In Situ Lattice-Resolution Revelation of the Origins of Unexplored Anisotropic Sodiation Kinetics and Phase Transition in the Niobium Sulfide Anode.

Layered transition metal dichalcogenides (TMDs) have exhibited huge potential as anode materials for sodium-ion batteries. Most of them usually store sodium via an intercalation-conversion mechanism, but niobium sulfide (NbS2) may be an exception. Herein, through in situ transmission electron microscopy, we carefully investigated the insertion behaviors of Na ions in NbS2 and directly visualized anisotropic sodiation kinetics. Lattice-resolution imaging coupled with density functional theory calculations reveals the preferential diffusion of Na ions within layers of NbS2, accompanied by observable interlayer lattice expansion. Impressively, the Na-inserted layers can still withstand in situ mechanical testing. Further in situ observation vertical to the a/b plane of NbS2 tracked the illusive conversion reaction, which could result from interlayer gliding or wrinkling associated with stress accumulation. In situ electron diffraction measurements ruled out the possibility of such a conversion mechanism and identified a phase transition from pristine 3R-NbS2 to 2H-NaNbS2. Therefore, the NbS2 anode stores Na ions via only the intercalation mechanism, which conceptually differs from the well-known intercalation-conversion mechanism of typical TMDs. These findings not only decipher the whole sodiation process of the NbS2 anode but also provide valuable reference for unraveling the precise sodium storage mechanism in other TMDs.

Inherent Behavior of Electrode Materials of Lithium-Ion Batteries.

For independency from the fossil fuels and to save environment, we need to move toward the green energies, which requires better energy storage devices, especially for usage in electric vehicles. Li-ion and beyond-lithium insertion batteries are promising to this aim. However, they suffer from some inherent limitations which must be understood to allow their development and pave the way to find suitable energy storage alternatives. It is found that each positive or negative electrode material (cathode or anode) of the intercalation batteries has its own behavioral (charge-discharge) properties. The modification of preparation parameters (composition, loading density, porosity, particle size, etc.) may improve some aspects of the electrode performance, but cannot change the intrinsic property of the electrode itself. Accordingly, these properties are called as the "inherent behavior characteristics" of the active material. It is concluded that the behavior of a specific electrode substance, even following different preparation routes, depends only on diffusion mechanisms. This work shows that the inherent electrode properties can be visualized by representation of current density vs. capacity.

Alkali-Ion Intercalation Chemistry and Phase Evolution of Sn4P3.

Despite its high theoretical capacities, Sn4P3 anodes in alkali-ion batteries (AIBs) have been plagued by electrode damage and capacity decay during cycling, mainly rooted in the huge volume changes and irreversible phase segregation. However, few reports endeavor to ascertain whether these causes bear relevance to phase evolution upon cycling. Moreover, the phase evolution mechanism for alkali-ion intercalation remains imprecise. Herein, the structural transformations and detailed mechanisms upon various alkali-ion intercalation processes are systematically revealed, utilizing both experimental techniques and theoretical simulations. The results reveal that the energy storage of Sn4P3 occurs in a two-stage process, starting from an insertion process, followed by a transition process. As the cycle proceeds, the final delithiated/desodiated/depotassiated components gradually trap alkali ions (Li+, Na+, and K+), which is attributed to the incomplete electrochemical transition and difficulty in Sn4P3 regeneration due to the kinetic limitations in removing M (M = Li, Na, and K). Furthermore, Sn4P3 anode obeys the "shrinking core mechanism" in potassium-ion batteries (KIBs), wherein a minor fraction of Sn4P3 in the outer layer of the particles is initially involved in the potassiation/depotassiation processes, followed by a gradual participation of the inner parts until the entire particle is involved. It is worth mentioning that K-Sn alloys are not found to exist during the transition process of KIBs; instead, K-Sn-P phases are found, which makes it differ from that in lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs). These findings are expected to deepen the understanding of the reaction mechanism of Sn4P3 and enlighten the material designs for improved performance.

Energy Storage Mechanism of C12-3-3 with High-Capacity and High-Rate Performance for Li/Mg Batteries.

The low specific capacity and Mg non-affinity of graphite limit the energy density of ion rechargeable batteries. Here, we first identify that the monolayer C12-3-3 in sp2-sp3 carbon hybridization with high Li/Mg affinity is an appropriate anode material for Li-ion batteries and Mg-ion batteries via the first-principles simulations. The monolayer C12-3-3 can achieve high specific capacities of 1181 mAh/g for Li and 739 mAh/g for Mg, higher than those of most previous anodes. The Li storage reaction is an "adsorption-conversion-intercalation mechanism", while the Mg storage reaction is an "adsorption mechanism". The 2D carbon material of C12-3-3 displays fast diffusion kinetics with low diffusion barriers of 0.41 eV for Li and 0.21 eV for Mg. As a new carbon-based anode material, the monolayer C12-3-3 will promote the practical application of batteries with high-capacity and high-rate performance.

Understanding the Predominant Potassium-Ion Intercalation Mechanism of Single-Phased Bimetal Oxides by in Situ Magnetometry.

The electrochemical performance of electrode materials is largely dependent on the structural and chemical evolutions during the charge-discharge processes. Hence, revealing ion storage chemistry could enlighten mechanistic understanding and offer guidance for rational design for energy storage materials. Here, we investigate the mechanisms of potassium (K)-ion storage in the promising bimetal oxide materials by in situ magnetometry. We focus on a single-phased hollow FeTiO3 (SPH-FTO) hexagonal prism synthesized through a complexing-reagent assisted approach and find that the K-ion storage in this compound occurs predominantly with an intercalation mechanism and fractionally a conversion mechanism. We also demonstrate a K-ion hybrid capacitor assembled with the prepared SPH-FTO hexagonal prism anode and activated carbon cathode, delivering a high energy density and high power density as well as extraordinary cycling stability. This new understanding is used to showcase the inherently high K-ion storage properties from the earth-abundant FeTiO3.

Electrolyte Design Enabling Stable Solid Electrolyte Interface for High-Performance Silicon/Carbon Anodes.

Silicon (Si)-based materials have been considered as one of the most promising anodes for the development of high-energy-density lithium-ion batteries (LIBs). However, poor interfacial stability and structural degradation are critical challenges for the successful application of Si-based anodes in LIBs. Herein, the use of a novel fluorinated carbonate (trifluoropropylene carbonate, TFPC) with high reduction potential and rapid film-forming ability as an electrolyte cosolvent is reported, which overcomes the deterioration of the electrode structure that hinders the battery quality. X-ray photoelectron spectroscopy combined with Fourier transform infrared spectroscopy technology investigated the composition and distribution of the solid electrolyte interface (SEI) layer formed on the Si/C anode. Notably, a stable SEI with an organic and inorganic bilayer structure was formed in this electrolyte design, and excellent mechanical properties and ionic conductivity were achieved. Moreover, the Li intercalation mechanism is elucidated by in situ Raman characterization. Benefited from this unique SEI, the Si/C-based batteries exhibit compelling cycling and rate performance. This work provides an in-depth understanding of the Li intercalation mechanism of the Si/C electrode, as well as a novel electrolyte, for high-performance LIBs.

Anisotropic Angstrom-Wide Conductive Channels in Black Phosphorus by Top-down Cu Intercalation.

Intercalation in black phosphorus (BP) can induce and modulate a variety of the properties including superconductivity like other two-dimensional (2D) materials. In this perspective, spatially controlled intercalation has the possibility to incorporate different properties into a single crystal of BP. We demonstrate anisotropic angstrom-wide (∼4.3 Å) Cu intercalation in BP, where Cu atoms are intercalated along a zigzag direction of BP because of its inherent anisotropy. With atomic structure, its microstructural effects, arising from the angstrom-wide Cu intercalation, were investigated and extended to relation with macrostructure. As the intercalation mechanism, it was revealed by in situ transmission electron microscopy and theoretical calculation that Cu atoms are intercalated through top-down direction of BP. The Cu intercalation anisotropically induces transition of angstrom-wide electronic channels from semiconductor to semimetal in BP. Our findings throw light on the fundamental relationship between microstructure changes and properties in intercalated BP, and tailoring anisotropic 2D materials at angstrom scale.

Progressive "Layer to Hybrid Spinel/Layer" Phase Evolution with Proton and Zn2+ Co-intercalation to Enable High Performance of MnO2-Based Aqueous Batteries.

Manganese oxides are promising host materials in rechargeable aqueous batteries due to their low cost and high capacity; however, their practical applications have long been restricted by their sluggish reaction kinetics and poor cycling stability. Herein, the layered K0.36H0.26MnO2·0.28H2O (K36) with a proton and Zn2+ cointercalation mechanism leads to a progressive phase evolution from layer-type K36 to hybrid layer-type KxHyZnzMnO2·nH2O and spinel-type ZnMn2O4 nanocrystal after a long-term cycle. Accordingly, K36 shows a high specific capacity (∼329.8 mAh g-1 at 0.1C), a superior rate performance (∼100.1 mAh g-1 at 10C), and a remarkable cycling stability (capacity retention of ∼93.4% over 3000 cycles at 4C). This work provides a new viewpoint of enhancing electrode performance via generating hybrid phases under electrochemical driving and will be a benefit to developing the next-generation aqueous batteries.

Three-Dimensional Topotactic Host Structure-Secured Ultrastable VP-CNO Composite Anodes for Long Lifespan Lithium- and Sodium-Ion Capacitors.

Performance degradation of lithium/sodium-ion capacitors (LICs/SICs) mainly originates from anode pulverization, particularly the alloying and conversion types, and has spurred research for alternatives with an insertion mechanism. Three-dimensional (3D) topotactic host materials remain much unexplored compared to two-dimensional (2D) ones (graphite, etc.). Herein, vanadium monophosphide (VP) is designed as a 3D topotactic host anode. Ex situ electrochemical characterizations reveal that there are no phase changes during (de)intercalation, which follows the topotactic intercalation mechanism. Computational simulations also confirm the metallic feature and topotactic structure of VP with a spacious interstitial position for the accommodation of guest species. To boost the electrochemical performance, carbon nano-onions (CNOs) are coupled with 3D VP. Superior specific capacity and rate capability of VP-CNOs vs lithium/sodium can be delivered due to the fast ion diffusion nature. An exceptional capacity retention of above 86% is maintained after 20 000 cycles, benefitting from the topotactic intercalation process. The optimized LICs/SICs exhibit high energy/power densities and an ultrastable lifespan of 20 000 cycles, which outperform most of the state-of-the-art LICs and SICs, demonstrating the potential of VP-CNOs as insertion anodes. This exploration would draw attention with regard to insertion anodes with 3D topotactic host topology and provide new insights into anode selection for LICs/SICs.

Uniformly coating ZnAl layered double oxide nanosheets with ultra-thin carbon by ligand and phase transformation for enhanced adsorption of anionic pollutants.

The increasing severity of water pollution has strongly urged to develop green and efficient adsorbents for waste-water treatment. In this work, ZnAl layered double oxide nanosheets uniformly coated with ultra-thin amorphous carbon shells (ZnAl-LDO@C) were fabricated by modifying ZnAl layered double hydroxides (LDHs) with molecular ligands followed by calcination. Compared with their counterparts derived from the pristine ZnAl-LDH, ZnAl-LDO@C nanosheets exhibit higher specific surface area with abundant and highly accessible active sites. The adsorption performance of the ZnAl-LDO@C nanosheets for methyl orange (MO) and hexavalent chromium [Cr(VI)] ions was investigated in detail. It is found that the channel-like hydrophilic carbon shells facilitate the diffusion of water molecules and ions, leading to the fast adsorption rate. In addition, the rich oxygen-containing functional groups in the amorphous carbon shells can efficiently improve the adsorption capacity through multiple interactions. As a result, ZnAl-LDO@C nanosheets exhibit superior adsorption performance for MO and Cr(VI), outperforming most LDH- or LDO-based adsorbents reported previously. Meanwhile, a new oriented overlapping intercalation mechanism for MO adsorption was proposed for the first time to clarify how MO molecules arrange at the interlayer space.

Efficient Sodium-Ion Intercalation into the Freestanding Prussian Blue/Graphene Aerogel Anode in a Hybrid Capacitive Deionization System.

In this study, we introduced an efficient hybrid capacitive deionization (HCDI) system for removal of NaCl from brackish water, in which Prussian blue nanocubes embedded in a highly conductive reduced graphene oxide aerogel have been used as a binderfree intercalation anode to remove Na+ ions. The combination of redox-active nanocubes and the three-dimensional porous graphene network yielded a high salt removal capacity of 130 mg g-1 at the current density of 100 mA g-1. Moreover, energy recovery and energy consumption upon different desorption voltages of the HCDI system were investigated and the result showed a notably low energy consumption of 0.23 Wh g-1 and a high energy recovery of 39%. Furthermore, the real-time intercalation process was verified by in situ X-ray powder diffraction measurements, which confirmed the intercalation and deintercalation processes during charging and discharging, respectively. Eventually, a perfect stability of the desalination unit was confirmed through the steady performance of 100 cycles. The improved efficiency as well as ease of fabrication opens a shiny horizon for our HCDI system toward commercialization of such technology for brackish water desalination.

Dynamic Impedance Spectroscopy of Nickel Hexacyanoferrate Thin Films.

Dynamic multi-frequency analysis (DMFA) is capable of acquiring high-quality frequency response of electrochemical systems under non-stationary conditions in a broad range of frequencies. In this work, we used DMFA to study the kinetics of (de-)intercalation of univalent cations (Na+ and K+) in thin films of nickel hexacyanoferrate (NiHCF) during cyclic voltammetry. For this system, the classic stationary electrochemical impedance spectroscopy fails due to the instability of the oxidized form of NiHCF. We are showing that such spectra can be fitted with a physical model described by a simple two-step intercalation mechanism: an adsorption step followed by an insertion step. The extracted kinetic parameters are depending on the state of charge as well on the nature of the inserted cation.

Electrochemical Exchange Reaction Mechanism and the Role of Additive Water to Stabilize the Structure of VOPO4 ⋠2 H2 O as a Cathode Material for Potassium-Ion Batteries.

VOPO4 ⋅2 H2 O is demonstrated as a cathode material for potassium-ion batteries in 0.6 m KPF6 in ethylene carbonate/diethyl carbonate, and its distinct exchange reaction mechanism between potassium and crystal water is reported. In an anhydrous electrolyte, the cathode shows an initial capacity of approximately 90 mAh g-1 , with poor capacity retention (32 % after 50 cycles). In contrast, the capacity retention dramatically improved (86 % after 100 cycles) in a wet electrolyte containing 0.1 m of additive water. VOPO4 ⋅2 H2 O contains two types of water (structural and crystal). Upon discharge, potassium ions are intercalated whereas the crystal water is simultaneously de-intercalated from the structure. Upon charging, a completely reverse reaction takes place in the wet electrolyte, resulting in high stability of the host structure and excellent cyclability. However, in the anhydrous electrolyte, some portion of the extracted crystal water molecules cannot be reinserted into the host structure because they are distributed over the anhydrous electrolyte. Keeping some concentration of water in the electrolyte turns out to be was the key to achieving such high reversibility. The potassium ions (90 %) and proton or hydronium ions (10 %) seem to be co-intercalated in the wet electrolyte. This work provides a general insight into the intercalation mechanism of crystal-water-containing host materials.

Controllable Design of MoS2 Nanosheets Anchored on Nitrogen-Doped Graphene: Toward Fast Sodium Storage by Tunable Pseudocapacitance.

Transition-metal disulfide with its layered structure is regarded as a kind of promising host material for sodium insertion, and intensely investigated for sodium-ion batteries. In this work, a simple solvothermal method to synthesize a series of MoS2 nanosheets@nitrogen-doped graphene composites is developed. This newly designed recipe of raw materials and solvents leads the success of tuning size, number of layers, and interplanar spacing of the as-prepared MoS2 nanosheets. Under cut-off voltage and based on an intercalation mechanism, the ultrasmall MoS2 nanosheets@nitrogen-doped graphene composite exhibits more preferable cycling and rate performance compared to few-/dozens-layered MoS2 nanosheets@nitrogen-doped graphene, as well as many other reported insertion-type anode materials. Last, detailed kinetics analysis and density functional theory calculation are also employed to explain the Na+ - storage behavior, thus proving the significance in surface-controlled pseudocapacitance contribution at the high rate. Furthermore, this work offers some meaningful preparation and investigation experiences for designing electrode materials for commercial sodium-ion batteries with favorable performance.

Investigation of the Na Storage Property of One-Dimensional Cu2- xSe Nanorods.

In this study, one-dimensional Cu2- xSe nanorods synthesized by a simple water evaporation-induced self-assembly approach are served as the anode material for Na-ion batteries for the first time. Cu2- xSe electrodes express outstanding electrochemical properties. The initial discharge capacity is 149.3 mA h g-1 at a current density of 100 mA g-1, and the discharge capacity can remain at 106.2 mA h g-1 after 400 cycles. Even at a high current density of 2000 mA g-1, the discharge capacity of the Cu2- xSe electrode still remains at 62.8 mA h g-1, showing excellent rate performance. Owing to the excellent electronic conductivity and one-dimensional structure of Cu2- xSe, the Cu2- xSe electrodes manifest fast Na+ ion diffusion rate. Moreover, detailed Na+ insertion/extraction mechanism is further investigated by ex situ measurements and theoretical calculations.

The Mechanism of the Interfacial Charge and Mass Transfer during Intercalation of Alkali Metal Cations.

Intercalation of alkali metal cations, like Li+ or Na+, follows the same three-stage mechanism of the interfacial charge and mass transfer irrespective of the nature of the electrolyte, electrolyte composition or electrode material.