A general strategy for preparing pyrrolic-N4 type single-atom catalysts via pre-located isolated atoms.

Single-atom catalysts (SACs) have been applied in many fields due to their superior catalytic performance. Because of the unique properties of the single-atom-site, using the single atoms as catalysts to synthesize SACs is promising. In this work, we have successfully achieved Co1 SAC using Pt1 atoms as catalysts. More importantly, this synthesis strategy can be extended to achieve Fe and Ni SACs as well. X-ray absorption spectroscopy (XAS) results demonstrate that the achieved Fe, Co, and Ni SACs are in a M1-pyrrolic N4 (M= Fe, Co, and Ni) structure. Density functional theory (DFT) studies show that the Co(Cp)2 dissociation is enhanced by Pt1 atoms, thus leading to the formation of Co1 atoms instead of nanoparticles. These SACs are also evaluated under hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and the nature of active sites under HER are unveiled by the operando XAS studies. These new findings extend the application fields of SACs to catalytic fabrication methodology, which is promising for the rational design of advanced SACs.

Unveiling the Nature of Pt Single-Atom Catalyst during Electrocatalytic Hydrogen Evolution and Oxygen Reduction Reactions.

Single-atom catalysts (SACs) have attracted significant attention due to their superior catalytic activity and selectivity. However, the nature of active sites of SACs under realistic reaction conditions is ambiguous. In this work, high loading Pt single atoms on graphitic carbon nitride (g-C3 N4 )-derived N-doped carbon nanosheets (Pt1 /NCNS) is achieved through atomic layer deposition. Operando X-ray absorption spectroscopy (XAS) is performed on Pt single atoms and nanoparticles (NPs) in both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The operando results indicate that the total unoccupied density of states of Pt 5d orbitals of Pt1 atoms is higher than that of Pt NPs under HER condition, and that a stable Pt oxide is formed during ORR on Pt1 /NCNS, which may suppress the adsorption and activation of O2 . This work unveils the nature of Pt single atoms under realistic HER and ORR conditions, providing a deeper understanding for designing advanced SACs.

Metal Halide Superionic Conductors for All-Solid-State Batteries.

ConspectusRechargeable all-solid-state Li batteries (ASSLBs) are considered to be the next generation of electrochemical energy storage systems. The development of solid-state electrolytes (SSEs), which are key materials for ASSLBs, is therefore one of the most important subjects in modern energy storage chemistry. Various types of electrolytes such as polymer-, oxide-, and sulfide-based SSEs have been developed to date and the discovery of new superionic conductors is still ongoing. Metal-halide SSEs (Li-M-X, where M is a metal element and X is a halogen) are emerging as new candidates with a number of attractive properties and advantages such as wide electrochemical stability windows (0.36-6.71 V vs Li/Li+) and better chemical stability toward cathode materials compared to other SSEs. Furthermore, some of the metal-halide SSEs (such as the Li3InCl6 developed by our group) can be directly synthesized at large scales in a water solvent, removing the need for special apparatus or handling in an inert atmosphere. Based on the recent advances, herein we focus on the topic of metal-halide SSEs, aiming to provide a guidance toward further development of novel halide SSEs and push them forward to meet the multiple requirements of energy storage devices.In this Account, we describe our recent progress in developing metal halide SSEs and focus on some newly reported findings based on state-of-the-art publications on this topic. A discussion on the structure of metal-halide SSEs will be first explored. Subsequently, we will illustrate the effective approaches to enhance the ionic conductivities of metal halide SSEs including the effect of anion sublattice framework, the regulation of site occupation and disorder, and defect engineering. Specifically, we demonstrated that proper structural framework, balanced Li+/vacancy concentration, and reduced blocking effect can promote fast Li+ migration for metal halide SSEs. Moreover, humidity stability and degradation chemistry of metal halide SSEs have been summarized for the first time. Some examples of the application of metal halide SSEs with stability toward humidity have been demonstrated. Direct synthesis of halide SSEs on cathode materials by the water-mediated route has been used to eliminate the interfacial challenges of ASSLBs and has been shown to act as an interfacial modifier for high-performance all-solid-state Li-O2 batteries. Taken together, this Account on metal halide SSEs will provide an insightful perspective over the recent development and future research directions that can lead to advanced electrolytes.

Temperature-Dependent Chemical and Physical Microstructure of Li Metal Anodes Revealed through Synchrotron-Based Imaging Techniques.

The Li metal anode has been long sought-after for application in Li metal batteries due to its high specific capacity (3860 mAh g-1 ) and low electrochemical potential (-3.04 V vs the standard hydrogen electrode). Nevertheless, the behavior of Li metal in different environments has been scarcely reported. Herein, the temperature-dependent behavior of Li metal anodes in carbonate electrolyte from the micro- to macroscales are explored with advanced synchrotron-based characterization techniques such as X-ray computed tomography and energy-dependent X-ray fluorescence mapping. The importance of testing methodology is exemplified, and the electrochemical behavior and failure modes of Li anodes cycled at different temperatures are discussed. Moreover, the origin of cycling performance at different temperatures is identified through analysis of Coulombic efficiencies, surface morphology, and the chemical composition of the solid electrolyte interphase in quasi-3D space with energy-dependent X-ray fluorescence mappings coupled with micro-X-ray absorption near edge structure. This work provides new characterization methods for Li metal anodes and serves as an important basis toward the understanding of their electrochemical behavior in carbonate electrolytes at different temperatures.

Origin of Superionic Li3Y1-xInxCl6 Halide Solid Electrolytes with High Humidity Tolerance.

The high ionic conductivity, air/humidity tolerance, and related chemistry of Li3MX6 solid-state electrolytes (SSEs, M is a metal element, and X is a halogen) has recently gained significant interest. However, most of the halide SSEs suffer from irreversible chemical degradation when exposed to a humid atmosphere, which originates from hydrolysis. Herein, the function of the M atom in Li3MX6 was clarified by a series of Li3Y1-xInxCl6 (0 ≤ x < 1). When the ratio of In3+ was increased, a gradual structural conversion from the hexagonal-closed-packed (hcp) anion arrangement to cubic-closed-packed (ccp) anion arrangement has been traced. Compared to hcp anion sublattice, the Li3MX6 with ccp anion sublattice reveals faster Li+ migration. The tolerance of Li3Y1-xInxCl6 towards humidity is highly improved when the In3+ content is high enough due to the formation of hydrated intermediates. The correlations among composition, structure, Li+ migration, and humidity stability presented in this work provide insights for designing new halide-based SSEs.

Site-Occupation-Tuned Superionic LixScCl3+xHalide Solid Electrolytes for All-Solid-State Batteries.

The enabling of high energy density of all-solid-state lithium batteries (ASSLBs) requires the development of highly Li+-conductive solid-state electrolytes (SSEs) with good chemical and electrochemical stability. Recently, halide SSEs based on different material design principles have opened new opportunities for ASSLBs. Here, we discovered a series of LixScCl3+x SSEs (x = 2.5, 3, 3.5, and 4) based on the cubic close-packed anion sublattice with room-temperature ionic conductivities up to 3 × 10-3 S cm-1. Owing to the low eutectic temperature between LiCl and ScCl3, LixScCl3+x SSEs can be synthesized by a simple co-melting strategy. Preferred orientation is observed for all the samples. The influence of the value of x in LixScCl3+x on the structure and Li+ diffusivity were systematically explored. With increasing x value, higher Li+, lower vacancy concentration, and less blocking effects from Sc ions are achieved, enabling the ability to tune the Li+ migration. The electrochemical performance shows that Li3ScCl6 possesses a wide electrochemical window of 0.9-4.3 V vs Li+/Li, stable electrochemical plating/stripping of Li for over 2500 h, as well as good compatibility with LiCoO2. LiCoO2/Li3ScCl6/In ASSLB exhibits a reversible capacity of 104.5 mAh g-1 with good cycle life retention for 160 cycles. The observed changes in the ionic conductivity and tuning of the site occupations provide an additional approach toward the design of better SSEs.

Water-Mediated Synthesis of a Superionic Halide Solid Electrolyte.

To promote the development of solid-state batteries, polymer-, oxide-, and sulfide-based solid-state electrolytes (SSEs) have been extensively investigated. However, the disadvantages of these SSEs, such as high-temperature sintering of oxides, air instability of sulfides, and narrow electrochemical windows of polymers electrolytes, significantly hinder their practical application. Therefore, developing SSEs that have a high ionic conductivity (>10-3  S cm-1 ), good air stability, wide electrochemical window, excellent electrode interface stability, low-cost mass production is required. Herein we report a halide Li+ superionic conductor, Li3 InCl6 , that can be synthesized in water. Most importantly, the as-synthesized Li3 InCl6 shows a high ionic conductivity of 2.04×10-3  S cm-1 at 25 °C. Furthermore, the ionic conductivity can be recovered after dissolution in water. Combined with a LiNi0.8 Co0.1 Mn0.1 O2 cathode, the solid-state Li battery shows good cycling stability.

Highly Stable Lithium Metal Anode Interface via Molecular Layer Deposition Zircone Coatings for Long Life Next-Generation Battery Systems.

Herein, molecular layer deposition is used to form a nanoscale "zircone" protective layer on Li metal to achieve stable and long life Li metal anodes. The zircone-coated Li metal shows enhanced air stability, electrochemical performance and high rate capability in symmetrical cell testing. Moreover, as a proof of concept, the protected Li anode is used in a next-generation Li-O2 battery system and is shown to extend the lifetime by over 10-fold compared to the batteries with untreated Li metal. Furthermore, in-situ synchrotron X-ray absorption spectroscopy is used for the first time to study an artificial SEI on Li metal, revealing the electrochemical stability and lithiation of the zircone film. This work exemplifies significant progress towards the development and understanding of MLD thin films for high performance next-generation batteries.

Insight into the Microstructure and Ionic Conductivity of Cold Sintered NASICON Solid Electrolyte for Solid-State Batteries.

Li1.3Al0.3Ti1.7(PO4)3 (LATP) is a popular solid electrolyte used in solid-state lithium batteries due to its high ionic conductivity. Traditionally, the densification of LATP is achieved by a high-temperature sintering process (about 1000 °C). Herein, we report the compaction of LATP by a newly developed cold sintering process and post-annealing. LATP pellets are first densified at 120 °C and then annealed at 650 °C, yielding an ionic conductivity of 8.04 × 10-5 S cm-1 at room temperature and a relative density of 93% with a low activation energy of 0.37 eV. High-resolution transmission electron microscopy of the cold sintered pellets is investigated as well, showing that the particles are interconnected with some nanoprecipitates at the grain boundaries. Such nanocrystalline-enriched grain boundaries are beneficial for lithium-ion transportation, which leads to higher ionic conductivity of the cold sintered sample. This new sintering process can direct new horizons for development of all solid-state batteries due to its simplicity.

A Novel Organic "Polyurea" Thin Film for Ultralong-Life Lithium-Metal Anodes via Molecular-Layer Deposition.

Metallic Li is considered as one of the most promising anode materials for next-generation batteries due to its high theoretical capacity and low electrochemical potential. However, its commercialization has been impeded by the severe safety issues associated with Li-dendrite growth. Non-uniform Li-ion flux on the Li-metal surface and the formation of unstable solid electrolyte interphase (SEI) during the Li plating/stripping process lead to the growth of dendritic and mossy Li structures that deteriorate the cycling performance and can cause short-circuits. Herein, an ultrathin polymer film of "polyurea" as an artificial SEI layer for Li-metal anodes via molecular-layer deposition (MLD) is reported. Abundant polar groups in polyurea can redistribute the Li-ion flux and lead to a uniform plating/stripping process. As a result, the dendritic Li growth during cycling is efficiently suppressed and the life span is significantly prolonged (three times longer than bare Li at a current density of 3 mA cm-2 ). Moreover, the detailed surface and interfacial chemistry of Li metal are studied comprehensively. This work provides deep insights into the design of artificial SEI coatings for Li metal and progress toward realizing next-generation Li-metal batteries.

In Situ Li3 PS4 Solid-State Electrolyte Protection Layers for Superior Long-Life and High-Rate Lithium-Metal Anodes.

A thin and adjustable Li3 PS4 (LPS) solid-state electrolyte protection layer on the surface of Li is proposed to address the dynamic plating/stripping process of Li metal. The LPS interlayer is formed by an in situ and self-limiting reaction between P4 S16 and Li in N-methyl-2-pyrrolidone. By increasing the concentration of P4 S16 , the thickness of the LPS layer can be adjusted up to 60 nm. Due to the high ionic conductivity and low electrochemical activity of Li3 PS4 , the intimate protection layer of LPS can not only prevent the formation of Li dendrites, but also reduces parasitic side reactions and improves the electrochemical performance. As a result, symmetric cells with the LPS protection layer can deliver stable Li plating/stripping for 2000 h. Full cells assembled with the LPS-protected Li exhibit two times higher capacity retention in Li-S batteries (≈800 mAh g-1 ) at 5 A g-1 for over 400 cycles compared to their bare Li counterparts. Furthermore, high rate performances can be achieved with Li-LPS/LiCoO2 cells, which are capable of cycling at rates as high as 20 C. This innovative and scalable approach to stabilizing the Li anode can serve as a basis for the development of next-generation high-performance lithium-metal batteries.

Stabilizing the Interface of NASICON Solid Electrolyte against Li Metal with Atomic Layer Deposition.

Solid-state batteries have been considered as one of the most promising next-generation energy storage systems because of their high safety and energy density. Solid-state electrolytes are the key component of the solid-state battery, which exhibit high ionic conductivity, good chemical stability, and wide electrochemical windows. LATP [Li1.3Al0.3Ti1.7 (PO4)3] solid electrolyte has been widely investigated for its high ionic conductivity. Nevertheless, the chemical instability of LATP against Li metal has hindered its application in solid-state batteries. Here, we propose that atomic layer deposition (ALD) coating on LATP surfaces is able to stabilize the LATP/Li interface by reducing the side reactions. In comparison with bare LATP, the Al2O3-coated LATP by ALD exhibits a stable cycling behavior with smaller voltage hysteresis for 600 h, as well as small resistance. More importantly, on the basis of advanced characterizations such as high-resolution transmission electron spectroscope-electron energy loss spectroscopy, the lithium penetration into the LATP bulk and Ti4+ reduction are significantly limited. The results suggest that ALD is very effective in improving solid-state electrolyte/electrode interface stability.

Novel High-Energy-Density Rechargeable Hybrid Sodium-Air Cell with Acidic Electrolyte.

Low-cost, high-energy-density, and highly efficient devices for energy storage have long been desired in our society. Herein, a novel high-energy-density hybrid sodium-air cell was fabricated successfully on the basis of acidic catholytes. Such a hybrid sodium-air cell possess a high theoretical voltage of 3.94 V, capacity of 1121 mAh g-1, and energy density of 4418 Wh kg-1. First, the buffering effect of an acidic solution was demonstrated, which provides relatively long and stable cell discharge behaviors. Second, the catholytes of hybrid sodium-air cells were optimized systematically from the solutions of 0.1 M H3PO4 + 0.1 M Na2SO4 to 0.1 M HAc + 0.1 M NaAc and it was found that the cells with 0.1 M H3PO4 + 0.1 M Na2SO4 displayed a maximum power density of 34.9 mW cm-2. The cell with 0.1 M H3PO4 + 0.1 M Na2SO4 displayed higher discharge capacity of 896 mAh g-1. Moreover, the fabricated acidic hybrid sodium-air cells exhibited stable cycling performance in ambient air and they delivered a low voltage gap around 0.3 V when the current density is 0.13 mA cm-2, leading to a high energy efficiency up to 90%. Therefore, the present study provides new opportunities to develop highly cost-effective energy storage technologies.

Designing High-Performance Nanostructured P2-type Cathode Based on a Template-free Modified Pechini Method for Sodium-Ion Batteries.

Layered oxides are promising cathode materials for sodium-ion batteries because of their high theoretical capacities. However, many of these layered materials experience severe capacity decay when operated at high voltage (>4.25 V), hindering their practical application. It is essential to design high-voltage layered cathodes with improved stability for high-energy-density operation. Herein, nano P2-Na2/3(Mn0.54Ni0.13Co0.13)O2 (NCM) materials are synthesized using a modified Pechini method as a prospective high-voltage sodium storage component without any modification. The changes in the local ionic state around Ni, Mn, and Co ions with respect to the calcination temperature are recorded using X-ray absorption fine structure analysis. Among the electrodes, NCM fired at 850 °C (NCM-850) exhibits excellent electrochemical properties with an initial capacity and energy density of 148 mAh g-1 and 555 Wh kg-1, respectively, when cycled between 2 and 4.5 V at 160 mA g-1 along with improved cyclic stability after 100 charge/discharge cycles. In addition, the NCM-850 electrode is capable of maintaining a 75 mAh g-1 capacity even at a current density of 3200 mA g-1. In contrast, the cell fabricated with NCM obtained at 800 °C shows continuous capacity fading because of the formation of an impurity phase during the synthesis process. The obtained capacity, rate performance, and energy density along with prolonged cyclic life for the cell fabricated with the NCM-850 electrodes are some of the best reported values for sodium-ion batteries as compared to those of other p2-type sodium intercalating materials.

Atomic Layer Deposition of Lithium Niobium Oxides as Potential Solid-State Electrolytes for Lithium-Ion Batteries.

The development of solid-state electrolytes by atomic layer deposition (ALD) holds unparalleled advantages toward the fabrication of next-generation solid-state batteries. Lithium niobium oxide (LNO) thin films with well-controlled film thickness and composition were successfully deposited by ALD at a deposition temperature of 235 °C using lithium tert-butoxide and niobium ethoxide as Li and Nb sources, respectively. Furthermore, incorporation of higher Li content was achieved by increasing the Li-to-Nb subcycle ratio. In addition, detailed X-ray absorption near edge structure studies of the amorphous LNO thin films on the Nb L-edge revealed the existence of Nb as Nb5+ in a distorted octahedral structure. The octahedrons in niobium oxide thin films experienced severe distortions, which could be gradually alleviated upon the introduction of Li atoms into the thin films. The ionic conductivities of the as-prepared LNO thin films were also measured, with the highest value achieving 6.39 × 10-8 S cm-1 at 303 K with an activation energy of 0.62 eV.