Recent Advances in the Preparation and Performance of Porous Titanium-Based Anode Materials for Sodium-Ion Batteries

Abstract

Sodium-ion batteries (SIBs) are among the most cost-effective and environmentally benign electrical energy storage devices required to match the needs of commercialized stationary and automotive applications. Because of its excellent chemical characteristics, infinite abundance, and low cost, the SIB is an excellent technology for grid energy storage compared with others. When used as anodes, titanium compounds based on the Ti⁴⁺/Ti³⁺ redox couple have a potential of typically 0.5–1.0 V, which is far from the potential of dangerous sodium plating (0.0–0.1 V). This ensures the operational safety of large-scale SIBs. Low lattice strain, usually associated with Ti-based materials, is also helpful for the longevity of the cycling of SIBs. Numerous Ti-based anode materials are being developed for use in SIBs. In particular, due to adequate electrode–electrolyte interaction and rapid charge transportation, hierarchical porous (HP) Ti-based anode materials were reported as having high specific capacity, current density, and cycling stability. HPTi-based anode materials for SIBs have the potential to be used in automobiles and portable, flexible, and wearable electronic devices. This review addresses recent developments in HPTiO₂-based SIBs and their preparation, properties, performance, and challenges.

1. Introduction

The shortage of fossil fuels is increasing and an alternative is necessary to switch from nonrenewable to renewable energy sources, such as ocean energy, wind, solar, thermal, and geothermal [1,2,3]. Moreover, the climate has been changing due to pollution, resulting in an increase in pollution as well as global warming [4]. Carbon and harmful gases emitted from automobiles and industries are inducing global warming [5,6], which is considered a potential threat to the world, causing health issues, floods, and droughts. Batteries, a major energy storage device, have been considered as having the potential to control and reduce the damage being caused to the environment [7]. The most recent example is electric vehicles equipped with batteries [8]. Based on usage, batteries are categorized into two types: (1) primary batteries and (2) secondary batteries [9,10]. Primary batteries are nonrechargeable, but the secondary batteries are rechargeable and can be used for prolonged periods of time. This review deals with secondary batteries.

Based on the material source used, the secondary batteries are further divided into several types: lead-acid [11], aluminium-ion [12], dual carbon [13], vanadium redox flow [14], magnesium-ion [15], lithium-ion [16,17], sodium-ion [18,19], etc. Sodium (Na)-ion batteries are being developed as an economic replacement, as lithium batteries cost more [20]. The foremost advantage of sodium-ion batteries is that they are more abundant naturally and are less expensive than lithium. In the crust of the earth, sodium is present at 2.6%, while lithium is present at a much lower 0.06%. Compared with Li, which has an extraction cost of USD 5000 per ton, Na has a much lower cost of USD 150 per ton. Additionally, the cathode materials for Na-ion batteries contain abundant and environmentally safe materials instead of cobalt and nickel [21]. The most prominent anode material for batteries is graphite, which has a distinct layered structure, good cycling stability, a flat potential profile, high conductivity, and high coulombic efficiency [22]. However, its applicability in sodium-ion batteries is constrained by the small interlayer spacing and poor rate capability [23]. In addition, they are limited by the inability of reversible insertion/extraction of sodium ions [24]. Moreover, sodium-ion batteries have been intensively researched to replace expensive charge carriers with less expensive alternatives [25]. In order to improve the efficiency and energy storage in Na-ion batteries, titanium (Ti)-based materials and nanostructures have been synthesized. When used as anodes, titanium compounds based on the Ti⁴⁺/Ti³⁺ redox couple have a potential of typically 0.5–1.0 V, which is far away from the potential of dangerous sodium plating (0.0–0.1 V). This ensures the operational safety of large-scale SIBs.

In the Earth’s crust, Ti is widely distributed because it is one of the rock-forming elements. TiO₂ is nontoxic, stable, and inexpensive [26]. In nature, TiO₂ exists in four crystal structures, namely brookite, rutile, anatase, and bronze [27]. The sodiation/desodiation mechanism for different TiO₂ polymorphs is different and still unclear. It was recently studied only for anatase [28,29,30]. The conductivity of TiO₂ is low, which affects the performance of Na-ion batteries [31]. To overcome these disadvantages, multiple methods are being developed to prepare materials with varying morphology, control, size, and doping with heteroatoms [32,33,34,35].

2. TiO₂ as Anode Materials

For the first time, in 1991, Sony established commercialized Li-ion batteries (LiBs) in the energy storage market, whereas hard carbon and layered LiCoO₂ were utilized as anode and cathode materials, respectively [36]. Graphite was used as an anode material for LiBs instead of hard carbon. Due to increasing power demands, researchers focused on developing advanced batteries with higher storage capacity, power, and safety.

Graphite has safety issues in high-performance batteries [37,38]. To overcome the safety issues, graphite has been successfully replaced by lithium titanium oxide (LTO) [39] in the anode terminal. TiO₂ was proposed to replace LTO to increase the energy density of LIBs [40]. In this regard, the properties of TiO₂, such as electrochemical stability, low cost, and large-scale availability, are advantageous for battery applications [41]_. Among the valve materials, TiO₂ possesses the highest stability [42,43]. Ti surfaces always have a thin, formed, natural oxide layer (TiO₂) due to their easy reaction with oxygen in the atmosphere [44]. Hence, the formation of TiO₂ can be achieved via simple, cost-effective, and eco-friendly procedures [45,46,47]. Upon calcination, TiO₂ with varying surface and crystalline features can be obtained. High-performance batteries require high storage density, which is associated with a short diffusion distance. To minimize the diffusion distance, Lewis et al. proposed a strategy of increasing the effective surface area of anode materials by using nanostructured TiO₂ [48]. The nanostructures rendered a remarkable Li storage value of 1.01–0.5 Li⁺ [48]. Additionally, the effectiveness of several TiO₂ nanostructured morphologies as LiB anode materials was explored. Wu et al. achieved a maximum performance of 150 mAhg⁻¹ and acycle performance of 100 mAhg⁻¹ at the 70th cycle by using TiO₂ nanocrystals as anode materials [49]. Su et al. used TiO₂ anatase hollow nanospheres as anode materials in SIBs, and they obtained a maximum performance of 265 mAhg⁻¹ [50]. TiO₂ nanorods were used as anode materials, and the maximum performance was about 220 mAhg⁻¹. Only nanosized particles were interacting with the electrolyte, resulting in another electrochemical reaction [51]. It may be the reason for the difficulties associated with reaching high power in SIBs. Therefore, the mixed micro/nano structure of TiO₂ has received attention. Table 1 demonstrates the electrochemical performance of hierarchically micro/nanoporous TiO₂ anode materials for SIBs.

Figure 1 shows the structure of nanoporous and hierarchically micro/nanoporous TiO₂ anodes and associated preparation methods. The material’s active surface area is important to obtain batteries with high charge storage parameters. Thus, porous materials, especially nanoporous ones, were developed. Table 2 compares the performance of nanoporous TiO₂ made using different methods for use in Na-ion batteries. In this material, nanopores are connected by a very thin wall of TiO₂. Thin walls of TiO₂ are highly prone to react with electrolytes and gradually detach from the anode terminal. Disintegration of the anode structure decreases the overall performance of SIBs. Additionally, hierarchically structured micro/nanoporous (HMNP) surfaces endow better binding energy and a high surface-to-volume ratio and prevent the undesirable electrochemical reaction between the electrode and TiO₂. Thereby, the performance of SIBs can be maintained over a prolonged usage cycle. HMNP TiO₂ can be fabricated by several methods, including the sol-gel route, coprecipitation, metal–organic framework, vacuum filtration process, and electrospray.

3. Methods of Preparing Hierarchical Micro/Nanoporous Structured Ti-Based Materials

3.1. Sol-Gel Route

Low cost, improved power density, and efficiency are major requirements for an anode. For example, Li₄Ti₅O₁₂ [52] monolith was synthesized by treating TiO₂ gels (tunable macro-porous) in aqueous LiOH, and subsequent calcinations at 700–800 °C. The resultant flower-like nanostructures exhibited a performance of 146 mAhg⁻¹ at 10C and 105 mAhg⁻¹ at 30C, without carbon coating. When applied as an anode in SIBs, this material improved the operating temperature. Figure 2 displays the morphology of the obtained nanostructures after each step. As previously stated, hydrous lithium titanate (LHLT) particles are produced on the surface of macropores by treating porous TiO₂ gels in aqueous LiOH under moderate conditions. Though subjected to a high-temperature calcination process, crack-free, monolithic, flower-like structures with interconnected macroporous structure are preserved. The flower-like structures have a thickness of ~30 nm and have granular crystallites of 30–80 nm in diameter. The rod-like structures were obtained at 800 °C due to sintering-induced densification. An increase in the number of active sites increased the rate-capability performance.

3.2. Coprecipitation

The phosphates in NASICON (Na superionic conductor) have the general formula AxMM’(PO4)_3; for example, KTi₂(PO₄)₃@C has a 3D-framework structure. This material structure provides fast ion interactions and fast extractions. Because of its low cost and high safety compared with aqueous Na-ion-based batteries, superionic conductor structures of sodium titanium phosphate (NaTi₂(PO₄)₃) were synthesized [54] and subjected to a carbon coating process. NaTi₂(PO₄)₃ was formed as a NASICON structure with good crystallinity and high phase purity. For electrochemical evaluations, under ambient conditions, a 1 M Na₂SO₄ aqueous electrolyte, NTP5h/C anode, and Na_0.44 MnO₂ cathode were assembled.

The initial charge and discharge capacities based on NTP mass were 131 and 121 mAh g⁻¹, respectively, as shown in Figure 3a,b, equating to a coulombic efficiency of 92%. The remarkable reversibility was ascribed to its open 3D framework inside the NTP structures, better aqueous electrolyte kinetics, and rapid Na ion transport. Discharge capabilities at different rates were measured. When the rate was increased to 2C, the discharge capacity was determined to be around 103 mAh g⁻¹, which is 85% of the value reported. In terms of cycling performance, Figure 3c shows that after 300 cycles at 1C at voltages ranging from 0.7 to 1.3 V, the cell had around 75% cyclic stability and >99.5% coulombic efficiency. Electrochemical impedance spectra between 1 MHz and 10 mHz, measured after 10, 30, and 50 cycles of 100% discharge, each had an amplitude of 10 mV (Figure 3d). In comparison with the fitted RCT values (i.e., [R_s + R_CT]—[R_s]), it can be noticed that the RCT value of the NTP 5h/C slightly decreased upon cycling, i.e., 2.3 Ω at the 10th cycle and 2.1 Ω at the 50th cycle. This promoted electronic conductivities. In addition, they possessed rate capability, high reversible capacity, and cycling performance. Apart from these advantages of NASICON, however, there are some disadvantages, such as its low electronic conductivity (>10⁻¹² Scm⁻¹) and slow kinetic property. Therefore, it failed to reach its theoretical capacity [60].

3.3. Hydrolysis Route

Hydrolysis of a precursor mixture containing graphite oxide, hydrogen peroxide, NH₃ H₂O [55], different high-tap-density TiO₂ spheres, and nanograins with hierarchical porous structure were synthesized, the morphology of which is shown in Figure 4. Hydrolysis at 80 °C and annealing in an Ar atmosphere at 450 °C resulted in crystalline TiO₂ spheres having a typical size of 500 nm. The size was further decreased to 100–150 nm TiO₂. The addition of ethanol and changes in the concentration of Ti(OH)₄ yielded different sizes of TiO₂ spheres. As the volume ratio of the precursor solution to the ethanol reduced from 2:1, 1:1, 1:2, to 1:4, the average diameter of the TiO₂ spheres steadily shrunk from approximately 1.5 µm to 100 nm. Many nanospheres, each with a diameter of 90 nm from the TiO₂ sample, contained a few microspheres; this is schematically explained in Figure 5. The as-synthesized material had improved storage and transportation properties. At 1 C, the Na-ion storage specific capacity was found to be 184 mAh g⁻¹ with a capacity retention of 90.5% after 200 cycles.

3.4. Metal-Organic Framework (MOF)

Simple synthesis methods are still required to improve the properties and to make it cost-effective. TiO₂ with a porous cake structure [56] was synthesized by simple annealing of Ti-based metal-organic templates. All the corresponding tests were conducted such as X-ray diffraction, nitrogen adsorption-desorption, galvanic charge, or discharge tests. The maximum charge capacity was to be found to be 250 mAhg⁻¹ after 50 cycles, and 50 mAhg⁻¹ of charge density provided stable reversible capacity. An excellent electrochemical performance was observed (Figure 6).

TiO₂ nanopills were synthesized using a titanium metal-organic framework [58]. These nanopills had a high specific surface area of 102 m²/g. When the material was employed as an anode in SIBs, it provided good results with a discharge capacity of 196.4 mAh g⁻¹ at a current density of 0.1 Ag⁻¹. The capacitive retention was 90% after 3000 cycles. Figure 7 shows the Na⁺-ion storage and diffusion in hierarchically porous TiO₂ nanopills.

3.5. Vacuum Filtration Process

Different types of hierarchical structures were designed to determine their performance. Liu et al. prepared freestanding HP assemblies from TiO₂ nanocrystals and multiwalled nanotubes (MWCNTs) [57]. Figure 8 shows a schematic representation of commercially viable and low-cost methods for obtaining free-standing TiO₂-MWCNT assemblies. TiO₂ nanocrystals (60 mg) were dispersed in 0.5% bovine serum albumin, then the BSA-coated TiO₂ nanocrystals were mixed with 1 mg/mL of oxidized MWCNTS. This composite was redispersed in 1 mg/mL NH₄HCO₃ and stirred to yield a fluffy TiO₂/MWCNTs intermediate. Upon filtration and two-step heat treatment (600 °C for 6 h and 350 °C for 2 h) annealing at free-standing TiO₂-MWCNTs assembly was obtained. The packing density of this assembly was 0.63 g cm⁻³. The TiO₂-MWCNTs assembly was found to be mechanically stable and possess high electrical conductivity. Apart from flower-like structures, nanotubes and nanoparticles were also explored.

3.6. Combined Techniques of Solvothermal and Annealing Process

Ultrathin nanosheets with a flower structure assembly consisting of anatase/bronze type TiO₂ in carbon were reported [59]. The synthesis process consisted of a solvothermal process and subsequent annealing at high temperatures. Excellent electrochemical properties were observed. Apart from structure-driven performance improvement, the carbon matrix and the interface between anatase and carbon structure also enhanced the overall efficiency. Figure 9 shows the hypothesis model sodium storage at the TiO₂(A)/TiO₂(B) interface.

3.7. Coating Techniques

In order to attain efficient energy storage in SIBs, various materials were coated on TiO₂-based hierarchical nano and microporous anode materials. Li et al. prepared nitrogen-doped graphene nanosheets and TiO₂ composites with an interconnected structure having a layered network [61]. This composite functioned as both a self-sacrificial template and as a hybrid carbon source and provided the longest cyclabilities. Wei et al. [60], fabricated potassium titanyl phosphate and a carbon-based porous composite (KTP@C). The schematics of the synthesis and morphology of this composite are shown in Figure 10. Because charged droplets were present throughout the electrospray procedure, microspheres could be observed and were clearly dispersed in the TEM and SEM pictures displayed. The KTP@C composite was heated for four hours at 800 °C. In Figure 10d,e, nanoscale particles (between 50 and 300 nm) are depicted, and the average spherical size ranged from 1 to 4 m (see the inset in Figure 10d). The spheres were self-assembled aggregates of KTP nanoparticles, according to the TEM images in Figure 10f,g.

HP anatase TiO₂ microparticles were synthesized by using supercritical methanol with organic surface-modifying substances such oleic acid. Porous anatase TiO₂ was also prepared by typical solvothermal methods as it improved electronic or ionic transport, cyclic stability, and other properties [62,63]. As mentioned earlier, in terms of the Na⁺-ion storage mechanism in the carbon-coated TiO₂ particles (CPC-TiO₂, which was prepared by using citric acid (CA) and polyethylene glycol methyl ether (PEGME)) electrode, electronic impedance spectra were obtained during the first processes of sodiation and desodiation (Figure 11a,b). When sodiation and desodiation first began, the electrolyte impedance (Re) levels were low. When the insertion of the Na⁺ ion occurred at 0.005 V, the RSEI values steadily climbed to 123.97 Ω. Then, they gradually dropped to 39.27 Ω upon electrode desodiatation. The potential profile depicted in Figure 11a was produced by the solid electrolyte interphase layer formation at close to 0.5 V during sodiation.

Ex situ XRD analysis was used to better explore CPC-sodium TiO₂’s storage behavior (Figure 11c). Prior to cycling shifting to 25.130, the (101) plane’s apex was visible at 25.250. Prior to cycling, the peak of the (200) plane was at 48.210, it changed to 48.260 at 0.005 V during sodiation, and it returned to the initial values at 2.5 V after the electrode was fully desodiated. The Na⁺-ion uptake by the CPC-TiO₂ and the Na⁺-ion insertion into the TiO₂ phase are both influenced by the interfacial storage.

3.8. Template-Assisted Spray Pyrolysis

The cow electronic conductivity and drastic volume of the cycles of batteries lead to decaying capacity and poor structural stability. So, the intercalation TiO₂ materials or hybrid structure materials for anode have gained popularity due to their affordable price and strong stability. In the case of hybrid nanoflowers and nanosheets, the structure of TiO₂/MoS₂ has improved the mobility of Na⁺ ions during charge and discharge cycles. These materials can be synthesized by the template-supported spray pyrolysis method or by the hydrothermal and calcination method [64,71].

The morphology of TiO₂/MoS₂ is shown in Figure 12a–c. The samples of TiO₂:MoS₂ with lattice spacings of 1:0.5, 1:1, and 1:1.5 had respective lattice spacings of 0.35, 0.37, and 0.35 nm. Figure 12d indicates the XRD patterns of these three samples. Two identified peaks at 464.46 eV and 458.71 eV, corresponding to the Ti 2p_3/2 and Ti 2p_1/2 orbitals, respectively, and two peaks of binding energy with an interval of 5.75 eV are visible in the 1:1 sample of the Ti 2p XPS spectra seen in Figure 12e. The aforementioned findings demonstrated that the electron density around Ti⁴⁺ was reduced, leading to a decrease in the binding energy.

3.9. Biotemplate Route

TiO₂-based anode materials were also prepared using biotemplates. For example, a yeast biotemplate [53] was used to prepare a porous TiO₂ anode material. Figure 13 shows the significant FESEM images of the biotemplate yeast cells. Spheres with a diameter of 2.0–3.0 μm were formed. The surface of yeast cells was coated with spherical TiO₂ particles; the solid, porous TiO₂ was typically 1.5 to 2.5 microns in diameter and had pore walls that were 0.2 to 0.7 microns thick. According to Figure 13d, the very small primary nanoparticles filled the outer surface of the micropore. The produced porous TiO₂ material had a discharge capacity of 255.98 mAh g⁻¹, and after 100 cycles, capacity retention was around 80%. Ion diffusion was excellent due to the electrolyte’s access to the electrode.

4. Conclusions

The selection of electrode material was found to be one of the main factors important for obtaining high-performance SIBs. Hierarchical TiO₂ nanostructures-based anode materials were found to be useful for developing SIBs with high grid storage. HP TiO₂ anode materials were utilized for achieving long cycle ability without a sudden drop in rate performance in the SIBs. Generally, TiO₂-based anodes possess merits such as relatively high performance, cost-effectiveness, and higher stability than other materials used so far. In order to decrease the diffusion length by HP TiO_2,anodes in SIBs for various synthesis routes were used, such as sol-gel route, coprecipitation, hydrolysis route, vacuum filtration process, combined method of solvothermal and annealing process and coating techniques. These multiple developments of porous titania-based anode materials were focused on SIBs, and its anode material will improve performance and can be used for wide applications, completely replacing LIBs in the future. However, a few disadvantages are still present with the available fabrication route. They are expensive, time-consuming, use hazardous chemicals, and complicate the process to develop prototypes for mass production; these required parameters need to be studied by researchers. In general, all of those will be addressed and resolved by laser sintering 3D-printing technology in the future. Moreover, by preventing dangerous sodium plating, TiO₂-based anodes endow operational safety to SIBs. TiO₂ nanostructures were rendering SIBs with high current density, cyclic stability, and specific capacity along with less durability. In the case of SIBs with HP TiO₂ anodes, they have the potential to be used in wearable electronic devices and automobiles for achieving high mileage and long life in the near future.

Author Contributions

Conceptualization, A.B.; Writing—Original draft, A.B., S.E.A. and S.R.; Writing—review & editing, S.R., T.O. and K.A.; Figures and Visualization, P.S. and S.A.; Formal analysis, G.S. and M.D.K.; Methodology, G.S.; Validation, G.S.; Visualization, M.D.K. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Schematic of HPTiO2 anode materials, existing preparation methods, and advantages.

Figure 2. (a) Appearance of the monolithic Li4Ti5O12 calcined at 700 °C. (b–f) SEM image samples calcined at 700 and 800 °C: (b,c,e) samples calcined at 700 °C; and (d,f) samples calcined at 800 °C. Insets in (e) and (f) are respective cross-sectional images [52].

Figure 3. (a) Capacity, (b) rate capability, (c) cycle performance, and (d) electrochemical impedance spectra of the NTP-5h/C. For (a) and (c), the C rates employed were 0.2 C and 1 C, respectively. The analogous circuit model utilized for the parameter-fitting is shown in (d) [54].

Figure 4. Four TiO2 spheres were imaged using SEM: (a) TiO2 (2:1), (b) TiO2 (1:1), (c) TiO2 (1:2), and (d) TiO2 (1:4). Parentheses indicate the ratios of the precursor to the absolute alcohol in each TiO2 sphere [55].

Figure 5. Diagrammatic representation of the synthesis of four TiO2 spheres with different sizes by adjusting the volume ratios of the precursor solution consisting of GO, H2O2, and NH3 H2O to absolute ethanol [55].

Figure 6. (a) Rate performance at various current densities, and (b) long-life cycling performance of powder of porous TiO2 at 1Ag−1 [56].

Figure 7. Na+-ion storage and diffusion in hierarchically porous TiO2 nanopills.

Figure 8. An example of a schematic showing the steps involved in creating a free-standing TiO2 MWCNT electrode with routes for salt storage. “D” and “C” represent sodium storage behavior that is diffusion-controlled and capacitive (due to the interfacial Na storage process) [57].

Figure 9. (a) A hypothesis about the storage of sodium at the TiO2(A)/TiO2(B) interface. Relaxed interfacial structure doped with Na in the [100] and [001] directions of TiO2(A) and TiO2(B), or in the [010] and [100] directions of TiO2(A) and TiO2(B) (b). (c,d) The change in charge density at the interface caused by Na doping. The red zone shows an accumulation of density, the blue region shows a depletion of electron density. (A) and (B) correspond to the anatase and bronze crystal structures of TiO2, respectively; a, b, and c stand for lattice parameters [59].

Figure 10. (a) Synthesis diagram for KTP@C nanocomposites; (b–g) morphology of SEM of KTP@C; (b,c) precursor as sprayed; different resolutions of KTP@C nanocomposites are shown in (d,e). The KTP@C’s grain size distribution is shown in the inset in (d), and the KTP@C’s TEM image is shown in (f,g) [60].

Figure 11. (a) Galvanostatic voltage pattern, (b) Nyquist plot, and (c) ex situ XRD plots during the initial sodiation and desodiation processes of CPC-TiO2 [62].

Figure 12. (a–c) High-resolution TEM images of the 1:0.5, 1:1, and 1:1.5 (TiO2:MoS2) samples, respectively; (d) XRD patterns and (e) Ti 2p XPS spectra of the above three samples [71].

Figure 13. FESEM micrographs of yeast cells (a), yeast@TiO2 precursor (b), and its calcined sample TiO2 (c,d) in air at 450 °C for 3 h. Points I and II in (b) and points III and IV in (c) were the points in which energy dispersive spectroscopy was studied for the calcined products [53].

Table 1

Electrochemical performance of hierarchically micro/nanoporous TiO₂ anode materials for SIBs.

Characteristics	Maximum Rate Performance(mAhg−1)	Cycle Performance (mAhg−1)	Strategy	Ref.
HP Li4Ti5O 12	165	121 at 30C (500 cycles)	Temperature-dependent rate capability of SIBs	[52]
Porous anatase TiO2-HP structure	255.98	112.93 at 5C (100 cycles)	Low-cost yeast cells used as bio-templates	[53]
NaTi2(PO4)3, nanoparticles	121	103 at 2C (300 cycles)	Cost-effective hydrothermal method without calcination	[54]
HP and high-tap-density TiO2 spheres with controllable size	189	184 at 1C (200 cycles)	Hydrolysis method for producing different types of high-tap-density TiO2	[55]
TiO2 porous cake-like	250	173 (2500 cycles)	Annealing Ti-based metal-organic frameworks templates	[56]
TiO2 free-standing HP nanocrystals	100	150 (500 cycles)	Mixing of multiwall carbon nanotubes and free-standing TiO2 nano crystals by ubes (designated as TiO2 MWCNTs) for Na storage by free-drying, annealing, and modified vacuum filtering	[57]
TiO2 HP nanopills	196.4	115.9 (3000 cycles)	MIL-125(Ti) titanium metal–organic framework as a precursor	[58]
Flower-like ultrathin TiO2 nanosheets composed of anatase and bronze	120	104 at 100C (6000 cycles)	Simple solvothermal reaction and high temperature annealing	[59]
Self-assembled hierarchical spheroid-like KTi2(PO4)3@C nanocomposites	283.2	136.1 (5000 cycles)	Electrospray method	[60]
TiO2 nanoparticles linked in consistent pattern to compose HP hybrid nanosheet	146	129 at 10C (20,000 cycles)	Adding nitrogen-doped graphene layer networks	[61]
HP anatase TiO2 microparticles	275	40 at 10C (450 cycles)	Incorporation of organic surface modifiers with supercritical methanol (scMeOH)	[62]
Hierarchical architecture of porous anatase TiO2 microspheres	207.3	140.6 (10,000 cycles)	Solvothermal reaction, combining ether-based electrolyte with porous structure	[63]
TiO2/MoS2 to form a nanoflower structure	616	460 (350 cycles)	Construction of hybrid architecture composed of MoS2 and TiO2 nanosheets	[64]

Table 2

The electrochemical performance of nanoporous TiO₂ as anode materials for SIBs.

Characteristics	Maximum Rate Performance(mAhg−1/Ag−1)	Cycle Performance (mAh g−1)	Strategy	Reference
Porous TiO2 nanospheres	123.1/4.0	208 (over 500 cycles)	Graphene-supported TiO2 nanospheres by using hydrothermal method	[65]
Nanoporous anatase TiO2	179/6.7	145 at 1C (over 3000 cycles)	Combination of uniforme nanopores and tiny nanocrystals	[66]
TiO2 nanotubes	257/0.05	103 (over 700 cycles)	Sn doping	[67]
Mesoporous TiO2(B)	For Zn-doping 173/0.05For nickel-doping 104/1.8	For Zn-doping 151 (over 100 cycles)For nickel-doping 97 (over 50 cycles)	Zn doping, nickel doping	[68]
Mesoporous TiO2	150/2	135–150 (over 100 cycles)	Anode material prepared by using anatase TiO2 nanocrystals	[69]
Nanoporous NaTi2(PO4)3//Na3V2(PO4)3	85/2.4	64 (over 1000 cycles)	Scalable sol-gel method	[70]

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