Carbon-Enhanced Electrochemical Performance for Spinel

Dec 5, 2016 - The design and facile fabrication of CrTi-based anode materials with long-life, good safety, and low cost are strongly desired for lithi...
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Research Article pubs.acs.org/journal/ascecg

Carbon-Enhanced Electrochemical Performance for Spinel Li5Cr7Ti6O25 as a Lithium Host Material Lei Yan, Shangshu Qian, Haoxiang Yu, Peng Li, Hua Lan, Nengbing Long, Ruifeng Zhang, Miao Shui, and Jie Shu* Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, Zhejiang Province, P. R. China

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ABSTRACT: The design and facile fabrication of CrTi-based anode materials with long-life, good safety, and low cost are strongly desired for lithium-ion batteries. In this study, we adopt the sol−gel method using glucose as carbon source to synthesize carbon-coated Li5Cr7Ti6O25. The effects of a carbon layer in Li5Cr7Ti6O25/C on electrochemical properties are focused through comparing carboncoated Li5Cr7Ti6O25 with carbon-free Li5Cr7Ti6O25. Electrochemical tests show that Li5Cr7Ti6O25/C possesses better cycling and rate performances than those of carbon-free Li5Cr7Ti6O25. Cycled at 500 mA g−1, Li5Cr7Ti6O25/C delivers a high specific capacity of 111.6 mAh g−1 with 13% capacity loss after 200 cycles. In contrast, Li5Cr7Ti6O25 only exhibits a specific capacity of 93.6 mAh g−1 at 500 mA g−1 with 19% capacity loss after 200 cycles. The preeminent electrochemical property of Li5Cr7Ti6O25/C is ascribed to the amorphous carbon coating layer. This carbon layer can remarkably facilitate the transportation of electrons and ions in Li5Cr7Ti6O25. Furthermore, the lithiation/delithiation behavior of Li5Cr7Ti6O25/C is also investigated by advanced in situ X-ray diffraction technique, and the results verify that Li5Cr7Ti6O25/C possesses a highly reversible structural change during the charge/discharge process. KEYWORDS: Li5Cr7Ti6O25, Carbon coating, Anode material, In situ X-ray diffraction, Lithium-ion batteries

1. INTRODUCTION The development of high safe and low-cost energy storage devices for electric vehicles and hand-held electronics has been a major goal in electrochemical fields over the last several years.1−7 Lithium-ion batteries (LIBs) are deemed as the best potential candidate because of having better safety than fossil oils and better reproducibility than coal. Thus, LIBs can bridge the gap between fossil energy and renewable energy.8−13 In addition, LIBs possess larger energy density and higher output power to meet the demand of various technological applications. As an essential section of LIBs, the anode material is vital to the determination of safety and the life cycle. As is well known, graphite is generally chosen as the commercialized anode material for LIBs. Unfortunately, graphite as the anode material may cause a serious safety issue at high rate for the formation of a solid-electrolyte interphase (SEI) and lithium dendrite below 0.8 V versus Li+/Li.14−19 Subsequently, spinel Li4Ti5O12 is seen as a hopeful alternative material due to its excellent Li+ intercalation/deintercalation reversibility, high voltage plateau (1.55 V) avoiding SEI film formation, and appreciable theoretical capacity (175 mAh g−1). Compared with graphite, Li4Ti5O12 is demonstrated to be of high safety. However, the Li4Ti5O12 anode material suffers from poor electronic conductivity,20−26 which hampers its large-scale application. © 2016 American Chemical Society

So far, a lot of effort has been put toward improving Li 4 Ti 5 O 12 from reducing the particle size to surface modifications to solving the problem described above. Among them, surface coating is seen as an effective surface modification method to improve electronic conductivity. For instance, Zhang synthesized Li4Ti5O12/C by a solid-state method using lithium citrate as the carbon source.27 Lin prepared Li4Ti5O12@C by a hydrothermal process using glucose as the carbon source.28 Xu prepared Li4Ti5O12/C through an ingenious electrospinning technique using polyvinylpyrrolidone as the carbon source.29 All of these carbon-coated Li4Ti5O12 composites show increased electronic conductivity. Apart from graphite and Li 4Ti5 O 12 , the CrTi-based compounds are another kind of well-recognized anode material owing to their high reversibility and small volume change during the charge/discharge process. These compounds mainly include LiCrTiO4, Li5Cr9Ti4O24, and derivative compounds with a high voltage plateau of around 1.5 V.30−38 Recently, a novel CrTi-based compound Li5Cr7Ti6O25 is reported as a hopeful anode material for LIBs. Yi successfully synthesized Li5Cr7Ti6O25 via the sol−gel process using lithium acetate, Received: September 21, 2016 Revised: November 23, 2016 Published: December 5, 2016 957

DOI: 10.1021/acssuschemeng.6b02280 ACS Sustainable Chem. Eng. 2017, 5, 957−964

Research Article

ACS Sustainable Chemistry & Engineering chromium nitrate, and tetrabutyl titanate as raw materials.39 Cycled at the voltage window of 0.0−2.5 V, the lithium storage capacity of Li5Cr7Ti6O25 is 176 mAh g−1 at 5 C and 132 mAh g−1 at 10 C, respectively. This indicates that Li5Cr7Ti6O25 is a good anode candidate for LIBs. However, its electrochemical performance is still inferior to Li4Ti5O12 due to shortcomings in electronic/ionic conductivity. For the sake of enhancing the electrochemical property of Li5Cr7Ti6O25, the method of coating a conductive material on the titanates as described above can also be used to enhance the conductivity of Li5Cr7Ti6O25 particles. Herein, we synthesize Li5Cr7Ti6O25/C through the sol−gel method using glucose as the carbon source. We give a detailed introduction regarding electrochemical properties of Li5Cr7Ti6O25/C as a hopeful anode material for LIBs. It can be found that Li5Cr7Ti6O25/C shows prominent enhancements in rate performance, cycling stability, and conductivity compared to those of carbon-free Li5Cr7Ti6O25, which may be relative to the formation of a thin amorphous carbon layer coated on the Li5Cr7Ti6O25 particles. In addition, the structural evolution of Li5Cr7Ti6O25/C during the lithiation/delithiation process is first evidenced through in situ X-ray diffraction measurements, which presents a negligible lattice volume during the charge/discharge cycle. This indicates that the interconnected carbon layer not only enhances the conductivity but also performs as a solid and flexible buffer to hold volume change of Li5Cr7Ti6O25.

diffractometer (Bruker D 8 Focus) with Cu Kα radiation for 2θ ranging from 10° to 80°. For evaluating the carbon content, thermogravimetric analysis (TG) for Li5Cr7Ti6O25/C was conducted on a Seiko TG/DTA 7300 instrument from 25 to 650 °C at a heating rate of 5 °C min−1. The micromorphology of the samples was examined by scanning electron microscopy (SEM, Hitachi SU-70) combined with EDS (Oxford Inca-300). The microstructure of the samples was studied by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010) and selected-area electron diffraction (SAED, JEOL JEM-2010). 2.3. Electrochemical Measurements. The electrochemical performances were measured with coin-type electrochemical cells assembled in a glovebox. The Li5Cr7Ti6O25/C working electrode was fabricated via mixing the synthesized sample with carbon conductive black and polyvinylidene fluoride at a weight ratio of 82.83:7.18:10 (80:10:10 for the Li5Cr7Ti6O25 electrode). The average mass loading of the two samples was around 1.8 mg cm−2. In a typical cell, pure lithium foil served as counter electrode, and microporous polypropylene films served as separator. All coin-type cells were measured on a battery test system (LANHE CT2001A). Moreover, cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) tests were performed using an electrochemical workstation (BioLogic VSP-300).

3. RESULTS AND DISCUSSION The powder XRD images of as-prepared Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C are displayed in Figure 2a. All the diffraction

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Synthesis of Li5Cr7Ti6O25 was performed as follows. Lithium nitrate (LiNO3), chromium nitrate (Cr(NO3)3), and tetrabutyl titanate (Ti(C4H9O)4) were used as starting materials, and citric acid was taken as the chelating agent. The above starting materials were added to 100 mL of deionized water and stirred to obtain a transparent blue gel. Then, the gel was decomposed by heating at 160 °C in air for 24 h. Finally, the product was preheated at 500 °C for 6 h and further calcined at 800 °C in air for 10 h to obtain the Li5Cr7Ti6O25 sample. Li5Cr7Ti6O25/C was prepared by mixing the Li5Cr7Ti6O25 obtained above with glucose at a weight ratio of 5:1. The mixture was ball-milled for 120 min to obtain a homogeneous distribution. This precursor was transferred to a porcelain boat and then sintered at 600 °C for 4 h under an argon atmosphere to obtain the desired product. Herein, we labeled the as-obtained product as Li5Cr7Ti6O25/C. The schematic preparation process is described in Figure 1. 2.2. Material Characterization. X-ray diffraction (XRD) images of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C were collected on an X-ray

Figure 2. (a) XRD patterns and (b, c) corresponding Rietveld refinement results of (b) Li5Cr7Ti6O25/C and (c) Li5Cr7Ti6O25.

peaks are in accordance with standard diffraction peaks of Li5Cr7Ti6O25 from the literature.39 In addition, no apparent impurity phase can be observed in the two patterns, which indicates the carbon coating does not alter the crystal orientation and composition of Li5Cr7Ti6O25. This result also exhibit that glucose is an appropriate carbon source to remain amorphous carbon in Li5Cr7Ti6O25/C. All of the Li5Cr7Ti6O25/ C powders are dark gray in color in sharp contrast to the yellow color of Li5Cr7Ti6O25 powders as displayed in the inset of Figure 2a, which verifies that the carbon layer is successfully coated on the surface of Li5Cr7Ti6O25 particles. For further understanding the purity and structure of as-obtained samples, Rietveld refinements are performed using the crystal data of spinel as the initial crystal data as shown in Figure 2b and c. The refined results indicate the successful preparation of high pure Li5Cr7Ti6O25 phase before and after carbon coating in this work. According to the refinement results, the lattice parameters of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C are 8.315 and 8.323 Å, respectively, which is line with previous work.39 The detailed surface morphology and crystal structure of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C are illustrated in Figure 3. Panels a−d in Figure 3 display the SEM images of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C. These pictures reveal that the as-received samples consist of irregular polyhedrons. By careful observa-

Figure 1. Schematic illustration of the synthesis process for Li5Cr7Ti6O25/C. 958

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Figure 3. (a, c, e) SEM, TEM, HRTEM, and SAED images of Li5Cr7Ti6O25. (b, d, f−h) SEM, TEM, HRTEM, TG, and EDS images of Li5Cr7Ti6O25/C. (i−m) Elemental mapping images of Cr, Ti, O, and C elements in Li5Cr7Ti6O25/C.

For the carbon content in Li5Cr7Ti6O25/C to be determined, thermogravimetric analysis is conducted under an air atmosphere. The TG curve of Li5Cr7Ti6O25/C performed at a heating rate of 5 °C min−1 from 25 to 650 °C is shown in Figure 3g. The oxidation reaction of carbon mainly occurs from 300 to 420 °C. After the range of decomposition temperatures, the weight fading is nearly zero, indicating that the reaction between carbon and oxygen is complete. The total weight loss of the carbon layer coated on Li5Cr7Ti6O25 particles is calculated to be 3.42 wt %, which is close to the expected value. According to previous literature,40−44 this thin carbon layer is beneficial for strengthening the conductivity of active particles and suppressing the volume change of particles during electrochemical cycles. Thus, it is desired that Li5Cr7Ti6O25/C can be a potential ultrahigh power anode candidate for LIBs. For further confirming the presence of element composition, energy-dispersive X-ray spectroscopy is performed as displayed in Figure 3h. A clear peak of carbon suggests the existence of a carbon element in the as-received Li5Cr7Ti6O25/C. Additionally, homogeneous distribution of C, Cr, Ti, and O elements in Li5Cr7Ti6O25/C is displayed via element mapping images as shown in Figure 3i−m. These images demonstrate that

tion, the average diameter of a Li5Cr7Ti6O25 particle is approximately 50−200 nm. After carbon coating, Li5Cr7Ti6O25 shows no obvious difference in surface morphology and particle size. Additionally, as the inset TEM image shows in Figure 3d, it is easy to find that the surface of the Li5Cr7Ti6O25/C particle is wrapped by amorphous carbon from the carbonization of glucose. For comparison, the surface of the Li5Cr7Ti6O25 particle is relatively smooth as the inset TEM image shows in Figure 3c. For close observation of the carbon-coated layer, high-resolution transmission electron microscopy (HRTEM) is also performed to characterize the structures of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C. Compared with the HRTEM images of two samples in Figure 3e and f, the amorphous carbon layer can be clearly observed on the Li5Cr7Ti6O25 surface. As can be seen in Figure 3f, the HRTEM image of the Li5Cr7Ti6O25/C particle reveals a carbon coating layer with a thickness of around 7−9 nm on the surface. The inset selected-area electron diffraction (SAED) image in Figure 3e also verifies good crystallinity and purity of the Li5Cr7Ti6O25 particles, which is in accordance with the results from XRD analysis. The lattice spacing of 0.48 nm in the HRTEM images of the carbon-free and -coated samples corresponds to the (111) crystal plane of Li5Cr7Ti6O25. 959

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Figure 4. (a, b) Charge/discharge curves, (c) cyclic voltammograms, (d) rate performances, and (e, f) long-term cycle properties of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C.

Upon the current density increasing to 500 mA g−1, as shown in Figure 4b, the voltage plateau of two samples becomes shorter and shows a large voltage drop. Despite this, the redox polarization of Li5Cr7Ti6O25/C is still lower than that of carbon-free Li5Cr7Ti6O25. Cycled at 500 mA g−1, the initial discharge/charge capacities of Li5Cr7Ti6O25/C are 145.2 and 127.9 mAh g−1, respectively. In contrast, the initial discharge/ charge capacities of Li5Cr7Ti6O25 are only 132.8 and 115.0 mAh g−1 at the same current density, respectively. Obviously, the initial Coulombic efficiency of Li5Cr7Ti6O25/C (88%) is also higher than that of carbon-free Li5Cr7Ti6O25 (86%) and previous reported result (78%).39 After 200 cycles, the Li5Cr7Ti6O25/C still maintains the discharge/charge capacities of 111.9 and 111.6 mAh g−1, which are 77 and 87% of their initial capacities, respectively. Alternatively, the carbon-free Li5Cr7Ti6O25 only maintains discharge/charge capacities of 95.4 and 93.6 mAh g−1, which are 72 and 81% of their initial capacities, respectively. From these results, it is clear that the electrochemical properties of Li5Cr7Ti6O25/C are significantly improved after carbon coating. Cyclic voltammograms of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C between 1.0 and 3.0 V at 0.1 mV s−1 are displayed in Figure 4c. As can be seen, the CVs of Li5Cr7Ti6O25/C show a main reduction peak at around 1.45 V and an oxidation peak at

Li5Cr7Ti6O25/C is composed of Li5Cr7Ti6O25 particles with a glucose-derived amorphous carbon layer. To identify the impact of carbon-coating on the electrochemical properties of Li5Cr7Ti6O25, we conduct electrochemical evaluation using Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C as working electrodes. Panels a and b in Figure 4 show the charge/discharge curves of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C. As seen in Figure 4a, Li5Cr7Ti6O25/C displays high initial discharge/charge capacities of 159.8 and 142.3 mAh g−1 at 200 mA g−1, respectively. In contrast, the carbon-free Li5Cr7Ti6O25 only delivers the initial discharge/charge capacities of 148.7 and 132.2 mAh g−1 at the same current density, respectively. After 200 cycles, the discharge/charge capacities of the carbon-free Li 5 Cr 7 Ti 6 O 25 decrease to 112.9 and 112.5 mAh g −1 , respectively, whereas the Li5Cr7Ti6O25/C still maintains high discharge/charge capacities of 128.5 and 128.4 mAh g−1, respectively. Seen from the curves, it is obvious that the two samples exhibit a flat operating voltage plateau at around 1.5 V corresponding to the insertion/extraction of Li + in Li5Cr7Ti6O25. By careful observation, we can find that the redox polarization of Li5Cr7Ti6O25 is larger than that of Li5Cr7Ti6O25/C, indicating that the existence of carbon layer can reduce electrode polarization. This decrease in polarization is also confirmed by the following cyclic voltammetry results. 960

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Figure 5. (a) EIS patterns and (b) corresponding Z′ versus ω−1/2 relationships in the low frequency region for Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C. (c, e) Cyclic voltammograms of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C recorded at different scan rates and (d, f) corresponding relationships between peak current and square root of the scan rate.

Table 1. Diffusion Coefficients of Li+ in Li5Cr7Ti6O25 Calculated from CVs

Li5Cr7Ti6O25/C possesses superior electrochemical performance. Figure 4d shows the rate capabilities of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C from 50 to 600 mA g−1. It is reasonably clear that Li5Cr7Ti6O25/C delivers better rate stability than that of Li5Cr7Ti6O25. The charge capacities of Li5Cr7Ti6O25/C are 145, 142, 132, 125, 120, 115, and 109 mAh g−1 at 50, 100, 200, 300, 400, 500, and 600 mA g−1, respectively. In contrast, carbon-free Li5Cr7Ti6O25 delivers charge capacities of 140, 137, 122, 110, 100, 90, and 79 mAh g−1 at the same current densities. Upon the current density turning back to 50 mA g−1, the charge capacity of Li5Cr7Ti6O25/C can recover to ∼142 mAh g−1 after 70 cycles, whereas the charge capacity of Li5Cr7Ti6O25 is barely recovered to 136 mAh g−1. Noticeably, Li5Cr7Ti6O25/C possesses a much higher reversible capacity than that of carbon-free Li5Cr7Ti6O25, especially at a high current density. In addition, the rate performance of Li5Cr7Ti6O25/C is also better than the previous reported result for Li5Cr7Ti6O25.39 The cycling performances for Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C at 200 and 500 mA g−1 are shown in Figure 4e and f, respectively. At 200 mA g−1, the charge capacities of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C are 112.5 and 128.4 mAh g−1 after 200 cycles, corresponding to 85 and 90% of capacity retention, respectively. More importantly, even cycled at a higher current

DLi+ (×10−12 cm2 s−1) sample

anodic peak

cathodic peak

Li5Cr7Ti6O25 Li5Cr7Ti6O25/C

2.39 9.51

2.37 6.96

around 1.62 V. For Li5Cr7Ti6O25, a main reduction peak at around 1.43 V and an oxidation peak at around 1.64 V can be observed. It suggests that Li5Cr7Ti6O25/C has a smaller electrode polarization (0.17 V) than that of Li5Cr7Ti6O25 (0.21 V), which is in line with the better reversibility of Li5Cr7Ti6O25/C. Additionally, no other redox peaks are observed in Figure 4c, indicating that the two samples are pure phases. This is in accordance with the Rietveld refined result. Furthermore, the intensity of the redox peaks of Li5Cr7Ti6O25/C is much higher than that of Li5Cr7Ti6O25. The higher redox peak current of Li5Cr7Ti6O25/C suggests its higher redox kinetic activity due to enhanced electronic conductivity by carbon coating. This phenomenon is in good agreement with that of Li4Ti5O12@C using glucose as the carbon source.28 The above results further confirm that 961

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Figure 6. (a) In situ XRD patterns and (b) corresponding lattice parameter and volume evolutions of Li5Cr7Ti6O25/C during the first charge/ discharge cycle.

density of 500 mA g−1, Li5Cr7Ti6O25/C still delivers a high charge capacity of 111.6 mAh g −1 after 200 cycles, corresponding to 87% of capacity retention. For comparison, the Li5Cr7Ti6O25 only delivers the charge capacity of 93.6 mAh g−1 at 500 mA g−1 with capacity retention of 81% after 200 cycles. These improved lithium storage performances of Li5Cr7Ti6O25/C are owing to the carbon-coating membrane on the active particles. The thin carbon membrane provides a pathway for electrons to migrate rapidly during cycling. For the impact of carbon coating on enhancement of conductivity and electrochemical kinetics to be further validated, electrochemical impedance spectroscopy tests of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C are carried out as displayed in Figure 5a. The EIS spectra consist of a semicircle in the highmiddle frequency region and a low frequency inclined line. The semicircle at the high-middle frequency region represents charge transfer resistance (Rct) between the electrolyte and electrodes. According to the equivalent circuit in the inset of Figure 5a, the Rct of Li5Cr7Ti6O25/C is calculated to be 42.12 Ω, which is much smaller than that of Li5Cr7Ti6O25 (62.96 Ω). The inclined line at the low frequency region represents the Warburg resistance (W), which is connected with the Li+ diffusion in the electrode. The Li+ diffusion coefficients (DLi+) of two electrodes can be roughly calculated from the equations45−48 D Li+ =

R2T 2 2A n F C σ

2 4 4 2 2

Z′ = R e + R ct + σω−1/2

process is quite reversible. Additionally, each redox peak current (ip) shows a linear relationship with the square root of scan rate (v1/2) as shown in Figure 5d and f. This relationship can be described through the Randles−Sevcik equation49−52 i p = (2.69 × 105)n3/2 AD1/2 C +ν1/2 Li+ Li

(3)

According to this equation, the Li+ diffusion coefficients of two electrodes can be obtained. Here, the DLi+ values of Li5Cr7Ti6O25 before and after carbon coating are calculated and presented in Table 1. It is clear that the DLi+ values of Li5Cr7Ti6O25/C are larger than those of Li5Cr7Ti6O25, suggesting that Li + can migrate more rapidly in the Li5Cr7Ti6O25/C electrode, and the ionic conductivity of Li5Cr7Ti6O25/C is markedly improved. Figure 6a exhibits the in situ XRD pattern of Li5Cr7Ti6O25/C during initial cycling. In a typical in situ cell, the beryllium disk is used as an X-ray diffraction window. The cell is assembled with Li5Cr7Ti6O25/C electrode as the working electrode and lithium foil as the counter electrode in a glovebox filled with pure argon. All the observed peaks can conform to the standard diffraction peaks of Li5Cr7Ti6O25/C, except for three peaks at 38.4°, 41.3°, and 44.0°, which are related to peaks of BeO.53−55 It is easy to see that the (111), (311), (222), and (400) reflections corresponding to the peaks at 18.2°, 35.7°, 37.4°, and 43.4° slowly shift toward lower angles when the in situ cell is discharged to 1.0 V. Upon the charge process to 3.0 V, all these reflections can gradually move back to original Bragg positions. At the same time, it can also be observed that the (111) and (311) planes gradually weaken in intensity, and the (400) plane gradually strengthens in intensity during the lithiation process. During the delithiation process, they can gradually recover to the initial intensity. A detailed description of in situ evolution can also be found in Figure S1. These results confirm that Li5Cr7Ti6O25/C possesses good structural stability and reversibility for lithium storage. Therefore, there is no doubt that Li5Cr7Ti6O25/C would be a good anode material for LIBs. By performing refinement on in situ XRD patterns, the evolution of lattice parameter for Li5Cr7Ti6O25/C during the charge/discharge cycle is obtained as shown in Figure 6b. Upon the discharge process, the lattice parameter rises from 8.3364 to 8.3689 Å, and lattice volume rises from 579.35 to 586.14 A3, which are evident as lithium ions inserted into the structure of Li5Cr7Ti6O25/C. It is worthwhile to note that the changes of

(1) (2)

Here, all the parameters have specific constant values except for the Warburg factor σ, which can be gained on the basis of the slope of Z′ relative to the ω−1/2 line in Figure 4b. On the basis of the above equations, the value of DLi+ is calculated to be 2.55 × 10−13 and 8.53 × 10−13 cm2 s−1 for Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C, respectively. It is clear that carbon coating can effectively strengthen the electronic/ionic conductivity of Li5Cr7Ti6O25 and then enhance its lithium storage capability. Cyclic voltammograms cycled at different scan rates are another method to study the electrochemical kinetics of Li5Cr7Ti6O25 and Li5Cr7Ti6O25/C. As shown in Figure 5c and e, the anodic peaks shift toward large voltage, and cathodic peaks shift toward small voltage in the wake of the increase of the scan rate, suggesting that the Li+ insertion/extraction 962

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lattice parameter and volume are completely reversible during the lithiation/delithiation process, indicating perfect structural stability of Li5Cr7Ti6O25/C. It can be also found that the maximum volume expansion of Li5Cr7Ti6O25/C is only 1.17% after full lithiation, which is smaller than that of TiNb2O7 (7.22%).56 The above results further demonstrate that the structure of Li5Cr7Ti6O25/C maintains high stability during lithium ion insertion into and extraction from the electrode.

4. CONCLUSIONS We prepare Li5Cr7Ti6O25/C through a versatile sol−gel method and evaluate it as a possible anode material for LIBs. Electrochemical evaluations reveal that Li5Cr7Ti6O25/C has high electronic/ionic conductivity, exceptional rate performance, and stable cycle life. Compared with carbon-free Li5Cr7Ti6O25, Li5Cr7Ti6O25/C exhibits higher initial charge capacities of 142.3 and 127.9 mAh g−1 at 200 and 500 mA g−1, respectively. Additionally, Li5Cr7Ti6O25/C also shows outstanding long-term cycle stability with 128.4 mAh g−1 of charge capacity at 200 mA g−1, ∼90% capacity retention after 200 cycles, and 111.6 mAh g−1 of charge capacity at 500 mA g−1, ∼87% of capacity retention after 200 cycles. The superior lithium storage property of Li5Cr7Ti6O25/C is ascribed to the enhanced lithium ion diffusion coefficient and rapid electron transportation pathway after carbon coating. Moreover, it should be noted that the phase transition and lattice volume change of Li5Cr7Ti6O25/C is highly reversible as proven by advanced in situ XRD measurements during initial cycling. This ensures that Li5Cr7Ti6O25/C can be a hopeful anode material for high-performance lithium storage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02280. Detailed in situ XRD patterns of Li5Cr7Ti6O25/C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Jie Shu E-mail: [email protected]. ORCID

Jie Shu: 0000-0002-2326-6157 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is sponsored by National Natural Science Foundation of China (U1632114), Ningbo Key Innovation Team (2014B81005), Ningbo Natural Science Foundation (2016A610068), and K.C. Wong Magna Fund in Ningbo University.

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REFERENCES

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DOI: 10.1021/acssuschemeng.6b02280 ACS Sustainable Chem. Eng. 2017, 5, 957−964

Research Article

ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.6b02280 ACS Sustainable Chem. Eng. 2017, 5, 957−964