Na-ion Diffusion Research Articles

A novel phase of superionic conductor β'-Na3PS4 with a large band gap and a low migration barrier.

Na3PS4 crystals with high ionic conductivity are promising solid-state electrolytes. Here, a novel phase of Na3PS4 (β'-NPS) crystallizing in a cubic lattice with a space group of P4̄3m was systematically investigated using first-principles calculations. First of all, β'-NPS is determined to be thermodynamically, dynamically and mechanically stable. The phase transition from tetragonal Na3PS4(α-NPS) to a cubic β'-NPS system occurs at approximately 480 K, suggesting high feasibility of experimental access. Moreover, the β'-NPS is an insulator with a large band gap of 4.05 eV and a low migration energy barrier of 0.10 eV for an interstitial Na ion. Significantly, a novel Na ion diffusion mechanism, that is, interstitial diffusion, is proposed, in contrast to traditional vacancy diffusion or kick-off diffusion as observed in most solid electrolytes. This work proposes β'-NPS as a promising superionic conductor for sodium ion batteries and provides theoretical guidance towards designing future ideal solid-state electrolytes.

Structure–transport correlations in Na11Sn2SbSe12 and its sulfide solid solutions

We report a new ion conductor—Na11Sn2SbSe12—as a possible candidate for a solid catholyte in composite cathodes for all-solid state Na-batteries, which exhibits a room temperature ionic conductivity of 0.15 ± 0.03 mS cm−1 and an activation energy of 0.39 ± 0.02 eV. The sulfide solid solutions of Na11Sn2SbSe12, namely, Na11Sn2SbS12−xSex (x = 1 and 6), were also investigated through a combination of Rietveld refinement against powder x-ray diffraction data and electrochemical impedance spectroscopy to reveal the complex structure–property relationships governing ion transport in this class of materials. Meanwhile, broadening of the Na-ion diffusion pathways in Na11Sn2SbSe12 is expected to facilitate Na-ion transport compared to the sulfide-rich member of the solid solution, the opposite holds: increasing the Se fraction in Na11Sn2SbS12−xSex leads to a little change in the activation energy but a reduction in the ionic conductivity. We ascribe this to the lowering of the prefactor, σ0, in the Arrhenius relationship with increasing lattice “softening” as a function of higher Se content.

Molten salt synthesis of disordered spinel CoFe2O4 with improved electrochemical performance for sodium-ion batteries.

Sodium-ion (Na-ion) batteries are currently being investigated as an attractive substitute for lithium-ion (Li-ion) batteries in large energy storage systems because of the more abundant and less expensive supply of Na than Li. However, the reversible capacity of Na-ions is limited because Na possesses a large ionic radius and has a higher standard electrode potential than that of Li, making it challenging to obtain electrode materials that are capable of storing large quantities of Na-ions. This study investigates the potential of CoFe2O4 synthesised via the molten salt method as an anode for Na-ion batteries. The obtained phase structure, morphology and charge and discharge properties of CoFe2O4 are thoroughly assessed. The synthesised CoFe2O4 has an octahedron morphology, with a particle size in the range of 1.1-3.6 μm and a crystallite size of ∼26 nm. Moreover, the CoFe2O4 (M800) electrodes can deliver a high discharge capacity of 839 mA h g-1 in the first cycle at a current density of 0.1 A g-1, reasonable cyclability of 98 mA h g-1 after 100 cycles and coulombic efficiency of ∼99%. The improved electrochemical performances of CoFe2O4 can be due to Na-ion-pathway shortening, wherein the homogeneity and small size of CoFe2O4 particles may enhance the Na-ion transportation. Therefore, this simple synthetic approach using molten salt favours the Na-ion diffusion and electron transport to a great extent and maximises the utilisation of CoFe2O4 as a potential anode material for Na-ion batteries.

Designing Solid Electrolyte Interfaces towards Homogeneous Na Deposition: Theoretical Guidelines for Electrolyte Additives and Superior High-Rate Cycling Stability.

Metallic Na is a promising metal anode for large-scale energy storage. Nevertheless, unstable solid electrolyte interphase (SEI) and uncontrollable Na dendrite growth lead to disastrous short circuit and poor cycle life. Through phase field and ab initio molecular dynamics simulation, we first predict that the sodium bromide (NaBr) with the lowest Na ion diffusion energy barrier among sodium halogen compounds (NaX, X=F, Cl, Br, I) is the ideal SEI composition to induce the spherical Na deposition for suppressing dendrite growth. Then, 1,2-dibromobenzene (1,2-DBB) additive is introduced into the common fluoroethylene carbonate-based carbonate electrolyte (the corresponding SEI has high mechanical stability) to construct a desirable NaBr-rich stable SEI layer. When the Na||Na3 V2 (PO4 )3 cell utilizes the electrolyte with 1,2-DBB additive, an extraordinary capacity retention of 94 % is achieved after 2000 cycles at a high rate of 10 C. This study provides a design philosophy for dendrite-free Na metal anode and can be expanded to other metal anodes.

Designing Solid Electrolyte Interfaces towards Homogeneous Na Deposition: Theoretical Guidelines for Electrolyte Additives and Superior High‐Rate Cycling Stability

AbstractMetallic Na is a promising metal anode for large‐scale energy storage. Nevertheless, unstable solid electrolyte interphase (SEI) and uncontrollable Na dendrite growth lead to disastrous short circuit and poor cycle life. Through phase field and ab initio molecular dynamics simulation, we first predict that the sodium bromide (NaBr) with the lowest Na ion diffusion energy barrier among sodium halogen compounds (NaX, X=F, Cl, Br, I) is the ideal SEI composition to induce the spherical Na deposition for suppressing dendrite growth. Then, 1,2‐dibromobenzene (1,2‐DBB) additive is introduced into the common fluoroethylene carbonate‐based carbonate electrolyte (the corresponding SEI has high mechanical stability) to construct a desirable NaBr‐rich stable SEI layer. When the Na||Na3V2(PO4)3 cell utilizes the electrolyte with 1,2‐DBB additive, an extraordinary capacity retention of 94 % is achieved after 2000 cycles at a high rate of 10 C. This study provides a design philosophy for dendrite‐free Na metal anode and can be expanded to other metal anodes.

CoSe2 nanoparticles anchored on porous carbon network structure for efficient Na-ion storage

Cobalt selenide, as a star material in battery industry, has attracted much attention. However, when it is applied solely in sodium ion batteries, it will cause large volume expansion and material agglomeration, which will seriously affect the overall performance of batteries. In this work, we use ice bath impregnation to combine CoSe2 nanoparticles with porous nitrogen-doped carbon networks (NC) as advanced anodes for ultra-long cycle life sodium ion batteries (SIBs). CoSe2 nanoparticles are evenly attached to NC with strong interfacial contacts in CoSe2@NC. The strong contact of CoSe2 on the porous carbon network, along with the carbon network's unique network cross-linking structure, results in rapid electron transfer and Na ion diffusion kinetics of CoSe2@NC, resulting in superior electrochemical performance. Besides, we have synthesized CoSe2@NC with different loading by changing Co2+ concentration. The results show that CoSe2@NC anode thus provides a high reversible capacity of 406 mAh/g. In addition, at high current 5 A/g, it can keep a reversible capacity of 300 mAh/g after 4500 cycles with an average capacity loss of less than 0.01 % per cycle. The excellent anchoring structure enables it to form stable solid electrolyte film (SEI) and reduce the amount of dead sodium in the first charge–discharge process, showing high Initial Coulombic Efficiency (ICE) (89.2 %). Finally, CoSe2@NC and Na3V2(PO4)3 (NVP) are assembled into a full cell and the results shows an ultra-long cycle stability at 0.1 A/g. This strategy will facilitate the application of transition metal selenides in next-generation energy storage systems.

Enhanced Electrochemical Performance of the Na3V2(PO4)3/C Cathode Material upon Doping with Mn/Fe for Na-Ion Batteries.

Research studies on Na-ion batteries (NIBs) are receiving significant scientific and commercial attention recently owing to the availability of low-cost, safe, and abundant materials in comparison to the conventional Li-ion batteries. The cathode material in a battery plays a crucial role in determining its cell capacity and cycle life. NASICON-based Na3V2(PO4)3, NVP, is known to be a favorable cathode material for NIBs due to its structural stability with high Na-ion mobility. The present work shows the structural and electrochemical properties of bare NVP/C and NVP/C partially doped with low-cost and much abundant transition element Fe/Mn at the toxic and expensive V site. The bare NVP/C as well as the transition-metal ion-doped NVP/C materials are prepared by the sol-gel method. XRD and FTIR studies confirm the formation of materials exhibiting the rhombohedral NVP structure (R3̅c) without any trace of impurities. The presence of a carbon layer in the investigated cathode materials is confirmed by the HRTEM micrographs; furthermore, the oxidation states of different transition-metal elements present are evaluated by X-ray photoelectron spectroscopy. Electrochemical studies reveal that the moderate doping of Fe/Mn in NVP/C results in an enhancement in discharge capacities in the doped materials at different C rates compared to the bare NVP/C sample. The differences in their electrochemical results are explained with respect to their Na-ion diffusion coefficient values obtained using the Randles-Sevcik equation. A Mn-doped NVP/C material exhibits an enhanced discharge capacity of 107 mA h g-1 at 0.1C with 90% capacity retention even after 100 cycles at 1C current rate. At the end, a Na-ion full cell (NVMP/C||HC) comprising a Mn-doped NVP/C cathode with the commercial hard carbon anode delivering a discharge capacity of 90 mA h g-1 is demonstrated.

Investigation of physical and electrochemical properties of Ni-doped Tunnel/P2 hybrid Na0.44MnO2 cathode material for sodium-ion batteries

The increasing demand for energy in recent years has accelerated the efforts to increase the efficiency of energy storage systems. Although lithium-ion batteries are very popular in energy storage systems, the dramatic increase in costs due to the decrease in lithium resources has greatly increased the interest in sodium-ion batteries. Na0.44MnO2 has recently received increasing attention due to the fact that the tunnel structures in the crystal structure are suitable for the diffusion of Na ions. However, rapid structural degradation is an important problem that must be overcome to move into practical applications. In this study, the tunnel/P2 hybrid type Na0.44MnO2 was synthesized by a one-step heat treatment with the Ni substitution to Mn sites for improving cyclic performance. It was demonstrated by various physical analyses, that biphasic hybrid material starts forming with Ni substitution, and Ni occupied the Mn sites in the P2 phase. Electrochemical measurements provide that after 100 cycles at 0.3C, while Na0.44MnO2 has 77% capacity retention, 1% and 5% Ni substituted samples have 86.4% and 77.3%, respectively. The results show that tunnel-P2 hybrid cathode materials can be developed for practical applications in sodium-ion batteries.

Elucidating the optimized P2 type Na0.67Mn1−xCuxO2 cathode active material via operando XAS

Here we report on the structural and electrochemical properties of P2-type Na0.67Mn1−xCuxO2 (where x = 0.20–0.50) via various techniques. X-ray diffraction (XRD) reveals a reduction of the unit cell volume upon substitution of Cu elucidated through detailed Rietveld analysis. The cyclic voltammetry (CV) behavior is also affected by the Cu substitution indicating new redox reactions stemming from Cu substitution. Galvanostatic cycling measurements at room temperature show that when x = 0.35 in a P2-type Na0.67Mn1−xCuxO2 cathode active material, the best electrochemical performance is obtained. The Na-ion diffusion rate is found to be strongly dependent upon the environmental temperature. Changes in the valence state and the local structures of P2-type Na0.67Mn1−xCuxO2 during the charge/discharge are investigated through the operando X-ray absorption spectroscopy (XAS) technique.

Electrochemical Performance of High-Valence Mo6+ and Low-Valence Mn2+ Doped- Na3 V2 (PO4 )3 @C Cathode for Sodium-Ion Batteries.

The sodium superionic conductor (NASICON)-Na3 V2 (PO4 )3 (NVP) is an attractive cathode for sodium-ion batteries, which is still confronted with limited rate performance due to its low electronic conductivity. In this paper, a chemical strategy is adopted to partially replace V3+ of the NVP framework by low-valence Mn2+ and high-valence Mo6+ substitution. The crystal structure, sodium-ion diffusion coefficient and electrochemical performance of Mn-Mo-doped [Na3.94 V0.98 Mo0.02 Mn(PO4 )3 @C] cathode were investigated. X-ray diffraction confirmed the NASICON-type structure and XPS analysis confirmed the oxidation state of Mn and Mo in doped NVP cathode. The Na ion diffusion processes were inferred from Cyclic Voltammetry (CV), Galvanostatic intermittent titration technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) measurement, which clearly show rapid Na-ion diffusion in NASICON-type cathode materials. The Mn-Mo-substituted NVP shows smoother charge-discharge profiles, improved rate performance (64.80 mAh/g at 1 C rate), better energy density (308.61 mWh/g) and superior Na-ion kinetics than that of unsubstituted NVP@C cathode. Their enhanced performance is attributed to large interstitial volume mainly created by high valence Mo6+ and enhanced capacity is attributed to the low valence Mn2+ doping. These results demonstrate that Mn-Mo-doped NVP cathode is strongly promising cathode material for sodium-ion batteries.

Effects of ball milling on Structure and Electrochemical Properties of Na4MnV(PO4)3

Na4MnV(PO4)3 is regarded as a promising candidate as cathode material for Na-ion batteries (SIB) due to its low cost and high energy density. However, it suffers from fast capacity decay upon cycles, hindering its further commercialization. The ball milling technology was adopted in this study to control the particle size of Na4MnV(PO4)3, and the correlation of electrochemical properties was investigated. With increasing ball milling time, particle size was greatly reduced. The material with 20 minutes of ball milling displayed the best cycling stability and rate capability. It delivered a reversible capacity of 77.1 mAh/g at 0.5 C after 80 cycles, much higher than that of pristine Na4MnV(PO4)3. The decreased particle size might be to blame for the enhanced electrochemical performance, which effectively increased the interfacial contact with the electrolyte and greatly reduced the length of Na-ion diffusion in active particles.

Step-by-step desolvation enables high-rate and ultra-stable sodium storage in hard carbon anodes

Hard carbon is regarded as the most promising anode material for sodium-ion (Na-ion) batteries, owing to its advantages of high abundance, low cost, and low operating potential. However, the rate capability and cycle life span of hard carbon anodes are far from satisfactory, severely hindering its industrial applications. Here, we demonstrate that the desolvation process defines the Na-ion diffusion kinetics and the formation of a solid electrolyte interface (SEI). The 3A zeolite molecular sieve film on the hard carbon is proposed to develop a step-by-step desolvation pathway that effectively reduces the high activation energy of the direct desolvation process. Moreover, step-by-step desolvation yields a thin and inorganic-dominated SEI with a lower activation energy for Na+ transport. As a result, it contributes to greatly improved power density and cycling stability for both ester and ether electrolytes. When the above insights are applied, the hard carbon anode achieves the longest life span and minimum capacity fading rate at all evaluated current densities. Moreover, with the increase in current densities, an improved plateau capacity ratio is observed. This step-by-step desolvation strategy comprehensively enhances various properties of hard carbon anodes, which provides the possibility of building practical Na-ion batteries with high power density, high energy density, and durability.

Cobalt doped ovoid magnetite (Fe3O4) nanocomposites incrusted on sheets of reduced graphene oxide as anode for sodium-ion batteries

Fe3O4 could be a better choice as anode for sodium-ion batteries (NIBs) due to high sodium ion storage capability and abundant resource. Yet, the low conductivity and severe volume expansion during cycling have fetched large defies towards practicality. In this work, an ovoid Fe3O4/rGO and (5%, 10%) cobalt doped Fe3O4/rGO nanocomposites were synthesized through one-step hydrothermal route. Structural, morphological chemical, vibrational, and compositional analysis have been employed using X-ray diffraction, Scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results indicate the successful substitution of cationic Co2+ ions in Fe3O4 lattice. Hence, integration of optimal cobalt doping in Fe3O4 lattice can alter the charge/discharge specific capacity as compared to undoped Fe3O4-rGO anode. Due to sufficient production of oxygen vacancies in the structure 5% Co- Fe3O4/rGO reveals the highest initial charge capacity of 434 mAh g−1 at 0.05 C and exhibits stable prolonged cycling and delivers upto 303 mAh g−1 of charge capacity during 100 cycles (with 100% of CE and 72% of remaining capacity retention)as compared to 10% Co- Fe3O4/rGO (414 mAh g−1) and Fe3O4/rGO (357 mAh g−1) nanocomposite. Moreover, the Nyquist plot reveals the rapid transfer of charge for 5% Co- Fe3O4/rGO anode due to lowest charge transfer impedance of 56.5 Ω as compared to 10% Co- Fe3O4/rGO (114.3 Ω) and pristine Fe3O4/rGO (177.8 Ω). Therefore, the presence of rGO layers and the optimum doping of Co atoms in Fe3O4 crystal can curtail volume expansion, generate more contact sites for electrode/electrolyte, deep penetration of electrolyte into the electrode and can yield superior electrochemical performance via fast Na-ion diffusion rate. Sodium-ion batteries are a promising technology for electric vehicles, the energy grid and other applications.

Influence of low content metal intermixing on electrochemical stability and sodium ion transport in Na2(Mg2−xZnx)TeO6 (x = 0.125, 1.875) for solid state sodium-ion batteries

Based on solid electrolyte instead of traditional liquid electrolyte, all-solid-state battery technology leads to the next generation of high performance sodium ion batteries. In this work, we propose a metal intermixing in the P2-type layered compounds Na2X2TeO6 (X = Mg, Zn), resulting in Na2(Mg2-xZnx)TeO6 (x=0,0.125,1.875,2), for advanced solid-state electrolyte and Na ion conductor, based on first-principles calculations of electrochemical stability and Na ion diffusion. Our calculations reveal that Zn intermixing in a low content with Mg in Na2Mg2TeO6 can effectively improve the electrochemically stable window and Na ion conductivity, whereas Mg intermixing with Zn in Na2Zn2TeO6 slightly reduces the performance. The atom-resolved electronic density of states and electron density differences are analyzed to give an understanding of performance enhancement or deterioration by intermixing. This work highlights the possibility of improving the performance of metal oxide-based solid electrolyte by intermixing metals.

Design of Perovskite-Type Fluorides Cathodes for Na-ion Batteries: Correlation between Structure and Transport

Transition metal-based sodium fluoro-perovskite of general formula NaMF3 (M = Fe, Mn, and Co) were investigated as cathode materials for rechargeable Na-ion batteries. Preliminary results indicated Na-ion reversible intercalation but highlighted the need to find optimization strategies to improve conductivity and to modulate the operating voltages within experimentally accessible electrolytes’ stability windows, in order to fully exploit their potential as high-voltage cathodes. In this study, we combined experimental and computational techniques to investigate structures, defects, and intercalation properties of the NaFe1-xMnxF3 and NaCo1-xMnxF3 systems. Through the use of a simple solvothermal synthesis, we demonstrated the possibility to modulate the sample’s morphology in order to obtain fine and dispersed powder samples. The structural results indicated the formations of two solid solutions with a perovskite structure over the entire compositional range investigated. Atomistic simulations suggested that Na-ion diffusion in these systems was characterized by relatively high migration barriers and it was likely to follow three-dimensional paths, thus limiting the effect of anti-site defects. The correlation between structural and computational data highlighted the possibility to modulate both ionic and electronic conductivity as a function of the composition.

Cu-substitution P2-Na0.66Mn1-xCuxO2 sodium-ion cathode with enhanced interlayer stability

P2-type Mn-based layered oxides are viewed as promising cathode materials for sodium ion battery by virtue of their high theoretical capacity. Considering that pure Na2/3MnO2 suffers from poor cycling performances, Cu-substitution strategy is proposed to effectively alleviate this issue. However, the structural evolution mechanism of the Cu-containing samples still remains unclear. Herein, we propose that Cu-Substitution P2-type Na0.66Mn1-xCuxO2 with enlarged lattice parameters are conducive to improving the interlayer structure stability through mitigating TMO2 slabs distortion. Proved by synchrotron XAS spectra and ex/in situ XRD, the expansion/contraction of MnO6 octahedron is dramatically reduced with the increased Cu content, showing the facilitated Na ion diffusion. Furthermore, when the ratio of Cu to Mn reaches 1:4, the phase transition from P2 to P'2 type at the end of discharge can be suppressed, resulting in the improved interlayer skeleton stability. The Cu-containing samples with stable interlayer structure exhibit high capacity retention and outstanding rate performances (a capacity of 79.9 mAh g−1 at 5 C). This Cu-substitution strategy provides a promising approach to designing highly stable cathodes.

Insights into the Reaction Mechanisms of Nongraphitic High-Surface Porous Carbons for Application in Na- and Mg-Ion Batteries.

The fabrication of low-cost carbon materials and high-performance sodium- and magnesium-ion batteries comprising hierarchical porous electrodes and superior electrolytes is necessary for complementing Li-ion energy storage. In this work, nongraphitic high-surface porous carbons (NGHSPCs) exhibited an unprecedented formation of n-stages (stage-1 and stage-2) due to the co-intercalation of sodium (Na(dgm)2C20) with diglyme. X-ray diffraction patterns, Patterson diagram, Raman spectra, and IR spectra suggested the presence of n-stages. This phenomenon implies an increase of the initial capacity (∼200 mAh g-1) and good Na-ion diffusion (2.97 × 10-13 cm2 s-1), employing diglyme as compared to standard electrolytes containing propylene carbonate and fluoroethylene carbonate. Additionally, the current approach is scalable to full Na- and Mg-ion cells by using t-Na5V(PO4)2F2 and MgMnSiO4 cathodes, respectively, reaching 250 and 110 W h kg-1 based on the anode mass. The simultaneous Mg (de)insertion from/into MgMnSiO4 and the adsorption/desorption of bistriflimide ions on the NGHSPC surface is responsible for capacity enhancement.

Electrochemically Active Mn-Doped Iron Hexacyanoferrate as the Cathode Material in Sodium-Ion Batteries

In this work, for the performance enhancement of iron hexacyanoferrate, an electrochemically active Mn-doped iron hexacyanoferrate cathode is fabricated via a bottom-up approach. It is found that the pre-treatment of interstitial water and appropriate Mn doping are two keys to achieving higher capacity and higher stability. The interstitial water has a trade-off effect between the alleviation of volume expansion upon Na+ (de)intercalation and the retardation of Na-ion diffusion. The moisture-tailored iron hexacyanoferrate with appropriate Mn doping exhibits a high initial Coulombic efficiency of 94.8%, enhanced capacity and rate performance, and excellent cycling stability. These results benefit from the fact that the extraction/insertion of Na ions from/into the lattice via a solid-solution mechanism correspond to both the slight volume expansion and fast sodium diffusion rate; otherwise, the removal of interstitial water and a higher Mn content might lead to poor cycling stability due to excessive volume expansion resulting from rhombohedral to cubic phase transformation. Finally, the less demand on the control of air humidity for the fabrication of electrodes and the potential for the full cell coupled with hard carbon are also demonstrated, which shows great potential for practical applications.

Constructing abundant oxygen vacancies in NaVPO4F@C for boosting sodium storage kinetics

NaVPO4F is attracting extensive attention as cathode material for sodium ion batteries owing to its advantages of stable structure, high voltage platform and high theoretical capacity. However, the poor electrical conductivity of NaVPO4F hinders its practical application. In this work, we propose a facile strategy to construct abundant oxygen vacancies in NaVPO4[email protected] by the introduction of Cr2+ to solve the problem associated with poor electronic conductivity. Density functional theory calculations and experimental studies indicate that the suitable Cr doping and abundant oxygen vacancies can significantly promote the electronic conductivity and Na-ion diffusion kinetics. Benefiting from the synergistic effects, the optimized Cr-doped NaVPO4[email protected] displays a high capacity of 121.3 mA h g−1 at 1 C and a high rate capacity of 98.8 mA h g−1 at 10 C with the capacity retention of 79.3% after 4000 cycles. Moreover, it also shows outstanding cycling performance and rate capability when assembled into full batteries (NaTi2(PO4)3@G film as the anode). This facile method proposed in our work may present a feasible approach to design the oxygen vacancies into other cathode materials for greatly improving the performance of sodium ion batteries.

Tailoring Nitrogen Terminals on MXene Enables Fast Charging and Stable Cycling Na-Ion Batteries at Low Temperature

HighlightsAn interlayer confined strategy to realize the substitution of nitrogen terminals between Ti3C2 MXene layers is proposed.The targeted Ti3C2-Nfunct anode exhibits fast-charging ability and great superiority in cycle life at − 25 °C.Ti3C2-Nfunct not only possesses a lower Na-ion diffusion energy barrier and charge transfer activation energy, but also exhibits Na+-solvent co-intercalation behavior at low temperature.