Nano/micro structured porous Li₄Ti₅O₁₂ synthesized by a polyethylene glycol assisted hydrothermal method for high rate lithium-ion batteries

Abstract

In this study, a new polyethylene glycol (PEG)-assisted hydrothermal method was proposed to prepare nano/micro structured porous Li₄Ti₅O₁₂ for high rate lithium-ion batteries. The fabrication process involves a modified hydrothermal synthesis using PEG as a structure directing agent and a subsequent calcination procedure. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy and nitrogen adsorption–desorption analysis were used to characterize their morphologies and structures. Such Li₄Ti₅O₁₂ shows spheroidic morphology with a size of 1–4 μm, assembled by well-crystalline nanoparticles of ca. 15 nm. The nano/micro structured Li₄Ti₅O₁₂ presents rich pores of 10–70 nm with a specific surface area of 80.1 m² g⁻¹ and a pore volume of 0.45 cm³ g⁻¹. The Li₄Ti₅O₁₂ annealed at 600 °C has the perfect crystallization and developed porosity, and exhibits ultrahigh rate capability with a reversible capacity of 132 mA h g⁻¹ at 20 C, and excellent capacity retention over 100 cycles.

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Introduction

Lithium-ion batteries, as the most important energy storage and conversion device with prominent advantages of high energy and power density as well as long cycle life, have witnessed large-scale application in portable electronic devices, communication facilities, stationary energy storage systems and ever-enlarging markets of electric vehicles (EV) and hybrid electric vehicles (HEV).^1–3 Graphite is a widely used anode material, but the formation of dendritic lithium during charging due to its low Li-intercalation potential and poor reversibility cannot satisfactorily meet the safety and rate requirements for future applications in EV and HEVs.^4,5 Recently, spinel Li₄Ti₅O₁₂ has been extensively studied as a good alternative anode material for lithium-ion batteries.^1,6–10 Compared with graphite, the spinel Li₄Ti₅O₁₂ has a high stable operating voltage at approximately 1.55 V (vs. Li), which avoids the battery short circuit triggered by the formation of lithium dendrites to achieve safe operation. Moreover, the spinel structure Li₄Ti₅O₁₂, as a zero-strain insertion material with negligible volume change in the charge–discharge process, possesses excellent reversibility, structural stability and excellent cycling performance. Combined with virtues of low cost and environmental friendliness, the spinel Li₄Ti₅O₁₂ material has been demonstrated as the most promising anode material in practical energy application.^11,12

However, the inherent poor electrical conductivity and sluggish lithium-ion diffusion of Li₄Ti₅O₁₂ seriously limits its high rate capability.^13–15 Therefore, current researches on spinel Li₄Ti₅O₁₂ are mostly focused on improving its rate performance, which can be classified into two general strategies. One is to accelerate charge-transfer reaction by enhancing ionic diffusion and electronic conductivity via surface modification or ion doping, such as carbon coating,^14–16 doping with other cations,^17–19 and decorated on carbon nanotubes,²⁰ carbon nanofibers²¹ or graphene.²² The other strategy is to design nanoscale Li₄Ti₅O₁₂ with various morphology such as nanoparticle,^23,24 nanofiber²⁵ or nanosheet,²⁶ which facilities to improved kinetic performance by reducing the transport path lengths of lithium ions and electrons. However, these nanomaterials often suffer from agglomeration, low tap density and difficulties in the electrode film coating process. To overcome such a barrier, fabricating nano/micro structured materials is an effective way to take advantage of both the nanoscale primary building blocks and microscale secondary assemblies, which has become an important trend for developing high performance Li-ion batteries. Zhu et al. prepared microsized Li₄Ti₅O₁₂ particles (10–20 μm) accumulated by nano-sized primary particles (∼200 nm) by the spray drying method.²⁷ Jung et al. prepared microscale (1–2 μm) secondary Li₄Ti₅O₁₂ particles composed of nanoscale (<100 nm) primary particles by solid-state reaction.²⁸ Hierarchical hollow microspheres assembled from N-doped carbon coated Li₄Ti₅O₁₂ nanosheets²⁹ and Li₄Ti₅O₁₂ nanosheets stacked by ultrathin nanoflakes³⁰ have also been synthesized. These Li₄Ti₅O₁₂ materials all display much improved rate capability and excellent cycle durability, indicating the superiority of the unique nano/micro structure for high performance lithium-ion batteries.

Herein, we report a simple method to prepare nano/micro structured, spherical-like Li₄Ti₅O₁₂ assembled by well-crystalline nanoparticles via a modified hydrothermal synthesis using polyethylene glycol (PEG) as a structure directing agent and a subsequent calcination procedure. The unique microstructure endows the as-prepared Li₄Ti₅O₁₂ exhibits excellent rate performance with a high reversal capacity up to 132 mA h g⁻¹ at 20 C and excellent capacity retention over 100 cycles, making it a promising anode material for high rate lithium-ion batteries.

Experimental

Synthesis of spinel Li₄Ti₅O₁₂

All of the reactants and solvents were of analytical grade and used without further purification. A certain stoichiometric amount of PEG with a molecular weight of 2000 (0.25 g, 0.5 g, 0.75 g) was dispersed in LiOH ethanol solution (40 ml ethanol, 0.45 g LiOH·H₂O) respectively. Tetrabutyl titanate (TBT) ethanol solution (20 ml ethanol, 4 ml TBT) and 20 ml deionized water were simultaneously added dropwise to above mixed solutions of LiOH and PEG under vigorous stirring in ice bath condition. After stirring for 4 h, the colloidal suspension was transferred into 150 ml Teflon-lined autoclave and kept at 120 °C for 24 h. Then the white precipitate was separated by filtration, washed with ethanol and dried at 60 °C for 6 h. Subsequently, the white powder was annealed at 500–700 °C for 2 h in air to obtain Li₄Ti₅O₁₂ materials. The Li₄Ti₅O₁₂ samples prepared with PEG of 0.25 g, 0.5 g and 0.75 g are denoted as PLTO-1, PLTO-2 and PLTO-3, respectively. For comparison, the Li₄Ti₅O₁₂ denoted as LTO was prepared at 600 °C in a similar manner except for the absence of PEG.

Characterization

The crystal structure of the obtained samples were characterized by X-ray powder diffraction (XRD) using an X'Pert-pro MPD (PANalytical, The Netherlands) diffractometer with monochromatic Cu Kα radiation of wavelength λ = 0.1541 nm and at a scanning speed of 10° min⁻¹. The morphology of the samples was observed by field emission scanning electron microscopy (FESEM, S4800) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010). The N₂ adsorption–desorption measurements were performed with ASAP2020 (Micromeritic, USA) instrument. Before measurement, the sample was outgassed under vacuum at 250 °C for ca. 12 h until the pressure less than 5 μm Hg. The specific surface area (S_BET) was calculated by the conventional BET (Brunauer–Emmett–Teller) method while the pore size distribution was calculated by the Barrett–Joyner–Halenda (BJH) method.

Electrochemical measurements

The electrochemical performances of the as-prepared Li₄Ti₅O₁₂ were evaluated with simulated cell in which a lithium metal foil was used as the counter electrode. For electrodes preparation, Li₄Ti₅O₁₂ powder (80 wt%), acetylene black (10 wt%) and polyvinylidene fluoride binder (10 wt%) were homogeneously mixed in N-methyl pyrrolidinone solvent and then coated uniformly on a copper foil current collector. The electrolyte employed was 1 M solution of LiPF₆ in ethylene carbonate and dimethyl carbonate (EC + DMC) (1 : 1 in volume). Simulated cells were assembled in an argon-filled glove box (German, M. Braun Co., O₂ < 1 ppm, H₂O < 1 ppm). The simulated cells were cycled under different current densities between the cut-off voltages of 2.5 and 1.0 V on a CT2001A cell test instrument (LAND Electronic Co.) at room temperature.

Results and discussion

Fig. 1a shows the XRD patterns of the Li₄Ti₅O₁₂ samples prepared with different amounts of PEG calcinated at 600 °C. The diffraction peaks of LTO, PLTO-1, PLTO-2, PLTO-3 are all indexed to the pure-phase spinel Li₄Ti₅O₁₂ (JDPDS no. 49-0207), indicating that the addition of PEG do not affect the phase composition of the Li₄Ti₅O₁₂. Fig. 1b shows the effect of calcination temperature on the XRD patterns of the Li₄Ti₅O₁₂ hydrothermal synthesized with a PEG addition of 0.5 g. The PLTO annealed at 500 °C is mainly Li₄Ti₅O₁₂ accompanied with a small amount of Li₂Ti₃O₇ (JCPDS no. 40-0303). As the temperature increases, the diffraction peaks of PTLO annealed at 600 °C and 700 °C in XRD patterns are absolutely indexed to the cubic spinel Li₄Ti₅O₁₂ (JDPDS no. 49-0207). Notably the diffraction peaks of Li₄Ti₅O₁₂ gradually sharpened as the sintering temperature increased from 500 to 700 °C, which indicates increased crystallinity with the anneal temperature due to the ordering of the local structure and release of the lattice strain.³¹ The grain sizes of LTO annealed at 600 °C without PEG addition calculated by Scherrer's formula based on the (111) peak (2θ = 18.331°) is 28.6 nm. With PEG addition, the calculated grain size of the PLTO samples shows decreasing tendency (Table S1†). The crystalline size of PLTO-1 with a PEG addition of 0.25 g decreases to 22.3 nm, and then 14.9 nm for PLTO-2 as PEG addition increases to 0.5 g, only half of that of the LTO. This proves that the PEG acts as a structure directing agent for Li₄Ti₅O₁₂ synthesis. However, as the PEG addition further increased to 0.75 g, the grain size of PLTO-3 turns to increase to 22.9 nm.

Fig. 1 (a) XRD patterns of hydrothermal synthesized LTO calcinated at 600 °C with and without PEG addition; (b) XRD patterns of PLTO with a PEG addition of 0.5 g calcinated at different temperatures.

SEM and TEM were employed to observe the morphology and crystallite structure of the hydrothermal synthesized Li₄Ti₅O₁₂ with a calcination temperature of 600 °C without and with PEG addition. SEM observation indicates the LTO has an anomalous granular morphology (Fig. 2a) with different sizes assembled by many nanoplates (Fig. 2e). TEM observation (Fig. 3a) indicates the morphology and size of LTO nanocrystallites are uniform. With PEG addition, the PLTO samples show microscale second Li₄Ti₅O₁₂ spheroidic particles composed of nanoscale primary particles. The PLTO-2 sample with a PEG addition of 0.5 g has a more uniform spheroidic morphology with diameters of 1–4 μm (Fig. 2c), which was assembled by many fine nanoparticles with much decreased nanocrystallite of ca. 15 nm (Fig. 3b), in good agreement with that estimated from XRD patterns. As the PEG addition increased to 0.75 g, the morphology of PLTO-3 becomes irregular and the particle size varied in a wide range of 2–8 μm (Fig. 2d), indicating that excess PEG addition has adverse effect for nano/micro structured Li₄Ti₅O₁₂ synthesis. The interplanar distances between adjacent lattice lanes for the Li₄Ti₅O₁₂ without or with PEG are all about 0.48 nm (Fig. 3c and d), corresponding to the (111) lattice fringe of spinel Li₄Ti₅O₁₂, confirming that PEG addition has not influenced the well-crystallized spinel phase of the Li₄Ti₅O₁₂. That is to say, with the addition of PEG, nano/micro structured Li₄Ti₅O₁₂ can be obtained by hydrothermal reaction and subsequent calcination.

Fig. 2 SEM images of LTO (a and e), PLTO-1 (b), PLTO-2 (c and f) and PLTO-3 (d).

Fig. 3 TEM images of LTO (a and c) and PLTO-2 (b and d).

To gain further insight into the porous structure of the nano/micro structured Li₄Ti₅O₁₂, nitrogen adsorption–desorption isotherms of LTO, PLTO-1, PLTO-2, PLTO-3 were measured. As shown in Fig. 4a, all the samples exhibit type IV adsorption–desorption isotherms according to IUPAC classification,³² indicating mesoporous materials. Similar to other hydrothermal synthesized Li₄Ti₅O₁₂, LTO has a high BET surface area (50.7 m² g⁻¹) and large pore volume (0.34 cm³ g⁻¹) due to its small nanocrystallite size of 28.6 nm by XRD. By adding PEG as a structure directing agent, the Li₄Ti₅O₁₂ samples with much increased surface area and pore volume were obtained (Table S1†). The BET surface area of PLTO-1 is 55.3 m² g⁻¹, which increased to as high as 80.1 m² g⁻¹ with a large pore volume of 0.45 cm³ g⁻¹ for PLTO-2. The BET surface area of PLTO-3 is 67.6 m² g⁻¹, larger than LTO but not as high as PLTO-2. As shown in Fig. 4b, the pore size of the Li₄Ti₅O₁₂ samples distribute in 10–70 nm. It is known that a more developed porous structure and higher surface area can provide more transport channels for lithium ion to insert into the electrode material, increase the electrode-electrolyte interfacial area and decrease the irreversible capacity loss associated with the concentration polarization at higher current density, which is critical for high rate lithium ion battery applications.³³

Fig. 4 The N₂ (77 K) adsorption–desorption isotherms (a) and pore size distribution (b) of the LTO, PLTO-1, PLTO-2 and PLTO-3.

It is presumed that the PEG act as a structure directing agent during synthesis of the nano/micro structured porous Li₄Ti₅O₁₂.³⁴ Tetrabutyl titanate dissolves in aqueous solution of ethanol to produce titanium precursor sol particles. The adsorption of PEG on the titanium precursor surface can not only reduce the crystal particles' surface energy and restraining the particles' growth, but also make the nanoparticles assemble to form microspheres. During calcination, PEG will entirely decompose and a significant number of mesopores are created inside the Li₄Ti₅O₁₂ microspheres.³⁴

The electrochemical performances of the PEG-assistant hydrothermal synthesized Li₄Ti₅O₁₂ as anode material for lithium ion battery were evaluated in detail. Fig. 5a presents the first cyclic voltammograms of PLTO-2 synthesized with a PEG addition of 0.5 g and a calcination temperature of 600 °C at 0.2 mV s⁻¹. Only one pair of oxidation/reduction peak is observed, proving the phase purity of PLTO-2. The cathodic and anodic peaks are at 1.37 V and 1.74 V, respectively, which can be regarded as the signature of lithium insertion and extraction from the spinal Li₄Ti₅O₁₂ framework.³⁵ The initial three discharge–charge curves of PLTO-2 cycled at 0.2 C are shown in Fig. 5b. There was about 10% irreversible capacity loss at the first cycle, which might be ascribed to the breakdown process in the electrolyte solution, such as reduction of trace water, which is more pronounced for high-surface-area electrodes and for fresh electrodes during the initial cycles.³⁶ The second and third cycle curves almost overlapped, the capacities are 167 mA h g⁻¹, which demonstrates the highly reversibility of PLTO-2 during discharge–charge process.

Fig. 5 The first cyclic voltammogram at a scan rate of 0.2 mV s⁻¹ (a) and galvanostatic discharge–charge curves at 0.2 C (b) of PLTO-2.

Fig. 6 shows the first discharge–charge curves of the LTO and PLTO samples prepared at 600 °C between 1.0 and 2.5 V at the rates of 1–20 C. From the view of rate capability, it is known the Li₄Ti₅O₁₂ obtained by hydrothermal synthesis is generally superior to those prepared by other methods due to its nanostructured morphology. As shown in Fig. 6a, the charge capacity of LTO is 160 mA h g⁻¹ at 1 C, which slightly decreases to 154 mA h g⁻¹ at 2 C and then 140, 120 and 91 mA h g⁻¹ at the rates of 5 C, 10 C and 20 C, respectively, proving the good rate performance of hydrothermal synthesized LTO. By adding PEG in hydrothermal reaction as the structure directing agent, the much decreased nanocrystallite and nano/micro porous structure enable the PLTO samples show further much enhanced rate performance. The capacity of PLTO-1 (Fig. 6b) is 156 mA h g⁻¹ at 1 C, which remains 150, 143, 134 and 111 mA h g⁻¹ at rates of 2 C, 5 C, 10 C and 20 C, respectively. The PLTO-2 (Fig. 6c) shows the best rate performances. As the rate increases, the charge capacity decreases very slightly from 160 mA h g⁻¹ at 1 C to 157, 152 and 144 mA h g⁻¹ at rates of 2 C, 5 C and 10 C, respectively. Even when the rate increases to 20 C, the discharge–charge platforms are very stable with a weak polarization and a capacity as high as 132 mA h g⁻¹ can be remained, which is a very attractive value for Li₄Ti₅O₁₂. The rate performance of PLTO-2 is not only much better than LTO prepared without PEG, but also higher than some previous reported Li₄Ti₅O₁₂ samples, such as sol–gel synthesized Li₄Ti₅O₁₂ hollow microspheres (115.6 mA h g⁻¹ at 10 C),³⁷ one-dimensional Li₄Ti₅O₁₂ nanofibers (138 mA h g⁻¹ at 10 C),²⁵ Li₄Ti₅O₁₂ thin film (125 mA h g⁻¹ at 20 C),³⁸ nitrogen-doped carbon coated Li₄Ti₅O₁₂ (128.2 mA h g⁻¹ at 20 C).³⁹ The excellent rate performance of PLTO-2 may be attributed to its small nanocrystallite (∼15 nm), nano/micro spheroidic structure with developed porosity, which benefit for the diffusion of lithium ion and improve the interfacial effect and decrease the polarization. However, as the PEG addition is further increased to 0.75 g, the rate capability of PLTO-3 shows a little decay (Fig. 6d), which may be ascribed to its increased nanocrystallite (22.9 nm) and secondary particle size (2–8 μm), and decreased surface area (67.6 m² g⁻¹).

Fig. 6 Charge–discharge curves at different rates for LTO (a), PLTO-1 (b), PLTO-2 (c) and PLTO-3 (d).

The PEG-assistant hydrothermal synthesized Li₄Ti₅O₁₂ also exhibits good cycling stability. As shown in Fig. 7, the sample PLTO-2 shows high capacity retention upon cycling except for the initial first cycle loss. After 100 discharge–charge cycles, the capacity of PLTO-2 can remain 150 mA h g⁻¹ at 2 C and 148 mA h g⁻¹ at 5 C, better than LTO prepared without PEG addition.

Fig. 7 Cycling performance of PLTO-2 and LTO.

Conclusions

Nano/micro structured porous Li₄Ti₅O₁₂ were synthesized by hydrothermal reaction using polyethylene glycol as structure directing agent. The sample PLTO-2 prepared with a PEG addition of 0.5 g shows spheroidic morphology with size of 1–4 μm, assembled by nanocrystallite with a size of about 15 nm. The surface area of the PLTO-2 reaches 80.1 m² g⁻¹ with a pore volume of 0.45 cm³ g⁻¹. The unique microstructure enable the sample presents a reversible capacity as high as 132 mA h g⁻¹ at 20 C, making it a potential anode material for high-rate lithium ion batteries.

Acknowledgements

This work was financially supported by the National Key Basic Research and Development Program (2015CB251100), the National Science Foundation of China (21073233 and 50802112) and the Production, Teaching and Research Combination Project of Guangdong Province (2012B090500019).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07786e

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