Electrodes For Na-ion Batteries Research Articles

Operando Stress Evolution in Hard Carbon Anodes – Insight into the Mechanical Degradation Induced Failure Mode in Rechargeable Sodium Ion Batteries

Microstructural instabilities in electrodes arising from cycling induced stress is a primary failure mode in rechargeable batteries. Therefore, a quantitative knowledge of the cycling induced stress evolution in rechargeable battery electrodes is important for understanding the electrode microstructural instabilities during cycling that can potentially inform the electrode design to prolong battery cycle life. The cycling induced stress in electrodes typically arises from the volume changes associated with insertion/extraction of the electroactive species in the electrode matrix. For example, operando measurement of lithiation-delithiation induced stress in graphite electrodes that typically show 10% volume change has previously resulted in useful information that correlates well with the lithiation-delithiation mechanism in graphite anodes1. The size of the shuttling-electroactive species can be taken as an indicator of the cycling induced stress magnitude that drives the electrode microstructural instabilities. In this regard, quantification of the cycling induced stress in rechargeable Na-ion battery electrodes is warranted for understanding mechanical degradation induced failure modes owing to the larger size of the electroactive species, i.e., Na ions (vs. Li-ion).Hard carbons as potential anodes in rechargeable sodium ion batteries has gained considerable attention in the recent past due to their superior Na-ion storage capability (vs. graphite that is a canonical anode in rechargeable lithium-ion batteries). However, hard carbon anodes typically show significant capacity fade that can potentially arise from cycling induced mechanical degradation of the anodes. Operando stress measurements coupled with comprehensive electrode microstructural characterization will provide valuable insights in this context. Here we report on the operando stress evolution in hard carbon anodes in rechargeable sodium ion batteries as probed by Multiple Optical-beam Sensing (MOS) technique that basically involves monitoring the spacings among a group of laser spot (Fig. 1). Preliminary measurements have shown that the stress correlates with the potential with a maximum around 12 MPa in compression during sodiation and tensile stress of ~ 2MPa during desodiation. Operando stress measurements are performed in hard carbon anodes in two different electrode configurations – typical composite anodes (with binder and conductive agents) and hard carbon thin film anodes (without binder and conductive agents). The thin film configuration is considered to evaluate intrinsic stress response in hard carbon anodes in absence of any secondary phase. A comparison of operando stress response in these two types of hard carbon anodes along with comprehensive electrode microstructural characterization will be presented. Additionally, attempts will be made to shed light on the current debate in the mechanism of sodium storage in hard carbon anodes by comparing operando stress response of hard carbons from multiple sources along with their structural characterization (Raman, XPS, surface area etc.). V. A. Sethuraman, N. Van Winkle, D. P. Abraham, A. F. Bower, and P. R. Guduru, Journal of Power Sources, 206, 334–342 (2012). Figure 1

Mechanically Exfoliated Graphite for Highly Reversible Na-Storage with Carbon Coated Na3V2(PO4)3 Cathode with Low-Temperature Conditions.

Tremendous efforts are put forward to develop novel high-performance electrodes for Na-ion batteries (SIBs) in order to replace commercial Li-ion batteries (LIBs). Graphite, the most versatile anode for LIBs, fails to accommodate Na+ions owing to the poor thermodynamic stability of the binary graphite intercalation compound. This study aims to exfoliate the layers of graphite through a simple mechanical process at different time intervals (1, 5, 10, 20, 40, and 80h) and examine the potential candidate for Na-storage in the presence of carbonate-based electrolytes. This study reports that ball milling plays a vital role in the performance of the graphite in Na-storage. The graphite exfoliated for 80h (EG-80h) rendered an excellent reversible capacity of 209mAhg-1 with coulombic efficiency of >99% after 100 cycles in EC-based electrolyte. In situ impedance analysis is performed to validate the charge storage mechanism and Na-ion kinetics. The performance of EG-80h in a full-cell assembly is evaluated with a carbon-coated Na3V2(PO4)3 cathode, which exhibited an initial capacity of ≈75mAhg-1 andenergy density of 201Whkg-1. In addition, the adaptability of the NVPC/EG-80h cell at different temperatures is examined from -10 to 50°C, displaying excellent performance in both high and low-temperature conditions.

Practical Alloy-Based Negative Electrodes for Na-ion Batteries

The volumetric capacity of typical Na-ion battery (NIB) negative electrodes like hard carbon is limited to less than 450 mAh cm−3. Alloy-based negative electrodes such as phosphorus (P), tin (Sn), and lead (Pb) more than double the volumetric capacity of hard carbon, all having a theoretical volumetric capacity above 1,000 mAh cm−3 in the fully sodiated state. These alloy materials have massive volume expansion, with P expanding by almost 300% and both Sn and Pb expanding to about 400% of their initial volumes when fully sodiated. This work shows that despite this large volume change, Sn and Pb have excellent half-cell cycling performance, including high Coulombic efficiency. A blended P-hard carbon cell shows promising initial performance despite P having far lower electronic conductivity than either Sn or Pb. Overall, it is expected that these Na-alloying elements will be needed to increase the volumetric energy density of NIBs to compete with low-cost lithium iron phosphate-based cells. In fact, one company, UNIGRID, has recently demonstrated cylindrical Na-ion cells that use a pure tin negative electrode, dramatically increasing the volumetric energy density of Na-ion technology.

Flexible Centrifugally Spun N, S-Doped SnS2-Including Porous Carbon Nanofiber Electrodes for Na-Ion Batteries.

Carbon nanofibers are promising for various applications such as energy storage, sensors, and biomedical applications; however, the brittle structure of nanofibers limits the usage of carbon nanofibers. For the first time, a facile and effective strategy is reported to fabricate flexible carbon nanofibers via a fast and safe nanofiber fabrication technique, centrifugal spinning, followed by heat treatment. Moreover, sulfidization was employed to fabricate high-performance flexible N, S-doped SnS2-including porous carbon nanofiber electrodes for sodium ion batteries via centrifugal spinning. Binder-free flexible centrifugally spun N, S-doped SnS2-including porous carbon nanofiber electrodes delivered a high reversible capacity of over 460 mAh/g at 100 mA/g. Furthermore, at a high current density of 1 A/g, the electrodes showed a reversible capacity of over 300 mAh/g with excellent cycling stability in 2000 cycles. This work reports a fast, low-cost, and facile way to prepare flexible carbon nanofibers with tunable morphology and various compositions, which could be applicable for different energy storage applications.

Templating Nanostructured Aromatic Based Materials as Possible Anode Electrodes for Na-ion Batteries: A Computational DFT Approach

Templating Nanostructured Aromatic Based Materials as Possible Anode Electrodes for Na-ion Batteries: A Computational DFT Approach

Unraveling the Synthesis of Positive Electrodes for Na-Ion Batteries by in Situ XRD

In situ diffraction studies can capture transient crystalline phases forming during chemical reactions. Whether the reaction is a chemical solid-state synthesis, or an electrochemical intercalation process within typical active compounds used as battery electrodes, proper sample environments allow nowadays to perform in situ diffraction experiments with high temporal and angular resolution even when using laboratory diffractometers. In both cases, the time-resolved nature of the experiments allows to obtain a greatly increased amount of information. For example, in the synthesis of inorganic materials, reactions often yield non-equilibrium kinetic byproducts instead of the thermodynamic equilibrium phase [1]. On the other hand, often stable compounds cannot be synthesized. To rationalize that, the competition between thermodynamics and kinetics occurring during the process need to be investigated in real time. Fully determining the reaction pathway is a key requirement to achieve the rational synthesis of target materials [2, 3]. In this presentation, recent unpublished examples will be reported from our work applying in situ XRD to understand the synthesis of cathode materials for Na-ion batteries. We focus on layered materials and on the relationship between P2 and P3 polymorphs in series of samples of the same composition NaxDyNizMn1-y-zO2 (where D is a substitutional element and Ni and Mn are in divalent and tetravalent oxidation state, respectively) and highlight the transformation of a phase into the other during calcination. Moreover, we provide electrochemical and morphologic characterizations of the target compounds, aiming to answer the question: does one or the other polymorph offer better performances? The charge compensation mechanism, involving Ni redox but also O redox, will be reported. Finally, the presentation will also briefly cover the investigation of novel solid-state synthesis routes for cathodes of the fluorophosphate Na3V2(PO4)2F3-xOx family.

Structure and evolution of disordered deep-charge phases in NaxCrO2 Na-ion battery electrodes

Layered sodium chromium oxide, O3–NaCrO2, is a promising electrode material for Na-ion batteries due to the high thermal stability, high rate-capability, and good coulombic efficiency of the material. However, it is well-known that the material loses crystallinity and the ability to store Na-ions reversibly when taken to high states of charge. While the structural identity of the disordered state has been investigated, a bulk structural model has so far been lacking, as has the mechanism for the order-disorder transition from the P′3-NaxCrO2 (the last crystalline phase observed during charge) to the disordered state. In this work, we combine pair distribution function analysis from total X-ray scattering and powder X-ray diffraction to obtain a complete structural model for the disordered O3-like NaxCrO2 forming at high states-of-charge, and from operando pair distribution function analysis and X-ray absorption spectroscopy, we elucidate the structural evolution of the order-disorder transition and shed new light on the Cr-migration in the disordered state, which is the cause of the irreversibility of the Na-ion storage.

Morphology and Crystallinity Effects of Nanochanneled Niobium Oxide Electrodes for Na-Ion Batteries.

Niobium pentoxide (Nb2O5) is a promising negative electrode for sodium ion batteries (SIBs). By engineering the morphology and crystallinity of nanochanneled niobium oxides (NCNOs), the kinetic behavior and charge storage mechanism of Nb2O5 electrodes were investigated. Amorphous and crystalline NCNO samples were made by modulating anodization conditions (20-40 V and 140-180 °C) to synthesize nanostructures of varying pore sizes and wall thicknesses with identical chemical composition. The electrochemical energy storage properties of the NCNOs were studied, with the amorphous samples showing better overall rate performance than the crystalline samples. The enhanced rate performance of the amorphous samples is attributed to the higher capacitive contributions and Na-ion diffusivity analyzed from cyclic voltammetry (CV) and the galvanostatic intermittent titration technique (GITT). It was found that the amorphous samples with smaller wall thicknesses facilitated improved kinetics. Among samples with similar pore size and wall thickness, the difference in their power performance stems from the crystallinity effect, which plays a more significant role in the resulting kinetics of the materials for Na-ion batteries.

Fabrication and characterization of conductive polyaniline-decorated Na3V2(PO4)3 cathode for Na-ion batteries

NASICON-structured Na3V2(PO4)3 (NVPO) has been regarded as an advanced electrode for Na-ion storage due to the high theoretical capacity. Unfortunately, the pristine NVPO possesses a bad electrical conductivity and a slow Na+ mobility. Herein, polyaniline-enhanced NVPO (NVPO@PANI) nanocomposite is firstly prepared through the sol-gel way followed by an in-situ aniline polymerization process. Benefiting from the small NVPO crystals and constructed polyaniline film, the transportation of Na+ and electron in NVPO@PANI has been greatly improved. Therefore, the fabricated NVPO@PANI cathode displays outstanding sodium storage performance. It delivers an initial capacity of 114.9 mAh g−1 at 0.1C, and it also present a good cyclic-life with a capacity ratio of about 95.1% at 20C over 500 cycles. The present study offers a prospective strategy for improving the electrochemical property of other electrodes for Na-ion batteries.

The Impact of Graphene in Na2FeP2O7/C/Reduced Graphene Oxide Composite Cathode for Sodium-Ion Batteries

This study presents a thorough investigation of Na2FeP2O7 (NFP) cathode material for sodium-ion batteries and its composites with carbon and reduced graphene oxide (rGO). Our findings demonstrate that rGO sheets improve cycling performance in NFP/C/rGO composite in the absence of solid electrolyte interphase (SEI)-stabilizing additives. However, once SEI is stabilized with the help of fluoroethylene carbonate electrolyte additive, NFP with carbon additive (NFP/C) exhibits a superior electrochemical performance when compared to NFP/rGO and NFP/C/rGO composites. The decreases in capacity and rate capability are proportional to the amount of rGO added, and lead to an increase in overvoltage and internal resistance. Based on our results, we attribute this effect to worsened sodium kinetics in the bulk of the electrode—the larger ionic radius of Na+ hinders charge transfer in the presence of rGO, despite the likely improved electronic conductivity. These findings provide a compelling explanation for the observed trends in electrochemical performance and suggest that the use of rGO in Na-ion battery electrodes may present challenges associated with ionic transport along and through rGO sheets.

Electrochemical Response of 3D-Printed Free-Standing Reduced Graphene Oxide Electrode for Sodium Ion Batteries Using a Three-Electrode Glass Cell.

Graphene and its derivatives have been widely used to develop novel materials with applications in energy storage. Among them, reduced graphene oxide has shown great potential for more efficient storage of Na ions and is a current target in the design of electrodes for environmentally friendly Na ion batteries. The search for more sustainable and versatile manufacturing processes also motivates research into additive manufacturing electrodes. Here, the electrochemical responses of porous 3D-printed free-standing log-type structures fabricated using direct ink writing (DIW) with a graphene oxide (GO) gel ink are investigated after thermal reduction in a three-electrode cell configuration. The structures delivered capacities in the range of 50-80 mAh g-1 and showed high stability for more than 100 cycles. The reaction with the electrolyte/solvent system, which caused an initial capacity drop, was evidenced by the nucleation of various Na carbonates and Na2O. The incorporation of Na into the filaments of the structure was verified with transmission electron microscopy and Raman spectroscopy. This work is a proof of concept that structured reduced GO electrodes for Na ion batteries can be achieved from a simple, aqueous GO ink through DIW and that there is scope for improving their performance and capacity.

Engineering of Conformal Electrode Coatings by Atomic Layer Deposition for Aqueous Na-ion Battery Electrodes

The application of atomic layer deposition on active material particles or as conformal layers directly on electrodes is an effective and viable approach for protecting the battery materials from degradation. Al2O3, TiO2, and HfO2 coatings are applied on NaTi2(PO4)3, which is among the most studied negative electrode materials for aqueous Na-ion batteries. The coated electrodes are characterized in terms of electrochemical kinetics, charge capacity retention, and electrochemical impedance spectra. Al2O3, a widely used protective coating in non-aqueous batteries, is shown to be insufficient to suppress parasitic processes and is eventually dissolved by reaction with hydroxide during extended cycling in aqueous Na2SO4. However, this process provides a local buffering effect making the protective action of this coating mainly of chemical nature. TiO2 is found to be very resistant to increase in pH and remains almost intact during electrochemical cycling. However, we provide strong evidence that TiO2 itself is electrochemically active in aqueous electrolytes at negative potentials. The protonation of TiO2 leads to an additional increase in local pH which is detrimental to NaTi2(PO4)3 and results in even faster capacity loss than in uncoated electrodes. Only HfO2 is found to be sufficiently stable and electrochemically inert ALD coating for negative NaTi2(PO4)3 electrodes operating in aqueous electrolytes.

Pyrolysis temperature dependence of sodium storage mechanism in non-graphitizing carbons

Hard carbons are the most investigated materials as negative electrode for Na-ion batteries, although the exact mechanism of sodium storage remains under debate. This work is focused on the study of the sodiation mechanism of non-graphitizing carbons (NGC) prepared in a wide range of pyrolysis temperatures (1000 °C–2500 °C), thus covering the whole range from hard carbons (HC) to glassy carbons (GC). Structural and textural characterizations show that increasing the pyrolysis temperature leads to NGCs with a more ordered structure, containing fewer heteroatoms and structural defects, and particularly leads to a developed closed microporosity. In this work, in situ Raman spectroscopy at different excitation wavelengths is used to clarify the mechanism of electrochemical sodiation: it is revealed that the intercalation of Na+ between the graphene layers occurs mainly during the sloping part of the galvanostatic profile, whereas the plateau at low voltage (below 0.1 V vs. Na+/Na) can be associated with the filling of micropores with sodium.

Solvothermal Engineering of NaTi2(PO4)3 Nanomorphology for Applications in Aqueous Na-Ion Batteries.

Aqueous Na-ion batteries are among the most discussed alternatives to the currently dominating Li-ion battery technology, in the area of stationary storage systems because of their sustainability, safety, stability, and environmental friendliness. The electrochemical properties such as ion insertion kinetics, practical capacity, cycling stability, or Coulombic efficiency are strongly dependent on the structure, morphology, and purity of an electrode material. The selection and optimization of materials synthesis route in many cases allows researchers to engineer materials with desired properties. In this work, we present a comprehensive study on size- and shape-controlled hydro(solvo)thermal synthesis of NaTi2(PO4)3 nanoparticles. The effects of different alcohol/water synthesis media on nanoparticle phase purity, morphology, and size distribution are analyzed. Water activity in the synthesis media of different alcohol solutions is identified as the key parameter governing the nanoparticle phase purity, size, and shape. The careful engineering of NaTi2(PO4)3 nanoparticle morphology allows control of the electrochemical performance and degradation of these materials as aqueous Na-ion battery electrodes.

MnP4 electrode for Na-ion batteries: a complex and effective electrochemical mechanism

MnP4 has recently been identified as a possible negative electrode for Li-ion batteries.

Multiphase Transformation of NaFeF3During Desodiation and Sodiation

The orthorhombic NaFeF3, which is envisioned to be an auspicious positive electrode for Na-ion batteries, has drawn considerable interest as an environmentally benign energy material with exceptionally high theoretical capacity. Despite these prospects, the reaction mechanism(s) during orthorhombic NaFeF3 operations are still not well understood. Thus, in a bid to expound on this research space, we report the reaction mechanism(s) of a carbon-coated orthorhombic NaFeF3 prepared through high-energy ball-milling and heat treatment processes. A thermally stable ionic liquid electrolyte at elevated temperatures is employed to maximize the utilization of NaFeF3. The orthorhombic NaFeF3 exhibits high electrochemical activity and long-term cycling stability of up to 400 cycles at 90 °C. Through a combination of galvanostatic intermittent titration technique and synchrotron X-ray powder diffraction measurements, we discover that the (de)sodiation processes are facilitated by a multiphase transformation mechanism. Further, we experimentally identify, for the first time, an orthorhombic Na0.5FeF3 compound as an intermediate phase of the transformations. The results discussed in this work are expected to provide invaluable insights for the future advancement of the Na–Fe–F system.

Glass-ceramic route to NASICON-type Na Ti2(PO4)3 electrodes for Na-ion batteries

Glass-ceramic processes to prepare NASICON-type Na x Ti 2 (PO 4 ) 3 , which is an anode candidate for sodium-ion batteries, from 30Na 2 O–40TiO 2 –30P 2 O 5 glass have been investigated with varied atmospheric conditions. By annealing in the inert (N 2 ) atmosphere, the glass started crystalizing into NASICON-type NaTi 2 (PO 4 ) 3 with rhombohedral symmetry at ∼600 °C followed by crystallization of other phases to be multiphase mixtures at higher temperatures. In contrast, in the reducing (5% H 2 /Ar) atmosphere, NASICON-type Na 3 Ti 2 (PO 4 ) 3 with triclinic symmetry is crystallized at >∼800 °C. The formation of Na 3 Ti 2 (PO 4 ) 3 is associated with a loss of excess oxygens in the initial glass composition and a reduction of Ti from Ti 4+ to Ti 3+ . The reduction process upon the glass-to-ceramic conversion was traced by in-situ observation during the thermogravimetric analysis. It is also revealed that the electrochemical Na + -storage capabilities of the glass-ceramic electrodes are correlated with the Na-ion occupancy between these two phases, and their phase fractions affect the charge-discharge properties of Na-ion cells. Finer glass-ceramic powders could improve the electrochemical properties and achieve almost 80% of its theoretical capacity.

Ru-Doping of P2-NaxMn0.75Ni0.25O2-Layered Oxides for High-Energy Na-Ion Battery Cathodes: First-Principles Insights on Activation and Control of Reversible Oxide Redox Chemistry

Design and development of high-energy, efficient, and structurally stable positive electrodes for Na-ion batteries (NIBs) have recently been focusing on layered transition metal oxides (NaxTMO2, 0 < x < 1). When doped with late transition metals, the redox reactions ensuring the charge compensation upon Na+ removal can rely on the direct participation of anionic chemistry, with encouraging outcomes in terms of both energy density and power. However, the control of reversible O2–/O– reactions is still a major issue, as the undesired release of O2 can lead to detrimental effects on cathode capacity and overall cell stability. The fine-tuning of metal–oxygen bond covalency has recently emerged as a promising strategy toward the reversible access of oxygen redox. Following the route paved by Ru-based Li-rich cathode materials, we hereby present a first-principles investigation of a Ru-doped NaxTMO2 (TM = Ru, Ni, and Mn) system and the related structural and electronic properties of interest for NIB applications. We aim to dissect the specific role of each element sublattice in compensating the electronic charge along desodiation, with a major focus on the anionic contribution. The oxygen activity is addressed in the high-voltage range (i.e.,xNa = 0.25), and the underlying mechanism is derived from PBE + U(-D3BJ) calculations. We also discuss the effects of Mn deficiency as a suitable site for the formation of low-energy superoxide species via preferential breaking of the Ni–O bond. Conversely, breaking a Ru–O bond is unlikely to occur, which assesses the key role of the Ru dopant in stabilizing the oxide lattice and enabling the desired reversible conditions. Our results also highlight the oxygen vacancy formation energy as an effective descriptor for different activities toward the O2–/O–/O2 evolution. All these theoretical insights can be useful to drive further experimental efforts toward the optimal design of efficient and high-energy NIB cathodes with enhanced practical reversible capacity.

Operando X-ray diffraction studies of NASICON-type positive electrodes for Na-ion batteries

<i>Operando</i> X-ray diffraction studies of NASICON-type positive electrodes for Na-ion batteries

Critical overview of polyanionic frameworks as positive electrodes for Na-ion batteries

Na-ion batteries (NIBs) are an important technological alternative to Li-ion batteries (LIBs) for developing energy storage systems that are cost-effective and less constrained by geographical supply chains, with similar energy densities. Analogous to LIBs, cathodes play a critical role in determining the energy density of NIBs, and layered transition metal oxide compounds are widely used as NIB cathodes. However, the instability-driven irreversible phase transitions in layered frameworks, especially at large degrees of Na removal, has necessitated the exploration of rigid structural frameworks that are resistant to structural changes with Na exchange. Thus, polyanionic frameworks, which primarily consist of transition metal polyhedra interconnected by PO4, SiO4, and/or SO4 units, with or without the presence of fluorine, have been investigated as NIB cathodes. This review provides a critical overview of recent studies using polyanion cathodes, which will be useful in guiding the scientific community towards the development of better NIB cathodes.Graphical abstract