Electrochemistry of sodium titanate nanotubes as a negative electrode for sodium-ion batteries

Journal Pre-proof Electrochemistry of sodium titanate nanotubes as a negative electrode for sodium-ion batteries Marina M. Leite, Vitor L. Martins, Flavio M. Vichi, Roberto M. Torresi PII:

S0013-4686(19)32294-7

DOI:

https://doi.org/10.1016/j.electacta.2019.135422

Reference:

EA 135422

To appear in:

Electrochimica Acta

Received Date: 17 October 2019 Revised Date:

28 November 2019

Accepted Date: 29 November 2019

Please cite this article as: M.M. Leite, V.L. Martins, F.M. Vichi, R.M. Torresi, Electrochemistry of sodium titanate nanotubes as a negative electrode for sodium-ion batteries, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135422. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Electrochemistry of sodium titanate nanotubes as a negative electrode for sodium-ion batteries Marina M. Leite, Vitor L. Martins, Flavio M. Vichi, Roberto M. Torresi* Depto. Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil.

Abstract Sodium-ion batteries are a promising alternative to lithium-ion devices, but the development of proper negative electrode materials is still challenging. Here, the properties of a low-voltage sodium titanate material are evaluated. Sodium titanate nanotubes (NTO) were produced by an alkalyne alkaline hydrothermal treatment with TiO2 and consisted of a hydrated Na1.4H0.6Ti3O7 with a surface area of 128 m2 g1

. NTO electrode kinetics were studied by cyclic voltammetry, electrochemical

impedance spectroscopy, and galvanostatic intermittent titration techniques. The (de)intercalation of Na+ ions involved two redox pairs at 0.3/0.5 V and 1.0/1.2 V, associated with the present mixture of nanotubes and nanosheets. Surface processes had a 95% coulombic efficiency and a high contribution even at low scan rates, accounting for 47% of the total capacity at 0.5 mV s-1. Upon Na+ removal, the electronic resistance and the semiconductor capacitance increased. Battery tests performed on Na|NTO half-cells showed a reversible capacity of 90 mA h g-1 at 10 mA g-1 and near 100% coulombic efficiency at current rates ranging from 10 mA g-1 to 10 A g-1. Additionally, NTO presented a good capacity retention of 92% after 170 cycles at 100 mA g-1. Keywords Na-ion batteries, sodium trititanate nanotubes, electrode kinetics.

*Corresponding author e-mail: [email protected] 1. Introduction

Energy storage technology has become indispensable for our society. On the one hand, the need to use renewable energy sources requires efficient stationary, grid-scale energy storage to compensate for the discontinuous power generation intrinsic to those sources. On the other hand, the emergence of electric vehicles and the concept of smart energy push for the development of even more efficient and high-power devices. So far, Li-ion batteries (LIBs) are the most developed portable energy storage technologies, possibly reaching their intrinsic limits of 300 W h kg-1 within the next few years.[1] However, the mass production of LIBs could cause a shortage of raw materials, which include not only lithium but also cobalt reserves, making it important to develop new technologies based on other elements. Na-ion batteries (SIBs) have come back into the spotlight in the last decade as a promising alternative to diversify the type of resources used for energy storage.[2,3] When comparing Na+ to Li+, one of the main advantages of using Nabased systems is that Na is more abundant and more uniformly distributed around the world. Some of the drawbacks include the following: a) the higher ionic radius of Na+ can cause sluggish kinetics and enhanced structural transformation of the electrode material upon sodiation and desodiation, b) the higher atomic mass implies a smaller specific capacity, and c) the higher reduction potential, -2.7 V vs SHE, can directly affect the energy density of the device.[1] Another characteristic of SIBs is that graphite, a state-of-the-art negative electrode for LIBs, shows a very low capacity toward Na+ intercalation. This has led to the search for other negative materials such as hard carbons,[4,5] which show a potential plateau below 0.1 V that promotes sodium insertion at an oxidation state

near zero.[6] At high rates, these low insertion potentials could promote Na plating, causing safety concerns, cycle life problems due to electrolyte decomposition with Na+ consumption, and capacity fading.[5] Sodium trititanate, Na2Ti3O7 (NTO), has drawn attention for its low intercalation voltage of 0.3 V vs Na/Na+ without the metallic Na-plating concern.[7] The compound consists of a layered zig-zag structure of [TiO6] octahedra with Na+ occupying sites in the interlayer region.[8] Similar to other sodium titanates, NTO is interesting due to its low toxicity, high availability of precursor materials (TiO2 and Na compounds), and easy fabrication. NTO is considered to have a relatively high theoretical capacity of 177 mA h g-1 by taking two Na+ per formula unit (Eq. 1) and reducing 2/3 of Ti4+ to Ti3+. However, it shows low electronic conductivity and high capacity fading.[9] Strategies to overcome these issues include using a carbon coating[10–12] and nanostructuring[13–16].


   + 2
  + 2 ⇌
   (. 1)

Nanostructured NTO, in particular, can be easily obtained by hydrothermally treating TiO2 in a strong alkaline environment[17], forming nanosheets, nanotubes or nanowires.[18] Despite the simplicity of the alkaline hydrothermal method, the obtained nanosheets and nanotubes can be quite complicated to characterize. The exact crystalline structure of these materials is still under debate, in part due to the small size and curvature of the particles,[19] and the easy ion-exchange from Na+ H+ that can take place during the washing step of the process.[20] Although some of these nanostructured materials have been tested as negative electrodes for Na-ion batteries[21–23] and pseudocapacitive electrodes in supercapacitors,[24–26] a

thorough investigation of the electrochemical behavior of nanostructured NTOs is still needed. In this study, we report the results from cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT) experiments to provide insights about the kinetics of NTO electrodes and their performance in Na half-cell batteries. 2. Experimental Methods

2.1.

Material Synthesis

The synthesis of NTO was based on a hydrothermal method described by Kasuga et al.[17] TiO2 P25 (Evonik) was dispersed in 30 wt% NaOH (Synth) solution at a ratio of 0.18 g of TiO2 per 1 mL of the alkaline solution. The suspensions were stirred for 2 h at room temperature before being transferred into a PTFE-lined stainless-steel autoclave, filling 80% of its volume. The reaction was carried out for 24 h at 140 °C, and the obtained white precipitates were filtered, washed with deionized water, and dried at 80 °C. 2.2.

Physico-chemical characterization

NTO samples were characterized by powder X-ray diffractometry (XRD) using a Shimadzu XRD7000 diffractometer under Cu-Kα radiation generated at 40 kV and 30 mA. The scans were performed at 2° per minute wi th a step of 0.02° (2 θ). Raman spectra were collected with a Renishaw InVia Raman microspectrometer using 633 nm laser radiation. Surface area analysis was performed at 77 K using N2 as the adsorbate in a Quantachrome 1000e surface area analyzer. The samples were degassed at 120 °C under vacuum overnight prior to analysis. The Brunauer-EmmetTeller (BET) method was used to calculate the surface area of the material. The morphology of the NTO particles was studied by transmission electron microscopy (TEM). A suspension of the powder in isopropanol was dripped on a Formvar/C

copper grid, and the images were acquired using a Jeol JEM 2100 microscope. Thermogravimetric analysis (TG) was used to determine the amount of water in the material. The sample was dried at 120 °C under vacu um and kept in a dry box before being analyzed in a TA Instruments TGAQ500 using a dynamic highresolution mode under N2 gas flow. Elemental analysis of the same dried sample batch was carried out by two techniques: Ti and Na were quantified by inductively coupled plasma optical emission spectrometry (ICP-OES) in a Spectro Arcos spectrometer, and the H content was determined by the Pregl-Dumas method using a Perkin Elmer 2400 series II analyzer. For the ICP analysis, the sample was digested in concentrated HF/HNO3 solution. 2.3.

Electrochemical methods

For the fabrication of the electrodes, NTO powders were mixed with a polyvinylidene fluoride (PVDF, MTI) binder and Timical Super C65 (MTI) conductive carbon black in a weight proportion of 8:1:1 using n-methyl-2-pyrrolidone (NMP, Aldrich) as a solvent. Cu foils were coated with the suspension using a doctor blade (wet thickness of 150 µm) and dried at 120 °C under vacuum. The electrodes were cut into 10- and 12-mm-diameter disks with a typical active mass of 1.6 mg cm-2. Three-electrode Swagelok-type cells were assembled with NTO as the working electrode and metallic sodium as the counter and quasi-reference electrodes. Coincells were assembled with NTO and Na as positive and negative electrodes, respectively. In both configurations, 1 mol L-1 NaClO4 solution in a 1:1 weight ratio of diethylcarbonate/propylcarbonate (IoLiTec) was used as the electrolyte, and glass fiber (Whatman, GF/F) disks were used as separators. The cells were assembled in a dry box filled with Argon, and each NTO electrode was individually weighed after drying (120 °C, vacuum). Cyclic voltammetry (CV) an d electrochemical impedance

spectroscopy (EIS) were performed in three-electrode cells using an Autolab M101 potentiostat/galvanostat (Metrohm). To take EIS at different states of charge, after a full discharge, a current of 50 mA g-1 was applied for 625 s (equivalent to 31.25 C g1

). The cell was set to rest for 30 minutes. Then, the open circuit potential was held

for 5 minutes. An EIS analysis was carried out, ranging from 68 kHz to 10 mHz. Impedance spectra data analysis was performed using RelaxIS 3 (rhd instruments GmbH & Co. KG, Germany) impedance data analysis software. The capacitance values associated with constant-phase elements (CPE) were calculated using Eq. 2 [27], where R is the resistance of the CPE element, Rsum is the sum of the previous resistances, Q is the CPE admittance, and alpha is the angle parameter. 

  =   ! (. 2)

 ) ( +  Battery tests and galvanostatic intermittent titration technique (GITT) experiments were conducted in a BioLogic BCS 805 battery tester. For GITT measurements, a 10 mA g-1 current was applied for 10 minutes followed by a rest period of 1 h. 3. Results and Discussion

3.1.

Physico-chemical characterization

The NTO material produced by the hydrothermal treatment of TiO2 consists of thick agglomerates of open-ended nanotubes and partially rolled nanosheets (Fig. 1a). Close-ups of loose nanotube particles show that they are multiwalled but with varying thickness throughout the tube length, as shown in Fig. 1b. The distance between the walls is approximately 0.7 nm. The inner diameter, on the other hand, is constant along the tubes and was found to be approximately 6 nm. A contrast pattern can be observed on the nanotube walls (Fig. 1c), showing the crystalline nature of the material, and the fringes have an average spacing of 0.4 nm.

a

c

b

50 nm

10 nm

5 nm

Fig. 1. TEM images of NTO powders at different magnifications. Fig. 2a shows the XRD pattern of the powder sample, with 2θ reflections at 9.9°, 24.4°, 28.4° and 48.7°. This set of peaks is

a common result for titanate

nanosheets and nanotubes produced by a hydrothermal method and has been assigned to different layered sodium/hydrogen titanates, with the peak around 10° being related to the interlayer (wall spacing) of the nanotubes.[28] The anisotropic shape and nanometric size of the particles could be responsible for changing the diffraction pattern of bulk titanates, thus making it difficult to specifically assign one titanate phase.[29] The same situation is found in the Raman spectrum (Fig. 2b). The intense band at 277 cm-1 is characteristic of titanates obtained by lowtemperature hydrothermal processes, but its origin is still unclear.[30] Other works on sodium titanate nanotubes assigned the bands at 153, 192 and 277 cm-1 to lattice modes and Ti-O-Ti modes[29], although others suggested that the band at 277 cm-1 is related to Ti-O vibrations affected by near Na+.[31][32] The low intensity bands at 121 and 815 cm-1 have been related to Na-O vibrations.[32] The band at 447 cm-1 has been assigned to vibrations of TiO6 octahedra [29]. The bands at 667 and 910 cm-1 are more prominent in Na-rich nanotube titanates, being assigned to Ti-O-Na vibrations [33][34]. It is important to mention that no XRD peaks or Raman bands for anatase or rutile TiO2 phases were observed. Based on elemental analysis of Na, Ti

and H and the water content of the sample from the TG analysis (Fig. S1), an empirical formula of Na1.6H0.4Ti3O7⋅H2O was obtained. Therefore, a trititanate model was assumed, and XRD peaks were indexed according to the Na2Ti3O7 pattern (JCPDS 31-1329). The hydration water seems to be reversibly inserted into the interlayer region of the nanotubes, since the peak around 10° shifts to lower values (increased interlayer distance) when the sample is exposed to humid atmosphere, and shifts back to higher values (decreased interlayer distance) when the sample is dried at 120 °C under vacuum (Fig. S2). Moreover, t he hydration water was completely removed at 160 °C during TG analysis, wi th a peak at 120 °C on the DTG profile (Fig. S1). Therefore, it is assumed that only a minimal amount of water would still be present after transference of the electrodes to the sealed glove box, thus it should not affect the electrochemical behavior of NTO.

Fig. 2. a) XRD profile of NTO powder, showing (hkl) indices of Na2Ti3O7 phase (blue bars correspond to JCPDS 31-1329). b) Raman spectrum of NTO powder. One of the major advantages of using nanostructure compounds as electrode materials is their high surface area compared to their bulk counterparts. In the present case, NTO powders have a surface area of 128 m2 g-1, as calculated using the BET method applied to N2 adsorption isotherms (Fig. 3a). The isotherms present

hysteresis between approximately 0.4 and 1.0 P/P0, which is typical of mesoporous materials. The Barrett-Joyner-Halenda (BJH) method was applied to the desorption branch, leading to the narrow pore size distribution shown in Fig. 3b, which ranged from 4 to 12 nm. The peak at 2.5 nm is an artifact due to the tensile strength effect [35] caused by the interconnection of the nanotube pores in the aggregates. b

180

1.0 Artifact

160 -1

Pore volume (cm g )

140

0.8

3

3 -1

Volume Adsorbed (cm g )

a

120 100 80 60 2

40

SBET = 128 m g

-1

0.6 0.4 0.2 0.0

20 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

2

4

6

8 10

20

40

60 80 100

Pore width (nm)

Fig. 3. a) N2 adsorption (circles) and desorption (squares) of NTO nanotubes (powdered sample). b) BJH pore size distribution calculated from desorption data. 3.2.

Electrochemical behavior

Fig. 4a shows the first two CV cycles for NTO electrodes in a 3-electrode cell arrangement. The 1st cycle presents a reduction peak at 0.5 – 0.6 V vs Na/Na+, which is not seen in further cycles. This is due to the reduction of the electrolyte forming a solid-electrolyte interphase (SEI) layer. The H+ ions in NTO material are probably irreversibly removed in the first reduction process, as observed by Zarrabeitia et al. in bulk NaxH2-xTi3O7[36]. This process cannot be distinguished from the formation of the SEI layer in the case of NTO nanotubes due to the nanometric shape of the particles and its consequent broadening of voltammetry peaks. The other reduction peak starting at 0.3 V is the reversible intercalation of Na+ in the titanate structure with the reduction of Ti4+ to Ti3+. If the potential is reduced to below

0.1 V, the reduction of Na+ to Na0 on the positive electrode surface can be observed (Fig. S3). From the second cycle on, even though the peak at 0.5 V is not present, there is a continuous reduction process from 1.4 V to 0.5 V centered at 1.0 V. The oxidation pathway has two peaks at 0.5 V and 1.2 V. These two pairs of redox potentials (0.3-0.5 V and 1.0-1.2 V) indicate different Na+-ion intercalation mechanisms. Although bulk Na2Ti3O7 has only a lower voltage process below 0.5 V [7], Ko et al. [37] observed a second pair of redox potentials at 1.1-1.2 V when nanoplatelets from exfoliated Na2Ti3O7 were present. Wang et al.[16] also had two pairs of redox peaks in CVs from NTO nanotube samples similar to those in the present work. Because thin, partially rolled nanosheets are usually present in nanotube NTO samples produced with a hydrothermal method, the material shows both redox peaks, with the nanotubes having the same intercalation potential as bulk NTO. To estimate the contribution of surface processes to the overall current, the total current can be described as the sum of a capacitive-like current related to surface processes and a diffusion-controlled current related to bulk processes. For the former, as capacitance varies linearly with the potential sweep rate (ν), it can be expressed by the term k1ν in Eq. 3. [38] The latter depends on the root square of the sweep rate, corresponding to the second term k2ν1/2 in Eq. 3. (#) = $ % + $ % / (. 3) Eq. 3 can be rearranged into Eq. 4: [33] (#) = $ % / + $ (. 4) % / From linear fits of the current i/v1/2 versus the sweep rate ν, k1 and k2 values can be found for each potential and then multiplied by a potential sweep rate, [39]

giving separate CVs for surface and bulk processes. An example of the calculation is shown in Fig. S5. Fig. 4b shows the result of these calculations for 0.5 mV s-1, where surface processes account for 47% of the total current. It also shows that the sum of the calculated values is very similar to the experimental CV performed at 0.5 mV s-1. Because this model assumes that the charge of surface processes is independent of the scan rate, the surface capacity is constant and calculated to be 16.2 mA h g-1. As the scan rate is increased, the total capacity decreases because the bulk capacity – a diffusion-controlled process – decreases. Therefore, the capacity contribution of surface processes increases and reaches 55% at 1 mV s-1, as shown in Fig. 3c. This is considered a high value for such a low scan rate, and it is due to the small size of the particles and the high surface area of the NTO material. Other works have used the same method to estimate the surface contribution in different types of nanoparticles and have obtained similar results. For comparison, NTO nanosheet arrays showed 41% surface contribution at 1 mV s-1 [40] and 55% surface contribution at 0.8 mV s-1 in NTO@CNT nanocables.[14] NTO nanotube and reduced graphene oxide composites presented a 77% surface contribution at 1 mV s-1.[41] The coulombic efficiency of surface and bulk processes was calculated by the ratio of positive to negative charge from the calculated CVs (Fig. 3c). It is noticeable that the surface process has over 95% efficiency, while the efficiency of the global process increases with the scan rate. This must be so, since the bulk process shows some irreversibility with an efficiency of approximately 75%. As the scan rate is increased, there is less bulk capacity and higher surface contribution, so the global efficiency approaches the 95% value of the surface process.

Fig. 4. a) Cyclic voltammetry of the 1st (black line) and 2nd (red line) cycles recorded at 50 µV s-1 in a 3-electrode cell arrangement. b) Experimental (solid black line) and calculated (green squares) CVs at 0.5 mV s-1, separately showing the estimated surface (red up triangles) and bulk (blue circles) process contributions. c) Capacity contribution for surface (red) and bulk (blue) processes and their coulombic efficiency (global efficiency in green squares). The diffusion of Na+ ions through the electrode during the charge (desodiation) process was studied using GITT.[42] The diffusion coefficient D can be calculated using the simplified Eq. 5: )=

4 - #  ∆  , / , / (. 5) . ∆1 * +

provided that a) the steady-state is achieved after the relaxation time and b) only the potential region of the pulse with constant dE/dt0.5 is used.[42] However, some considerations must be made when 3D mixed-substance electrodes are used. In the equation, τ is the pulse time in seconds, ∆Es is the difference between the steadystate potential before and after the pulse, and ∆Et is the potential difference due to the pulse current, as shown in the inset of Fig. 5a. The values of nm, Vm and S correspond to the number of moles of the electroactive substance (mass of active

material divided by the molar mass of NTO), its molar volume and the contact area between the electrode and electrolyte, respectively. In this study, the Vm used was that of bulk Na2Ti3O7 calculated from its cell parameters: Na2Ti3O7 has a monoclinic crystal system with 2 formula units per unit cell, with a total cell volume of 299.381 Å3, [43] which gives Vm = 90.14 cm3 mol-1. S was considered to be the geometric area of the electrode. Fig. 5b shows the calculated diffusion coefficient with varying amounts of intercalated Na+ (x). In the region where Eq. 5 can be applied, from 0.17 to 1.12 V, there is no significant change in the diffusion coefficient value. Above 1.12 V, there is no more Na+ deintercalation, and the potential changes abruptly with the application of charge. Moreover, only 1.04 moles of Na+ are removed from the structure. This is related to the low capacity of the material in comparison to that of the theoretical capacity, which takes into account the intercalation of two Na+ ions per formula unit.

Fig. 5. a) GITT experiment for the charge (desodiation) process. The inset shows an enlarged image of the pulse with the corresponding ∆Et and ∆Es as an example. b) Diffusion coefficient (log scale) calculated from GITT data with varying x. To further understand the desodiation process, EIS was performed at different states of charge. Nyquist plots are shown in Fig. 6a. There are three time constants,

with two semicircles in the high- and medium-frequency ranges and a sloping line at lower frequencies. Upon Na+ deintercalation, the semicircle at the medium frequency expands, but the first semicircle remains practically constant throughout the whole desodiation process. The equivalent circuit shown in the inset of Fig. 6a was chosen to determine the resistance values and capacitance values (Fig. 6b).

Fig. 6. a) Nyquist plots and the electrochemical fit results (solid lines) for different states of charge, indicated by x. The inset shows the equivalent circuit model used to fit EIS data. b) Change in resistance and c) capacitance values throughout the desodiation process. The first element accounts for the resistance of the electrolyte, Rel, and it is approximately 7 Ω throughout the desodiation process. For the first semicircle, there is the resistance of the SEI layer, RSEI, in parallel with the CPESEI, which is associated with the SEI layer capacitance, CSEI. Both are constant at 50 Ω and 4 µF, respectively. Therefore, once the SEI layer is formed during the first reduction (sodiation) process, it remains stable throughout the cycles and during the oxidation (desodiation) of NTO. At lower frequencies, the polarization resistance RP includes the charge transfer resistance and the electronic resistivity of the material. Upon desodiation,

there is an increase in RP until it reaches very high values (11 kΩ) for the completely desodiated material. At this point, the resistance is so high that the diffusion of Na+ in the solid cannot be observed at the same frequency domain (Fig. S6). Zarrabeitia et al.[44] also observed an increase in the polarization resistance in desodiated bulk Na2Ti3O7 and concluded that there was a transition from conducting to insulating after the removal of intercalated Na+ ions. In parallel with the polarization resistance, there is a CPE element related to a capacitance CT, a serial combination of the double-layer capacitance (CDL) and the capacitance (CSC) related to the semiconducting property of NTO (1/CT = 1/CSC + 1/CDL).[45–47] Because CDL does not depend on the composition of the material, it should not change with potential. Meanwhile, CSC is larger when the resistivity of the NTO is higher, that is, at higher potentials when the Na+ content is smaller and Ti is fully oxidized to Ti4+. As CSC increases, its contribution to the total CT is smaller. Fig. 6c shows the variation of calculated CT with varying x, and when the sodium content decreases below 2 mol per formula unit, there is a decrease in CT. That should be the point when CDL becomes the major contributor to the total CT, with a value close to 100 µF. The diffusion element was adjusted using a finite-space diffusion model with a Warburg-open WO element, as expressed by Eq. 6, where Z is the admittance, ω is the frequency, i is the time constant, and α is the angle parameter.[48] The phase angle in the low frequency domain is higher than 45°, so a semi-infinite Warburg element is not appropriate. However, it is below 90° ( α < 0.5) due to a distribution of particle sizes and diffusion lengths. 4 =

45 789ℎ (6+) (. 6) (6+)

For the different stages of charge, α values remained almost constant, with a value between 0.28 and 0.36. The time constant τ ranged from 0.1 to 0.6 s. The small variations in these parameters are related to the fact that the diffusion coefficient is essentially constant during desodiation, as shown by GITT (Fig. 5b). The formation of the SEI layer during the first discharge (sodiation) can also be observed during a galvanostatic discharge (Fig. 7a) of a two-electrode cell, where the process has a specific capacity of approximately twice that of the theoretical one. Due to the nanometric size of the titanate particles, there are no potential plateaus for the (de)intercalation of Na+ in the charge/discharge profiles. However, there are changes in the slope of the discharge (sodiation) curve at approximately 1.2 V and at 0.4 V. For the charge (sodiation) process, the slope changes at 0.6 V and again at 1.3 V. These potentials are the same as those observed in the CV curves, which reinforces the mechanism of (de)intercalation involving two different sites. When increasing the desodiation rate from 10 mA g-1 to 1 A g-1, there is a decrease in the specific capacity from 93 mA h g-1 to 59 mA h g-1 (Fig. 7b), but the initial capacity is recovered after a higher rate test. Even at a very high rate of 10 A g-1, NTO shows a capacity of 14 mA h g-1 and recovers a capacity of 80 mA h g-1 when returned to a low current density (Fig. 7c). The coulombic efficiency remained close to 100% at the different charge currents. To evaluate the NTO electrode cyclability, the cell was cycled at two different rates, showing a good capacity retention of 87% after 190 cycles at 50 mA g-1 and 92% after 170 cycles at 100 mA g-1 (Fig. 7d).

Fig. 7. a) Galvanostatic charge/discharge profiles at different rates, including the first cycle where there is the formation of the SEI layer. Black circles, red triangles and yellow open triangles are for the 1st, 5th and 35th cycles, respectively, all at 10 mA g-1; green squares are for the 15th cycle, at 100 mA g-1; purple asterisks are for the 25th cycle, at 1 A g-1. b) Rate capability showing current densities from 10 mA g-1 to 1 A g1

from a fresh cell. c) High-rate capability from 10 mA g-1 to 10 A g-1 from a

previously cycled cell. d) Cyclability of the NTO electrode at 50 and 100 mA g-1. Blue squares are for sodiation, red circles for desodiation and green diamonds for efficiency.

4. Conclusions

Sodium titanate nanotubes with an empirical formula of Na1.4H0.6Ti3O7 have been studied as electrodes in Na-ion half-cells. In addition to a formation of a SEI layer at 0.5 – 0.6 V vs Na/Na+ in the first cycle, Na+ (de)intercalation occurs at two different potentials (0.3-0.5 V and 1.0-1.2 V vs Na/Na+), instead of just one peak at 0.3 V, as reported in the literature for bulk NTO. The existence of two redox potential pairs is probably due to the presence of nanosheet-like particles mixed with the nanotubes. The small nanoparticles and high surface area of the material account for a high contribution of surface processes even at low scan rates (47% at 0.5 mV s-1). Upon desodiation, there is no significant variation in the diffusion coefficient, but the electrical resistance of the material increases because of the oxidation of Ti3+ to Ti4+. NTO nanotubes present a low specific capacity in comparison to that of the theoretical value, even at low rates (93 mA h g-1 at 10 mA g-1). However, the electrodes showed good capacity retention (92% after 170 cycles at 100 mA g-1) and good response at high rates (14 mA h g-1 at 10 A g-1) with a coulombic efficiency close to 100%, even at low rates. The present study provides insights about Na+ intercalation kinetics into a nanosized material, which can contribute to the improvement of this material type when applied as electrodes in Na-ion devices. Acknowledgements This work was supported by the CNPq, CAPES and FAPESP (2015/26308-7). VLM and MML wish to thank the FAPESP fellowship 2015/26308-7, 2018/13492-2, and 2019/02669-1. Additionally, we would like to thank Prof. Romulo Ando (Lab. Molecular Spectroscopy – IQUSP) for the Raman analysis.

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Electrochemical behavior of sodium titanate nanotubes relate to sodium-ion batteries Marina M. Leite, Vitor L. Martins, Flavio M. Vichi, Roberto M. Torresi* Depto. Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, São Paulo, SP, Brazil.

Highlights: •

Surface process contribution is 47% at 0.5 V s-1 with 95% efficiency

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Finite-space diffusion model to fit EIS data considering particle-size distribution

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(Semi)conductive properties of NTO change with (de)sodiation

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Electrode capacity of 93 mA h g-1 at 10 mA g-1

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92% capacity retention after 170 cycles at 100 mA g-1

Sample CRediT author statement

Marina M. Leite: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Review & Editing. Vitor L. Martins: Methodology, Visualization, Writing- Original draft preparation. Flavio M. Vichi: Conceptualization, Resources. Roberto M. Torresi: Conceptualization, Supervision, Review.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: