Diffusion Coefficient Of Sodium Ions Research Articles

Binder-free Cu-Sb-Sn-Zn-P alloys as high-efficiency anodes for sodium-ion batteries via double-pulse electrodeposition

Alloy anodes are widely recognized for their high theoretical specific capacity and have garnered significant attention among the anode materials studied for sodium-ion batteries. In this study, we synthesize Cu68.6Sb23.0Sn3.2Zn0.3P4.9 anodes using a double-pulse electrodeposition method. This binder-free alloy anode, comprising five high-capacity elements, significantly enhances specific capacity and improves cycling stability, while also demonstrating excellent sodium storage capability. Specifically, the Cu68.6Sb23.0Sn3.2Zn0.3P4.9 anode achieves an initial charge capacity of 368.8 mAh∙g−1 and remains highly stable over 100 cycles, with a capacity decay of only 0.1 %. In addition, the anode delivers 290.3 mAh∙g−1 even at a current density of 3200 mA∙g−1. The outstanding electrochemical performance can be attributed to improved electronic transfer efficiency due to the introduction of Cu into Cu68.6Sb23.0Sn3.2Zn0.3P4.9 alloy via reverse potential, as well as the porous structure that facilitates electrolyte penetration and sodium ion transport. The galvanostatic intermittent titration technique reveals a high sodium ion diffusion coefficient, further highlighting the potential of this alloy anode in advancing sodium-ion batteries.

A general methodology for the in situ construction of MxSy@N,S-C composites from L-cysteine for sodium-ion battery applications

High-capacity metal sulfides have been extensively investigated as cathode materials for secondary batteries. Despite their high capacity, the redox potential of metal sulfides is limited to 2.7 V. Therefore, researchers explored its application as a negative electrode material and discovered its exceptional electrochemical performance. However, the conventional methods for synthesizing transition metal sulfides (TMS) inevitably require an external source of sulfur for secondary sulfidation, resulting in energy dissipation. In this study, L-cystine was adopted as a polydentate ligand for the synthesis of MxSy@N, SC metal-organic frameworks, which can confine the desired carbon, nitrogen, and sulfur sources within the framework. This approach facilitates one-step sintering to obtain nitrogen-doped transition metal sulfides. The structure composition and electrochemical reaction mechanism of FeS@N,S-C as the negative electrode in Na-ion batteries were studied through scanning electron microscopy (SEM)/transmission electron microscopy (TEM) and X-ray diffraction (XRD)/X-ray photoelectron spectroscopy (XPS) analysis. The electrochemical properties of FeS@N,S-C materials were test as an example. The FeS@N, SC composite exhibited a reversible Na+ storage capacity of 724.4 mAh g−1 at a current density of 200 mA g−1, and still maintained a reversible capacity of 600.7 mAh g−1 at a current density of 2000 mA g−1, demonstrating excellent rate performance. After 200 cycles, the material was still able to retain 97.3 % of the discharge specific capacity, demonstrating fine cycle performance. It shows that the FeS@N, SC composite has rapid electron conduction ability and high sodium ion diffusion coefficient. At the same time, a rapidly charge-discharge process can be achieved with a large capacitive contribution, indicating the great potential of this material as a sodium-ion battery.

Prussian White/Reduced Graphene Oxide Composite as Cathode Material to Enhance the Electrochemical Performance of Sodium-Ion Battery.

Prussian white (PW) is considered a promising cathode material for sodium-ion batteries. However, challenges, such as lattice defects and poor conductivity limit its application. Herein, the composite materials of manganese-iron based Prussian white and reduced graphene oxide (PW/rGO) were synthesized via a one-step in situ synthesis method with sodium citrate, which was employed both as a chelating agent to control the reaction rate during the coprecipitation process of PW synthesis and as a reducing agent for GO. The low precipitation speed helps minimize lattice defects, while rGO enhances electrical conductivity. Furthermore, the one-step in situ synthesis method is simpler and more efficient than the traditional synthesis method. Compared with pure PW, the PW/rGO composites exhibit significantly improved electrochemical properties. Cycling performance tests indicated that the PW/rGO-10 sample exhibited the highest initial discharge capacity and the best cyclic stability. The PW/rGO-10 has an initial discharge capacity of 128 mAh g-1 at 0.1 C (1 C = 170 mA g-1), and retains 49.53% capacity retention after 100 cycles, while the PW only delivers 112 mAh g-1 with a capacity retention of 17.79% after 100 cycles. Moreover, PW/rGO-10 also shows better rate performance and higher sodium ion diffusion coefficient (DNa+) than the PW sample. Therefore, the incorporation of rGO not only enhances the electrical conductivity but also promotes the rapid diffusion of sodium ions, effectively improving the electrochemical performance of the composite as a cathode material for sodium-ion batteries.

Enhanced ion separation by amine grafted graphene oxide-tripolyphosphate anionic composite membrane based on polyvinylidene fluoride

The amine functionalized graphene oxide (GO) was prepared via a chemical reaction. This was achieved by reacting GO with a mixture of ethanolamine, N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride, and sodium tripolyphosphate (STTP). The amine-functionalized graphene oxide (EGOS) with varying STTP:GO weight ratios was then used to fabricate cation-exchange membranes (CEMs) based on polyvinylidene fluoride. The characterizations of the modified EGOS samples were investigated using various techniques, such as Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), Raman spectroscopy, and energy dispersive spectroscopy (EDS). Furthermore, the influence of the STTP:GO weight ratio in the EGOS samples on the properties of the fabricated CEMs was investigated using different experimental methods. More desirable characteristics were recorded for the EGOS-based CEMs compared to GO-containing membranes. The top-performance membrane comprising EGOS0.5 (STTP: GO weight ratio of 0.5:1) demonstrated a remarkable water content of 45.6 ± 1.3 %, and an ion exchange capacity of 3.6 ± 0.3 meq g−1. The electrochemical impedance spectroscopy results showed that this membrane has the lowest resistance of 1.5 Ω cm2, which was considerably (86.96 %) less than the value obtained for the unmodified GO-containing reference membrane (11.5 Ω cm2). The diffusion coefficient of sodium ions through the fabricated CEMs was investigated via electrochemical cyclic voltammetry (CV) and the Randles-Sevceks equation and the highest value was obtained for EGOS0.5-based CEMs.

Novel in-situ encapsulation of tin phosphide particles in MXene conductive networks as anode materials of the durable sodium-ion battery

Sodium-ion battery (SIB) is one of potential alternatives to lithium-ion battery, because of abundant resources and lower price of sodium. High electrical conductivity and long-term durability of MXene are advantageous as the anode material of SIB, but low energy density restricts applications. Tin phosphide possesses high theoretical capacity, low redox potential, and large energy density, but volume expansion reduces its cycling stability. In this study, tin phosphide particles are in-situ encapsulated into MXene conductive networks (SnxPy/MXene) by hydrothermal and phosphorization processes as novel anode materials of SIB. MXene amounts and hydrothermal durations are investigated to evenly distribute SnxPy in MXene. After 100 cycles, SnxPy/MXene reaches high specific capacities of 438.8 and 314.1 mAh/g at 0.2 and 1.0 A/g, respectively. The capacity retentions of 6.0% and 73.6% at 0.2 A/g are respectively obtained by SnxPy and SnxPy/MXene. The better specific capacity and cycling stability of SnxPy/MXene are attributed to less volume expansion of SnxPy during charge/discharge processes and relieved self-stacking of MXene by encapsulating SnxPy particles between MXene layers. Electrochemical impedance spectroscopy and Galvanostatic intermittent titration technique are also applied to analyze the charge storage mechanism in SIB. Higher sodium ion diffusion coefficient and smaller charge-transfer resistance are obtained by SnxPy/MXene.

Determination of Sodium Ion Diffusion Coefficient in Tin Sulfide@Carbon Anode Material Using GITT and EIS Techniques

The electroanalytical behavior of SnSx (x = 1, 2) encapsulated into a carbon phase was studied using the galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). These techniques are widely utilized in battery systems to investigate the diffusion of alkali metal cations in anode and cathode materials depending on the concentration of ions in the host material. Here, we report different calculation methods showing how the applied model affects the derived diffusion coefficient. The calculated value of the apparent chemical diffusion coefficient of sodium ions (DNa+) is in the range of 1 × 10−10 to 1 × 10−15 cm2/s depending on the technique, mathematical protocol, geometry of the electrode material, and applied potential.

Optimizing interfacial modification for enhanced performance of Na3V2(PO4)3 cathode in sodium-ion batteries

A novel cathode material, Na3V2(PO4)3C@CNT (NVP/C@CNT), was designed and synthesized by a low-temperature solid-phase drying ball milling method. The nanoparticles are covered by an amorphous carbon layer, and simultaneously enveloped and embedded by carbon nanotubes on the surface. Consequently, a carbon network consisting of carbon nanotubes and amorphous carbon layers is formed in the material. Notably, no phase transition during the intercalation process of sodium ions, confirming the stable crystal structure, which ensures the stability and reversibility of the large-capacity cathode in the large-volume change. The addition of CNTs can regulate the size of NVP particles, increase the contact area between NVP and electrolyte, leading to an enhancement in the sodium ion diffusion coefficient. The NVP/C@CNT electrode exhibits a capacity of 114.5 mA h g−1 at 0.1 C. After 2650 cycles, the discharge capacity retention rate is 98.2 % at 10 C. Even at 20 C, the discharge capacity is still 93.3 mA h g−1 after 2710 cycles, with a retention rate of 99.5 %. This work provides a feasible approach for the design of low-cost, long-life, high-performance cathode materials for sodium-ion batteries.

Hollow ultra-microporous carbon spheres as high-rate anode nanomaterials for sodium-ion batteries

Hollow porous carbon spheres are considered as promising anode nanomaterials for sodium-ion batteries (SIBs), but suffer from the problems of poor rate and cycling performances. Typical improvement strategies by creating micropores or mesopores are used to boost cycling and rata performances. Herein, hollow porous carbon spheres (HPCS) with ultra-micropores are prepared as anode nanomaterials for SIBs. The optimized HPCS-2 delivers a decent capacity of 272.3 mAh g−1 at 0.1 A g−1, a capacity retention rate of 96.62% over 500 cycles at 2 A g–1, and, in particular, a remarkable rate performance of 233.64 mAh g–1 at 10 A g–1. The great enhancement is elucidated predominantly by the abundant ultra-micropores that provide rapid ion-transport pathways and accelerated sodium-ion diffusion coefficient. The results promise rational design and preparation of high-rate carbon anode nanomaterials for sodium storage.

Stable Structure and Fast Ion Diffusion: A Flexible MoO2@Carbon Hollow Nanofiber Film as a Binder-Free Anode for Sodium-Ion Batteries with Superior Kinetics and Excellent Rate Capability.

Designing innovative anode materials that exhibit excellent ion diffusion kinetics, enhanced structural stability, and superior electrical conductivity is imperative for advancing the rapid charge-discharge performance and widespread application of sodium-ion batteries. Hollow-structured materials have received significant attention in electrode design due to their rapid ion diffusion kinetics. Building upon this, we present a high-performance, free-standing MoO2@hollow carbon nanofiber (MoO2@HCNF) electrode, fabricated through facile coaxial electrospinning and subsequent heat treatment. In comparison to MoO2@carbon nanofibers (MoO2@CNFs), the MoO2@HCNF electrode demonstrates superior rate capability, attributed to its larger specific surface area, its higher pseudocapacitance contribution, and the enhanced diffusion kinetics of sodium ions. The discharge capacities of the MoO2@HCNF (MoO2@CNF) electrode at current densities of 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g-1 are 195.55 (155.49), 180.98 (135.20), 163.81 (109.71), 144.05 (90.46), 121.16 (71.21) and 88.90 (44.68) mAh g-1, respectively. Additionally, the diffusion coefficients of sodium ions in the MoO2@HCNFs are 8.74 × 10-12 to 1.37 × 10-12 cm2 s-1, which surpass those of the MoO2@CNFs (6.49 × 10-12 to 9.30 × 10-13 cm2 s-1) during the discharging process. In addition, these prepared electrode materials exhibit outstanding flexibility, which is crucial to the power storage industry and smart wearable devices.

Preparation of Na3V2(PO4)3 Cathode Materials by Hydrothermal Assisted Sol-Gel Method for Sodium -Ion Batteries

Na3V2(PO4)3 (NVP) cathode material of the sodium ion battery (1 C=117 mAh g-1) has a NASICON-type structure, which not only facilitates the rapid migration of sodium ions, but also has a small volume deformation during sodium ion de-intercalation and the main frame mechanism remains unchanged, and thus is seen as an energy storage material for a wide range of applications, but has a limited electronic conductivity due to its structure. In this paper, NVP cathode materials with finer primary particles are successfully prepared using a simple hydrothermal treatment-assisted sol-gel method. The increased pore size of the NVP materials prepared under the hydrothermal process allows for more active sites and more effective resistance to the volume deformation of sodium ions during insertion/extraction processes, effectively facilitating the diffusion of ions and electrons. The Na3V2(PO4)3 material obtained by the optimized process exhibited good crystallinity in XRD characterization, as well as superior electrochemical properties in a series of electrochemical tests. A specific capacitance of 106.3 mAh g-1 at 0.2 C is demonstrated, compared to 96.5 mAh g-1 for Na3V2(PO4)3 without hydrothermal treatment, and cycling performance is also improved with 93% capacity retention. The calculated sodium ion diffusion coefficient (DNa = 5.68 × 10-14) obtained after EIS curve fitting of the improved sample illustrates that the pore structure is beneficial to the performance of the Na3V2(PO4)3 cathode material.

Fe doping mechanism of Na0.44MnO2 tunnel phase cathode electrode in sodium-ion batteries

Due to its stability and low cost, the tunnel-style sodium-manganese oxide (Na0.44MnO2) material is deemed a popular cathode choice for sodium-ion rechargeable batteries. However, the Jahn-Teller effect caused by Mn3+ in the material results in poor capacity and cycling stability. The purpose of this experimental study is to partially replace Mn3+ with Fe3+, in order to reduce the Jahn-Teller effect of the material during charging and discharging process. The results of Raman spectroscopy and X-ray photoelectron spectroscopy confirmed that the content of Mn3+ decreased after Fe3+ doping. Electrochemical studies show that the Na0.44Mn0.994Fe0.006O2 cathode has better rate performance (exhibits a reversible capacity of 87.9 mAh/g at 2 C) and cycle stability in sodium-ion batteries. The diffusion coefficient of sodium ions increases by Fe3+ doping. The excellent rate performance and capacity improvement are verified by density functional theory (DFT) calculation. After doping, the band gap decreases significantly, and the results show that the state density of O 2p increases near the Fermi level, which promotes the oxidation–reduction of oxygen. This work provides a straightforward approach to enhance the performance of Na0.44MnO2 nanorods, and this performance improvement has guiding significance for the design of other materials in the energy storage domain.

Fine phase adjustment optimizes structure stability and sodium ion diffusion ability of Na0.6Mn0.67–xNi0.22Fe0.11+xO2 via chemical environment variation

In this work, Na0.6Mn0.67−xNi0.22Fe0.11+xO2 (x = 0, 0.05, 0.1, 0.15) cathode materials were synthesized by a simple solid-state method, and the influence of Fe/Mn ratio on its fine phase structure and electrochemical performance was emphatically investigated. Firstly, XRD Rietveld refinement demonstrates that proportion of P2 phase and the distance between Na layers increases via increase of Fe/Mn ratio, which is beneficial to improve the diffusion rate of sodium ions. Secondly, reduced proportion of Mn3+ and Ni3+ in the material with the increase of Fe/Mn ratio is verified via XPS data analysis, which weakens Jahn-Teller effect and improves the cyclic stability of the material. Among them, the designed Na0.6Mn0.62Ni0.22Fe0.16O2 exhibits the initial discharge specific capacity 139 mAh g-1 at 0.5 C. The reversible specific capacity of 95 mAh g-1 can be maintained even at a higher current density of 2 C. The high electrochemical performance can be contributed to the lower charge transfer resistance and the higher sodium ion diffusion coefficient, which are evidenced by EIS and GITT analysis.

Designing CoHCF@FeHCF Core-Shell Structures to Enhance the Rate Performance and Cycling Stability of Sodium-Ion Batteries.

Prussian blue analogs are well suited for sodium-ion battery cathode materials due to their cheap cost and high theoretical specific capacity. Nax CoFe(CN)6 (CoHCF), one of the PBAs, has poor rate performance and cycling stability, while Nax FeFe(CN)6 (FeHCF) has better rate and cycling performance. The CoHCF@FeHCF core-shell structure is designed with CoHCF as the core material and FeHCF as the shell material to enhance the electrochemical properties. The successfully prepared core-shell structure leads to a significant improvement in the rate performance and cycling stability of the composite compared to the unmodified CoHCF. The composite sample of core-shell structure has a specific capacity of 54.8 mAh g-1 at high magnification of 20 C (1C = 170mA g-1 ). In terms of cycle stability, it has a capacity retention rate of 84.1% for 100 cycles at 1 C, and a capacity retention rate of 82.7% for 200 cycles at 5 C. Kinetic analysis shows that the composite sample with the core-shell structure has fast kinetic characteristics, and the surface capacitance occupation ratio and sodium-ion diffusion coefficient are higher than those of the unmodified CoHCF.

Cellulose as a novel precursor to construct high-performance hard carbon anode toward enhanced sodium-ion batteries

Cellulose, as it has high content and low cost along with the absence of heteroatoms, can well balance the cost and structural consistency issues raised by mass production. Herein, the cellulose-derived hard carbon as an anode material for sodium-ion batteries with high performance has been prepared by a one-step direct carbonization process using pure cellulose as a precursor in the temperature range of 900–1400 °C. Moreover, the effect of carbonization temperature on the microstructure and the sodium battery storage properties of cellulose-derived hard carbon was investigated by structural and morphological characterization, as well as electrochemical performance testing. The results show that the CHC-1300 electrode material displays a high reversible capacity of 373.2 mAh g−1 at 30 mA g−1, and exhibits a desirable cycling stability with 89.7 % capacity retention even after 200 cycles. Furthermore, cyclic voltammetry technique (CV) at different scan rates and galvanostatic intermittent titration technique (GITT) have been employed to explore the sodium storage mechanism and sodium storage kinetics of cellulose-derived hard carbon. The results exhibit that the rate performance of the electrode material would be affected by the low apparent diffusion coefficient of sodium ions in the plateau region. Besides, a full-cell has been assembled with CHC-1300 as the anode and Na0.83Mg0.11Ni0.22Mn0.63O2 (NMNM) as the cathode. The CHC-1300//NMNM full-cell shows high energy density and good cycling stability, which could verify the practical application of the cellulose-derived hard carbon.

Kinetic Analysis of Sodium-Ion Intercalation Reaction into Graphene-Like Graphite by Electrochemical Impedance Spectroscopy

Graphene-like graphite (GLG) is a promising anode material for sodium-ion batteries, which is believed to have unique kinetic properties compared to hard carbon due to its different intercalation mechanism. In this study, electrochemical impedance spectroscopy was used to investigate the kinetic properties of sodium-ion intercalation in GLG. Our results indicated that the activation energies for interfacial sodium-ion transfer of GLGs were nearly identical to those reported for graphite, regardless of the heat treatment temperature of the GLGs. Furthermore, these activation energies were lower than those observed for hard carbon, suggesting better sodium-ion intercalation kinetics. In addition, the diffusion coefficient of sodium ions in the GLG was similar to that of graphite, with the highest value observed for GLG800, the GLG heat-treated at the highest temperature of 800 °C. This may indicate that the diffusion coefficient increases with the presence of nanopores in the graphene layer of GLG. It has also been reported that GLG800 is superior in terms of reversible capacity and working potential compared to GLGs synthesized at other temperatures. Consequently, the results clearly demonstrated that GLG800 has the best electrochemical properties in terms of both thermodynamics and kinetics among the GLGs investigated in this study.

Iron-based Prussian blue coupled with polydopamine film for advanced sodium-ion batteries

In this study, high-performance polydopamine (PDA) coated Na1.33FeFe(CN)6·1·16H2O (PBA) cathode materials for sodium-ion batteries (SIBs) are successfully synthesized by using a single source method with Na4Fe(CN)6 as sole iron source. The PDA coating does not alter the structure or morphology of the PBA sample, but greatly improves its conductivity and Na ion diffusion ability because of the good conductivity. Consequently, we observe enhanced reversible capacity, rate capability and cycle stability during cycling. Our results indicate that PBA-2 has the best electrochemical performance and sodium ion diffusion coefficient among all samples. Even at 1 C rate, PBA-2 has a high initial discharge capacity of 145 mAh g−1 with a capacity retention of 90% over 100 cycles. The introduction of a suitable amount of PDA represents an effective strategy for enhancing the electrochemical activity of PBA for advanced SIBs. Such design strategies can be also used to modify other cathode materials for SIBs.

High-Voltage Layered Manganese-Based Oxide Cathode with Excellent Rate Capability Enabled by K/F Co-doping

P-type layered manganese-based materials are prone to undergo lattice oxygen oxidation accompanied by oxygen layer slipping and even unfavorable phase transitions at around 4.2 V, giving rise to rapid discharge capacity decline and inferior structural stability, which restricts their operating voltage and energy density. Here we propose a modification strategy for layered Na0.67MnO2 material with K/F co-doping so as to reach the optimization of lattice oxygen oxidation behavior and structural stability at a high cut-off voltage of 4.4 V. Combining X-ray powder diffraction refinement results, ex situ X-ray photoelectron spectrometry analysis, high-resolution transmission electron microscopy, and electrochemical characterization, our work demonstrates that an appropriate amount of K/F co-doping has a favorable effect on the formation of well-reversible lattice oxygen oxidation behavior and the improvement of Na layer spacing. Owing to the synergetic effect of potassium and fluorine atoms, the initial discharge capacity is up to 210.2 mA h g–1 at 0.1 C and 140.2 mA h g–1 at 1 C with 73% capacity retention after 100 cycles and high discharge capacity of 115.0 mA h g–1 at 5 C, with 68.1 mA h g–1 retained after 200 cycles. The kinetic analysis shows that the optimal K0.05Na0.62MnO1.95F0.05 sample exhibits the largest sodium ion diffusion coefficient and the smallest electrochemical polarization, which paves a novel path for the application of layered manganese-based oxides in high-voltage cathode materials for sodium-ion batteries.

In-situ synthesis of Fe7S8 on metal sites of MOFs as high-capacity and fast-kinetics anodes for sodium ion batteries

Low cost and high theoretical capacity are the main directions for the development of anode materials for sodium-ion batteries in the future, while iron sulfide fits in with this expectation. In this paper, nano-size porous Fe7S8 anode (Fe7S8 @MC) material with its Fe and mesoporous carbon (MC) derived from the metal-organic framework (MOF) MIL-53 (Fe) for the sodium ion battery is successfully prepared by hydrothermal reaction combined with carbonization and vulcanization process. The iron ion sites are fully utilized to form carbon coated Fe7S8 basic monomers, and the monomers are connected with the carbon skeleton to form a porous nanostructure, which can not only maintain high energy storage capacity, but also ensures the stability of the electrode structure, so as to obtain stable cycling and rate performance. At the current density of 500 mA g-1, the reversible specific capacity of the obtained material is still 413.9 mAh g-1, even after 100 cycles. In particular, the reversible capacity of 390.2 mAh g-1 can still be obtained at a high current density of 1 A g-1. Simultaneously, the obtained material can recover 97% of the initial capacity after being charged and discharged at the high specific current of 1 A g-1. The total sodium storage capacity is contributed by a combination of capacitance and diffusion. In addition, the calculated result for the apparent activation energy (Ea = 30.89 kJ mol-1), the diffusion apparent activation energy (EaD = 14.41 kJ mol-1), and the sodium ion diffusion coefficient (DNa+ = 2.55 × 10-12 cm2 s-1) show that the obtained materials have rapid reaction kinetics. Therefore, synthetic materials have excellent research potential and are expected to be used in high-performance sodium ion batteries with practical efficacy.

Effect of Ti doping on the structure and electrochemical properties of Na3V2(PO4)2F3 as anode material for sodium ion batteries

In this article, Co-doped Na3V2- x Ti x (PO4)2F3(NVTPF x = 0, 0.1, 0.5, 0.7) sodium ion battery anode material was prepared by one-step hydrothermal method and it was coated with graphene. The crystal structure and morphology of NVTPF cathode material were characterized by scanning electron microscope and X-ray diffractometer. The results show that the particle diameter of the sample becomes smaller and the shape gradually becomes regular with the increase of Ti doping amount. The graphene coating did not change the crystal structure of NVTPF, but it was helpful to improve its electrochemical performance. The substitution of Ti4+ for V3+ contributes to improve the electronic conductivity and diffusion coefficient of sodium ions, reduce the charge transfer impedance, and accelerate the reaction kinetics of NVTPF electrode, thus significantly improving the cycling stability of NVTPF electrode. Among the four groups of materials, Na3v1.5Ti0.5(PO4)2F3 has the best electrochemical performance. At 1C, the specific capacity of initial discharge is 53.4 mAh/g, and the capacity retention rate of 35 cycles is 72.3%. The graphene coated NVTPF has a discharge capacity of 125.9mAh/g for the first cycle and 44.8mAh/g for 35 cycles at 1C.

TiO2-coated MoP/phosphorus doped carbon nanorods for ultralong-life sodium ion batteries with high capacity.

How to green synthesize and construct MoP anode electrode materials with advanced structures for sodium-ion batteries still faces great challenges. Herein, a TiO2-coated MoP/phosphorus doped carbon (MoP@TiO2/P-C) nanorod with a new structure is constructed using TiO2-coated Mo-MOF as the precursor through an in situ topological conversion technique. In the synthesis process, the traditional highly toxic PH3 phosphorus is avoided. TEM results reveal that TiO2 nanoparticles are distributed at the interface between the MoP core and the P-doped carbon shell, which breaks the density of the carbon layer and facilitates ion transport. The GITT results demonstrate the fact that the diffusion coefficient of sodium ions is remarkably improved by two orders of magnitude due to the presence of TiO2. Notably, TiO2 can effectively cushion volume expansion with an almost negligible rate of 13%, allowing cells to manifest a high discharge capacity of 419 mA h g-1 at 0.5 A g-1 current density and exceptional stability where the specific capacity remains constant for 10 000 cycles at a high density of 10 A g-1 (∼81 seconds for one charging). The results indicate that MoP@TiO2/P-C possesses promising capabilities as an anode substance for SIBs. This also establishes a foundation for future investigations and practical use of this material in the field.