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The insertion of guest species in graphite is the key feature utilized in applications ranging from energy storage and liquid purification to the synthesis of graphene. Recently, it was discovered that solvated-Na-ion intercalation can occur in graphite even though the insertion of Na ions alone is thermodynamically impossible; this phenomenon enables graphite to function as a promising anode for Na-ion batteries. In an effort to understand this unusual behavior, we investigate the solvated-Na-ion intercalation mechanism using in operando X-ray diffraction analysis, electrochemical titration, real-time optical observation, and density functional theory (DFT) calculations. The ultrafast intercalation is demonstrated in real time using millimeter-sized highly ordered pyrolytic graphite, in which instantaneous insertion of solvated-Na-ions occurs (in less than 2 s). The formation of various stagings with solvated-Na-ions in graphite is observed and precisely quantified for the first time. The atomistic configuration of the solvated-Na-ions in graphite is proposed based on the experimental results and DFT calculations. The correlation between the properties of various solvents and the Na ion co-intercalation further suggests a strategy to tune the electrochemical performance of graphite electrodes in Na rechargeable batteries.

We report a 3.8 V manganese-based mixed-phosphate cathode material for applications in sodium rechargeable batteries; i.e., Na4Mn3(PO4)2(P2O7). This material exhibits a largest Mn2+/Mn3+ redox potential of 3.84 V vs. Na+/Na yet reported for a manganese-based cathode, together with the largest energy density of 416 W h kg−1. We describe first-principles calculations and experimental results which show that three-dimensional Na diffusion pathways with low-activation-energy barriers enable the rapid sodium insertion and extraction at various states of charge of the Na4−xMn3(PO4)2(P2O7) electrode (where x = 0, 1, 3). Furthermore, we show that the sodium ion mobility in this crystal structure is not decreased by the structural changes induced by Jahn–Teller distortion (Mn3+), in contrast to most manganese-based electrodes, rather it is increased due to distortion, which opens up sodium diffusion channels. This feature stabilizes the material, providing high cycle stability and high power performance for sodium rechargeable batteries. The high voltage, large energy density, cycle stability and the use of low-cost Mn give Na4Mn3(PO4)2(P2O7) significant potential for applications as a cathode material for large-scale Na-ion batteries.

Ion exchange is a ubiquitous phenomenon central to wide industrial applications, ranging from traditional (bio)chemical separation to the emerging chimie douce synthesis of materials with metastable structure for batteries and other energy applications. The exchange process is complex, involving substitution and transport of different ions under non-equilibrium conditions, and thus difficult to probe, leaving a gap in mechanistic understanding of kinetic exchange pathways toward final products. Herein, we report in situ tracking kinetic pathways of Li

The increasing use of lithium-ion batteries (LIBs) in high-power applications requires improvement of their high-temperature electrochemical performance, including their cyclability and rate capability. Spinel lithium manganese oxide (LiMn2O4) is a promising cathode material because of its high stability and abundance. However, it exhibits poor cycling performance at high temperatures owing to Mn dissolution. Herein we show that when stoichiometric lithium manganese oxide is coated with highly doped spinels, the resulting epitaxial coating has a hierarchical atomic structure consisting of cubic-spinel, tetragonal-spinel, and layered structures, and no interfacial phase is formed. In a practical application of the coating to doped spinel, the material retained 90 % of its capacity after 800 cycles at 60 °C. Thus, the formation of an epitaxial coating with a hierarchical atomic structure could enhance the electrochemical performance of LIB cathode materials while preventing large losses in capacity.

Defect-engineered carbon nanotubes (CNTs) were prepared by KOH activation and subsequent nitrogen doping. Controlled KOH activation of the CNTs enlarged the specific surface area to 988 m 2 g −1 , which is about 4.5 times greater than that of pristine CNTs. In addition, a hierarchical pore structure and a rough surface developed at high degrees of activation, which are advantageous features for fast ion diffusion. The subsequent nitrogen doping changed the band structure of the CNTs, resulting in improved electrical properties. Symmetric supercapacitors fabricated using these nitrogen-doped and activated CNTs (NA-CNTs) successfully worked across a wide potential range (0–3.5 V) and exhibited a high capacitance of 98 F g −1 at a current density of 1 A g −1 . Furthermore, a low equivalent series resistance (2.2 Ω) was achieved owing to the tailored nanostructure and electrical properties of the electrode materials. Over the voltage range from 0 to 3.5 V, supercapacitors based on NA-CNTs exhibited a high specific energy of 59 Wh kg −1 and a specific power of 1750 W kg −1 . In addition, a specific power of 52,500 W kg −1 with a 3-s charge/discharge rate was achieved with a specific energy of 26 Wh kg −1 . Moreover, the supercapacitors showed stable performance over 10,000 charge/discharge cycles.

Herein, we explore the capacity degradation of dilithium rhodizonate salt (Li_2C_6O_6) in lithium rechargeable batteries based on detailed investigations of the lithium de/insertion mechanism in Li_2C_6O_6 using both electrochemical and structural ex situ analyses combined with first-principles calculations. The experimental observations indicate that the Li_xC_6O_6 electrode undergoes multiple two-phase reactions in the composition range of 2 ≤ x ≤ 6; however, the transformations in the range 2 ≤ x ≤ 4 involve a major morphological change that eventually leads to particle exfoliation and the isolation of active material. Through first-principles analysis of Li_xC_6O_6 during de/lithiation, it was revealed that particle exfoliation is closely related to the crystal structural changes with lithium deinsertion from C_6O_6 interlayers of the Li_xC_6O_6. Among the lithium ions found at various sites, the extraction of lithium from C_6O_6 interlayers at 2 ≤ x ≤ 4 decreases the binding force between the C_6O_6 layers, promoting the exfoliation of C_6O_6 layers and pulverization at the electrode, which degrades capacity retention.

Lithium-ion batteries based on intercalation compounds have dominated the advanced portable energy storage market. The positive electrode materials in these batteries belong to a material group of lithium-conducting crystals that contain redox-active transition metal and lithium. Materials without lithium-conducting paths or lithium-free compounds could be rarely used as positive electrodes due to the incapability of reversible lithium intercalation or the necessity of using metallic lithium as negative electrodes. These constraints have significantly limited the choice of materials and retarded the development of new positive electrodes in lithium-ion batteries. Here, we demonstrate that lithium-free transition metal monoxides that do not contain lithium-conducting paths in their crystal structure can be converted into high-capacity positive electrodes in the electrochemical cell by initially decorating the monoxide surface with nanosized lithium fluoride. This unusual electrochemical behaviour is attributed to a surface conversion reaction mechanism in contrast with the classic lithium intercalation reaction. Our findings will offer a potential new path in the design of positive electrode materials in lithium-ion batteries. Positive electrode materials for lithium-ion batteries feature lithium element and lithium-ion conduction paths. Here the authors report transition metal monoxides that contain neither the intrinsic lithium nor conduction channels for high-capacity positive electrode materials.

The development of a high-performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water-splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost-effective catalysts remain elusive. Herein, an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a room-temperature precipitation is introduced as a new OER catalyst; this material exhibits a remarkably low overpotential (η ≈ 367 mV for a current density of 10 mA cm-2 ). A structural investigation using X-ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also reveal that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox-inert groups may be a new unexplored avenue for the design of novel high-performance catalysts.

Metallic lithium (Li) is a promising anode candidate for high-energy-density rechargeable batteries because of its low redox potential and high theoretical capacity. However, its practical application is not imminent because of issues related to the dendritic growth of Li metal with repeated battery operation, which presents a serious safety concern. Herein, various aspects of the electrochemical deposition and stripping of Li metal are investigated with consideration of the reaction rate/current density, electrode morphology, and solid electrolyte interphase (SEI) layer properties to understand the conditions inducing abnormal Li growth. It is demonstrated that the irregular (i.e., filamentary or dendritic) growth of Li metal mostly originates from local perturbation of the surface current density, which stems from surface irregularities arising from the morphology, defective nature of the SEI, and relative asymmetry in the deposition/stripping rates. Importantly, we find that the use of a stripping rate of Li metal that is slower than the deposition rate seriously aggravates the formation of disconnected Li debris from the irregularly grown Li metal. This finding challenges the conventional belief that high-rate stripping/plating of Li in an electrochemical cell generally results in more rapid cell failure because of the faster growth of Li metal dendrites. Figure 1

Cobalt-free layered lithium-rich nickel manganese oxides, Li[LixNiyMn1−x−y]O2, are promising positive electrode materials for lithium rechargeable batteries because of their high energy density and low materials cost. Utilization of the oxygen anionic redox in this series of materials enables realization of a high capacity beyond that achieved via the conventional transition metal cationic redox when charging above 4.5 V vs. Li/Li+. However, substantial voltage decay is inevitable upon electrochemical cycling, which makes this class of materials less practical. The undesirable voltage decay has been proposed to be linked to irreversible structural rearrangement involving irreversible oxygen loss and cation migration. Herein, we demonstrate that the voltage decay of the electrode is correlated to the activation of Mn4+/Mn3+ redox and subsequent cation disordering, which is able to be remarkably suppressed via simple compositional tuning to induce the formation of Ni3+ in the pristine material. By implementing our new strategy, an alternative redox reaction involving the use of this pristine Ni3+ as a redox buffer, which has been designed to be widened from Ni3+/Ni4+ to Ni2+/Ni4+, subdued the Mn4+/Mn3+ reduction without compensation for the capacity in principle. Negligible change in the voltage profile of the modified lithium-rich nickel manganese oxide electrode is observed upon extended cycling, and manganese migration into the lithium layer is significantly suppressed. Based on these findings, we propose a general strategy to suppress the voltage decay of Mn-containing lithium-rich oxides to achieve long-lasting high energy density from this class of materials.

The development of high‐performance oxygen evolution reaction (OER) catalysts is crucial to achieve the clean production of hydrogen via water splitting. Recently, Co‐based oxides have been intensively investigated as some of the most efficient and cost‐effective OER catalysts. In particular, compositional tuning of Co‐based oxides via doping or substitution is shown to significantly affect their catalytic activity. Nevertheless, the origin of this enhanced catalytic activity and the reaction mechanism occurring at catalytic active sites remain controversial. Theoretical investigations are performed on the electrocatalytic properties of pristine and transition metal (Fe, Ni, and Mn)‐substituted Co oxides using first‐principle calculations. A comprehensive evaluation of the doping effects is conducted by considering various oxygen local environments in the crystal structure, which helps elucidate the mechanism behind the doping‐induced enhancement of Co‐based catalysts. It is demonstrated that the local distortion induced by dopant cations remarkably facilitates the catalysis at a specific site by modulating the hydrogen bonding. In particular, the presence of Jahn–Teller‐active Fe(IV) is shown to result in a substantial reduction in the overpotential at the initially inactive catalysis site without compromising the activity of the pristine active sites, supporting previous experimental observations of exceptional OER performance for Fe‐containing Co oxides.

Author(s): Xu, ZL; Park, J; Yoon, G; Kim, H; Kang, K | Abstract: Lithium-ion batteries (LIBs) have dominated the energy storage market for more than two decades; however, the quest for lower-cost battery alternatives is rapidly expanding, especially for large-scale applications. Sodium-ion batteries (SIBs) have recently experienced an impressive resurgence owing to the earth's abundance of sodium resources and the similar electrochemistry of SIBs and the well-established LIBs. Nonetheless, whereas cost-effective and reliable graphite anodes have served as a cornerstone in current LIB technology, one of the major limitations of SIBs has been the inability to exploit graphite as an electrode because of its negligible sodium storage capability. Recently, however, clear progress has been made in preparing high-performance graphitic carbon anodes for SIBs with new findings on the mechanisms of sodium storage. Herein, this paper aims to review the progress made in understanding the sodium storage mechanisms in graphitic carbon materials and comprehensively summarize the start-of-the-art achievements by surveying the correlations among the type of graphitic material, microstructure, sodium storage mechanisms, and electrochemical performance in SIBs. In addition, perspectives related to practical applications, including the electrolyte, coulombic efficiency, and applicability in sodium-ion full cells, are also presented.

Elemental lithium metal has received a great attention as an ideal high-energy-density negative electrode material for lithium batteries, owing to the highest theoretical specific capacity (3860 mAh g-1) and the lowest negative electrochemical potential (-3.040V vs. standard hydrogen electrode). While these latent merits have been significantly plagued by the catastrophic safety issues of lithium metal anodes, which are tied to chronic problems such as (1) lithium dendritic growth and (2) the high reactivity of metallic lithium in electrolyte, recent intense demands for rechargeable batteries with higher energy density has made lithium metal come into the spotlight again. Moreover, various next-generation batteries including lithium-sulfur (Li-S) and lithium-oxygen (Li-O2) systems with exceptionally high energy density heavily rely on the stable cycling of lithium metal electrode. Thus, the utilization of lithium metal anode in rechargeable system is pivotal not only for the further development of lithium ‘ion’ batteries employing high energy lithium metal anode, but also for the realization of the next-generation battery chemistries. Herein, we propose a facile and cost-effective approach to stabilize lithium metal anode using gas phase doping to the electrolyte. We find that the use of gaseous sulfur dioxide as an electrolyte additive is particularly effective for enhancing the chemical/electrochemical stability of the lithium metal surface among various gas additives. The surface analyses reveal that the reduction of sulfur dioxide gas occurs on the lithium metal and produces a thin layer of inorganic Li-S-O compounds, which aids in the development of artificially uniform SEI layers. It also leads to the homogeneous supply of lithium ions to the lithium electrode throughout the uniform SEI layer. Furthermore, the applicability of sulfur dioxide-induced protection layer is examined in several electrolyte systems including acetonitrile based electrolytes which are highly reactive with lithium metal, confirming the simple and effective gas doping strategy to stabilize lithium metal electrode for various types of lithium batteries. This finding of the enhancement of lithium metal stability using sulfur dioxide gas also suggests the underlying reason for the remarkably long calendar life and high efficiency of lithium-sulfur dioxide primary batteries that have been developed in 1970s. This revisit demonstrates that stable and homogeneous protection layers on the lithium metal induced by sulfur dioxide additives efficiently suppress the selective growth of lithium and provides with a detailed mechanistic study using the combined experimental and computational investigations.

The development of a high-performance oxygen evolution reaction (OER) catalyst is pivotal for the practical realization of a water-splitting system. Although an extensive search for OER catalysts has been performed in the past decades, cost-effective catalysts remain elusive. Herein, an amorphous cobalt phyllosilicate (ACP) with layered crystalline motif prepared by a roomtemperature precipitation is introduced as a new OER catalyst; this material exhibits a remarkably low overpotential (η ≈ 367 mV for a current density of 10 mA cm−2). A structural investigation using X-ray absorption spectroscopy reveals that the amorphous structure contains layered motifs similar to the structure of CoOOH, which is demonstrated to be responsible for the OER catalysis based on density functional theory calculations. However, the calculations also reveal that the local environment of the active site in the layered crystalline motif in the ACP is significantly modulated by the silicate, leading to a substantial reduction of η of the OER compared with that of CoOOH. This work proposes amorphous phyllosilicates as a new group of efficient OER catalysts and suggests that tuning of the catalytic activity by introducing redox-inert groups may be a new unexplored avenue for the design of novel high-performance catalysts.

Rechargeable batteries have been regarded as leading candidates for energy storage systems to satisfy soaring energy demands and ensure efficient energy use, and intensive efforts have thus been focused on enhancing their energy densities and power capabilities. First-principles calculations based on quantum mechanics have played an important role in obtaining a fundamental understanding of battery materials, thus providing insights for material design. In this feature article, the theoretical approaches used to determine key battery properties, such as the voltage, phase stability, and ion-diffusion kinetics, are reviewed. Moreover, the recent contribution of first-principles calculations to the interpretation of complicated experimental characterization measurements on battery materials, such as those obtained using X-ray absorption spectroscopy, electron energy-loss spectroscopy, nuclear magnetic resonance spectroscopy, and transmission electron microscopy, are introduced. Finally, perspectives are provided on the research direction of first-principles calculations for the development of advanced batteries, including the further development of theories that can accurately describe the dissolved species, amorphous phases, and surface reactions that are integral to the operation of future battery systems beyond Li-ion batteries.

The intercalation of lithium ions into graphite electrode is the key underlying mechanism of modern lithium-ion batteries. However, co-intercalation of lithium-ions and solvent into graphite is considered undesirable because it can trigger the exfoliation of graphene layers and destroy the graphite crystal, resulting in poor cycle life. Here, it is demonstrated that the [lithium–solvent]+ intercalation does not necessarily cause exfoliation of the graphite electrode and can be remarkably reversible with appropriate solvent selection. First-principles calculations suggest that the chemical compatibility of the graphite host and [lithium–solvent]+ complex ion strongly affects the reversibility of the co-intercalation, and comparative experiments confirm this phenomenon. Moreover, it is revealed that [lithium–ether]+ co-intercalation of natural graphite electrode enables much higher power capability than normal lithium intercalation, without the risk of lithium metal plating, with retention of ≈87% of the theoretical capacity at current density of 1 A g−1. This unusual high rate capability of the co-intercalation is attributed to the (i) absence of the desolvation step, (ii) negligible formation of the solid–electrolyte interphase on graphite surface, and (iii) fast charge-transfer kinetics. This work constitutes the first step toward the utilization of fast and reversible [lithium–solvent]+ complex ion intercalation chemistry in graphite for rechargeable battery technology.

The use of reduced graphene oxide (rGO) suffers from irreparable damage because of topological defects and residual heteroatoms, which degrade the inherent properties of graphene. To restore its electrical transport properties, charge-transfer chemical doping with d-electron-rich heteroatoms has been proposed. Herein, we report the effects of atomic-level selenium doping in rGO. Using first-principles calculations, we found that selenium atoms could be selectively bonded in particular locations, such as the pseudo-edge sites of hole-cluster defects in the basal plane and edge defect sites of graphene; however, we found that the intrinsic topological defects of the basal plane were unfavorable for bonding. Numerous selenium atoms were introduced on the fully amorphorized rGO surface, inducing a dramatic change of its electrical transport properties by electron doping. The large metallic regions formed by the selenium atoms on rGOs led to the enhancement of electrical conductivity by 210 S cm–1 at 300 K. Moreover, the temperature-dependent conductivities (σ)/σ20K of selenium-doped rGOs (Se-rGOs) were almost constant in the temperature range of 20–300 K, indicating that the carrier mobility of Se-rGOs becomes temperature-independent after selenium doping, similar to that of pure graphene. Atomic-thin layers of selenium can turn microscale graphene oxide sheets into enhanced energy-storage devices, reports a new study. Graphene's extraordinary conductivity has attracted interest for applications such as battery anodes, but manufacturers seeking sizeable quantities often use reduced graphene oxide – chemically synthesized carbon films containing oxygen and other defects that hinder charge transport. Hyoung-Joon Jin from Inha University in South Korea and colleagues now demonstrate that heating graphene oxide with elemental selenium returns metal-like conductivity to the carbon sheet through a process called surface transfer doping. Scanning electron microscopy and first-principles calculations revealed that selenium atoms attach to surface edge defect sites and transfer electrons to graphene oxide. These dopants enabled graphene oxide to achieve a similar battery capacity and longevity as pure graphene when incorporated in a prototype lithium-ion device. Selenium atoms were selectively introduced in particular locations such as the pseudo-edge sites of hole-cluster defects in the basal plane and edge defect sites of the fully amorphorized surface of reduced graphene oxide (rGO), inducing a dramatic change of electrical transport properties of rGO by electron doping.

Graphite is the most widely used anode material for Li-ion batteries and is also considered a promising anode for K-ion batteries. However, Na+, a similar alkali ion to Li+ or K+, is incapable of being intercalated into graphite and thus, graphite is not considered a potential electrode for Na-ion batteries. This atypical behavior of Na has drawn considerable attention; however, a clear explanation of its origin has not yet been provided. Herein, through a systematic investigation of alkali metal graphite intercalation compounds (AM-GICs, AM = Li, Na, K, Rb, Cs) in various solvent environments, it is demonstrated that the unfavorable local Na-graphene interaction primarily leads to the instability of Na-GIC formation but can be effectively modulated by screening Na ions with solvent molecules. Moreover, it is shown that the reversible Na intercalation into graphite is possible only for specific conditions of electrolytes with respect to the Na-solvent solvation energy and the lowest unoccupied molecular orbital level of the complexes. It is believed that these conditions are applicable to other electrochemical systems involving guest ions and an intercalation host and hint at a general strategy to tailor the electrochemical intercalation between pure guest ion intercalation and cointercalation.

Grid-scale energy storage systems (ESSs) that can connect to sustainable energy resources have received great attention in an effort to satisfy ever-growing energy demands. Although recent advances in Li-ion battery (LIB) technology have increased the energy density to a level applicable to grid-scale ESSs, the high cost of Li and transition metals have led to a search for lower-cost battery system alternatives. Based on the abundance and accessibility of Na and its similar electrochemistry to the well-established LIB technology, Na-ion batteries (NIBs) have attracted significant attention as an ideal candidate for grid-scale ESSs. Since research on NIB chemistry resurged in 2010, various positive and negative electrode materials have been synthesized and evaluated for NIBs. Nonetheless, studies on NIB chemistry are still in their infancy compared with LIB technology, and further improvements are required in terms of energy, power density, and electrochemical stability for commercialization. Most recent progress on electrode materials for NIBs, including the discovery of new electrode materials and their Na storage mechanisms, is briefly reviewed. In addition, efforts to enhance the electrochemical properties of NIB electrode materials as well as the challenges and perspectives involving these materials are discussed.

Graphite is a universal host materials that can accommodate various guest species in its galleries. Its capability of reversible lithium de/intercalation facilitates graphite as a standard anode for lithium ion batteries, while the thermodynamic instability of sodium intercalation made it not suitable for sodium ion batteries. We recently discovered that reversible solvated-Na ion intercalation enables graphite to become a promising anode for sodium ion batteries. As a continuous effort to exploit the graphite as an anode in sodium ion batteries, here we investigate the solvated-Na intercalation mechanism through in operando X-ray diffraction analysis, electrochemical titration, real-time observation of graphite expansion and density functional theory-based calculations. This work demonstrates the formation of first-stage graphite intercalation compound where the solvated-Na ions are doubly stacked in parallel with graphene layers. We further provide insights on tuning energy storage potentials of solvated ion intercalation electrodes by introducing various solvents.