Sodium Diffusion Coefficient Research Articles

Effect of increasing alkali layer spacing on the performance of O3-NaNi1/3Fe1/3Mn1/3O2 cathode materials

O3-Na1-xCax/2Ni1/3Fe1/3Mn1/3O2 (NaCNFMx, x=0, 0.05, 0.1, 0.15) was prepared by doping calcium ions and replacing the sodium sites in the NaNi1/3Fe1/3Mn1/3O2 (NaNFM) material using ball milling spray. XRD, SEM and XPS results demonstrated that the calcium element was successfully embedded in the lattice. The electrochemical performance of NaCNFM0.1 is the best when the doping amount of calcium ion is 0.1. The sodium diffusion coefficient of NaCNFM0.1 cathode material after 3 cycles is 1.67×10-14 cm2/s. The first specific discharge capacity at 1C is 121.1mAh/g. Even at the high current density of 5C, the 200cycles capacity of 103.6mAh/g is much higher than NaNFM's 95.2mAh/g, and the capacity retention rate is as high as 91.5%. Ex-situ XRD and SEM analyses manifested that the moderate amount of calcium doping improved the stability of the layered structure. These results show that the cathode material has great application potential.

Experimental investigation of the impact of additives in low clinker cementitious materials on multi-ion transference numbers and diffusion coefficients

In this paper, multi-ion transference numbers were determined throughout the chloride migration testing of low clinker cement-based materials. For this purpose, five cement pastes were studied: pure Portland paste (as a reference) and four other pastes, based on limestone filler, fly ash, slag or silica fume. The transference numbers were calculated from the concentration evolution in the three zones of the migration cell (catholyte, anolyte and sample), considering that: (i) ions moved in the catholyte and anolyte; (ii) ions leached from the sample; (iii) ions were generated from the electrode processes, and (iv) the pore solution of the sample evolved during the test. The chloride transference numbers of pastes with pure Portland cement, limestone filler, fly ash, slag or silica fume are almost zero at the beginning of the migration test but 0.23; 0.18; 0.06; 0.05 and 0.20 at the end of the test, respectively. The ion transference numbers obtained were used for the calculation of diffusion coefficients of chlorides, sodium and potassium using the Nernst-Einstein equation.

Real-Time Dialysis Dose: Ionic Dialysis Versus Classical Urea Kinetic Modeling Indices.

Introduction Worldwide, over 850 million people need renal replacement therapy, and over 80% of them are on hemodialysis. The term "dialysis adequacy" is most often associated with the achievement of minimally acceptable indices - single pool Kt/V (spKt/V) and urea reduction ratio (URR%) and largely does not take into account other clinical indicators in patients. In addition, it should be taken into account that in the conditions of clinical practice, their measurement is carried out according to standards controlled by the regulatory structures and is carried out with a frequency between one and three months, and not during each dialysis procedure. Accordingly, based on the obtained value, we assume that it refers to the urea clearance of the entire dialysis prescription. A new methodology for measuring dialysis adequacy through ionic dialysis allows dose estimation in real time without the need for additional blood testsby recording the difference in sodium ion conductivity at the dialyzer inlet and outlet. The latter corresponds to the dialysis dose delivered and can be measured at each dialysis session, enabling timely therapeutic intervention at minimal cost to the community. The aim of this original article is to present ion dialysis to the community as a reliable and inexpensive alternative to classical urea kinetic modeling (UKM), evaluating the comparability of the two methods and the possible sources of error in the result. Material and methods This was a retrospective study of 32 patients undergoing hemodialysis at the Clinic of Nephrology and Dialysis at the University Hospital St. Marina (Varna, Bulgaria) for the period between January and December 2020.For each of the patients, four measurements (every three months) of the delivered dialysis dose were performed by ion dialysis and classical urea kinetic modeling in order to assess the comparability and reliability of the two methods. Results The analysis of the results proved a high correlation between the validated indicators of dialysis adequacy (spKt/V; URR) and those registered with online monitoring (online clearance monitor) by ionic dialysis - online Kt/V (onKt/V)while at the same time reporting a significant difference in the two methods - primarily based on the anthropometric formulas used to estimate the volume. The established regression models confirmed the high predictive value of ionic dialysis in relation to the actually delivered dose. Discussion Despite the high bond strength, our recorded values ​​for onKt/V are 8% lower compared to results using UKM. Therefore, onKt/V as an assessment metric has the ability to underestimate the dialysis dose received. Various factors as a possible source of error and its cause have been reported by some authors. Several studies have reported differences of 2-5% in instantaneous conductance measurements, which they associate mainly with differences in the diffusion coefficients of urea and sodium, as well as different effects of dialysis membrane charge or inadequate ultrafiltration correction. Conclusions Our study confirms that online clearance monitoring (OCM) is a practical non-invasive tool for daily use that complements the classic performance of OCM by helping to deliver an adequate dialysis dose with increased patient benefit and minimal cost of financial resources.

Boosting the Sodium‐Ion Storage Performance of Prussian Blue Analogs by Crystalline Regulation

AbstractIn the development of practical sodium‐ion batteries, electrochemical performance and cost efficiency of the electrode materials are vital considerations. Prussian blue analogs (PBAs) have emerged as promising cathode materials due to their affordability, high theoretical capacity, and cycling stability. However, low crystallinity can negatively impact their performance. This study introduces a straightforward “chelating agent‐assisted” method for producing highly crystalline PBAs. The resulting cathode boasts excellent characteristics, including a high capacity of 150 mAh g−1, an initial Coulombic efficiency of 87.2 %, a long cycling lifespan of 1000 cycles, and superior rate performance. Furthermore, the study shows that the low structure distortion and high sodium diffusion coefficient can be attributed to the suppression of structural defects, as evidenced by in situ synchrotron powder diffraction. We believe these findings have the potential to enable the widespread use of PBAs in sodium‐ion batteries for an affordable and large‐scale energy storage system.

Structural Engineering of Prussian Blue Analogues Enabling All-Climate and Ultralong Cycling Sodium-Ion Batteries.

The development of cost-efficient, long-lifespan, and all-climate sodium-ion batteries is of great importance for advancing large-scale energy storage but is plagued by the lack of suitable cathode materials. Here, we report low-cost Na-rich Mn-based Prussian blue analogues with superior rate capability and ultralong cycling stability over 10,000 cycles via structural optimization with electrochemically inert Ni atoms. Their thermal stability, all-climate properties, and potential in full cells are investigated in detail. Multiple in situ characterizations reveal that the outstanding performances benefit from their highly reversible three-phase transformations and trimetal (Mn-Ni-Fe) synergistic effects. In addition, a high sodium diffusion coefficient and a low volume distortion of 2.3% are observed through in situ transmission electron microscopy and first-principles calculations. Our results provide insights into the structural engineering of Prussian blue analogues for advanced sodium-ion batteries in large-scale energy storage applications.

Improving electrochemical sodium storage performance and insight into the sodium ion diffusion in the high-pressure polymorph β-V2O5

The high-pressure form of vanadium pentoxide, β-V2O5, possesses promising sodium storage properties, featuring reversible sodium intercalation of one Na+ per formula unit, yielding an appealing capacity of 147 mAh g−1. However, its short cycle life in conventional carbonate-based electrolytes remains a significant drawback. In this work, we demonstrate that using non-carbonate-based electrolytes markedly enhances key electrochemical performances. Additionally, reducing the particle size of β-V2O5 through milling significantly increases the specific capacity, particularly at high current rates. Milled V2O5 maintains a respectable capacity of 73 mAh g−1 at 1 C rate, compared to the negligible capacity observed in non-milled V2O5. The milling process also alters the energy storage mechanism. Interestingly, after milling, sodium diffusion coefficient (DNa+) increased from 1.78 × 10−13 to 1.73 × 10−11 cm2 s−1, likely due to induced near-surface defects. Sodium storage exhibits dominant faradaic behavior at slow current rates, while, at high current rates, capacitive processes predominate. The synergy of improved sodium diffusion and additional capacitive charge storage leads to significantly improved electrochemical performance at high current rates. Furthermore, ionic liquid-based electrolytes promote long-life cycling, advancing this material toward practical application as a cathode in sodium-ion cells.

Understanding the Phase Transitions in Tin Foil Electrodes for Sodium-Ion Batteries through Light Microscopy and Kinetic Analysis

Group IVA elements exhibit some interesting sodium capabilities via electrochemical alloying processes (except carbon). Among them, tin is believed to be the most promising anode candidate for sodium-ion batteries (SIBs) due to its simple fabrication and monolithic form (e.g., metallic foils).[1] Nevertheless, tin foil anodes usually fail prematurely after only a handful of cycles. This poor reversibility is reported to be largely associated with the structural collapse of tin electrodes caused by huge volume differences among various Na-Sn phases (up to ~420%).[2][3] The volume change and the number of stress-induced cracks during different phase transitions can be different, given that these intermediate sodium-containing tin phases have different crystal structures and mechanical properties.[4] To prolong the lifetime of tin foil electrodes, it is crucial to determine which phase transitions have a greater impact on crack formation in the tin electrode and deliberately mitigate the problematic phase transitions.In this work, the mechanisms and kinetics of the phase transitions that occur in the sodiation of tin foil are extensively investigated using operando light microscopy and robust electrochemical techniques. Ex situ X-ray diffraction (XRD) (Figure 1) reveals that the initial sodiation of tin foil begins with a significant crystalline to amorphous transition (i.e., c-Sn to a-NaSn), followed by the a-NaSn to c-Na9Sn4 and the c-Na9Sn4 to c-Na15Sn4 transitions. To further understand these transitions, we introduce a new battery configuration for light microscopy to observe the dynamic change of the cross-section macrostructure of tin foil electrodes during cycling. It is expected that the observation could partly quantify the volume expansion during different phase transitions, directly visualize the formation of cracks, and accurately estimate the sodium diffusion coefficient in tin foil electrodes. Moreover, the change of charge transfer resistance as a function of sodiation depth is measured using electrochemical impedance spectroscopy (EIS), shedding light on the degradation mechanism. Besides, potentiostatic intermittent titration technique (PITT) is also employed to determine the role of diffusion or interfacial charge transfer on the whole kinetics of tin anode at different depths of discharge. These investigations could yield fundamental insights into the sodiation mechanisms and kinetics of tin foil electrodes, and more importantly, provide guidance for the utilization of binder- and additive-free tin foil as a reliable SIB anode. Figure 1

Etch-driven N, P co-doped hierarchical porous carbon embedded with Ni nanoparticles as an efficient dynamic carrier for room-temperature Na[sbnd]S battery

Shuttle effect in Room temperature sodium‑sulfur (RT NaS) batteries in ether-based electrolyte is difficult to avoid and reduces the cycle performance. Small molecule sulfur with short chain structure (e.g. S2~4) can avoid shuttle effect but low conductivity. In this paper, a N, P co-doped hierarchical porous carbon embedded with Ni nanoparticles (Ni@NPC) is developed by heat-treatment method using commercially phosphorus-containing chelating resin for waste water as the precursor, which adsorb Ni2+ by both physical and chemical process. The micro-pores in the Ni@NPC composite provide the matrix for S2~4, the mesoporous-macropore are conductive to the transmission of electrolyte and the carbon skeleton together with Ni nano-particles facilitate the electron transport. As a result, Ni@NPC/S cathode exhibits a sodium diffusion coefficient of the order of 10−7– 10−9, which is better than reported literatures. Similarly, Ni@NPC/S has excellent cycling performance (maintained at a specific capacity of 231.5 mAh g−1 after 7000 cycles at 5 A g−1, with an average decay rate of 0.004 % per cycle) and rate performance (258.3 mAh g−1 at a high current density of up to 10 A g−1).

Cost-effective microwave-assisted O3- type sodium-based layered oxide cathode materials for sodium-ion batteries

In this work, phase pure and highly crystalline O3-type layered oxide material (Na1Ni0.33Mn0.33Fe0.33O2-NNMF) was developed using; (i) a conventional solid-state synthesis route and (ii) a facile microwave-assisted sol–gel technique. A comparison of structural, thermal, and electrochemical properties is presented to elucidate the usefulness of the microwave-assisted sol–gel synthesis technique. A remarkable reduction in the sintering process time is noticed in the microwave-assisted sol–gel synthesis technique without compromising on the structural, thermal and electrochemical properties when compared to the conventional solid-state synthesis route confirming its decent cost-effectiveness. It is further noticed that NNMF developed through microwave-assisted sol–gel synthesis technique demonstrates superior thermal stability and comparable electrochemical performance as compared to the same material produced through the conventional sintering process. The decent electrochemical properties induced in NNMF during the microwave-assisted sol–gel synthesis technique can be attributed to the efficient diffusion of Na+ ions into/from the host structure during the intercalation/de-intercalation process as indicated by the high value of sodium diffusion coefficient (1 × 109-3.58 × 109m2s−1). The Potentiostatic Intermittent Titration Technique (PITT) analysis confirms single-phase Na+ intercalation/deintercalation in the host structure, regardless of the synthesis process. Finally, EIS analysis confirms the capacity fading of the developed materials during the cycling process is essentially due to an increase in the resistance with the increasing number of cycles due to the gradual thickening of formed SEI layer. The microwave-assisted sol–gel synthesis technique can be effectively employed for the production of many families of cathode materials at competitive cost facilitating their commercialization.

Evaluating Electrochemical Properties of Layered NaxMn0.5Co0.5O2 Obtained at Different Calcined Temperatures

P-type layered oxides recently became promising candidates for Sodium-ion batteries (NIBs) for their high specific capacity and rate capability. This work elucidated the structure and electrochemical performance of the layered cathode material NaxMn0.5Co0.5O2 (NMC) with x~1 calcined at 650, 800 and 900 °C. XRD diffraction indicated that the NMC material possessed a phase transition from P3- to P2-type layered structure with bi-phasic P3/P2 at medium temperature. The sodium storage behavior of different phases was evaluated. The results showed that the increased temperature improved the specific capacity and cycling stability. P2-NMC exhibited the highest initial capacity of 156.9 mAh·g−1 with capacity retention of 76.2% after 100 cycles, which was superior to the initial discharge capacity of only 149.3 mAh·g−1 and severe capacity fading per cycle of P3-NMC, indicating high robust structure stability by applying higher calcination temperature. The less stable structure also contributed to the fast degradation of the P3 phase at high current density. Thus, the high temperature P2 phase was still the best in sodium storage performance. Additionally, the sodium diffusion coefficient was calculated by cyclic voltammetry (CV) and demonstrated that the synergic effect of the two phases facile the sodium ion migration. Hard carbon||P2-NMC delivered a capacity of 80.9 mAh·g−1 and 63.3% capacity retention after 25 cycles.

Multifunctional low temperature-cured PVA/PVP/citric acid-based hydrogel forming microarray patches: Physicochemical characteristics and hydrophilic drug interaction

The characteristics of multifunctional polymeric hydrogel-forming microarray patches based on poly(vinyl alcohol)/poly(vinylpyrrolidone)/citric acid composite crosslinked at 80 °C were investigated. The swelling study showed that this composite possesses a higher swelling degree than the same polymer heated at 130 °C due to a lower crosslink density, which was then confirmed by FTIR examination. Solid-state studies revealed that lower-temperature crosslinking does not provide enough energy for the polymer to rearrange itself into a crystalline form. However, this composite polymer was shown to possess acceptable mechanical strength to insert/penetrate into the skin. The patch can function both as a means to sample model hydrophilic drugs from the skin and to deliver them when combined with a melt-type polyethylene glycol reservoir. The hydrophilic interaction between the hydrogel and drugs was investigated. A drug with a higher diffusion coefficient, modelled by theophylline (diffusion coefficient = 16.17 × 10-6 cm2/s), can be delivered more efficiently than fluorescent sodium (diffusion coefficient = 2.32 × 10-6 cm2/s) or cyanocobalamin (diffusion coefficient = 7.31 × 10-6 cm2/s). This is mainly due to theophylline’s high permeability (permeability coefficient = 7.40 × 10-5 cm/s) and weak ability to interact with the hydrogel (coefficient of partitioning = 1.3). These results indicated that the diffusion coefficient could be a useful predictive parameter to determine the delivery efficiency of the system. Furthermore, the results provide insight into how to select a suitable hydrogel for drug monitoring or delivery involving hydrophilic compounds based on the hydrophilic interaction between the polymer and the drug.

A Disordered Rubik's Cube‐Inspired Framework for Sodium‐Ion Batteries with Ultralong Cycle Lifespan

AbstractSodium‐ion batteries (SIBs) with fast‐charge capability and long lifespan could be applied in various sustainable energy storage systems, from personal devices to grid storage. Inspired by the disordered Rubik's cube, here, we report that the high‐entropy (HE) concept can lead to a very substantial improvement in the sodium storage properties of hexacyanoferrate (HCF). An example of HE‐HCF has been synthesized as a proof of concept, which has achieved impressive cycling stability over 50 000 cycles and an outstanding fast‐charging capability up to 75 C. Remarkable air stability and all‐climate performance are observed. Its quasi‐zero‐strain reaction mechanism and high sodium diffusion coefficient have been measured and analyzed by multiple in situ techniques and density functional theory calculations. This strategy provides new insights into the development of advanced electrodes and provides the opportunity to tune electrochemical performance by tailoring the atomic composition.

A Disordered Rubik's Cube-Inspired Framework for Sodium-Ion Batteries with Ultralong Cycle Lifespan.

Sodium-ion batteries (SIBs) with fast-charge capability and long lifespan could be applied in various sustainable energy storage systems, from personal devices to grid storage. Inspired by the disordered Rubik's cube, here, we report that the high-entropy (HE) concept can lead to a very substantial improvement in the sodium storage properties of hexacyanoferrate (HCF). An example of HE-HCF has been synthesized as a proof of concept, which has achieved impressive cycling stability over 50 000 cycles and an outstanding fast-charging capability up to 75 C. Remarkable air stability and all-climate performance are observed. Its quasi-zero-strain reaction mechanism and high sodium diffusion coefficient have been measured and analyzed by multiple in situ techniques and density functional theory calculations. This strategy provides new insights into the development of advanced electrodes and provides the opportunity to tune electrochemical performance by tailoring the atomic composition.

Unraveling Water and Salt Transport in Polyamide with Nuclear Magnetic Resonance Spectroscopy

Quantifying water and salt transport properties in polyamide is of growing importance as the use of reverse osmosis membranes grows in many industries. Here, using solid-state nuclear magnetic resonance (NMR) spectroscopy, we measure the translational diffusion coefficients using pulsed-field gradient NMR, examine ion dynamics with NMR relaxometry, and determine the activation energy barriers of hydrogen and sodium ions in ion-exchanged polyamide using variable-temperature NMR. We identify two predominant diffusion components within the spectra associated with bound and unbound hydrogen and sodium ions. We show that the diffusion coefficient of the bound hydrogen ions decreases by ∼50% while the unbound hydrogen diffusion coefficient remains constant as the salinity of the mixture doubles. Conversely, the bound sodium diffusion coefficient remained constant while the unbound sodium diffusion coefficient decreased by ∼40% as the salinity of the mixture doubled. Through examining the spin–lattice relaxation time (T1) we also report the associated activation energy for sodium and hydrogen. We believe these molecular-scale measurements can aid in extending physics-based models of salt and water transport in high-pressure reverse osmosis applications.

Cobalt-free copper-substituted Na(Ti,Mn,Ni,Cu)O2 layered oxide cathode materials for Na-ion batteries

This work presents the results of an investigation of the structural, transport (electronic and ionic components of electrical conductivity, chemical diffusion coefficients of sodium) and electrochemical properties of P2-Na0.67Ti0.67Ni0.33-yCuyO2 (y ​= ​0, 0.1) and P2-Na0.67Ti0.33Mn0.33Ni0.33-yCuyO2 (y ​= ​0, 0.1, 0.2) cathode materials. Both families of compounds possess structural and electronic transport properties that change significantly upon copper substitution. Substitution with 0.1 ​mol of copper makes it possible to increase the electronic component of conductivity by more than one order of magnitude. A correlation between transport properties and electrochemical performance is demonstrated. The most desirable electrochemical behaviour was observed for the copper-substituted Na0.67Ti0.67Ni0.23Cu0.1O2 and Na0.67Ti0.33Mn0.33Ti0.33Ni0.23Cu0.1O2 cathode materials. The initial capacity of the former was 98 mAh g−1 with 96% capacity retention after 60 cycles with varying current rate, while the latter was characterized by 96 mAh g−1 with 99% capacity retention.

Insight to Se-doping effects on Fe7S8/carbon nanotubes composite as anode for sodium-ion batteries

Iron based sulfides are considered as promising candidates for sodium-ion battery anodes. However, the inferior cyclic stability and poor capability hinder their practical application. In this study, a series of Se-doped Fe7S8 modified by carbon nanotubes (Fe7S8-xSex/CNTs) are synthesized through heat treatment and following selenization process. The composition of hybrids can be easily controlled by tuning the proportion of precursors. Electrochemical tests reveal that Fe7S8-xSex/CNTs with optimized nano-structure and composition displays improved sodium storage performances. Specifically, a discharge capacity of 303.4 mAh g−1 can be obtained at a high current of 4 A g−1. In addition, a remarkable capacity of 313.6 mAh g−1 is obtained after 1000 cycles at 2 A g−1, corresponding to a capacity retention of 67.7%. Sodium storage kinetic analysis suggests that Fe7S7Se1/CNTs electrode exhibit an enhanced capacitive-controlled process and improved sodium diffusion coefficient. Density functional theory (DFT) calculation results manifest that Fe7S7Se1 present a stronger binding energy with Na atoms and a decreased Na + diffusion barriers, which are responsible for the superior electrochemical performances. These results demonstrate that Se-doping of iron based sulfides may be a feasible strategy to develop high performance sodium-ion battery anodes.

Ice-Assisted Synthesis of Highly Crystallized Prussian Blue Analogues for All-Climate and Long-Calendar-Life Sodium Ion Batteries.

For practical sodium-ion batteries, both high electrochemical performance and cost efficiency of the electrode materials are considered as two key parameters. Prussian blue analogues (PBAs) are broadly recognized as promising cathode materials due to their low cost, high theoretical capacity, and cycling stability, although they suffer from low-crystallinity-induced performance deterioration. Herein, a facile "ice-assisted" strategy is presented to prepare highly crystallized PBAs without any additives. By suppressing structure defects, the cathode exhibits a high capacity of 123 mAh g-1 with initial Coulombic efficiency of 87.2%, a long cycling lifespan of 3000 cycles, and significantly enhanced high/low temperature performance and calendar life. Remarkably, the low structure distortion and high sodium diffusion coefficient have been identified via in situ synchrotron powder diffraction and first-principles calculations, while its thermal stability has been analyzed by in situ heated X-ray powder diffraction. We believe the results could pave the way to the low-cost and large-scale application of PBAs in all-climate sodium-ion batteries.

Glucosamine derived hydrothermal carbon electrodes for aqueous electrolyte energy storage systems.

Nitrogen-doped porous hard carbons are synthesized by hydrothermal carbonization method (HTC) using glucosamine as biosource and treated at different carbonization temperatures in nitrogen environment (500, 750, 1000 °C). The electrochemical performances of hard carbons electrode materials for aqueous electrolyte sodium ion batteries are examined to observe the effect of two different voltage ranges (-0.8-0.0) V and (0.0-0.8) V in 1.0 M Na2SO4 aqueous electrolyte. The best electrochemical performances are acquired for the 1000 °C treated glucosamine (GA-1000) porous carbon sample that provides ~96 F/g capacitance value in the negative voltage range (between -0.8 and 0.0) V. The sodium diffusion coefficient of the GA-1000 carbon calculated by electrochemical impedance measurements is found to be 1.5 × 10-14 cm2/s.

On the benefits of Cr substitution on Na4MnV(PO4)3 to improve the high voltage performance as cathode for sodium-ion batteries

Several samples of the Na4-xMn1-xCrxV(PO4)3 (0 ≤x ≤ 1) series are prepared by a sol-gel method that favors the deposition of an in-situ carbon conductive phase to improve the electronic conductivity of these electrode materials. XRD diffraction, electron microscopy, and Raman and XPS spectroscopies are applied to determine their structural, chemical, and morphological properties. Apparent diffusion coefficients of sodium are calculated by different techniques such as cyclic voltammetry, galvanostatic intermittent titration, and impedance spectroscopy. The results reveal the benefit of the partial replacement of Mn by Cr to improve sodium diffusivity, which leads to a net gain in the reversible extraction of sodium at either low and high rates and low interface resistance values for samples with x = 0.6 and 0.8.

High rate and cyclic performance of Na3–2xMgxV2(PO4)3/C cathode for sodium-ion batteries

Na3–2xMgxV2(PO4)3/C (0 ≤ x ≤ 0.2) were synthesized by a sol–gel method. According to X-ray diffraction (XRD) Rietveld refinement and energy dispersive spectroscopy (EDS) mapping results, Mg ions were approved to occupy the Na2 (18e) site in Na3–2xMgxV2(PO4)3/C for the first time. The amount of Mg substitution in Na3–2xMgxV2(PO4)3/C was optimized. The specific discharge capacities of Na2.9Mg0.05V2(PO4)3/C at 10 C, 20 C, and 30 C were 98.9%, 91.5%, and 76.2% of the value obtained at 1 C, respectively. Na2.9Mg0.05V2(PO4)3/C exhibited the highest capacity retention of 80.3% with a discharge capacity of 77.3 mAh·g−1 after 1200 cycles at 10 C. Na2.9Mg0.05V2(PO4)3/C exhibited superior rate performance, resulting from a high sodium diffusion coefficient (DNa) of 3.91 × 10–12 cm2·s−1 and the excellent electrical conductivity (5.7 × 10–2 s·cm−1). Combining the X-ray photoelectron spectroscopy (XPS) and Electrochemical impedance spectroscopy (EIS) results, the mechanism of Mg substitution was investigated. Since two Na+ were replaced by one Mg2+, $${\mathrm{V}}_{\text{Na}}^{^{\prime}}$$ was produced to maintain valence equilibrium. This phenomenon benefited Na+ migration by broadening the diffusion channel in the structure of Na3V2(PO4)3. According to the EPR results, the signal intensity of Na2.9Mg0.05V2(PO4)3/C was much stronger than that of Na3V2(PO4)2/C, thus confirming the existence of $${\mathrm{V}}_{\text{Na}}^{\mathrm{^{\prime}}}$$ after Mg substitution. When the amount of Mg substitution was further increased to 0.1, the DNa decreased because of the small radius of Mg2+, which led to severe lattice distortion and volumetric shrinkage of the unit cell.