Spinel Li4Ti5O12, exhibiting excellent reversibility during discharge-charge processes for its intrinsic zero- strain property[
Journal of Inorganic Materials, Volume. 36, Issue 9, 999(2021)
Surface coating has long been an effective means to improve the electrochemical performance of electrode materials for Li-ion batteries. In this study, the traditional micron-sized Li4Ti5O12 anodes are coated with amorphous lithium phosphate by radio frequency (RF) magnetron sputtering. The characterized morphology indicates that the electrodes are covered by a thin amorphous lithium phosphate coating layer, showing a very smooth surface. Both the rate performance and cycle performance of the electrodes are enhanced markedly by the coating layer in the voltage range of 0.01-3.00 V. When the discharge-charge currents are 35 and 1750 mA∙g-1, the capacities are elevated to 265 and 151 mAh∙g-1, respectively, which are higher than those of the uncoated ones (240 and 22 mAh∙g-1). After further discharge-charged for 200 cycles at 88 mA∙g-1, the coated electrodes still maintain a high reversible capacity of 238 mAh∙g-1. This result clearly suggests that the thin layer can stabilize the solid electrolyte interface, maintain the integrity of the interparticle electronic passage and form a cross-linked ionic conducting network for Li4Ti5O12 grains on the surface.
Spinel Li4Ti5O12, exhibiting excellent reversibility during discharge-charge processes for its intrinsic zero- strain property[
Recently, Lei and his coworkers[
Amorphous lithium phosphate (Li3PO4), a stable Li+ conductor, is a potential coating material for its strong glass forming character and ease of preparation[
1 Experimental
1.1 Preparation and characterization of amorphous Li3PO4 coated Li4Ti5O12 electrode
Li4Ti5O12 powder was prepared by a simple solid-state reaction method described in previous work[
1.2 Characterization
The structure and morphology information were collected by XRD diffraction (XRD, χ’pert pro MPD, Cu Kα radiation, 0.03 (°)/s, 2θ=10°-85°), field emission scanning electron microscope (FE-SEM, Hitachi, S3400N) and Raman spectroscope (Renishaw inVia Reflex Raman Microscope, 0.5 mW, 514 nm). As for electrochemical test, two-electrode half cells were assembled with 1 mol∙L-1 LiPF6 solution in ethylene carbonate (EC)-diethyl carbonate (DEC) (1 : 1 in volume) as the electrolyte and Li foil as the counter and reference electrode. The mass loading was approximately 1.3 mg∙cm-2. A LAND series battery testing system (CT2001A/ CT1001C, Wuhan Kingou Electronics Co., Ltd.) was used to evaluate the rate performance and cycling behavior. Electrochemical impedance spectroscopy (EIS) (0.1 Hz-10 MHz, 1.56 V) was conducted on an electrochemical analyzer (Solartron Model 1287/1260A, Solartron Analytical).
2 Results and discussion
2.1 Structure and morphology characterization
XRD patterns of the prepared electrodes are displayed in Fig. 1. The peaks located at 2θ=18.3°, 35.6°, 43.2°, 57.2° and 62.9° can be assigned to Li4Ti5O12. Alternatively, those peaks positioned at 2θ=43.3°, 50.4° and 74.1° belong to the copper substrate. All of the diffraction peaks of amorphous Li3PO4 coated electrodes match well with those of pristine electrodes, which confirms that surface coating treatment has no serious impact on the crystal structure of the active material. No Li3PO4 diffraction peaks are detected, suggesting the deposited lithium phosphate layer is in amorphous state. To further illustrate the amorphous behavior, the FT-IR absorption spectrum of Li3PO4 deposited on copper foil for 60 min is shown in Fig. S1.
Figure 1.XRD patterns of pristine electrode LTOLPO00 and coated electrodes LTOLPO10, LTOLPO20 and LTOLPO40
Figure S1.FT-IR absorption spectra of Li3PO4 sputtered on copper foil for 60 min
SEM images of the amorphous Li3PO4 coated Li4Ti5O12 electrodes are shown in Fig. 2. As for LTOLPO00, Li4Ti5O12 active materials with the particle size of hundreds of nanometers disperse in the matrix of carbon black nanoparticles. With the coating time increasing, the particle corners of the active materials become blunt. As can be seen, the surface morphology of LTOLPO40 is totally changed. Both of the active species and the surrounding carbon black particles are covered by a thin layer, showing very smooth surface, which is another evidence of amorphous state for the coating layer. Moreover, The EDS result of LTOLPO40 in Fig. S2 indicated the existence of Li3PO4 on the modified Li4Ti5O12 electrode. The neighboring particles are wrapped together with the coating layer, leading to enlarged particles and the cross- linked network.
Figure 2.SEM images of pristine electrode LTOLPO00 (a) and coated electrodes LTOLPO10 (b), LTOLPO20 (c), and LTOLPO40 (d)
Figure S2.EDS result of Li4Ti5O12 electrode sputtered for 40 min (LTOLPO40)
2.2 Electrochemical performance
The first two discharge-charge curves of LTOLPO00 and LTOLPO20 at the current of 35 mA∙g-1 are given in Fig. 3(a, c), separately. Checking the discharge profiles, the plateau at about 1.54 V indicates the coexist of spinel Li8a[Li1/3Ti5/3]16d[O4]32e phase and rock-salt [Li2]16c[Li1/3Ti5/3]16d[O4]32e phase[
Figure 3.(a, c) First two discharge-charge curves at 35 mA∙g-1, 22nd discharge-charge curve at 175 mA∙g-1 and (b, d) the corresponding differential capacity plots of (a, b) pristine electrode LTOLPO00 and (c, d) coated electrode LTOLPO20
Fig. 3(b, d) exhibit the plots of differential capacities versus the voltages. The reduction peaks located at 1.53 and 1.54 V, respectively, in the first cycle for LTOLPO20 and LTOLPO00, which shift to 1.52 V in the second cycle. As for the oxidation peaks, an unchanged voltage (1.60 V) is detected for LTOLPO20. On the contrary, a small shift from 1.61 to 1.62 V can be distinguished for LTOLPO00. It can be concluded that the polarization of LTOLPO00 tends to intensify even in the first two cycles, which is another sign of the preliminary formation of passivated surface layer[
The rate performances of the pristine and coated electrodes exhibit in Fig. 4(a). All of the coated electrodes behave better than the pristine electrode. LTOLPO20 is the best among them. The discharge capacities for LTOLPO20 are 265, 223, 207, 196, 179 and 151 mAh∙g-1, respectively, when discharge-charged at the current density of 35, 88, 175, 350, 875 and 1750 mA∙g-1. When the discharge-charge current density is set back to 35 mA∙g-1, reversible capacity retains 238 mAh∙g-1, comparable to the 10th discharge capacity at the same current rate. The improved rate performance is attributed to the coverage of the amorphous layer which can prevent the formation of SEI film on the interface of lithium phosphate and the particles of active material/acetylene black. The SEM images shown in Fig. S3 demonstrates that the primary electrode is wrapped with a thick coating material (SEI film), while the surface of the modified electrode maintains clear after galvanostatic discharge- charge tests (100 times). A resultant smooth path is thus reserved for electronic transfer[
Figure 4.(a) Rate performance and (b) long cycle performance of LTOLPO00-LTOLPO40
Figure S3.SEM images of (a)primary electrode and (b)Li3PO4 modified electrode (LTOLPO40) after being cycled for 100 times
Figure S4.Cycle performance and galvanostatic discharge-charge test profiles of LTOLPO60
The comparison of XRD diffraction and Raman spectra between the as-prepared electrodes and the electrodes after 100 electrochemical cycles are collected in Fig. 5. No obvious peak shift and impurity peaks are detected in XRD patterns (Fig. 5(a)) for both pristine and coated electrodes, demonstrating the intrinsic stability of the general structure of Li4Ti5O12, which agrees with the literature[
Figure 5.XRD patterns and (b) Raman spectra of LTOLPO00 and LTOLPO20 before and after discharge-charge tests with inset in (b) showing magnification of Raman shift at 233 cm-1
To further understand the role of the amorphous Li3PO4 coating layer in the electrochemical performance, the AC impedance spectra of the pristine LTO and the coated electrodes tested after electrochemically cycling for 10 and 100 times are shown in Fig. 6. The inset equivalent circuits are employed to fit the EIS plots[
Figure 6.EIS plots and the fitted data of (a) LTOLPO00 and (b) LTOLPO20 after discharge-charged for 10 and 100 cycles with insets showing the corresponding equivalent circuits
Fitted data for EIS plots of LTOLPO00 and LTOLPO20 at given cycles
Fitted data for EIS plots of LTOLPO00 and LTOLPO20 at given cycles
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In general, all of the outstanding electrochemical performances achieved by electrode coating is contributed by the follows: firstly, the coating layer forms a good ionic conducting network and alleviates the possible impact of anisotropy of Li+ diffusion, which reduces polarization and results in better rate performance; secondly, the coating layer protects most particles of Li4Ti5O12 and acetylene black from being separated by SEI film unavoidably formed at a low potential of 0.01 V, which can undoubtedly maintain the integrity of an electronic passage; thirdly, the amorphous Li3PO4 can accommodate numerous transition metal ions accompanied with enhanced electronic conductivity, which is suggested to be the possible reason for the gradually increasing capacity during the long cycle tests.
3 Conclusion
Traditionally fabricated Li4Ti5O12 electrodes were coated with amorphous Li3PO4 by RF magnetron sputtering. The electrodes covered by amorphous thin films show a very smooth surface. At 20 min of deposition, the electrodes show a maximum improvement of the rate capability and cycle stability. The capacity remains a high level of 151 mAh∙g-1 when discharge- charged at 1750 mA∙g-1. A high capacity of 238 mAh∙g-1 is achieved after discharge-charged at 88 mA∙g-1 for more than 200 cycles. The milder Rct increase of the modified electrodes during charge-discharge process demonstrates that the amorphous Li3PO4 coating layer is very effective in maintaining the integrity of the electrode structure, enhancing the stability of the solid electrolyte interface and promoting the conducting network among the particles. The magnetron sputtering is an effective approach for surface coatings of electrodes, and using Li3PO4 as a coating layer can be broadened to other electrode materials.
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Performance of Amorphous Lithium Phosphate Coated Lithium Titanate Electrodes in Extended Working Range of 0.01-3.00 V
WANG Ying1, ZHANG Wenlong1, XING Yanfeng1, CAO suqun2, DAI Xinyi3, LI Jingze4
(1. School of Mechanical and Automobile Engineering, Shanghai University of Engineering Science, Shanghai 201620, China; 2. Faculty of Electronic Information Engineering, Huaiyin Institute of Technology, Huaian 223003, China; 3. College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China; 4. School of Microelectronics and Solid-state Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China)
The spectrum reveals the following characteristics. The bands in the range 420-570 cm-1 are related to the bending modes of the (PO4)3- anion[
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Ying WANG, Wenlong ZHANG, Yanfeng XING, suqun CAO, Xinyi DAI, Jingze LI.
Category: RESEARCH LETTER
Received: Sep. 30, 2020
Accepted: --
Published Online: Dec. 9, 2021
The Author Email: XING Yanfeng (smsmsues@163.com), LI Jingze (lijingze@uestc.edu.cn)