bSchool of Resources and Materials, Northeastern University at Qinghuangdao Branch, Qinghuangdao 066004, China;
cHebei Provincial Laboratory of Dielectric and Electrolyte Functionals Materials, Qinghuangdao 066004, China
The spinel Li4Ti5O12 is regarded as an ideal anode material with long cycling stability due to zero strain or small volume change during the lithium insertion and extraction process . Its advantages are low toxicity,low cost,easy preparation and lithium intercalation/deintercalation [2, 3, 4]. Li4Ti5O12 has a theoretical specific capacity of 170 mAh/g and a voltage plateau at 1.55 V vs. Li+/Li. Li-ions are intercalated at lower negative potentials than in the usual carbon-based anodes [5, 6]. The synthetic method has a great influence on the electrochemical properties of the materials [7, 8]. A variety of synthetic methods have been chosen including solid-state reactions ,the hydrothermal route [10, 11],the sol-gel process and the molten salt method [12, 13]. However,most of these methods involve several steps and some are quite expensive to operate.
The combustion method has been known as a simple,fast,and energetically economic method that yields high purity products [14, 15]. In recent years,many research groups have focused their efforts on using different kinds of fuel to synthesize Li4Ti5O12 ,and the choice of fuel has a significant impact on structure,morphology and properties of the synthesized materials. PVP was employed due to its low toxicity and high aqueous solubility. It has been extensively used as a stabilizer and a structure-directing agent in nanotechnology because of its excellent absorption ability . Chen prepared PVP/LiCoO2 nanofibers using an electrospinning route . It is hypothesized that PVP also can be used to prepare lithium titanate powders with high electrochemical performance.
In this paper,Li4Ti5O12 was synthesized by a gel-combustion method (GCM) using polyvinylpyrrolidone (PVP) as the fuel and complexing agent. PVP was employed because its amide groups can coordinate metal ions and the winding function of macromolecule chains can form gel. The sample prepared by a solid-state reaction (SSR) with Li2CO3 and TiO2 was investigated as a reference. 2. Experimental
Li4Ti5O12 powders were prepared by a PVP-assisted gel combustion method. PVP (AR),LiCH3COO2·2H2O (AR) and TiO2 (AR) were mixed in deionized water (the molar ratio of PVP to total metal ions was fixed at 2.0) and pH 3 was achieved by adding HNO3. The mixture was stirred and heated at 100°C in air to obtain a dried gel,and then the gel was dried at 110°C for 24 h. The dried gel was ignited on a hot iron pot for several minutes to induce a combustion process. The black as-combusted powers were calcined at 800°C for 8 h in air,and then cooled to room temperature naturally. The obtained sample is denoted as GCM LTO. As a comparator,Li4Ti5O12 powders were synthesized by the solid-state reaction (SSR) method using Li2CO3(AR) and TiO2(AR), which were mixed through ball-milling and calcined at 800°C for 8 h.
The thermal decomposition behavior of the Li4Ti5O12 dried-gel precursor was measured by the thermogravimetry and differential thermal analysis (TG/DTA) (Hengjiu Instrument Co.,Ltd.,HCT-2, Beijing,China) from room temperature to 1000°C at a heating rate of 10°C/min in air. The crystal structures of the synthesized powers were examined by X-ray diffraction (XRD) (Haoyuan DX-2500,Dandong Instrument Co.,Ltd.,China) analysis with nickelfiltered Cu Kα radiation (λ= 1.5418 Å) over the 2θ range from 108 to 858. The particle morphology was observed using a ZEISS SUPRA55 scanning electron microscope (SEM). The electrochemical performance of GCM LTO and SSR LTO were measured using a 2025-type coin cell. LTO electrodes were prepared using the LTO materials,carbon black,and polyvinylidene fluoride (PVDF),which were dissolved inN-methyl pyrrolidinone (NMP) with a weight ratio of 80:10:10. The as-prepared slurry was coated on a Cu foil and dried at 120°C for 10 h to remove NMP. The electrodes were punched into discs with a diameter of 10 mm. Finally,the electrodes were dried under vacuum over at 80°C for 10 h. These anodes were assembled into coin cells in an argon-filled glove box with a metallic lithium counter electrode,an electrolyte of 1 mol/L LiPF6/ethylene carbonate-dimethyl carbonate-methyl ethyl carbonate (1:1:1,v/v/v) and Celgard 2400 separator. Cyclic voltammetry (CV,scanning range: 0.5-2.7 V,scanning rate: 0.1 mV/s) and electrochemical impedance spectroscopy (EIS,scanning range: 10-1 -106 Hz) were performed using an electrochemical analyzer (Solartron 1260 + 1287). Galvanostatic charge/discharge measurements were conducted using a Land Battery Test System (CT2001A, Wuhan Jinnuo Eletronic Co.,Ltd.,Wuhan China) in a potential range of 0.5-3.0 V (vs.Li+/Li) under different rates between 0.5 C and 5 C at room temperature. 3. Results and discussion
Gels are usually considered the intermediates in processing reactions. The characterization of precursor gels gives a better understanding of,and allows a control of,the whole process of the synthesis. The mechanism of the thermal decomposition of dried gel precursor was studied in air by TG/DTA measurements in the temperature range of 20-1000°C as shown in Fig. 1(a). A slight initial weight loss in TG and endothermic peak in DTA observed below 110°C are mainly attributed to the evaporation of H2O along with the decomposition of some other salts. The small exothermic peak at 350-400°C in the DTA curve with the second weight loss (2%) in the TG can be attributed to the pyrolysis of PVP. Subsequently,a strong exothermic peak at 454°C with a drastic weight loss can be assigned to the transformation from the driedgel precursor to Li4Ti5O12. Afterwards,there are no distinct changes in TG/DTA,indicating the completion of PVP-chain decomposition.
|Fig. 1. (a) TG and DTA curves for the dried-gel precursor under an ambient atmosphere (heating rate = 10°C/min); (b) XRD patterns of the samples.|
The XRD patterns of the as-combusted LTO and the LTO powders calcined at 800°C are shown in Fig. 1(b). It can be seen from Fig. 1(b) that the as-combusted powers consist of spinel LTO (JCPDS 26-1198),anatase TiO2 (JCPDS 21-1272) and rutile TiO2 (JCPDS 21-1276). The distinct peaks stand for the decomposition product of PVP in as-combusted product,which is not observed, indicating that the addition of PVP does not have any effect on the formation of spinel LTO. For the as-combusted sample calcined at 800°C for 8 h,several sharp reflections located at 18.48,35.68, 43.38,57.28,62.88,66.78,74.38,79.38 and 82.38,which are corresponding to (1 1 1),(3 1 1),(4 0 0),(3 3 1),(5 1 1),(4 4 0), (5 3 1),(5 3 3),(4 4 4) and (5 5 1) planes of a face-centered cubic spinel LTO with an Fd 3m space group,respectively. Thus,XRD proved that the synthesized spinel LTO have high purity and good crystallinity.
Fig. 2 shows the SEM morphologies of the Li4Ti5O12 oxides prepared by the gel-combustion method and the solid-state reaction (SSR). On the left image,the GCM sample consists uniform spheres with a coverage grain size of 200-500 nm. The GCM LTO particles have a well-crystallized structure with relatively uniform particle size. It can be seen from the right image that the SSR increased the particle size to 1mm with a roasted agglomerate appearance,which is interspersed with a small number of nanoscale-sized particles. The particle size of GCM LTO is smaller than that of SSR LTO,which suggests that the GCM can decrease the particle size and prevent the aggregation. This indicates that the PVP can reduce particle size to hinder the growth of LTO during combustion and the subsequent annealing. The carbon layer in close contact with the as-combusted LTO particles provides a better connection between adjacent LTO particles and prevents their further aggregation. Based on the previous results,the smaller particle size of the Li4Ti5O12 could introduce more active sites for the charge transfer processes and reduce the diffusion resistance of Li+and electron,thus a further improvement of the battery’s electrochemical performance could be expected.
It was expected that the GCM could be very effective in enhancing the electrochemical activity of Li4Ti5O12 . Fig. 3 shows the rate performance of the GCM LTO and the SSR LTO electrodes at different current densities from 0.5 C to 5 C. For each stage,the test is performed for 20 cycles. The GCM LTO shows much better cycle stability and much better rate capability than SSR LTO at the same discharge rate. The discharge capacity decreases as the rate increases,which is caused by polarization. GCM LTO delivered a discharge capacity of 167.6 mAh/g at 0.5 C,166.3 mAh/g at 1 C, 152.9 mAh/g at 3 C,and 142.2 mAh/g at 5 C,respectively. The GCM LTO anode material exhibits an outstanding performance in theCrate test. The reversible capacity of GCM LTO at 5 C was as high as that of SSR LTO at 1 C,indicating that the rate capability could be significantly enhanced by changing the synthetic method. Additionally,the GCM LTO at 5 C could still deliver a stable capacity of 142.2 mAh/g,reaching nearly 86% of the capacity at 0.5 C. The excellent cycling stability at highC-rates was originated from the associated sub-microscale. Accordingly,sub-microscale by GCM could provide a relatively short lithium transportation path,facilitating lithium diffusion during cycling,especially at high cycling rates. In contrast,theC-rate capability test of SSR LTO is illustrated in Fig. 3. As compared to theC-rate performance in GCM LTO,the 1st discharge capacity at 0.5 C was around 161.2 mAh/g. At increased current rates,SSR LTO demonstrated a rather poor cycling behavior,especially at high charge/discharge rates. The capacities of SSR LTO at the current rate of 1,3,and 5 C were around 150.7,136.8,and 114 mAh/g,respectively. This might be attributed to the larger particle size and roasted agglomerates can be seen from the SEM image in Fig. 2. During the lithium insertion process, lithium ions migrated into the material. The lithium transportation path was long in SSR LTO,owing to the structure. Eventually, lithium intercalation/deintercalation reactions in the center of lithium titanate failed to occur. SSR LTO could not sustain stable capacity at high charge/discharge rates,also leading to relatively low capacity. It should be noted that the cycling stability under high current rates was significantly affected by the synthetic route selection. Herein,lithium titanate by GCM delivered a better cycling performance,owing to its shorter lithium diffusion route.
|Fig. 3. Rate performance of GCM LTO and SSR LTO at various cycling rates of 0.5 C,1C,3C and 5C.|
To further understand the higher rate performance of Li4Ti5O12 , electrochemical impedance spectrum (EIS) measurements are carried out at the complete state of discharge for GCM and SSR LTO samples (see Fig. 4(a)) The Nyquist plot consists of a semicircle at high-to-medium frequency region and a sloping line at low frequency region,which are referred to the charge-transfer reaction and Li+ion diffusion in the solid electrode material. The EIS is simulated using the equivalent circuit as shown in the inset of Fig. 4,in whichRsis the resistance of electrolyte,Rctreflects the charge-transfer resistance and CPE stands for the double layer capacitance that has taken the roughness of the particle surface into consideration. The slope line corresponding to the Warburg impedance is represented by W1. The fitting results indicate that theRctof GCM LTO is 48V,which is much smaller than that of SSR LTO (95V). This indicates that the GCM LTO has a better lithium ion transfer than the SSR LTO. The larger lithium ion diffusion coefficient means that a better capability to meet the lithium ion transfer at high charge/discharge rates.
|Fig. 4. (a) EIS spectra of GCM LTO and SSR LTO electrodes with the frequency range of 10-1 -106 Hz; (b) cyclic voltammograms of cell using the GCM and SSR LTO at the scanning rate of 0.1 mV/s.|
The cyclic voltammograms of Li4Ti5O12 prepared by GCM and SSR are shown in Fig. 4(b). As seen in the figure,only one pair of oxidation/reduction peaks are observed in the CV curves of both GCM LTO and SSR LTO,which corresponds to an oxidation and reduction process of the Ti4+/Ti3+couple in the cubic structure. In the GCM LTO cycle,the cathodic and anodic peaks are at 1.46 V and 1.69 V,respectively. These peaks are very sharp and fairly symmetrical to each other,indicating an excellent reversibility of Li ion insertion into and deinsertion from the LTO spinel and a fast kinetics of the electrochemical process. This is in good agreement with the redox potential of the electrochemical process (Li4Ti5O12 + 3Li3++3e=Li7Ti5O12E = 1.55 Vvs.Li/Li+) determined from charging/discharging tests. The difference between the anodic and cathodic peak potential is about 0.27 V. The general shape of the CV voltammogram is similar to those from previous reports . Synthesized SSR LTO by SSR correspond to the lithium insertion/extraction in spinel LTO lattice,but the peak area is smaller than that of the peak of GCM LTO. It further supports the high reversibility and capacity from the GCM LTO anode material. 4. Conclusion
In summary,we developed a facile approach to produce pure sub-microscale Li4Ti5O12 by a novel gel-combustion method using PVP as the polymer chelating agent and fuel. The introduction of PVP plays a crucial role in the synthetic process since it helps to control the grain size of Li4Ti5O12 and improves the electrochemical properties significantly. In terms of the electrochemical properties,the GCM LTO exhibits better electrochemical performance. It can deliver a discharge capacity of 168 mAh/g at 0.5 C and reach a discharge capacity of 143 mAh/g even at a high rate of 5 C. The cycling performance is also exciting and the capacity can remain at 153 mAh/g and the capacity retention is 94.1% after 20 cycles at 3 C. The excellent rate and cycling performance is attributed to the smaller particle size,lower charge transfer resistance (48V) and the larger lithium ion diffusion coefficient. We believe that the sub-micron Li4Ti5O12 synthesized by the PVP assisted gel-combustion method could be a promising anode material for high rate lithium ion batteries.Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51374056),the support program for hundreds of outstanding innovative talents in Higher Education Institutions of Hebei Province (II) (No. BR2-127),Natural Science Foundation of Hebei Province (No. E2013501135),program for New Century Excellent Talents in University (No. NCET-10-0304), The Special Fund for Basic Scientific Research of Central Colleges, Northeastern University (Nos. N100123003 and N120523001).
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