Aqueous sol–gel synthesized anatase TiO₂ nanoplates with high-rate capabilities for lithium-ion and sodium-ion batteries

Abstract

Anatase nanoplates were successfully synthesized by a sol–gel method and exhibit superior electrochemical performance in LIBs and SIBs. Nanoplates with 10 nm particle sizes were produced by thermally decomposed titanium–terephthalate hybrid materials. In LIBs, the anatase nanoplates deliver a 100 mA h g⁻¹ capacity at 10C, maintaining 150 mA h g⁻¹ capacity at 5C for 500 cycles. Anatase with high crystallinity exhibits fairly good rate capability in SIBs, delivering 53 mA h g⁻¹ capacity at 30C. Using ex situ X-ray diffractometry and X-ray photoelectron spectroscopy, anatase was discovered to trap Na, forming metallic Ti⁰ and amorphous sodium titanate in the chemical states of Ti³⁺ and Ti²⁺. The sodium titanate is structurally stable and electrochemically reversible with the redox couple of Ti⁴⁺/Ti³⁺. It is suggested that pseudocapacitance comprises most of the capacity in the first cycle. After the activation, anatase gradually stores more capacity by insertion. Therefore, it is demonstrated that anatase nanoplates with this promising insertive/extractive host structure are promising anode materials for LIBs and SIBs.

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Introduction

Rechargeable batteries have attracted worldwide attention due to the demand for electric vehicles, portable electronic devices and stationary energy storage. To date, Li-ion batteries (LIBs) are still the most promising system because of their long cycle life, high energy, power density and safety. However, global lithium resources cannot meet the growth in consumption, especially for the application of large-scale systems. Na, the sixth most abundant element, is a cost-efficient secondary light metal that is recognized as a suitable charge carrier for numerous materials. For anode materials, graphite has been widely used in commercial LIBs; however, it has insufficient interlayer spacing for Na-ion intercalation. Although several methods have been discovered to expand the interlayers, graphite still delivers low rate capability.¹ Recently, Na-ion in disordered carbon (also called hard carbon) was expected to have a similar storage mechanism as Li-ion, yet it also presents rate limitation due to obstructed mass transportation.^2,3 Titanium-based materials, such as Na₂Ti₃O₇, Ti₂(SO₄)₃ and Na₄Ti₅O₁₂, display electrochemical activity in sodium-ion batteries (SIBs); however, their low capacity and cycle life limit their future development.^4–7

TiO₂ has been recognized as a promising anode material for both LIBs and SIBs.^8,9 Several types of TiO₂ have been reported as anode materials for LIBs, such as anatase, brookite, rutile and TiO₂(B). Anatase and TiO₂(B) can deliver superior rate capability and cycle life owing to their open Li-ion diffusion channels.^10–12 In anatase, the Li-ion insertion behavior is accompanied by a two-phase transformation from a tetragonal (space group I4₁/amd) to an orthorhombic structure (space group Imma). Li accommodation at the interstitial sites is expressed by the following equation:

TiO₂ + 0.5Li⁺ + 0.5e⁻ ↔ Li_0.5TiO₂ (1)

LiTiO₂ (space group I4₁/amd) is only observed in nano-sized TiO₂ and appears below 1.5 V vs. Li/Li⁺. Recently, it has been claimed that the [100] and [010] channels are the probable paths for Na-ion transportation.^13,14 Na-ion is predicted to be stored in the interstitial sites, but is restricted to the lattice due to its large radius. Although several investigators have attempted to determine the Na-ion storage process in anatase, different results were revealed. Kim et al. and Oh et al. reported that Na-ion does not undergo a phase transformation but intercalates into the anatase lattice with Ti⁴⁺/Ti³⁺ redox reactions.^14,15 Contrastingly, Wu et al. observed that Na-ion would distort anatase to an amorphous structure with metallic titanium formation. They discovered that about 41% sodium could reversibly diffuse out of anatase, indicating a strong insertion behavior in the first cycle.¹⁶ This observation was contrary to the results from González et al., who claimed a pseudocapacitive behavior based on NMR analysis.¹⁷ Thus, it is still critical to study the Na-ion storage process in anatase.

The construction of two-dimensional nanostructures is expected to be a promising method for enhancing the performance of SIBs, since a short diffusion path assists Na-ion to diffuse inside the particles.^18,19 In this study, high and low crystallinity anatase nanoplates based on the concept of metal–organic frameworks (MOFs) were prepared. MOFs are highly porous materials which have wide applications, such as gas storage, catalysis and sensing.²⁰ However, they are often prepared by a costly and time-consuming solvothermal process with toxic solvents (DMF, DEF, etc.), thus limiting their applications in industry.²¹ In this study, the as-synthesized anatase nanoplates from a facile aqueous sol–gel method exhibit uniform nanopores and particle sizes. These structures contribute to superior performance, especially rate capability, in LIBs and SIBs. Ex situ XPS analysis also reveals that the Na-ion storage process in the first cycle is dominated by a pseudocapacitive behavior. After the activation, the storage process will involve insertion behavior.

Experimental section

Materials synthesis

The method of synthesizing anatase nanoplates was self-designed and inspired by the microstructure of MOFs. The molar ratio of titanium oxide sulfate sulfuric acid hydrate to terephthalic acid was 1 : 2. Initially, 2.29 g titanium oxide sulfate sulfuric acid hydrate (TiOSO₄·xH₂O + H₂SO₄, Alfa Aesar) was dissolved in 40 ml deionized water. The other solution was composed of 1 g sodium dodecyl sulfate (CH₃(CH₂)₁₁OSO₃Na, Sigma-Aldrich), 2.38 g terephthalic acid (p-C₆H₄(COOH)₂, SHOWA) and 40 ml DI-water. The two solutions were mixed with continuous stirring. After the terephthalic acid was dissolved by the titanium oxide sulfate, the solution was heated at 90 °C for 2 h. The obtained precipitates (terephthalate–titanium hybrid materials) were harvested via centrifugation. The terephthalate–titanium hybrid materials were further dried at 70 °C for 1 day and then calcined at 500 °C for 3 h.

The low crystallinity anatase nanoplates were synthesized by filtering the undissolved terephthalic acid from the mixed solution. The filtered solution was heated at 90 °C for 2 h, and then the product was harvested via centrifugation.

Materials characterization

The as-synthesized samples were identified by X-ray diffractometry (XRD, Bruker D2-phaser, Cu Kα) at 30 kV and 10 mA. The morphologies and microstructures of the as-synthesized products were characterized by field-emission SEM (JSM-7600F, JEOL) and spherical-aberration corrected field emission TEM (JEM-ARM200F, JEOL) operated at 200 kV. The BET specific surface area was determined by N₂ adsorption/desorption analysis at 77 K (Autosorb-1, Quantachrome).

Ex situ XRD and XPS. The cycled electrodes were identified by X-ray diffractometry (Rigaku, TTRAXIII, Cu Kα) at 50 kV and 300 mA in a high angle mode. High-resolution X-ray photoelectron spectroscopy (HR-XPS, PHI-5000 Versaprobe-II, ULVAC-PHI) was also used to derive the oxidation states of the cycled electrodes and was carried out using a monochromatic Al kα source. The analyzed area on the sample was about 500 × 500 μm and was etched by C₆₀⁺ sputtering at 20 kV 20 nA for 8 min. To perfectly fit the spectra for mixed oxidation states, some restrictions should be followed. The fitting parameters were edited from the NIST database using the C 1s signal (284.5 eV) as the internal standard. The oxidation states of Ti⁴⁺, Ti³⁺, Ti²⁺ and Ti⁰ were fixed at 458.80, 457.27, 455.48 and 454.00 eV, respectively. The integrated area of Ti 2p_1/2 to 2p_3/2 was in the ratio of 1 : 2 and was split into 5.66, 5.60, 5.73 and 6.13 eV. It was noted that the FWHM value of each fitting spectrum was highly dependent on the crystallinity. The structure in disorder was slightly broader than that in order. In this study, anatase gradually transformed to an amorphous structure during discharge. Thus, all the fitting spectra were constrained to have identical widths that were broader than the fitting spectrum of the as-synthesized electrode.

Electrochemical measurements

Electrochemical tests were performed using 2032 type coin cells. The cells contained TiO₂ anode, metallic Li or Na, and polypropylene as a separator. The electrolytes were composed of 1.0 M LiPF₆ in EC/DMC (1 : 1, vol%) for LIBs and 1.0 M NaPF₆ in EC/PC (1 : 1, vol%) for SIBs. The cells were assembled in an argon glove box where both moisture and oxygen content were less than 1 ppm. The anode materials containing TiO₂ powder, carbon black, and polyvinylidene fluoride (PVDF) in a weight ratio of 70 : 20 : 10 were mixed together in N-methyl pyrrolidinone (NMP) and coated on pure Cu foil. Before cycling, the cells were aged for 12 h to ensure that the electrodes were completely soaked in electrolyte. The long cycling and C-rate tests were performed using an Arbin battery tester. In this work, the 1C rate was defined as 167 mA g⁻¹ for both LIBs and SIBs. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were carried out using an electrochemical workstation (Ametek 263A, USA). The Nyquist plot was in the range between 10 mHz and 100 kHz.

Results and discussion

The procedure of synthesizing TiO₂ nanoplates is shown in Fig. 1a. Terephthalate–titanium hybrid materials (PT–Tis) were constructed by self-assembling terephthalic acids and Ti⁴⁺, forming organic–inorganic arrays. From the XRD patterns (Fig. 1b), the diffraction of PT–Tis is similar to that of terephthalic acids, but is rearranged from (110)-rich to (200)-rich, changing from triclinic phase to monoclinic phase.²² It was discovered that the proportion of (200)/(110) reached a maximum value of 2.3 when the molar ratio of titanium oxide sulfate to terephthalic acids was 1 : 2. After the PT–Tis were calcined at 500 °C for 3 h, the obtained anatase nanoplates labelled as AN-500 exhibited high crystallinity. Compared with AN-500, the low crystallinity anatase nanoplates were labelled as AN. Additionally, as shown in the SEM images (Fig. 1d), the morphology of the PT–Tis is irregular. After removing the organic compounds from the PT–Tis, AN-500 showed nanoplate morphology.

Fig. 1 (a) Schematics of synthesized anatase nanoplates. (b) XRD patterns of terephthalic acid, PT–Ti hybrid materials, AN, and AN-500. SEM images of (c) terephthalic acid, (d) PT–Tis, (e) AN, and (f) AN-500.

In TEM images (Fig. 2a), AN shows some anatase clusters distributed in the amorphous structure, which is coincident with the XRD results. As is evident in Fig. 2b, the structure of AN-500 is around 50 nm in thickness; however, the widths are hundreds of nanometers to micrometers. The primary particles of AN-500 are around 10 nm and are connected to each other with uniform arrays (Fig. 2c and d). To identify the whole microstructures, the anatase nanoplates were analyzed by N₂ adsorption–desorption isotherms, as shown in Fig. 2e. The specific surface area of AN (254.9 m² g⁻¹) is greater than that of AN-500 (91.6 m² g⁻¹). The pore size distributions derived from the Barrett–Joyner–Halenda method reveal that AN and AN-500 have average pore sizes of 2.93 and 3.69 nm, respectively. The narrow pore size distributions and small particle sizes demonstrate that terephthalic acid successfully restrains the aggregation of anatase nanoparticles, retaining the nanopores instead of destroying them.

Fig. 2 TEM images of (a) AN and (b)–(d) AN-500. (e) The pore size distribution calculated from the BJH method. The inset shows the N₂ adsorption–desorption isotherms of AN and AN-500.

AN and AN-500 were tested as anodes in LIBs and SIBs using the corresponding metals as counter electrodes. In LIBs, both materials delivered higher rate capability than the commercial p25 (Fig. 3a). In the discharge curves (Fig. 3b), AN-500 shows distinct phase transformation plateaus at 1.72 V, contributing more capacity (90 mA h g⁻¹) than AN (30 mA h g⁻¹) at 0.5C. Although AN does not exhibit distinct phase transformation plateaus due to its low crystallinity, it delivers rather high capacity above 5C. Most of the capacity comes from the slope between 1.72 and 1 V. A slope without significant capacity fading is associated with pseudocapacitive behavior (as will be discussed later). This indicates that about 70% of the capacity (136 mA h g⁻¹) comes from Li-ion that is stored near the surface due to the higher surface area. However, the large surface area also results in 190 mA h g⁻¹ capacity loss in the first cycle, greater than that of AN-500 (60 mA h g⁻¹).

Fig. 3 Electrochemical properties of p25, AN, and AN-500 in LIBs: (a) rate capability in which the former 2 cycles are pre-cycled at 0.2C; the galvanostatic voltage profiles for (b) AN-500 and (c) AN measured from 0.5 to 20C. Electrochemical properties of p25, AN, and AN-500 in SIBs: (d) the galvanostatic voltage profiles in the first cycle of 0.5 and 0.05C, (e) the rate capability, the galvanostatic voltage profiles for (f) AN-500 and (g) AN measured from 0.5 to 30C.

In SIBs, the cells were operated at the rates of 0.5 and 0.05C. The voltage was driven to 0.01 V to promote more Na-ion diffusing into TiO₂ (Fig. S1†). In the first cycle (Fig. 3d), AN shows oblique voltage curves at both 0.5 and 0.05C, exhibiting a typical solid–solution reaction.⁹ In contrast, AN-500 delivers a voltage plateau at 0.5C, and an additional plateau appears between 0.80 and 0.60 V as the rate decreases to 0.05C. At the beginning of sodiation, the voltage quickly drops from 2.60 to 0.65 V, contributing 65 mA h g⁻¹ capacity. These voltage drops, which are independent of the current rate, are associated with the solid–solution reaction and SEI formation with electrolyte decomposition, independent of the insertion of Na-ion into the anatase lattice.^14,23 The two subsequent plateaus located at 0.65 and 0.27 V are greatly elongated at 0.05C. Although there are trace amounts of S in AN-500, it shows no electrochemical behavior (Fig. S2†).^24,25 It is interesting to note that AN-500 reveals similar coulombic efficiencies of 51.3% and 50.5% at 0.5 and 0.05C, respectively, indicating that the plateaus do not involve electrolyte decomposition, which will enhance the portion of irreversible capacity. Thus, the plateaus are associated with the reaction of phase transformation. However, it was also discovered that the lower plateau may involve additional formation of dendrites, which will be discussed later.

In rate capability tests (Fig. 3e), AN-500 shows comparable capacity to LIBs, delivering 200, 174, 153, 134, 104, 77, and 53 mA h g⁻¹ at the rates of 0.5, 1, 2, 5, 10, 20, and 30C, respectively. In contrast to lithiation/delithiation, Na-ion shows no distinct phase transformation plateau (Fig. 3f). It is logical that the larger Na-ion radius (1.02 Å) cannot easily diffuse into the anatase lattice as Li-ion (0.76 Å) does.²⁶ Na-ion is preferentially stored on the surface rather than in the lattice. The oblique voltage curves in Fig. 3f should be identified as a pseudocapacitor rather than a double-layer capacitor, which delivers a sloping voltage drop.^11,27 Pseudocapacitors, such as TiO₂, CeO₂ and Nb₂O₅, are generally metal oxides with high specific surface areas that can exhibit high rate capability.^28,29 However, in Fig. 3g, AN shows a low rate capability, even lower than that of commercial p25, despite having a large specific surface area. Thus, in comparing two galvanostatic voltage profiles (Fig. 3f and g); although AN shows high polarization with increasing current rate, the curves have similar shapes to those of AN-500, indicating a similar electrochemical mechanism. The same tendency is also observed in the CV:AN and AN-500 exhibit cathodic/anodic peaks after sufficient cycling. Therefore, any investigation into the poor rate capability of AN should focus on the first cycle.

To examine the rate limitation in AN, the CV at 0.2 mV s⁻¹ for 10 cycles was tested, recording the Nyquist plot at the end of each cycle, as shown in Fig. 4. In the first cycle of AN, high concentrations of Na are trapped in TiO₂ defects, leading to a huge cathodic peak at 0.26 V. On the other hand, AN-500 shows two cathodic peaks at 0.35 and 0.12 V, which are identified as the two plateaus in Fig. 3d. The relatively low voltages compared to the discharge curves are attributed to polarization at the high current rate (0.2 mV s⁻¹ ≈ 0.24C). Following the irreversible cathodic peaks, a slight anodic peak located at 0.10 V represents the desodiation of super-P, which shows no electrochemical reaction above 1 mV s⁻¹.³⁰ It is evident that neither sample has a reversible redox couple in the first cycle. The couple gradually appearing at 0.60 and 0.84 V is recognized as the activation.³¹ Therefore, EIS is operated at 3 V to realize the cell resistance. In the Nyquist plot (Fig. 4c and d), the curves can be divided into sections of R_s, R_ct and the Warburg coefficient. R_s is caused by the resistance of the cell, electrolyte and SEI. The semi-arc, which represents R_ct, is associated with the charge-transfer resistance between the interfaces. In this case, the interface is equal to the region between the TiO₂ surface and the electrolyte. After the first cycle, the R_ct of AN-500 decreases and remains at 350 ohm, forming stable interfaces on the surface. In contrast, AN shows a gradual increase in R_ct. It is demonstrated that Na is trapped by surface defects and residual organics (Fig. S3†), restraining the subsequent sodiation in AN.³²

Fig. 4 Cyclic voltammetry of (a) AN and (b) AN-500 at 0.2 mV s⁻¹ in SIBs; AC-impedance of (c) AN and (d) AN-500 at the end of each cycle of cyclic voltammetry.

To understand the phase transformation of AN-500, ex situ XRD at different states of charge in the first cycle was carried out, as shown in Fig. 5a and b. The electrode was cycled between 3 and 0.01 V at 0.05C. When the cell was discharged to 0.45 V, the diffractions broadened and (101) slightly shifted to lower angles compared to the as-synthesized electrode. It is thus confirmed that the plateau at 0.65 V belongs to the phase transformation from crystalline to amorphous structure. Following the plateau at 0.27 V, the XRD pattern at 0.01 V is totally amorphous, without any anatase diffraction. This disappearance of the diffraction is attributed to the insertion of Na-ion into the anatase lattice, which is accompanied by an anisotropic structural distortion.³³ We also discovered that anatase retained poor crystallinity at 0.5C, implying that the length of the plateau was associated with the degree of structural distortion. However, AN-500 still retains an amorphous structure as it is charged to 3 V, demonstrating that the irreversible Na is still trapped by the lattice. Ex situ SEM images reveal both the cross-section and top view of the electrode (Fig. 5c–f and S4†). It is interesting that as it is discharged to 0.01 V, dendrites cover the top of electrode. These dendrites are formed below 0.45 V and dissolve above 0.9 V in the first cycle. After that, the dendrites experience high polarization and do not reappear in the next cycle. According to the EDX mapping (Fig. S5†), the dendrites are present in the formula of sodium superoxide (NaO₂). However, even low-angle XRD cannot detect any diffraction related to the dendrites, which seem to be amorphous. It is thus difficult to clarify whether NaO₂ was formed upon cycling or was oxidized by air when the cell was opened.³⁴ To the best of our knowledge, the dendrite structures initially form in crystalline Na and are then destroyed by the air. This gives a reasonable explanation as to why the dendrites have regular shapes.^35,36

Fig. 5 Ex situ analysis of the cycled AN-500 electrodes in SIBs at different states of charge: (a) ex situ XRD, (b) enlarged ex situ XRD of (101) diffraction, (c)–(f) SEM images of the cross-section cycled electrode, the green dots in the EDX mapping representing the distribution of Na, (g) ex situ XPS of Ti spectra as using C₆₀⁺ to sputter electrodes for 12 min. The pristine electrode exhibits some reduced Ti³⁺ and Ti²⁺ by sputtering C₆₀⁺.

Ex situ XPS was further explored for the cycled electrodes to reveal the oxidation states of anatase. In the literature, it was observed that Ti⁴⁺ would be reduced to Ti³⁺ and metallic Ti⁰ during sodiation.^14–16 Wu et al. removed the SEI on the electrode by Ar⁺ sputtering and quantified a large insertion capacity within the first cycle.¹⁶ However, Ar⁺ is a strong reductive source that will highly destroy the structure as well as reduce Ti⁴⁺. This demonstrates that Ar⁺ sputtering is not suitable for quantifying different Ti oxidation states. The effect of sputtering is also revealed in Fig. S6.† Therefore, C₆₀⁺ was chosen instead of Ar⁺ to sputter the electrode surface. Additionally, the accuracy of curve-fitting is another critical point for spectra involving several oxidation states. The fitting principles have been described in the Experimental section. The Ti spectra and the calculated results are shown in Fig. 5g and Table 1. It is revealed that as it is discharged to 0.45 V, Ti⁴⁺ is reduced to Ti³⁺ and Ti²⁺. The formation of Ti²⁺ is reported for the first time in this study. If it is assumed that both oxidation states are reduced by Na-ion, forming NaTiO₂ and Na₂TiO₂, the total capacity calculated from XPS is consistent with the discharge capacity. Ti²⁺ comprises a 0.75 portion of the capacity at 0.45 V and does not increase at lower voltage, implying that the side effect of the solid–solution reaction above 0.45 V involves strong surface reduction. As the charge reached 0.01 V, a further 0.12 portion of Ti³⁺ formed, followed by a 0.06 portion of Ti⁰. The conversion of TiO₂ is reported, yet the value in this study is not as high as that in the literature.¹⁶ It was revealed that the total capacity calculated from XPS is 217.1 mA h g⁻¹, incompatible with the discharge capacity of 622.0 mA h g⁻¹. The only explanation is the formation of dendrites at low voltage. As they are charged to 3 V, Ti²⁺ and Ti⁰ are fully electrochemically irreversible. Only a 0.10 portion of Ti³⁺ re-oxidizes to Ti⁴⁺, leading to 33.4 mA h g⁻¹ capacity. The difference in charge capacity between XPS and the galvanostatic voltage profiles gives rise to an assumption that, in the first cycle, most of the reversible Na is stored on the TiO₂ surface, forming the pseudocapacitance. When the cell is opened, Na on the TiO₂ surface can be washed out by PC. Thus, Ti³⁺ without Na is re-oxidized by the atmosphere.

Table 1 The concentration of different Ti oxidation states calculated from ex situ XPS

Oxidation states	Ti^4+ (458.80 eV)	Ti^3+ (457.27 eV)	Ti^2+ (455.48 eV)	Ti^0 (454.00 eV)	Total capacity (ex situ XPS)	Total capacity (galvanostatic voltage profiles of 0.05C)
a The chemical state was calculated from the pristine electrode sputtered with C60^+ for 12 min.b The discharge capacity (mA h g^−1) was calculated from the portion of Ti oxidation states and has been already subtracted the portion of Ti reduction from C60^+.c The charge capacity (mA h g^−1) was calculated from the portion of Ti at higher voltage to subtract the portion at 0.01 V.
Before cycle^a	0.91	0.07	0.02	—	—	—
0.45 V	0.58	0.20 (43.4)^b	0.22 (133.6)	—	177.0	180.0
0.01 V	0.40	0.32 (83.5)	0.22 (133.6)	0.06 (80.1)	217.1	622.0
0.9 V	0.50	0.25 (23.4)^c	0.21 (−)	0.04 (−)	23.4	129.2
3 V	0.52	0.22 (33.4)	0.21 (−)	0.05 (−)	33.4	315.0

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To understand the particularity of the anatase rate capability, AN-500 was cycled at several scan rates (Fig. 6a and c) which are described as a power-law relationship with redox peaks. The equation follows:

i(redox peaks) = aν^b (2)

where ‘i’ is the peak current, ‘ν’ is the scan rate and ‘a’ and ‘b’ are adjustable parameters. In fact, the b-value can be identified from the slope of the curve. For AN-500 in LIBs, the b-values are close to 0.5, as shown in Fig. 6b. This demonstrated that the lithiation/delithiation process is diffusion-controlled. However, the redox peaks in the SIBs show that the b-value is close to 0.87, demonstrating a mixed diffusion-controlled (i ∝ ν^0.5) and surface-controlled (i ∝ ν) process. In the previous section, it is mentioned that this phenomenon is called pseudocapacitive behavior. However, as is evident in Fig. 6d, the b-values systematically deviate to 0.7 and 0.59 for cathodic and anodic peaks above 2 mV s⁻¹. In fact, this compression is concluded to be due to the increases in ohm resistance (active material resistance, interface resistance, diffusion constraints).²⁸ To further quantify the mixing process, the peak current is formulated by the equation^37,38

i(ν) = i_a + i_b = C₁ν + C₂ν^0.5 (3)

where i_a (pseudocapacitance) and i_b (insertion capacity) can be calculated from each scan voltage, as shown in Fig. 6e and f. It appears that the capacity of AN-500 is divided by the insertion capacity and pseudocapacitance. The insertion capacity occupies a lower voltage, suggesting that Na-ion is preferentially inserted into the anatase lattice at voltages close to 0.01 V. At 2 mV s⁻¹, the higher pseudocapacitance dominates the anatase. In Fig. 6g, the graph records the capacity contribution for each scan rate. It shows that pseudocapacitive behavior is dominant at higher scan rates, delivering a 0.75 portion of pseudocapacitance at 2 mV s⁻¹. This suggests that pseudocapacitive behavior is the mechanism by which AN-500 delivers high rate capability in SIBs.

Fig. 6 Elucidating the Li-ion/Na-ion storage process by cyclic voltammetry of AN-500 in at various scan rates: (a) LIBs operating in the range of 0.1 to 2.5 mV s⁻¹, (b) b-values from the slope of log(ν) versus log(i_p) representing the Li-ion storage process, (c) SIBs operating in the range of 0.2 to 2 mV s⁻¹, (d) b-values representing the Na-ion storage process, (e) and (f) graphs of capacity distribution composed of the insertion capacity and pseudocapacitance, (g) comparison of the capacity contribution of SIBs from 0.2 to 2 mV s⁻¹.

Finally, AN and AN-500 were tested for long cycling stability. In LIBs (Fig. 7a), AN and AN-500 show comparable performance up to 500 cycles, maintaining coulombic efficiencies near 99%. AN shows activation in the first 100 cycles that may be associated with the rearrangement of the amorphous structure during lithiation/delithiation.⁹ When the SIBs were cycled at 0.5C (Fig. 7b), despite the poor rate capability delivered by AN, it still maintained 143 mA h g⁻¹ capacity with 97% coulombic efficiency after 150 cycles. On the other hand, AN-500 shows good cycling stability within 150 cycles, maintaining 230 mA h g⁻¹ capacity without decay. A comparison of this work with the literature is also represented in Table. S1.† In this study, the anatase nanoplates with 10 nm particle size exhibit greater capacity at low current. Among all the results, the anatase nanoplates are a promising anode which exhibits good cycling stability in LIBs and SIBs. However, operating the cell near 0 V to enhance the capacity causes a large amount of irreversible Na formation, forming the irreversible Ti³⁺, Ti²⁺, Ti⁰, and the Na-containing dendrites. Although the formation mechanism of the dendrites is still unclear, it is fortunate that they disappear at the end of the first cycle and do not reappear. In the aspect of a full battery, the supply of Na is limited by the cathode materials. Therefore, it is suggested that surface modification might be a suitable way to eliminate the dendrites, improving the coulombic efficiency of anatase (50.5%) in the first cycle.

Fig. 7 Long cycling tests of AN and AN-500: (a) cycling at the rate of 5C for 500 cycles in LIBs, (b) cycling at the rate of 0.5C for 150 cycles in SIBs.

Conclusions

Anatase nanoplates with uniform nanopores and particle sizes were successfully synthesized by a facile aqueous sol–gel method. Different anatase crystallinities exhibit distinct electrochemical performances in LIBs and SIBs. The high crystallinity anatase nanoplates with 10 nm particle sizes especially show excellent rate capability in SIBs, retaining a low surface resistance after numerous cycles. The low crystallinity anatase nanoplates, which have more defects and organic residues, result in higher surface resistance. Through systematic analysis of the cycled electrodes by ex situ XPS and XRD, the reversible capacity of SIBs was found to be due to the redox couple of Ti⁴⁺/Ti³⁺, with components such as amorphous sodium titanate, metallic Ti⁰ and dendrites forming in the first cycle. The pseudocapacitance was found to be the major Na storage process in the first cycle. However, after activation, CV revealed that anatase undergoes a mixed process involving insertion capacity, which suggests that the anatase lattice is a suitable host for accommodating Na. Therefore, these findings indicate that anatase is an attractive choice as a potential anode material for both LIBs and SIBs.

Acknowledgements

Financial support from the National Science Council, Taiwan, R. O. C, under Contract no. NSC-103-2622-E-007-001-CC1 is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27814g

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