Improving electrochemical performance of Na3(VPO4)2O2F cathode materials for sodium ion batteries by constructing conductive scaffold

Achieving a Rapid Na+ Migration and Highly Reversible Phase Transition of NASICON for Sodium-Ion Batteries with Suppressed Voltage Hysteresis and Ultralong Lifespan

Sodium ion batteries have attracted great attention for large scale energy storage devices to replace lithium-ion batteries. As a promising polyanionic cathode material of sodium-ion batteries, Na3V2(PO4)2F3 (NVPF) belonging to NASICON exhibits large gap space and excellent structural stability, leading to a high energy density and ultralong cycle lifespan. To improve its stability and Na ion mobility, K+ cations are introduced into NVPF crystal as in situ partial substitution for Na+. The influence of K+ in situ substitution on crystal structure, electronic properties, kinetic properties, and electrochemical performance of NVPF are investigated. Through ex situ examination, it turns out that K+ occupied Na1 ion, in which the K+ does not participate in the charge-discharge process and plays a pillar role in improving the mobility of Na+. Moreover, the doping of K+ cation can reduce the bandgap energy and improve the electronic conductivity. Besides, the optimal K+ doping concentration in N0.92K0.08VPF/C is found so as to achieve rapid Na+ migration and reversible phase transition. The specific capacity of N0.92K0.08VPF/C is as high as 128.8 mAh g-1 at 0.2 C, and at 10 C its rate performance is excellent, which shows a capacity of 113.3 mAh g-1.

Polyaniline-Coated Na3V2(PO4)2F3 Cathode Enables Fast Sodium Ion Diffusion and Structural Stability in Rechargeable Batteries

Na3V2(PO4)2F3 (NVPF), a typical sodium superionic conductor (NASICON) type structure, has attracted much interest as a potential positive electrode in sodium-ion battery. However, the inherently poor electronic conductivity of phosphates compromises the electrochemical properties of this material. Here, we develop a general strategy to improve the electrochemical performance by preparing a new composite material "polyaniline (PANI)@NVPF" using a Pickering emulsion method. The X-ray diffraction and Raman results indicated a successful PANI coating without affecting the NASICON-type structure of NVPF, and they enhanced the interfacial bonding between the two components. Also, thermogravimetric analysis and scanning electron microscopy analyses revealed that the PANI content influenced the thermal stability and morphology of the nanocomposites. As a result, the sodium test cells exhibited multielectron reactions and a better rate performance for PANI@NVPF nanocomposites as compared to NVPF. Specifically, 2%PANI@NVPF maintained 70% of its initial capacity at 5C. Ex-situ electron paramagnetic resonance revealed the existence of mixed valence states of vanadium (V4+/V3+) in both discharge and charge processes. Consequently, the successful PANI coating into the sodium superionic conductor framework improved the sodium diffusion channels with a measurable increase of diffusion coefficients with cycling (ca. 3.25 × 10-11 cm2 s-1). Therefore, PANI@NVPF nanocomposites are promising cathode candidates for high-rate sodium-ion battery applications.

Revealing the Multi-Electron Reaction Mechanism of Na3 V2 O2 (PO4 )2 F Towards Improved Lithium Storage

Na₃ V₂ O₂ (PO₄ )₂ F (NVOPF) as an attractive electrode material has received much attention based on the one-electron reaction of V⁴⁺ /V⁵⁺ . However, the electrochemical reactions involving lower vanadium valences were not investigated till now. Herein, a composite of graphene decorated nanosheet-assembled NVOPF microflowers (NVOPF/G) was synthesized and the multi-electron reaction of NVOPF/G was conducted by controlling the operation voltage windows. The reaction mechanism, structural changes, and vanadium valences during the insertion/extraction of Li ions (from 2 to 6) were elucidated clearly by in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy. Theoretical computations also revealed the Li-ion locations in the structure of NaV₂ O₂ (PO₄ )₂ F. Due to the additional redox couple of V³⁺ /V⁴⁺ , NVOPF/G displayed a much higher initial capacity of 183.3 mAh g^-1 in the wider voltage window of 1.0-4.8 V than that of 2.5-4.8 V (129.3 mAh g^-1 ). Moreover, excellent Li-storage performance of NVOPF/G at a lower voltage (≤2.5 V) with the active reaction of V²⁺ /V³⁺ /V⁴⁺ was obtained for the first time, demonstrating the high potential of NVOPF/G as an anode material for Li ion storage.

Ultrathin vanadium hydroxide nanosheets assembled on the surface of Ni-Fe-layered hydroxides as hierarchical catalysts for the oxygen evolution reaction

Developing state-of-the-art non-noble metal catalysts for the oxygen evolution reaction holds a key to the production of electrolytic hydrogen. Herein, self-supported hierarchical NiFe LDH/VO(OH)2 nanoflowers/nanosheets grown on a Ni foam have been synthesized via a two-step hydrothermal method. Numerous fine VO(OH)2 nanosheets grown on NiFe LDH nanoflowers enlarge the contact area for the electrolyte penetration and facilitate ion diffusion, while the three-dimensional structure of the material also provides an extensive active surface area and plentiful accessible active sites. Moreover, the strong synergistic interaction between VO(OH)2 and NiFe LDHs subtly modulates the electronic environment, accelerating the electron/charge transfer. As a result, the catalyst exhibits excellent electrochemical performance for OER giving a voltage of 1.51 V to achieve the current density of 100 mA cm-2 and possessed a Tafel slope of 65 mV dec-1 in 1.0 M KOH. In addition, the material exhibited remarkable long-term durability and stability during the 40 h measurement. This investigation provides a promising strategy for rationally designing high-efficiency metal electrocatalysts with hierarchical multi-dimensional nanostructures for OER.

Aliovalent-doped sodium chromium oxide (Na0.9Cr0.9Sn0.1O2 and Na0.8Cr0.9Sb0.1O2) for sodium-ion battery cathodes with high-voltage characteristics

NaCrO2 with high rate-capability is an attractive cathode material for sodium-ion batteries (NIBs). However, the amount of reversibly extractable Na+ ions is restricted by half, which results in relatively low energy density for practical NIB cathodes. Herein, we describe aliovalent-doped O3-Na0.9[Cr0.9Sn0.1]O2 (NCSnO) and O3-Na0.8[Cr0.9Sb0.1]O2 (NCSbO), both of which show high-voltage characteristics that translate to an increase in energy density. In contrast to NaCrO2, NCSnO and NCSbO can be reversibly charged to 3.80 and 3.95 V, respectively, delivering 0.5 Na+ along with Cr3+/4+ redox alone. The reversible chargeability to Na0.4[Cr0.9Sn0.1]O2 and Na0.3[Cr0.9Sb0.1]O2 is not associated with the suppression of Cr6+ formation. Both compounds show concentrations of Cr6+ that are higher than that of Na0.3CrO2, with an absence of O3' phases. This implies that aliovalent-doping contributes to a suppression of the Cr6+ migration into tetrahedral sites in the interslab space, which reduces the possibility of irreversible comproportionation. NCSnO and NCSbO deliver capacities comparable to that of NaCrO2, but show a higher average discharge voltage (2.94 V for NaCrO2; 3.14 V for NCSnO; 3.21 V for NCSbO), which leads to a noticeable increase in energy densities. The high-voltage characteristics of NCSnO and NCSbO are also validated via density-functional-theory calculations.

Construction of reduced graphene oxide wrapped yolk-shell vanadium dioxide sphere hybrid host for high-performance lithium-sulfur batteries

Owing to the considerable theoretical energy density, lithium-sulfur batteries have been deemed as a competitive candidate for the next-generation energy storage devices. However, its commercialization still depends on the moderation of the shuttle effect and the conductivity improvement of the sulfur cathode. Herein, a novel reduced graphene oxide (rGO) wrapped yolk-shell vanadium dioxide (VO2) sphere hybrid host (rGO/VO2) is reported to simultaneously tackle these barriers. In particular, the polar VO2 sphere can chemically anchor and catalyze the conversion of polysulfides effectively both on the yolk and the shell surfaces. Meanwhile, the highly conductive 3D porous rGO network not only allows sufficient penetration of electrolyte and provides efficient transport pathways for lithium ions and electrons, but also buffers the volume variation during the lithiation process. Besides, the dissolution of the polysulfides can also be alleviated by physical confinement via the interconnected carbon network. Benefiting from these synergistic features, such designed rGO/VO2/S cathode delivers outstanding cycle stability (718.6 mA h g-1 initially, and 516.1 mA h g-1 over 400 cycles at 1C) with a fading rate of 0.07% per cycle. Even at 3C, a capacity of 639.7 mA h g-1 is reached. This proposed unique structure could provide novel insights into high-energy batteries.

Synthesis of polyvalent ion reaction of MoS2/CoS2-RGO anode materials for high-performance sodium-ion batteries and sodium-ion capacitors

Metal sulfide is the most promising anode material for sodium storage devices due to its high theoretical capacity and low cost. However, the practical application of metal sulfide is largely hindered by huge capacity fading during the sodiation/desodiation process. Here mixed bimetallic sulfides grown on reduced graphene oxide (MoS₂/CoS₂-RGO) are prepared via a facile hydrothermal method. MoS₂/CoS₂-RGO displays a unique 2D structure which provides large specific surface area for pseudocapacitive charge storage, polyvalent ion reaction for ultrahigh capacity, and a heterostructure to high Na-ion diffusion rate. The optimized MoS₂/CoS₂-RGO shows a considerable reversible capacity of 593.6 mA h g^-1 at 100 mA g^-1 over 50 cycles and a high rate capability of 215.8 mA h g^-1 even at a high specific current of 5000 mA g^-1. A reaction kinetics and galvanostatic intermittent titration technique analysis indicates that MoS₂/CoS₂-RGO possesses fast pseudocapacitive charge storage and high Na-ion diffusion rate, benefiting the kinetics balance between anode and cathode. With this special structure, SICs containing the anode deliver a high specific energy of 152.98 W h kg^-1 at 562.5 W kg^-1. Similarly, the SIB exhibits a good capacities of 64 mA h g^-1 at the high rates of 5C over 100 cycles.