Na-ion Battery Technology Research Articles

Theoretical prediction of two-dimensional metallic AM2B8 (AM = K, Rb, Cs) as anode materials for Na-ion batteries

The development of two-dimensional anode materials with superior performance is becoming a critical undertaking for the advancement of Na-ion battery technology. Using isoelectronic substitution strategies, we predicted three alkali metal borides AM2B8 (AM = K, Rb, Cs) based on the SrB8 monolayer. Ab initio molecular dynamics simulations and phonon dispersion relationship calculations demonstrated their stability. Using density functional theory, we predict the feasibility of these three new metal nanosheets as anodes for Na-ion batteries. The AM2B8 monolayer retains its metallicity after the insertion of Na ions, ensuring that the anode has good electrical conductivity. In addition, AM2B8 has a low diffusion barrier and open circuit voltage (K2B8 is 0.08 eV and 0.26 V) during charge/discharge. In the voltage range that inhibits dendrite growth, K2B8 can reach the maximum theoretical specific capacity (326 mAh/g). These results indicate that AM2B8, especially K2B8, is a promising anode material for Na-ion batteries.

Achieving high-capacity and durable sodium storage by constructing a binder-free nanotube array architecture of iron phosphide/carbon

The conversion-type anode material of iron phosphide (FeP) promises enormous prospects for Na-ion battery technology due to its high theoretical capacity and cost-effectiveness. However, the poor reaction kinetics and large volume expansion of FeP significantly degrade the sodium storage, which remains a daunting challenge. Herein, we demonstrate a binder-free nanotube array architecture constructed by FeP@C hybrid on carbon cloth as advanced anodes to achieve fast and stable sodium storage. The nanotubular structure functions in multiple roles of providing short electron/ion transport distances, smooth electrolyte diffusion channels, and abundant active sites. The carbon layer could not only pave high-speed pathways for electron conductance but also cushion the volume change of FeP. Benefiting from these structural virtues, the FeP@C anode receives a high reversible capacity of 881.7 mAh/g at 0.1 A/g, along with a high initial Coulombic efficiency of 90% and excellent rate capability and cyclability in half and full cells. Moreover, the sodium energy reaction kinetics and mechanism of FeP@C are systematically studied. The present work offers a rational design and construction of high-capacity anode materials for high-energy–density Na-ion batteries.

Sodium-Bismuth-Titanium glass–ceramic network: A high capacity anode network for Na+ ion storage

This article discusses 95[35Na2O-65Bi2O3]: 5TiO2 (mol%) glass-ceramic anode material system (abbreviated as GC-NBT5.0) in order to overcome major issues with Na-ion battery technology, such as inadequate reversible capacity, rate capability, electrical conductivity, and safety. After heat treatment for different heat treating regimes, the production of intense intermediate domains Na3Bi and Na3Ti results in flexible active centers with which heavier Na+ ions may react very easily for Na+ ion storage in the glass-ceramic anode network. As evaluated at 50mA/g over a longer cycle life of up to 3000 cycles, the GC-NBT5.0-8h glass ceramic anode beats both glass anode and traditional graphite anodes with a specific discharge capacity of 801mAh/g. However, the cell design is still backed by the best electrical conductivity because of the good interfacial contact between the glass-ceramic anode and electrolyte, and the lowest cycle-dependent complex impedance characteristics (Rct and Zw), which guarantee consistent capacity retention and the highest Columbic efficiency over longer cycle durations.

Monolayer TiSiP as a high-performance anode for Na-ion batteries

Exploring anode materials with overall excellent performance remains a great challenge for rechargeable Na-ion battery technologies. Herein, we have identified that monolayer TiSiP is just such a prospective anode candidate via first-principles calculations. It is showed to be dynamically, thermally, mechanically, and energetically stable, which provides feasibility for experimental realization. The Na diffusion on the its surface is proved to be ultrafast, with a migration energy barrier as low as 73 meV. Electronic structure confirms that the pristine system undergoes a transition from the semiconductor to metal during the whole sodiation process, which is a significant advantage to the electrode conductivity. More excitingly, monolayer TiSiP can accommodate up to double-sided five-layer adatoms, resulting in an ultrahigh theoretical capacity of 1176 mA h g−1 and a low average open-circuit voltage of 0.195 V. Moreover, the maximally sodiated electrode monolayer yields rather small in-plane lattice expansion of only 1.40%, which ensures reversible deformation and excellent cycling stability as further corroborated by structural relaxation and ab initio molecular dynamics simulation. Overall, all of these results point to the potential that monolayer TiSiP can serve as a promising anode candidate for application in high-performance low-cost Na-ion batteries.

Thermal Stability of Na3–xV2(PO4)2F3–yOy: Influence of F– for O2– Substitution and Degradation Mechanisms

The Na-ion battery technology appears as a reliable, sustainable, and environmentally friendly alternative to the Li-ion one, especially for stationary energy storage. As for the Li-ion technology, the safety aspect is of high importance to ensure large-scale development. In this work, we studied the thermal stability and decomposition mechanisms of carbon-coated Na3V2(PO4)2F3 and two fluorine-rich phases belonging to the solid-solution Na3V3+2–yV4+y(PO4)2F3–yOy (y = 0.07 and y = 0.12), that family of compounds being often considered among the most promising positive electrode materials for Na-ion batteries. This study shows the good thermal stability of these polyanionic materials and reveals that a low O2– for F– substitution has a very limited effect on the thermal stability of fully reintercalated materials recovered in the discharged state of the battery, whereas it has a beneficial impact for highly deintercalated ones, obtained by in-depth charges. Furthermore, whatever the state of charge and the oxygen content in NaxV2(PO4)2F3–yOy (1 ≤ x ≤ 3 and y = 0, 0.07 and 0.12), the thermal degradation leads, quite unexpectedly, to the formation of crystalline Na3V3+2(PO4)2F3 in addition to an amorphous phase. The fluorination of the partial oxygen for fluorine substituted material was clearly demonstrated by X-ray diffraction (XRD) and solid state nuclear magnetic resonance spectroscopy (NMR) on materials recovered after differential scanning calorimetry (DSC) analyses. The formation of a fully sodiated crystalline phase from the thermal degradation of the material obtained in charged states of the battery, with or without the presence of electrolyte, was never reported before.

(Europe Section Alessandro Volta Award) The Journey Towards the Large-Scale Commercialization of Low-Cost and High Energy Density Na-ion Batteries

Na-ion batteries based on non-aqueous electrolytes represent an inexpensive and sustainable alternative to their Li-ion counterparts [1,2]. The cost advantage is particularly apparent at the present time as the prices of battery grade Li and Co precursor salts have spiraled upwards in the last 18 months. Faradion Limited is a UK-based company, founded in 2011 and from December 2021, part of Reliance Industries Limited of India*. It is commercializing its Na-ion battery technology in a number of large format applications. It has identified and developed a wide range of inexpensive and proprietary active materials and non-aqueous electrolyte systems which offer low manufacturing costs as well as outstanding electrochemical performance and intrinsic safety. Over the past 10 years the company has incorporated these materials into full-scale Na-ion cells to a point where battery performance characteristics such as energy density, rate capability and cycle life are competitive with commercial Li-ion technologies.The Faradion Na-ion prototype cells demonstrate low-capacity fade on cycling, coupled to low polarization and excellent columbic and energy (round-trip) efficiency and may be configured for both energy and power applications. The use of Al for both current collectors serves as an additional and significant cost and safety benefit and allows the cells to be stored and transported at 0 V (i.e. physically shorted) [3]. The Faradion Na-ion cells are manufactured on commercial Li-ion production lines using proven battery designs [4,5]. Pouch, cylindrical and prismatic cell designs have all been demonstrated successfully [6].Faradion has worked with its commercial partners to scale-up its Na-ion cell chemistry to the 40 Wh and 90 Wh pouch cell level – see for example, figure 1. These cells deliver a cell level specific energy of over 150 Wh/kg and have been incorporated into a range of demonstrator energy storage applications, including E-bike, residential, renewables, telecoms and automotive [6]. Faradion’s technology roadmap indicates that a specific energy in excess of 190 Wh/kg will be accessible in the near future. Other key attributes such as low precursor costs, material sustainability and excellent temperature range, confirm that Faradion’s Na-ion battery technology will prove commercially successful in a range of large format applications [7]. Reference s: [1] J. Barker, M.Y. Saidi and J. Swoyer, Electrochem. Solid-State Chem. 6 (2003) A1[2] K. Kubota and S. Komaba, J. Electrochem. Soc., 162 (2015) A2538. doi.org/10.1149/2.0151514jes[3] (a) A. Rudola, C.J. Wright and J. Barker, Energy Materials Advances, 2021 Article ID 9798460. doi.org/10.34133/2021/9798460 (b) J. Barker and C.J. Wright, Assignee: Faradion Limited. US Patent #11159027[4] A. Bauer, J. Song, S. Vail, W. Pan, J. Barker and Y. Lu, Adv. Energy Materials, 1 2018, 1702869. doi.org/10.1002/aenm.201702869[5] For example, J. Barker and R.J. Heap, Assignee: Faradion Limited, US Patent#9774035, US Patent #9917307, US Patent #1019628, US Patent #10115966, US Patent #10050271, US Patent #10399863[6] (a) American Chemical Society, Chemical & Engineering News, July 20, 2015, Vol. 93, Issue 29. (b) American Chemical Society, Chemical & Engineering News, May 24, 2022, Vol. 100, Issue 19[7] A. Rudola et al. J. Mater Chem A, 2021, 9, 8279-8302. doi.org/10.1039/D1TA00376CFootnote:[*] In late 2021, Faradion Limited was acquired by Reliance New Energy Systems Limited (RNESL), a wholly owned subsidiary of Reliance Industries Limited (RIL) of India. Figure 1

Controllable Insertion Mechanism of Expanded Graphite Anodes Employing Conversion Reaction Pillars for Sodium-Ion Batteries.

Controlling the structural and reaction characteristics of carbonaceous anode materials is essential to realizing alternative alkali-ion batteries. In this study, we report on expanded graphite material employing MoSx conversion reaction pillars (EG-MoSx) inserted into the interlayers and assess them as potential anode candidates for Na-ion batteries. We succeed in a tailored control of the insertion characteristics between one-phase reaction and two-phase reaction by modifying the crystal structure of EG-MoSx under different thermal treatment conditions. EG-MoSx-900 anode with an enlarged interlayer of ∼5.38 Å delivers an exceptionally high capacity of 501 mAh g-1. We successfully solve the irreversible capacity issues of the expanded graphite materials by forming chemical preformation of the solid electrolyte interface (SEI) layer on the electrode surface, thereby significantly increasing coulombic efficiencies of thermally tuned EG-MoSx (52.20 → 97.25%). We elucidate the electrochemical mechanism and structural properties of the EG-MoSx anode materials by ex situ characterizations. Inserting active sulfide pillars enables us to overcome the performance limitations of existing Na-ion battery technologies, and we expect that this strategy will be applied to realize another family of alkali-ion batteries.

Group VI metallic pillars for assembly of expanded graphite anodes for high-capacity Na-ion batteries

Despite the economic viability and promising potential of Na-ion batteries, their commercialization remains unrealized because of the limited intercalation of Na+ ions into graphite anodes due to the large ionic radius of Na and instability of Na+ ions on the interstitial sites, which result in a poor cell performance. Herein, we report a synthetic strategy for increasing the graphite interlayer distance along c-axis to facilitate the intercalation of Na+ ions by embedding Group VI W metallic pillars between the graphene layers. The strong electrostatic attraction between the positively charged W6+ ions and the negatively charged graphene oxide (GO) layers enables the assembly of the expanded graphite layers by W pillars (W-rGO) via a subsequent chemical reduction. The interlayer spacing of the reconstructed W-rGO increased to 11.1 Å, which is three-fold larger than that of graphite (3.34 Å). Consequently, the W-rGO anodes delivered an exceptionally high capacity of 678 mAh g−1 for a Na-ion battery compared with that of a pristine rGO anode (240 mAh g−1). Further, we elucidate the structural characteristics and electrochemical reaction mechanisms of the W-rGO anodes. This work presents a simple and effective strategy for developing high-performance carbon-based anode materials for the realization of Na-ion battery technology.

Greener, Safer, and Sustainable Batteries: An Insight into Aqueous Electrolytes for Sodium-Ion Batteries

Rechargeable Na-ion battery technology has successfully emerged as a replacement to the current Li-ion battery technology, especially in large-scale storage applications. Aqueous electrolytes in re...

Exploring Na-ion technological advances: Pathways from energy to power

Abstract Currently Na-ion battery (NIB) technology is proving a popular area of investigation, in part due to the considerable interest in developing ‘beyond lithium’ battery technology to augment -or in certain situations replace- lithium ion batteries. Consequently, NIBs have begun to gain prominence –in part due to their flexibility and low cost. An examination of the ongoing work in this field reveals significant advances have been made, which provides impetus to explore and understand the topic in detail. Moreover, as NIBs are currently beginning to see commercialization, there is considerable interest in targeting future advances, including recent research into nascent Na-ion capacitors (NICs) which consist of a coupled supercapacitor- and battery-type electrode. The goal of this review will, therefore, be to provide an overview of the state-of-the-art in NIB technology, as well as the development of future Na-based technological advances (including NICs). The most promising advances will be discussed, including remaining challenges and future developments. This will help highlight not only the potential benefits in terms of environmental impact, performance and cost, but also the flexibility and versatility of these systems -as well as future pathways for Na-based technological development.

Structural Instability Driven by Li/Na Competition in Na(Li1/3Ir2/3)O2 Cathode Material for Li-Ion and Na-Ion Batteries.

With increasing hopes placed on the Na-ion battery technology to complement the current Li-ion battery systems, it is important to improve the energy density of Na-based cathode materials. Na-rich rocksalt oxides, Na1+xM1-xO2 (M = transition metal), combining cationic and anionic redox activity, could provide the necessary increase in capacity to achieve this goal, but their synthesis remains challenging compared to the Li analogues. As an alternative, mixed compounds Na(AxM1-x)O2, with A being an electropositive cation such as Li, Mg, or Zn sitting in the transition-metal layer, have been reported. As a continuation, we herein prepared the mixed Na(Li1/3Ir2/3)O2 phase and compared its structure and electrochemical properties with the well-known Li2IrO3 and Na2IrO3 parent materials. By mixing Na and Li in the material, the stacking sequence of the transition-metal honeycomb layers in Na(Li1/3Ir2/3)O2 is modified compared to the two parent materials, resulting in the presence of extra superstructure peaks in X-ray diffraction data. Using electrochemical characterization and an in-situ X-ray diffraction technique, the mixed Na(Li1/3Ir2/3)O2 was found to be unstable both in Li and Na batteries and to separate into Na-rich NaxIrO3 and Li-rich LixIrO3 phases due to the competition between electrochemical (de)insertion, cation exchange with the electrolyte, and segregation of Na and Li in the material. These findings highlight important challenges and offer useful insight into guide the design of new mixed Na(AxM1-x)O2 cathode materials with high capacity for Na-ion batteries.

Exploring the Role of Manganese on Structural, Transport, and Electrochemical Properties of NASICON-Na3Fe2-yMny(PO4)3-Cathode Materials for Na-Ion Batteries.

Given the extensive efforts focused on protecting the environment, eco-friendly cathode materials are a prerequisite for the development of Na-ion battery technology. Such materials should contain abundant and inexpensive elements. In the paper, we present NASICON-Na3Fe2-yMny(PO4)3 (y = 0, 0.1, 0.2, 0.3, and 0.4) cathode materials, which meet these requirements. Na3Fe2-yMny(PO4)3 compounds were prepared via a solid-state reaction at 600 °C, which allowed to obtain powders with submicron particles. The presence of manganese in the iron sub-lattice inhibits phase transitions, which occurs at ∼95 and ∼145 °C in Na3Fe2(PO4)3, changing the monoclinic structure to rhombohedral and affecting the structural and transport properties. The chemical stability of Na3Fe2-yMny(PO4)3 was thus higher than that of Na3Fe2(PO4)3, and it also exhibited enhanced structural, transport, and electrochemical properties. The observed correlation between the chemical composition and electrochemical properties proved the ability to precisely tune the crystal structure of NASICONs, allowing cathode materials with more desirable properties to be designed.

(Invited) The Scale-up and Commercialization of a High Energy Density Na-Ion Battery Technology

Na-ion batteries represent an attractive alternative to their Li-ion counterparts [1,2]. Faradion Limited, a UK-based startup, has developed this battery technology to a point where its performance characteristics, such as specific energy, rate and cycle life, are competitive with commercial Li-ion cells. In addition, the company has built an extensive patent portfolio [3]. Prototype Na-ion cells are manufactured on existing, commercial Li-ion production lines using readily available equipment and proven battery designs. Cylindrical, prismatic and pouch cells are all under development [4]. In this presentation we will demonstrate the performance characteristics of prototype Na-ion cells constructed using a proprietary layered oxide cathode NaaNi(1-x-y-z)MnxMgyTizO2 and a hard carbon anode material [5]. In this configuration the cathode material delivers a reversible specific capacity of over 150 mAh/g and the cell generates an average discharge cell voltage of around 3.2 V. The cell cycles with low polarization and with excellent charge and energy (round-trip) efficiency. To demonstrate the commercial viability of Na-ion batteries, Faradion has worked with its commercial partners to scale-up its Na-ion cell chemistry to the 40 Wh and 80 Wh pouch cell level. These prototype cells deliver a specific energy of around 140 Wh/kg under full depth-of-discharge conditions and have been incorporated successfully into E-Bike, E-Scooter (>750 Wh) and other demonstrator applications [6,7]. These developments will encourage rapid penetration into new market opportunities. Further performance, cost and safety characteristics of the Faradion Na-ion cells and batteries will be presented. Detailed 3-electrode cell performance data will also be discussed.

Sodium intercalation into α- and β-VOSO4

Na-ion battery is one of the best alternatives to Li-ion battery. Abundance of sodium on earth is three orders of magnitude higher than lithium, which should make Na-ion battery technology cheaper. But alkaline-ion battery prices, which tend to increase because of the massive world demand, also depend on the choice of electrode materials. Therefore, cost-effective electrode development remains an important subject of research because this will allow Na-ion battery to be even more competitive. Electrochemical performances of anhydrous VOSO4 as electrode for Na-ion battery are reported in this letter. Two anhydrous phases of vanadyl sulfate have been studied. The first one, α-VOSO4, shows that up to 0.8 sodium per formula unit (Na/f.u.) can be intercalated in this phase, and a reversible intercalation of 0.4 Na/f.u. has been observed with a strong polarization. The second one, β-VOSO4, can intercalate up to 0.9 Na/f.u. with a reversible intercalation of 0.4 Na/f.u. leading to a reversible capacity of 64 mAh/g.

Sodiation and Desodiation via Helical Phosphorus Intermediates in High-Capacity Anodes for Sodium-Ion Batteries

Na-ion batteries are promising alternatives to Li-ion systems for electrochemical energy storage because of the higher natural abundance and widespread distribution of Na compared to Li. High capacity anode materials, such as phosphorus, have been explored to realize Na-ion battery technologies that offer comparable performances to their Li-ion counterparts. While P anodes provide unparalleled capacities, the mechanism of sodiation and desodiation is not well-understood, limiting further optimization. Here, we use a combined experimental and theoretical approach to provide molecular-level insight into the (de)sodiation pathways in black P anodes for sodium-ion batteries. A determination of the P binding in these materials was achieved by comparing to structure models created via species swapping, ab initio random structure searching, and a genetic algorithm. During sodiation, analysis of 31P chemical shift anisotropies in NMR data reveals P helices and P at the end of chains as the primary structural components in amorphous Na xP phases. X-ray diffraction data in conjunction with variable field 23Na magic-angle spinning NMR support the formation of a new Na3P crystal structure (predicted using density-functional theory) on sodiation. During desodiation, P helices are re-formed in the amorphous intermediates, albeit with increased disorder, yet emphasizing the pervasive nature of this motif. The pristine material is not re-formed at the end of desodiation and may be linked to the irreversibility observed in the Na-P system.

Arrhenius Behavior of the Bulk Na-Ion Conductivity in Na3Sc2(PO4)3 Single Crystals Observed by Microcontact Impedance Spectroscopy.

NASICON-based solid electrolytes with exceptionally high Na-ion conductivities are considered to enable future all solid-state Na-ion battery technologies. Despite 40 years of research the interrelation between crystal structure and Na-ion conduction is still controversially discussed and far from being fully understood. In this study, microcontact impedance spectroscopy combined with single crystal X-ray diffraction, and differential scanning calorimetry is applied to tackle the question how bulk Na-ion conductivity σbulk of sub-mm-sized flux grown Na3Sc2(PO4)3 (NSP) single crystals is influenced by supposed phase changes (α, β, and γ phase) discussed in literature. Although we found a smooth structural change at around 140 °C, which we assign to the β → γ phase transition, our conductivity data follow a single Arrhenius law from room temperature (RT) up to 220 °C. Obviously, the structural change, being mainly related to decreasing Na-ion ordering with increasing temperature, does not cause any jumps in Na-ion conductivity or any discontinuities in activation energies Ea. Bulk ion dynamics in NSP have so far rarely been documented; here, under ambient conditions, σbulk turned out to be as high as 3 × 10–4 S cm–1 at RT (Ea, bulk = 0.39 eV) when directly measured with microcontacts for individual small single crystals.

Will Sodium Layered Oxides Ever Be Competitive for Sodium Ion Battery Applications?

The Na-ion battery technology is rapidly developing as a possible alternative to Li-ion for massive electrochemical energy storage applications because of sustainability and cost reasons. Two types of technologies based either on sodium layered oxides NaxMO2 (x ≤ 1, M = transition metal ion(s)) or on polyanionic compounds such as Na3V2(PO4)2F3 as positive electrode and carbon as negative electrode are presently being pursued. Herein, we benchmark the performance of full Na-ion cells based on several sodium layered oxide materials against Na3V2(PO4)2F3/hard carbon cells. Although several studies report more attractive capacities for sodium layered oxides vs. Na metal (∼200 mAh g−1) than for polyanionic phases (∼120 mAh g−1), we find that such advantages are not maintained when assembling practical full Na-ion cells; the opposite of what is found for Li-ion technology. The reasons for such a loss of supremacy of the layered oxides against polyanionic compounds are discussed in terms of materials structural stability and composition so as to identify fundamental challenges that impede their practical applications. Finally, a few perspectives are given to design better sodium layered oxide electrode materials that could outweigh the performance of today's stellar Na3V2(PO4)2F3.

The Na3V2(PO4)2F3/Carbon Na-Ion Battery: Its Performance Understanding as Deduced from Differential Voltage Analysis

The Na3V2(PO4)2F3/Carbon Na-ion battery technology is appealing because of its high energy and power density, hence the need to assess its viability under practical operation. Here we provide a comprehensive understanding of its storage and cycling performances in various electrolytes and at different temperatures via a through differential voltage (dV/dQ) analysis of the data obtained from coin and pouch cells. The cells show excellent long-term cycling performance (capacity decay less than 0.020 mAh g−1 per cycle at 1C rate whatever the electrolytes), but suffer at 55°C from copious self-discharge stemming from either reduction or oxidization of electrolytes. We could deduce, from monitoring side reactions rate as a function of temperature, the activation energies of 34 and 29 kJ mol−1 for the parasitic reactions proceeding at the anode and cathode side, respectively. We hope our analysis protocol to be widely implemented for the fundamental investigation of side reactions between electrode and electrolyte interface in Na-ion battery; a must for optimizing battery performances via the screening of various electrolyte formulations.

Strategy for Metal Anode Protection in Advanced Energy Storage Systems

To meet the demand for higher capacity longer life batteries in a “next-generation batteries” technology, anodes with substantially higher energy density than current graphite are needed. One class - metal anodes (particularly Li, Na) - offers far higher energy density, but to date their utilization has been impeded by their high surface reactivity (especially to the organic electrolyte) and tendency to form dendrites (a shorting and safety hazard). We have used atomic layer deposition (ALD) to create highly controlled, thin ceramic type coatings as an artificial solid electrolyte interphases (SEIs) to protect these anode surfaces by preventing dendrite formation and electrolyte decomposition. The ultrathin Al2O3 protective coating on Na stabilizes the Na-electrolyte interface and extends its cycle life under high current density (i.e., high power) cycling conditions. We have also developed a hybrid (elastomer + ceramic-type) protective coating on Li metal by combining ALD ceramic (LiPON) and electrochemical deposition of a polymeric elastomer. The hybrid layer achieves high Li conductivity while suppressing corrosion and due to electrolyte breakdown. The elastomer component provides mechanical flexibility to accommodate volume change both may play roles in the observed mitigation of dendrite growth at high current density. These results demonstrate promise for achieving high energy density anodes with significantly enhanced chemical stability, electrochemical cyclability, and dendrite protection as needed for a viable beyond Li or Na ion battery technology.

Interface Storage and Diffusion in Titania-Based Na-Ion Battery Negative Electrodes

Ordered, nanostructured transition metal oxides are currently in the spotlight of battery research. To exploit, however, the full potential of self-assembled titania nanotube layers in Na-ion battery technology we need to close the gap in knowledge about the underlying storage mechanisms as well as ion dynamics. For this purpose, amorphous TiO2 nanotubes (70-130 nm in diameter, 4.5 – 17 µm in length) were fabricated by anodization, i.e., electrochemical oxidation. The electrochemical behaviour with respect to sodium insertion was studied by cyclic voltammetry and galvanostatic cycling with potential limitation. It turned out that the nanotubes are very resilient to cycling, some being able to withstand more than 300 charge-discharge cycles without significant loss of capacity. Compared to classical insertion reactions, the mechanism of reversible electrochemical storage of Na+ in the nanotubes seems to differ significantly. There is strong evidence that a large amount of the Na+ ions is stored on the surface or in the surface-influenced regions while only a smaller fraction is inserted in the walls of the tubes. The interfacial Na+ storage is governed by a faradaic adsorption phenomena, which is essentially different from the (pseudo-)capacitive model hitherto used in the literature. The finding that, in the case of sodium, interfacial storage is more important than ion insertion into the crystal lattice is supported by the very low Na+ diffusion coefficients determined at various electrode potentials (in the order of 4 · 10-20 - 10-21 cm2/s). This dominant interfacial storage mechanism may explain the high specific capacity and the very good cycling performance recently reported for some nanostructured titania phases as well as for other nanosized titanate materials. Figure A. Chemical diffusion coefficient of Na+ into amorphous TiO2 at room temperature (23 ±1°C) after subtracting the interfacial storage contributions for different sample types. Figure B. Pseudo-capacitance values corresponding to the interfacial storage of Na+ ions on various nanotube layers plotted as a function of the electrode potential. Figure 1