Electrodes For Na-ion Batteries Research Articles

High Power Sodium-Ion Batteries and Hybrid Electrochemical Capacitors Using Mo or Nb-Doped Nano-Titania Anodes

Nano-sized anatase (TiO2) and doped anatase (Mo0.1Ti0.9O2 and Nb0.25Ti0.75O2) materials (ca. 5 nm) were synthesized using continuous hydrothermal flow synthesis and evaluated as negative electrodes in Na-ion batteries and hybrid capacitors. Na-ion half-cells (vs. Na metal counter electrodes) for the Mo-doped titania (Mo0.1Ti0.9O2) and Nb-doped titania (Nb0.25Ti0.75O2) electrodes both showed significantly higher specific discharge capacities than undoped anatase (ca. 75 mAh g−1 compared to only 30 mAh g−1 for undoped TiO2 at 1 A g−1). This improved performance was attributed to higher pseudocapacitive contributions to charge storage, as well as improved sodium ion diffusion and lower charge transfer resistance. Na-ion hybrid electrochemical capacitors (Na-HECs) were made from the electrodes with activated carbon positive electrodes. As expected, Na-HECs using doped titania showed superior performance to the undoped anatase, with power densities up to 10.5 kW kg−1 or energy densities of over 60 Wh kg−1 (based on the weight of active material in both anode and cathode). The Mo0.1Ti0.9O2/AC Na-ion hybrid capacitor also showed excellent specific capacitance retention of ca. 75% over 3000 cycles at 5 mA cm−2 (1 A g−1).

Electronic density distribution of Mn-N bonds by a tuning effect through partial replacement of Mn by Co or Ni in a sodium-rich hexacyanoferrate and its influence on the stability as a cathode for Na-ion batteries.

This study evaluates the effect of equimolar substitution of manganese by cobalt or nickel in hexacyanoferrate (HCF) open frameworks as electrode materials for Na-ion batteries. As the stability of Mn-N bonds is crucial to obtain long term stability and cyclability of manganese (Mn-HCF), the samples were analyzed thoroughly using several spectroscopic and structural methods. The XPS and infrared experiments reveal that the charge density around Fe is modulated by the presence of Co or Ni, which is associated with their high polarizing power, leading to decreased cell distortion as revealed by XRD. The Rietveld refinement demonstrated that the octahedra built by 3d metals and the cyanide nitrogen were distorted with the axial bond distances being larger than the equatorial distances. This octahedral distortion promotes the spin behavior of 3/2 for Mn2+ confirmed by magnetic experiments; the arising of this spin state is attributed to d orbital splitting determined by UV-Vis experiments. Therefore, the changes upon Mn substitution are related to the modification of the covalent character of Mn-N bonds, modulated by the effect of the Ni and Co polarizing power. All these properties improve the electrochemical stability of the Ni or Co substituted materials as Na-ion batteries, leading to higher capacity retention even at higher C-rates (5C) and good capacity recovery, in comparison with those obtained for Mn-HCF.

Impact of the Morphology of V2O5 Electrodes on the Electrochemical Na+-Ion Intercalation

The development of high performance electrodes for Na-ion batteries requires a fundamental understanding of the electrode electrochemistry. In this work, the effect of the morphology of vanadium oxide on battery performance is investigated. First, the phase transitions upon sodiation/de-sodiation of NaxV2O5 cathodes in standard battery solvents are explored by cyclic voltammetry and X-Ray diffraction. At potentials 1.5 V positive of Na/Na+ the insertion of the first Na+ into pristine V2O5 is completed and α’-NaV2O5 is formed. A discharge to 1.0 V results in the introduction of a second Na+ and after a deep discharge to 0 V a third Na+ is intercalated. When cycled as an intercalation electrode, the Na-content x in NaxV2O5 varies between x = 1 (charged) and x = 2 (discharged). For studying the effect of electrode morphology on the battery performance, several types of V2O5 (hollow V2O5 microspheres, V2O5 nanobundles and V2O5 nanobundles blended with 10%wt TiO2) were prepared and compared to a commercially available V2O5-micropowder. The nanobundles were prepared by a facile sonochemical process. In comparison to the microsized V2O5 morphologies, the potential plateaus in the charge/discharge curves of the V2O5 nanobundles are at more positive potentials and the capacity loss in the first cycle is suppressed. The V2O5 nanobundles showed the best battery performance with a reversible capacity of 209.2 mAh g−1 and an energy density of 571.2 mWh kg−1 (2nd cycle). After an initial capacity fading, which can be slightly suppressed by blending the V2O5 with TiO2, the pure V2O5 nanobundles have a practical capacity of 85 mAh g−1, an operation potential of 2.4 V, an energy density of 266.5 mWh kg−1 and a capacity retention of 83% after 100 cycles. The best battery performance of the nanomaterial is ascribed in this study to the amorphous character of the electrode, favoring faster electrode kinetics due to a (pseudo-) capacity dominated charging/discharging, reducing diffusion lengths and preventing further amorphization, which all is beneficial in terms of lifetime, capacity, operation voltage, energy density and energy efficiency.

Design of Roughened Current Collector by Bottom-up Approach Using the Electroplating Technique: Charge–Discharge Performance of a Sn Negative-Electrode for Na-Ion Batteries

The use of high capacity electrode materials based on alloying and dealloying reactions with Na is very effective for improving energy density of batteries. However, their application brings on electrical isolation such as detachment of the electrode mixture layer from a current collector, causing rapid capacity fading. We previously found that Cu electrochemically grows in sheet form by electroplating in a CuSO4-based aqueous solution with poly(acrylic acid) (PAA). In the present study, our goal was to elucidate the formation mechanism of Cu sheets by characterization using scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and electron scatter diffraction patterns (EBSD) mapping. Then, the cycling performance of a Sn negative electrode for Na-ion batteries was significantly upgraded by the application of a roughened-Cu substrate with optimized sheet thickness. The STEM images and EBSD maps revealed that the Cu sheet was a single crystal, and the results obtained from XRD...

Electrochemical Na-Insertion/Extraction Property of Ni-Coated Black Phosphorus Prepared by an Electroless Deposition Method.

Electrical conductivity is one of the properties required for an active material, and it is extremely essential to exert its potential. In the present study, the strategy of coating a metal at a single particle level by an electroless deposition method was applied to enhance the cycling performance of phosphorus-based negative electrodes for Na-ion batteries. The deposition morphology and composition of the Ni coating layer were characterized by field-emission scanning electron microscopy, scanning transmission electron microscopy, and X-ray diffraction. In the 5 wt % Ni coating, an amorphous Ni layer of several nanometers thickness was homogeneously formed on the phosphorus surface, whereas a shell layer having a 200 nm thickness was formed in the order of Ni12P5, NiP2, NiP3, and metallic Ni from the surface toward the center in the 30 wt % Ni coating. Electrochemical impedance spectroscopic measurements clarified that the good electron transport proceeded throughout the developed conduction pathway to promote the phase transition to trisodium phosphide (Na3P), leading to a high reversible capacity for phosphorus; the as-prepared black phosphorus showed only a reversible capacity of 140 mA h g–1 at the 60th cycle, whereas the 30 wt % Ni-coated composite delivered a relatively high capacity of 780 mA h g(P)–1. In addition, the expansion ratio of the electrode after the 30th desodiation was the lowest among the three kinds of electrodes. By contrast, cracks and exfoliation of the active material layer from the current collector were confirmed in the as-prepared black phosphorus. These results demonstrate that the upgraded performance accomplished using the 30 wt % Ni-coated composite with the Ni/Ni–P layer is due to the synergetic effects of the electron conduction channel and a buffer matrix against a large volumetric change (∼400%) in phosphorus during the charge–discharge reactions.

Structural Evolution and Redox Processes Involved in the Electrochemical Cycling of P2–Na0.67[Mn0.66Fe0.20Cu0.14]O2

We report the synthesis and structural evolution of P2–Na0.67[Mn0.66Fe0.20Cu0.14]O2 during charge and discharge as a positive electrode for Na-ion batteries. Operando X-ray diffraction analysis revealed the existence of two phase transitions in the voltage window between 1.5–4.3 V. Ex situ pair distribution function analysis was used to characterize the local structure of the high-voltage phase which is disordered along the c-axis and is derived by the migration of a fraction of iron ions into the interlayer space. Operando X-ray absorption spectroscopy was employed to monitor the local structural evolution of the transition metals and shows the existence of Mn3+/4+ redox below ∼3.4 V, followed by the redox activity of Cu and Fe ions at higher voltages. However, no change was observed in the K-edge X-ray absorption near-edge spectrum of any of the transition metals at voltages above 4.1 V, where the growth of the high-voltage phase is initiated. The combination of these results implies the reversible cont...

CuS Microspheres as High-Performance Anode Material for Na-ion Batteries

In this work, CuS microspheres are synthesized by a facile microwave synthesis without any template or additive. As-prepared CuS microspheres display steady reversibility by adopting triethylene glycol dimethyl ether (TEGDME) as the electrolyte solvent and adjusting cut-off voltage to 0.6 − 3.0V. The discharge capacity stands at 162mAhg−1 after 200 cycles with the capacity retention of 95.8%. Electrochemical kinetics of CuS electrodes is investigated by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) tests. Beyond that, electrochemical reactions of CuS electrodes are explored using ex-situ X-ray diffraction (XRD). As far as we know, it is not only the best performance of CuS electrodes in Na-ion batteries (NIBs) but also the first time that the kinetics and reaction mechanism of CuS in NIBs are investigated.

Chemical, Morphological, and Structural Tuning of Tunnel Manganese Oxides for Intercalation Batteries

In this work, we investigate the effect of tunnel dimension, material morphology, and structural water content on the performance of two tunnel structured manganese oxides in intercalation batteries. Tunnel manganese oxides represent an attractive family of materials for intercalation-based energy storage due to their open crystal structures consisting of well-defined structural tunnels that allow for the reversible insertion/extraction of charge-carrying ions, as well as their low cost, low environmental impact, and high electrochemical activity. Facile hydrothermal methods can be used to synthesize tunnel manganese oxides, which have the general formula AxMnO2, where “A” is a stabilizing cation present within the structural tunnels built from MnO6 octahedra. We focus on comparing the performance of two tunnel manganese oxides with square tunnel structures, α-MnO2 (K0.11MnO2), which contains tunnels of 2x2 octahedra dimension, and todorokite MnO2 (Mg0.20MnO2), which contains tunnels of 3x3 octahedra dimensions. When tested as intercalation electrodes in Na-ion batteries, we find that the material with smaller structural tunnels, α-MnO2, achieved a higher initial capacity of 330 mAh g-1­, while todorokite MnO2, which contains larger 3x3 octahedra tunnels, achieved a capacity of 157 mAh g-1. However, the larger tunnel structure proved to be more stable in a Na-ion battery, demonstrating superior capacity retention, as well as superior rate performance. These results suggest that although smaller tunnels may initially show higher capacity, larger tunnels are preferable for superior electrochemical stability. Further, the effect of material morphology is also shown by synthesizing todorokite MnO2 with both a platelet and a nanowire morphology. In Li-ion batteries, the nanowire morphology exhibited a 30% higher capacity retention after 100 cycles, demonstrating the advantage of a nanowire morphology in terms of enhanced cycling stability. Moreover, the effect of structural water in todorokite MnO2 (0.42 H2O molecules per Mn) is discussed, elucidating the role of structural water on ion insertion of into tunnel structured materials. Lastly, we discuss the effect of controlling the electrochemical potential window used during cycling, the trade off between capacity and capacity retention, and how the stability of these materials can be improved. This work describes the role of tunnel size, ionic content, and material morphology on the behavior of tunnel manganese oxides and thus provides insight into tailoring of tunnel structured materials for intercalation batteries.

Improving Fluorophosphates Na3V2(PO4)2F3 As Na-Ion Cathodes Beyond the 2 Sodium Limit

In the last 30 years the world of portable electronics has been powered by Ni-Cd, Ni-MH and then Li-ion batteries, which rapidly became the technology of choice for such applications. More recently, great interest has been given to transport and storage applications. Since concerns have been raised about the future availability of lithium resources, much research has been addressed to alternatives such as Na-ion batteries. Materials for such technology could develop rapidly because of the essential similarity between Li+ and Na+ intercalation chemistry. A significant fraction of recently proposed electrodes are phosphate-based polyanionic materials which possess stable structural frameworks and whose potential can be tailored thanks to the inductive effect. Among them, Na3V2(PO4)2F3 is a material attracting great interest. Its crystal structure had been determined from single crystal X-Ray diffraction in 1999 by Le Meins et al. in the tetragonal space group P42/mnm at 298K [1]. Very recently high resolution diffraction techniques led to new insight on Na3V2(PO4)2F3, both from the crystal structure and electrochemical points of view, and explaining the significant amount of discrepancies found throughout the literature [2-6], which we believe to be the result of a partial substitution of oxygen for fluorine. Synchrotron radiation powder diffraction data revealed a small but significant orthorhombic distortion in Na3V2(PO4)2F3 (b/a=1.002). This led to an indexation in the Amam space group and to a new structure preserving the framework, but strongly impacting the sodium distribution [7]. Moreover, a subsequent operando synchrotron radiation XRD allowed to determine the complicated phase diagram observed upon cycling of Na3V2(PO4)2F3, consisting of several phase transition between intermediate phases, leading to the fully charged state NaV2(PO4)2F3 [8]. Na3V2(PO4)2F3 has the potential to deintercalate 3 sodium ions, but only two have been experimentally observed to be electrochemically active. The extraction of the third one would require a potential too high, where common electrolytes are not stable. Moreover, Na3V2(PO4)2F3 actually belongs to a full solid solution Na3V2(PO4)2F3-2xO2x [3], where the whole family delivers good performances as positive electrodes for Na-ion batteries, thanks to the active redox couple shifting from V3+/V4 to V4+/V5+. This means a very high degree of flexibility is present in this class of materials. To improve their performances even further, we considered a doping strategy with different transition metals to replace part of the vanadium into the structure, in the attempt at lowering such potential; in this presentation we will show the results of our doping from the predictions of ab initio calculations to the actual synthesis. The best strategies will be presented and the improvement they bring in terms of energy density and/or Na ion kinetics highlighted.

Electrochemical Behaviour and Phase Evolutions during Sodiation/Desodiation of Na2Ti3O7 nanoparticles, Synthesized Via Sol-Gel Route, Decorating Carbon Nanotubes

The emerging technology of sodium ion batteries has attracted a great deal of attention due to the abundant availability and cost efficiency of the concerned raw materials. The technology still needs improvements, primarily in terms of finding suitable anode material with superior Na-capacity, cyclic stability and also rate capability. This is mainly caused by graphitic carbon not being able to intercalate sodium, unlike in the case of lithium. Layered Na2Ti3O7, because of its low potential against Na/Na+and decent theoretical Na-capacity (~200 mAh/g), has emerged as a promising anode material among other Na-based intercalation compounds [1]. However, in addition to its poor rate capability owing to inherently low electronic conductivity in pristine form, its cyclic stability is also far from being satisfactory. Poor structural stability during sodiation/desodiation is one of the reasons for the same. Against this backdrop, the present work focuses on improving the overall electrochemical behaviour, not only by reducing size of the Na2Ti3O7 particles to nano-sized regime, but also by incorporating conducting reinforcements like multi-walled carbon nanotubes (MWCNTs). However, the challenges associated with synthesizing active materials having nanoscaled dimensions via facile and cost-effective wet-chemical synthesis routes, while at the same time achieving intimate contact with carbon nanotubes usually pose practical hindrances towards improving rate capability and cyclic stability of promising ceramic electrode materials. Developing MWCNTs-incorporated composite electrode slurry via the usual physical mixing route may not allow intimate contact; rather may lead to contamination. Growth of MWCNTs on electrode-active particles via vapour deposition is not a cost-effective and scalable process. These disadvantages have been overcome in our work by developing an innovative, but facile sol-gel based route, which helped in synthesizing Na2Ti3O7 nanoparticles with near-perfect control over stoichiometry and also in intimate contact with MWCNTs. In more precise terms, the sol-gel based route has been tuned for successfully developing Na2Ti3O7 nanoparticles with MWCNTs uniformly wrapped around them allowing intimate contact with the MWCNTs, as was clearly evident from the high-resolution TEM images. The results once again highlight the success of this route based on direct addition of MWCNTs into the ‘matrix’ sol, followed by rapid gelation and subsequent crystallization in presence of MWCNTs (for Na-ion battery electrode), as recently demonstrated also for LiFePO4-MWCNT electrodes (for Li-ion batteries) [2]. The specific Na-capacities, as recorded during galvanostatic cycling, were considerably higher upon incorporation of the MWCNTs, as compared to those with the active particles of the same size/composition, prepared using the same route, but sans MWCNTs. More importantly, the cyclic stability improved significantly in the presence of the MWCNTs. The differences were more notable at the higher current densities, signifying considerably improved rate capability. In more specific terms, for over a period of 100 discharge/charge cycles, the capacity retention for Na2Ti3O7/MWCNT was superior to that for Na2Ti3O7, sans MWCNTs, by a factor of ~16. Another important observation has been the significant reduction in voltage hysteresis in presence of the MWCNTs. In addition to increasing the overall electrical conductivity, the anchoring/buffering action provided by the MWCNTs against the high volume changes during phase transformation reactions, might possibly be some of the reasons for the higher capacity and improved cyclic stability in the case of MWCNTs-containing Na2Ti3O7 nanoparticles. The work, to be presented, also includes in-situ synchrotron X-Ray diffraction studies for obtaining better insights into some of the fundamental aspects concerning phase transformations during charge/discharge at such ultra-fine dimension, in the presence/absence of MWCNTs (in intimate contact) and also in the presence of impurity phases (as deliberately introduced). The results provide clarity on the sodiation/de-sodiation mechanisms and correlations between the electrochemical performances, phase assemblage and phase evolutions of Na2Ti3O7 and Na2Ti3O7/MWCNT anodes for Na-ion batteries. Keywords: Na2Ti3O7/MWCNT, Na-ion batteries, Cyclic stability, In-situ synchrotron X-Ray diffraction

Electrochemically activated MnO as a cathode material for sodium-ion batteries

Besides classical electrode materials pertaining to Li-ion batteries, recent interest has been devoted to pairs of active redox composites having a redox center and an intercalant source. Taking advantage of the NaPF6 salt decomposition above 4.2V, we extrapolate this concept to the electrochemical in situ preparation of F-based MnO composite electrodes for Na-ion batteries. Such electrodes exhibit a reversible discharge capacity of 145mAhg−1 at room temperature. The amorphization of pristine MnO electrode after activation is attributed to the electrochemical grinding effect caused by substantial atomic migration and lattice strain build-up upon cycling.

Elucidation of the reaction mechanisms of isostructural FeSn2and CoSn2negative electrodes for Na-ion batteries

FeSn2and CoSn2are tested as suitable negative electrode materials to improve the energy density of Na-ion batteries.

Transmission electron microscopy investigation of the LiMn2O4/NaxMnO2 interface as a model study of a Na-ion battery electrode

The phase transformation from spinel LiMn2O4 to layered rock-salt NaxMnO2 via Na insertion-extraction cycles is crucial for a LiMn2O4 positive electrode in a Na-ion battery. To reveal the atomic-scale mechanism of the structural conversion, we applied advanced techniques of analytical electron microscopy to a Na-containing LiMn2O4 specimen, formed by lithiation of a thin MnO wafer containing Na impurity. Scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) analyses revealed that Na and Li are separately distributed in the two phases of the specimen, which are layered NaxMnO2 and spinel LiMn2O4 phases confirmed by annular bright field (ABF)-STEM observation. The large difference in the ionic radii of Na and Li is considered to be the reason for the clear phase separation without atomic-scale mixture. EDX analysis showed that the layered NaxMnO2 phase with P3 structure exhibits local variations in Na composition, with the maximum value of x = 0.6. High-resolution transmission electron microscopy (HRTEM) and ABF-STEM imaging clearly showed that an epitaxial LiMn2O4/NaxMnO2 interface is formed without any lattice dislocations, however, the interface has a low inclination angle due to the lattice mismatch, and local distortions are induced in the NaxMnO2 phase. The epitaxial lattice transition suggests smooth generation of a Na-inserted phase with a layered structure in a spinel LiMn2O4 crystal, which should contribute to the superiority of the LiMn2O4 electrode in a Na-ion battery.

Strong Impact of the Oxygen Content in Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 0.5) on Its Structural and Electrochemical Properties

Polyanionic materials such as Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 2) are of high interest as positive electrode for Na-ion batteries since they offer competitive electrochemical performances compared to sodiated transition metal oxides. The composition Na3V2(PO4)2F3 (y = 0) has the highest theoretical energy density among the series, but surprisingly a lot of discrepancies are reported throughout the literature considering its structure and its electrochemical properties. We will show that most of the compounds reported as being Na3VIII2(PO4)2F3 are in fact slightly oxidized due to synthesis conditions resulting in a partial oxygen substitution for fluorine. To get an in-depth understanding of this system, a series of compositions Na3V2(PO4)2F3–yOy (0 ≤ y ≤ 0.5, i.e., near the fluorine-rich composition) was synthesized and characterized combining synchrotron X-ray diffraction, X-ray absorption spectroscopy, solid state nuclear magnetic resonance spectroscopy, and galvanostatic electrochemical tests. The structural...

Self-Assembled Synthesis of Mesocrystalline TiO2@C-rGO Hybrid Nanostructures for Highly Reversible Sodium Storage

With the aim of enhancing the electrochemical kinetics and capacity of the TiO2 electrode for Na ion batteries (NIBs), we have designed a hybrid material of carbon-coated TiO2 mesocrystals anchored on reduced graphene oxide (TiO2@C-rGO). Such hybrid nanostructures are fabricated through a facile one-step route including in situ growth of oriented self-assembly of TiO2 mesocrystals on GO. TiO2@C-rGO possesses a very large surface area (279 m2 g–1), mesoporous nature, and single-crystal-like structure. It is also found that the capacity of TiO2 electrode for NIBs could be improved by carbon coating at a low current rate, but pure TiO2 shows better rate performance than that of TiO2@C. Remarkably, the enhanced electrochemical kinetics and large capacity can be simultaneously achieved by designing hybrid material. The hybrid nanostructures exhibit a highly reversible capacity of 300 mAh g–1 at 100 mA g–1, superior rate capability, and long-term cycling stability (a stable capacity of 159 mAh g–1 can be mainta...

Direct observation of electronic conductivity transitions and solid electrolyte interphase stability of Na2Ti3O7 electrodes for Na-ion batteries

Abstract This communication reports the first experimental evidence of an interesting change of transport properties, and particularly of electron conductivity, during the Na + insertion/extraction process in Na 2 Ti 3 O 7 negative electrodes. Probed by electrochemical impedance spectroscopy, for 0.0 ≤ x 2+x Ti 3 O 7 the material exhibits insulator behaviour, the bulk electronic conductivity being the limiting factor in the insertion process. After further Na + insertion, the material becomes electronic conductor and at around 0.13 V vs. Na + /Na the rate of interfacial charge-transfer becomes the limiting factor. The observed conductivity transition is reversible upon cycling. Additionally, this impedance study sheds new light on the solid electrolyte interphase layer performance which is found to be unstable upon electrochemical cycling and negatively contributes on the capacity fading observed for this electrode material.

Structural Properties and Phase Transition of Na Adsorption on Monolayer MoS2.

First-principles calculations are performed to investigate the structural stability of Na adsorption on 1H and 1T phases of monolayer MoS2. Our results demonstrate that it is likely to make the stability of distorted 1T phase of MoS2 over the 1H phase through adsorption of Na atoms. The type of distortion depends on the concentration of adsorbed Na atoms and changes from zigzag-like to diamond-like with the increasing of adsorbed Na atom concentrations. Our calculations show that the phase transition from 1H-MoS2 to 1T-MoS2 can be obtained by Na adsorption. We also calculate the electrochemical properties of Na adsorption on MoS2 monolayer. These results indicate that MoS2 is one of potential negative electrodes for Na-ion batteries.

Layered Na2V6O16 nanobelts as promising cathode and symmetric electrode for Na-ion batteries with high capacity

Layer structured Na2V6O16 nanobelts (NVON) are synthesized by a facile, effective, low cost, one-pot and template free method. Inspired by its structure advantages, the NVON are tested as cathode material for Na-ion batteries (NIBs), which show a high reversible capacity of 194.6 mAh g−1. Furthermore, the NVON can maintain its initial structure after repeated charge/discharge which could result in good cyclic stability. Most importantly, the NVON can be used to prepare symmetric NIBs electrode, which exhibit a high discharge capacity of 132.9 mAh g−1 and a higher energy density of 82 Wh kg−1. The obtained results demonstrate the promising potential of NVON as cathode materials for NIBs, which highlight the importance of nanocrystallization of layered structured for maximum utilization of electrochemically active sites in a high performance NIBs.

Probing the Rate Limiting Steps in Conversion Reactions of Nanostructured Iron Oxide in Li- and Na-Ion Batteries

Na-ion batteries have gained a remarkable interest during the last decade due to their attractive features of reduced costs and possible use of more environmentally friendly materials. These characteristics could promote the development of this type of electrochemical storage technology as a possible alternative to Li-ion cells for a cost-effective large-scale stationary storage of electricity. Yet, the advancement of suitable Na-ion batteries for such a purpose has to face more severe challenges than those occurred during the initial development of Li-ion battery technology. In fact, the noticeable differences in mass and size between Li+ and Na+ ions have considerable effects on the associated electrochemical performances for the corresponding battery systems. These factors heavily affect also the choice of convenient materials for Na-ion cells, especially for negative electrodes. In fact, graphite cannot be employed as insertion host [1], although possible Na+ incorporation via solvent co-intercalation phenomena [2] has been reported. The most suitable materials for negative electrodes in Na-ion batteries so far are disordered carbons [3], since they can offer a convenient trade-off in terms of specific capacities, rate capabilities and cycle performances. Nevertheless, such specifications could be advanced and definitely there is room for further improvements. In this regard, more abundant and cheaper materials, which can be synthesized in simpler ways at lower temperatures, have also been proposed. Among these, iron oxide is certainly one of the most attractive because of its earth-abundance and environmental friendliness. For these reasons, this compound has been investigated in a number of preliminary studies [4-6] in Na-half cells. The reaction pathway of iron oxide upon its initial phases of Na+ uptake resembles to a good extent that occurring upon similar Li+ incorporation. However, noticeable differences characterize the respective electrochemical mechanisms, being the subsequent conversion reaction of iron oxide into Fe and Na2O clearly hampered [4-6] compared to its analogue mechanism upon lithiation, in which the extensive formation of Fe nanoparticles surrounded by a Li2O matrix is involved [7]. The objectives of this study are to identify the limiting factors occurring during the conversion reaction of iron oxide upon sodiation and to assess its feasibility for possible enhancement of Na storage. These aims are pursued by comparing its features to those displayed by its analogous mechanism driven by Li+. A systematic analysis via cyclic voltammetry (CV) has been conducted on different types of iron oxide nanostructures (e.g. nanopowders, nanowires and thin films) cycled in parallel in Li- and Na-half cells containing analogous electrolytes (i.e. LiClO4 and NaClO4 respectively). In this approach, the characteristic behaviours of these particular electrochemical systems have been compared. The roles played by their intrinsic structures, textures, as well as by the different electrical wiring throughout the associated electrodes embedding these active materials, will be presented and highlighted. The limiting factors influencing the conversion reactions of these iron oxide nanostructures upon their respective lithiation and sodiation will be discussed and the issues arising from such limitations will be identified and addressed. It will be shown that the porosity of the active materials and their mutual electrical contact, as well as that with the current collector, play a crucial role in the electrochemical processes. Finally, Na storage in iron oxide via conversion reactions is demonstrated to be intrinsically limited, irrespectively of the type of nanostructure employed for this specific purpose, thus posing restrictions on the ultimate usage of this type of material in combination with Na+ ions.

Transport Properties and SEI Stability of Na2Ti3O7 electrodes for Na-Ion Batteries: An EIS Study

Na2Ti3O7 is a promising negative electrode for Na-ion batteries (NIBs) with a very low insertion voltage (0.3 V vs. Na+/Na) and high specific capacity (178 mAh/g) [1, 2]. However, Na2Ti3O7 shows poor capacity retention when synthesized from Na2CO3 as sodium precursor, reaching only 50-70% of capacity retention after 10 cycles [3, 4, 5]. The capacity fading is correlated, among other factors, with the presence of Na2CO3 on the particles [3], which is originated by the interaction of Na2Ti3O7 particles with atmospheric H2O and CO2. Another important factor to take into account is the formation of a stable solid-electrolyte interphase (SEI) layer. In fact, the reversible Na+ insertion/extraction reaction occurs at low potential and, therefore, electrolyte reduction occurs. The stability and composition of this SEI layer has been previously studied by X-ray photoelectron spectroscopy (XPS) [6], concluding that the SEI layer formed upon Na+insertion is partially dissolved during extraction. In order to better understand the reasons behind the poor capacity retention, an electrochemical impedance spectroscopy (EIS) study was carried out to determine the electronic and ionic transport properties of Na2Ti3O7 electrodes. An interesting change of transport properties, and particularly of electron conductivity, during Na+ insertion/extraction process is revealed for Na2Ti3O7negative electrodes by EIS. Na2Ti3O7 was synthesized via a ceramic route from precursors: TiO2 anatase and Na2CO3·H2O in excess. Three electrode Swagelok type cells were tested using metallic sodium disks as counter and reference electrodes; electrochemical measurements were performed at room temperature in the voltage window 0.05-1.6 V vs. Na+/Na. EIS measurements were performed by controlling the electrode potential through PITT (potentiostatic intermittent titration technique). The EIS study here presented is the first experimental demonstration of a transition from electronic insulator to conductor in Na2Ti3O7 electrodes for NIBs [Fig. 1]. This reversible transition is originated by Na+ insertion/extraction and was recently predicted by DFT calculations. Moreover, the instability of the SEI layer has been also observed, in agreement with previous XPS studies, contributing to the capacity fading widely reported for this material. This confirms that prior to Na+ insertion the Na2Ti3O7 is an insulator and the ionic transport kinetics are limited by the electronic conductivity, but when the intercalated Na+ increases the Na2Ti3O7behaves as a metallic conductor and the kinetics are limited by the interfacial charge-transfer step. Acknowledgments M. Zarrabeitia thanks the Government of the Basque Country for funding through a PhD Fellowship. Financial support from the Basque Government (Etortek 10 CIC Energigune) and from the Ministerio de Economía y Competitividad of the Spanish Government (ENE2013-44330-R) is also acknowledged.