High-performance thin-film Li₄Ti₅O₁₂ electrodes fabricated by using ink-jet printing technique and their electrochemical properties

Abstract

Li₄Ti₅O₁₂ thin-film anode with high discharge capacity and excellent cycle stability for rechargeable lithium ion batteries was prepared successfully by using ink-jet printing technique. The prepared Li₄Ti₅O₁₂ thin film were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, cyclic voltammograms, and galvanostatic charge–discharge measurements. It was found that the average thickness of 10-layer Li₄Ti₅O₁₂ film was about 1.7~1.8 μm and the active material Li₄Ti₅O₁₂ in the thin film was nano-sized about 50–300 nm. It was also found that the prepared Li₄Ti₅O₁₂ thin film exhibited a high discharge capacity of about 174 mAh/g and the discharge capacity in the 300th cycle retained 88% of the largest discharge capacity at a current density of 10.4 μA/cm² in the potential range of 1.0–2.0 V.

Introduction

Spinel Li₄Ti₅O₁₂ is well-known for its excellent electrochemical properties and has found its versatile potential applications in many fields till now. Firstly, spinel Li₄Ti₅O₁₂ has been demonstrated as a promising anode material for lithium-ion batteries because it has good Li-ion intercalation and de-intercalation reversibility and exhibits no structural change during charge-discharge cycling [1]. It exhibits a working potential of 1.55 V vs. Li⁺/Li and its theoretical discharge capacity is 175 mAh/g [2–3].This anode, when coupled with the 4 V available cathodes such as LiCoO₂ [4–7] or LiMn₂O₄ [8–11], can provide a cell with an operating voltage of 2.5 V which is twice that of Ni/MH or Ni/Cd. Secondly, the improved safety and reliability of the spinel compared with that of carbon electrodes make the lithium-ion batteries using Li₄Ti₅O₁₂ material as anode suitable for electric vehicle and power storage batteries. Thirdly, it can also be used in hybrid supercapacitors by combination with active carbon or some conductive polymers just because of its superior cycleability and safety [12–16].

Meanwhile, many studies have been carried out on rechargeable lithium microbatteries because the microscale batteries will be utilized in various application fields related to microsystems, such as microsensors, micromechanics, microelectronics, and so on [17]. Li₄Ti₅O₁₂ in the form of thin film is also finding its potential application as anode in all solid-state lithium ion microbatteries. Many techniques such as spray pyrolysis, magnetron sputtering, electrostatic spray pyrolysis, pulsed laser deposition, soft solution processing, hydrothermal method, sol-gel method and so on have been applied to study the LiCoO₂ thin film cathode. By comparison, fewer techniques have been reported to investigate Li₄Ti₅O₁₂ thin film anode. Up to now, thin film Li₄Ti₅O₁₂ has been fabricated for anode in lithium ion batteries by the following three techniques [17–21] according to some related reports as far as we know. Firstly, magnetron sputtering [18] is a typical methods for thin film preparation. C.-L. Wang et al fabricated spinel-phase Li₄Ti₅O₁₂ thin films by radio frequency magnetron sputtering on an Au/Ti/SiO₂/Si substrate followed by an annealing process at 500~700 °C [18]. Besides, electrostatic spray deposition [19] and spin-coating method [17, 20–21] have been also employed to obtain Li₄Ti₅O₁₂ thin films. Y. Yu et al reported porous thin films of Li₄Ti₅O₁₂ prepared by electrostatic spray deposition technique with lithium acetate and titanium butoxide as the precursors. They found that the capacity retained 150 mAh/g after 70 cycles when the heating process was carried out at 700 °C [19]. Thin films of Li₄Ti₅O₁₂ with a thickness of 1 μm were prepared by Y. H. Rho et al. from precursors obtained by a sol–gel process using poly(vinylpyrrolidone) followed by a heat treatment at 600–800 °C for 1 h [20].

At the same time, the application of ink-jet printing technology has been successfully extended from its traditional areas into more extensive fields such as fabrication of field-effect transistors [22–23], protein [24], high-density bacterial colony [25], even cell and organs [26] in recent years. Thin-film electrets [27] and polymer films [28] are also reported by ink-jet printing method. MnO₂ thin film electrode for primary battery and SnO₂ thin film electrode for rechargeable lithium ion batteries have been introduced in our previous papers [29–30]. Since ink-jet printing technique has many advantages such as simple device, low-cost, and facile fabrication process, ease of mass fabrication and so on, it is finding increasing application fields and a more promising future is on its way. Here, we reported Li₄Ti₅O₁₂ thin film electrode successfully fabricated by a simple ink-jet printing technique. The prepared Li₄Ti₅O₁₂ thin film electrode displayed high discharge capacity and excellent cycle stability.

Experimental

Preparation of Li₄Ti₅O₁₂ ink

The specific preparation process of Li₄Ti₅O₁₂ ink is very similar to our previous work, as shown in Fig. 1 [30]. Prior to the Li₄Ti₅O₁₂ ink preparation, the electrochemically active Li₄Ti₅O₁₂ was home-made by solid-state reaction process aided with a wet-ball milling process (powder Li₄Ti₅O₁₂ for short hereafter). Then the ink solution was prepared as follows: firstly, Li₄Ti₅O₁₂ suspension was prepared by adding 50 mg polymeric hyperdispersant CH10B (Shanghai Sanzheng Polymer Material Co. Ltd) in 10 ml mixture solvent (distilled water/absolute ethanol/diethylene glycol/triethanolamine/isopropylalcohol = 56:18:5:1:1 by mass ratio) and then followed by addition of 400 mg Li₄Ti₅O₁₂ powder. The obtained mixture was ball-milled at 450 rpm for 6 h and then the stable Li₄Ti₅O₁₂ suspension was achieved. The other kind of polymeric hyperdispersant CH12B (Shanghai Sanzheng Polymer Material Co. Ltd) was employed to disperse the conductive agent acetylene black (AB). Following the same procedure, CH12B 100 mg was dissolved in 10-ml mixture solvent and then followed by addition of 40 mg AB. The as-prepared mixture was ball-milled at 450 rpm for 6 h and then the stable AB suspension was attained. Finally, the stable suspension Li₄Ti₅O₁₂ and the stable suspension AB were mixed by a certain volume ratio and then about 5% water-soluble sodium of carboxymethyl cellulose (CMCS) was added as binder, which did not destroy the stability of the dispersion system. Then the mixture suspension containing Li₄Ti₅O₁₂, AB, and CMCS was ultrasonically dispersed for 20 min, kept alone for 3~5 h, discarded small amount of sediment at the bottom and finally the upper stable part formed the print suspension named as Li₄Ti₅O₁₂ ink.

Fig. 1

Flow chart showing preparation of Li₄Ti₅O₁₂ ink for subsequent ink-jet printing process

Preparation of thin film Li₄Ti₅O₁₂ electrodes

The Li₄Ti₅O₁₂ ink 10~15 ml was transferred into a well-cleaned black Canon BC-03 cartridge and then ink-jet printed through Canon 1000SP printer onto Au substrate followed by an annealing process at 550 °C for 90 min according to the pre-designed electrode size and shape. In our experiment, we repeated the printing process ten times in order to achieve enough capacity for subsequent electrochemical measurements. The as-printed Li₄Ti₅O₁₂ films were dried under infrared light at 80 °C for 2 h, compressed at 5 MPa, annealed at 550 °C for 90 min (as-prepared Li₄Ti₅O₁₂ thin film for short hereafter) and finally used for the subsequent characterization and measurements.

Characterizations

The morphology and thickness of Li₄Ti₅O₁₂ film were examined by means of scanning electron microscopy (SEM, Philips XL 30) and transmission electron microscopy (TEM, Joel JEM 2010). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)) were employed to investigate the structure and composition of as-prepared Li₄Ti₅O₁₂ thin film. XRD patterns were recorded on a Bruker D8 advanced diffractometer with Cu Kα radiation, operated at 40 mA and 40 kV. Cyclic voltammograms (CV) and galvanostatic charge–discharge measurements were performed on a CHI660 electrochemical workstation (CHI, USA) with conventional three-electrode cell. Two lithium plates were used as both the reference and counter electrode. All the electrodes were dried under vacuum at 90 °C for 12 h before they were constructed in a dry-air filled box. One molar LiPF₆ in ethyl carbonate (EC) and dimethyl carbonate (DMC) (EC, DMC = 1:1 by volume) was used as the electrolyte. CV measurements were conducted at different sweep rates over the potential range of 1.0–2.0 V. Galvanostatic charge–discharge measurements were performed over the potential range of 1.0–2.0 V. The exact mass of Li₄Ti₅O₁₂ in the thin films was obtained as follows: firstly, the as-prepared thin-film was assembled into three-electrode glass cell for charge–discharge measurements. When the measurements were finished, the cells were disassembled. The Li₄Ti₅O₁₂ content in the thin film was determined by inductively coupled argon plasma (ICP) method (IRIS Intrepid, Thermo Elemental Company) after the sample was solubized in concentrated HNO₃–HCl solution. The mass of Li₄Ti₅O₁₂ was calculated on the basis of the mass of titanium achieved by ICP result.

Results and discussion

Electron micrographs of as-prepared Li₄Ti₅O₁₂ thin films on Au substrate are presented in Fig. 2. It can be seen that the distribution of Li₄Ti₅O₁₂ thin film is very uniform (Fig. 2a) and the particles are nano-sized (Fig. 2b). TEM image of Li₄Ti₅O₁₂ particles in the thin film (inset in Fig. 2b) shows that the particle size of nano-sized Li₄Ti₅O₁₂ ranges from 30~300 nm. The cross-sectional view of the as-prepared Li₄Ti₅O₁₂ thin films on Au substrate reveals that the average thickness of 10-layer film is about 1.7~1.8 μm, which was used for subsequent electrochemical measurements.

Fig. 2

SEM images for Li₄Ti₅O₁₂ thin films on Au substrate annealed at 550 °C for 90 min. a surface view with low magnification (×1000); b a cross-sectional view; c surface view with high magnification (×10,000), Inset is TEM image for nano-sized Li₄Ti₅O₁₂ in the thin film

The X-ray diffraction of as-prepared Li₄Ti₅O₁₂ thin film (b) is similar to that of powder Li₄Ti₅O₁₂ (a) are shown in Fig. 3. The main diffraction peaks of Li₄Ti₅O₁₂ (111, 311.400) are indexed based on a face-centered cubic spinel structure with an Fd3m space group, which is in good agreement with the JCPDS reference (26–1198) and corresponds to typical pure-phased spinel structure [31–32]. In addition, the peaks attributed to the substrate Au for as-prepared Li₄Ti₅O₁₂ thin films are also clearly observed and indexed.

Fig. 3

X-ray diffraction patterns of a powder Li₄Ti₅O₁₂ and b the as-prepared Li₄Ti₅O₁₂ thin film annealed at 550 °C for 90 min

The survey XPS spectra corresponding to Li₄Ti₅O₁₂ thin film on Au substrate annealed at 550 °C for 90 min (a) and powder Li₄Ti₅O₁₂ (b) are presented in Fig. 4a. The Ti2p, O1s, and Li1s photoelectron peaks are clearly observed. Carbon peaks present in the spectra of powder Li₄Ti₅O₁₂ are attributed to a thin contamination layer whereas that in the as-prepared Li₄Ti₅O₁₂ thin film comes from conductive agent AB, the polymeric dispersants CH10B, CH12B and the binder CMCS. The high resolution spectra of the Ti2p corresponding to both the as-prepared Li₄Ti₅O₁₂ thin film (a) and powder Li₄Ti₅O₁₂ (b) are shown in Fig. 4b. The spectrum corresponding to Li₄Ti₅O₁₂ thin film in Fig. 4b (a) is characterized by a main doublet composed of two symmetric peaks situated at Eb (Ti2p_3/2) = 459.0 eV and Eb (Ti2p_1/2) = 464.8 eV. This main doublet is assigned to titanium in the IV oxidation state. In addition, it is necessary to take into account the difference between the two samples. The binding energy for Li₄Ti₅O₁₂ thin film increased about 0.5 eV compared with powder Li₄Ti₅O₁₂. The main reason for this phenomenon is the particle size difference. Powder Li₄Ti₅O₁₂ was prepared by solid state reaction process and composed of both nano-sized and micro-sized particles whereas Li₄Ti₅O₁₂ thin film was composed of only nano-sized particles ranged from 30–300 nm shown in Fig. 3. The Li₄Ti₅O₁₂ particles with larger size than 300 nm are not effectively dispersed during the ink preparation and therefore do not exist in the as-prepared thin film.

Fig. 4

XPS survey spectra (A) and the Ti2p region (B); a Li₄Ti₅O₁₂ thin film on Au substrate annealed at 550 °C for 90 min; b powder Li₄Ti₅O₁₂ home-made by solid state reaction process

Figure 5 shows CV curves of as-prepared Li₄Ti₅O₁₂ thin film electrode compared with Li₄Ti₅O₁₂ powder at a scan rate of 0.1 mV/s in a mixed solvent of ethylene carbonate and dimethyl carbonate containing 1.0 mol/dm³ LiPF₆. Two sharp redox peaks for Li₄Ti₅O₁₂ thin film were observed at 1.55 and 1.60 V vs. Li⁺/Li for cathodic and anodic scans respectively, which indicates that as-prepared Li₄Ti₅O₁₂ thin film exhibits a typical behavior for two-phase reaction during Li⁺ ion extraction and insertion processes [11, 22–23]. It can be also clearly found that the redox peaks were sharper and the peak separation was smaller for as-prepared Li₄Ti₅O₁₂ thin film compared with those for powder Li₄Ti₅O₁₂ electrode.

Fig. 5

Comparison of cyclic voltammograms at a scan rate of 0.1 mV/s in a mixed solvent of ethylene carbonate and dimethyl carbonate containing 1.0 mol/dm³ LiPF₆ between as-prepared Li₄Ti₅O₁₂ thin film electrode and powder Li₄Ti₅O₁₂ electrode

CV curves of Li₄Ti₅O₁₂ thin film on Au substrate annealed at 550 °C for 90 min at different scan rates from 0.1 to 1.0 mV/s are shown in Fig. 6a. Accordingly, the anodic and cathodic peak current as a function of scan rate are presented in Fig. 6b. It can be seen from Fig. 6 that the anodic and cathodic peak separation was 74 mV when the scan rate was 0.1 mV/s for as-prepared Li₄Ti₅O₁₂ thin film. The peak separation increases slightly with the increase of scan rate. The anodic peaks show a positive shift from 1.625 to 1.676 V (64 mV shifted) versus Li⁺/Li with the increase of scan rate from 0.1 to 1.0 mV/s. It is easy to understand that the peak shift with the increasing scan rate was caused by the polarization of charge transfer reaction. The peak current is linearly dependent on the sweep rate, which indicates that the diffusion of lithium ion inside the electrode is not the dominating factor for as-prepared Li₄Ti₅O₁₂ electrode. The charge transfer process is the rate-determining step. This phenomenon is characteristic of thin film electrode.

Fig. 6

a Cyclic voltammograms of as-prepared Li₄Ti₅O₁₂ thin film electrode at different scan rates of (1) 0.1, (2) 0.2, (3) 0.3, (4) 0.4, (5) 0.5, (6) 0.6, (7) 0.7, (8) 0.8, (9) 0.9, (10) 1.0 mV/s in a mixed solvent of ethylene carbonate and dimethyl carbonate containing 1.0 mol/dm³ LiPF₆; b anodic and cathodic peak current as a function of scan rate

Figure 7 shows discharge–charge curves in the second, tenth, 20th, 30th, 40th, and 50th cycle of Li₄Ti₅O₁₂ film prepared on Au substrate annealed at 550 °C for 90 min at a current density of 10.4 μA/cm² in a mixed solvent of ethylene carbonate and dimethyl carbonate containing 1.0 mol dm⁻³ LiPF₆. On one hand, the shape of charge-discharge curves of the as-prepared Li₄Ti₅O₁₂ film is similar to that of powder Li₄Ti₅O₁₂, which indicates the as-prepared Li₄Ti₅O₁₂ film has the same charge-discharge mechanism as powder Li₄Ti₅O₁₂ electrode. On the other hand, the charge-discharge behavior for as-prepared Li₄Ti₅O₁₂ thin film is very different from powder Li₄Ti₅O₁₂ electrode. An electrochemical activation process existed in as-prepared Li₄Ti₅O₁₂ films annealed at 550 °C for 90 min. The discharge capacity is small during the first several cycles and increases with the cycle process are carried through. The discharge capacity in the second, tenth, 20th, 30th, 40th, and 50th cycle of as-prepared Li₄Ti₅O₁₂ film is 117, 157, 166, 170, 172 and 174 mAh/g respectively. The reason for this phenomenon is not clear yet and further investigation is required.

Fig. 7

Discharge and charge curves of Li₄Ti₅O₁₂ film prepared on Au substrate annealed at 550 °C for 90 min at a current density of 10.4 µA/cm² in a mixed solvent of ethylene carbonate and dimethyl carbonate containing 1.0 mol/dm³ LiPF₆

Figure 8 displays the specific discharge capacity and coulumbic efficiency of as-prepared Li₄Ti₅O₁₂ thin film annealed at 500 °C for 90 min as a function of cycle number during the first 300 cycles at a constant current density of 10.4 µA/cm² in potential range of 1.0–2.0 V versus Li⁺/Li. The initial discharge capacity is 97 mAh/g, but the discharge capacity increases with the process of cycling and the discharge capacity in the 42nd cycle reaches 174 mAh/g, which is almost near the theoretical value of spinel Li₄Ti₅O₁₂ material (175 mAh/g). The discharge capacity in the 300th cycle (153 mAh.g⁻¹) is 88% of the largest discharge capacity, which shows the excellent cycle stability of as-prepared Li₄Ti₅O₁₂ thin films. The initial coulumbic efficiency is 16.8% and the efficiency also increases with the increase of cycle number. The coulumbic efficiency maintains 98% or so after the 30th cycle. With regard to the irreversible capacity of Li₄Ti₅O₁₂ thin film during the first several cycles, it can be mainly attributed to parasitic cathodic reactions due to enhanced adsorption of reducible impurities such as humidity and the quality of the spinel crystalline lattice itself [2].

Fig. 8

Discharge capacity and coulumbic efficiency of as-printed thin film Li₄Ti₅O₁₂ electrode as a function of cycle number during the first 300 cycles at a constant discharge current density of 10.4 µA/cm² in potential range of 1.0–2.0 V

Conclusion

In this work, Li₄Ti₅O₁₂ thin films on Au substrate fabricated by using ink-jet printing technique followed by an annealing process at 550 °C for 90 min as an anode for rechargeable lithium ion batteries were investigated in detail. The as-prepared Li₄Ti₅O₁₂ thin films annealed at 550 °C for 90 min displayed its largest discharge capacity about 174 mAh/g. The discharge capacity in the 300th cycle (153 mAh g⁻¹) is 88% of the largest discharge capacity. The excellent electrochemical properties of the as-prepared Li₄Ti₅O₁₂ thin films suggest the promising application of ink-jet printing technique to thin film electrodes for lithium ion batteries.