In recent years, with huge demand for renewable energy and pollution to the environment problem is attracting more and more attention, the secondary batteries (rechargeable battery or battery), this energy storage technology, which can convert other forms of energy into electrical energy and store it in the form of chemical energy in advance, has ushered in a new development opportunity in the new round of energy reform^[1-4]. With the deepening of the study, the researchers found that SIBs not only have abundant resources, widely distributed, low cost, no development bottleneck, environment friendly and compatible with lithium-ion battery the advantage of existing production equipment, also have good adaptability power characteristics, wide temperature range, the safety performance and the advantages of the problem without discharge. At the same time, by virtue of the characteristic that both positive and negative electrodes can use aluminum foil to collect fluid to construct bipolar battery, the energy density of sodium ion battery can be further improved, so that the sodium ion battery will move towards the direction of low cost, long life, high specific energy and high safety^[5].

The challenges for the SIBs include finding suitable materials and construction of advanced electrodes. Due to the different properties of sodium and lithium (polarity, ion radius, interaction with the lattice of host, and so on), many complexes that work well in lithium-ion battery (LIB) may not adapt to SIBs at all, with graphite as a good example^[6]. Meanwhile, the electrode materials storing Na ion through alloying or conversion reaction mechanism, such as bismuth (Bi), antimony (Sb) and molybdenum sulfide (MoS₂), etc., have suffered severe volume changes during Na ion insertion and extraction. This phenomenon will cause the active material to crush and fall off in the collector, and eventually lead to a serious decline in battery capacity^[7]. Therefore, it is essential to alleviate the volume expansion or search for materials with stable structure during cycling to achieve long-term cycling performance for SIBs.

In this regard, the intercalation-based materials are considered to be the most promising negative materials for sodium ion batteries, because these materials can reversibly insert and exact foreign sodium ions while maintaining structural integrity^[8]. The typical intercalation-based anode materials mainly include carbon based material, titanium oxide and niobium pentoxide and so on. Since 2011, when titanium oxide materials were first used as cathode materials for SIBs, researchers have made great efforts to design different strategies for preparing polycrystalline TiO₂ structures. However, the capacity, rate performance and cyclic stability of titanium dioxide anode materials, especially the high-rate performance, need to be further improved to meet the needs of high-energy equipment such as electric vehicles. In addition, for the intercalation-based materials, we should fully balance the dependence between electrochemical properties and structural stability^[9].

With A₂Ti _n O_2n+1 as the molecular formula, alkali titanate is composed of titanium oxide layer and interlayer cation, and has a unique layered and tunnel-like crystal structure^[10]. Due to its stability, non-toxicity, low cost and abundance, as well as interesting ion exchange/interlayer characteristics, it has attracted more and more attention. Particularly, the larger layer spacing of titanate is beneficial to the diffusion of electrolyte and the storage of charge. Notwithstanding, there are still several disadvantages of titanate for SIBs: ① The nature of low conductivity for titanate due to the large band gap results in low charge transport efficiency, which limits the rate performance of SIBs; ② the large surface energy can lead to the overlapping and aggregation of titanate layers, which will result to the fast capacity decay during sodiation/desodiation process; and ③ The stability and life problems caused by volume expansion during sodium absorption remain an obstacle to the large-scale application of titanates^[11-13]. In order to solve the above issues, a series of effective methods have been put forward, including expansion the interlayer spacing, controlling the morphology and size of the titanates, and using carbonaceous materials as a conductive matrix.

Sodium titanate (Na₂Ti₃O₇) is a typical Ti-based compound, consisting of a zigzag layer of oxygen and titanium octahedral. Each formula unit can easily insert 3.5 Na ions into the interlayer space, leading to a theoretical capacity of 310 mAh/g^[14-15]. In addition, Na₂Ti₃O₇ had the lowest average Na insertion potential at 0.3 V, but higher than the voltage generated by Na dendrites, compared with the reported inserted oxide, which was conducive to achieve high operating voltage and energy density in the full battery. Notably, the larger layer spacing is ～0.83 nm, which is favorable for charge storage performance and electrolyte ion diffusion. In spite of this, there are still some disadvantages of Na₂Ti₃O₇ anode for SIBs: ① the low electronic conductivity stemmed from the large bandgap (3.7 eV) leads to lower charge transport efficiency and thus restricts the rate property; ② Due to high surface energy, the aggregation and overlap of Na₂Ti₃O₇ layers leads to rapid capacity decay during Na⁺ insertion/extraction; and ③ The stability and life problems caused by volume expansion during sodium absorption remain an obstacle to the large-scale SIBs application of Na₂Ti₃O₇. In order to solve these problems, a series of effective methods have been developed, including expanding the interlayer spacing of Na₂Ti₃O₇, regulating the morphology at the nanoscale, and using carbonaceous materials as conductive matrix. Dou and et al ^[16]reported double-shell sodium titanate microspheres constructed from 2D ultrathin nanosheets via sulfur doping to enhance the sodium ion storage. In this work, the S-doped sodium titanate microspheres are designed via a self -templating sacrifice strategy followed with a low-temperature sulfurization route. The unique double-shell microspheres deliver a high reversible capacity of　～222 mAh/g at 1 C, and superior rate capability of 122 mAh/g at 50 C, when used as anode for SIBs. The enhanced electrochemical performance was attributed to the synergistic effects between sulfur element doping and the unique double-shell nanostructures built from 2D nanosheets architecture. Qiao and co-worker^[17]prepared Na₂Ti₃O₇@N-doped carbon hollow spheres for sodium-ion batteries with excellent rate performance. In this work, they reported the first preparation of Na₂Ti₃O₇ hollow spheres assembled from ultrathin Na₂Ti₃O₇ nanosheets with N-doped carbon coating. The unique hollow structure not only offer a larger specific surface area facilitates the penetration of the electrolyte but also provide shorter diffusion path for ions and electron. Li and et al^[18]reported hydrogenation driven conductive Na₂Ti₃O₇ nanoarrays as robust binder-free anodes for sodium-ion batteries. They provide a rational and general strategy to prepare highly affordable and accessible anodes for sodium-ion battery by engineering 3D hydrogenated Na₂Ti₃O₇ nanoarrays supported on flexible Ti foil. The H-Na₂Ti₃O₇ nanoarrays can derive a high reversible capacity of 227 mAh/g and retain at 65 mAh/g after 10,000 cycles at a high rate of 35 C. Therefore, through the synergistic effect between array structure and hydrogenation reaction, a large number of anodes can be designed to store Na⁺ ions reversibly and rapidly. Pan and et al^[19]explored sodium storage and transport properties in layered Na₂Ti₃O₇ for room-temperature sodium-ion batteries. Na₂Ti₃O₇, as a typical layered sodium titanium oxide, is obtained by a simple solid-state reaction strategy as a candidate anode for sodium-ion batteries. The Na ion storage performance in layered Na₂Ti₃O₇ has been studied from kinetic and thermodynamic aspects. To meet the requirement for cycling performance, they are trying to reduce the nanometer size of the particles, although the nanosized Na₂Ti₃O₇ shown better capacity, but still with unsatisfied cyclic performance. The solid phase layer on Na₂Ti₃O₇ electrode was analyzed. A Zero current overpotential associated with thermodynamic factors was observed in both nano- and micro-scale Na₂Ti₃O₇. They have employed several technologies to investigate the Na⁺ ion transport, electronic structure and conductivity of Na₂Ti₃O₇ such as first-principles calculation and electrochemical characterizations. According to the vacancy-hopping mechanism, they have proposed a quasi-3D energy favorable trajectory for Na₂Ti₃O₇. Chen and et al ^[20]studied the effects of F-doping on the electrochemical performance of Na₂Ti₃O₇ as an anode for sodium-ion batteries. The results showed that the specific capacity of Na₂Ti₃O₇ increased by 30% through F-doping due to the enhanced Na+ diffusion coefficient. In addition, better rate capability and cycle property were also obtained. When the Na₂Ti₃O₇ nanotubes used as an anode material for sodium ion battery, it exhibited an excellence reversible capacity of 126.2 mAh/g at the current density of 100 mAh/g, this discharge capacity can still remain at 109 mAh/g after 2000 cycles. Although, various Na₂Ti₃O₇ anode materials have been engineered, but they demonstrate rather poor sodium-storage performance.

Herein, P-doped and S-doped NTO nanosheets were prepared by treating graphene-loaded NTO nanosheets arrays with H₂P/Ar or H₂S/Ar mixed gas and homogeneously replacing with P or S atom (defined as P-NTO or S-NTO). Moreover, unlike sulfidation, which can reduce the bandgap of NTO by mixing of the delocalized S 3p states with the valence band, phosphorylation does not lead to narrowing of the bandgap in NTO. It plays an important role on the surface, but not on the whole. This provides an ideal platform for establishing the correlation between surface functionalization and reactivity, which is essential for achieving efficient sodium storage. Hence, when used as the anode material of SIBs, we found that P-NTO showed better cycling capacity and rate performance than S-NTO. As expected, the P-NTO nanosheets manifest excellent electrochemical sodium storage performance with enhanced rate capability (144 mAh/g at 5 A/g ), high specific capacity (～156 mAh/g at 0.5 A/g for 1300 cycles), and excellent long cycling stability.

Synthesis of TiO₂ nanosheets: A TiO₂ nanosheets precursor was prepared through a simple hydrothermal method. 0.5 mL of triethylenetetramine(TETA) was slowly added into 30 mL ethanol solution and stirred for 30 min. Subsequently, tetraisopropoxytitanium (Ⅳ) (0.9 mL) was dropped into the above solution. After vigorous stirring for 30 min, the above mixture solution was transferred to a Teflon-lined stainless steel hydrothermal autoclave (50 mL, Anhui Kemi Machinery Technology Co., Ltd.) and maintained at 200 ℃ for 12 h. Finally, the samples were separated by centrifugation, washed with deionized water for several times, and dried at 80 ℃ for 10 h.

Synthesis of NTO, S-NTO, and P-NTO: The as-prepared TiO₂ nanosheet precursor (0.5 mg) was dispersed into 10 mL of deionized water, and then mixed with 10 mL of 0.6 mol/L NaOH solution. The solution was vigorous stirring for 30 min and then transferred to a Teflon-lined autoclave and maintained at 160 ℃ for 6 h. Finally, the product was washed with deionized water for three times and dried at 60 ℃ for 12 h. The as-obtained sample was annealed at 350 ℃ for 4 h under Ar atmosphere, and the product was denoted as NTO. For the sulfidation of NTO, sulfur powder and NTO was placed on both ends and middle of a magnetic boat, and finally annealed at 350 ^oC for 4 h under Ar atmosphere. The ratio of sulfur powder to NTO was 10∶1 by weight, and the final product was washed with carbon disulfide and ethanol, and denoted as S-NTO. For the phosphation of NTO, except that the sulfur powder is replaced by NaH₂PO₂, the other conditions are the same as vulcanization. and the final product was denoted as P-NTO.

The morphology and microstructure of the NTO, S-NTO, and P-NTO were explored by scanning electron microscopy (SEM, GeminiSEM 300) and transmission electron microscopy (TEM, JEOL ARM-200F field-emission transmission electron microscope), respectively. The crystal phase structure of samples was obtained by Bruker D8 Advance X-ray diffractometer equipped with Cu K_α radiation. Raman spectra were collected on a NEMUS670 Raman spectrometer with an excitation wavelength of 632 nm at room temperature. The surface chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (XPS), which was performed on an X-ray photoelectron spectrometer (Thermo ESCALAB 250Xi) with Al K_α (hν=1486.6 eV) as the excitation source. And the binding energies using the typical C 1s peak (284.8 eV) as a reference.

The electrochemical performance was measured by two-electrode CR2032 type coin cells. The self-made sodium tablet used as the counter electrode. The working electrode was made by mixing active material, conductive agent (acetylene black) and binder (carboxymethyl cellulose, CMC) with a weight ratio of 7∶2∶1 into a slurry, which was coated onto a copper foil and dried at 60 ℃ for 12 h.The foil was cut into disks with a diameter of 12 mm. The weight of the active material on each piece was ～0.9 mg/cm². The electrolyte use 1 mol/L NaClO₄ in ethylene carbonate (EC) and diethylene carbonate (DEC) (vol%=1∶1) with 5% volume percent of fluoroethylene carbonate (FEC) added as an additive. The battery assembly in a glove box filled (both O₂ and H₂O contents were less than 1 ppm, 1 ppm=10^-6). The separator use the Whatman glass fiber. The galvanostatic charge/discharge cycle tests were recorded on LAND-CT2001A with the voltage window of 0.01—3.0 V versus Na⁺/Na. The CV tests were performed using an electrochemical workstation (CHI760D) in the voltage range 0.01—3.0 V versus Na⁺/Na.

The NTO, P-NTO and S-NTO nanosheets arrays were hydrothermal synthesized in a 1 mol/L NaOH solution, and then annealed with NaH₂PO₂∙H₂O vapor or H₂S (Produced by pyrolytic thiourea) at 500 ℃ for 1 h in an Ar flow gas. The details of the synthesis process were described in the experiment section. The X-ray diffraction (XRD) patterns was corresponded to the NTO phase. The main peaks of NTO, P-NTO, S-NTO are in good agreement with the characteristic peaks of NTO (JCPDS NO. 31-1329)^[23-24]. The morphology and microstructure of the NTO, P-NTO and S-NTO nanosheets arrays were characterized at great length by the field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM)[Fig. 1(a)-(d)].A panoramic image of both the precursor and P-NTO or S-NTO was composed of uniform flower-like nanosheets with the thickness of ~10 nm in Fig. 1(c), (d). TEM further confirms the structure of the as-synthesized P-NTO nanosheets. As can be seen from Fig. 1(c), (d), the TEM images were consistent with the SEM observations. Fig. 1(e) displays the high-resolution TEM (HRTEM) image of P-NTO, exhibiting the obvious lattice fringe with high crystallinity^[21-22]. The lattice space of 0.84 nm matches well with the (001) planes of NTO [Fig. 1(f)]. This indicates that proper heating temperature produces high crystallinity of NTO. The corresponding energy dispersive spectrometer (EDS) elemental mapping displays the homogeneous elemental distribution of P, O, Na, and Ti in the P-NTO sample, the element P is also evenly distributed in the P-NTO sample [Fig. 1(g)].The energy dispersion spectrometer (EDS) element mapping of S-NTO shows the uniform distribution of Na, Ti, O, S elements, and the uniform distribution of S elements in S-NTO samples.[Fig. 1(h)]

The chemical states and chemical composition of the synthesized products are investigated by X-ray photoelectron spectroscopy (XPS). Based on the survey spectra [Fig. 2(a)], the signals of P and S are observed in the P-NTO and S-NTO samples, proving the successful doping of P and S.Fig. 2(b) shows the high-resolution Ti 2p XPS, after S and P doping, the Ti 2p_3/2 and Ti 2p_1/2 peaks of the S-NTO and P-NTO samples are also slightly shifted to lower energy compared to NTO. This phenomenon is because of the substitution of S and P with lower electronegativity for oxygen. The binding energy of Ti 2p in S-NTO and P-NTO shifts to lower energy. This result may orginate from the fact that Ti⁴⁺ is partially reduced to Ti³⁺ under inert atmosphere^[25-27]. In combination with Fig. 2(c), the 1s peak of O also moves towards low energy, indicating that O is replaced by S and P with low electronegativity^[28]. The O1s peaks of NTO, S-NTO and P-NTO samples can be deconvolved to 532.2 eV and 530.4 eV, corresponding to the O—Ti—O and O—Ti bonds, respectively.Two additional peaks (531.9 and 530.6 eV) are detected in the P-NTO sample, which are assigned to the O—P and P—O—Ti bonds^[29]. In the S-NTO sample, two more peaks (531.8 and 530.1 eV) are detected, which are assigned to O—S and S—O—Ti bonds ^[27].It can be seen from Fig. 2(a) and 2(d) that the 2p peak of P corresponds to a lower binding energy than the 2p peak of S, which was attributed to the electronegativity of P is lower than that of S. Electron paramagnetic resonance (EPR) tests are also used to study the defect structure of the set of samples[Fig. 2(e)]. Both S-NTO and P-NTO have stronger g-factor signals than NTO, indicating that sulfur doping and phosphorus doping can increase defects. P-NTO has the hightest g-factor signal intensity, suggesting that phosphorus doping will make anionic defects larger. All these indicate that P-doping plays a significant role in increasing atomic defects.

The Raman spectra of the samples have two typical peaks at 1322 and 1586 cm^-1, which are attributed to the D and G bands of carbon, respectively [Fig. 2(f)].The ratio of I_D/I_G is usually used to evaluate the degree of graphitization of the sample. Peak D represents the defects of the lattice of carbon atoms, and peak G represents the in-plane stretching vibration of sp² hybridization of carbon atoms. The larger the ratio of I_D/I_G, the more defects in the lattice of carbon atom.Herein, the I_D/I_G values of P-NTO, S-NTO and NTO are 1.2, 1.1 and 1.0, respectively.This reflects the highly disordered carbon structure and high defect concentration in P-NTO, which can provide a large number of electrochemical active sites, increase the adsorption of the sodium ions, and accelerate the electrochemical reaction kinetics^[30]. Therefore, phosphating can not only accelerate the electrochemical reaction kinetics by providing electrons to balance the adsorption/desorption of sodium ions, but also increase the surface functionalization of samples, achieve higher chemical reaction activity to improve ionic kinetics and conductivity, achieve fast charge transfer, and thus obtain better electrochemical performance.

The electrochemical properties of NTO, P-NTO and S-NTO were studied to explore the effect of different element doping on Na₂Ti₃O₇ storage of sodium ion. Cyclic voltammetry (CV) technique is used to study the mechanism of sodium storage in samples.In the CV curve of P-NTO in Fig. 3(a), the reduction peak centered at 1.2 V is due to the insertion of sodium ions into Na₂Ti₃O₇ and the simultaneous reduction of Ti⁴⁺ to Ti^3+[31]. In subsequent cycles, this peak shifts to 1.6 V, possibly due to the reduced overpotential of reversible sodium storage. However, in the second and third cycles, the oxidation peak was more pronounced at 0.5 V, possibly due to the increase in sodium storage sites created during phosphating. In the first cathodic scan, the reduction peak at 0.5 V belongs to the irreversible uptake of sodium-ions with the formation of solid electrolyte interphase (SEI). Impressively, the anodic peaks keep stable for the P-NTO but gradually weaken for the S-NTO in the subsequent cycles [Fig. 3(d)], these results show that P-NTO electrode has better electrochemical reversibility than S-NTO electrode. In the initial cyclic voltammetry (CV) profiles, the larger the gap between the first cycle and the second cycle, the lower the coulombic efficiency. Compared with P-NTO, the coulombic efficiency of S-NTO and NTO are lower, indicating the excellent reversibility of P-NTO electrode[Fig. 3(d), (g)].The charge/discharge curves under different current densities are shown in Fig. 3(b), (e) and 3(h). As the current density increased from 0.2 to 20 A/g, the specific capacity of the P-NTO electrode decreased from 227.5 to 102.4 mAh/g. As a contrast, the specific capacity of the S-NTO electrode decreased from 131.5 to 37.2 mAh/g, while the specific capacity of the NTO electrode decreased from 149.6 to 15.2 mAh/g.These results indicate that doping can improve cyclic stability.The charge and discharge platform can be clearly seen in the P-doped curve, but it is difficult to see the platform in the S-doped curve, and even less visible in the NTO curve.These results indicate that P-NTO has better sodium storage performance than S-NTO and NTO.

Fig. 3(c) shows the rate performances of the two samples from 0.2 to 20 A/g. While for the comparable S-NTO electrode, the rate performance is less desirable than the P-NTO. The P-NTO electrode displays average reversible capacities of 215, 180, 167, 150, 137 and 125 mAh/g at 0.2, 0.5, 1, 2, 5 and 10 A/g, respectively. Even at a current densities as high as 20 A/g, the P-NTO still achieves a stable capacity of 120 mAh/g, which is much higher than that of S-NTO (58 mAh/g). Notably, when the test currents are reduced to 0.5 A/g and 0.2 A/g, P-NTO returns to high capacities of 180 mAh/g and 210 mAh/g, indicating a significant reversibility. The cycling performance of P-NTO and S-NTO at a current density of 0.5 A/g are shown in Fig. 3(f). The irreversible reduction of P-NTO specific capacity in early stage can be attributed to electrolyte decomposition and formation of solid electrolyte interface (SEI) film. After the initial capacity decay, the P-NTO electrode exhibits a reversible capacity of 166 mAh/g for the second charge process and maintains a stable capacity of 157 mAh/g in the 1350th cycle with capacity retention of 94.6%, while the average specific capacity of S-NTO is only 108.3 mAh/g. The cycling performance of NTO at a current density of 0.5 A/g is shown in Fig. 3(i), and the average specific capacity of NTO over 400 cycles is 99.8 mAh/g.The results show that P-NTO has high specific capacity and excellent cyclic stability.The P-NTO electrode is further cycled at a high current density of 2 A/g, and maintained 98.8% of its initial capacity after 6500 cycles, showing remarkable long-term stability [Fig. 3(k)].

In order to study the kinetic properties of P-NTO, S-NTO and NTO electrodes, CV curves are measured at different scanning rates (Fig. 4).The shape of CV curves of P-NTO and S-NTO show high reproducibility as the scan rate increased from 0.1 to 2 mV/s [Fig. 4(a), (b)], indicating excellent persistence and reversibility of sodium ions stored procedures. In contrast, the peak intensity of the CV curve of NTO shows a sustained attenuation [Fig. 4(c)], illustrating a decline in reactivity and irreversible structural change. Impressively, as the scanning rate increases, the CV curve of P-NTO has a distinct peak at 3 V, which may be due to the storage of sodium in some of the lower states of Ti atoms.

The relationship between peak current (i) and scanning rate (ν) is quantitatively analyzed based on Formula (1)^[32]. The values of a and b can be fitted with the slope of the log (i) -log (ν) graph, as shown in Formula (2).

(i: current density; ν: sweep rate; a, b: constants)

The value of slope b describes the diffusion- or capacitance-dominated process in the redox reaction. When b approaches 0.5, it means diffusion control; when b approaches 1, it means surface capacitance control.The b value of P-NTO (0.97)and S-NTO (0.89) is higher than that of NTO (0.79)[Fig. 4(d)-(f)], indicates that the capacity of P-NTO and S-NTO is more controlled by capacitance, and the b value of P-NTO is as high as 0.97, indicating that the slow kinetics can be improved significantly by capacitance-controlled redox reaction. By further analyzing the relationship between sweep rate and current response, the relative contributions of pseudocapacitance control and diffusion control at a particular voltage can be quantitatively distinguished, as shown in Formula (3). At a given potential (v), the current response (i) is a mixture of ionic-diffusion controlled (k₂v^1/2) and capacitive-controlled process (k1ν), where ν is the scanning rate and k₁ and k₂ are adjustable parameters^[32].

Fig. 4(h) shows that with the scanning speed from 0.1 to 1 mV/s, the capacitance control increases from 43% to 86%. The relative value of capacitance contribution increases with the increase of sweep rate, which can be considered as the reason for its remarkable reaction kinetics and rate performance.

In summary, P-doped and S-doped NTO nanosheet arrays are prepared. Element doping increases ion defects and provides superior nanoadsorption capacity, among which P-NTO shows better performance. CV kinetic analysis shows that the sodium storage process is a pseudocapacitive process, and the pseudocapacitance contribution can reach 86% of the total charge storage at the scanning rate of 1 m/V. P-NTO provides a reversible capacity of 100 mAh/g when the current density is as high as 2000 mA/g. After 1300 constant current cycles at a current density of 500 mA/g, the electrode retains a capacity of 150 mAh/g. This study provides new insights for the practical application of improving the performance of Na₂Ti₃O₇ for storing sodium ions.