[email protected] composite as high performance anode materials for lithium ion battery application

[email protected] composite as high performance anode materials for lithium ion battery application

Electrochimica Acta 329 (2020) 135139 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electa...

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Electrochimica Acta 329 (2020) 135139

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

[email protected] composite as high performance anode materials for lithium ion battery application Hong-Li Ding a, Hai-Tao Yu a, Chen-Feng Guo a, Ying Xie a, *, Ting-Feng Yi b, ** a

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, PR China b School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao, 066004, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 September 2019 Received in revised form 15 October 2019 Accepted 23 October 2019 Available online 24 October 2019

Titanate anode materials are usually considered as zero strain materials because of the very small volume change during cycling. SrLi2Ti6O14, one material of this family with a low potential plateau, is a promising candidate. In this work, a pure phase SrLi2Ti6O14 was successfully synthesized by a simple sol-gel method. To further improve the overall performance of this material, different amount of AlF3 was coated on its surface. Relying on different techniques, the structural details of [email protected] were revealed. The electrochemical tests showed that [email protected], [email protected], [email protected], and [email protected] samples can deliver a reversible capacity of about 193.3, 246.1, 280.9, and 223.7 mA h g1 at 0.05 A g1, which decrease to 74.2, 143.3, 155.7 and 107.6 mA h g1 at the rate of 2.62 A g1. Furthermore, the rate capability and cycling stability of the coated samples are all improved obviously. The [email protected] sample exhibit the best performance, and it shows a high specific capacity (>100 mA h g1) and a good retention rate (45.2%) after 2000 cycles at an ultrahigh current density of 5.24 A g1. Our experiments confirmed that AlF3 coating is an effective method to boost the electrochemical performance of SrLi2Ti6O14, which also provides some hints for the optimization of the titaniate-based materials. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Anode material AlF3 coating Lithium ion battery SrLi2Ti6O14

1. Introduction In the past two decades, lithium-ion batteries have long been widely used as portable electronic products, main power source for electric vehicles, and large-scale power storages due to their high energy density, long cycling life and environmental benignity [1,2]. However, classic lithium-ion batteries have safety problems originated from the applications of flammable liquid electrolytes and unsafe electrode materials [2]. To solve this problem, one effective strategy is to find an anode material with excellent structure/ thermal stabilities. In recent years, spinel-type Li4Ti5O12 has been extensively studied because of its higher discharge/charge plateau (~1.55 V vs. Liþ/Li) with respect to graphite anode, excellent thermodynamic stability, and zero strain characteristics [3e8]. However, the very low electronic conductivity has significantly restricted its high rate performance. At the same time, it is also expected that the plateau of the anode should be as low as possible,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Xie), [email protected] (T.-F. Yi). https://doi.org/10.1016/j.electacta.2019.135139 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

which will produce a higher open-circuit voltage (OCV) and thus the energy density of the battery. In this context, investigations on the anode materials with a lower working potential as well as a longer cycle stability and a higher conductivity have become rather important, and MLi2Ti6O14 (M ¼ Sr, Ba, Pb, and Na) materials were thereby focused on [9]. After comparison, it was reported that the electrochemical property and stability of lithium strontium titanate are better [10e12]. In fact, MLi2Ti6O14 can be considered as a Li2O-MO-TiO2 ternary oxide system [13e17], which provide another viewpoint for designing titanate-based anode materials. Relying on the mesoporous TiO2, Li2SrTi6O14 was successfully prepared, and it was reported that the LiMn2O4/Li2SrTi6O14 full cell exhibits a good cycle stability and an excellent rate performance [18]. This implied that MLi2Ti6O14 is a good candidate for constructing high-power LIBs [9,19e21]. Furthermore, to improve the electrochemical performance of MLi2Ti6O14, doping strategy is usually applied. To date, the doping effects of Yi3þ, Cr3þ, La3þ [22], Al3þ [23], and F [24] have been revealed, and the substitution of Ti sites by other elements (Li2Na2Ti5.9X0.1O14, X ¼ Al, Zr, and V) was also evaluated. It was confirmed that Al3þ and Zr4þ doping can effectively improve the

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electron/ion conductivity of Li2Na2Ti6O14, leading to the reduction of the charge transfer resistance and the enhancement of the electrochemical performances [17]. At the same time, coating of a conductive layer on the electrode materials surface or constructing nano-scale composites are also helpful for the increase of the electrochemical performance [8,25e29]. For example, Sun and Ding have studied different coating layers respectively on the cathode and anode surface and proved the improvement of cycle stability [30], while AlF3 coating was reported to be helpful for suppressing the oxygen release and thermal performance of the cathode at high temperature [31]. In comparison to other coating substances, fluoride coating can effectively protects the electrode material from the attack of acidic species in the electrolyte and maintain the structural integrity of the electrode material, leading to the improvement of the electrochemical stability of battery. Nowadays, many fluoride coating works (SrF2 [32], MgF2 [33], LaF3 [34], CaF2 [35e37], FeF3 [38], ZrFx [39], AlF3 [31,40e44], BiOF [45], CeF3 [46]) have been mainly focused on the cathode materials, and its effect on the MLi2Ti6O14 anode materials was rather absent and still unclear. The coating thickness and uniformity are also crucial for the performance optimization of the materials, which is also unknown and should be discussed in detail. On the other hand, when discharging down to a very low potential (~0.0 V vs. Liþ/Li), the safety issue for SrLi2Ti6O14 is also important. If this process is available and highly reversible, the energy density and discharge capacity of a full cell constructed by such an anode will be improved significantly. With the above motivations in mind, the present manuscript is aim at revealing relevant mechanism via surveying the relationship between the structural changes of the coating layer and the electrochemical performance of the materials. Our result will be helpful for identify the effect of coating and thus helpful for the optimization of the electrochemical properties of the MLi2Ti6O14 family.

composition, and element distribution) of the samples were characterized by XRD, SEM, HRTEM, and EDS mapping. Electrochemical tests were carried out with the CR2025 coin cells. Further details can also be obtained in the SI.

3. Result and discussion The XRD patterns and structures of SLTO and [email protected] samples are shown in Fig. 2. It can be found that all observed diffraction peaks are well conformed to the featured peaks of SLTO, and the XRD pattern of the AlF3 coating layer in [email protected] samples is unable to be observed clearly. According to the sharp and well-defined reflection peaks in Fig. 2, it can be determined that obtain samples belong to the Cmca space group, which is consistent with the standard card of SrLi2Ti6O14 (ICSD #152566) [18,19,47e49]. Furthermore, the XRD peaks exhibit sharp and well defined characteristics, suggesting that the purity and crystallinity of the asprepared samples are good. SrLi2Ti6O14 is a three-dimensional structure formed by the connection of TiO6 octahedrons. Each titanium atom is surrounded by 6 oxygen atoms and lithium atoms are located in the channels

2. Experimental section The schematic diagram for the preparation of SrLi2Ti6O14 and [email protected] samples is shown in Fig. 1. Relying on the sol-gel method, pure SrLi2Ti6O14 was synthesized first. The synthesized sample is simply referred to SLTO. The mass loading of the active materials is at the range of 0.32e0.56 mg. Further preparation details were summarized in the Supplemental Information (SI). The structural details (i.e. crystal information, morphology,

Fig. 2. (a) XRD patterns of [email protected], [email protected], [email protected], and [email protected] samples.

Fig. 1. Schematic diagram for the preparation of SrLi2Ti6O14 (SLTO) and [email protected] ([email protected]) samples.

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form by TiO6 octahedrons. Furthermore, to obtain the lattice parameters of the samples, Rietveld refinement was performed based on the crystal structure of the SrLi2Ti6O14. The refinement results were shown in Fig. S1, and relevant crystal model for SrLi2Ti6O14 was depicted Fig. S2. The results clearly showed that the simulated pattern well matches the experimental data, and the Rwp. is smaller than 8.34%. According to the lattice constants listed in Table S1, it can be found that the lattice volume of the [email protected] sample increases slightly, while those of other samples are all reduced. The XRD result has clarified the structures of the samples, being consistent with previous reports [18,19,47e49]. The SEM images of the [email protected], [email protected], [email protected], and [email protected] samples are shown in Fig. 3(aed). As can be seen from the figure, pure SLTO has an average particle size of about 0.5e0.8 mm, and coating different amount of AlF3 does not significantly affect the particle size. Fig. 3(e) shows the EDS mapping of the [email protected] sample, and the results indicate that strontium (red), titanium (cyan), and oxygen (green) are distributed uniformly. Furthermore, aluminum (blue) and fluorine (purple) elements also

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exhibit a nearly identical distribution within the whole area, which implies the existence of aluminum fluoride. Indicated that uniform element distribution, preliminary indication that aluminum fluoride is evenly coated on the surface of SLTO sample. To visualize the structural details of the samples, the TEM images for [email protected] and SLTO @AF5 samples were obtained and depicted in Fig. 4. It can be seen clearly that bare SLTO (Fig. 4(a)) particles exhibit a smooth surface. In contrast, a rough thin layer (Fig. 4(b)) can be observed on the SLTO surface in the [email protected] sample, which also implies the formation of aluminum fluoride. This is consistent with the SEM result. Furthermore, Fig. 4(b) also showed that the coating layer is amorphous with a thickness of about 5 nm. The high resolution images indicated that two sets of lattice fringe can be seen clearly and the values are calculated to about 4.27 Å for the [email protected] and [email protected] samples. The values well match the stripe spacing of the (221) facet of the crystal in standard card (ICSD # 152566). It should be noted that the formation of aluminum fluoride on the surface of SLTO can block the attack of HF released from the electrolyte and thereby protect the

Fig. 3. SEM images of the (a) [email protected], (b) [email protected], (c) [email protected], and (d) [email protected] samples, and (e) elemental mapping for the [email protected] sample.

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Fig. 4. (a), (b) TEM and (c) HRTEM for [email protected] sample and (c), (d) TEM and (e) HRTEM for [email protected] sample.

electrode material from corrosion. Furthermore, the amorphous nature of the coating layer will facilitate the transport of lithium ions in between the SLTO and the electrolyte [43]. As a result, the electrochemical performance of the sample should be enhanced.

The rate capabilities of different samples at different current densities ranging from 0.05 A g1 to 2.62 A g1 were summarized and compared in Fig. 5(a). For a large discharging rate, the charging rate is set to be 0.05 A g1. It was found from Fig. 5(a) that

Fig. 5. Rate capability during (a) delithiation, (b) lithiation processes at different rates. Cyclic performance during (c) delithiation, (d) lithiation processes at 0.05 A g1 for the [email protected], [email protected], [email protected], and [email protected] samples.

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[email protected], [email protected], [email protected], and [email protected] samples can deliver a reversible capacity of about 193.3, 246.1, 280.9, and 223.7 mA h g1 at 0.05 A g1. With increasing rate, the capacity of each sample was reduced gradually. At the rate of 2.62 A g1, the capacities became 74.2, 143.3, 155.7 and 107.6 mA h g1. When the current density was reset to 0.05 A g1, The charge/discharge capacities of the [email protected], [email protected], [email protected], and [email protected] samples can be recovered to 159.9/162.3, 217.2/245.6, 259.6/262.4 and 171.6/172.6 mA h g1, suggesting that the cycling stability of the as-prepared materials is excellent. Although due to the polarization the capacities of the samples decrease with increasing rates, it is no doubt that the rate performances of [email protected] samples are all superior to pure SLTO, and [email protected] is indeed much better than other samples. This demonstrated that coating a certain amount of aluminum fluoride does have a positive influence on the rate

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performance of the electrode material. The cyclic performances of different samples were depicted in Fig. 5(c) and (d). The charge/ discharge capacities of different samples after 100 cycles are 118/ 120, 145/149, 165/170, and 132/134 mA h g1, suggesting that the cycling stability of the as-prepared materials is good. As AlF3 is very stable in fluoride-containing electrolyte, the introduction of a thin coating layer on the surface of the electrode material thus avoids the direct contact between the electrode material and the electrolyte, as illustrated in Fig. 6. Therefore, the structural stability of the electrode will be enhanced obviously. The validity of coating AlF3 layer on the surface of other materials has also been confirmed in previous literatures [30,31,40]. To evaluate the Li þ insertion/extraction kinetics, the cyclic voltammetry (CV) tests were carried out. The data for different samples at a scan rate of 50 mV/s were shown in Fig. 7. In the cyclic

Fig. 6. Schematic diagram of the effect of coated aluminum fluoride on the electrode surface of electrode material.

Fig. 7. Cyclic voltammetry curves for (a) [email protected], (b) [email protected], (c) [email protected], and (d) [email protected] samples at a scanning rate of 0.01 mV/s.

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voltammetry curves, the cathode (C) peak in the negative scan is attributed to Li þ intercalation to SrLi2Ti6O14, while the anode (A) peak in the positive scan is due to Liþ delithiation from the SrLi2Ti6O14. The scanning curve of SrLi2Ti6O14 has one major pair of reversible redox peaks, which corresponds to the redox reaction between Ti3þ and Ti4þ. There is also a pair of small redox peaks at a lower potential. This behavior is opposite to Na2Li2Ti6O14, which exhibits only one redox pair in the same potential window. The substitution of two Naþ by one Sr2þ not only kept the charge neutrality of the materials but also provided addition interstitial positions for lithium. As a result, when Ti4þ was fully reduced to Ti3þ, continuous intercalation of lithium will make the reduction of Ti3þ to Ti(3d)þ possible, leading to the occurrence of the secondary redox peaks. Furthermore, Fig. 7 also showed that the average peak currents of [email protected], [email protected], [email protected], and [email protected] samples are 0.24/-0.19, 0.23/-0.19, 0.35/-0.29, and 0.32/0.26 mA mg1, and the corresponding peak potentials are 1.48/1.30, 1.50/1.36, 1.46/1.38, and 1.50/1.30 V. During the charge and discharge processes, the electrochemical reactions of the materials can be expressed as,

SrLi2 Ti6 O14 þ4Liþ Discharge

þ 4e ƒƒƒƒƒƒ ƒ!SrLi6 Ti6 O14 ðFrom 3:0 to 0:5 VÞ SrLi6 Ti6 O14 þ ð2 þ dÞLiþ þ þð2  Discharge þ d e / SrLi8þd Ti6 O14 ðFrom 0:5 to 0:0 VÞ

Charge

SrLi8þd Ti6 O14 ƒƒƒƒƒ!SrLi2 Ti6 O14 þ ð6 þ dÞLi ðFrom 0:0 to 3:0 VÞ To evaluated the cyclic reversibility, Eq. (1) and Eq. (2) were introduced [50],

Ipa ¼1 Ipc jEpa  Epcjz

(1)

2:3RT nF

(2)

where R the gas constant, T the absolute temperature, n the number of electrons transferred in the half-reaction of the redox pair, and F the Faraday constant. If the two equations are satisfied perfectly, the materials thus have the best cyclic reversibility.The ipa/ipc values for [email protected], [email protected], [email protected], and [email protected] samples are calculated to be 1.26, 1.21, 1.21, 1.23, while the |Epa Epc| for these samples are 0.18, 0.14, 0.08, 0.20 V. Therefore, [email protected] sample thus has the best cyclic reversibility. Furthermore, the peaks current of the [email protected] are also the highest, implying that its impedance should be smaller than other samples. The data above also suggested that suitable thickness of the coating layer is crucial for the optimization of the electrochemical performance of the electrode. The charge-discharge curves for the different samples at different rates were shown in Fig. 8. All charge and discharge data were collected from the second cycle under different rates, while the initial charge-discharge profiles can be found in Fig. S3 of the SI. Our tests showed that during the first cycle the irreversible

Fig. 8. Charge-discharge curves for (a) [email protected], (b) [email protected], (c) [email protected], and (d) [email protected] samples at different rates.

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capacities of different samples are large, and it is a common phenomenon for Ti-based anode materials, such as SrLi2Ti6O14 [47,48], Li2BaTi6O14 [51] and Na2Li2Ti6O14 [52e54]. Due to the coating, the Coulombic efficiencies increase from 57% to 62%, 58%, and 67% respectively. It was reported that the high irreversible capacity loss can be attributed to the side reactions, such as the formation of solid electrolyte interface, lithium adsorption in the conductive additives, and the irreversible electrochemical decomposition of the electrolyte [51e54]. However, all samples possess an obvious voltage plateau at about 1.35e1.42 V, followed by a short plateau at 1.11 V. This indicated the existence of a continuous phase transition after the reduction of Ti4þ to Ti3þ. During the charging process, another short platform at 0.9e1.1 V plus one major delithiation plateau at 1.41 V were observed. The data confirmed that the two redox processes are highly reversible, which is also consistent with the CV in Fig. 7. To further demonstrate the effect of coating, Fig. 9 depicted the charge-discharge profiles for different samples at 1st, 10th, 30th, 50th, and 100th in a constant current density of 0.05 A g1. As can be seen from the curve, the major plateau at about 1.35e1.42 V for [email protected], [email protected], and [email protected] samples are nearly vanished after 100 cycles. However, this plateau for [email protected] sample still persisted in the 100th cycle. In comparison to the [email protected], [email protected], and [email protected] samples, [email protected] shows a much better cycle stability and higher reversible capacity. Fig. 9(f) provided

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the decay rate of the discharge capacity of the four considered samples. The result again showed that the value for [email protected] sample is around 10%, which is the smallest among all samples. Our experiments indicated that 5 wt % mounts of AlF3 coating would be the best for optimization of the electrochemical performance of SLTO. Thanks to the coating, the suppression of the aggressive byproducts of the electrolyte decomposition and of the HF attack will improve the cyclic stability and cycle life of the material [30]. Furthermore, the median voltages as a function of cycle number were also measured. From Fig. 9(g), it can be seen that the [email protected] sample shows the maximum value. Since the plateau voltage of the electrode material is around 1.35 V, the median voltage of the material is about 1.2 V, which is much larger than the one (~0.8 V) of other sample. According to Fig. 9(gei), the chargedischarge specific energy efficiencies of the four samples were calculated to be 39%, 44%, 71%, and 43% after 100 cycles. As a result, suitable amount of AlF3 coating also improves the energy efficiency during cycling. Testing the cycle performance of batteries under an ultra-high current density is also helpful for exemplifying the stabilizing effect caused by the coating. Fig. 10 showed that [email protected], [email protected], and [email protected] samples have a specific capacity of about 24.1, 43.6, 52.4, 41.2 mA h g1 after 2010 cycles at the current density of 5.24 A g1, respectively, And the battery reversible capacity retention rates after 2010 cycles are about 33.5%, 42.1%,

Fig. 9. Charge-discharge curves for the [email protected], [email protected], [email protected], and [email protected] samples at (a) 1st, (b)10th, (c)30th, (d)50th, and (e)100th at a constant current density of 0.05 A g1. And (f) the decay rate of discharge capacity for different samples. (g) Median voltage, specific energy of (h) delithiation and (i) lithiation process at 0.05 A g1 for thedifferent samples.

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Fig. 10. Cyclic performance over 2010 times at the current density of 5.24 A g1 for (a) the [email protected], (b) [email protected], (c) [email protected], and (d) [email protected] samples.

45.2%, 40.4%. The electrochemical test show the [email protected], [email protected], and [email protected] samples exhibit the good cycle life. It should be noted that when the cut-off voltage was raised from 0.0 to 1.0 V, the side reactions can be relieved and the electrochemical reactions will become more stable. In this context, it can be expected that the cycling performance will be more stable. To investigate the dynamic behavior of different samples during the electrochemical reaction, the CV curves at different scanning rates were analyzed. As shown in Fig. 11, two reduction peaks at about 1.35 V and 1.15 V and two oxidation peaks at about 1.13 V and 1.45 V were observed. Relying on Eq. (3),

logðiÞ ¼ b  logðyÞ þ logðaÞ

(3)

where a, b are controllable parameter, I the peak current, and y the scanning rate, a linear relationship between log (i) and log (y) was obtained. It was known that when b is about 0.5, the reaction kinetics is controlled by the electrochemical reaction. When b equals 1, the ion diffusion kinetics is controlled by the tantalum capacitance. According to our calculations, b values for [email protected], [email protected], [email protected], and [email protected] samples were calculated to be 0.401, 0.533, 0.477, 0.472, which was shown in Fig. 12(a), indicating a similar mechanism for all samples. These results implied that the ion diffusion in the electrode material are mainly determined by the electrochemical reaction rather than the pseudo capacity process [55e57]. To determine the impedance and lithium ion diffusion coefficient, Electrochemical Impedance Spectroscopy (EIS) tests were performed for all samples, as shown in Fig. 12 (b). Furthermore, Zre as a function of u0.5 at the low frequency region was obtained and depicted in Fig. 12(c). The Nyquist plots of the four samples are all composed of a concave semicircle from the high frequency to the intermediate frequency region and a straight line in the low

frequency region. The high frequency intercept on the real axis corresponds to the Ohmic resistance (Rs.). The intercept in the intermediate frequency region represents the charge transfer resistance (Rct.), which is related to the charge transfer resistance of the material/electrolyte interface. The equivalent circuit used to fit the curve is shown in the inset of Fig. 12(b). Based on the experimental data, the kinetic parameters were obtained and listed in Table S2. It is obvious that [email protected] has the smallest charge transfer resistance, According to the imitating results of Zivew Software, the charge transfer resistance (Rct.) of [email protected], [email protected], [email protected], and [email protected] are 137.10U, 104.70U, 84.34U, 107.10U. The lowest Rct of [email protected] suggested that it has the best diffusion kinetics. In addition, the lithium ion diffusion coefficient was also estimated by Ref. [27],

Zre ¼ Rct þ Rs þ su1=2

DLi þ ¼

R 2 T2 2A nF4 c2 s2 2

(4)

(5)

where s is the Warburg factor, R the gas constant, T the absolute temperature, A the surface area of the electrode, n the number of electrons transferred in the half-reaction of the redox pair, F the Faraday constant, and C Liþ the concentration of lithium ions in the solid. As shown in Fig. 12(c), the relationship between Zre and u0.5 is a straight line, and the slope represents Warburg factor (s). According to Eq. (5), the lithium ion diffusion coefficient were also calculated and listed in Table S2. The value for [email protected] sample is 3.11  1017 cm2 s1, which the largest one among all samples. Our data confirmed that the Rs and Rct of the [email protected] samples are reduced by 9.91% and 38.48% respectively in comparison to the un-

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Fig. 11. Cyclic voltammetry curves for (a) [email protected], (b) [email protected], (c) [email protected], and (d) [email protected] samples.

Fig. 12. (a) log(i)/log(y) plots and (b) EIS patterns of the [email protected], [email protected], [email protected], and [email protected] samples in a frequency range of 102-105 Hz with an amplitude of 5 mV and (c) Zre as a function of u0.5 at low frequency region for different samples.

coated sample. Therefore, our experiment showed that suitable amount of AlF3 coating is helpful for improving the structural stability and reducing the impedance of the materials, being responsible for the enhancement of the specific capacity, rate capability, and cyclic performance. 4. Conclusion SrLi2Ti6O14 was successfully prepared by a simple sol-gel method, and then different amount of AlF3 was coated on the surface of the as-prepared samples. Relying on the structural characterization technique and electrochemical tests, it was confirmed that the electrochemical performances of the anode materials are changed significantly depending on the thickness and

uniformity of the coating layer. For pure SLTO sample, the reversible capacity is measured to be ~193.3 at 0.05 A g1, and the values increase to 246.1, 280.9, and 223.7 mA h g1 after coating. Further analysis indicated that the cyclic stability and impedance of the battery was also obviously improved along with the extension of the charge/discharge plateau. The ultra-thin protecting layer against fluoride corrosion and its good ionic conductivity are responsible for the improved electrochemical performance of the anode materials.

Declaration of competing interest The authors declare that they have no conflict of interest.

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