Roles of Ti in Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries offer a promising alternative to lithium-ion batteries due to their low cost, environmental friendliness, high abundance of sodium, and established electrochemical process. However, problems, such as low capacity, low storage voltage and capacity fade of electrode materials, must be resolved for the applications of sodium ion batteries. Many Ti-containing compounds were reported as cathode and anode materials, but very few studies focus on the role of Ti in electrodes used in sodium-ion batteries. This paper systemically reviews the roles of Ti in electrodes of sodium ion batteries. The Ti⁴⁺/Ti³⁺ redox couple is a good choice for anodes due to its low potential and it exhibits different storage voltages in different structures. Although Ti⁴⁺ does not participate in charge transfer in cathodes, it can indirectly enhance the capacity, cycling life and rate performance via structure change, cation order-disorder transition, and its interaction with the crystal lattice structure. This review will provide a new insight in designing and understanding novel high-performance electrodes.

Introduction

Due to environmental concerns associated with gasoline engines, electric vehicles (EVs) powered by lithium-ion batteries are becoming increasingly popular (Armand and Tarascon, 2008). However, the high cost and safety issues of lithium-ion batteries (LIB) are limiting their practical applications (Goodenough and Kim, 2009) and provide an opportunity for other batteries to meet the needs of energy storage (Dunn et al., 2011; Yang et al., 2011; Wang et al., 2018a). Sodium-ion batteries (SIB) have the advantage of an abundant sodium resource and compatibility with an aluminum current collector for both cathodes and anodes, which reduce battery cost and make them a promising alternative to lithium ion batteries (Pan et al., 2013a; Slater et al., 2013).

Research on sodium-ion batteries began in the early 1980's (Delmas et al., 1980), but the successful commercialization of lithium ion batteries in 1990 distracted the attention from research and development of SIB (Ellis and Nazar, 2012). Since 2010, numerous novel electrode materials for sodium-ion batteries have been reported. The cathodes are classified according to their structure, namely, Prussian blue, NASICON, olivine, tunnel-type, and oxides. Prussian blue analogs are attractive cathodes for sodium-ion batteries due to their open channel-like structure, compositional and electrochemical tenability. But water control during synthesis and the potential toxicity from cyanide release limit their applications (Wessells et al., 2011a,b). NASICON compounds, such as Na₃V₂(PO₄)₃ and Na₃V₂(PO₄)₂F₃, were also used as electrodes for sodium-ion batteries. Na₃V₂(PO₄)₃/C cathode has a voltage plateau of 3.4 V with capacity of ~100 mAh/g, excellent rate capacity and cycle stability (Jian et al., 2012), whereas Na₃V₂(PO₄)₂F₃ compounds possess two plateaus at 3.6 and 4.1 V as well as a capacity of 100 mAh/g in the 2.5–4.3 V voltage range. Although these cathodes exhibit promising Na storage performance, the highly toxic V³⁺/V⁵⁺ and the possibility of releasing fluorine gas becomes an environmental issue in large-scale production (Gover et al., 2006). Unlike LiFePO₄, the olivine NaFePO₄ is thermally unstable in ambient environment and has to be produced (Kim et al., 2015) by the ion exchange of Li⁺ in olivine LiFePO₄ with Na⁺, which complicates the production and increases cost (Oh et al., 2012). The commercial success of layered-LiCoO₂ in lithium-ion batteries (Mizushima et al., 1980) has prompted extensive investigation of its sodium counterpart Na_xTMO₂ (TM: Ni, Mn, Co, Ti) which crystallize into layered or tunneled structures depending on sodium content. The layered-structure consists of TM-O and Na-O layers. Compared to Li⁺, Na⁺ has a larger radius and stronger interaction with O²⁻. The sodium-layered structure is further divided into O3, P3, P2, and O2. The O and P represent the octahedral and trigonal prismatic coordination environment of alkali ions, respectively; 3 and 2 describe the number of TM layers of repeated stacking (Delmas et al., 1980). The oxides P2/O3-Na_xCoO₂, Na_xMnO₂ Na_xVO₂ and Na_xNiMnO₂ etc. are also investigated (Delmas et al., 1981; Braconnier et al., 1982; Maazaz et al., 1983; Miyazaki et al., 1983; Jeong and Manthiram, 2001; Lu and Dahn, 2001), and some of the plateaus in the charge/discharge profile are attributed to Na⁺/vacancy ordering, which affects the rate and cycling performance. Electrodes-containing Ti, such as Na_0.6Cr_0.6Ti_0.4O₂, Na_2/3Ni_1/3Mn_1/2Ti_1/6O2, Na_0.8Ni_0.4Ti_0.6O₂, and Na_0.67Co_0.33Ti_0.67O₂, etc., show a smooth sloping curve during sodium insertion and extraction, suggesting that Ti-substitution interrupted the Na⁺/vacancy ordering (Yoshida et al., 2014; Yu et al., 2014; Guo et al., 2015a; Wang et al., 2015c). Tunnel-type Na_0.44MnO₂ was first proposed as a cathode by Doeff et al. (1994). It has a unique structure that prevents interaction with H₃O⁺, which is useful as a cathode or anode for non-aqueous and aqueous sodium-ion batteries (Whitacre et al., 2010; Hosono et al., 2012). Ti- substitution for Mn in Na_0.44MnO₂ smooth's its multi-plateaus discharge-charge curve (Wang et al., 2015a).

The carbon-based compounds (Alcántara et al., 2001; Li et al., 2015, 2016b; Zheng et al., 2017; Wang et al., 2018b), alloys (Xiao et al., 2012; Farbod et al., 2014; Lim et al., 2017), organic compounds (Wang et al., 2014; Wu et al., 2015d), sulfides (Newman and Klemann, 1980), phosphates (Wu et al., 2013; Pang et al., 2014), and oxides (Senguttuvan et al., 2011; Trinh et al., 2012; Pan et al., 2013b; Sun et al., 2013; Wang et al., 2013; Xu et al., 2014; Zhao et al., 2015; Ding et al., 2017) are widely investigated as anodes. Hard carbon, obtained at high temperatures (1,600°C), exhibits a high reversible capacity of 330 mAh/g at a rate of 0.1C in the range of 0–2.5 V (Lu et al., 2018; Wang et al., 2018b; Zhao et al., 2018b). The near zero voltage vs. Na⁺/Na may result in sodium metal deposition on a hard carbon surface caused by improper battery operation or fast charging, which is a major safety concern (Chevrier and Ceder, 2011). In addition, low initial columbic efficiency and relatively high cost need to be resolved for practical application (Zhang et al., 2016). Although alloys, such as Sb/C (610 mAh/g) (Darwiche et al., 2012; Wu et al., 2014), SnSb/C (544 mAh/g) (Xiao et al., 2012), P (1,800 mAh/g) (Ramireddy et al., 2015), deliver high reversible capacities, the large volume change during cycling leads to electrode instability, a common problem in lithium-ion batteries too (Goodenough and Kim, 2009; Chevrier and Ceder, 2011). Organic compounds, such as Na₂C₈H₄O₄, Na₂C₁₆H₁₀O₄ etc., are also actively explored for sodium-ion batteries due to their low cost and high performance (Zhao et al., 2012b; Wang et al., 2014; Wu et al., 2015d). But the poor electronic conductivity, low initial coulombic efficiency, and poor cyclic stability still need to be improved. Sulfide electrodes, for example, TiS₂ and MoS₂, with ~200 mAhg⁻¹ at the storage voltage of 2.0 V, have been investigated since 1976, but the voltage is slightly higher for the anode and the samples are sensitive to air (Silbernagel and Whittingham, 1976; Newman and Klemann, 1980; Hu et al., 2014). NASICON-type NaTi₂(PO₄)₃, obtained by Ti⁴⁺/Ti³⁺ conversion, exhibits a reversible Na storage capacity of ca. 120 mAh/g at 2.1 V vs. Na⁺/Na, which is suitable as an anode for aqueous sodium-ion batteries (Park et al., 2011; Li et al., 2013; Wang et al., 2015b). Metal-oxides, e.g., Fe₂O₃, Sb₂O₄ and MnO₂ etc. demonstrate a capacity around 200 mAh/g; however, the conversion reaction involved in charge transfer may result in low initial coulombic efficiency and electrode instability (Reddy and Reddy, 2004; Sun et al., 2011; Jian et al., 2014). Ti-containing oxides, such as Li₄Ti₅O₁₂, attracted a lot of attention due to their high reversibility in lithium-ion batteries (Ferg et al., 1994; Wagemaker et al., 2008). Research on Ti-containing oxides has expanded since 2010. For example, monoclinic Na₂Ti₃O₇ (Senguttuvan et al., 2011); spinel Li₄Ti₅O₁₂ (Zhao et al., 2012a), P2-layered Na_0.66Li_0.22Ti_0.78O₂ (Wang et al., 2013), O3-type Na_0.68Mg_0.34Ti_0.66O₂ (Zhao et al., 2018a), Anatase/Rutile TiO₂ (Wu et al., 2015b; Lan et al., 2017), NaTiOPO₄ (Mu et al., 2016) and NaTiO₂ (Wu et al., 2015a). All of these oxides exhibit Ti³⁺/Ti⁴⁺ charge transfer, but the sodium storage voltages are very different.

Herein, we focused on the roles of Ti in electrodes for sodium-ion batteries. Various Ti-containing compounds are shown in Figure 1. In anode materials, Ti⁴⁺/Ti³⁺ participate in charge transfer and demonstrate different electrochemical performances corresponding to their different structures. In the cathode, the doped Ti⁴⁺ in Na_xTMO₂ oxides does not participate in charge transfer. However, Ti-substitution results in a structural change that interrupts the Na⁺/vacancy orderings, which in turn affects the capacity, cycling and rate performance. This review provides a deep understanding of the Ti's roles in the development of novel materials for sodium-ion batteries.

FIGURE 1

Figure 1. A comparison of Ti-based electrode materials in sodium-ion batteries.

Anode Materials

Titanium dioxides with different polymorphs, such as anatase, rutile, TiO₂ (B) and amorphous, have been explored as anode materials for sodium ion batteries due to their high theoretical capacity of 335 mAh/g, high rate performance, good cyclability, non-toxicity and low cost (Xiong et al., 2011; Wu et al., 2015b; Lan et al., 2017; Li et al., 2017; He et al., 2018). In addition, Density Function Theory (DFT) calculations show that the energy required for insertion of two Na⁺ into unit cells of anatase and rutile are 11.10 and 20.65 eV, respectively. The difference in the energy values indicate an easier and faster Na⁺ intercalation as well as diffusion in anatase than in rutile, which is due to the presence of 2D tunnels in the anatase structure (Su et al., 2015). As such, the electrochemical performance of anatase is better than rutile. On the other hand, DFT calculations predict that introduction of defects in the structure can narrow the energy gap in rutile, increase the electronic conductivity and Na⁺ diffusivity, as well as the rate performance, which are proved by experimental results (Chen et al., 2015; Usui et al., 2015; He et al., 2017).

TiO₂

Anatase

Anatase TiO₂ has tetragonal unit cell with space group (S.G.) I41/amd. Its crystal structure is built on corner and edge sharing of TiO₆ octahedra. There are two dimensional tunnels along its a and b axes, which facilitate Na⁺ diffusion and accommodation. Despite the reduction of titanium ion to metal (Ti⁰) detected on the electrode surfaces, it is attributed to the side reactions at the electrolyte/electrode interface, whereas the Ti⁴⁺ ↔ Ti³⁺ redox reaction is considered as the contribution to the reversible capacity of the electrode. Although different shapes of sodium discharge/charge curves (with no obvious potential plateaus and with mulita-plateaus) are reported, the investigations on the reaction mechanism suggest that the capacity observed at the high potential section (> ~0.8 V) during discharge is mainly associated to the pseudo-capacitive process as well as the side reactions and SEI layer formation. Further discharge to 0.01/0.1 V, the crystallinity of anatase decreases greatly due to the insertion of Na⁺ in the octahedral sites in the crystal structure. This crystallinity to amorphous/partial amorphous transition is irreversible upon the extraction of Na ions during charge (Wu et al., 2015b; Li et al., 2017). In fact, certain amounts of Na ions still rest in the host crystal structure after charge, the estimated reversible Na⁺ insertion/extraction is ~0.41 Na⁺ per TiO₂ (V_{cutoffdischarge} = 0.1 V) corresponding to ~140 mAh/g (Wu et al., 2015c). On the other hand, the preservation of anatase's crystallinity during Na⁺ insertion is also reported, even to a low voltage of 0.01 V (Kim et al., 2014) and 0 V (González et al., 2014). In spite of the discrepancies concerning the crystallinity of anatase during cycling, the working mechanism of the electrode is agreed to be the redox reaction between Ti⁴⁺ and Ti³⁺ introduced by Na⁺ insertion/extraction, see Figure 2A.

FIGURE 2

Figure 2. The structure and charge–discharge profiles of (A) Anatase TiO₂, with permission from ACS; (B) Rutitle TiO₂, with permission from ACS; (C) TiS₂, With permission from Wiley; (D) Na₂Ti₃O₇; with permission from RSC. (E) Na_0.66Li_0.22Ti_0.78O₂; with permission from Springer. (F) Li₄Ti₅O₁₂; with permission from Springer. (G) NaTiO₂; with permission from ACS. (H) NaTiOPO₄; with permission from RSC. (I) NaTi₂(PO₄)₃. With permission from ECS.

Rutile

The rutile TiO₂ crystal has a tetragonal unit cell with S.G. P42/mnm, with the Ti⁴⁺ ions surrounded by an octahedron of 6 oxygen atoms. The TiO₆ octahedrons are connected with edge and corner sharing. The rutile structure has a one-dimensional tunnel that enables the Na⁺ to diffuse through and accommodates them, making rutile a candidate anode material for Na-ion batteries. Despite this, rutile has good cycling stability and high sodium storage performance, and it is rarely studied compared to its counterpart, anatase, due to slow sodium diffusion (Usui et al., 2015) and its very low electronic conductivity (Chen et al., 2015). Substitution of Ti⁴⁺ with non-tetravalent cations is considered an effective way to enhance Na⁺ diffusivity and electric conductivity owing to the increased amount of vacancies. In fact, the measured electronic conductivities of doped rutile are 3–4 orders higher than the un-doped one, which agrees to the decreased band gap energy predicated by DFT calculation and leads to the high rate performance. The Fe and Nb doped rutile anodes were investigated and they showed high capacity (Fe-TiO₂, 327.1 mAh/g at 16.8 mA/g) and high rate performance (Nb-TiO₂, 120 mAh/g at 16.75A/g) (Usui et al., 2015; He et al., 2017). Their galvanostatic discharge-charge profiles show rounded potential shoulders below 1 V which are attributed to the Ti⁴⁺/Ti³⁺ redox reaction due to reversible Na⁺ insertion and extraction in the rutile structure, whereas the part above is assigned to the pseudo-capacitive sodium ion adsorption as well as the side reactions and SEI formation. XRD analysis demonstrated that (1) crystal structure change of rutile is reversible and its crystallinity is maintained during cycling; Na ions rest in the octahedral sites, formed by corner-sharing of the octahedrons, during discharge leading to the lattice expansion in a axis direction; whereas a axis contraction is observed during charge; (2) no Na₂O and Ti metal are observed during discharge. Based on the analysis, the working mechanism of rutile is considered not based on conversion reaction, but based on intercalation/de-intercalation of Na⁺, similar to the case of anatase, Figure 2B.

TiS₂

TiS₂ has been investigated as an anode material for sodium ion battery for similar reasons as TiO₂. Four electrons can be transferred through electrochemical conversion of TiS₂ to metallic Ti, leading to a theoretical capacity of 957 mA h/g (Tao et al., 2018).

TiS₂ has a layered structure with a van der Waals force bounding the TiS₂ layers. Every layer contains two hexagonally closed packed S atoms sublayers with Ti⁴⁺ sandwiched in between. Ti⁴⁺ is located at the center of S octahedron and each sulfide is connected to three Ti⁴⁺ ions. TiS₂ crystallizes into a hexagonal lattice with space group P$\overline{3}m1.$As an anode material of sodium ion battery; the weak van der Waals force between TiS₂ layers facilitates the Na⁺ intercalation/de-intercalation and diffusion during discharge/charge. The working mechanism of TiS₂ electrode has been studied in the voltage window of 0.3 to 3 V. The phase transformation is proposed as a reversible stepwise intercalation followed by a conversion reaction:

$\begin{array}{l}{\text{TiS}}_{\text{2}}{\text{+Na}}^{\text{+}}\text{+}e\leftrightarrow {\text{NaTiS}}_{\text{2}}{\text{+Na}}^{\text{+}}\text{+}e\leftrightarrow {\text{Ti}}_{\text{0}.\text{77}}{\text{S+Na}}_{\text{2}}\text{S}\\ \text{ initial} \text{cycle}\end{array}$

$\begin{array}{l}{\text{TiS}}_2+{\text{Na}}^{+}+e\leftrightarrow {\text{NaTiS}}_2+{\text{Na}}^{+}+e\leftrightarrow {\text{Ti}}_{0.77}\text{S}+{\text{Ti}}_{\text{metal}}+{\text{Na}}_2\text{S}\\ \text{ subsequent} \text{cycles}\end{array}$

Some intermediate compounds also appeared before forming NaTiS₂, such as Na_0.22TiS₂ and Na_0.55TiS₂. Obviously, after activation of the electrode material in the initial cycle, Ti⁴⁺ was reduced to a lower valence state (Ti⁰) in the subsequent cycles, whereas conversion of NaTiS₂ to Ti_metal and Na₂S was not seen with the cut off voltage of 1 V (Liu et al., 2016). DFT calculations proposed that TiS₂ is a semi-conductor in nature whereas NaTiS₂ and Ti_0.77S are more metallic, which can enhance the electronic conductivity of the electrode, leading to the excellent rate capacity of 621 mAh/g at 40 A/g (Tao et al., 2018), Figure 2C.

Spinel-Li₄Ti₅O₁₂

The spinel Li₄Ti₅O₁₂, a well-known “zero-strain” anode material for long-life lithium-ion batteries (Ferg et al., 1994; Chen et al., 2001; Wagemaker et al., 2008), is considered a promising anode for sodium ion batteries. The spinel Li₄Ti₅O₁₂ follows a typical bi-phase reaction mechanism and exhibits a flat voltage plateau of 1.55 V vs. Li⁺/Li and high rate performance in lithium-ion batteries.

Hu et al. first investigated Li₄Ti₅O₁₂ as anode material for sodium-ion batteries in 2013 (Sun et al., 2013). The spinel-Li₄Ti₅O₁₂ (shown in Figure 2F) has a cubic lattice; its symmetry is described by Fd-3m space group with eight formula units of (Li)^8a[Li_1/3Ti_5/3]^16d(O₄)^32e. Each unit cell consists of three sub-lattices sites, 8a, 16d, and 32e. The Li and O ions completely occupy the tetrahedral 8a sites and the octahedral 32e sites, respectively, while the octahedral 16d sites are randomly occupied by Li and Ti ions at the ratio of 1:5. During Li intercalation, the Li⁺ ions originally reside on the 8a sites, together with the newly incorporated Li⁺ ions, fill the 16c sites (Samin et al., 2015). In sodium-ion batteries, Li₄Ti₅O₁₂ exhibits a stable specific capacity of 155 mAh/g with coulombic efficiency of >99% at a sodium storage voltages of 0.93 V and current rate of 0.1C. This storage voltage is higher than the sodium metal deposition voltage, making it safer than hard carbon. Hu et al. combined density DFT calculations, in-situ synchrotron XRD and scanning transmission electron microscope (STEM) imaging techniques to predict and confirm the three-phase separation mechanism, which was different from the bi-phase reaction mechanism in lithium-ion batteries. Upon discharge, Na⁺ ions will occupy the 16c sites exclusively to form a Na₆Li phase, and at the same time, Li^8a ions are pushed to the nearest neighbor Li₄Ti₅O₁₂ (Li₄) phase to form Li₇Ti₅O₁₂ (Li₇) phase which is equivalent to a lithium insertion process. In this manner two new phases, Na₆Li and Li₇, are created. As discharge continues without further nucleation of Na₆Li, sodium insertion will proceed on the Na₆Li/Li₇ boundary, resulting in the transformation of Li to a Na₆Li phase and pushing this boundary forward. Meanwhile, the Li^16c ions from the initial Li₇ phase will diffuse into the nearby Li₄ phase to form Li₇ and thus the Li₇/Li₄ boundary proceeds. Although the operating voltage and high capacity are attractive, three-phase separation causes electrode cracking and delaminating from the current collector, adversely affecting cycle life. Furthermore, a full battery cell with the spinel-Li₄Ti₅O₁₂ as an anode and Na₃V₂(PO4)₃/C as a cathode has been tested. It displays an average operating voltage plateau at ~2.4 V and delivers 135 mAh/g based on mass of anode. The electrochemical performance can be further enhanced by optimizing both electrodes and their weight ratio (Sun et al., 2013).

P2-Type Layered Oxides

Hu et al. explored a new “zero-strain” anode Na_0.66[Li_0.22Ti_0.78]O₂ (as shown in Figure 2E) with a P2-layered structure, which is an analog to Li₄Ti₅O₁₂, as a promising electrode material for room-temperature sodium-ion batteries (Wang et al., 2013). In this structure, the large sodium ions occupy the trigonal prismatic sites in one layer, whereas smaller Li and Ti ions co-occupy the neighboring layer. In addition, some of the Na ions (0.425) occupy the 2d site sharing the edges with the TiO₆ octahedra, whereas others (0.211) occupy the 2b site sharing two faces with the TiO₆ octahedra. This situation is similar to other P2 materials. The reversible capacity of Na_0.66[Li_0.22Ti_0.78]O₂ is ca. 110 mAh/g, and the average sodium storage voltage is ca. 0.75 V. The volume change during sodium insertion and extraction was only 0.77%, indicative of “zero-strain” characteristics; the material exhibits over 1,200 cycles with a capacity retention of 75%. The measured Na⁺ ion diffusion coefficient is ca. 1 × 10⁻¹⁰ cm²/s. It is also interesting to find out that the final discharge product is a mixture of several phases that retained the P2-structure, as confirmed by in-situ and ex-situ X-ray diffraction (XRD). The differences in these P2-phases lie in the sodium compositions and occupations (2b, 2d). These results demonstrate that P2-layered oxides are promising anode materials for long-life rechargeable sodium-ion batteries. In addition, P2-layered Na_0.6Cr_0.6Ti_0.4O₂ (Wang et al., 2015c), P2-Na_0.66Co_0.33Ti_0.67O₂ (Guo et al., 2015b), Na_0.67Ni_0.33Ti_0.67O₂ (Shanmugam and Lai, 2014), and Na_2/3V_2/3Ti_1/3O₂ (Fielden et al., 2017) showed similar sodium storage voltage around 0.75 V and capacity ca. 100 mAh/g with low volume change during sodium intercalation/extraction.

In some P2 layered oxides, such as Na_0.6Cr_0.6Ti_0.4O₂, Na_0.67Ni_0.33Ti_0.67O₂ and Na_2/3V_2/3Ti_1/3O₂, the valence states of Cr/Ni/V are 3⁺/2⁺/3⁺, which can be oxidized to 4⁺/3⁺/4⁺ on Na de-intercalation, enabling them as possible positive electrodes. Additionally, the valence state of Ti in these compounds is 4⁺; it can be reduced to 3⁺ on Na intercalation, making it a possible negative electrode. On the basis of these results, a sodium-ion full cell has been demonstrated using the same material as both negative and positive electrodes, for example, P2-Na_0.6[Cr_0.6Ti_0.4]O₂. The greatest advantage of using the same materials as both negative and positive electrodes is reducing the process cost of electrodes significantly. The symmetric full cell using P2-Na_0.6[Cr_0.6Ti_0.4]O₂ delivers an average operating voltage plateau at ~2.53 V. The energy densities of this system are calculated to be 82 and 94 Wh/kg at current rates of 1C and C/5 based on the mass of positive and negative electrodes.

O3-Layered Oxides

In 1983, NaTiO₂ was first investigated as an electrode for sodium ion batteries by Maazaz et al. (1983). NaTiO₂ (shown in Figure 2G) adopts the α-NaFeO₂, structure which is an ordered variant of the rock-salt structure. The layers of edge-sharing TiO₆ octahedra and edge-sharing NaO6 octahedra alternate along the rock-salt [111] direction (i.e., there are alternating Na-O and Ti-O slabs) (Clarke et al., 1998). They evaluated its potential as a Na⁺ insertion/extraction. This material delivers ~75 mAh/g, corresponding to 0.3 Na⁺, during multiple phases' transformation from O3 to O'3. It has an average sodium storage voltage of 1 V, while used as an anode in SIB. In 2014, Ceder et al. re-investigated O3-NaTiO₂ as an anode material for sodium-ion batteries (Wu et al., 2015a). Approximately 0.5 Na is reversibly intercalated in NaTiO₂, corresponding to a reversible capacity of 152 mAh/g. Special Na⁺/vacancy ordering was observed and accompanied with an unusual lattice parameter variation, yielding a constant inter-slab distance and slight change in the in-plane Ti–Ti distance in the O'3 phase. The O3-type Na_0.8Ni_0.4Ti_0.6O₂ was investigated as an anode by Zhou and co-workers (Guo et al., 2015a). The reversible capacity of Na_0.8Ni_0.4Ti_0.6O₂ is 107 mAh/g with a distinct step around 0.7 V. Na_0.8Ni_0.4Ti_0.6O₂-based symmetric cell has a voltage of 2.8 V, a reversible discharge capacity of 85 mAh/g, 75% capacity retention after 150 cycles. In our opinion, the maximum sodium content in Na_0.8Ni_0.4Ti_0.6O₂ is 1, so insertion and extraction of 0.2 mol Na⁺ corresponds to 52 mAh/g, and the extra capacities are attributed to the participation of conductive additives, such as carbon. Recently, the Hu Group reported an O3-type oxide with low sodium content, Na_0.66Mg_0.34Ti_0.66O₂, as an anode for sodium-ion batteries (Zhao et al., 2018a). This material delivers a capacity of about 98 mAh/g in a voltage range of 0.4–2.0 V and exhibits a better cycling stability (ca. 94.2 % of capacity retention after 128 cycles). In-situ XRD reveals a single-phase reaction in the discharge–charge process, which is different from the common phase transitions reported in O3-type electrodes; this single-phase mechanism ensures long-term cycling stability and high rate capacity.

Monoclinic Na₂Ti_xO_2x+1

Na₂Ti₃O₇

In 2011, Palacin et al. reported that Na₂Ti₃O₇ consisting of zigzag-type layers are formed by stacking TiO₆ octahedra ribbon, the Na⁺ cations locate between the TiO₆ ribbons. Na₂Ti₃O₇ takes up 2 Na⁺ in a formula unit at a low storage voltage of 0.3 V vs. Na⁺/Na (Senguttuvan et al., 2011). The structure and electrochemical performance was shown in Figure 2D. However, the initial coulombic efficiency was rather low and only 10 cycles were shown. Hu et al. found that the sodium storage behavior was affected by the particle size of Na₂Ti₃O₇ (Pan et al., 2013b). A nano-sized Na₂Ti₃O₇ anode exhibited higher storage capacity than a micro-sized one. After optimization of the electrolyte and binder, the Na₂Ti₃O₇ electrode exhibited a reversible capacity of 188 mAh/g in 1 M NaFSI/PC electrolyte and sodium alginate as a binder at a current rate of 0.1C in a voltage range of 0.0–3.0 V. Unfortunately, the cycling properties are not satisfactory. In order to improve cycle life, Na₂Ti₃O₇@MWCNTs (Wei et al., 2014), Na₂Ti₃O₇/C (Ding et al., 2017), Na₂Ti₃O₇@N-Doped Carbon Hollow Spheres (Xie et al., 2017), Na₂Ti₃O₇/Titanium Peroxide (Zhao et al., 2015) and F-Doping Na₂Ti₃O₇ (Chen et al., 2018) were investigated. However, their performances were not satisfactory due to the low electronic conductivities and structural instability which needs to be further improved. In addition, full sodium-ion batteries have been tested based on advantageous electrochemical features of nanostructured Na₂Ti₃O₇. –for example, by using VOPO₄ material as a cathode, the full cell showed the operating voltages close to 2.9 V and delivered a reversible capacity of 114 mAh/g at a rate of 0.1C. It also shows outstanding rate capability (~74 mA h/g at 2C rate) and excellent cycling stability (92.4% capacity retention after 100 cycles) (Li et al., 2016a).

Na₂Ti₆O₁₃

Na₂Ti₆O₁₃ crystallizes in a monoclinic crystalline structure with continuous tunnel channels along the c axis, offering space to accommodate the alkali metals (Dominko et al., 2006). In the crystal structure of Na₂Ti₆O₁₃, all of the oxygen atoms belong to at least two octahedra. Transformation of the crystal structure of Na₂Ti₃O₇ to that of Na₂Ti₆O₁₃ is described as a condensation of the two-dimensional layers of octahedra to a three-dimensional structure by sharing these one-coordinated oxygen atoms. It means that there is no terminal oxygen atom in the crystal structure of Na₂Ti₆O₁₃; all oxygen atoms are linearly coordinated by two titanium atoms. The TiO₆ octahedra in Na₂Ti₆O₁₃ are more regular, there are no very long or very short Ti-O distances. In 2011, T. Brousse et al. investigated Na₂Ti₆O₁₃ as the negative electrode for sodium-ion batteries (Trinh et al., 2012). The electrode showed a 0.8 V plateau vs. Na/Na⁺, with an initial discharge capacity and initial columbic efficiency of only 22 mAh/g and 27%, respectively, Shen et al. subsequently performed DFT simulation and showed that a maximum of 4 Na⁺ are inserted into this structure (Shen and Wagemaker, 2014). They obtained 196 mAh/g by lowering the cut-off voltage from 0.3 to 0 V.

Na₂Ti₄O₉

Kataoka et al. reported a tunnel-type Na₂Ti₄O₉ as a negative electrode for sodium-ion batteries (Kataoka and Akimoto, 2016). The crystal structure of Na₂Ti₄O₉ shows all of the TiO₆ octahedra are strongly distorted. The remarkably defective occupations for all of three sodium sites in the Na₂Ti₄O₉ sample; 72% for Na₁, 69% for Na₂ and 58% for Na₃ sites, should be noted. The electrochemical measurements of Na₂Ti₄O₉ showed the reversible sodium insertion and extraction reactions at 1.1 V, 1.5 V, and 1.8 V vs. Na/Na⁺. The reversible capacity for the sodium cell was 100 mAh/g at a current density of 12 mA/g, which progressively faded to 45 mAh/g after 60 cycles.

ATiOPO₄ (A = NH₄, K, Na)

KTiOPO₄ is a well-known excellent non-linear optical material. Its crystal structure contains polytitanate chains interconnected by PO₄ tetrahedral units and belongs to the Pna21 space group; K⁺ ions occupy two different sites, K1 and K2. The K1 site is located near the center of a passage and the K2 site is near the point of intersection of TiO₆ and PO₄, as shown in Figure 2H. The preparation of NaTiOPO₄ by the traditional solid-state reaction is difficult. Fortunately, it was found that NaTiOPO₄ is easily synthesized using a hydrothermal method and KTiOPO₄ can be prepared by ion-exchange of K⁺ with ${\text{NH}}_4^{+}$ and Na⁺ due to the difference in their ionic radii. These materials were investigated by electrochemical discharge/charge, with average charge voltages of 1.45 V (NH4TiOPO₄), 1.4 V (KTiOPO₄), and 1.5 V (NaTiOPO₄), and capacities of 100 mAh/g (NH4TiOPO₄), 60 mAh/g (KTiOPO₄), 80 mAh/g (NaTiOPO₄), respectively (Mu et al., 2016). In-situ XRD was performed to understand the reaction, and the results revealed a bi-phase reaction mechanism in NaTiOPO₄ during sodium intercalation and de-intercalation.

NASICON Compounds

The crystal structure of NASICON compounds was determined in 1968. In general, the NASICON-type compounds (Na_xM1M2(XO₄)₃ (M = V, Ti, Fe, Tr or Nb etc.; X = P, or S, x = 0–4) exhibit an open three-dimensional structure with two types of interstitial positions (M1, M2), where the conductor cations are distributed. The matrix can be broken down into fundamental groups of 2MO₆ octahedra separated by 3 XO₄ tetrahedra that share common corner oxygen's. As there are no shared edges or shared faces in the matrix, all of the large sodium sites are connected. The NASICON-type NaTi₂(PO₄)₃ (shown in Figure 2I) exhibits a reversible Na storage capacity of ca.120 mAh/g at ca. 2.1 V vs. Na⁺/Na via Ti⁴⁺/Ti³⁺ conversion (Park et al., 2011). The storage voltage is high for organic sodium-ion batteries, but suitable for aqueous ion batteries. Reported examples of NASICON compounds include Na_0.44MnO₂/NaTi₂(PO₄)₃ (Kim et al., 2013), Na₂CuFe(CN)₆/NaTi₂(PO4)₃ (Wessells et al., 2011a), Na₂NiFe(CN)₆/ NaTi₂(PO4)₃ (Wessells et al., 2011b), Na₃V₂(PO₄)₃/ NaTi₂(PO₄)₃ (Song et al., 2014).

The discussion above demonstrates that the sodium storage voltages of Ti- containing anodes depend on their crystal structures, especially the local environments of Ti ions, although the redox reaction always involving Ti⁴⁺ and Ti³⁺. Their structures, voltages and electrochemical performances are summarized in Figure 3A in the order of voltage increase.

FIGURE 3

Figure 3. Roles of Ti in the electrodes. (A) The voltage of Ti³⁺/Ti⁴⁺ in the different structure as anode for sodium-ion batteries; (B) Ti-doping resulting in the structure change from the P2-layered to tunnel-type. (C) Upper: For the three temperature regions, there is a clear evolution from static Na+ ions to quasi-1D ion diffusion along a axis to finally fully 2D ion diffusion; With permission from APS. Lower: The mean square displacements (MSDs) of Na+ ions in P2-Na_0.6[Cr_0.6Ti_0.4]O₂ are displayed at 1,200, 1,500, 1,800, 2,400, and 2,700 K; With permission from Springer. (D) The schematic diagram of Ti-substitution increases the crystal lattice of P2-layered oxides.

The average Na⁺ ion insertion potentials of electrode materials can also be estimated by Density Function Theory calculation which plays an important role in new material design. The equilibrium intercalation voltage depends on the chemical potential difference of sodium in the electrode materials and can be approximated by equation (1) without considering the small contributions of entropy and volume changes to the cell voltage (Aydinol and Ceder, 1997; Meng and Arroyo-De Dompablo, 2009).

$\begin{array}{ll}{V}_{ave}=\frac{E_{total}(N{a}_{x2}final)-\left[E_{total}{(Na)}_{x1}pristine\right)+(x_2-x_1)E_{total}(Na)}{(x_2-x_1)zF}& (1)\end{array}$

where x₁ and x₂ are the Na⁺ contents in the pristine and final electrodes; F is the Faraday constant; z is electronic charge of sodium ions, i.e., 1; E_total is the total energy per formula unit which is a function of the crystal structure. In the above mentioned Ti containing compounds, despite all Ti ions situated at the center of TiO₆ octahedra, the differences in the connections between TiO₆ octahedra as well as between TiO₆ and other components constitute the different environments of Ti in various structures, as a consequence, the total energies are different, and therefore have different average cell voltages. For example, Na_0.66[Li_0.22Ti_0.78]O₂ (0.47 V), Li₄Ti₅O₁₂ (0.91 V), Na2Ti3O7 (0.3 V) (Rousse et al., 2013). In general, Ti-containing anodes have low electronic conductivity. Among the six structures mentioned in this section, layered oxides showed good cycle life due to their single-phase reaction mechanism and low volume change, but their capacities must increase from the present value of ca. 100 mAh/g. Therefore, future work should focus on increasing the capacities (200–300 mAh/g) and electronic conductivities of the “zero strain” layered oxides with storage voltages at ca. 1 V.

Function of Ti in the Cathode

In contrast to the anode, the cathode materials include the redox couples Ni²⁺/Ni³⁺/Ni⁴⁺, Mn²⁺/Mn³⁺/Mn⁴⁺, Co³⁺/Co⁴⁺, V³⁺/V⁴⁺, Cu²⁺/Cu³⁺, Cr³⁺/Cr⁴⁺, etc. Although Ti⁴⁺-dopant does not participate in charge transfer involving Na⁺ extraction and insertion, it affects the crystal structure, including cation-ordering and lattice structure.

Structure Change

In the case of P2-type materials, the value of “x” in Na_xMO₂ is typically 2/3, leaving many vacancies in the alkali layer. Na_xMnO₂ (0.6 < × < 0.72) is a typical P2-layered oxide. Caballero reported a layered sodium manganese oxide (Na_0.6MnO₂) synthesized by a sol–gel method (Caballero et al., 2002). This material delivered a constant specific capacity of 140 mAh/g at 0.1 mA/cm² in a voltage window of 2.0–3.8V. However, the continuous strains and distortions resulting from insertion and extraction of Na⁺ caused the host structure to gradually collapse and yielded an amorphous material after eight cycles. This led to a progressive reduction of the cell capacity, regardless of the voltage window used. In 2014, DFT calculations by Ceder et al. revealed that Na⁺ ordering in Na_xMnO₂ was controlled by the underlying combination of electrostatic and electronic structure interactions via Jahn–Teller effect, enabling some Na to occupy the highly distorted octahedral site. This led to Na and Mn charge-ordered stripes, which yielded a fascinating low-temperature magnetic ordering (Li et al., 2014). In 2015, Wang et al. reported a tunnel-type structure, Na_x[Mn_xTi_1−x]O₂ (x = 0.44, 0.50, 0.55, 0.60, 0.63, 0.66), which has a large S-shaped tunnel and a small hexagonal tunnel formed by MnO₆ and MnO₅ polyhedrals (Wang et al., 2015b). The structure of these materials was similar to tunnel-type Na_0.44MnO₂ (S.G.: Pbam) (as shown in Figure 3B) with sodium ions in the S-shaped and hexagonal tunnels. Unlike layered Na_xMnO₂, tunnel-type Na_x[Mn_xTi_1−x]O₂ is stable in an aqueous solution. Hence, Na_0.66[Mn_0.66Ti_0.34]O₂ can be used as a positive electrode material for aqueous sodium-ion batteries. In particular, it showed the highest reversible capacity (76 mAh/g) at a current rate of 2C among all the oxide electrode materials, with an average operating voltage of 1.2 V when coupled with a NaTi₂(PO₄)₃/C negative electrode. The Na_0.66[Mn_0.66Ti_0.34]O₂|NaTi₂(PO₄)₃/C aqueous sodium-ion cell demonstrates excellent cycle performance with minimal capacity decay after 300 cycles.

The O3-type NaFeO₂ attracted a lot of attention due to its inexpensive and non-toxic features, and is expected to have a reversible Fe⁴⁺/Fe³⁺ conversion (Yabuuchi et al., 2012). Recent studies showed a reversible Na storage in NaFeO₂ at 3.3 V with a small polarization by limiting the cut-off voltage. In contrast, the Li analog, O3-LiFeO₂ (Kanno et al., 1996), is nearly electrochemically inactive due to a low energy barrier for Fe⁴⁺ migrating to the alkali layer. The charge capacity in the half cell was ascribed to the dominant reaction of oxygen removal at the solid/electrolyte interface rather than Fe⁴⁺/Fe³⁺ conversion as the Fe³⁺ 3d-orbital is strongly hybridized with the oxygen 2p orbital in the Li system; oxygen removal is more favorable energetically than the oxidation to Fe⁴⁺. Substitution of Ti for Fe in NaFeO₂ changes the structure from the O3-phase to a tunnel type. For example, the tunnel-type Na_0.875Fe_0.875Ti_1.125O₄ delivered a capacity of 45 mAh/g corresponding to the extraction/insertion of 0.37 Na⁺ ions. Compared to NaFeO₂, Na_0.875Fe_0.875Ti_1.125O₄ exhibits the low capacities due to the lower quantity of sodium participates in the redox reaction, but the sodium storage voltage improves from 3.2 to 3.6 V (Pan et al., 2013a; Bhange et al., 2017).

Cation-Disorder

Na_xCoO₂ is known as the first layered oxide investigated as a Na intercalation host (Delmas et al., 1981). Depending on the sodium content and temperature of this compound, an interplay between Na⁺-Na⁺ and Na⁺-Co^3+/4+ electrostatic interactions coupled with Co^3+/4+ charge ordering results in the formation of numerous Na⁺/vacancy-ordered superstructures (see the Figure 3C; Medarde et al., 2013). These ordered superstructures have distinct voltage plateaus, lowering the Na⁺ ion diffusion coefficient and even reducing the dimensionality of ionic transport. The combination of these characteristics, in turn, result in rapid capacity-fading during cycling, as in the case of Na_xCoO₂ (Berthelot et al., 2011), Na_xMnO₂ (Jeong and Manthiram, 2001), Na_xVO₂ (Guignard et al., 2013), Na_2/3Ni_1/3Mn_2/3O₂ (Lu and Dahn, 2001), and Na_2/3Co_2/3Mn_1/3O₂ (Carlier et al., 2011). In 2015, Wang et al. reported a critical difference in ionic radii between M1 and M2 is ~15% in layered oxides P2-NaM1M2O₂ (M1, M2 are transitional metals) which governs cation order-disorder transition (Wang et al., 2015c). If the radius difference is higher than 15% and M1/M2 content is close to a rational ratio, then an ordered structure is expected; otherwise M1 and M2 is disordered. The presence of charge ordering is determined by the redox potentials (that is, Fermi level of M1 and M2). Small differences are favorable for charge ordering, even if M1 and M2 are chemically disordered. Finally, Na⁺/vacancy ordering and charge ordering are coupled to each other. There would be no charge ordering without Na⁺/vacancy ordering and vice versa. The ratio of ionic radii of Ti⁴⁺/Ni²⁺, Ti⁴⁺/Co²⁺, Ti⁴⁺/Cr³⁺, Ti⁴⁺/Mn³⁺ are 1.14, 1.04, 1.02, 1.14, respectively, which are <1.15. This means that the transition layered oxides are all disordered. In addition, there are large differences in redox potentials (or Fermi level) of Ti⁴⁺/Ni²⁺, Ti⁴⁺/Co²⁺, Ti⁴⁺/Cr³⁺, Ti⁴⁺/Mn³⁺ which leads to charge disordering, and hence Na⁺/vacancy disordering. The behavior of charge/discharge profiles of Na_2/3Ni_1/3Ti_2/3O₂ (Shanmugam and Lai, 2014), Na_2/3Co_1/3Ti_2/3O₂ (Guo et al., 2015b) and Na_0.6Cr_0.6Ti_0.4O₂ exhibit a smooth slope without the plateau that is attributed to the Na⁺/vacancy ordering. Wang et al. analyzed neutron powder diffraction data collected from Na_0.6[Cr_0.6Ti_0.4]O₂ on cooling to 3 K; the results showed no evidence of Cr/Ti and Na+/vacancy orderings, and charge and Na⁺/vacancy orderings were also not observed in the tunnel-type Ti-substituted Na_0.44MnO₂ In contrast, Na⁺/vacancy ordering was found in Na_0.7CoO₂ which exhibits the different transport channels at the different temperatures (see in Figure 3C).

Distance Between Layers

A larger crystal lattice enhances ionic transport of Na⁺, thus improving rate and cycle performance (As shown in Figure 3D). Normally, Ti-doping will lead to an increase in the alkali-layer distance of 1–3%, such as in NaCrO₂ (Li et al., 2019), Na_2/3CoO₂ (Sabi et al., 2017) and Na_0.67Fe_0.5Mn_0.5O₂ (Park et al., 2018). Recently, Lee et al. reported enhanced rate capability and cycle performance by substitution of Ti for Fe in P2-Type Na_0.67Fe_0.5Mn_0.5O₂ cathode for sodium-ion batteries (Park et al., 2018). They found 5% Ti substitution increases the crystal lattice from 3.528 to 3.571 Å. This small change promotes Na-ion diffusion and prevents the phase transition from the P2 to the OP4/“Z” structure. In addition, Du's group reported that with 5% Ti-substitution in NaCrO₂, the crystal lattice expanded 1%, which resulted in an increase of 25% in capacity retention at the current rate of 30C and enhancement in cycle life (Li et al., 2019).

Summary

The role of Ti in the cathode and anode for sodium-ion batteries are reviewed in the present work. In the anode, the redox couple, Ti⁴⁺/Ti³⁺, exhibits different storage voltages and imparted properties that are beneficial in designing different materials for various applications, such as NaTi₂(PO₄)₃ for aqueous batteries, P2-Na_0.66Li_0.22Ti_0.78O₂ for long cycle batteries, etc. The substitution of Ti disrupts the structure ordering and expands the crystal lattice which prolongs the cycle life and enhances rate performance. In addition, Ti-substituted Na_2/3MnO₂ prevents H₃O⁺ intercalation, making it a perfect cathode material for aqueous sodium-ion batteries. Developing practical cathode and anode materials for sodium-ion batteries is a big challenge. In spite of the advances discussed in this review, there is still substantial room for further improvement.

Although the above-mentioned challenges still remain, the recent progress in the development of Ti-compounds is significant. There is now an improved understanding of the effects of Ti addition on the crystal structure, including the lattice expansion and distortion, the cation and vacancy orderings, as well as on the electronic configurations of the redox couples. This knowledge helps researchers to identify other dopants and materials for designing and developing new high-performance electrode materials. We believe that further studies will lead to more exciting results. This review highlights the importance of designing and exploring disordered transition metal/charge in electrodes as an effective strategy to improve Na-storage properties, including rate capacity, cycle stability and high-power performance.

Author Contributions

YW wrote the draft. All authors contributed to the writing, discussion, and revision of the final version.

Funding

Centre d'excellence en électrification des transports et stockage d'énergie, Québec, Canada.

Conflict of Interest Statement

YW, WZ, AG, CK, and KZ are employed by Hydro Quebec.

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