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The octahedral structure of 2D molybdenum disulfide (1T-MoS2 ) has attracted attention as a high-efficiency and low-cost electrocatalyst for hydrogen production. However, the large-scale synthesis of 1T-MoS2 films has not been realized because of higher formation energy compared to that of the trigonal prismatic phase (2H)-MoS2 . In this study, a uniform wafer-scale synthesis of the metastable 1T-MoS2 film is performed by sulfidation of the Mo metal layer using a plasma-enhanced chemical vapor deposition (PE-CVD) system. Thus, plasma-containing highly reactive ions and radicals of the sulfurization precursor enable the synthesis of 1T-MoS2 at 150 °C. Electrochemical analysis of 1T-MoS2 shows enhanced catalytic activity for the hydrogen evolution reaction (HER) compared to that of previously reported MoS2 electrocatalysts 1T-MoS2 does not transform into stable 2H-MoS2 even after 1000 cycles of HER. The proposed low-temperature synthesis approach may offer a promising solution for the facile production of various metastable-phase 2D materials.

Summary The realization of lithium-ion battery (LIB) anodes with high volumetric energy densities and minimal Li plating at high rates remains a key challenge for emerging technologies, including electric vehicles and grid-level energy storage. Here, we present graphene-functionalized Li2TiSiO5 (G-LTSO) as a high volumetric energy and power density anode for LIBs. G-LTSO forms a dense electrode structure with electronically and ionically conductive networks that deliver superior electrochemical performance. Upon lithiation, in situ transmission electron microscopy reveals that graphene functionalization yields minimal structural changes compared with pristine LTSO, resulting in high cycling stability. Furthermore, G-LTSO exhibits not only high charge and discharge capacities but also low overpotentials at high rates with minimal voltage fading due to reduced formation of a solid-electrolyte interphase. The combination of highly compacted electrode morphology, stable high-rate electrochemistry, and low operating potential enables G-LTSO to achieve exceptional volumetric energy and power densities that overcome incumbent challenges for LIBs.

Flexibile biosensors have a lot of applications in measuring the concentration of target bioanalytes. In combination with its flexibility, electrochemical sensors containing 2D materials have particular advantages such as enlarged area compatibility, transparency, and high scalability. A flexible biosensor was fabricated by direct synthesis of molybdenum disulfide (MoS2) on a polyimide (PI) substrate, which can be used as the working electrode in electrochemistry platforms. The direct formation of 2D-MoS2 on the PI was achieved using plasma-enhanced chemical vapor deposition (PE-CVD). Since the MoS2 provides higher electrical conductivity, the MoS2-Au-PI flexible sensor is able to provide highly sensitive detection of target proteins with a relatively fast response via cyclic voltammetry. To evaluate the high performance of the fabricated sensor, we selected the endocrine-related hormones parathyroid hormone (PTH), triiodothyronine (T3), and thyroxine (T4) as analytes because they are one of the most important markers for the determination of endocrinopathy, however, they are very difficult to quantify. The newly developed biosensor achieved highly sensitive detection of the hormones and could determine their location with high accuracy. In addition, we performed electrochemical measurements of hormones obtained from 30 clinical patients' sera with confirmed agreement and compared with the measurements performed with standard immunoassay equipment (E 170, Roche Diagnostics, Germany).

Lithium-rich layered oxides (LLOs) are considered promising cathode materials for lithium-ion batteries because of their high reversible capacity, which is attributed to the exploitation of the novel anionic redox in addition to the conventional cationic redox process. Transition metal (TM) migration, which is known to be the main cause of the voltage decay in LLOs, is now understood to also be the critical factor triggering anionic redox, although this origin is still under debate. A better understanding of the specific TM migration behavior and its effect during charge/discharge would thus enable further development of this class of materials. Herein, we demonstrate that the unique TM migration during charge/discharge significantly alters the lithium diffusion mechanism/kinetics of LLO cathodes. We present clear evidence of the much more sluggish lithium diffusion occurring during discharge (lithiation) than during charge (de-lithiation), which contrasts with the traditional lithium diffusion model based on simple topotactic lithium intercalation/deintercalation in the layered framework. The reversible but asymmetric TM migration in the structure, which originates from the non-equivalent local environments around the TM during the charge and discharge processes, is shown to affect the lithium mobility. This correlation between TM migration and lithium mobility led us to propose a new lithium diffusion model for layered structures and suggests the importance of considering TM migration in designing new LLO cathode materials.

Makers of point-of-care devices and wearable diagnostics prefer flexible electrodes over conventional electrodes. In this study, a flexible electrode platform is introduced with a WS

Co-intercalation reactions make graphite as promising anodes for sodium ion batteries, however, the high redox potentials significantly lower the energy density. Herein, we investigate the factors that influence the co-intercalation potential of graphite and find that the tuning of the voltage as large as 0.38 V is achievable by adjusting the relative stability of ternary graphite intercalation compounds and the solvent activity in electrolytes. The feasibility of graphite anode in sodium ion batteries is confirmed in conjunction with Na1.5VPO4.8F0.7 cathodes by using the optimal electrolyte. The sodium ion battery delivers an improved voltage of 3.1 V, a high power density of 3863 W kg−1both electrodes, negligible temperature dependency of energy/power densities and an extremely low capacity fading rate of 0.007% per cycle over 1000 cycles, which are among the best thus far reported for sodium ion full cells, making it a competitive choice in large-scale energy storage systems., Graphite is a promising anode material for sodium-ion batteries but suffers from the high co-intercalation potential. Here, the authors examine the factors influencing this potential and tailor the stability of graphite intercalation compound, realizing high energy and power densities.

Increasing demands for performance beyond the limit of current lithium ion batteries for higher energy densities have rejuvenated research using lithium metal as an anode. However, commercial implementation has still been hampered due to safety issues. Herein, we introduce a lithium rechargeable battery system with an auxiliary electrode that can detect the potential signs of an internal short-circuit and simultaneously prevent cell failure by inhibiting further dendritic growth of lithium metal. Based on this working principle, we provide guidelines for an auxiliary electrode design and demonstrate that it can act as both a safety sensor and a lithium scavenger. Finally, we show that our in-house designed cell, using a flexible and self-standing auxiliary electrode, can effectively alert the danger of a short circuit in real-time without additional dendrite growth. We expect that this finding will open up unexplored opportunities utilizing various auxiliary electrode chemistries for safe rechargeable lithium metal batteries.

The formation of lithium sulfides as discharge products imparts high specific energy density to lithium sulfur batteries (LSBs), however, the involvement of soluble intermediates in the battery reaction makes it challenging to achieve it reversibly for extended cycles. The precise understanding of phase transitions from the soluble intermediates to solid discharge products would aid in fundamentally resolving practical issues involving the intermediates, and thus allow the realization of long-lived high-energy-density LSBs. Herein, we utilize liquid in situ transmission-electron-microscopy (TEM) to probe detailed liquid–solid reaction processes. It reveals that the surface nature of the host materials of the polysulfides critically influences the growth mechanism of lithium sulfides. It is elucidated that polar hosts induce instantaneous nucleation of lithium sulfides, followed by diffusion-controlled-to-reaction-limited growth kinetics and a crystalline-to-amorphous phase transition. Moreover, it is verified that polysulfides are better immobilized in polar hosts, whereas polysulfide diffusion through nonpolar hosts is evidently observed, leading to the eventual degradation of cells. Based on these findings, an optimal host structure for sulfur is proposed, where the dual walls of polar (inner)/nonpolar (outer) spheres confine the polysulfides. The new cathode exhibits remarkable electrochemical performance, retaining a capacity of 4.3 mA h cm−2 over 400 cycles at a low electrolyte/sulfur ratio of 6.8 ml g−1, which rivals state-of-the-art LSBs. This work contributes the first liquid in situ TEM study of liquid–solid phase evolution for high energy electrode materials.

Background Various public health measures have been implemented globally to counter the coronavirus disease 2019 (COVID-19) pandemic. The purpose of this study was to evaluate respiratory virus surveillance data to determine the effectiveness of such interventions in reducing transmission of seasonal respiratory viruses. Method We retrospectively analysed data from the Respiratory Virus Detection Surveillance System in Canada, before and during the COVID-19 pandemic, by interrupted time series regression. Results The national level of infection with seasonal respiratory viruses, which generally does not necessitate quarantine or contact screening, was greatly reduced after Canada imposed physical distancing and other quarantine measures. The 2019–2020 influenza season ended earlier than it did in the previous year. The influenza virus was replaced by rhinovirus/enterovirus or parainfluenza virus in the previous year, with the overall test positivity remaining at approximately 35%. However, during the 2019–2020 post-influenza period, the overall test positivity of respiratory viruses during the COVID-19 was still low (7.2%). Moreover, the 2020–2021 influenza season had not occurred by the end of February 2021. Conclusion Respiratory virus surveillance data may provide real-world evidence of the effectiveness of implemented public health interventions during the current and future pandemics.

The octahedral structure of 2D molybdenum disulfide (1T-MoS

The optical properties of colloidal quantum dots (QDs) are controllable through introduction of excess electrons or holes into their delocalized bands. Crucial to robust and energy-efficient electronic doping of QDs is suitable charge compensation. Compensation by surface modification and substitutional impurities are however not sufficiently controllable to enable effective doping of QDs. This article describes electrochemical n-type doping of CdSe QDs where injected electrons are compensated by interstitial Li

Flexibile biosensors have a lot of applications in measuring the concentration of target bioanalytes. In combination with its flexibility, electrochemical sensors containing 2D materials have particular advantages such as enlarged area compatibility, transparency, and high scalability. A flexible biosensor was fabricated by direct synthesis of molybdenum disulfide (MoS

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

The electrochemical properties and performances of lithium-ion batteries are primarily governed by their constituent electrode materials, whose intrinsic thermodynamic and kinetic properties are understood as the determining factor. As a part of complementing the intrinsic material properties, the strategy of nanosizing has been widely applied to electrodes to improve battery performance. It has been revealed that this not only improves the kinetics of the electrode materials but is also capable of regulating their thermodynamic properties, taking advantage of nanoscale phenomena regarding the changes in redox potential, solid-state solubility of the intercalation compounds, and reaction paths. In addition, the nanosizing of materials has recently enabled the discovery of new energy storage mechanisms, through which unexplored classes of electrodes could be introduced. Herein, we review the nanoscale phenomena discovered or exploited in lithium-ion battery chemistry thus far and discuss their potential implications, providing opportunities to further unveil uncharted electrode materials and chemistries. Finally, we discuss the limitations of the nanoscale phenomena presently employed in battery applications and suggest strategies to overcome these limitations.

Lithium-ion batteries are expected to serve as a key technology for large-scale energy storage systems (ESSs), which will help satisfy recent increasing demands for renewable energy utilization. Besides their promising electrochemical performance, the low self-discharge rate (

Makers of point-of-care devices and wearable diagnostics prefer flexible electrodes over conventional electrodes. In this study, a flexible electrode platform is introduced with a WS2 /graphene heterostructure on polyimide (WGP) for the concurrent and selective determination of dopamine and serotonin. The WGP is fabricated directly via plasma-enhanced chemical vapor deposition (PECVD) at 150 °C on a flexible polyimide substrate. Owing to the limitations of existing fabrication methods from physical transfer or hydrothermal methods, many studies are not conducted despite excellent graphene-based heterostructures. The PECVD synthesis method can provide an innovative WS2 /graphene heterostructure of uniform quality and sufficient size (4 in.). This unique heterostructure affords excellent electrical conductivity in graphene and numerous electrochemically active sites in WS2 . A large number of uniform qualities of WGP electrodes show reproducible and highly sensitive electrochemical results. The synergistic effect enabled well-separated voltammetric signals for dopamine and serotonin with a potential gap of 188 mV. Moreover, the practical application of the flexible sensor is successfully evaluated by using artificial cerebrospinal fluid.

Increasing demands for advanced lithium batteries with higher energy density have resurrected the use of lithium metal as an anode, whose practical implementation has still been restricted, because of its intrinsic problems originating from the high reactivity of elemental lithium metal. Herein, we explore a facile strategy of doping gas phase into electrolyte to stabilize lithium metal and suppress the selective lithium growth through the formation of stable and homogeneous solid electrolyte interphase (SEI) layer. We find that the sulfur dioxide gas additive doped in electrolyte significantly improves both chemical and electrochemical stability of lithium metal electrodes. It is demonstrated that the cycle stability of the lithium cells can be remarkably prolonged, because of the compact and homogeneous SEI layers consisting of Li–S–O reduction products formed on the lithium metal surface. Simulations on the lithium metal growth process suggested the homogeneity of the protective layer induced by the ga...

TiO2@SnO2@TiO2 triple-shell nanotubes are fabricated using electrospun polyacrylonitrile (PAN) nanofiber template and plasma-enhanced atomic layer deposition (PEALD). The triple-shell nanotubes have a uniform diameter of ~200 nm, and the thickness of each shell is ~10 nm. The triple-shell nanotube electrode exhibits high reversible capacity of 550 mA g−1 after 60 cycles at the current density of 50 mA g−1, stable cyclability, and high-rate performance (296 mA g−1 at high current density of 5 A g−1) as an anode for lithium-ion batteries. The excellent electrochemical properties are attributed to the structural robustness of the triple-shell nanotubes against pulverization.

Li-O 2 batteries have a high theoretical energy density; however, their current cathode system based on a heavy metal framework strikingly diminishes their real energy density. Herein, we report the fabrication of all-carbon-based cathodes composed of conventional active carbon and a carbon mesh (CM) framework produced from waste silk fabric by simple pyrolysis. CM frameworks show a high electrical conductivity of ∼150 S cm −1 , good tensile strength of 34.1 ± 5.2 MPa, and a Young's modulus of 4.03 ± 0.7 GPa, as well as a well-ventilated ordered macroporous structure. These all-carbon-based cathodes exhibit stable cycling and high energy densities of ∼2600 Wh kg −1 based on total electrode weight, which are 4–15 times higher than those of conventional air cathodes.

An in-depth understanding of electrode reactions is essential to achieve a breakthrough in lithium-ion battery technology, the new ‘engine’ for electric vehicles. Recent studies have continued to reveal unexpected electrode behaviors, providing a more refined view of the operating mechanisms of electrodes from the atomistic to particle level and offering new perspectives to design better battery systems. Herein, it is observed for the first time that the history of applied current densities is memorized in electrode materials that operate via a two-phase reaction and systematically induces a transient galvanostatic profile variation of the electrode. These unforeseen profile changes can be explained by a new proposed intercalation model in which active particle sub-groupings are intermittently generated with a non-uniform chemical potential distribution at the end of charge or discharge. The types of active particle groupings are determined by the current density of the prior charge or discharge, resulting in distinct signatures in the electrochemical profile in the subsequent galvanostatic process. Our proposed intercalation model affords a more comprehensive view of the behavior of electrodes containing many-body particles by elucidating the effect of the applied current densities.

Recent discovery of high-concentration electrolyte systems has opened a new avenue toward the high-voltage, safe, and low-cost aqueous rechargeable batteries. However, the need for generally high-cost organic solutes in the high-concentration electrolyte has become another major obstacle. Herein, we revisited all the commonly used low-cost solutes for high-concentration system and discovered that the use of NaClO4 solute effectively results in a wide electrochemical stability window by suppressing water decomposition and induces stable solid-electrolyte interphase (SEI) layer formation without involving the reduction of salt anions. The SEI layer, composed of Na2CO3 and Na O compounds including NaOH, guarantees the excellent electrochemical storage stability of the full-cell composed of Na4Fe3(PO4)2(P2O7) cathode and NaTi2(PO4)3 anode for the extended period of time. This new class of electrolyte systems provides remarkable cycle stability and a coulombic efficiency of ∼99% at 1C for over 200 cycles, which outperforms the state-of-the-art super-concentrated systems based on NaCF3SO3.

In eusocial ants, aggressive behaviors require the ability to discriminate between chemical signatures such as cuticular hydrocarbons that distinguish nestmate friends from non-nestmate foes. It has been suggested that a mismatch between a chemical signature (label) and the internal, neuronal representation of the colony odor (template) leads to aggression between non-nestmates. Moreover, a definitive demonstration that odorant receptors are responsible for the processing of the chemical signals that regulate nestmate recognition has thus far been lacking. To address these issues, we have developed an aggression-based bioassay incorporating highly selective modulators that target odorant receptor functionality to characterize their role in nestmate recognition in the formicine ant Camponotus floridanus. Electrophysiological studies were used to show that exposure to either a volatilized antagonist or an agonist eliminated or dramatically altered signaling, respectively. Administration of these compounds to adult workers significantly reduced aggression between non-nestmates without altering aggression levels between nestmates. These studies provide direct evidence that odorant receptors are indeed necessary and sufficient for mediating aggression towards non-nestmates. Furthermore, our observations support a hypothesis in which rejection of non-nestmates depends on the precise decoding of chemical signatures present on non-nestmates as opposed to the absence of any information or the active acceptance of familiar signatures.

Solid-state electrolytes based on ionic liquids and a gelling matrix are promising for rechargeable lithium-ion batteries due to their safety under diverse operating conditions, favorable electrochemical and thermal properties, and wide processing compatibility. However, gel electrolytes also suffer from low mechanical moduli, which imply poor structural integrity and thus an enhanced probability of electrical shorting, particularly under conditions that are favorable for lithium dendrite growth. Here, we realize high-modulus, ion-conductive gel electrolytes based on imidazolium ionic liquids and exfoliated hexagonal boron nitride (hBN) nanoplatelets. Compared to conventional bulk hBN microparticles, exfoliated hBN nanoplatelets improve the mechanical properties of gel electrolytes by 2 orders of magnitude (shear storage modulus ∼5 MPa), while retaining high ionic conductivity at room temperature (1 mS cm

Layered, nickel-rich lithium transition metal oxides have emerged as leading candidates for lithium-ion battery (LIB) cathode materials. High-performance applications for nickel-rich cathodes, such as electric vehicles and grid-level energy storage, demand electrodes that deliver high power without compromising cell lifetimes or impedance. Nanoparticle-based nickel-rich cathodes seemingly present a solution to this challenge due to shorter lithium-ion diffusion lengths compared to incumbent micrometer-scale active material particles. However, since smaller particle sizes imply that surface effects become increasingly important, particle surface chemistry must be well characterized and controlled to achieve robust electrochemical properties. Moreover, residual surface impurities can disrupt commonly used carbon coating schemes, which result in compromised cell performance. Using x-ray photoelectron spectroscopy, here we present a detailed characterization of the surface chemistry of LiNi0.8Al0.15Co0.05O2 (NCA) nanoparticles, ultimately identifying surface impurities that limit LIB performance. With this chemical insight, annealing procedures are developed that minimize these surface impurities, thus improving electrochemical properties and enabling conformal graphene coatings that reduce cell impedance, maximize electrode packing density, and enhance cell lifetime fourfold. Overall, this work demonstrates that controlling and stabilizing surface chemistry enables the full potential of nanostructured nickel-rich cathodes to be realized in high-performance LIB technology.

The development of long-lasting and low-cost rechargeable batteries lies at the heart of the success of large-scale energy storage systems for various applications. Here, we introduce Fe- and Mn-based Na rechargeable battery cathodes that can stably cycle more than 3000 times. The new cathode is based on the solid-solution phases of Na4MnxFe3–x(PO4)2(P2O7) (x = 1 or 2) that we successfully synthesized for the first time. Electrochemical analysis and ex situ structural investigation reveal that the electrodes operate via a one-phase reaction upon charging and discharging with a remarkably low volume change of 2.1% for Na4MnFe2(PO4)(P2O7), which is one of the lowest values among Na battery cathodes reported thus far. With merits including an open framework structure and a small volume change, a stable cycle performance up to 3000 cycles can be achieved at 1C and room temperature, and almost 70% of the capacity at C/20 can be obtained at 20C. We believe that these materials are strong competitors for large-s...

To achieve better supercapacitor performance, efforts have focused on increasing the specific surface area of electrode materials to obtain higher energy and power density. The control of pores in these materials is one of the most effective ways to increase the surface area. However, when the size of pores decreases to a sub-nanometer regime, it becomes difficult to apply the conventional parallel-plate capacitor model because the charge separation distance (d-value) of the electrical double layer has a similar length scale. In this study, ultramicroporous carbonaceous materials (UCMs) containing sub-nanometer-scale pores are fabricated using a simple in situ carbonization/activation of cellulose-based compounds containing potassium. The results show that alkali cations act as charge carriers in the ultramicropores (

Olivine electrodes have been extensively studied as an important class of cathode materials for lithium-ion batteries. Although LiFePO4 has shown promise as a low-cost and high-power electrode thereby leading to its commercialization, recently, binary and ternary olivines such as Li(Fe,Mn,Co)PO4 have also been considered as alternatives to overcome the low operating voltage and low energy density of LiFePO4. Herein, we investigate the structural evolution of Li(Mn1/3Fe1/3Co1/3)PO4, which is a promising multi-component olivine cathode material, using combined in situ high-temperature X-ray diffraction and flux neutron diffraction analyses at various states of charge. The phase stability map of the electrode shows that delithiation/lithiation occurs via a one-phase reaction from room temperature to ∼500 °C with excellent thermal stability. Maximum entropy method analysis reveals anisotropic lattice expansion along each lattice direction at elevated temperature, which corresponds well with the lattice variations observed upon delithiation in the electrochemical cell.

Lithium iron phosphate (LFP) has attracted tremendous attention as an electrode material for next-generation lithium-rechargeable battery systems due to the use of low-cost iron and its electrochemical stability. While the lithium diffusion in LFP, the essential property in battery operation, is relatively fast due to the one-dimensional tunnel present in the olivine crystal, the tunnel is inherently vulnerable to the presence of FeLi anti-site defects (Fe ions in Li ion sites), if any, that block the lithium diffusion and lead to inferior performance. Herein, we demonstrate that the kinetic issue arising from the FeLi defects in LFP can be completely eliminated in lithium-excess olivine LFP. The presence of an excess amount of lithium in the Fe ion sites (LiFe) energetically destabilizes the FeLi-related defects, resulting in reducing the amount of Fe defects in the tunnel. Moreover, we observe that the spinodal decomposition barrier is notably reduced in lithium-excess olivine LFP. The presence of LiFe and the absence of FeLi in lithium-excess olivine LFP additionally induce faster kinetics, resulting in an enhanced rate capability and a significantly reduced memory effect. The lithium-excess concept in the electrode crystal brings up unexpected properties for the pristine crystal and offers a novel and interesting approach to enhance the diffusivity and open up additional diffusion paths in solid-state ionic conductors.

The SnO 2 material has been considered as a promising lithium-ion battery anode candidate, and recently, the importance has been increased due to its high performance in sodium-ion batteries. Remarkably, the SnO 2 lithium-ion battery anode usually shows extra specific capacity that greatly exceeds the theoretical value. Partial reversibility of conversion reaction has been commonly considered to contribute the extra capacity, however, this has not clearly solved due to the indirect experimental evidences. Here, a realistic ex situ transmission electron microscopy (TEM) analysis technique was developed to reveal the origin of the extra capacity. We demonstrate that reactions of Li 2 O phase contribute to the extra capacity and the reverse conversion reaction of SnO 2 hardly occurs in the real battery system. This work provides significant implications for establishing an accurate electrochemical reaction mechanism of SnO 2 lithium-ion battery anode, which may lead to inspiration on enhancing performance of the SnO 2 anode in lithium- and sodium-ion batteries as well. Furthermore, the robust ex situ TEM experimental approach we have introduced is extensively applicable to analyses of various battery electrode materials.

Pyroprotein-based carbon nanoplates are fabricated from self-assembled silk proteins as a versatile platform to examine sodium-ion storage characteristics in various carbon environments. It is found that, depending on the local carbon structure, sodium ions are stored via chemi-/physisorption, insertion, or nanoclustering of metallic sodium.

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.

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.

Defects critically affect the properties of materials. Thus, controlling the defect concentration often plays a pivotal role in determining performance. In lithium rechargeable batteries, the operating mechanism is based on ion transport, so large numbers of defects in the electrode crystal can significantly impede Li ion diffusion, leading to decreased electrochemical properties. Here, we introduce a new way to heal defects in crystals by a room-temperature electrochemical annealing process. We show that defects in olivine LiFePO4, an important cathode material, are significantly reduced by the electrochemical recombination of Li/Fe anti-sites. The healed LiFePO4 recovers its high-power capabilities. The types of defects in LiFePO4 and recombination mechanisms are discussed with the aid of first-principles calculations.

The lithium-oxygen battery has the potential to deliver extremely high energy densities; however, the practical use of Li-O2 batteries has been restricted because of their poor cyclability and low energy efficiency. In this work, we report a novel Li-O2 battery with high reversibility and good energy efficiency using a soluble catalyst combined with a hierarchical nanoporous air electrode. Through the porous three-dimensional network of the air electrode, not only lithium ions and oxygen but also soluble catalysts can be rapidly transported, enabling ultra-efficient electrode reactions and significantly enhanced catalytic activity. The novel Li-O2 battery, combining an ideal air electrode and a soluble catalyst, can deliver a high reversible capacity (1000 mAh g(-1) ) up to 900 cycles with reduced polarization (about 0.25 V).

A low-cost and high-performance energy storage device is a key component for sustainable energy utilization. Recently, sodium (Na) ion batteries have been highlighted as a possible competitor to lithium (Li) ion batteries due to their potential merit in the cost effectiveness. Na resources are earth-abundant, and Na electrochemistry shares many similarities with Li. However, their relatively low energy/power densities and unreliable cycle stability need to be addressed. Herein, we propose a novel high-performance cathode for Na rechargeable batteries based on mass-scalable functionalized graphite nanoplatelets. This new class cathode material can deliver a high energy of ~500 W h kg −1 without noticeable capacity decay after 300 cycles. Furthermore, it can retain an energy of ~100 W h kg −1 at a power of ~55 kW kg −1 (less than 10-s charge/discharge), which is the highest among cathodes for Na ion batteries. This transition-metal-free high-performance cathode is expected to lead to the development of low-cost and high-performance Na rechargeable batteries.

Li/O2 battery has the highest theoretical energy density among any battery systems reported to date. However, its poor cycle life and unacceptable energy efficiency from a high charging overpotential have been major limitations. Recently, much higher energy efficiency with low overpotential was reported for a new metal/oxygen system, Na/O2 battery. This finding was unexpected since the general battery mechanism of the Na/O2 system was assumed to be analogous to that of the Li/O2 cell. Furthermore, it implies that fundamentally different kinetics are at work in the two systems. Here, we investigated the reaction mechanisms in the Na/O2 cell using first-principles calculations. In comparative study with the Li/O2 cell, we constructed the phase stability maps of the reaction products of Na/O2 and Li/O2 batteries based on the oxygen partial pressure, which explained why certain phases should be the main discharge products under different operating conditions. From surface calculations of NaO2, Na2O2, and Li2O...

A novel, Na-pillared LiMnPO4 (Li0.78Na0.22MnPO4) with an alluaudite structure is successfully prepared for the first time and proposed as a new positive electrode for Li rechargeable batteries. Approximately 0.8 Li+ ions are reversibly extracted from and reinserted into LiMnPO4via Mn2+/Mn3+ redox reactions. The alluaudite LiMnPO4 (a-LiMnPO4) structure is sufficiently robust during repeated Li extraction/insertion such that ∼92% of the initial capacity is retained after 50 charge/discharge cycles. An ex situ XRD study of the electrochemical cell during cycling indicates that Li de/intercalation in alluaudite LiMnPO4 occurs via the one-phase (solid–solution) reaction instead of the two-phase reaction observed in olivine LiMnPO4.

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.

Hybridizing battery and supercapacitor technologies have the potential to overcome the limitations of the currently prevailing energy-storage systems. Combining high-power capacitive electrodes from supercapacitors with the high-energy intercalation electrodes in lithium-ion batteries provides the opportunity to create a single device that can deliver both high energy and high power. Although energy densities in such hybrid systems easily exceed those found in supercapacitors, the kinetic imbalance between capacitive and intercalation electrodes remains a bottleneck to achieving the desired performance. This imbalance is eliminated through the use of graphene-wrapped Li4Ti5O12 from a simple, one-step process as a high-power anode in a new hybrid supercapacitor. The new hybrid supercapacitors are capable of delivering a high specific energy of up to 50 Wh kg−1 and can even maintain an energy of approximately 15 Wh kg−1 at a 20 s charge/discharge rate.

Compounds with a mixed polyanion framework have recently gained attention as a new class of compounds for material exploration. The potential tunability of the structure by using various combinations of polyanions can potentially lead to a novel cathode. However, the redox reaction in complex structures often involves complex structural evolutions during the electrochemical reaction, which require careful analysis. We investigated the electrochemical mechanism of NaxFe3(PO4)2(P2O7) (1 ≤ x ≤ 4), which was recently proposed as a promising mixed-polyanion cathode for Na rechargeable batteries, using first principles calculations and experiments. We discovered that the de/sodiation of the NaxFe3(PO4)2(P2O7) electrode occurs via a one-phase reaction with a reversible Fe2+/Fe3+ redox reaction and accompanies an exceptionally small volumetric change of less than 4%. Na ion intercalation usually induces a large volumetric change in conventional systems; therefore, this small volume change is unusual and was attri...

A hybrid supercapacitor with high energy and power densities is reported. It comprises a composite anode of anatase TiO2 and reduced graphene oxide and an activated carbon cathode in a non-aqueous electrolyte. While intercalation compounds can provide high energy typically at the expense of power, the anatase TiO2 nanoparticles are able to sustain both high energy and power in the hybrid supercapacitor. At a voltage range from 1.0 to 3.0 V, 42 W h kg−1 of energy is achieved at 800 W kg−1. Even at a 4-s charge/discharge rate, an energy density as high as 8.9 W h kg−1 can be retained. The high energy and power of this hybrid supercapacitor bridges the gap between conventional batteries with high energy and low power and supercapacitors with high power and low energy.