(111) Peak Evolution during Cycling for High-Performance Lithium Ion

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LiCrTiO4 Nanowires with the (111) Peak Evolution during Cycling for High-Performance Lithium Ion Battery Anode Minghe Luo, Haoxiang Yu, Xing Cheng, Haojie Zhu, Wuquan Ye, Lei Yan, Shangshu Qian, Miao Shui, and Jie Shu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02567 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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ACS Sustainable Chemistry & Engineering

LiCrTiO4 Nanowires with the (111) Peak Evolution during Cycling for High-Performance Lithium Ion Battery Anode

Minghe Luo 1, Haoxiang Yu 1, Xing Cheng, Haojie Zhu, Wuquan Ye, Lei Yan, Shangshu Qian, Miao Shui, Jie Shu * Faculty of Materials Science and Chemical Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, Zhejiang Province, People’s Republic of China

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These authors contributed equally to this work.

** Corresponding author: Jie Shu Tel.: +86-574-87600787 Fax: +86-574-87609987 E-mail: [email protected]

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Abstract LiCrTiO4 is a lithium insertion material which is isostructural with Li4Ti5O12. By modified its morphology, LiCrTiO4 nanowires exhibit a high charge capacity of 154.6 mAh g-1 at 100 mA g-1, and this value can be maintained at 121.0 mAh g-1 even at a high current density of 700 mA g-1. Furthermore, the cycling performance shows that LiCrTiO4 nanowires can also deliver a reversible capacity of 120.0 mAh g-1 with 95.6 % capacity retention of the first cycle after 550 cycles. The excellent electrochemical properties are revalidated by the cyclic voltammetry and electrochemical impedance spectroscopy measurements. The most interesting feature in this work is the relationship between the periodic variation of the (111) peak intensities and the migration of lithium ions during cycling. It proves that LiCrTiO4 nanowires are zero-strain insertion material which can be a promising anode material for lithium ion batteries.

Keyword: Lithium Chromium titanate; Nanowires; Electrospun; Structural evolution; In situ X-ray diffraction; Lithium ion batteries.

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Introduction Lithium-ion batteries (LIBs) have been widely used in the diverse range of applications, from cell phones to electric vehicles since the first commercialization by Sony in the 1990s.1-4 A lithium-ion battery mainly comprises of a positive electrode, a separator, a negative electrode and electrolyte. Therefore, anode materials with high performance have a positive influence on the further application of LIBs in electrochemical energy storage devices and systems. Among various anode materials, graphite is a traditional anode material in commercial LIBs. It exhibits excellent electrochemical properties with a low operating potential (< 1.0 V vs. Li+/Li), high energy density and eco-friendiness.5-6 However, poor safety issues limit its application in LIBs. To satisfy the requirement of the alternative insertion hosts with good cyclability and outstanding safety for LIBs, lots of efforts have been paid in the last several years.7-12 Several alternative materials for negative electrode, such as Li4Ti5O123,4, Na2Li2Ti6O1413-15 and SrLi2Ti6O14,11,16-18 have been proposed and demonstrated so far. Among these proposed materials, spinel-type metal oxide is a popular type of anode materials for LIBs. For example, Li4Ti5O12 is regarded as an anode material for the practical LIBs. It owns a theoretical capacity of 175 mAh g-1 and shows the very small overall evolution of the volume during the cycles.19-26 LiCrTiO4 also belongs to the spinel-framework structure family.27 In 2000, LiCrTiO4 was firstly reported by Ohzuku.26 It demonstrates that LiCrTiO4, as a zero-strain insertion host, can be easy to be prepared and offers high energy density with a theoretical capacity of 157 mAh g-1. Then, Arillo, Sun, Mukai, Martín had

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reported the electrochemical performance of LiCrTiO4 by changing Li/Cr/Ti atomic ratio or the metal elements, respectively.27-30 After that, many methods, such as carbon-coating, metal doping, and size reducing have been applied by many battery researchers and material chemists to improve the electrochemical properties of LiCrTiO4 for its commercial application.31-36 Those approaches exhibit decent enhancement in electrochemical performance. Nevertheless, further improvement is still satisfied. To our knowledge, modifying with morphologies or shapes of anode materials is a good way to improve the electrochemical performance for LIBs. Hence, LiCrTiO4 has been synthesized by a simple electrospun method to form the one-dimensional nanowires in this work. Electrospun is thought to be a simple and facile process for the fabrication of wires in nano-scale range.12 Nanowires not only show smaller particle size and larger surface, but also keep away from self-aggregation.12,37 As a control, LiCrTiO4 also has been obtained via a sol-gel route using the precursor sol for electrospun of LiCrTiO4 nanowires. By comparing electrochemical performance of LiCrTiO4 nanowires and bulks, we can identify the impact on electrochemical performance of LiCrTiO4 with different morphologies and describe it in detail. Furthermore, the in situ XRD technique is also applied to observe the structural evolution of LiCrTiO4 nanowires during cycling.

Experimental Material Preparation

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In this work, LiCrTiO4 nanowires are obtained via a traditional electrospun technique. In a typical synthetic procedure, the precursor sol is prepared by dissolving 0.33 g of CH3COOLi·2H2O, 0.99 g of Ti(OC4H9)4, 1.17 g of Cr(NO3)3·9H2O and 2.10 g of PVP (Mw = 1,300,000) in a solution containing 3 mL of CH3COOH, 1 mL of H2O, 5 mL of N,N-dimethylformamide and 40 mL of C2H5OH. After vigorous stirring overnight, the prepared precursor sol is poured into a 10 mL plastic syringe with a 21-gauge stainless steel needle. An applied voltage of 12 kV is used between the needle and Al foil at a distance of about 15 cm. The electrospun process is conducted at a feeding rate of 14 µm min-1. Finally, the as-collected precursor for LiCrTiO4 nanowires is sintered at 800 oC for 4 hours in air to acquire the single-phase LiCrTiO4 nanowires. As a control, bulk LiCrTiO4 is synthesized by a simple sol-gel method. The homogeneous LiCrTiO4 precursor sol is preheated at 160 oC in the oven to form a uniform dried gel. Then, this dried gel is calcined at 800 oC for 4 hours under air atmosphere to obtain bulk LiCrTiO4. Samples Characterization The crystal structures of LiCrTiO4 nanowires and bulk LiCrTiO4 are evaluated using a powder X-ray diffraction analyzer (XRD, Bruker D8 Focus). The in situ XRD structural evolution of LiCrTiO4 nanowires is also characterized by the same XRD analyzer, and the cell for in situ XRD observation is a homemade cell which has already been reported in our previous reports.38,39 The surface morphologies and particle sizes of LiCrTiO4 nanowires and bulk LiCrTiO4 are determined by scanning electron microscope (SEM, Hitachi SU-70) and transmission electron microscopy

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(TEM, JEOL JEM-2010). Electrochemical Measurement CR2032 coin type cells are comprised of working electrode, lithium metal counter electrode, whatman glass fiber separator and organic electrolyte. It shall be noted that working electrode consists of the active materials, acetylene black and polyvinylidene fluoride binder with a weight ratio of 8:1:1. All the cells are assembled in an argon-filled glove box. Cyclic voltammetry (CV) studies and electrochemical impedance spectroscopy (EIS) measurements are both recorded by using CHI660D electrochemical workstation at 25 oC. The electrochemical performances of samples are investigated on multichannel battery tester (LANHE CT2001A).

Results and Discussion X-ray diffraction (XRD) pattern is an effective method which can reflect the structure of the as-prepared samples. Figure 1a illustrates the XRD patterns of LiCrTiO4 nanowires and bulk LiCrTiO4. All the patterns match the standard card (JCPDS card No. 47-0139) well that the characteristic diffraction reflections are located at 2θ = 18.47°, 35.79°, 37.44° and 43.50°, corresponding to the (111), (311), (222) and (400) planes, respectively. It indicates good crystallinity of LiCrTiO4 and no impurity existing in these samples. In addition, the XRD pattern of LiCrTiO4 nanowires is also refined by Rietveld method, as displayed in Figure 1b. The Rietveld refinement result gives the lattice parameters a = b = c = 8.296 Å with a space group of Fd3m. This result is close to the experimental data obtained from the previous

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literatures.26-29,31-33 The detail data of lattice parameters for LiCrTiO4 are listed in Table S1 (Supporting information). Figure 1c presents crystal structure of LiCrTiO4. In this spinel structure, lithium ions are located at the tetrahedral 8a sites, and transition metal ions, such as trivalent chromium and tetravalent titanium ions, are randomly distributed in a atomic ratio of 1:1 at the octahedral 16d sites with oxygen ions forming a cubic close-packed array at the 32e sites.31-34 Furthermore, this LiCrTiO4 contains vacant 16c sites to accommodate additional inserted lithium ions. This is the reason for LiCrTiO4 to be a promising anode material.

Figure 1. (a) XRD patterns of LiCrTiO4 nanowires and bulk LiCrTiO4; Rietveld-refined XRD pattern of electrospun LiCrTiO4 nanowires; (c) the crystal structural model of LiCrTiO4.

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Figure 2. SEM images of (a) the as-prepared precursor fibers for LiCrTiO4 wires, (b, c) LiCrTiO4 nanowires; (d) bulk LiCrTiO4. (e) TEM image of LiCrTiO4 nanowires; inset: HRTEM images of LiCrTiO4 nanowires; (f) the selected area electron diffraction (SAED) image of LiCrTiO4 nanowires.

SEM images are to determine the size and morphology of each material. Figure 2a depicts the SEM image of the as-electrospun LiCrTiO4 precursor wires. The precurosor wires present the smooth surfaces and the independent wire skeleton with a diameter ranging from 0.7 to 4.5 µm. The huge difference in diameter of each independent precursor wire may be related to the precursor sol electrophoresis. During electrospinning, the PVP component can be migrated by the influence of high voltage (12 kV), leading to the different concentration of PVP in the same syringe so that the sizes of precursor wires show different with each other. After calcined at 800 o

C in air for 4 hours, LiCrTiO4 nanowires are obtained (Figure 2b-c). Most of

nanofibers show a diameter of about 550 nm, fabricating by plenty of nanoparticles (50 nm). The reduction of the diameter for LiCrTiO4 nanowires can be attributed to 8


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the removal of PVP and the decomposition of the starting material. In addition, the huge difference in diameter of LiCrTiO4 nanowires cannot be seen due to the probable homogeneousness of the starting material in the precursor sol. In contrast, the average particle size of bulk LiCrTiO4 is measured to be about 330 nm with the serious self-aggregation and irregular shapes (Figure 2d). LiCrTiO4 nanowires are also observed by TEM and high-resolution TEM measurements. As displayed in Figure 2e, the individual LiCrTiO4 nanowire is composed of many nanoparticles. This result is also confirmed by the SAED image of LiCrTiO4 nanowires (Figure 2f).

Figure 3. (a, b) Typical galvanostatic discharge-charge curves of LiCrTiO4 nanowires and bulk LiCrTiO4 at different current densities, respectively; (c) the corresponding rate performance of LiCrTiO4 nanowires and bulk LiCrTiO4; (d) comparison of rate capability of LiCrTiO4 nanowires with other LiCrTiO4 and Ti-based materials reported. (e) long-life cycling stability of LiCrTiO4 nanowires and bulk LiCrTiO4 at a current density of 600 mA g-1.

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To investigate the electrochemical performance of LiCrTiO4 nanowires and bulk LiCrTiO4 as the anode materials, the cycling tests are performed at various current densities ranging from 100 to 700 mA g-1 between 1.0 and 3.0 V, as shown in Figure 3a-c. LiCrTiO4 nanowires show the charge capacity of 154.6 mAh g-1 at the current densities of 100 mA g-1, it is slightly lower than the theoretical capacity of 157mAh g-1 but close to the previous results obtained from Ohzuku et al. (Figure 3a).26 With the increase of current densities, a slight capacity fading can be observed. The charge capacities of LiCrTiO4 nanowires are 150.0, 143.1, 135.9, 129.9, 124.4 and 121.0 mAh g-1 at 200, 300, 400, 500, 600 and 700 mA g-1, respectively. Furthermore, LiCrTiO4 nanowires can still exhibit a high charge capacity of 150.5 mAh g-1 as the current density returning from 700 to 100 mA g-1. In contrast, the charge capacities of bulk LiCrTiO4 fades dramatically as shown in Figure 3b. When the current density increases to 200, 300, 400, 500, 600 and 700 mA g-1, the charge capacities are 140.8, 115.7, 101.2, 95.1, 91.5 and 90.3 mAh g-1, respectively. Meanwhile, bulk LiCrTiO4 shows higher polarization compared to LiCrTiO4 nanowires as displayed in Figure 3a-b. In addition, the corresponding Coulombic efficiency (CE) of rate performance shows that the CE of LiCrTiO4 nanowires is higher than that of bulk LiCrTiO4 during cycling (Figure S1, Supporting information). It is worth noting that the CE in the initial state of cycling for LiCrTiO4 nanowires and bulk LiCrTiO4 can only reach to about 67%. This situation can also be found in previous studies.40,41 The reason for the low Coulombic efficiency in the initial state of cycling may be related to the

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electrolyte reduction decomposition on the titanate electrode.40-42 Obviously, LiCrTiO4 nanowires exhibit the excellent rate capability. The rate capabilities of LiCrTiO4 nanowires and bulk LiCrTiO4 are also compared with those of recently reported LiCrTiO4 anodes and other Ti-based electrodes (Figure 3d). It is found that LiCrTiO4 nanowires in this paper display the highest specific capacity, suggestive of a promising lithium insertion host.7,17,28,35,43 Further texting results of cycling performances at a current density of 600 mA g-1 are displayed in Figure 3e. The cell of LiCrTiO4 nanowires delivers an initial charge capacity of 125.5 mAh g-1. After 550 cycles, the capacity remains stable and finally keeps at 120.0 mAh g-1 with 95.6 % capacity retention of the first cycle. The coulombic efficiency of the 550th cycle approaches 99.8 %, nearly close to 100 %. As a control, bulk LiCrTiO4 shows a fast capacity fade. The reversible capacity is only 58.9 mAh g-1 with capacity retention of 65.4 % after 550 cycles.

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Figure 4. (a, b) CV curves of LiCrTiO4 nanowires and bulk LiCrTiO4 at various scan rates, respectively; (c) the linear relationship of the peak current (Ip) and the square root of the scan rate (ʋ-0.5) for LiCrTiO4 nanowires and bulk LiCrTiO4; (d) the chemical diffusion coefficients of Li+ ions calculated from CV.

In addition, the better electrochemical properties of LiCrTiO4 nanowires can also be evaluated by the determination of the diffusion coefficients for lithium ions (DLi) in LiCrTiO4 nanowires and bulk LiCrTiO4. Figure 4a-b depicts the CV curves of LiCrTiO4 nanowires and bulk LiCrTiO4 at various scan rates from 0.7 to 1.5 mV s-1. Each CV curve shows a couple of redox peaks during cycling, indicative of the insertion/extraction of lithium ions into/from LiCrTiO4. Meanwhile, the intensity of the redox peaks goes up with the increase of the scan rate and it can be found that the linear relationship appears between the peak current intensity (Ip) and the square root 12


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of the scan rate (ν-0.5). Based on this phenomenon, the classical Randles-Sevcik equation (eq 1) can be applied and the equation is followed as:44-46 0.5

I p = ( 2.69 × 10 5 ) n 1.5 AD Li+ C Li+ υ 0.5

(1)

The terminologies for the above equation (1) are shown in the Supporting Information. According to the equation (1) and the slope values obtained from Figure 4c and Figure S2 (Supporting information), the DLi for anodic peak and cathodic peak of LiCrTiO4 nanowires and bulk LiCrTiO4 can be calculated and are displayed in Figure 4d. The detail data are listed in Table S2 (Supporting information). Clearly, LiCrTiO4 nanowires exhibit the higher DLi than that of bulk LiCrTiO4. Meanwhile, the DLi of LiCrTiO4 nanowires and bulk LiCrTiO4 are also revalidated by the EIS measurement. Figure 5a presents the Nyquist plots of LiCrTiO4 nanowires and bulk LiCrTiO4 at open circuit voltage state. It is obvious that all the curves can be divided into two distinct parts. The front-part is a semicircle and the back-part is an inclined line. The semicircle and the inclined line correspond to the charge transfer resistance (Rct) and the Warburg diffusion impedance,44 respectively. Such the curves can be simply explained by an equivalent circuit47-48 as shown in inset of Figure 5a. By utilizing an equivalent circuit, the charge transfer resistance of samples can be calculated and the fitted results are listed in Table S3 (Supporting information). It can be found that the Rct of LiCrTiO4 nanowires is lower than that of bulk LiCrTiO4, suggesting that the LiCrTiO4 nanowires have higher electrochemical activity than of bulk LiCrTiO4. And this result is further supported by the DLi obtained from EIS, as shown in Figure 5b. The DLi can be calculated according to the following equations (2,

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3):47-49 Z re = R ct + R e + σω−0.5

D Li+ =

( RT ) 2 2( An 2 F 2 C Li+ σ) 2

(2)

(3)

Figure 5. (a) The impedance spectra of LiCrTiO4 nanowires and bulk LiCrTiO4, respectively; inset figure: the enlargement of the impedance spectra; (b) the linear relationship of Im(Z) and ω-0.5 for LiCrTiO4 nanowires and bulk LiCrTiO4; inset figure: the chemical diffusion coefficients of Li+ ions obtained from EIS.

The terminologies for the above equations (2), (3) are also displayed in the Supporting Information. Based on the calculation, the DLi of LiCrTiO4 nanowires and bulk LiCrTiO4 are obtained. Obviously, the DLi of LiCrTiO4 nanowires (1.02 × 10-12 cm2 s-1) is higher than that of bulk LiCrTiO4 (9.67 × 10-13 cm2 s-1). According to the previous reports,47,48 the lithium-ion diffusion is the control step during cycling. The higher DLi can show the higher diffusion dynamics. Also, these results are close to the

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results obtained from the CV measurement. Hence, All the above results obtained show that LiCrTiO4 nanowires exhibit a better electrochemical performance than bulk LiCrTiO4.

Figure 6. (a) The overall in situ XRD patterns of LiCrTiO4 nanowires; (b) the charge-discharge curve of LiCrTiO4 nanowires during the in situ XRD test; the evolution of the (c) (111), (d) (311), (e) (222) and (f) (400) reflection for LiCrTiO4 nanowires during the in situ XRD test.

In situ XRD is a texting method which is usually used to detect the insertion/extraction mechanism of the electrode material upon cycling. Here the observation of in situ XRD is carried out on a homemade cell. Figure 6 shows the 15



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collected in situ XRD spectra of LiCrTiO4 nanowires during the charge/discharge process. It can be seen that four reflections are observed in the in situ XRD patterns. The (111), (311), (222) and (400) reflections are located at 18.25°, 35.71°, 37.37° and 43.44°, respectively. They can confirm spinel LiCrTiO4 nanowires with space group Fd-3m.26-29,31-33 During cycling, the (111), (311), (222) and (400) reflections show a regular variation. In addition, the position and intensity of reflections do not change after cycling at 65 and 130 mA g-1. It suggests that LiCrTiO4 nanowires show a stable structure upon cycling. By refining the in situ XRD patterns, the lattice parameters of LiCrTiO4 nanowires are obtained. Since LiCrTiO4 has a spinel structure, its lattice parameters (a, b, and c) show the same value, suggesting that its lattice parameters can be calculated by just processing one reflection. Here, the (111) reflection is selected and the variation in the position of the (111) reflection are plotted in Figure 7a, and the corresponding evolution of lattice parameter a and lattice volume V for LiCrTiO4 nanowires are presented in Figure 7b-c. The trend in Figure 7b is similar to that of Li4Ti5O12 reported from previous studies.50-52 Meanwhile, the overall evolution of the lattice volume changed from 570.97 Å3 to 573.32 Å3 during the first cycle is confined to 5‰, suggesting that LiCrTiO4 is also a zero-strain insertion material which is the same as Li4Ti5O12.

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Figure 7. (a) The enlargement of the (111) reflection evolution for LiCrTiO4 nanowires during the in situ XRD test; the transformation of (b) the lattice parameter a and (c) the lattice volume V for LiCrTiO4 nanowires during the test; (d) the intensity evolution of the (111) reflection for LiCrTiO4 nanowires during the test; (e) the charge-discharge curve of LiCrTiO4 nanowires during the in situ XRD test.

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Figure 8. (a-c) The evolution of the (111) crystal plane during cycling; (d-f) schematic diagram of the transformation for the intensity of the (111) crystal plane during cycling.

Previous study about Li4Ti5O1251 has reported that Li4Ti5O12 has a periodic change of the reflection intensities during cycling. In this work, a similar periodic variation of the (111) reflection intensities is also displayed in LiCrTiO4 nanowires (Figure 7d) during the discharge and charge process (Figure 7e). Moreover, this periodic variation of the (111) reflection intensity occurs with the migration of the (111) reflection (Figure 7a). As mentioned above, the migration of the (111) reflection has a relationship with the evolution of lattice parameter. On the other side, the periodic variation of the (111) reflection intensity can be related to the 18


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intercalation/deintercaltion of lithium ions into/from the host lattice of LiCrTiO4. For better understanding the relationship between the periodic variation of the (111) reflection intensity and the migration of lithium ions during cycling, a schematic diagram we depicted is selected in Figure 8. In our previous study,52 the lithiation process of LiCrTiO4 during discharging is proposed as followed: Li(8a)[Cr3+Ti4+](16d)O4(32e) + e- + Li+ ＝ Li2(16c)[Cr3+Ti3+](16d)O4(32e) This formula suggests that lithium ions at the 8a sites will migrate to the 16c sites and the extra lithium ions will also take the 16c sites via 8a sites upon discharging. Therefore, 8a sites are vacant when LiCrTiO4 nanowires are discharged to 1.0 V. The corresponding schematic diagram is shown in Figure 8a-b. Before discharging, the (111) plane is occupied fully with lithium ions (Figure 8a), which looks like a mirror (Figure 8d). When X-ray irradiates to this mirror, the mirror can reflect the X-ray well, so that the intensity of the (111) reflection is strong (Figure 7d). After the in situ XRD cell is discharged to 1.0 V, there is no lithium ions “hanging” on the (111) plane (Figure 8b). At this time, the (111) plane seems to be a dense net (Figure 8e), which can only reflect partial X-ray. This result leads to the lower intensity of the (111) reflection than the intensity of the (111) reflection before discharging (Figure 7d). When the cell is recharged to 3.0 V, the migrated lithium ions come back to their original sites. The intensity of the (111) reflection are recovered. Hence, this phenomenon indicates that this spinel structure is stable, which shows that LiCrTiO4 nanowires have a high electrochemical reversibility.

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Conclusion In summary, LiCrTiO4 nanowires are successfully synthesized by a facile electrospun method and evaluated its possibility as an anode material for LIBs. The obtained LiCrTiO4 nanowires exhibit high rate performance and stable cyclability. Moreover, LiCrTiO4 nanowires are revalidated by the determination of the diffusion coefficients for lithium ions via the CV and EIS measurements. It indicates that the excellent electrochemical properties of LiCrTiO4 nanowires are attributed to the improved lithium ions diffusion coefficients and the lower charge transfer resistance. Finally, LiCrTiO4 nanowires are also measured by using a novel in situ XRD technique. The result shows that LiCrTiO4 nanowires are a zero-strain insertion material. After these systematic experiments, it suggests that LiCrTiO4 nanowires as a high-performance anode can be a good choice for LIBs.

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.

Supporting Information Supplementary data associated with this article can be found in this submission, including the Coulombic efficiency, linear relationship of Ip versus ʋ-0.5, refined lattice parameters, lithium ions diffusion coefficients, electrolyte resistance, charge transfer

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resistance of LiCrTiO4 nanowires and bulk LiCrTiO4, the terminologies for the equations (1), (2) and (3).

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Table of Contents Graphic

For Table of Contents Use Only

Electrospun LiCrTiO4 nanowires exhibit superior lithium storage performance in long-life high-rate rechargeable batteries.

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(111) Peak Evolution during Cycling for High-Performance Lithium Ion

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