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C prepared by one-step solid-state reaction
Electrochimica Acta 56 (2011) 5046–5053
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effects of carbon source and carbon content on electrochemical performances of Li4 Ti5 O12 /C prepared by one-step solid-state reaction Xuebu Hu a,b,∗∗ , Ziji Lin c , Kerun Yang b , Yongjian Huai d , Zhenghua Deng b,∗ a
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, Sichuan 610066, PR China Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, Sichuan 610041, PR China China National Quality Supervision & Inspection Center for Alcoholic Beverage Products and Processed Food, Luzhou, Sichuan 646100, PR China d China Aviation Lithium Battery Co. Ltd., Luoyang, Henan 471009, PR China b c
a r t i c l e
i n f o
Article history: Received 8 January 2011 Received in revised form 23 March 2011 Accepted 24 March 2011 Available online 2 April 2011 Keywords: Li4 Ti5 O12 /C Carbon source Carbon content Solid-state reaction Electrochemical performances
a b s t r a c t Li4 Ti5 O12 /C composites were synthesized by one-step solid-state reaction method using four commonly used organic compounds or organic polymers as carbon source, i.e., polyacrylate acid (PAA), citric acid (CA), maleic acid (MA) and polyvinyl alcohol (PVA). The physical characteristics of Li4 Ti5 O12 /C composites were investigated by X-ray diffraction, electron microscopy, Raman spectroscopy, particle size distribution and thermogravimetry-derivative thermogravimetry techniques. Their electrochemical properties were characterized by cyclic voltammograms, electrochemical impedance spectra, constant current charge–discharge and rate charge–discharge. These analyses indicated that the carbon source and carbon content have a great effect on the physical and electrochemical performances of Li4 Ti5 O12 /C composites. An ideal carbon source and appropriate carbon content effectively improved the electrical contact between the Li4 Ti5 O12 particles, which enhanced the discharge capacity and rate capability of Li4 Ti5 O12 /C composites. PAA was the best carbon source for the synthesis of Li4 Ti5 O12 /C composites. When the carbon content was 3.49 wt.% (LiOH·H2 O/PAA molar ratio of 1), as-prepared Li4 Ti5 O12 /C showed the maximum discharge capacity. At 0.2 C, initial capacity of the optimized sample was 168.6 mAh g−1 with capacity loss of 2.8% after 50 cycles. At 8 and 10 C, it showed discharge capacities of 143.5 and 132.7 mAh g−1 , with capacity loss of 8.7 and 9.9% after 50 cycles, respectively. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to unique properties, spinel Li4 Ti5 O12 as a new anode material has attracted widespread attention in lithium-ion battery industry and is regarded as one of the most possible substitutes for commercial graphite materials [1,2]. It shows zero-strain structure characteristic and a stable operating voltage of about 1.55 V versus lithium, and has good cycling stability, safety, low cost and simple synthesis [3–5]. Therefore, Li4 Ti5 O12 for power lithium-ion battery has great value in scientific research and commercial applications. Despite many advantages associated with Li4 Ti5 O12 , low electric conductivity prohibits its use in large-scale applications. Therefore, many methods have been attempted to improve the electric conductivity of Li4 Ti5 O12 . Surface modification of Li4 Ti5 O12 particle using carbon has been proved as an effective way to improve the
∗ Corresponding author. Tel.: +86 28 85229252; fax: +86 28 85233426. ∗∗ College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, Sichuan 610066, PR China. E-mail address: [email protected] (Z. Deng). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.092
electric conductivity of the material. Gao et al. [6,7] synthesized a spherical Li4 Ti5 O12 /C and La3+ -doped Li4 Ti5 O12 /C composites via an “outer gel” method using carbon black. Gao et al. [8] also reported spherical Li4 Ti5 O12 /C composites by spray-drying-solidstate reaction method using acetylene carbon black. Yang and Gao [9] synthesized Li4 Ti5 O12 /C composites via a simple solid-state reaction using Super-P-Li conductive carbon black as reaction precursor. Chen et al. [10] prepared a carbon-coated nanostructured Li4 Ti5 O12 via a simple carbon pre-coating process using carboncoated TiO2 and Li2 CO3 . In addition, it has been reported that Li4 Ti5 O12 /C were prepared with other carbon sources such as sugar [11,12], polymers [13,14] and pitch [15]. The electrical contact of each Li4 Ti5 O12 particle is improved significantly attributed to the surface modification using carbon coating, thus enhancing electrochemical performances of Li4 Ti5 O12 . Currently two major synthesis methods of Li4 Ti5 O12 /C composites are solid-state sintering method and sol–gel method. The sol–gel method usually can provide uniform morphology with a narrow size distribution [7,14]. However, this method is unsuitable for industrial production due to a high synthetic cost and complexity of synthetic route. Compared with the sol–gel method, the solid-state sintering method has a simple and low-cost synthetic
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Fig. 1. TG-DTG curves of the Li4 Ti5 O12 /C precursors prepared by different carbon sources: (a) PAA, (b) CA, (c) MA, (d) PVA.
route [9,11,12,15]. Owing to low cost and low thermal decomposition temperature, organic compounds or organic polymers are ideal carbon sources for the synthesis of Li4 Ti5 O12 /C composites. In previous studies [11,12,15], the electrochemical performances of Li4 Ti5 O12 /C composites prepared by organic compounds or organic polymers have been greatly improved but still not very satisfying. In this paper, spinel Li4 Ti5 O12 /C composites were synthesized from four commonly used organic compounds or organic polymers via one-step solid-state reaction. The physical and electrochemical properties of the Li4 Ti5 O12 /C composites prepared by different carbon sources were examined. The effects of the carbon content on the electrochemical performance of Li4 Ti5 O12 /C composites were systematically studied. Furthermore, the electrochemical performances of the Li4 Ti5 O12 /C composites prepared at optimized synthesis conditions were tested. It is desirable that as-prepared Li4 Ti5 O12 /C under optimized conditions shows excellent electrochemical performances. 2. Experimental 2.1. Preparation of Li4 Ti5 O12 /C composites 2.1.1. Li4 Ti5 O12 /C derived from different carbon sources The different precursors were sintered at 800 ◦ C for 8 h in flowing nitrogen with heating rate of 5 ◦ C min−1 , followed by cooling down to room temperature slowly. The final Li4 Ti5 O12 /C samples were obtained by grinding. Li4 Ti5 O12 /C composites prepared with the different precursors as PAALi + TiO2 , CALi + TiO2 , MALi + TiO2 and PVA + LiOH + TiO2 were labeled as LCTO-A, LCTO-B, LCTO-C and LCTO-D, respectively. 2.1.2. As-prepared Li4 Ti5 O12 /C with different carbon contents using PAA A stoichiometric amount of LiOH·H2 O was dissolved into aqueous solutions of polyacrylic acid with different molar ratios. Then powdered TiO2 (Li:Ti = 4:5) was added into the PAALi solutions with continuous stirring for 6 h to yield homogeneous
PAALi + TiO2 slurries. Subsequently, the above mixtures were dried at 100 ◦ C, sintered at 800 ◦ C for 8 h in flowing nitrogen, followed by cooling down to room temperature slowly. The Li4 Ti5 O12 /C samples with different carbon contents were obtained by grinding.
2.2. Cells assembly and performance evaluation A Q500 was used for thermogravimetric analysis (TG) with a heating rate of 10 ◦ C min−1 from 25 to 800 ◦ C at a nitrogen flow rate. X-ray diffraction (XRD) was carried on a Rigaku D/Max 2550 powder diffractometer with Cu K␣ radiation of = 1.5418 A˚ in the range of 10◦ < 2 < 90◦ . The surface carbon structure in the samples was investigated by Raman spectroscopy (Invia Raman Microscope). Scanning electron microscopy (SEM) was conducted on a JEOL JSM-6700F scanning electron microscope. Transmission electron microscopy (TEM) was revealed by a JEOL JEM-3010 transmission electron microscope. The particle size distribution was identified by a BT-2003 laser particle size analyzer. Electrochemical impedance spectra (EIS) were performed by a Solartron 1260/1287 impedance analyzer in the frequency range from 10−1 to 106 Hz. Constant current charge–discharge and cycling performance of the cells were tested on a Neware Battery Tester and cyclic voltammograms (CVs) were investigated by an Arbin Instrument. Electrochemical properties of the Li4 Ti5 O12 /C composites were measured with 2032 coin cells, for which a lithium metal foil as the counter electrode. The electrolyte was a 1.0 mol L−1 LiPF6 solution in the mixture of ethylene carbonate, dimethyl carbonate and ethylene methyl carbonate (1:1:1 by volume). A Celgard 2400 polypropylene membrane was used as the separator. The Li4 Ti5 O12 /C working electrodes were prepared by mixing active materials, conductive carbon black (Super P, TIMCAL) and an aqueous binder LA132 (Indigo, China) in a weight ratio of 86:10:4, and then pasted uniformly onto a copper foil and dried to give the electrodes. The cells were assembled in an argon-filled dry glove box.
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Fig. 2. XRD patterns of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D (red line means the standard Li4 Ti5 O12 ).
3. Results and discussion 3.1. Influence of different carbon sources on electrochemical properties of Li4 Ti5 O12 /C 3.1.1. Thermal analysis Fig. 1 shows the TG-DTG curves of the precursors derived from different carbon sources. The figures clearly show a step-like pattern of weight loss for all the precursors studied. All TG curves from 25 to 200 ◦ C mainly correspond to the vaporization of water and the decomposition of small volatile molecules. When the temperature is above 600 ◦ C, the TG curves show nearly constant weight, indicating that the reaction is complete, which makes Li4 Ti5 O12 completely crystallized. It is different in the temperature range of 200–600 ◦ C that the TG curves of the precursors derived from different carbon sources. The weight loss from 400 to 550 ◦ C is attributable to the thermal decomposition of PAA in Fig. 1a. The weight loss from 300 to 500 ◦ C mainly corresponds to the thermal decomposition of CA in Fig. 1b, and the stage from 300 to 350 ◦ C is mainly because of decarboxylation of CALi. In Fig. 1c, the weight loss from 300 to 500 ◦ C is related to the thermal decomposition of MA. However, the weight loss from 350 to 600 ◦ C is attributable to the thermal decomposition of PVA and LiOH in Fig. 1d.
3.1.2. XRD, electron microscopy and particle size distribution analysis Fig. 2 presents the XRD patterns of Li4 Ti5 O12 /C samples sintered with different carbon sources. It is found that the crystal structure of the samples is slightly changed with different carbon sources. When using polymer carbon sources (Fig. 2a and d), the XRD diagrams of the Li4 Ti5 O12 /C are in good accordance with the standard Li4 Ti5 O12 pattern (PDF no. 26-1198), demonstrating that a single phase is obtained with no evidence of impurities. When using nonpolymer carbon sources (Fig. 2b and c), the samples are composed of spinel Li4 Ti5 O12 /C as the main phase together with anatase TiO2 as impurity phase. Moreover, Fig. 2c shows that there is weak diffrac-
tion peak intensity and an uneven baseline. The uneven baseline is attributable to the existence of amorphous samples. Because of its low content or amorphous state, there is no obvious diffraction peak of the carbon in all XRD patterns. Fig. 3 shows the SEM micrographs of Li4 Ti5 O12 /C synthesized by different carbon sources. As is seen in Fig. 3a, b and d, the Li4 Ti5 O12 /C samples display regular shape, uniform size distribution and a small particle size of 200–500 nm. By contrast, as revealed in Fig. 3c, although the average particle size is about 300 nm, the reunion phenomenon of Li4 Ti5 O12 /C particles is obvious and there are a large number of amorphous particles, which is consistent with the results of XRD patterns. Fig. 4 presents the TEM micrograph of Li4 Ti5 O12 /C derived from PAA. It can be clearly seen from the micrograph that the surface of Li4 Ti5 O12 particles is coated with an amorphous carbon layer, which is responsible for good electric conductivity. The size distribution of Li4 Ti5 O12 /C synthesized by different carbon sources is depicted in Fig. 5. All samples show a relatively sharp peak of size distribution in the range of 0.1–1 m, indicating that they exhibit a narrow size distribution. The average particle size of LCTO-A, LCTO-B, LCTO-C and LCTO-D is 0.30, 0.36, 0.68 and 0.44 m, respectively. Due to obvious reunion phenomenon, LCTO-C sample has the largest average particle size, which is consistent with the conclusions of SEM micrographs.
3.1.3. Raman spectroscopy analysis Fig. 6 shows the Raman spectra of Li4 Ti5 O12 /C synthesized by different carbon sources. The intense broad bands at about 1350 and 1592 cm−1 are assigned to the disorder (D) and graphene (G) bands of residual carbon in Li4 Ti5 O12 /C composites, respectively. The G band corresponds to one of the E2g modes, which has been assigned as the sp2 graphite-like structure, whereas the D band corresponds to one of the A1g modes, which is attributed to the sp3 type tetrahedral carbon. The value ID /IG (the peak intensity ratio) can be used to evaluate the content of sp3− and sp2− coordinated carbon in the sample, as well as the degree of disorder for the pyrolytic car-
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Fig. 3. SEM of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D. Table 1 ID /IG ratios, carbon contents and initial discharge capacities of the Li4 Ti5 O12 /C samples. Sample
Carbon source
Raman band (cm−1 )
ID /IG
Asp3 /Asp2
Carbon content (wt.%)
Initial discharge capacity (mAh g−1 )
LCTO-A
PAA
0.961
2.04
4.53
161.5
LCTO-B
CA
0.832
1.51
3.07
149.5
LCTO-C
MA
0.945
1.82
3.18
126.7
LCTO-D
PVA
sp3 sp2 sp3 sp2 sp3 sp2 sp3 sp2
0.966
1.45
3.74
148.1
1351 1587 1354 1594 1355 1592 1349 1594
Note: The charge–discharge rate is 0.2 C.
bon. Theoretically, the low values for the ID /IG and Asp3 /Asp2 (the area ratio of sp3− to sp2− coordinated carbon) parameters indicate a high degree of graphitization [16]. Table 1 lists some carbon-related properties of Li4 Ti5 O12 /C samples, such as Raman spectral analysis,
Fig. 4. TEM of the Li4 Ti5 O12 /C sample derived from PAA.
carbon content and cell performance. In general, the lower ID /IG and sp3 /sp2 values exhibit the higher discharge capacity and better rate capability. Moreover, carbon content also directly affects the electrochemical performances of Li4 Ti5 O12 /C samples. Therefore, the carbon content and the values for the ID /IG and Asp3 /Asp2 should be appropriate for Li4 Ti5 O12 /C samples in order that the samples show good electrochemical performances. As is seen in this table, the ID /IG ratios of all samples are from 0.832 to 0.966, indicating that the pyrolytic carbon is mainly in an amorphous structure. If maximum discharge capacity is defined as a standard, PAA can be regarded as the best carbon source because LCTO-A has the highest discharge capacity. 3.1.4. EIS analysis Fig. 7 presents EIS of Li4 Ti5 O12 /C composites and pristine Li4 Ti5 O12 . It can be seen that the order is LCTO-A, LCTO-B, LCTO-D, LCTO-C and Li4 Ti5 O12 according to the descending order of electric conductivity of all samples respectively, which is due to the presence of pyrolytic carbon. Furthermore, the increasing of the electric conductivity of Li4 Ti5 O12 /C composites can provide good rate capability. If the electric conductivity is defined as a standard, PAA is the best carbon source because LCTO-A has the largest electric conductivity. These facts are consistent with the results of Raman spectra.
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Fig. 5. The particle size distribution of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
3.1.5. CVs analysis The CVs of various Li4 Ti5 O12 /C electrodes cycled between 1.0 and 3.0 V with 0.2 mV s−1 are shown in Fig. 8. It is obvious that four samples show a pair of redox peaks, which are the characteristics redox peaks of Li4 Ti5 O12 (about 1.70 and 1.46 V). The redox peaks of the samples show good symmetry, small half-peak width and peak potential difference. These facts illustrate that Li4 Ti5 O12 /C composites can be obtained by appropriate heat treatment when different carbon sources are used. It can be expected that the Li4 Ti5 O12 /C samples give good cycling stability because of the absence of impurity phase.
Fig. 6. Raman spectra of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
3.1.6. Electrochemical analysis Fig. 9 shows the first charge–discharge curves and the cycle curves of the Li4 Ti5 O12 /C electrodes at 0.2 C. It can be clearly seen from the figure that all the profiles exhibit extremely flat operating voltage. LCTO-A sample synthesized by PAA has minimum electrode polarization and its initial discharge capacity reaches 161.5 mAh g−1 . After 50 cycles the capacity loss of LCTO-A, LCTO-B, LCTO-C and LCTO-D is 4.1%, 12.1%, 2.8% and 3.9%, respectively. The results show that all samples have a good cycling stability except LCTO-B sample. The above results show the physical and electrochemical performances of as-prepared Li4 Ti5 O12 /C are influenced greatly by different carbon sources. In the samples prepared by four selected carbon sources, LCTO-A has a smallest particle size, narrow size distribution and contains the appropriate carbon content and the ID /IG ratio, which causes the maximum electric conductivity, best discharge capacity and cycling stability. Therefore, the PAA is chosen as the optimum carbon source for the synthesis of Li4 Ti5 O12 /C composites. The carbon content is another important influence factor on the electrochemical performances of Li4 Ti5 O12 /C. Therefore, the effect of the carbon content on the electrochemical performances of LCTO-A sample was systematically studied. 3.2. Influence of different carbon contents on electrochemical properties of Li4 Ti5 O12 /C
Fig. 7. EIS of the Li4 Ti5 O12 /C composites and pristine Li4 Ti5 O12 .
3.2.1. Tap-density, electric conductivity and initial discharge capacity Table 2 summaries some properties of LCTO-A samples containing different carbon contents, such as tap-density, electric conductivity and initial discharge capacity. As seen, initial dis-
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Fig. 8. CVs of the Li4 Ti5 O12 /C electrodes prepared by different carbon sources: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
Fig. 9. The first charge–discharge curves and the cycle curves of Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
Table 2 Carbon contents, tap-densities, electric conductivities and initial discharge capacities of LCTO-A samples prepared by different molar ratios of LiOH·H2 O/PAA. Sample
Molar ratio (LiOH·H2 O/PAA)
Carbon content (wt.%)
Tap-density (g cm−3 )
Electric conductivity (×10−7 S cm−1 )
Initial discharge capacity (mAh g−1 )
LTO LCTO-A1 LCTO-A2 LCTO-A3 LCTO-A4 LCTO-A5
1:1 1:0.90 1:0.96 1:1 1:1.04 1:1.10
0 2.82 3.12 3.49 3.95 4.33
1.102 1.027 1.025 1.016 1.005 1.001
0.65 1.08 1.32 1.83 2.35 2.82
145.8 146.4 147.9 160.5 155.9 150.5
Note: The charge–discharge rate is 0.2 C.
charge capacities of five LCTO-A samples are higher than the pristine Li4 Ti5 O12 when there is a certain amount of pyrolytic carbon. With decreasing the LiOH·H2 O/PAA molar ratio, the tapdensity of LCTO-A composites decreases while the carbon content and the electric conductivity increase, which illustrates that the increase of the carbon content can improve the electric conductivity. It can be clearly seen from the table that too low or too high carbon content in the samples does not lead to highest discharge capacity. When the value of LiOH·H2 O/PAA is 1, a maximum discharge capacity for as-prepared LCTO-A3 with the carbon content of 3.49 wt.% is 160.5 mAh g−1 .
3.2.2. CVs analysis Fig. 10 shows CVs of the LCTO-A electrodes cycled between 1.0 and 3.0 V with 0.2 mV s−1 . It is clear that all samples show a pair of reversible redox peaks of Li4 Ti5 O12 . However, the redox peak positions of the samples have been slight changed due to the different carbon contents. The less in the potential difference between reduction and oxidation peaks potentials (E), the better in reversibility of the redox reaction. When increasing the carbon content from 2.82 to 3.49 wt.%, E decreases, whereas on further increase in the carbon content, E again increases. This results from the variation of the particle size [17], LCTO-A3 possessing a well-controlled mor-
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Fig. 10. CVs of the LCTO-A electrodes with different carbon contents.
phology with small particles and an appropriate carbon content for adequate electrical conductivity shows the best reversibility of the redox reaction. Thus, when the LiOH·H2 O/PAA molar ratio is 1:1, as-prepared LCTO-A3 has a little electrode polarization attributed to a minimum E of 0.22 V. These results are consistent with the conclusions of Table 2. 3.2.3. Electrochemical analysis The cycling behavior of the LCTO-A samples at 0.2 C are shown in Fig. 11. After 30 cycles the capacity loss of pristine Li4 Ti5 O12, LCTO-A1 , LCTO-A2 , LCTO-A3 , LCTO-A4 and LCTO-A5 is 6.9%, 4.9%, 3.2%, 0.3%, 1.0% and 0.4%, respectively. As shown in the figure, the cycling performances of five LCTO-A samples are better than that of
Fig. 11. Cycling performances of LCTO-A samples with different carbon contents.
the pristine Li4 Ti5 O12 when there is a certain amount of pyrolytic carbon. It is indicated that the added carbon with high conductivity enhances the intercalation reaction, reduces the surface polarization between electrode and electrolyte and improves the cycling performance. LCTO-A3 has the best cycling performance in all samples, which is consistent with the results of CVs. 3.3. Electrochemical properties of Li4 Ti5 O12 /C obtained under optimized conditions The above results show that the carbon source and the carbon content are two important factors on the electrochemical performances of Li4 Ti5 O12 /C. In order to maximize the discharge capacity and exhibit the best cycling stability, the Li4 Ti5 O12 /C composites with the optimized electrochemical performances were obtained in the following conditions, i.e., PAA as the carbon source, LiOH·H2 O/PAA molar ratio of 1 (carbon content of 3.49 wt.%), heat treatment temperature of 800 ◦ C and sintering duration of 8 h. Fig. 12 shows the first charge–discharge curves and the cycle curves of as-prepared Li4 Ti5 O12 /C under optimized conditions. As shown in the figure, with the increase of current density, the electrode polarization increases gradually and the discharge voltage plateau gradually decreases. The discharge voltage plateau drops from 1.55 V at 0.2 C to 1.48 V at 10 C, indicating that the discharge capacity of the sample decreases with increasing the charge–discharge rate. At 0.2 C, the initial capacity is 168.6 mAh g−1 with capacity retention of 97.2% after 50 cycles. At 8 and 10 C, the initial capacity is 143.5 and 132.7 mAh g−1 with capacity retention of 91.3 and 90.1% after 50 cycles, respectively. It is obvious that the discharge capacity and cycling capability of as-prepared Li4 Ti5 O12 /C under optimized conditions are better than those of
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Fig. 12. The first charge–discharge curves and the cycle curves of as-prepared Li4 Ti5 O12 /C sample under optimized conditions.
Li4 Ti5 O12 /C in the previous studies [10,11,14]. The results show that as-prepared Li4 Ti5 O12 /C under optimized conditions has high rate capability and high discharge capacity.
rate cycling performance, which make it become promising anode materials for lithium ion batteries. References
4. Conclusions Spinel Li4 Ti5 O12 /C was prepared by one-step solid-state reaction using four commonly used organic compounds or organic polymers (PAA, CA, MA and PVA) as carbon source. In our study, PAA is the best carbon source for the synthesis of Li4 Ti5 O12 /C composites. Moreover, the added carbon with high conductivity effectively improves the electrical contact between the Li4 Ti5 O12 particles and enhances the electrochemical performances of Li4 Ti5 O12 /C. The results demonstrate that as-prepared Li4 Ti5 O12 /C shows the maximum discharge capacity when the carbon content is 3.49 wt.% (LiOH·H2 O/PAA molar ratio of 1). Even at charge–discharge rate of 8 and 10 C, the optimized sample shows discharge capacities of 143.5 and 132.7 mAh g−1 , with capacity loss of 8.7 and 9.9% after 50 cycles, respectively. As a result, the Li4 Ti5 O12 /C composites derived from this synthetic route shows high discharge capacity and good
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Effects of carbon source and carbon content on electrochemical performances of Li4 Ti5 O12 /C prepared by one-step solid-state reaction Xuebu Hu a,b,∗∗ , Ziji Lin c , Kerun Yang b , Yongjian Huai d , Zhenghua Deng b,∗ a
College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, Sichuan 610066, PR China Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu, Sichuan 610041, PR China China National Quality Supervision & Inspection Center for Alcoholic Beverage Products and Processed Food, Luzhou, Sichuan 646100, PR China d China Aviation Lithium Battery Co. Ltd., Luoyang, Henan 471009, PR China b c
a r t i c l e
i n f o
Article history: Received 8 January 2011 Received in revised form 23 March 2011 Accepted 24 March 2011 Available online 2 April 2011 Keywords: Li4 Ti5 O12 /C Carbon source Carbon content Solid-state reaction Electrochemical performances
a b s t r a c t Li4 Ti5 O12 /C composites were synthesized by one-step solid-state reaction method using four commonly used organic compounds or organic polymers as carbon source, i.e., polyacrylate acid (PAA), citric acid (CA), maleic acid (MA) and polyvinyl alcohol (PVA). The physical characteristics of Li4 Ti5 O12 /C composites were investigated by X-ray diffraction, electron microscopy, Raman spectroscopy, particle size distribution and thermogravimetry-derivative thermogravimetry techniques. Their electrochemical properties were characterized by cyclic voltammograms, electrochemical impedance spectra, constant current charge–discharge and rate charge–discharge. These analyses indicated that the carbon source and carbon content have a great effect on the physical and electrochemical performances of Li4 Ti5 O12 /C composites. An ideal carbon source and appropriate carbon content effectively improved the electrical contact between the Li4 Ti5 O12 particles, which enhanced the discharge capacity and rate capability of Li4 Ti5 O12 /C composites. PAA was the best carbon source for the synthesis of Li4 Ti5 O12 /C composites. When the carbon content was 3.49 wt.% (LiOH·H2 O/PAA molar ratio of 1), as-prepared Li4 Ti5 O12 /C showed the maximum discharge capacity. At 0.2 C, initial capacity of the optimized sample was 168.6 mAh g−1 with capacity loss of 2.8% after 50 cycles. At 8 and 10 C, it showed discharge capacities of 143.5 and 132.7 mAh g−1 , with capacity loss of 8.7 and 9.9% after 50 cycles, respectively. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to unique properties, spinel Li4 Ti5 O12 as a new anode material has attracted widespread attention in lithium-ion battery industry and is regarded as one of the most possible substitutes for commercial graphite materials [1,2]. It shows zero-strain structure characteristic and a stable operating voltage of about 1.55 V versus lithium, and has good cycling stability, safety, low cost and simple synthesis [3–5]. Therefore, Li4 Ti5 O12 for power lithium-ion battery has great value in scientific research and commercial applications. Despite many advantages associated with Li4 Ti5 O12 , low electric conductivity prohibits its use in large-scale applications. Therefore, many methods have been attempted to improve the electric conductivity of Li4 Ti5 O12 . Surface modification of Li4 Ti5 O12 particle using carbon has been proved as an effective way to improve the
∗ Corresponding author. Tel.: +86 28 85229252; fax: +86 28 85233426. ∗∗ College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, Sichuan 610066, PR China. E-mail address: [email protected] (Z. Deng). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.092
electric conductivity of the material. Gao et al. [6,7] synthesized a spherical Li4 Ti5 O12 /C and La3+ -doped Li4 Ti5 O12 /C composites via an “outer gel” method using carbon black. Gao et al. [8] also reported spherical Li4 Ti5 O12 /C composites by spray-drying-solidstate reaction method using acetylene carbon black. Yang and Gao [9] synthesized Li4 Ti5 O12 /C composites via a simple solid-state reaction using Super-P-Li conductive carbon black as reaction precursor. Chen et al. [10] prepared a carbon-coated nanostructured Li4 Ti5 O12 via a simple carbon pre-coating process using carboncoated TiO2 and Li2 CO3 . In addition, it has been reported that Li4 Ti5 O12 /C were prepared with other carbon sources such as sugar [11,12], polymers [13,14] and pitch [15]. The electrical contact of each Li4 Ti5 O12 particle is improved significantly attributed to the surface modification using carbon coating, thus enhancing electrochemical performances of Li4 Ti5 O12 . Currently two major synthesis methods of Li4 Ti5 O12 /C composites are solid-state sintering method and sol–gel method. The sol–gel method usually can provide uniform morphology with a narrow size distribution [7,14]. However, this method is unsuitable for industrial production due to a high synthetic cost and complexity of synthetic route. Compared with the sol–gel method, the solid-state sintering method has a simple and low-cost synthetic
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Fig. 1. TG-DTG curves of the Li4 Ti5 O12 /C precursors prepared by different carbon sources: (a) PAA, (b) CA, (c) MA, (d) PVA.
route [9,11,12,15]. Owing to low cost and low thermal decomposition temperature, organic compounds or organic polymers are ideal carbon sources for the synthesis of Li4 Ti5 O12 /C composites. In previous studies [11,12,15], the electrochemical performances of Li4 Ti5 O12 /C composites prepared by organic compounds or organic polymers have been greatly improved but still not very satisfying. In this paper, spinel Li4 Ti5 O12 /C composites were synthesized from four commonly used organic compounds or organic polymers via one-step solid-state reaction. The physical and electrochemical properties of the Li4 Ti5 O12 /C composites prepared by different carbon sources were examined. The effects of the carbon content on the electrochemical performance of Li4 Ti5 O12 /C composites were systematically studied. Furthermore, the electrochemical performances of the Li4 Ti5 O12 /C composites prepared at optimized synthesis conditions were tested. It is desirable that as-prepared Li4 Ti5 O12 /C under optimized conditions shows excellent electrochemical performances. 2. Experimental 2.1. Preparation of Li4 Ti5 O12 /C composites 2.1.1. Li4 Ti5 O12 /C derived from different carbon sources The different precursors were sintered at 800 ◦ C for 8 h in flowing nitrogen with heating rate of 5 ◦ C min−1 , followed by cooling down to room temperature slowly. The final Li4 Ti5 O12 /C samples were obtained by grinding. Li4 Ti5 O12 /C composites prepared with the different precursors as PAALi + TiO2 , CALi + TiO2 , MALi + TiO2 and PVA + LiOH + TiO2 were labeled as LCTO-A, LCTO-B, LCTO-C and LCTO-D, respectively. 2.1.2. As-prepared Li4 Ti5 O12 /C with different carbon contents using PAA A stoichiometric amount of LiOH·H2 O was dissolved into aqueous solutions of polyacrylic acid with different molar ratios. Then powdered TiO2 (Li:Ti = 4:5) was added into the PAALi solutions with continuous stirring for 6 h to yield homogeneous
PAALi + TiO2 slurries. Subsequently, the above mixtures were dried at 100 ◦ C, sintered at 800 ◦ C for 8 h in flowing nitrogen, followed by cooling down to room temperature slowly. The Li4 Ti5 O12 /C samples with different carbon contents were obtained by grinding.
2.2. Cells assembly and performance evaluation A Q500 was used for thermogravimetric analysis (TG) with a heating rate of 10 ◦ C min−1 from 25 to 800 ◦ C at a nitrogen flow rate. X-ray diffraction (XRD) was carried on a Rigaku D/Max 2550 powder diffractometer with Cu K␣ radiation of = 1.5418 A˚ in the range of 10◦ < 2 < 90◦ . The surface carbon structure in the samples was investigated by Raman spectroscopy (Invia Raman Microscope). Scanning electron microscopy (SEM) was conducted on a JEOL JSM-6700F scanning electron microscope. Transmission electron microscopy (TEM) was revealed by a JEOL JEM-3010 transmission electron microscope. The particle size distribution was identified by a BT-2003 laser particle size analyzer. Electrochemical impedance spectra (EIS) were performed by a Solartron 1260/1287 impedance analyzer in the frequency range from 10−1 to 106 Hz. Constant current charge–discharge and cycling performance of the cells were tested on a Neware Battery Tester and cyclic voltammograms (CVs) were investigated by an Arbin Instrument. Electrochemical properties of the Li4 Ti5 O12 /C composites were measured with 2032 coin cells, for which a lithium metal foil as the counter electrode. The electrolyte was a 1.0 mol L−1 LiPF6 solution in the mixture of ethylene carbonate, dimethyl carbonate and ethylene methyl carbonate (1:1:1 by volume). A Celgard 2400 polypropylene membrane was used as the separator. The Li4 Ti5 O12 /C working electrodes were prepared by mixing active materials, conductive carbon black (Super P, TIMCAL) and an aqueous binder LA132 (Indigo, China) in a weight ratio of 86:10:4, and then pasted uniformly onto a copper foil and dried to give the electrodes. The cells were assembled in an argon-filled dry glove box.
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Fig. 2. XRD patterns of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D (red line means the standard Li4 Ti5 O12 ).
3. Results and discussion 3.1. Influence of different carbon sources on electrochemical properties of Li4 Ti5 O12 /C 3.1.1. Thermal analysis Fig. 1 shows the TG-DTG curves of the precursors derived from different carbon sources. The figures clearly show a step-like pattern of weight loss for all the precursors studied. All TG curves from 25 to 200 ◦ C mainly correspond to the vaporization of water and the decomposition of small volatile molecules. When the temperature is above 600 ◦ C, the TG curves show nearly constant weight, indicating that the reaction is complete, which makes Li4 Ti5 O12 completely crystallized. It is different in the temperature range of 200–600 ◦ C that the TG curves of the precursors derived from different carbon sources. The weight loss from 400 to 550 ◦ C is attributable to the thermal decomposition of PAA in Fig. 1a. The weight loss from 300 to 500 ◦ C mainly corresponds to the thermal decomposition of CA in Fig. 1b, and the stage from 300 to 350 ◦ C is mainly because of decarboxylation of CALi. In Fig. 1c, the weight loss from 300 to 500 ◦ C is related to the thermal decomposition of MA. However, the weight loss from 350 to 600 ◦ C is attributable to the thermal decomposition of PVA and LiOH in Fig. 1d.
3.1.2. XRD, electron microscopy and particle size distribution analysis Fig. 2 presents the XRD patterns of Li4 Ti5 O12 /C samples sintered with different carbon sources. It is found that the crystal structure of the samples is slightly changed with different carbon sources. When using polymer carbon sources (Fig. 2a and d), the XRD diagrams of the Li4 Ti5 O12 /C are in good accordance with the standard Li4 Ti5 O12 pattern (PDF no. 26-1198), demonstrating that a single phase is obtained with no evidence of impurities. When using nonpolymer carbon sources (Fig. 2b and c), the samples are composed of spinel Li4 Ti5 O12 /C as the main phase together with anatase TiO2 as impurity phase. Moreover, Fig. 2c shows that there is weak diffrac-
tion peak intensity and an uneven baseline. The uneven baseline is attributable to the existence of amorphous samples. Because of its low content or amorphous state, there is no obvious diffraction peak of the carbon in all XRD patterns. Fig. 3 shows the SEM micrographs of Li4 Ti5 O12 /C synthesized by different carbon sources. As is seen in Fig. 3a, b and d, the Li4 Ti5 O12 /C samples display regular shape, uniform size distribution and a small particle size of 200–500 nm. By contrast, as revealed in Fig. 3c, although the average particle size is about 300 nm, the reunion phenomenon of Li4 Ti5 O12 /C particles is obvious and there are a large number of amorphous particles, which is consistent with the results of XRD patterns. Fig. 4 presents the TEM micrograph of Li4 Ti5 O12 /C derived from PAA. It can be clearly seen from the micrograph that the surface of Li4 Ti5 O12 particles is coated with an amorphous carbon layer, which is responsible for good electric conductivity. The size distribution of Li4 Ti5 O12 /C synthesized by different carbon sources is depicted in Fig. 5. All samples show a relatively sharp peak of size distribution in the range of 0.1–1 m, indicating that they exhibit a narrow size distribution. The average particle size of LCTO-A, LCTO-B, LCTO-C and LCTO-D is 0.30, 0.36, 0.68 and 0.44 m, respectively. Due to obvious reunion phenomenon, LCTO-C sample has the largest average particle size, which is consistent with the conclusions of SEM micrographs.
3.1.3. Raman spectroscopy analysis Fig. 6 shows the Raman spectra of Li4 Ti5 O12 /C synthesized by different carbon sources. The intense broad bands at about 1350 and 1592 cm−1 are assigned to the disorder (D) and graphene (G) bands of residual carbon in Li4 Ti5 O12 /C composites, respectively. The G band corresponds to one of the E2g modes, which has been assigned as the sp2 graphite-like structure, whereas the D band corresponds to one of the A1g modes, which is attributed to the sp3 type tetrahedral carbon. The value ID /IG (the peak intensity ratio) can be used to evaluate the content of sp3− and sp2− coordinated carbon in the sample, as well as the degree of disorder for the pyrolytic car-
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Fig. 3. SEM of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D. Table 1 ID /IG ratios, carbon contents and initial discharge capacities of the Li4 Ti5 O12 /C samples. Sample
Carbon source
Raman band (cm−1 )
ID /IG
Asp3 /Asp2
Carbon content (wt.%)
Initial discharge capacity (mAh g−1 )
LCTO-A
PAA
0.961
2.04
4.53
161.5
LCTO-B
CA
0.832
1.51
3.07
149.5
LCTO-C
MA
0.945
1.82
3.18
126.7
LCTO-D
PVA
sp3 sp2 sp3 sp2 sp3 sp2 sp3 sp2
0.966
1.45
3.74
148.1
1351 1587 1354 1594 1355 1592 1349 1594
Note: The charge–discharge rate is 0.2 C.
bon. Theoretically, the low values for the ID /IG and Asp3 /Asp2 (the area ratio of sp3− to sp2− coordinated carbon) parameters indicate a high degree of graphitization [16]. Table 1 lists some carbon-related properties of Li4 Ti5 O12 /C samples, such as Raman spectral analysis,
Fig. 4. TEM of the Li4 Ti5 O12 /C sample derived from PAA.
carbon content and cell performance. In general, the lower ID /IG and sp3 /sp2 values exhibit the higher discharge capacity and better rate capability. Moreover, carbon content also directly affects the electrochemical performances of Li4 Ti5 O12 /C samples. Therefore, the carbon content and the values for the ID /IG and Asp3 /Asp2 should be appropriate for Li4 Ti5 O12 /C samples in order that the samples show good electrochemical performances. As is seen in this table, the ID /IG ratios of all samples are from 0.832 to 0.966, indicating that the pyrolytic carbon is mainly in an amorphous structure. If maximum discharge capacity is defined as a standard, PAA can be regarded as the best carbon source because LCTO-A has the highest discharge capacity. 3.1.4. EIS analysis Fig. 7 presents EIS of Li4 Ti5 O12 /C composites and pristine Li4 Ti5 O12 . It can be seen that the order is LCTO-A, LCTO-B, LCTO-D, LCTO-C and Li4 Ti5 O12 according to the descending order of electric conductivity of all samples respectively, which is due to the presence of pyrolytic carbon. Furthermore, the increasing of the electric conductivity of Li4 Ti5 O12 /C composites can provide good rate capability. If the electric conductivity is defined as a standard, PAA is the best carbon source because LCTO-A has the largest electric conductivity. These facts are consistent with the results of Raman spectra.
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Fig. 5. The particle size distribution of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
3.1.5. CVs analysis The CVs of various Li4 Ti5 O12 /C electrodes cycled between 1.0 and 3.0 V with 0.2 mV s−1 are shown in Fig. 8. It is obvious that four samples show a pair of redox peaks, which are the characteristics redox peaks of Li4 Ti5 O12 (about 1.70 and 1.46 V). The redox peaks of the samples show good symmetry, small half-peak width and peak potential difference. These facts illustrate that Li4 Ti5 O12 /C composites can be obtained by appropriate heat treatment when different carbon sources are used. It can be expected that the Li4 Ti5 O12 /C samples give good cycling stability because of the absence of impurity phase.
Fig. 6. Raman spectra of the Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
3.1.6. Electrochemical analysis Fig. 9 shows the first charge–discharge curves and the cycle curves of the Li4 Ti5 O12 /C electrodes at 0.2 C. It can be clearly seen from the figure that all the profiles exhibit extremely flat operating voltage. LCTO-A sample synthesized by PAA has minimum electrode polarization and its initial discharge capacity reaches 161.5 mAh g−1 . After 50 cycles the capacity loss of LCTO-A, LCTO-B, LCTO-C and LCTO-D is 4.1%, 12.1%, 2.8% and 3.9%, respectively. The results show that all samples have a good cycling stability except LCTO-B sample. The above results show the physical and electrochemical performances of as-prepared Li4 Ti5 O12 /C are influenced greatly by different carbon sources. In the samples prepared by four selected carbon sources, LCTO-A has a smallest particle size, narrow size distribution and contains the appropriate carbon content and the ID /IG ratio, which causes the maximum electric conductivity, best discharge capacity and cycling stability. Therefore, the PAA is chosen as the optimum carbon source for the synthesis of Li4 Ti5 O12 /C composites. The carbon content is another important influence factor on the electrochemical performances of Li4 Ti5 O12 /C. Therefore, the effect of the carbon content on the electrochemical performances of LCTO-A sample was systematically studied. 3.2. Influence of different carbon contents on electrochemical properties of Li4 Ti5 O12 /C
Fig. 7. EIS of the Li4 Ti5 O12 /C composites and pristine Li4 Ti5 O12 .
3.2.1. Tap-density, electric conductivity and initial discharge capacity Table 2 summaries some properties of LCTO-A samples containing different carbon contents, such as tap-density, electric conductivity and initial discharge capacity. As seen, initial dis-
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Fig. 8. CVs of the Li4 Ti5 O12 /C electrodes prepared by different carbon sources: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
Fig. 9. The first charge–discharge curves and the cycle curves of Li4 Ti5 O12 /C samples: (a) LCTO-A, (b) LCTO-B, (c) LCTO-C, (d) LCTO-D.
Table 2 Carbon contents, tap-densities, electric conductivities and initial discharge capacities of LCTO-A samples prepared by different molar ratios of LiOH·H2 O/PAA. Sample
Molar ratio (LiOH·H2 O/PAA)
Carbon content (wt.%)
Tap-density (g cm−3 )
Electric conductivity (×10−7 S cm−1 )
Initial discharge capacity (mAh g−1 )
LTO LCTO-A1 LCTO-A2 LCTO-A3 LCTO-A4 LCTO-A5
1:1 1:0.90 1:0.96 1:1 1:1.04 1:1.10
0 2.82 3.12 3.49 3.95 4.33
1.102 1.027 1.025 1.016 1.005 1.001
0.65 1.08 1.32 1.83 2.35 2.82
145.8 146.4 147.9 160.5 155.9 150.5
Note: The charge–discharge rate is 0.2 C.
charge capacities of five LCTO-A samples are higher than the pristine Li4 Ti5 O12 when there is a certain amount of pyrolytic carbon. With decreasing the LiOH·H2 O/PAA molar ratio, the tapdensity of LCTO-A composites decreases while the carbon content and the electric conductivity increase, which illustrates that the increase of the carbon content can improve the electric conductivity. It can be clearly seen from the table that too low or too high carbon content in the samples does not lead to highest discharge capacity. When the value of LiOH·H2 O/PAA is 1, a maximum discharge capacity for as-prepared LCTO-A3 with the carbon content of 3.49 wt.% is 160.5 mAh g−1 .
3.2.2. CVs analysis Fig. 10 shows CVs of the LCTO-A electrodes cycled between 1.0 and 3.0 V with 0.2 mV s−1 . It is clear that all samples show a pair of reversible redox peaks of Li4 Ti5 O12 . However, the redox peak positions of the samples have been slight changed due to the different carbon contents. The less in the potential difference between reduction and oxidation peaks potentials (E), the better in reversibility of the redox reaction. When increasing the carbon content from 2.82 to 3.49 wt.%, E decreases, whereas on further increase in the carbon content, E again increases. This results from the variation of the particle size [17], LCTO-A3 possessing a well-controlled mor-
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Fig. 10. CVs of the LCTO-A electrodes with different carbon contents.
phology with small particles and an appropriate carbon content for adequate electrical conductivity shows the best reversibility of the redox reaction. Thus, when the LiOH·H2 O/PAA molar ratio is 1:1, as-prepared LCTO-A3 has a little electrode polarization attributed to a minimum E of 0.22 V. These results are consistent with the conclusions of Table 2. 3.2.3. Electrochemical analysis The cycling behavior of the LCTO-A samples at 0.2 C are shown in Fig. 11. After 30 cycles the capacity loss of pristine Li4 Ti5 O12, LCTO-A1 , LCTO-A2 , LCTO-A3 , LCTO-A4 and LCTO-A5 is 6.9%, 4.9%, 3.2%, 0.3%, 1.0% and 0.4%, respectively. As shown in the figure, the cycling performances of five LCTO-A samples are better than that of
Fig. 11. Cycling performances of LCTO-A samples with different carbon contents.
the pristine Li4 Ti5 O12 when there is a certain amount of pyrolytic carbon. It is indicated that the added carbon with high conductivity enhances the intercalation reaction, reduces the surface polarization between electrode and electrolyte and improves the cycling performance. LCTO-A3 has the best cycling performance in all samples, which is consistent with the results of CVs. 3.3. Electrochemical properties of Li4 Ti5 O12 /C obtained under optimized conditions The above results show that the carbon source and the carbon content are two important factors on the electrochemical performances of Li4 Ti5 O12 /C. In order to maximize the discharge capacity and exhibit the best cycling stability, the Li4 Ti5 O12 /C composites with the optimized electrochemical performances were obtained in the following conditions, i.e., PAA as the carbon source, LiOH·H2 O/PAA molar ratio of 1 (carbon content of 3.49 wt.%), heat treatment temperature of 800 ◦ C and sintering duration of 8 h. Fig. 12 shows the first charge–discharge curves and the cycle curves of as-prepared Li4 Ti5 O12 /C under optimized conditions. As shown in the figure, with the increase of current density, the electrode polarization increases gradually and the discharge voltage plateau gradually decreases. The discharge voltage plateau drops from 1.55 V at 0.2 C to 1.48 V at 10 C, indicating that the discharge capacity of the sample decreases with increasing the charge–discharge rate. At 0.2 C, the initial capacity is 168.6 mAh g−1 with capacity retention of 97.2% after 50 cycles. At 8 and 10 C, the initial capacity is 143.5 and 132.7 mAh g−1 with capacity retention of 91.3 and 90.1% after 50 cycles, respectively. It is obvious that the discharge capacity and cycling capability of as-prepared Li4 Ti5 O12 /C under optimized conditions are better than those of
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Fig. 12. The first charge–discharge curves and the cycle curves of as-prepared Li4 Ti5 O12 /C sample under optimized conditions.
Li4 Ti5 O12 /C in the previous studies [10,11,14]. The results show that as-prepared Li4 Ti5 O12 /C under optimized conditions has high rate capability and high discharge capacity.
rate cycling performance, which make it become promising anode materials for lithium ion batteries. References
4. Conclusions Spinel Li4 Ti5 O12 /C was prepared by one-step solid-state reaction using four commonly used organic compounds or organic polymers (PAA, CA, MA and PVA) as carbon source. In our study, PAA is the best carbon source for the synthesis of Li4 Ti5 O12 /C composites. Moreover, the added carbon with high conductivity effectively improves the electrical contact between the Li4 Ti5 O12 particles and enhances the electrochemical performances of Li4 Ti5 O12 /C. The results demonstrate that as-prepared Li4 Ti5 O12 /C shows the maximum discharge capacity when the carbon content is 3.49 wt.% (LiOH·H2 O/PAA molar ratio of 1). Even at charge–discharge rate of 8 and 10 C, the optimized sample shows discharge capacities of 143.5 and 132.7 mAh g−1 , with capacity loss of 8.7 and 9.9% after 50 cycles, respectively. As a result, the Li4 Ti5 O12 /C composites derived from this synthetic route shows high discharge capacity and good
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