LiF as an Artificial SEI Layer to Enhance the High-Temperature Cycle

Article Cite This: Langmuir 2017, 33, 11164-11169

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LiF as an Artificial SEI Layer to Enhance the High-Temperature Cycle Performance of Li4Ti5O12 Lan Zhang,† Kaihang Zhang,‡ Zhaohui Shi,† and Suojiang Zhang*,† †

Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), Beijing 100190, PR China ‡ School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14850, United States S Supporting Information *

ABSTRACT: Li4Ti5O12 (LTO) is a promising anode material for electric vehicles (EVs) and electrochemical energy storage applications because of its safety, good rate capability, and long cycle life. At elevated temperature, such as 60 °C, it always shows poor cycle performance because of the instability between the electrode material and electrolyte, which may also lead to a serious gassing issue. In this article, a facile hydrothermal method is adopted to coat the LTO powder with a thin LiF layer, in which the LiF acts as an artificial solid electrolyte interface (SEI) layer to prevent the direct contact of LTO and electrolyte, thus improving the high-temperature cycle performance. Electrochemical tests prove that the LiF coating layer has no influence on the kinetics at ambient temperature and greatly enhances the high-temperature cycle stability, and the LTO@LiF composite material keeps 87% of its initial discharge capacity in 300 1C cycles at 60 °C. Moreover, the LiF coating layer exhibits a special self-driven reforming process during the initial cycles, which makes it uniform and more effective at enhancing the stability between electrode/electrolyte interfaces.

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INTRODUCTION

As one of the most important components of SEI, LiF is thermally and chemically stable and is formed in the reaction of lithium or lithium ion with F−.16,17 It was believed to be poorly conductive and thus may hinder the transport of lithium ions and decrease the rate performance of LIB when deposited on the graphite surface,18 whereas in Cheng’s work19 LiF not only suppressed the electrode/electrolyte side reactions such as LiPF 6 or carbonate decomposition and enhanced the uniformity of the SEI layer but also enabled fast Li ion transport at the interface between the electrode and electrolyte. Lu’s work20 also proves that the presence of LiF yields large improvements in the stability of Li electrodeposition and thus increased the cycle stability of lithium metal cells. In this consideration, LiF was adopted and coated onto LTO by a hydrothermal method, and the results prove that the coating layer protected the electrolyte from directly contact with LTO, thus improving the high-temperature cycle stability and reducing the gassing possibility.

Li4Ti5O12 (LTO) is a zero strain material with a theoretical capacity of 175 mA h g−1. Lithium ion batteries (LIBs) based on LTO show promising cycle stability as well as safety; they are an attractive anode material to use in high-powered LIBs1−6 for electric vehicles (EVs) and have long life span LIBs for energy storage applications. However, LIBs based on LTO always have poor high-temperature cycle stability and swelling problems with respect to the instability between LTO and electrolytes or, specifically, the so-called gassing phenomenon,6−10 which hinders their commercial utilization. There is still no widely accepted conclusion on the mechanism that leads to the poor high-temperature cycle stability and gassing issue, but the most common opinions are LTO catalysis6,7 and trace water from the LTO lattice,8−11 respectively. On the basis of the catalysis reduction opinions, researchers have been exploring the control of LTO crystal growth7 to reduce the reactivity between LTO and electrolytes or choosing an electrolyte that is less reactive with LTO.9,12−15 A new strategy was adopted in this work to enhance the hightemperature cycle stability and address the gassing issue, which is to form an artificial solid electrolyte interphase (SEI) layer on LTO to prevent the direct contact between it and the electrolyte, and the results prove that it is effective at improving the high-temperature cycle stability. Moreover, it should be helpful in suppressing the gassing phenomenon. © 2017 American Chemical Society

Special Issue: Tribute to Keith Gubbins, Pioneer in the Theory of Liquids Received: June 14, 2017 Revised: August 18, 2017 Published: August 29, 2017 11164

DOI: 10.1021/acs.langmuir.7b02031 Langmuir 2017, 33, 11164−11169

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EXPERIMENTAL SECTION

LTO was purchased from Xingneng New Materials (Chengdu, China), and LiF (99.99% metals basis) was produced by Aladdin Industrial Inc. (China). The LTO@LiF composite material was prepared by a simple hydrothermal method. First, LTO and LiF with a mass ratio of 98:2 were dispersed into deionized water with strong magnetic agitation to make a homogeneous slurry, and then the mixture was heated by a stepwise method, which is 100 and 130 °C each for 3 h, and the whole process was carried out with reflux and condensation to make sure that LiF was totally dissolved and gradually coated onto LTO. In fact, the water temperature is 100 °C during the whole process, and the aim of overheating is to increase the coating uniformity. In the third step, the mixture was filtered and washed with a large amount of deionized water to remove excess LiF, the residue was baked in a vacuum oven at 80 °C to remove the residual water, and finally a white powder, LTO@ LiF, was obtained. The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Focus X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15406 nm). The scanned 2θ range was between 10 and 80° at room temperature. The morphologies and microstructures of the samples were characterized by scanning electron microscopy (SEM) at 10.0 kV on a JEOL JSM-7001F microscope. Transmission electron microscopy (TEM) observations were performed by using a JEOL JEM-2100 microscope operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was obtained via an ESCALAB 250Xi. The LTO@LiF composite electrode was prepared by mixing poly(vinylidene fluoride) (PVDF, Solef 5130 by Solvay), Ketjen black, and the composite material (mass ratio of 1:1:8) together into Nmethyl pyrrolidone (NMP) with ball milling to make a homogeneous slurry, and then the slurry was cast onto an Al foil (20 μm) with a doctor blade and baked in an oven at 80 °C for 12 h to remove the residue NMP. Finally, the foil was cut into φ14 mm disks as the electrode. The semicell was assembled in a glovebox whose oxygen and water contents were lower than 1 ppm, with lithium foil as the anode, Celgard 2025 as the membrane, 1 M LiPF6 + 0.1 M LiTFSI/EC + EMC + DMC (4:3:3 w/w/w) as the electrolyte, and the as-prepared composite electrode as the cathode. The cells were charge/discharged in constant current mode with different C rates at room temperature or elevated temperature, such as 60 °C. Electrochemical impedance spectroscopy (EIS) was performed on a CHI 660e electrochemical workstation at open circuit potential, with a 5 mV perturbation between 100 kHz and 0.01 Hz. The cells were disassembled after certain cycles in the glovebox, and the electrode was washed with a large amount of DMC to remove the residue electrolyte or impurity and then baked in a vacuum oven before TEM or XPS tests.

Figure 1. XRD patterns of LTO, LiF, and LTO@LiF.

treatment, a thin layer whose thickness is about 10−20 nm was coated onto LTO as shown in Figure 2d, and size of the crystal lattice is about 0.20 nm, which is in agreement with the LiF (200) crystal lattice parameters. Therefore, this proves that LiF was successfully coated onto LTO. Both LTO- and LTO@LiF-based semicells were charge/ discharged at different C rates at room temperature, and the results are shown in Figure 3 and Figure S1 in the Supporting Information. The initial discharge capacity is 160.2 mA h g−1 based on the weight of LTO@LiF as shown in Figure 3a, similar to that of commercial LTO, and the capacity retention is 93.0% in 500 cycles, which proves that the coating layer has little influence on either the capacity and cycle stability of the LTO. As shown in Figure 3b, the rate performance of this LTO@LiF composite is also satisfying. Even at a 5 C rate and over 100 cycles, the reversible capacity is still higher than 110 mA h g−1. And when the C rate reduces to 1 C, the specific capacity could increase up to 158 mA h g−1. Therefore, the coating layer has no significant influence on the rate performance. It can be seen that the high rate-specific capacity of LTO@LiF is higher than that of pristine LTO, which may be caused by the better electrolyte uptake capability of the LTO@ LiF composite material. This phenomenon was also reflected in Figure 7 when we tested the impedance of both cells, where the Rs in the LTO@LiF cell is much smaller than that of LTO. Figure 4 shows the cycle performances of both LTO@LiF and LTO at 60 °C at a 1 C rate, and the charge/discharge profiles are also shown in Figure S2. It can be seen that both materials have an initial discharge capacity that is higher than the theoretical value, and the coulombic efficiency is a little far from 100%. This phenomenon is caused by the side reactions between LTO in a high charged state (Ti3+) and the electrolyte in which Ti3+ may react with carbonates to form electroinert rutile TiO2 (which is also proven by the XPS spectra as shown in Figure 6), and some carbonates may reduce to gases such as CO and CO2,6 on which some of the active material and Li+ was consumed. After several cycles, the coulombic efficiency gradually approached 100%, but this process was a little longer for LTO@LiF than for LTO, even though the LTO@LiF composite showed better long-term cycle stability. It shows a sharp capacity drop after about 190 cycles for LTO in Figure 4b, which is caused by the contact inferiorities between LTO

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RESULTS AND DISCUSSION XRD patterns of LTO, LiF, and the as-prepared composite material are shown in Figure 1. Line a is the typical XRD pattern of spinel LTO, in which the peaks at 18.33, 35.57, 43.24, 57.21, and 66.07 are assigned to the (111), (311), (333), and (531) crystal planes, respectively. Line c is the XRD pattern of LiF, and we can see that the most prominent peaks include the (111), (200), and (220) crystal planes. In the composite material, we can just barely see (200) and (220) characteristic peaks of LiF, but (111) is too weak to see. The reason is that the LiF content is less than 2%, and some of the diffraction peaks assigned to LiF are too weak to be detected. Therefore, to further prove the existence of LiF in the composite material, we performed SEM and TEM tests on the composite as shown in Figure 2. Figure 2 shows the typical SEM and TEM pictures of LTO (a, b) and the coated material (c, d), and it can be seen that they have similar morphology and the coating process did not change the uniformity of LTO. The mean size of both LTO and the composite is about 500 nm. After the hydrothermal 11165

DOI: 10.1021/acs.langmuir.7b02031 Langmuir 2017, 33, 11164−11169

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Figure 2. SEM and TEM pictures of the as-prepared LTO@LiF composite material.

Figure 3. Cycle (a) and rate (b) performance of as-prepared LTO@LiF at room temperature.

Figure 4. Cycle performance of LTO with (a) and without (b) a LiF coating (at 60 °C).

2 that the thickness is not uniform, when the cell was cycled at high temperature, some of the LiPF6 may be decomposed and react with the trace amount of water in the cell, thus producing HF. Unfortunately, LiF is dissolvable in HF; therefore, the electrolyte could still penetrate the places where the LiF layer is thin, thus causing the side reactions between LTO and the electrolyte and leading to the irreversible capacity loss in the initial cycles. Meanwhile, we also found that after several

and the current collector along with the gassing phenomenon other than for lithium dendrite, which still has a high coulombic efficiency of about 100%. For the LTO@LiF composite material, after the capacity loss in the initial cycles, there is a capacity enhancement in concert with the coulombic efficiency that gradually approaches 100%. Obviously, this phenomenon is closely related to the LiF coating layer. Although LiF has been coated onto the LTO surface and we can still see in Figure 11166

DOI: 10.1021/acs.langmuir.7b02031 Langmuir 2017, 33, 11164−11169

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Figure 5. TEM picture of the LTO@LiF and LTO after 10 cycles at 60 °C.

Figure 6. C 1s, O 1s, and Ti 2p spectra of LTO after 10 cycles at 60 °C.

proves that we need not work so hard to get a uniform coating layer. The facile hydrothermal process could enhance the cycle stability effectively. Meanwhile, LiF is also an effective LiPF6 stabilizer because it could stop eq 1 from moving to the right, especially when some of the LiF is dissolved in the electrolyte during hightemperature operation.

charge/discharge cycles the LiF layer gradually became uniform as shown in Figure 5, which is the TEM pictures of the anode material after 10 cycles in 60 °C. In fact, a similar phenomenon occurred in other systems, such as in Wang et al.’s work,21 in which a polypyrrole (PPy)-coated sulfur−carbon nanotube (S− CNT) composite material was prepared, and PPy functioned just like LiF in this article. The initial PPy coating layer is also not even because after about 5 cycles it becomes very uniform. This is a kind of self-driven SEI reforming process in which some of the LiF dissolved and broke away from LTO surface, especially those on the thick places; meanwhile, some HF reacts with lithium ions and the newly produced LiF that may possibly deposit onto the LTO surface where the LiF layer is thinner. After these processes, the LiF layer becomes very uniform as shown in Figure 5. This process also explained the capacity enhancement after 10 cycles in the LTO@LiF cell. As we calculate the specific capacity on the basis of the composite material, when the coating layer becomes thinner, it will be much easier for lithium ions to move between active material particles, thus increasing the capacity. This phenomenon also

LiPF6 ⇌ LiF + PF5

(1)

However, we also found that the outmost layer of the LTO@ LiF composite material has somewhat turned to TiO2, whereas even the total thickness of LiF and TiO2 is much less that that of TiO2, which is the coating on the surface of LTO as shown in Figure 5b. The TiO2 layer may hinder the transport of both ions and electrons and lead to slow kinetics as well as capacity loss because it is not only a poor electron conductor but also electrochemically inert. This slow kinetics is also reflected in Figure S2 because in the initial cycle the polarization of LTO@ LiF is larger than that of LTO (Figure S2-a), while after 10 cycles, LTO@LiF became the better one (Figure S2-b). 11167

DOI: 10.1021/acs.langmuir.7b02031 Langmuir 2017, 33, 11164−11169

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Figure 7. Impedance plots of (a) LTO- and (b) LTO@LiF-based cells before and after high-temperature cycles and the equivalent electrical circuit (c) of the cells.

Table 1. Simulation Results of the Equivalent Circuit Rs (Ω) LTO initial LTO cycled LTO@LiF initial LTO@LiF cycled a

9.16 3.04 4.67 4.05

C1 (F) 4.1 7.93 1.4 1.4

× × × ×

Rct (Ω)

C2 (F)

RSEI (Ω)

Y0 (Ω−1 cm−2 S−0.5)a

49.8 88.66 93.02 53.1

1.84 × 10−6 4.8 × 10−6 1.14 × 10−5

48.04 33.52 51.85

0.06 0.02 0.08 0.03

−6

10 10−6 10−6 10−6

Warburg = (jw)0.5/Y0, where j is the imaginary unit and w is the frequency.

It can be seen that compared to the LTO@LiF cell the LTO cell has a lower initial Rct, a little higher Rs, and no RSEI (or artificial SEI), which proves that no SEI was formed in the cell system that was adopted. The Rct in the initial state is higher for the one with the LiF coating, which proves that the coating layer hindered the transport of both ions and electrons whereas the relatively small Rs also proves that it facilitated the wetting process between the electrolyte and electrode. After 200 cycles in 60 °C, the Rct in the LTO cell increased from 49.8 to 88.66 Ω for the thick TiO2 layer formed on the LTO surface; meanwhile, a new SEI film was formed, which led to the RSEI in the circuit, whereas in the LTO@LiF cell, Rct decreases from about 93.02 to 53.1 Ω after 200 cycles for the SEI reforming process, which reduce the thickness of the LiF coating layer from about 20 to 10 nm. Also, the RSEI was also more stable than that of LTO, and it increased from 33.52 to 51.85 Ω. Therefore, the LTO@LiF cell has much better cycle stability than does LTO, and the LiF coating layer protected the cell effectively.

Therefore, we can say that the LiF could greatly enhance the high-temperature cycle stability of LTO. XPS tests were performed to further analyze the surface components of the high-temperature cycled-LTO electrode. C 1s, O 1s, and Ti 2p spectra are given in Figure 6, and the C 1s peak for the sp3-hybridized carbon was set at 284.8 eV22 and used as a reference for the calibration of the XPS peaks. The spectrum of C 1s in Figure 6a consists of four typical peaks in which the ones located at 248.8 and 290.5 eV are assigned to the binder in the electrode, PVDF. The peak located at 289.3 eV in the C 1s spectrum, together with the peak at 532.0 eV (O 1s spectrum, Figure 6b), verified the existence of the carboxyl groups,23 which is a side reaction product of LTO and electrolyte.18 Another peak detected in the O 1s spectrum is the one located at 529.55 eV, which is always assigned to metal oxide. This is combined with the Ti 2p1/2 and Ti 2p3/2 peaks located at 463.87 and 458.15 eV (Figure 6c), respectively, with an interpolation of 5.72 eV, which is in agreement with TiO2.22 Therefore, we could be sure that the outermost surface of LTO is fully covered with TiO2, and the thickness is more than 10 nm, which is the detection depth of XPS. These XPS spectra also further proved the decomposition of electrolyte during the LTO cell’s high-temperature operation, and it is also evidence that LTO in fact is not an SEI-free material.24 EIS of the cells before and after high-temperature cycling was also performed to further study the interphase transformation during the charge/discharge cycle as shown in Figure 7. In the meantime, an equivalent circuit25 was built based on the simulation result of ZsimWin software. In particular, there is no RSEI or C2 in the initial state of the LTO cell because there is no SEI on the active material. The simulation results are shown in Table 1.

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CONCLUSIONS An LTO@LiF composite material was successfully prepared by a facile hydrothermal method. The LiF coating layer, whose thickness is between 10 to 20 nm, could work as an artificial SEI layer to prevent direct contact between LTO and the electrolyte, thus reducing the chance of side reactions between them, for which the high-temperature cycle performance was greatly enhanced. Meanwhile, this material is very promising in resolving the gassing issue in LTO-based LIBs. A self-driven SEI reforming process was observed during the initial charge/ discharge process, which makes the LiF coating layer very uniform. This proves that the uniformity of the coating layer is 11168

DOI: 10.1021/acs.langmuir.7b02031 Langmuir 2017, 33, 11164−11169

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(8) Belharouak, I.; Koenig, G. M.; Tan, T.; Yumoto, H.; Ota, N.; Amine, K. Performance Degradation and Gassing of Li4Ti5O12/ LiMn2O4 Lithium-Ion Cells. J. Electrochem. Soc. 2012, 159 (8), A1165−A1170. (9) Wu, K.; Yang, J.; Liu, Y.; Zhang, Y.; Wang, C.; Xu, J.; Ning, F.; Wang, D. Investigation on gas generation of Li 4 Ti5 O12 /LiNi1/3Co1/3Mn1/3O2 cells at elevated temperature. J. Power Sources 2013, 237, 285−290. (10) Wu, K.; Yang, J.; Zhang, Y.; Wang, C.; Wang, D. Investigation on Li4Ti5O12 batteries developed for hybrid electric vehicle. J. Appl. Electrochem. 2012, 42 (12), 989−995. (11) Bernhard, R.; Meini, S.; Gasteiger, H. A. On-Line Electrochemical Mass Spectrometry Investigations on the Gassing Behavior of Li4Ti5O12 Electrodes and Its Origins. J. Electrochem. Soc. 2014, 161 (4), A497−A505. (12) He, Y.-B.; Liu, M.; Huang, Z.-D.; Zhang, B.; Yu, Y.; Li, B.; Kang, F.; Kim, J.-K. Effect of solid electrolyte interface (SEI) film on cyclic performance of Li4Ti5O12 anodes for Li ion batteries. J. Power Sources 2013, 239, 269−276. (13) Demeaux, J.; De Vito, E.; Le Digabel, M.; Galiano, H.; ClaudeMontigny, B.; Lemordant, D. Dynamics of Li4Ti5O12/sulfone-based electrolyte interfaces in lithium-ion batteries. Phys. Chem. Chem. Phys. 2014, 16 (11), 5201−12. (14) Kim, J.-H.; Song, S.-W.; Hoang, H.-V.; Doh, C.-H.; Kim, D.-W. Study on the Cycling Performance of Li4Ti5O12 Electrode in the Ionic Liquid Electrolytes Containing an Additive. Bull. Korean Chem. Soc. 2011, 32 (1), 105−108. (15) Wang, Q.; Pechy, P.; Zakeeruddin, S. M.; Exnar, I.; Grätzel, M. Novel electrolytes for Li4Ti5O12-based high power lithium ion batteries with nitrile solvents. J. Power Sources 2005, 146 (1−2), 813−816. (16) Xu, K. Electrolytes and Interphasial Chemistry in Li Ion Devices. Energies 2010, 3 (1), 135−154. (17) Tasaki, K.; Goldberg, A.; Lian, J.-J.; Walker, M.; Timmons, A.; Harris, S. J. Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents. J. Electrochem. Soc. 2009, 156 (12), A1019. (18) Andersson, A. M.; Edström, K. Chemical Composition and Morphology of the Elevated Temperature SEI on Graphite. J. Electrochem. Soc. 2001, 148 (10), A1100−A1109. (19) Wu, Z.-S.; Xue, L.; Ren, W.; Li, F.; Wen, L.; Cheng, H.-M. A LiF Nanoparticle-Modified Graphene Electrode for High-Power and HighEnergy Lithium Ion Batteries. Adv. Funct. Mater. 2012, 22 (15), 3290− 3297. (20) Lu, Y.; Tu, Z.; Archer, L. A. Stable lithium electrodeposition in liquid and nanoporous solid electrolytes. Nat. Mater. 2014, 13 (10), 961−969. (21) Wang, J.; Lu, L.; Shi, D.; Tandiono, R.; Wang, Z.; Konstantinov, K.; Liu, H. A Conductive Polypyrrole-Coated, Sulfur−Carbon Nanotube Composite for Use in Lithium−Sulfur Batteries. ChemPlusChem 2013, 78 (4), 318−324. (22) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Eden Prairie, MN, 1979. (23) Lu, Y.-C.; Mansour, A. N.; Yabuuchi, N.; Shao-Horn, Y. Probing the Origin of Enhanced Stability of “AlPO4” Nanoparticle Coated LiCoO2 during Cycling to High Voltages: Combined XRD and XPS Studies. Chem. Mater. 2009, 21 (19), 4408−4424. (24) Song, M.-S.; Kim, R.-H.; Baek, S.-W.; Lee, K.-S.; Park, K.; Benayad, A. Is Li4Ti5O12 a solid-electrolyte-interphase-free electrode material in Li-ion batteries? Reactivity between the Li4Ti5O12 electrode and electrolyte. J. Mater. Chem. A 2014, 2 (3), 631. (25) Westerhoff, U.; Kurbach, K.; Lienesch, F.; Kurrat, M. Analysis of Lithium-Ion Battery Models Based on Electrochemical Impedance Spectroscopy. Energy Technol-Ger 2016, 4 (12), 1620−1630.

not indispensable in the LTO case. Moreover, this facile hydrothermal method is also suitable for other electrode materials, such as LiMn2O4, and the LiF coating layer may prevent Mn3+ ions from dissolving in the electrolyte, thus improving its high-temperature cycle stability.

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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/acs.langmuir.7b02031. Room-temperature cycle and rate performance of LTO. High-temperature charge/discharge profiles of the initial cycle and the 10th cycle of LTO and the LTO@LiF cell. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suojiang Zhang: 0000-0002-9397-954X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research (973) Program of China (2014CB239700), the Key Program of National Natural Science Foundation of China (91434203), the National Key Research and Development Program of China (2016YFB0100100), the International Cooperation and Exchange of the National Natural Science Foundation of China (51561145020), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA09010103), the Ford-China University Research Program, and the CAS/SAFEA International Partnership Program for Creative Research Teams.

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b02031 Langmuir 2017, 33, 11164−11169