Dial the Mechanism Switch of VN from Conversion to Intercalation toward Long Cycling Sodium‐Ion Battery

Transition metal nitrides are promising energy storage materials in regard to good metallic conductivity and high theoretical specific capacity, but their cycling stability is impeded by the huge volume change caused by the conversion reaction mechanism. Here, a simple strategy to produce an intercalation pseudocapacitive‐type vanadium nitride (VN) by one‐step ammonification of V₂C MXene for sodium‐ion batteries is reported. Profiting from a distinctive layered structure pillared by Al atoms in the layer spacing, it delivers a high capacity of 372 mA h g^?1 at 50 mA g^?1 and a desirable rate performance. More importantly, it shows remarkably long cycling stability over 7500 cycles without capacity attenuation at 500 mA g^?1. As expected, it is found that the intercalation pseudocapacitance plays an important role in the excellent performance, by using in situ X‐ray diffraction and ex situ X‐ray absorption structure characterization. Even more remarkable, are the high energy and power density of the sodium‐ion capacitor after coupling with a carbon‐based cathode. The hybrid device possesses an energy density of 78.43 Wh kg^?1 at power density of 260 W kg^?1. The results clearly show that such a unique‐layered VN with outstanding Na storage capability is an excellent new material for energy storage systems.     

Mechanistic Insights of Zn2+ Storage in Sodium Vanadates

Rechargeable aqueous zinc‐ion batteries (ZIBs) with high safety and low‐cost are highly desirable for grid‐scale energy storage, yet the energy storage mechanisms in the current cathode materials are still complicated and unclear. Hence, several sodium vanadates with NaV₃O₈‐type layered structure (e.g., Na₅V₁₂O₃₂ and HNaV₆O₁₆·4H₂O) and β‐Na_0.33V₂O₅‐type tunneled structure (e.g., Na_0.76V₆O₁₅) are constructed and the storage/release behaviors of Zn²⁺ ions are deeply investigated in these two typical structures. It should be mentioned that the 2D layered Na₅V₁₂O₃₂ and HNaV₆O₁₆·4H₂O with more effective path for Zn²⁺ diffusion exhibit higher ion diffusion coefficients than that of tunneled Na_0.76V₆O₁₅. As a result, Na₅V₁₂O₃₂ delivers higher capacity than that of Na_0.76V₆O₁₅, and a long‐term cyclic performance up to 2000 cycles at 4.0 A g^?1 in spite of its capacity fading. This work provides a new perspective of Zn²⁺ storage mechanism in aqueous ZIB systems.     

Storage Capacity and Cycling Stability in Ge Anodes: Relationship of Anode Structure and Cycling Rate

Operando X‐ray diffraction (XRD) and X‐ray absorption spectroscopy (XAS) studies of Ge anodes are carried out to understand the effect of cycling rate on Ge phase transformation during charge/discharge process and to relate that effect to capacity. It is discovered that the formation of crystalline Li₁₅Ge₄ (c‐Li₁₅Ge₄) during lithiation is suppressed beyond a certain cycling rate. A very stable and reversible high capacity of ≈1800 mAh g^?1 can be attained up to 100 cycles at a slow C‐rate of C/21 when there is complete conversion of Ge anode into c‐Li₁₅Ge₄. When the C‐rate is increased to ≈C/10, the lithiation reaction is more heterogeneous and a relatively high capacity of ≈1000 mAh g^?1 is achieved with poorer electrochemical reversibility. An increase in C‐rate to C/5 and higher reduces the capacity (≈500 mAh g^?1) due to an impeded transformation from amorphous Li_xGe to c‐Li₁₅Ge₄, and yet improves the electrochemical reversibility. A proposed mechanism is presented to explain the C‐rate dependent phase transformations and the relationship of these to capacity fading. The operando XRD and XAS results provide new insights into the relationship between structural changes in Ge and battery capacity, which are important for guiding better design of high‐capacity anodes.     

Electrochemical and Structural Investigation of Calcium Substituted Monoclinic Li3V2(PO4)3 Anode Materials for Li‐Ion Batteries

In this work, the effect of Li⁺ substitution in Li₃V₂(PO₄)₃ with a large divalent ion (Ca²⁺) toward lithium insertion is studied. A series of materials, with formula Li_3?2xCa_xV₂(PO₄)₃/C (x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3–0.01 V versus Li⁺/Li. Synchrotron diffraction demonstrates that Li₃V₂(PO₄)₃/C has a monoclinic structure (space group P2₁/n), while Ca_1.5V₂(PO₄)₃/C possesses a rhombohedral structure (space group R‐3c). The intermediate compounds, Li₂Ca_0.5V₂(PO₄)₃/C and LiCaV₂(PO₄)₃/C, are composed of two main phases, including monoclinic Li₃V₂(PO₄)₃/C and rhombohedral Ca_1.5V₂(PO₄)₃/C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li₃V₂(PO₄)₃/C and Li₂Ca_0.5V₂(PO₄)₃/C electrodes. In contrast, LiCaV₂(PO₄)₃/C and Ca_1.5V₂(PO₄)₃/C have no obvious oxidation and reduction peaks but a box‐type voltammogram. This feature is the signature for capacitive‐like mechanism, which involves fast electron transfer on the surface of the electrode. Li₃V₂(PO₄)₃/C undergoes two solid‐solution and a short two‐phase reaction during lithiation and delithiation processes, whereas Ca_1.5V₂(PO₄)₃/C only goes through capacitive‐like mechanism. In operando X‐ray absorption spectroscopy confirms that, in both Li₃V₂(PO₄)₃/C and Ca_1.5V₂(PO₄)₃/C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li⁺ with larger divalent cations.     

Mechanism of Sodium Storage in Hard Carbon: An X‐Ray Scattering Analysis

Hard carbon is a standard anode material for Na‐ion batteries. However, its low crystallinity and diverse microstructures make obtaining a full understanding of the sodium storage mechanism challenging. Here, the results of a systematic ex situ small and wide angle X‐ray scattering study of a series of nanostructured hard carbons, which reveal clear evidence of sodium storage in the graphene–graphene interlayers and nanopores, are presented. Particularly, an emergence of a broad peak around q ≈ 2.0–2.1 Å^?1 in the low voltage region is suggested to be an indicator that sodium is densely confined in the nanopores. Thus, classical X‐ray scattering techniques are demonstrated to be effective in elucidating the overall reaction scheme of Na insertion into hard carbon.     

Reversible Multivalent (Monovalent,Divalent, Trivalent) Ion Insertion in Open Framework Materials

The reversible electrochemical insertion of multivalent ions into materials has promising applications in many fields, including batteries, seawater desalination, element purification, and wastewater treatment. However, finding materials that allow for the insertion of multivalent ions with fast kinetics and stable cycling has proven difficult because of strong electrostatic interactions between the highly charged insertion ions and atoms in the host framework. Here, an open framework nanomaterial, copper hexacyanoferrate, in the Prussian Blue family is presented that allows for the reversible insertion of a wide variety of monovalent, divalent, and trivalent ions (such as Rb⁺, Pb²⁺, Al³⁺, and Y³⁺) in aqueous solution beyond that achieved in previous studies. Electrochemical measurements demonstrate the unprecedented kinetics of multivalent ion insertion associated with this material. Synchrotron X‐ray diffraction experiments point toward a novel vacancy‐mediated ion insertion mechanism that reduces electrostatic repulsion and helps to facilitate the observed rapid ion insertion. The results suggest a new approach to multi­valent ion insertion that may help to advance the understanding of this complex phenomenon.     

Synthesis and Electrochemical Activity of Some Na(Li)‐Rich Ruthenium Oxides with the Feasibility to Stabilize Ru6+

The capacity of Li‐ion cathode materials has recently been greatly improved by the feasibility to trigger both cationic and anionic redox reactions within the same material. This concept has rapidly been implemented to Na‐ion batteries to boost their energy density. The electrochemical properties of Na₃RuO₄ with Li₃RuO₄ are reported and compared herein. Strikingly, it is found that 3 Na can be extracted from Na₃RuO₄ with the charge compensation mechanism enlisting first the oxidation of Ru⁵⁺ to Ru⁶⁺, leading to Na₂RuO₄, and then the oxidation of oxygen during the rest of the charge. This drastically contrasts with the behavior of the Li counterpart since Ru never reaches that high oxidation state during lithium removal. By comparing the phase diagrams of A_xRuO₄ (A being Li, Na, or K) together with density functional theory calculations, this finding is rationalized and it is demonstrated that this difference is mainly rooted in the size of the alkali cation. The bigger the alkali, the lower the coordination of Ru will be, stabilized by the same higher oxidation states. This work highlights the difference between Li and Na materials toward anionic redox and suggests the unusual coexistence of Ru⁶⁺ and (O₂)^n?, hence setting new challenges to theoreticians and opening opportunities for materials design.     

Na‐Ion Intercalation and Charge Storage Mechanism in 2D Vanadium Carbide

2D vanadium carbide MXene containing surface functional groups (denoted as V₂CT_x , where T_x are surface functional groups) is synthesized and studied as anode material for Na‐ion batteries. V₂CT_x anode exhibits reversible charge storage with good cycling stability and high rate capability through electrochemical test. The charge storage mechanism of V₂CT_x material during Na⁺ intercalation/deintercalation and the redox reaction of vanadium are studied using a combination of synchrotron based X‐ray diffraction, hard X‐ray absorption near edge spectroscopy (XANES), and soft X‐ray absorption spectroscopy (sXAS). Experimental evidence of a major contribution of redox reaction of vanadium to the charge storage and the reversible capacity of V₂CT_x during sodiation/desodiation process are provided through V K ‐edge XANES and V L _2,3‐edge sXAS results. A correlation between the CO₃^2? content and the Na⁺ intercalation/deintercalation states in the V₂CT_x electrode observed from C and O K ‐edge in sXAS results implies that some additional charge storage reactions may take place between the Na⁺‐intercalated V₂CT_x and the carbonate‐based nonaqueous electrolyte. The results of this study provide valuable information for the further studies on V₂CT_x as anode material for Na‐ion batteries and capacitors.     

Lithiation‐Induced Dilation Mapping in a Lithium‐Ion Battery Electrode by 3D X‐Ray Microscopy and Digital Volume Correlation

Recent advances in high‐resolution 3D X‐ray computed tomography (CT) allow detailed, non‐destructive 3D structural mapping of a complete lithium‐ion battery. By repeated 3D image acquisition (time lapse CT imaging) these investigations of material microstructure are extended into the fourth dimension (time) to study structural changes of the device in operando. By digital volume correlation (DVC) of successive 3D images the dimensional changes taking place during charge cycling are quantified at the electrode level and at the Mn₂O₄ particle scale. After battery discharging, the extent of lithiation of the manganese (III/IV) oxide grains in the electrode is found to be a function of the distance from the battery terminal with grains closest to the electrode/current collector interface having the greatest expansion (≈30%) and grains furthest from the current collector and closest to the counter electrode showing negligible dilation. This implies that the discharge is limited by electrical conductivity. This new CT+DVC technique is widely applicable to the 3D exploration of the microstructural degradation processes for a range of energy materials including fuel cells, capacitors, catalysts, and ceramics.     

Unfolding the Mechanism of Sodium Insertion in Anatase TiO2 Nanoparticles

It is frequently assumed that sodium‐ion battery chemistry exhibits a behavior that is similar to the more frequently investigated lithium‐ion chemistry. However, in this work it is shown that there are great, and rather surprising, differences, at least in the case of anatase TiO₂. While the generally more reducing lithium ion is reversibly inserted in the anatase TiO₂ lattice, sodium ions appear to partially reduce the rather stable oxide and form metallic titanium, sodium oxide, and amorphous sodium titanate, as revealed by means of in situ X‐ray diffraction, ex situ X‐ray photoelectron spectroscopy, scanning electron microscopy, and Raman spectroscopy. Nevertheless, once the electrochemical transformation of anatase TiO₂ is completed, the newly formed material presents a very stable long‐term cycling performance, excellent high rate capability, and superior coulombic efficiency, highlighting it as a very promising anode material for sodium‐ion battery applications.     

Structural Engineering of Multishelled Hollow Carbon Nanostructures for High‐Performance Na‐Ion Battery Anode

Hard carbon has long been considered the leading candidate for anode materials of Na‐ion batteries. Intensive research efforts have been carried out in the search of suitable carbon structure for an improved storage capability. Herein, an anode based on multishelled hollow carbon nanospheres, which are able to deliver an outstanding electrochemical performance with an extraordinary reversible capacity of 360 mAh g^?1 at 30 mA g^?1, is designed. An interesting dependence of the electrochemical properties on the multishelled structural features is identified: with an increase in the shell number of the model carbon materials, the sloping capacity in the charge/discharge curve remains almost unchanged while the plateau capacity continuously increases, suggesting an adsorption‐filling Na‐storage mechanism for the multishelled hollow hard carbon materials. The findings not only provide new perspective in the structural design of high‐performance anode materials, but also shed light on the complicated mechanism behind Na‐storage by hard carbon.     

Boosting the Stable Na Storage Performance in 1D Oxysulfide

Exploring new structure prototypes and phases by material design, especially anode materials, is essential to develop high‐performance Na‐ion batteries. This study proposes a new anode, Na₂Cu_2.09O_0.50S₂, with a 1D crystal structure and outstanding Na storage performance. In view of the crystal structure of Na₂Cu_2.09O_0.50S₂, [Cu₄S₄] chains act as electrically conducting units enabling conductivity as high as 0.5 S cm^?1. The residual Na₄[CuO] chains act as ionically conducting units forming rich channels for the fast conduction of Na ions as well as maintaining the structural stability even after Na ion extraction. Additional ball milling on the as‐prepared Na₂Cu_2.09O_0.50S₂ significantly decreases its grain size, achieving a capacity of 588 mA h g^?1 with a high initial Coulombic efficiency of 93% at 0.2 A g^?1. Moreover, the Na₂Cu_2.09O_0.50S₂ anode demonstrates outstanding rate capability (408 mA h g^?1 at 2 A g^?1) and extending cyclic performance (82% of capacity retention after 400 cycles). The general structural design idea based on functional units may offer a new avenue to new electrode materials.     

In Operando Strain Measurement of Bicontinuous Silicon‐Coated Nickel Inverse Opal Anodes for Li‐Ion Batteries

Elastic strains are measured in operando in a nanostructured silicon‐coated nickel inverse opal scaffold anode, using X‐ray diffraction to study the Si (de)lithiation‐induced Ni strains. The volume expansion upon lithiation of the Si in the anode is constrained by the surrounding Ni scaffold, causing mismatch stresses and strains in the Si and Ni phases during cycling. The Ni strains are measured in operando during (dis)charge cycles, using diffraction peak position and peak broadness to describe the distribution of strain in the Ni. During lithiation, compressive strains in the Ni first increase linearly with charge, after which a gradually decreasing strain rate is observed as the maximum lithiation state is approached; upon delithiation a similar process occurs. In‐plane average compressive strains on the order of 990 ± 40 με are measured in the Ni scaffold during lithiation, corresponding to compressive stresses of 215 ± 9 MPa. The decreasing strain rates and decreasing maximum and recovered strains suggest that plasticity in Ni and/or Si, as well as delamination between Ni and Si, may occur during cycling. Rate sensitivity in capacity is correlated with strain and a maximum Ni compressive stress of 230 ± 40 MPa is measured at the maximum state of lithiation.     

“Water‐in‐Salt” Electrolyte Makes Aqueous Sodium‐Ion Battery Safe,Green, and Long‐Lasting

Narrow electrochemical stability window (1.23 V) of aqueous electrolytes is always considered the key obstacle preventing aqueous sodium‐ion chemistry of practical energy density and cycle life. The sodium‐ion water‐in‐salt electrolyte (NaWiSE) eliminates this barrier by offering a 2.5 V window through suppressing hydrogen evolution on anode with the formation of a Na⁺‐conducting solid‐electrolyte interphase (SEI) and reducing the overall electrochemical activity of water on cathode. A full aqueous Na‐ion battery constructed on Na_0.66[Mn_0.66Ti_0.34]O₂ as cathode and NaTi₂(PO₄)₃ as anode exhibits superior performance at both low and high rates, as exemplified by extraordinarily high Coulombic efficiency (>99.2%) at a low rate (0.2 C) for >350 cycles, and excellent cycling stability with negligible capacity losses (0.006% per cycle) at a high rate (1 C) for >1200 cycles. Molecular modeling reveals some key differences between Li‐ion and Na‐ion WiSE, and identifies a more pronounced ion aggregation with frequent contacts between the sodium cation and fluorine of anion in the latter as one main factor responsible for the formation of a dense SEI at lower salt concentration than its Li cousin.