batteries with high safety is a challenge for practical use. All-solid-state batteries are attracting attention because they use non-flammable inorganic solid electrolytes. In particular, all-solid-state batteries using metal anode have been expected to be next-generation rechargeable batteries because of their high-energy density using metal anode. Nevertheless, they have not yet been used because metal dendrites cause short circuits during charge and discharge processes. Metal dendrites grow along grain boundaries in polycrystalline solid electrolytes.1) However, there is no report on direct observation of dendritic growth in amorphous solid electrolytes without grain boundaries. To elucidate the mechanism of dendrite formation in solid glass electrolytes, we observed direct three-dimensional images during electrochemical reactions of all-solid-state cell by using operando X-ray computed tomography (CT) with synchrotron radiation. Although lithium metal is generally used as anode, such a light element is difficult to be detected using X-ray absorption techniques. In this study, as a model case of dendrite formation in all-solid-state batteries using metal anode, a battery cell using silver-ion solid electrolytes was examined. Silver-ion conductors have the advantage that their solid electrolytes exhibit high conductivity at room temperature.2) 3) Also, the variation of glass composition is enough, which makes easy to design the battery cell for X-ray CT measurement. Study on silver-based glass electrolytes is mainly being proceeded on oxide-based and sulfide-based materials. Among them, AgI-Ag2O-P2O5 glass electrolyte was selected because it has high flexibility in composition and high chemical stability in air.4) AgI, Ag2O, and P2O5 were mixed at mole ratio of 1:1:1. The mixed powder was calcined at 520 °C for 2 hours. After that, glass electrolyte was obtained by quenching AgI-Ag2O-P2O5 melt to room temperature. X-ray diffraction measurements of the synthesized material revealed the amorphous phase. In this study, a vacuum pump and a quartz glass tube with an inner diameter of 0.24 mm were used to mold the glass electrolyte. First, AgI-Ag2O-P2O5 melt was quenched in a quartz glass tube. Second, an electrode material prepared by mixing silver and AgI-Ag2O-P2O5 glass powder at a mass ratio of 3:1 was added from both ends of the glass tube. Finally, a titanium wire with a diameter of 0.20 mm was connected to the electrode. In this way, we prepared Ag |AgI-Ag2O-P2O5| Ag cell in a diameter of 0.24 mm. Micro-X-ray CT measurements were performed at BL20XU in Spring-8. The X-ray energy was 20 keV. The CT scans were performed before and after the chronopotentiometry measurement.Before the electrochemical measurement, the observed CT images showed the micro-cracks in the glass solid electrolyte. After the chronopotentiometry measurement, it was confirmed that silver metal grew from the point of the initially presented crack. Also, the crack expanded based on the presented crack. It means that the silver metal formation induces additional cracks. The CT images obtained before and after the chronopotentiometry measurement indicate that the silver metal dendrite formation causes the stress in the solid electrolyte, resulting in additional crack formation.1) R. Sudo, Y. Nakata, K. Ishiguro, M. Matsui, A. Hirano, Y. Takeda, O. Yamamoto and N. Imanishi, Solid State Ionics, 262, 151 (2014).2) J. C. Burbano, J. E. Diosa, and D. Peña Lara, Molecular Simulation 45, 724-727 (2019).3) T. Takahashi, S. Ikeda, and O.Yamamoto, J. Electrochem. Soc. 120, 647 (1973).4) M. Tatsumisago and A. Hayashi, J. Chem. Soc. 35, 3 (2006).

A new ionic liquid-based electrolyte with both high conductivity and electrochemical stability