Effects of F-Doping on the Electrochemical Performance of Na2Ti3O7 as an Anode for Sodium-Ion Batteries

The effects of fluorine (F) doping on the phase, crystal structure, and electrochemical performance of Na2Ti3O7 are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical measurements. F-doping does not change the crystal structure of NTO, although it has an effect on the morphology of the resultant product. As an anode material for sodium-ion batteries, the specific capacity of Na2Ti3O7 exhibits a 30% increase with F-doping owing to the improved sodium ion diffusion coefficient. F-doped Na2Ti3O7 also displays an enhanced rate capability and favourable cycling performance for more than 800 cycles.


Introduction
At present, lithium-ion batteries (LIBs) are indispensable for power electronics [1,2]. In the coming decades, LIBs will be used on an increasing scale in the field of utility grid and a future "energy internet" [2]. However, the limited reserves and cost issues of lithium present great challenges for grid application of LIBs. Sodium-ion batteries (SIBs) have recently attracted interest because of their advantages such as low cost, environmentally friendliness, and the availability of rich sodium resources [3,4]. Various SIB electrode materials have been developed recently, including sodium metal phosphate [5], Na 0.44 MnO 2 [6], Na 0.67 Ni 0.23 Mg 0.1 Mn 0.67 O 2 [7,8], Na 3 [Ti 2 P 2 O 10 F] [9,10], NaMnFe 2 (PO 4 ) 3 [11], V 2 O 5 [12], NiCo 2 O 4 [13], Na 3 V 2 (PO 4 ) 3 [14], Sb 2 O 4 [15], Na 2 V 6 O 16 [16], NaFeF 3 [17], and Na 2 Ti 3 O 7 [18,19], as well as some hard carbon materials [20,21]. Among these materials, layered Na 2 Ti 3 O 7 is one of the most ideal choices as negative active materials for SIBs, with favorable electrochemical performance at a relatively low intercalation voltage of approximately 0.3 V as compared to Na + /Na and a low activation energy (~0.186 eV) for Na ion insertion [22]. Although the capacity of NTO reaches 200 mAh g −1 [23,24], Na 2 Ti 3 O 7 does not show a better rate capability or longer cycle life due to the large ionic radius of sodium ions (1.02 Å), which makes their Materials 2018, 11, 2206 2 of 8 diffusion much more difficult [25]. Moreover, the rapid capacity decay during cycling has been ascribed to the decomposition of Na 2 Ti 3 O 7 in electrolyte [26]. Therefore, improvement of the rate performance of Na 2 Ti 3 O 7 remains a great challenge [27]. Introducing additional vacancies to accommodate Na + is favorable for reversible and fast ion intercalation and deintercalation. Since fluorine (F) is the most electronegative element, F-doping has demonstrated a certain influence on the crystal structure and stability of Na x V 2 O 2 (PO 4 ) 2 F, leading to improved electrochemical performance [28].
Here, we studied the effects of F-doping on the electrochemical performance of Na 2 Ti 3 O 7 . The results demonstrated a 30% increase in the specific capacity of Na 2 Ti 3 O 7 through F-doping thanks to the improved Na + diffusion coefficient. Better rate capability and cycle performance were also observed.

The Preparation of Na 2 Ti 3 O 7 and F-Doping Na 2 Ti 3 O 7 Samples
All reagents in this study were purchased and used directly. Titanium (IV) oxide, anatase (99.6%) was purchased from Alfa Aesar (Shanghai, China) and anhydrous Na 2 CO 3 (99.8%) was bought from Sinopharm Chemical Regent Co., Ltd. (Shanghai, China). A solid-phase method was used to prepare Na 2 Ti 3 O 7 . Titanium oxide and sodium carbonate were mixed well at a molar ratio of 1:3. A 5% excess of Na 2 CO 3 was added to prevent the composition of the Na 2 Ti 16 O 13 impurity. The mixture was ground for 120 min. Then, the mixture powder was pressed to thin pellets, which were sintered at 800 • C for 10 h in a muffle furnace. After the muffle furnace was cooled to room temperature, the product, Na 2 Ti 3 O 7 (NTO), was obtained. Na 2 Ti 3 O 7 F x (NTOF x ) was prepared by the same procedure, except that the calculated amount of NaF was added and the stoichiometric ratio of titanium and sodium was retained. The Na 2 Ti 3 O 7 samples with different F-doping amounts were labeled as Na 2 Ti 3 O 7 F x (x = 0.1, 0.2, 0.3 and 0.4).

Phase Analysis and Morphology Characterization
The synthesized Na 2 Ti 3 O 7 and Na 2 Ti 3 O 7 F x samples were characterized by an X-ray diffraction analyzer (PANalytical X'Pert 3 Powder, Malvern Panalytical, Almelo, The Netherlands) with Cu/Kα (λ = 1.54178 Å) radiation in the 2 theta range from 5 • to 80 • . The morphologies of NTO and NTOF 0.3 were measured on a scanning electron microscope (SU8020) (HITACHI, Tokyo, Japan). High-resolution TEM (HRTEM) was tested on an FEI Tecnai, Model G2 F20S-Twin (Brno, Czech), at a working voltage of 200 kV.

Electrochemical Test
For the preparation of the anode electrode, 70 wt% active material (NTO or NTOFs), 20 wt% acetylene black (Alfa Aesar, Shanghai, China), 10 wt% polyvinylidene fluoride (PVDF, KE JING, Hefei, China) as a binder, and N-methylpyrrolodone (NMP, Alfa Aesar, Shanghai, China) as a solvent were mixed to form a slurry. Then the slurry was painted on cleaned carbon-coated aluminum foil and dried at 120 • C in a vacuum oven for 10 h. Coin-type cells were used to test the electrochemical performances. All cells were assembled in a glove box filled with argon in which oxygen and water contents were less than 0.1 ppm. The glass fiber 1822-047 membrane (Whatman, Shanghai, China) was used as the separator, 1 M NaClO 4 in diethyl carbonate (DEC)/dimethyl carbonate (DMC) (1:1 volume ratio) was used as the electrolyte (MJS, Nanjing, China), and metal sodium (Innochem, Beijing, China)was used as the counter electrode. Cyclic voltammetry experiments were carried out with scan rates at a certain range from 0.02 mV s −1 to 5 mV s −1 , with a voltage window between 0.01 V and 2.5 V (versus Na/Na + ) via CHI 640 E (B15536). Electrochemical impedance spectroscopy (EIS) was tested on an Autolab PGSTAT 302N instrument (Herisau, Switzerland) in a frequency range from 100 KHz to 100 mHz with a bias voltage of 5 mV.

Structural and Composition Characterization
As shown in Figure 1, the Na 2 Ti 3 O 7 samples with F-doping amounts of 0, 10%, 20%, 30%, and 40% are hereafter referred to as Na 2 Ti 3 O 7 , Na 2 Ti 3 O 7 F 0.1 , Na 2 Ti 3 O 7 F 0.2 , Na 2 Ti 3 O 7 F 0.3 , Na 2 Ti 3 O 7 F 0.1 , and Na 2 Ti 3 O 7 F 0.4 (x = 0.1, 0.2, 0.3, and 0.4), respectively. As the doping amount of F increases, the intensity of the diffraction peak first increases and then decreases. Several very sharp peaks indicate the high crystallinity of the samples.

Structural and Composition Characterization
As shown in Figure 1, the Na2Ti3O7 samples with F-doping amounts of 0, 10%, 20%, 30%, and 40% are hereafter referred to as Na2Ti3O7, Na2Ti3O7F0.1, Na2Ti3O7F0.2, Na2Ti3O7F0.3, Na2Ti3O7F0.1, and Na2Ti3O7F0.4 (x = 0.1, 0.2, 0.3, and 0.4), respectively. As the doping amount of F increases, the intensity of the diffraction peak first increases and then decreases. Several very sharp peaks indicate the high crystallinity of the samples.   Figure 3a shows a TEM image of NTOF 0.3 . The diameter of the nanorod is approximately 100 nm, and the length is several micrometres. Figure 3b displays a high-resolution TEM (HRTEM) image of a single nanorod. The interplanar spacing of the ordered stripes marked in Figure 3b is about 0.84 nm, which corresponds to the (001) lattice plane of Na 2 Ti 3 O 7 . The selected area electron diffraction (SAED) pattern further indicates that the nanorod is a monoclinic Na 2 Ti 3 O 7 . Na 2 Ti 3 O 7 nanorods grow along the (010) direction. To further examine the distribution of the F elements, energy-dispersive X-ray spectrometry (EDX) mapping analysis was employed. The results demonstrate that Na, O, Ti, and F elements are uniformly distributed in Figure 3d, which indicates that F was doped into Na 2 Ti 3 O 7 .  Figure 3a shows a TEM image of NTOF0.3. The diameter of the nanorod is approximately 100 nm, and the length is several micrometres. Figure 3b displays a high-resolution TEM (HRTEM) image of a single nanorod. The interplanar spacing of the ordered stripes marked in Figure 3b is about 0.84 nm, which corresponds to the (001) lattice plane of Na2Ti3O7. The selected area electron diffraction (SAED) pattern further indicates that the nanorod is a monoclinic Na2Ti3O7. Na2Ti3O7 nanorods grow along the (010) direction. To further examine the distribution of the F elements, energy-dispersive Xray spectrometry (EDX) mapping analysis was employed. The results demonstrate that Na, O, Ti, and F elements are uniformly distributed in Figure 3d, which indicates that F was doped into Na2Ti3O7.

Electrochemical Performance
In Figure 4a,b, the cell with an NTOF0.3 electrode exhibits the best electrochemical performance. Therefore, we mainly focused on the investigation of NTOF0.3. Figure 5a,b exhibits the charge and discharge curves of the NTO and NTOF0.3 electrodes at a current density of 20 mA g −1 . The initial reversible discharge and charge capacities of NTO and NTOF0.3 are 233.4 and 109.7 mAh g −1 ; 246.3 and 120.6 mAh g −1 , respectively. The initial coulombic efficiency is only 46.7% and 48.9% but it reaches almost 100% in subsequent cycles. The coulombic efficiency of the initial cycle may originate from the irreversible formation of a solid electrolyte interphase (SEI) film [29][30][31][32]. Due to the formation of the passivating layer on the surface and the reactive Ti-O that leads to electrolyte decomposition, the irreversible capacity loss in the titanium-based Na electrode is usually serious. However, after the initial cycle, the low irreversible capacity loss can be suppressed and thus high efficiency can be achieved when active Ti-O is passivated [33]. There is a sloping voltage plateau at approximately 0.46 V during the charge process, while a plateau is obtained at about 0.63 V during the discharge process. Furthermore, Figure 5c shows the rate capabilities of NTO and NTOF0.3 electrodes at current densities ranging from 20 mA g −1 to 500 mA g −1 . It can be seen that the specific capacity of the NTOF0.3 electrode is much higher than that of the NTO electrode. Figure 5d displays the cycling performance of NTO and NTOF0.3 electrodes. Both electrodes can run stably for more than 800 cycles at 100 mA g −1 . The discharge specific capacity of the NTOF0.3 electrode is about 30% higher than that of the NTO

Electrochemical Performance
In Figure 4a,b, the cell with an NTOF 0.3 electrode exhibits the best electrochemical performance. Therefore, we mainly focused on the investigation of NTOF 0.3 . Figure 5a,b exhibits the charge and discharge curves of the NTO and NTOF 0.3 electrodes at a current density of 20 mA g −1 . The initial reversible discharge and charge capacities of NTO and NTOF 0.3 are 233.4 and 109.7 mAh g −1 ; 246.3 and 120.6 mAh g −1 , respectively. The initial coulombic efficiency is only 46.7% and 48.9% but it reaches almost 100% in subsequent cycles. The coulombic efficiency of the initial cycle may originate from the irreversible formation of a solid electrolyte interphase (SEI) film [29][30][31][32]. Due to the formation of the passivating layer on the surface and the reactive Ti-O that leads to electrolyte decomposition, the irreversible capacity loss in the titanium-based Na electrode is usually serious. However, after the initial cycle, the low irreversible capacity loss can be suppressed and thus high efficiency can be achieved when active Ti-O is passivated [33]. There is a sloping voltage plateau at approximately 0.46 V during the charge process, while a plateau is obtained at about 0.63 V during the discharge process. Furthermore, Figure 5c shows the rate capabilities of NTO and NTOF 0.3 electrodes at current densities ranging from 20 mA g −1 to 500 mA g −1 . It can be seen that the specific capacity of the NTOF 0.3 electrode is much higher than that of the NTO electrode. Figure 5d displays the cycling performance of NTO and NTOF 0.3 electrodes. Both electrodes can run stably for more than 800 cycles at 100 mA g −1 .
The discharge specific capacity of the NTOF 0.3 electrode is about 30% higher than that of the NTO electrode. Figure 5e,f displays the Nyquist plots of NTO and NTOF 0.3 electrodes, respectively. All the plots show a depressed semicircle in the high-frequency region and a sloping line in the low-frequency region. It is believed that the former corresponds to the charge-transfer resistance (R CT ) while the latter corresponds to the Warburg diffusion process. It can be seen that the NTOF 0.3 electrode exhibits smaller semicircles (72.1 Ω) at high and medium frequencies when compared to those of the NTO electrode (147.7 Ω), indicating that F-doping can decrease the charge-transfer resistance. Compared with the resistances of the electrodes before and after the initial cycle, both the resistances of NTO and NTOF 0.3 after the first cycle are significantly decreased. This indicates that an activation process occurs during the cycling.
Materials 2018, 11, x FOR PEER REVIEW 5 of 8 electrode. Figure 5e,f displays the Nyquist plots of NTO and NTOF0.3 electrodes, respectively. All the plots show a depressed semicircle in the high-frequency region and a sloping line in the lowfrequency region. It is believed that the former corresponds to the charge-transfer resistance (RCT) while the latter corresponds to the Warburg diffusion process. It can be seen that the NTOF0.3 electrode exhibits smaller semicircles (72.1 Ω) at high and medium frequencies when compared to those of the NTO electrode (147.7 Ω), indicating that F-doping can decrease the charge-transfer resistance. Compared with the resistances of the electrodes before and after the initial cycle, both the resistances of NTO and NTOF0.3 after the first cycle are significantly decreased. This indicates that an activation process occurs during the cycling.  To reveal why the NTOF0.3 electrode exhibits enhanced performance compared with the NTO electrode, the electrochemical kinetics of Na + deintercalation and intercalation processes in Na2Ti3O7 Materials 2018, 11, x FOR PEER REVIEW 5 of 8 electrode. Figure 5e,f displays the Nyquist plots of NTO and NTOF0.3 electrodes, respectively. All the plots show a depressed semicircle in the high-frequency region and a sloping line in the lowfrequency region. It is believed that the former corresponds to the charge-transfer resistance (RCT) while the latter corresponds to the Warburg diffusion process. It can be seen that the NTOF0.3 electrode exhibits smaller semicircles (72.1 Ω) at high and medium frequencies when compared to those of the NTO electrode (147.7 Ω), indicating that F-doping can decrease the charge-transfer resistance. Compared with the resistances of the electrodes before and after the initial cycle, both the resistances of NTO and NTOF0.3 after the first cycle are significantly decreased. This indicates that an activation process occurs during the cycling.  To reveal why the NTOF0.3 electrode exhibits enhanced performance compared with the NTO electrode, the electrochemical kinetics of Na + deintercalation and intercalation processes in Na2Ti3O7 To reveal why the NTOF 0.3 electrode exhibits enhanced performance compared with the NTO electrode, the electrochemical kinetics of Na + deintercalation and intercalation processes in Na 2 Ti 3 O 7 were studied. Figure 6a,b shows the Cyclic voltammetry (CV) curves of the NTO and NTOF 0.3 electrodes. A cathodic peak at 0.16 V and an anodic peak at 0.68 V versus Na/Na + were observed, corresponding to typical Na + insertion/extraction in the NTO lattice, which are in consistent with the charge and discharge curves. Significantly, a redox pair developed at 0.68 V and gradually augmented with the scanning rate. This redox may be ascribed to the storage of sodium with low-valence-state titanium atoms. These two redox peaks were assigned to the redox couple of Ti 4+ /Ti 3+ over the discharge and charge processes. The reaction procedures can be depicted by Formula (1) [34]: Na 2 Ti 3 O 7 + x Na + + x e − → Na 2+x Ti 3 O 7 (0 < x < 3.5) (1) The redox peaks of NTOF 0.3 demonstrate its outstanding kinetic property. Figure 6c,d reveals the dependence of the logarithm of peak currents (log i) on the logarithm of the scan rates (log v). Furthermore, the reaction kinetics can be revealed by the formula of i = av b , which can also be expressed as log i = b × log v + log a, where i is the peak current, a and b denote related parameters, and ν represents the sweep rate [35,36]. When the value of b is close to 0.5, a battery behavior dominates the process; when the b value approaches 1.0, it shows the behavior of a capacitor. In Figure 6c, the log i versus the log v shows a linear relationship. Thus, the b values of the two peaks at 0.25 V and 0.16 V can be calculated as 0.58 and 0.45, respectively, while the values of b of the two redox peaks for NTOF 0.3 are 0.47 and 0.46, respectively. Therefore, these results indicate that a mixed process exists in the NTO and NTOF 0.3 anodes, although a diffusion-controlled process via a capacitive Na + storage mechanism should be more dominant.
According to Equation (2) [37]: where I p represents peak current (A), A is the electrode area (cm 2 ), n is the number of electrons transferred, C 0 is the concentration of Na + ion (cm 2 s −1 ), D is the diffusion coefficient of Na + (mol cm −3 ), and v is the sweeping rate (V s −1 ) in CV. Thus, the calculated diffusion coefficients of Na ions in NTO and NTOF 0.3 electrodes are 7.73 × 10 −9 cm 2 s −1 and 1.7 × 10 −8 cm 2 s −1 , respectively. This result is consistent with the EIS results. These results indicate that F-doping is favourable for improving electrochemical performance.
Materials 2018, 11, x FOR PEER REVIEW 6 of 8 were studied. Figure 6a,b shows the Cyclic voltammetry (CV) curves of the NTO and NTOF0.3 electrodes. A cathodic peak at 0.16 V and an anodic peak at 0.68 V versus Na/Na + were observed, corresponding to typical Na + insertion/extraction in the NTO lattice, which are in consistent with the charge and discharge curves. Significantly, a redox pair developed at 0.68 V and gradually augmented with the scanning rate. This redox may be ascribed to the storage of sodium with lowvalence-state titanium atoms. These two redox peaks were assigned to the redox couple of Ti 4+ /Ti 3+ over the discharge and charge processes. The reaction procedures can be depicted by Formula (1) [34]: Na2Ti3O7 + x Na + + x e − → Na2+xTi3O7 (0 < x < 3.5) The redox peaks of NTOF0.3 demonstrate its outstanding kinetic property. Figure 6c,d reveals the dependence of the logarithm of peak currents (log i) on the logarithm of the scan rates (log v). Furthermore, the reaction kinetics can be revealed by the formula of i = av b , which can also be expressed as log i = b × log v + log a, where i is the peak current, a and b denote related parameters, andν represents the sweep rate [35,36]. When the value of b is close to 0.5, a battery behavior dominates the process; when the b value approaches 1.0, it shows the behavior of a capacitor. In Figure 6c, the log i versus the log v shows a linear relationship. Thus, the b values of the two peaks at 0.25 V and 0.16 V can be calculated as 0.58 and 0.45, respectively, while the values of b of the two redox peaks for NTOF0.3 are 0.47 and 0.46, respectively. Therefore, these results indicate that a mixed process exists in the NTO and NTOF0.3 anodes, although a diffusion-controlled process via a capacitive Na + storage mechanism should be more dominant.
According to Equation (2) [37]: where Ip represents peak current (A), A is the electrode area (cm 2 ), n is the number of electrons transferred, C0 is the concentration of Na + ion (cm 2 s −1 ), D is the diffusion coefficient of Na + (mol cm −3 ), and v is the sweeping rate (V s −1 ) in CV. Thus, the calculated diffusion coefficients of Na ions in NTO and NTOF0.3 electrodes are 7.73 × 10 −9 cm 2 s −1 and 1.7 × 10 −8 cm 2 s −1 , respectively. This result is consistent with the EIS results. These results indicate that F-doping is favourable for improving electrochemical performance.

Conclusions
In summary, F-doped Na 2 Ti 3 O 7 nanorods were synthesized successfully by a solid-phase method. The effects of F-doping on the phase, morphology, and electrochemical performance of Na 2 Ti 3 O 7 were investigated. F-doping does not change the crystal structure of NTO, although it has an effect on the morphology of the resultant product. The specific capacity of Na 2 Ti 3 O 7 displays a 30% increase by