Lithium Titanate-Based Anode Materials

Abstract

Li₄Ti₅O₁₂ is a potential Li-ion battery anode material of for use in large-scale energy storage, considering its high safety, excellent cycling stability, environmental friendliness and low cost. It also presents attractive performance as anode material for Na-ion batteries. Nanostructuring and carbon coating endow Li₄Ti₅O₁₂ electrodes with excellent rate capability by overcoming its intrinsic sluggish Li-ion diffusivity and low electronic conductivity. The gassing issue of Li₄Ti₅O₁₂-based batteries is the main obstacle that hinders its practical application. Surface coatings on Li₄Ti₅O₁₂ electrode or Li₄Ti₅O₁₂ particles as well as employing effective additives for electrolyte are potential approaches to form stable film on Li₄Ti₅O₁₂ electrode to circumvent the gas generation problem. The cathode materials, electrolyte systems as well as capacity matching of the two electrodes impose important influences on the cycling performance of Li₄Ti₅O¹²-based batteries.

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Graphitic carbon is the most widely used anode material in commercial Li-ion batteries due to its low lithiation potential, long cycle life, abundant resources and low cost. However, Li-ion batteries using graphite as anode material give rise to rate, safety and life problems. During lithium intercalation/deintercalation process, graphite undergoes a considerable volume change (~10 % [1]), which could cause particle cracking and even peeling off of anode film from the current collector, leading to gradual capacity degradation of the electrode [2, 3]. Safety concerns arise when the cells experience fast charging, long-term cycling, or low temperature charging owing to the propensity of formation of lithium dendrites, which is induced by the low lithiation potential of the graphite anode (close to 0 V vs. Li/Li⁺) and the low lithium ion diffusivity in the graphite lattice [4, 5]. As an alternative anode material to carbon, Li₄Ti₅O₁₂ has been extensively studied for the potential use in large-scale Li-ion batteries. Li₄Ti₅O₁₂ shows stable charge/discharge platform at ca. 1.55 V versus Li/Li⁺, and possesses excellent cycling stability and unique safety characteristic owing to its negligible volume change and high redox potential upon Li-ion intercalation/deintercalation. However, coarse Li₄Ti₅O₁₂ exhibits poor rate performance because of its low electronic conductivity and sluggish lithium ion diffusivity [6, 7]. In some cases, especially when aging at elevated temperature or cycling in a long-term regime, gas generation frequently occurs in Li₄Ti₅O₁₂-based batteries [8]. In past decades, many efforts have been devoted to overcoming these problems and significant advancements have been achieved, which make Li₄Ti₅O₁₂ viable for practical application in batteries for various electrical energy storage, such as electric/hybrid electric/plug-in hybrid electric vehicles (EV/HEV/PHEV), grid load leveling, integration of renewable energy sources, etc.

1 Lattice Structure and Electrochemical Characteristics

The Li₄Ti₅O₁₂ compound has a defective spinel structure with cubic space group ${\text{Fd}}\overline{3} {\text{m}}$ [9], where the 32e positions are occupied by oxygen atoms, the tetrahedral 8a positions are taken by Li atoms and the octahedral 16d positions are shared by Ti and Li atoms in a ratio of 5:1. Upon lithiation, Li atoms at the 8a sites are moved to the 16c sites and the additional Li atoms fill the remaining 16c vacancies, resulting in phase transformation from spinel structure of Li₄Ti₅O₁₂ ([Li]_8a[Li_1/3Ti_5/3]_16dO₄) to rocksalt structure of Li₇Ti₅O₁₂ ([Li₂]_16c[Li_1/3Ti_5/3]_16dO₄) [9, 10]. The two crystal structures are illustrated in Fig. 1 [11]. The structural transformation generates only a slight lattice contraction, from 8.3595 Å to 8.3538 Å [9] for Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂, respectively, corresponding to a volume shrinkage of about 0.2 %. Due to the negligible volume change, Li₄Ti₅O₁₂ is widely considered to be a “zero strain” material for lithium insertion and removal. This gives active Li₄Ti₅O₁₂ superior structural stability and guarantees the mechanical integrity of the electrode by maintaining good Li₄Ti₅O₁₂ particle contact with binder and conductive carbon matrix during charge/discharge process, leading to an extremely long cycle stability.

Fig. 1

a Li₄Ti₅O₁₂ spinel structure. b Li₇Ti₅O₁₂, rock salt. Blue (dark) octahedra represent lithium, and green (light) octahedra represent disordered lithium and titanium (reproduced with permission from [11])

One mole of Li₄Ti₅O₁₂ can uptake three moles of Li ions, corresponding to a theoretical specific capacity of 175 mAh g⁻¹. The electrochemical lithiation of Li₄Ti₅O₁₂ is commonly regarded as a two-phase process between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂, which delivers a long and flat plateau at 1.55 V versus Li/Li⁺ [9, 10]. The typical charge/discharge curve of Li₄Ti₅O₁₂ is depicted in Fig. 2 [12]. The high redox potential makes lithium dendrite formation impossible, thereby averting the safety hazards of negative electrodes that operate close to the potential of metallic lithium [13]. Furthermore, the high operating potential of Li₄Ti₅O₁₂ can effectively avoid the formation of solid-electrolyte interface (SEI) film. Therefore, the consumption of lithium from cathode material for the generation of SEI film can be eliminated and the risk of SEI film decomposition at high temperature, which may release heat thereby triggering the reaction of cathode with electrolyte and consequently generating even large quantity of heat, can be decreased, making the electrode possess high coulombic efficiency and good thermal stability [14].

Fig. 2

Typical charge-discharge curves of Li₄Ti₅O₁₂ versus Li for 1st, 20th, 40th and 60th cycles at 0.5 C (reproduced with permission from [12])

2 Electronic Conductivity, Ionic Diffusivity and Lithiation/Delithiation Mechanism

Li₄Ti₅O₁₂ is insulating in character due to the large bandgap between the occupied oxygen p-states and the empty Ti d-states. Literature reports a wide-ranging bandgap width for Li₄Ti₅O₁₂ from 1.8 to 3.8 eV determined by experiments [15–18] to 1.7–2.3 eV calculated by ab initio [19–23]. The relatively low calculated value compared to that experimentally determined is not surprising because density functional theory (DFT) calculation tends to underestimate the bandgap of materials [19, 24]. The large bandgap endows Li₄Ti₅O₁₂ with a low electronic conductivity <10⁻¹³ S cm⁻¹ [25]. However, after lithium ions are inserted with concomitant incorporation of electrons, the Ti d-states become partly filled, and the electronic structure of Li₇Ti₅O₁₂ changes to be metallic [26]. This means that Li₄Ti₅O₁₂ is insulating only at the beginning of lithiation process.

Ionic diffusivity of active materials is another factor dominating electrode reaction kinetics. Li₄Ti₅O₁₂ has a low Li ion diffusion coefficient on the order of 10⁻⁸–10⁻¹⁵ cm² s⁻¹ [27–31]. The huge difference in the reported Li ion diffusivities of Li₄Ti₅O₁₂ is attributable to the different testing methods and the different lithiation depths of the employed electrode. The low electronic conductivity and poor ionic diffusivity lead to poor rate performance of Li₄Ti₅O₁₂ anode and thus limit the practical and direct application of coarse Li₄Ti₅O₁₂ in high-power batteries. To overcome this problem, numerous methods have been devised to enhance the electronic and/or ionic conductivities of Li₄Ti₅O₁₂, including lattice doping, surface coating, compositing and nanostructuring (see Sect. 3), with the eventual aim of improving the rate-capability.

It is imperative to understand the lithium insertion and extraction mechanism that affects the structural stability during cycling and the kinetic performance of the Li₄Ti₅O₁₂ electrodes. The electrochemical lithiation and delithiation of Li₄Ti₅O₁₂ is believed to occur through a two-phase reaction between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂, which is responsible for the very flat voltage plateau around 1.55 V versus Li/Li⁺ [9]. The electrochemical lithium insertion in the voltage range of 1.0–2.5 V versus Li/Li⁺ causes the filling of 16c sites and at the same time drives the original lithium to migrate from 8a to 16c sites, leading to the full occupation of 16c sites in Li₇Ti₅O₁₂. The evacuation of lithium ion from 8a sites is most likely due to the Coulombic repulsion between nearest Li ions occupying 8a–16c sites (separated at a distance of 1.81 Å) [32]. The lithiation/delithiation process of Li₄Ti₅O₁₂ can be described by Eq. (1). Further lithiation gives rise to the partial re-occupation of 8a site, resulting in the formation of Li_8.5Ti₅O₁₂ at a low potential (ca. 0.05 V vs. Li/Li⁺) accompanied by an approximately 0.4 % lattice expansion, which is confirmed by experiment and first principle calculation [26, 33, 34]. Although the framework of [Li₁Ti₅]_16dO₁₂ is not changed upon lithium insertion down to low potential, a structural distortion may be induced. The insertion of 4.5 mol lithium per Li₄Ti₅O₁₂ provides a theoretical capacity of ca. 262 mAh g⁻¹, 1.5 times higher than that of_. Li₇Ti₅O₁₂.

$$\begin{aligned} & [{\text{Li}}]^{8a} [\;]^{16c} [{\text{Li}}_{ 1 / 3} {\text{Ti}}_{ 5 / 3} ]^{16d} {\text{O}}_{4}^{32e} + {\text{x}}e^{ - } + x\,{\text{Li}}^{ + } \overset {{\text{ca}}.\;1.55\;{\text{eV}}\;{\text{vs}} .\;{\text{Li}}/{\text{Li}}^{ + } } \leftrightarrows \\ & (1 - x)[{\text{Li}}]^{8a} [\;]^{16c} [{\text{Li}}_{ 1 / 3} {\text{Ti}}_{ 5 / 3} ]^{16d} {\text{O}}_{4}^{32e} + x\text{ }[\;]^{8a} [{\text{Li}}_{2} ]^{16c} [{\text{Li}}_{ 1 / 3} {\text{Ti}}_{ 5 / 3} ]^{16d} {\text{O}}_{4}^{32e} \\ \end{aligned}$$

(1)

The observation of co-occupied 8a and 16c sites in the lithiated intermediate product Li_4+xTi₅O₁₂ has led to reconsideration of the lithiation/delithiation mechanism of Li₄Ti₅O₁₂. Wagemaker et al., based on neutron and X-ray diffraction measurements, suggested that the electrode reaction of Li₄Ti₅O₁₂ is not a two-phase reaction but a solid-solution process at room temperature, with 8a and 16c sites being co-occupied. With increasing lithiation depth, the occupancy at 16c sites increases while that at 8a sites decreases gradually. The real two-phase reaction between Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂ is only stable below 100 K [32]. The mixed occupation at 8a and 16c sites was also reported in a single-crystal study [35]. However, more detailed investigation revealed that the solid solution can be described as well dispersed distinct domains with either 8a or 16c Li occupation. The domain’s length scale is less than 10 nm at 373 K [36]. The two-phase character is clearly observed by aberration-corrected scanning transmission electron microscopy (STEM) [37], which shows a sharp, dislocation free coherent heterophase boundary (as illustrated in Fig. 3). This is beneficial for Li ion migration through the grain-boundaries and thereby contributes much to the good rate-capability of Li₄Ti₅O₁₂. The two-phase mechanism for lithiation/delithiation usually corresponds to a poor rate-capability due to the existence of strain and interfacial energy between the two phases.

Fig. 3

Interfacial structure in a chemically lithiated Li₄Ti₅O₁₂ sample with approximately 0.15 mol Li insertion per formula unit along the [110] direction. a ABF image near the interface between Li₄Ti₅O₁₂ phase (region 1) and Li₇Ti₅O₁₂ phase (region 2). The yellow (light) line indicates the boundary of the interface. b Colored ABF image of the two phases near the interface, where the 8a sites occupied in Li₄Ti₅O₁₂ and the 16c sites occupied in Li₇Ti₅O₁₂ are marked as yellow (light) and black dots, respectively (reproduced with permission from [37])

Curved charge-discharge voltage profiles in high potential range (>1.55 V) and extra specific capacity exceeding its theoretical value are commonly observed for nanosized Li₄Ti₅O₁₂ electrodes. This is believed to be associated with the energetically favorable Li occupation at 16c sites near the surface region and high accumulation of Li ions at the surface layer, because the surface environment helps relax the strain caused by the repulsion between Li ions co-occupied at 8a and 16c sites [38]. Deep lithiation can lead to a surface composition above that of Li_8.5Ti₅O₁₂ and therefore cause large structural distortion, or surface reconstruction or even mechanical failure of a thin surface layer. These changes will passivate the particle surface and deteriorate the electrochemical performance, especially the rate-capability of Li₄Ti₅O₁₂ electrodes. In addition, the surface structure variation of nanosized Li₄Ti₅O₁₂ can cause irreversible capacity loss, leading to a lower initial coulombic efficiency. The surface structure of nanoparticles contributes more fraction to the whole particle structure as compared to that of the bulk material. Therefore, from the point of view of overall performance, there is an optimum particle size for Li₄Ti₅O₁₂ electrodes depending on the voltage windows, although nanoparticles could significantly reduce the diffusion distance of Li ions. As lithium ion diffusion conducts along the 16c-8a-16c pathway [39, 40], therefore, Li occupation at 8a sites in addition to 16c sites will hinder the diffusion process and further restrict the rate performance of Li₄Ti₅O₁₂ electrodes.

3 Advances in Performance Improvement

The superior structural stability endowed by the “zero strain” characteristics upon lithiation/delithiation, the excellent safety feature ensured by the high redox potential and the resource abundance in raw materials render the Li₄Ti₅O₁₂ a promising anode material of lithium ion batteries used for stationary energy storage and electric vehicles. At the same time, the demand to further improve the unsatisfactory rate performance and resolve the gas generation issue of Li₄Ti₅O₁₂ electrodes has motivated the design and preparation of Li₄Ti₅O₁₂ materials with novel particle morphology or chemical composition. With these efforts, great progress has been made in optimization of electrochemical properties and better understanding of electrode reaction kinetics of Li₄Ti₅O₁₂ electrodes. Recently, Li₄Ti₅O₁₂ was found to be a good Na storage material, which make it be another research hot-spot as electrode material making it another source of research activity surrounding its possible use as electrode material for Na-ion batteries.

3.1 Rate-Capability

Rate-capability is one of the important electrochemical properties of batteries, which ensures a high and stable delivery of electrochemical capacity under high current density for the batteries. Many factors can affect the rate-capability of electrodes, including active material features (electronic and ionic conductivity, electrochemical activity, particle size and morphology), electrode recipe, and electrode geometric dimension, etc. Here, we place an emphasis on the intrinsic properties of Li₄Ti₅O₁₂ active material that affect the electrochemical performance under high current density environments.

3.1.1 Lattice Doping

Lattice doping is a common strategy to improve the electronic and ionic conductivity of materials by producing effectively charged point defects and/or altering lattice parameters. Donor doping with high valence elements substituting for Li (Mg²⁺, Al³⁺, La³⁺, Ca²⁺, Zn²⁺, Sn⁴⁺) [25, 41–46] or Ti (Nb⁵⁺, W⁶⁺,V⁵⁺, Mo⁶⁺, Ta⁵⁺) [47–51] or O²⁻ (F⁻, Br⁻) [43, 52, 53] can yield mixed valence Ti³⁺/Ti⁴⁺ as charge compensation and thereby generate n-type electronic conduction in Li₄Ti₅O₁₂. The Mg substitution increases the conductivity of Li_4−xMg_xTi₅O₁₂ by several orders of magnitude from 10⁻¹³ S cm⁻¹ for x = 0 to 10⁻² S cm⁻¹ for x = 1 [25]. The rate capability of Li₄Ti₅O₁₂ can be significantly improved by appropriate Ca substitution for Li sites, delivering a specific capacity of ~120 mAh g⁻¹ at 20 C in the cut-off voltages of 2.5–1.0 V [44]. The Nb-doped Li₄Ti_4.95Nb_0.05O₁₂ exhibits an enhanced rate capability with a reversible capacity of 135 mAh g⁻¹ at 10 C and 127 mAh g⁻¹ at 20 C [47]. The generation of electronic defects by aliovalent ion doping can enhance the electronic conductivity of materials by increasing the delocalized electron concentration, while the increase in lattice parameter by large size ion substitution or oxygen vacancy generation can facilitate the lithium ion diffusion in lattice, both of which are essential for a good rate-performance of electrode. It is reported that some dopants, such as Ru, W, Sr and Zr, increase the lattice parameter of Li₄Ti₅O₁₂, and therefore promote lithium ion diffusion [17, 48, 54, 55]. In addition, the isovalent doping of Na for Li [56, 57] and the Li substitution for Ti (Li excessive Li_4+xTi_5−xO₁₂) [58, 59] have a positive effect in improving the rate performance of Li₄Ti₅O₁₂, which can be related to the enlarged lattice parameter induced by large ion substitution or oxygen vacancy generation.

Because the lithium ion diffusion in the lattice of Li₄Ti₅O₁₂ during charge/discharge process occurs via 8a-16c-8a route, the occupancy of foreign ions at 8a site may affect the lithium insertion kinetics and even the specific capacity of doped Li₄Ti₅O₁₂ [41, 42, 60]. In spinel Li₄Ti₅O₁₂, the same Li ions take two different sites (8a and 16d) while the same 16d sites are occupied with two different ions (Li and Ti). To elucidate the exact occupying site of the dopant ions in Li₄Ti₅O₁₂, especially for Li substitution, is extremely important for understanding the doping mechanism and the resultant electrochemical performance variation. In this regard, many advanced techniques are employed to probe the structural details of the doped Li₄Ti₅O₁₂ that should be responsible for the variation of the electrochemical properties, including Electron energy-loss spectroscopy (EELS), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), ⁷Li nuclear magnetic resonance magic-angle spinning (NMR-MAS), inductive couple plasma-atomic emission spectrometry (ICP-AES), neutron diffraction (ND) as well as first principle calculation [17, 53, 60–63].

From the viewpoint of defect chemistry, either lower valence ion substitution for Ti site or charge compensation substitution for both Li and Ti sites (co-doping) cannot cause charged electronic point defects in Li₄Ti₅O₁₂ for improving the electronic conductivity. However, several reported works demonstrated that the rate-capability of Li₄Ti₅O₁₂ can be remarkably enhanced by low valence doping at Ti sites, such as Li₄Ti_5−xM_xO₁₂ (M = Mn²⁺ [63], Al³⁺ [64], Sc³⁺ [65]), and charge compensated co-doping for Li and Ti sites, such as Li_4-x/3M_xTi_5−2x/3O₁₂ (M = Cr [22], Al [66]), Li_3.9Ni_0.15Ti_4.95O₁₂ [54] and Li_3.95M_0.15Ti_4.9O₁₂ (M = Al, Ga, Co) [67]. The possible reason for the improvement is that these kinds of doping change the electronic structure and narrow the band-gap energy, leading to a decreased activation energy for electron conduction.

The substitution of elements with fixed valence for Ti in Li₄Ti₅O₁₂ can commonly enhance the structural stability against the shock of high current density, therefore, co-doping with two different elements, one with fixed valence for Ti and another acting as donor dopant for Li or O, can be expected to yield a good electrochemical performance under fast charge/discharge condition. The Mg, Zr co-doped system Li_3.95Mg_0.05Ti_4.95Zr_0.05O₁₂ displays excellent rate-capability and cycling stability [68].

Foreign element doping can sometimes cause the change of particle size because it usually alters the total energy of the lattice, the specific surface energy of crystals as well as the ion diffusion activities. Small particle size reduces the diffusion distance of Li ions and provides more surface area to come in contact with the electrolyte solution for electrode reaction, leading to an improved electrochemical property, especially the rate-capability. Doping with Sr, La and Zr was reported to decrease particle size and produce less particle agglomeration, which are part of the reasons that contribute to the improved rate performance of Li₄Ti₅O₁₂ [54, 69, 70].

In fact, the lattice ion doping may have influence on not just one aspect but simultaneously several aspects of the properties of the host material, such as electronic structure, point defect species and concentration, lattice distortion, lattice energy and specific surface energy, which in turn exert effect on the electronic conductivity, Li ion diffusivity, particle size and facet orientation. Therefore, the improvement of electrochemical performance by lattice doping is usually a synergistic effect of several factors. To effectively regulate the properties of Li₄Ti₅O₁₂, it is important to distinguish the dominant factors and establish the correlation among the dopant feature, electronic and lattice structure and electrochemical performance.

3.1.2 Surface Modification

Carbon coating is a common approach to improve the electrochemical performance of many electrode materials, including rate-capability and cycling stability. Owing to its high electronic conductivity and good chemical stability, carbon layers can remarkably enhance the electronic conductivity, increase interparticle contact, and help to form a uniform SEI layer on active particle surface and thus diminish the side reaction between the active material and electrolyte. In addition, the uniformly coated carbon layer can extend the effective reaction interface between the active particle and electrolyte, homogenize the current density and structural stress, and thereby improve the rate capability and cycling stability of the electrode. The carbon sources and carbon layer thickness have a strong impact on the physical and electrochemical performance of carbon coated Li₄Ti₅O₁₂/C electrodes [71–73]. The commonly used carbon sources are glucose, sucrose, pitch, epoxy, polyacrylate acid (PAA), citric acid (CA), maleic acid (MA), polyvinyl alcohol (PVA) and polyaniline (PANI) [73–78]. If the carbon coating is deposited via a CVD process, toluene vapor carried by an inert gas is usually employed as the carbon source [72, 79]. When the carbon coating process is required to carry out at low temperature to avoid the decomposition or vaporization of substance, acetylene is suggested.

The degree of graphitization of coated carbon has an effect on the electronic and ionic conductivity of the coated Li₄Ti₅O₁₂/C particles. Although high crystallinity of the carbon layer corresponds to a high electronic conductivity, it is actually unfavorable for lithium ion diffusion if the graphitic carbon grows with an orientated plane along the particle surface [72, 80], as illustrated in the scheme of Fig. 4. Therefore, there is a compromise between the electronic and lithium ionic conductivities when optimizing the carbon coating parameters. To avoid excessive graphitization, high calcination temperature for carbon source pyrolysis should be averted. Similarly, a thick carbon layer is not recommended from the point of view of lithium ion diffusion (insertion/extraction). The optimized carbon layer thickness is in the range of 0.7–5 nm [72, 80, 81]. Figure 5 shows the effect of carbon content on the rate performance of Li₄Ti₅O₁₂/C electrodes [77].

Fig. 4

Schematic illustration showing the effect of carbon coating on the electron and lithium ion conductions on a surface of carbon coated Li₄Ti₅O₁₂ (reproduced with permission from Ref. [80])

Fig. 5

Effect of carbon content on the rate performance of Li₄Ti₅O₁₂/C electrode (reproduced with permission from Ref. [77])

An in situ Raman study revealed that the defects and vacancies existing in the carbon coating layer provide passages for lithium ion diffusion, and thus can promote the interfacial electrode reaction [77]. The N-doped carbon, when compared with pristine carbon, shows better improvement in the rate capability and cycling stability of Li₄Ti₅O₁₂/C electrodes due to the enhanced electronic conductivity [79, 82, 83]. Other advantages of carbon coatings on Li₄Ti₅O₁₂ include (1) limiting the particle growth during calcination and thus shortening the lithium ion diffusion distance during charge/discharge process [74, 75]; (2) reducing Li₄Ti₅O₁₂ to generate Ti^3+/4+ mixed valency on the particle surface, enhancing the electronic conductivity [78, 84].

In order to achieve high tap density of Li₄Ti₅O₁₂ powders, large secondary particles composed of small primary particles are prepared. As shown in the work reported by Zhu et al., nano-TiO₂ was first coated with carbon by mixing with sugar and calcining at 600 °C and then ball-milled with Li₂CO₃, followed by spray drying and further calcining at 800 °C to obtain nanoprous micro-sphere LTO/C particles [75]. Shen et al. proposed a novel strategy for preparation of core/shell structured Li₄Ti₅O₁₂/C nanoparticles via a simple solid-state reaction method by using metal oxyacetyl acetonate as titanium and carbon sources [85]. Nanosized TiO₂ particles with a carbon coating was first formed during heating process and then lithium (Li₂CO₃) diffused through the carbon layer to react with TiO₂ in a limited space, forming nano-sized Li₄Ti₅O₁₂ coated with a thin and discrete carbon layer. This specially synthesized Li₄Ti₅O₁₂/C structure displays an excellent rate capability, ca. 53 % of the capacity at 0.2 C is delivered at 90 C. With a similar method, Wang et al. prepared nano-sized Li₄Ti₅O₁₂ particles with double surface modification layers of Ti³⁺ and carbon with polyaniline (PANI) as a carbon precursor, which prevents the Li₄Ti₅O₁₂ particle growth during heat treatment and simultaneously reduces the LTO particle surface to generate Ti³⁺-containing layer [78].

Besides carbon coating, metal nanoparticles are often employed to modify the surface of Li₄Ti₅O₁₂. The metal nanoparticles (e.g. Au [86], Ag [87, 88], Cu [89]) with a size range of 2–10 nm and highly dispersed on the Li₄Ti₅O₁₂ particle surface via a wet chemistry route can enhance the electrical contact between Li₄Ti₅O₁₂ particles and the current collector, promoting the electrode reaction kinetics and thereby improving the rate performance. As shown in Fig. 6, the Ag modified Li₄Ti₅O₁₂ nanocomposite delivers an excellent rate performance with a specific capacity of 131 mAh g⁻¹ at 30 C [88].

Fig. 6

TEM images and electrochemical performance of Li₄Ti₅O₁₂ and Li₄Ti₅O₁₂/Ag composite (reproduced with permission from Ref. [88])

Surface modification with oxides is another strategy to improve the rate performance of Li₄Ti₅O₁₂ anode material. Feng et al. [90] reported a modification of Li₄Ti₅O₁₂ with an aqueous CrO₃ solution, which leads to the generation of Li₂CrO₄, Cr₂O₅ and anatase TiO₂ on the Li₄Ti₅O₁₂ particle surface. The first two have a positive effect in improving the rate capability of Li₄Ti₅O₁₂, resulting in a capacity improvement of ca. 60 % from its original 80 to 130 mAh g⁻¹ at 30 C. CeO₂ is also suggested as a suitable coating oxide for improving the rate performance of Li₄Ti₅O₁₂ electrodes, which could enhance both electronic and lithium ionic conductivity because partial CeO₂ enters the Li₄Ti₅O₁₂ lattice as a dopant [91]. The nominal compositions Li₄Ti₅Cu_xO_12+x with two spinel phases Li₂CuTi₃O₈ and Li₄Ti₅O₁₂ were synthesized by Wang et al. [92]. The component of Li₂CuTi₃O₈ is decomposed into Cu, Li₂O and Li₄Ti₅O₁₂ during the first lithiation process, and the in situ generated Cu dispersing uniformly with Li₄Ti₅O₁₂ promotes the electron transport and improves the rate performance of the Li₄Ti₅O₁₂-based dual-phase electrode. TiN with high electronic conductivity is also employed as a surface modification material to facilitate the electron transport and promote the electrode reaction kinetics of Li₄Ti₅O₁₂, which can be generated on the particle surface of Li₄Ti₅O₁₂ by simply thermal treating in NH₃ atmosphere [93, 94]. Besides the formation of TiN, the surface of Li₄Ti₅O₁₂ particles is reduced in NH₃ atmosphere during heat treatment, leading to the generation of partial Ti³⁺, which can also contribute to the fast electrode reactions.

Heat-treating the sample in a hydrogen containing atmosphere at high temperature [6] or immersing the sample in formaldehyde aqueous solution at room temperature can also induce the reduction of Ti ions from Ti⁴⁺ to Ti³⁺, as confirmed by XPS examination, and thereby increase the electronic conductivity of Li₄Ti₅O₁₂ [95]. Wolfenstine’s work revealed that the electrical conductivity of Li₄Ti₅O₁₂ can be increased from less than 10⁻⁹ to 10⁻⁵ S cm⁻¹ after heat-treated in 3 vol.% H₂/Ar for 12 h at 800 °C [6]. Therefore, it is expected that high performance Li₄Ti₅O₁₂ could be obtained by preparing or post heat-treating the materials in a reducing atmosphere.

3.1.3 Compositing

Due to the extremely high electronic conductivity, graphene and carbon nanotubes (CNT) are frequently incorporated into electrode materials, including Li₄Ti₅O₁₂, to enhance the rate performance. Either directly mixing with CNT or in situ growing of Li₄Ti₅O₁₂ on CNT can create Li₄Ti₅O₁₂-based composites with a remarkably improved rate capability [96–98]. In order to ensure a high electrical contact area, the CNT is commonly employed together with amorphous carbon to enhance the electrode performance. The Li₄Ti₅O₁₂/C/CNT composite with a total carbon amount of 6 wt.% was reported to exhibit a reversible capacity of more than 140 mAh g⁻¹ at 10 C-rate [98]. Compared with CNT, graphene can deliver a even greater improvement in rate performance and cycling stability of Li₄Ti₅O₁₂ electrodes due to its high aspect ratio, especially the flexible feature, which enables good contact between graphene sheets and Li₄Ti₅O₁₂ particles, allowing a fast charge transfer process in the electrode reaction. Oh et al. [99] reported a graphene-wrapped Li₄Ti₅O₁₂ composite, which exhibits an excellent rate performance with a reversible capacity of 147 mAh g⁻¹ at 10 C and 105 mAh g⁻¹ at 100 C. For the synthesis process, as illustrated in Fig. 7, the graphene oxide wrapped TiO₂ nanoparticles are preferentially prepared via an electrostatic interaction between graphene oxide (GO) and P25 (TiO₂) nanoparticles in an acid environment, which are then mixed with Li₂CO₃ and calcined at 850 °C in 4 %H₂/Ar, leading to the formation of Li₄Ti₅O₁₂ particles tightly wrapped with graphene. In the synthesis step, the graphene acts as a buffer to prevent the Li₄Ti₅O₁₂ particle aggregation by entangling the particles within the graphene sheets, while in the electrode reaction step, it provides an electronic conducting network for fast electrode reaction. Another interesting work was conducted by Shen et al. [100], where the Li₄Ti₅O₁₂ particles were in situ formed on graphene sheets with a controlled size and a high loading density through a hydrothermal reaction. The Li₄Ti₅O₁₂ nanoparticles anchored onto graphene can effectively prevent the restacking of graphene sheets and provide void space for electrolyte well penetration. This kind of structure offers an excellent rate performance of ca. 85 mAh g⁻¹ at 60 C for 100 cycles (Fig. 8).

Fig. 7

Schematic for the effective graphene wrapping on individual LTO particles (reproduced with permission from Ref. [99])

Fig. 8

Capacity-voltage profile of Li₄Ti₅O₁₂ (a) and Li₄Ti₅O₁₂/GNS (b). Comparison of rate capabilities of Li₄Ti₅O₁₂/GNS with Li₄Ti₅O₁₂ (c). Cycle performance of Li₄Ti₅O₁₂/GNS electrode at different current densities (d). GNS means graphene nano sheets (reproduced with permission from Ref. [100])

The tight contact between Li₄Ti₅O₁₂ particles and reduced graphene oxide (rGO) sheets after solvothermal treatment was shown by micro-Raman and X-ray photoelectron spectroscopy studies, which revealed the formation of chemical bonds and internal electron transfer between Li₄Ti₅O₁₂ and graphene [101]. The transfer of a π electron from the C₆ unit of rGO to Li₄Ti₅O₁₂ will cause the partial occupation of the conduction band of Ti 3d t_2g in the particle surface area, leading to an enhanced charge transfer ability at the interface of Li₄Ti₅O₁₂ and graphene. This kind of hybridization between Li₄Ti₅O₁₂ and graphene was confirmed by first principle calculation based on DFT [79], which revealed the existence of strong bonding between the graphene coating layer and Ti-terminated Li₄Ti₅O₁₂ surface.

With the aim of increasing the specific capacity and enhancing the rate capability of Li₄Ti₅O₁₂, various metals and metal oxides with lithium storability are applied to composite with Li₄Ti₅O₁₂, such as Sn [102], SnO₂ [103], Fe₂O₃ [104] and CuO [105]. Most of those composites delivered a high initial specific capacity in concomitant with an unsatisfactory cycling stability, due to the large volume change of these active materials upon lithium insertion/extraction. A successful example for the compositing of Li₄Ti₅O₁₂ is the Li₄Ti₅O₁₂-TiO₂ composite, which delivers remarkably enhanced electrochemical properties in terms of specific capacity, cycling stability and rate capability when compared to pristine Li₄Ti₅O₁₂. The heterophase boundary between Li₄Ti₅O₁₂ and TiO₂ is considered to play an important role in improving the electrochemical performance of Li₄Ti₅O₁₂, which may help to store electrolyte and offer more channels for Li⁺ ion insertion/extraction reaction [106, 107]. A more rational mechanism of TiO₂ in improving the rate capability and reversible capacity of Li₄Ti₅O₁₂ is its lithium storability. TiO₂ in the form of rutile or anatase is an active material towards Li-ion storage with a theoretical capacity of ca. 336 mAh g⁻¹ and a pair of redox potential at ca. 1.7 and 2.0 V, respectively [108, 109]. It is reported that TiO₂ has fast lithium ion diffusivity [110]. During the first lithiation process, Li-ions can preferentially insert into TiO₂ to form Li_xTiO₂, in which the generated Ti³⁺ makes the Li_xTiO₂ phase a highly conductive phase and therefore capable of promoting the electrode reaction of Li₄Ti₅O₁₂/TiO₂ composites. Compared to the carbon coating layer, TiO₂ not only provides fast lithium ion and electron transportation, but also offers a high reversible capacity in the voltage range of 1.0–2.5 V, which endows the Li₄Ti₅O₁₂/TiO₂ composite with high specific capacity, excellent rate capability and stable cycling performance. Wang et al. prepared Li₄Ti₅O₁₂ nanosheets with rutile-TiO₂ at the edges via a facile solution-based route [111]. The rutile-TiO₂ as a coating layer enhances the lithium ion and electron conductivity of Li₄Ti₅O₁₂ and thus facilitates the electrode reaction kinetics, leading to an excellent electrochemical performance of high specific capacity and superior rate capability, as shown in Fig. 9.

Fig. 9

Electrochemical performance of LTO-RT-600 and LTO-600 NSs: a rate performance; b cycle performance at 5 and 20 C. LTO-600 NSs and LTO-RT-600 refer to pure LTO nanosheets and LTO nanosheets with a thin rutile-TiO₂ terminated layer at the edges (reproduced with permission from Ref. [111])

3.1.4 Nano-Structuring

The reversible capacity and cycling performance of Li₄Ti₅O₁₂ are greatly influenced by particle morphology. Reducing the geometrical size of particles is an important strategy to improve the rate performance of electrode materials. The small particle size provides a short distance for lithium ion diffusion and electron transport, and more surface area for electrode reaction, ensuring fast lithiation/delithiation kinetics and thereby a remarkably improved rate performance of the electrode. Additionally, nanoparticles usually have various defects and unsaturated coordination sites at the surface. As stated above, these defects sometimes induce new lithium storage mechanisms [38] or provide sites for the generation of conductive layers [79, 93, 94], and consequently contribute to some the improvement of electrochemical performance of Li₄Ti₅O₁₂. Nanostructured Li₄Ti₅O₁₂ with morphologies of nanosheets [112, 113], nanorods [114, 115], nanotubes [116], nanowires [117], nanoflakes [118] and nanoflowers [119–121], are designed and prepared via various routes, including solid-state, hydrothermal, sol-gel, microwave, combustion, molten salt, sonochemical, rheological phase, and spray pyrolysis, and improved electrochemical performance, especially rate-capability, was demonstrated.

The synthesis route and starting materials have strong influence on the crystal structure, particle morphology and therefore electrochemical performance of Li₄Ti₅O₁₂. Solvothermal techniques, including hydrothermal, is one of the important methods of preparing nanoparticles. Because no long distance diffusion of ions is required in the liquid phase, as is often encountered in solid-state reactions, the product can be usually finalized by a low-temperature post heat-treatment after the hydrothermal reaction, during which the intermediate product is subjected to the decomposition of remaining organic groups and lattice rearrangement of Li₄Ti₅O₁₂ particles. It is believed that the post heat treatment can improve the electrochemical stability of electrode materials [122]. By varying the species and concentration of surfactants and solvents, and controlling the pH value of solution, hydrothermal temperature and soaking time, various nanoparticles with distinct morphologies can be synthesized.

The Li₄Ti₅O₁₂ nanoparticles with homogenous spherical morphology in size of 10–20 nm were synthesized via a solvothermal route by using titanium tetra-isopropoxide and LiOH as the starting materials in a polyol medium at 235 °C, combined with a subsequent treatment of 500 °C for 5 h in air [123]. The solvent polyol is believed to play an important role in preventing particle agglomeration and creating well-dispersed nanoparticles. Compared to the microsized Li₄Ti₅O₁₂ (1–2 μm) prepared by solid state reaction, the synthesized nanoparticles exhibit high specific capacity and excellent rate capability with a reversible capacity of 159 and 137 mAh g⁻¹ at 30 C and 60 C, respectively. Feckl et al. reported a nanoscale porous framework of Li₄Ti₅O₁₂ composed of ultrasmall interconnected nanoparticles in the size range of a few nanometers (3–4 nm), which delivers a capacity of ca. 175 mAh g⁻¹ at 1–50 C and can maintain 74 % of the maximum capacity at an extremely high rate of up to 800 C (corresponding to 4.5 s charge/discharge time) without any decline for a thousand cycles (Fig. 10) [124]. Such a porously structured Li₄Ti₅O₁₂ was synthesized in tert-butanol by a solvothermal route with LiOtBu and Ti(OBu)₄ as the starting materials in the presence of Pluraonic polymer (P123). Their work demonstrates that besides the solvent, the similar reactivity of the precursors is essential for the formation of a stoichiometric compound via a solvothermal reaction.

Fig. 10

HR-TEM images of nanosized Li₄Ti₅O₁₂ heated at 400 °C (a) and 500 °C (b), respectively. Multicycling stability at different rates (c) and at a rate of 100 C (d). The gray and black symbols correspond to the samples heated at 400 °C and 500 °C, respectively. The open and the filled symbols correspond to charge and discharge cycles, respectively. The cut-off potentials are 1.0 V and 2.4 V versus Li. The thickness of the film is about 0.5 μm, corresponding to a loading of about 0.14 mg cm⁻² (reproduced with permission from Ref. [124])

Hollow structured Li₄Ti₅O₁₂ particles were frequently reported to have a remarkably enhanced rate performance and can be prepared by hydrothermal reaction [119] or SiO₂ microsphere-based template routes [125]. Electrospinning is a simple, versatile, fast and inexpensive technology to produce one-dimensional (1D) fibers at micro- or nanoscales. When combined with conventional sol-gel processing, it offers the possibility to produce ceramic or organic/inorganic composite fibers with either a solid, porous or hollow structure [126]. Li₄Ti₅O₁₂/C fibers were reported to be produced from the electrospinning technique [127, 128], in which the Li₄Ti₅O₁₂ nanopartices are coated with carbon or dispersed in carbon matrix to form Li₄Ti₅O₁₂/C 1D composite fibers. The unique structural characteristics of those fibers with well dispersed dual-phase structure, nanoscale diameters and high aspect ratios ensure a shortened distance for Li ion and electron transport and provide more contact area with the electrolyte for electrode reaction, leading to fast electrode reaction kinetics and therefore an excellent electrochemical performance, with a specific capacity of more than 140 mAh g⁻¹ achieved at a 10 C rate [128] (Fig. 11).

Fig. 11

a XRD pattern of 1D Li₄Ti₅O₁₂@C nanofibers, all the main diffraction peaks are indexed as spinel Li₄Ti₅O₁₂ with Fd3m space group; b and c Low and high magnification SEM images of the final annealed Li₄Ti₅O₁₂@C hierarchical nanofibers, these hierarchical nanofibers have a diameter of ca. 100–200 nm and are randomly oriented forming an interconnected fiber network; d charge/discharge capacities of Li₄Ti₅O₁₂@C hierarchical nanofibers over 1000 cycles at 10 C (reproduced with permission from Ref. [128])

Self-supported Li₄Ti₅O₁₂ nanosheet arrays grown directly on conductive Ti foil was prepared by hydrothermal reaction between Ti foil and LiOH solution [129]. The excellent stability of the well aligned self-supported Li₄Ti₅O₁₂ nanosheet enables the electrode to have flexibility while the advantages of good conductivity, high surface area and shortened Li ion diffusion distance endow the electrode with excellent electrochemical performance, a high capacity of 163 and 78 mAh g⁻¹ at 20 and 200 C, respectively, and a capacity of 124 mAh g⁻¹ after 3000 cycles at 50 C. Recently, self-supported Li₄Ti₅O₁₂/C nanotube arrays with uniform carbon layers on inner and outer tube surfaces was reported by Liu et al., which were directly grown on stainless steel foil via a template-based solution route [130]. The structure schematic, SEM images and electrochemical performance of such electrodes are shown in Fig. 12. The hollow structure expands the electroactive interface for electrode reaction and reduces the distance for Li-ion diffusion, while the carbon layer on the inner and outer surface of the tube enhances the electronic conductivity. Such electrode exhibits outstanding cycling performance of ca. 7 % capacity loss after 500 cycles at 10 C and excellent rate capability with a reversible capacity of 135, 105, and 80 mAh g⁻¹ at 30, 60 and 100 C, respectively.

Fig. 12

The structure schematic, SEM images and electrochemical performance of self-supported Li₄Ti₅O₁₂/C nanotube arrays electrode (reproduced with permission from Ref. [130])

Although nano-structured Li₄Ti₅O₁₂ particles could provide outstanding rate performance, the high specific surface area could induce high irreversible capacity loss and the low tap density reduce the volumetric energy density of batteries. Hollow structured particles especially cannot resist the high pressure rolling for practical electrode preparation. An effective way to overcome these problems while maintaining the excellent electrochemical performance is to fabricate hierarchical structures with microsized secondary particles composed of nanosized primary particles. The microsized particle increases the tap density while the nanosized particle and the connected pores inside the microsized particle provide not only a shortened distance for Li ion diffusion but also a percolative path for electrolyte penetration, thus maintaining all the advantages of nanoparticles. Such Li₄Ti₅O₁₂ particles with a nano-/micro-level combined structure were prepared by Amine et al. via a colloidal solution method, which deliver a high specific capacity and excellent rate-capability compared with microsized Li₄Ti₅O₁₂ particles, and show lower specific area impedance than microsized Li₄Ti₅O₁₂ and carbon electrodes when coupled with Li_1+xMn_2−xO₄ cathode material [131]. Lin et al. [132] and Shen et al. [133] prepared hierarchically porous Li₄Ti₅O₁₂ microspheres with nanosized primary particles and inside rich nanopores by using commercial TiO₂ powders with an average size of 10 nm and LiOH as starting materials via a hydro/solvothermal reaction route. These hierarchical particles exhibit excellent rate performance and high capacity retention over long-term cycles. Especially, a high tap density of 1.62 g cm⁻³ can be achieved for submicrospheres with a 60 mm secondary particle size and 20–100 nm primary particle size [132]. Such micro/nanoscale combined particles with high tap densities will find wide application in practical high energy batteries.

3.2 Gas Generation

Despite the various superiorities of Li₄Ti₅O₁₂, there is still one obstacle that hinders the practical application of Li₄Ti₅O₁₂ as anode material in lithium ion batteries. It suffers from continuous gas generation when aging or operating at elevated temperatures, which damages the cycle and calendar life and poses a serious safety issue for Li-ion batteries with Li₄Ti₅O₁₂-based anodes [8, 134–138]. Gas chromatography/mass spectrometry (GC/MS) examination reveals that the generated gas is composed primarily of H₂ with a minority of CO, CO₂, CH₄, C₂H₄, C₂H₆, etc. [136, 138, 139]. This phenomenon can occur in the beginning of the formation cycle period and can be observed after several hundred cycles. The gas generation in LTO-based chemistry is difficult to notice in small size cells or rigid packaged cells due to the small amount of gas released but becomes more noticeable in large scale soft-package cells. Three factors are considered to contribute to the gas generation of Li₄Ti₅O₁₂: lithiated Li₄Ti₅O₁₂, lithium salt and carbonate solvent [136, 140].

The lithium salt in electrolyte solvent was found to have an obvious impact on the gas generation. Compared with LiPF₆, LiBF₄ salt releases less gas in LTO-based batteries [136]. However, He et al. demonstrated that the lithium salt has little impact on the gassing reactions [8], whereas the solvent plays the more important role in the gas generation [8, 138]. They found that gassing reactions include decarboxylation, decaronylation and dehydrogenation of solvents, which are initiated not by PF₅, a reaction product of LiPF₆ with trace amount of water, but by the (111) plane on the outermost surface of Li₄Ti₅O₁₂ particles. The decomposition product of solvents depends on the molecular structure [138]. According to the results of IR and GC, linear carbonates produce mainly hydrogen and soluble species, while cyclic carbonates generates alkylene gas, Li₂CO₃ and dilithium alkyl carbonates. PC-contained electrolyte can form thicker/denser layer than the EC-contained on Li₄Ti₅O₁₂ surface, which can mitigate further decomposition of the solvents and thus generate minimum gases. By comparing different solvents with 1 mol L⁻¹ LiPF₆, Wu et al. revealed that DMC produces the maximum gases while PC + DMC (1:1 in vol.) the minimum gases. Besides the gases, anatase TiO₂ [8] and α-Li₂TiO₃ [141] are found as by-products on Li₄Ti₅O₁₂ particle surface, which may also exert influence on the cycling performance of Li₄Ti₅O₁₂ electrodes.

Among the various reasons for gas generation, the main suspect should be the lithiated Li₄Ti₅O₁₂ (Li_4+xTi₅O₁₂), which directly participates in the gas generation reaction by transferring electrons from LTO to the electrolyte resulting in the reduction of the electrolyte. The in situ XANES measurements on Li₄Ti₅O₁₂ electrodes at high temperature reveal the continuous shift of Ti K-edge energy to higher values during aging, implying the progressive increase in the average valence of Ti ions, which is caused by the electron loss due to the self-discharge of Li₄Ti₅O₁₂ electrodes. Lower temperature leads to a longer transition time from Ti³⁺ to Ti⁴⁺. Because the reduction reaction is carried out at the particle surface of Li₄Ti₅O₁₂, the particle surface chemistry [8] and particle morphology [137] have an important influence on the gas generation. Cutting off the transport path of electrons from Li₄Ti₅O₁₂ to electrolyte by forming a smooth surface layer on Li₄Ti₅O₁₂ should be an effective way to suppress the gas generation in LTO-based batteries. He et al. reported that addition of vinylene carbonate to the electrolyte of 1 M LiPF₆/EC + DMC + EMC could facilitate the rapid formation of a protective SEI film on Li₄Ti₅O₁₂ electrodes [137]. The chlorosilane additive also has an obvious effect in controlling the gas generation of Li₄Ti₅O₁₂ electrodes in electrolyte of LiPF₆ in EC/EMC [136]. Another approach to form a protective layer on Li₄Ti₅O₁₂ is to directly coat an inert layer onto Li₄Ti₅O₁₂ electrode or particles before cycling. The inert coating layer should be an electronic insulator but thin enough not to impede the lithium ion transport. Atomic layer deposition (ALD) [136, 140] and carbon coating techniques [142] are often employed to produce a protective thin layer on Li₄Ti₅O₁₂ electrode or particles, which can mitigate the gas generation problem. The carbon coating can help to form successive SEI film on Li₄Ti₅O₁₂ particles and thus prevent the gassing reaction between Li₄Ti₅O₁₂ and electrolyte. This has the added benefit in that the carbon coating can improve the rate-capability of Li₄Ti₅O₁₂ at the same time.

3.3 Performance as Anode Material for Na-Ion Battery

Apart from lithium-ion batteries, Li₄Ti₅O₁₂ can also be used as an electrode material for sodium-ion batteries. With the advantages of abundant and low cost of sodium sources, sodium-ion battery is deemed as an alternative of lithium-ion battery for large-scale energy storage applications [143]. Zhao et al. [144] first reported that Li₄Ti₅O₁₂ can be a Na-ion storage material, though the radius of Na ion (1.02 Å) is ca. 34 % larger than Li ion (0.76 Å). The sodiation behavior of Li₄Ti₅O₁₂ is much more complicated and quite different from the two-phase reaction of Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ system. It usually presents a three-phase separation mechanism [145], as described in Eq. (2), with a theoretical capacity of 175 mAh g⁻¹.

$$2 {\text{Li}}_{ 4} {\text{Ti}}_{ 5} {\text{O}}_{ 1 2} + 6 {\text{Na}}^{ + } + 6 {\text{e}}^{ - } \leftrightarrow {\text{Li}}_{ 7} {\text{Ti}}_{ 5} {\text{O}}_{ 1 2} + {\text{Na}}_{ 6} {\text{LiTi}}_{ 5} {\text{O}}_{ 1 2}$$

(2)

More detailed investigation reveals that the lithium insertion behavior into Li₄Ti₅O₁₂ is strongly particle size dependent [146]. A solid solution mechanism for sodium insertion is found in nano-sized Li₄Ti₅O₁₂ particles. The prepared Li₄Ti₅O₁₂ can deliver a reversible Na storage capacity about 155 mAh g⁻¹ with an average charge/discharge voltage of ~0.9 V versus Na⁺/Na, as illustrated in Fig. 13. Based on various in situ techniques, the Na⁺ ion apparent diffusion coefficient in Li₄Ti₅O₁₂ particles is estimated at around 10⁻¹⁶ cm² s⁻¹ [146, 147], which is consistent with the result of DFT calculation [145]. The sluggish diffusion kinetics of Na⁺ ions in Li₄Ti₅O₁₂ necessitates the preparation of nanosized Li₄Ti₅O₁₂ particles for suitable use as anode material in sodium-ion batteries.

Fig. 13

Elecrochemical performance of Li₄Ti₅O₁₂ in sodium-ion batteries. The Li₄Ti₅O₁₂ electrode with PVdF, NaAlg and Na-CMC as binder, respectively, was cycled in NaFSI/EC:DEC electrolyte at a current rate of C/10 (reproduced with permission from Ref. [145])

Liu et al. [128] prepared Li₄Ti₅O₁₂@C hierarchical nanofibers with tiny Li₄Ti₅O₁₂ nanoparticles embedded in carbon by an electrospinning technique, which exhibited high and stable reversible capacity of about 162.5 mA h g⁻¹ for 100 cycles at 0.2 C as anode material for Na-ion battery. The nanofiber shape combined with uniformly distributed nanosized Li₄Ti₅O₁₂ in carbon matrix offers short transport distance for Na ion and electron transport, high contact area with electrolyte, and good conducting phase percolation, ensuring excellent electrochemical properties.

A free-standing CNT/Li₄Ti₅O₁₂/C composite nanofiber with Li₄Ti₅O₁₂ nanoparticles and CNTs uniformly dispersing in a 1D carbon nanofiber matrix was also reported to deliver a good rate capability as anode material for Na-ion batteries [148]. Yu et al. [147] revealed the peseudocapacitive behavior of Li₄Ti₅O₁₂ upon Na ion insertion, especially for nanoparticles with sufficient surface defects, which contribute much of the specific capacity and rate-capability without trade-off of structural phase transformation.

4 Performance in Full-Cells

Because of the high safety, long cycling life and low cost, batteries with Li₄Ti₅O₁₂ as anode materials are ready for practical applications in the fields of electrical vehicles (EV) and large scale energy storage devices. However, the high redox potential of Li₄Ti₅O₁₂ results in some reduction of cell working voltage and thus the energy density. This issue can be overcome by coupling Li₄Ti₅O₁₂ anode with cathodes having relatively high redox potentials. Figure 14 displays the various voltages of Li-ion cells with Li₄Ti₅O₁₂ anode and different common cathode materials [149].

Fig. 14

Voltage of Li₄Ti₅O₁₂-based cells with different cathode materials. LMO—LiMn₂O₄, LCO—LiCoO₂, L333—LiNi_0.33Co_0.33Mn_0.33O₂, LFP—LiFePO₄, LCP—LiCoPO₄, LMP—LiMnPO₄ (reproduced with permission from Ref. [149])

Apparently, LiMn_1.5Ni_0.5O₄ (LMNO) electrode offers the highest voltage of 3.2 V when combined with Li₄Ti₅O₁₂ anode. The electrochemical performance of Li₄Ti₅O₁₂—containing batteries depends strongly on the match design of negative and positive electrodes. Wu et al. [150] compared the electrochemical performance of three cell designs with different LNMO-to-LTO (Li₄Ti₅O₁₂) loadings (positive-electrode limited, negative-electrode limited, and positive/negative capacity ratio = ~1) and demonstrated that the negative-limited LNMO/LTO full cells delivered the best electrochemical performance, with 98 % of the first cycle capacity after 1000 cycles. At the same time, the cells with Li₄Ti₅O₁₂ limiting capacity exhibits less limitation of electrolyte choice than the cells with LNMO limiting capacity [151]. Actually, due to the high redox potential of LMNO, the electrode/electrolyte interfacial reactivity at high potential is usually the dominating factor for the electrochemical performance of LMNO/LTO cells [152, 153]. Recently, Kim et al. [154] reported a Ti-substituted cathode material LiNi_0.5Mn_1.5−xTi_xO₄ (LMNTO), which delivers longer cycle life, higher cell operating voltage, higher coulombic efficiency and lower electrode polarization compared with Ti-free LMNO when paired with LTO negative electrode. Besides, the Ti substitution can improve the capacity retention of LMNTO/LTO cell at high temperature (45 °C). The improvement is mainly ascribed to the retardation of electrolyte oxidation at the cathode side.

Although LMNO provides high working voltage for LMNO/LTO cells, its high redox potential (ca. 4.8 V vs. Li⁺/Li) imposes stringent requirements on electrolyte systems. By contrast, LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂ and LiMn₂O₄ cathodes coupling with LTO can operate well in most of the conventional electrolytes and thus attracts more attention. In spite of the relatively lower working voltage, these battery systems deliver much stable cycling performance. Toshiba’s SCiB^™ rechargeable battery with LTO as anode and LiNi_1/3Co_1/3Mn_1/3O₂ as cathode exhibits excellent cycling stability with a capacity retention of 90 % after 10000 cycles and superior rate-capability taking only 6 min. to reach 80 % State of Charge level [155]. With respect to olivine LiFePO₄ (LFPO), owing to its intrinsic structural stability and electrochemical safety, the nanoengineered LFPO/LTO cell with a working voltage of 1.9 V presents especially promising performance with capacity loss rates of 0.003 %/cycle at 5 C rate after 200 cycles [156]. The mixed metal compound LiMn_yFe_1-yPO₄ (LMFP) has high redox potential at 3.6–4.1 V (vs. Li⁺/Li), while it is not high enough to decompose the conventional electrolyte solutions based on alkyl carbonate solvents. The full cell LMFP-LTO operates at two potentials of 2.5 V (80 %) and 2 V (20 %) and shows good rate capability, impressive cycling stability and high safety features, making it a competitive power system in load leveling applications [157].

5 Conclusion

Li₄Ti₅O₁₂ is a potential Li-ion battery anode material of for use in large-scale energy storage, considering its high safety, excellent cycling stability, environmental friendliness and low cost. Its intrinsic low electronic conductivity and sluggish Li-ion diffusivity has driven research to design and prepare nanosized particles with controlled morphologies with the aim of enhancing the rate-capability. The gassing issue of Li₄Ti₅O₁₂-based batteries is the main obstacle that hinders its practical application. Surface coatings on Li₄Ti₅O₁₂ electrode or Li₄Ti₅O₁₂ particles as well as employing effective additives for electrolyte are potential approaches to form stable film on Li₄Ti₅O₁₂ electrode to circumvent the gas generation problem. The cathode materials, electrolyte systems as well as capacity matching of the two electrodes impose important influences on the cycling performance of Li₄Ti₅O₁₂-based batteries.