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Room temperature surface-engineering enabling stability of high-energy-density lithium batteries
Materials Today Energy 17 (2020) 100415
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Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/
Room temperature surface-engineering enabling stability of highenergy-density lithium batteries C. Fang a, b, f, Y. Wang b, e, f, W. Liu b, R. Guo b, G. Dang c, e, Y. Zhang a, H. Pei b, Y. Li b, S.S. Mao d, S. Yu a, J. Xie b, * a
College of Environment and Chemistry Engineering, Yanshan University, Qinhuangdao 066004, China State Key Laboratory of Space Power-Sources Technology, Shanghai Institute of Space Power-Sources, Shanghai 200245, China Shanghai Institute of Technology, Shanghai 201620, China d Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA e Department of Chemical Engineering, Shanghai Electrochemical Energy Device Research Center (SEED), Shanghai Jiao Tong University, Shanghai 200240, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2019 Received in revised form 4 April 2020 Accepted 8 April 2020 Available online xxx
Lithium (Li) ion batteries have witnessed a great success in areas ranging from portable electronics to electrical vehicles. Among various materials used for battery anodes, Li metal has been considered as the “Holy Grail” due to the highest theoretical capacity and lowest electrochemical potential. However, unmodified Li metal anode suffers from uncontrolled dendrite growth as well as inherent ultrahigh reactivity. Here we report a facile and universal approach to engineering Li anode by passivating anode surface through reactions with CFx, which yields a uniform porous LiF layer on the surface of Li metal. Retaining a high theoretical specific capacity of 3680 mAh/g, a marked extension of the cycle life for Li anode after the formation of a LiF surface layer was achieved, and characterization of Li/Li4Ti5O12 (LTO) cells with surface-engineered Li anode showed capacity retention of 94.79% after 800 cycles at 2 C, which is much higher than those in the cells with pure Li anode (62.07%). Further, enhanced stability and suppressed overpotential augment after 300 cycles were realized in symmetric cells of surfaceengineered Li metal. In addition, Li/S cells with surface-engineered Li anodes also exhibit significantly improved initial columbic efficiency, cyclability, and specific capacity simultaneously. These results suggest that an engineered LiF surface layer would enable an ideal Li metal anode for high-energydensity batteries of the future. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Lithium anode Surface engineering Facile method Excellent stability
1. Introduction A growing number of applications was required based on the lithium batteries for portable devices. Owning the highest theoretical capacity (3860 mAh g1, or 2061 mAh cm3) and the lowest electrochemical potential (3.04 V versus the standard hydrogen electrode), lithium metal attracted much attention among all possible candidates [1e4]. Further, Li anode was the indispensable part for Li-S and Li-air systems, both of which were considered as the most potential systems for next-generation high energy storage devices [5e9].
* Corresponding author. E-mail address: [email protected] (J. Xie). f This author contributes equally. https://doi.org/10.1016/j.mtener.2020.100415 2468-6069/© 2020 Elsevier Ltd. All rights reserved.
However, suffering from dendrite, which rooted in the intrinsic properties of Li metal, has limited the broad application for battery systems. On the other hand, the highly reactive property of Li metal triggered spontaneously reaction with the organic electrolyte and formed a solid-electrolyte interphase (SEI) layer. During the cycling, the SEI layer will break and reform with the formation of Li dendrite, which consumed tremendous amount of Li and the electrolyte, and finally caused the failure of the batteries [10e14]. To solve these problems, there has been a tremendous amount of efforts aimed to optimize Li anode, with the following three typical methods. The first is to change the morphology of Li, such as with a 3D host or using a Li powder anode [15e18]. The second method is to form better SEI by additives or a self-healing electrostatic shield [19e22], using, e.g., N2 [23]. Recently, a thin (~50 nm), uniform Li3PO4 artificial SEI on Li was demonstrated,
2
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which exhibited improved Li-ion conductivity and high Young's modulus, and a smooth interface without obvious dendrites after 200 cycles [24]. The third approach is to stabilize Li anode by interface structuring [23e27]. For example, 3D electrodes could decrease the effective current density by increasing specific area. A conformal layer consisting of LiF coating and Li-rGO was recently introduced for constructing a 3D electrode [18], in which protrusions would enable 3D Li-ion diffusion, with faster Li deposition on the tips rather than 1D diffusion for conventional flat surfaces. This method improved cycling stability, and with little overpotential augment for over 200 cycles in symmetric cells. LiF was always considered as an important role because of its highly electrochemical stability, negligible solubility in electrolyte and adjustable surface tension [28,29]. Trace amount of HF or fluorinated compounds was proved to form LiF in SEI in previous work, which would be able to stabilize the SEI during cycling [30]. However, dispersive LiF layers introduced by the methods mentioned above would easily fractured. What's more, HF and other fluorinated compounds owned hazardous properties, which are not easily to be realized in industrial scale [31,32]. Proper structure of uniform and porous coating would be necessary to enable Li metal anode for better cyclic stability. A uniform coating can also provide a better-proportioned current distribution at Li anode, which could eliminate dendrite inherently. On the other hand, porous structure can ensure smooth Li ion diffusion. It is therefore desirable to form a uniform, porous LiF layer on Li anode, without the need of high-cost processes or hazardous reagents. Here we demonstrate a facile approach to engineering a uniform, porous LiF layer on Li metal anodes based on the reaction, CFx þ xLi / xLiF þ C. The whole preparation process is illustrated in Scheme 1. The surface-engineered Li anodes obtained after Li reaction with CFx for 1 h were labeled as 1 h CFx-Li. Considering the excellent cycle stability of LTO electrode, the lifespan of surface-engineered Li anodes was evaluated in Li/LTO batteries. The effects of surface-engineered layer on the electrochemical performances of Li/LTO, Li/Li and Li/S batteries will be discussed (on the one hand, considering that LTO was marked as one of the most stable materials and always set as the evaluation method for lithium metal, on the other hand, the Li/S battery was examined for the applicability verification for high-energy-density systems).
2. Results and discussion 2.1. Morphology and physical properties Fig. 1 (a, b) and (c, d) show the top surface of a bare-Li foil and an 1hCFx-Li foil, respectively. For comparison, bare-Li foil was dipped in the electrolyte for 1 h simultaneously. Particle products causing from the reaction between the Li foil and electrolyte can be obviously observed in Fig. 1 (a, b). It is noted that the surface of 1hCFx-Li foil exhibits dense and porous morphology that like a sponge structure, and magnification indicates that the surface after reaction has a flake/plate shape. Differently, the surface of bare-Li foil is nonuniform with irregular agglomerated clusters. The results indicate that the reaction between CFx and Li changed the surface structure. And the surface of the reacted CFx electrode (Fig. S1) was consisted of a large number of little particles. Dense and porous morphology would be in favor of Li deposition/stripping and inhibited the growth of Li dendrites. Cross-section morphologies of the surface-engineered Li metal anode were captured and shown in Figs. 2(a, b) and S2. We prepared bare-Li immersing in electrolyte for 1 h, which were parallel to that of 1hCFx-Li. It was found that the 1hCFx-Li has a thicker modified layer (Fig. 2(a)), about 100 mm. As we can see in the EDX images, oxygen, carbon and fluorine elements were found on the 1hCFx-Li. And the fluorine and carbon elements only existed in the modified layer of 1hCFx-Li, which indicated that the modified layer may consist of the compounds of fluoride and carbide. Meanwhile, the cross section of the bare-Li (Fig. S2 (a, b)) only has oxygen element, it had no obvious SEI layer. Depth analysis based on X-ray photoelectron spectroscopy (XPS) further demonstrated the uniform nature of the modified layer (Fig. 2(c, d); Fig. S3). As shown in Fig. 2(c), the atomic ratio of F dropped with the Li increased gradually when etching, which indicated the gradient contribution of fluoride. Fig. 2(d) shows the peak at around 685 eV in the F 1s spectra consisting of two peaks, the peak at around 685.9 eV can be attributed to LiF, while the peak at 685.0 eV is most likely the reduction product of LiPF6, e.g., LixPF6-x [33]. The dropping intensity of F 1s indicated a thickness decrease of LiF layer and it was also confirmed by Li 1s spectra, as shown in Fig. S4. These results indicate that the reaction between Li and CFx introduced a LiF layer on the Li anode.
Scheme 1. Illustration of the preparation of 1hCFx-Li foil.
C. Fang et al. / Materials Today Energy 17 (2020) 100415
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Fig. 1. Scanning electron microscopy (SEM) images of the surface of Li electrode, (a) bare Li metal after dipping in the electrolyte for 1 h, (b) magnified image of (a), (c) 1hCFx-Li, (d) magnified image of (c).
Fig. 2. Characterizations of the modified layer on Li foils. (a) Cross-section of the 1hCFx-Li, (b) EDX images of 1hCFx-Li. Color mappings of red, green and blue denote oxygen, carbon and fluorine elements, (c, d) XPS depth profiles of (c) the change rate of elements content with etching time and (d) F 1s.
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2.2. Electrochemical performance The enhancement of the LiF layer of Li anode was examined in the Li-LTO cells. Fig. 3(a) showed the capacity retention of Li-LTO cells at a constant charge/discharge rate of 2 C, where the CFx-Li electrodes consistently performed a superior stability comparing to the blank ones. The 1hCFx-Li/LTO had the highest capacity retention 94.79%, with only 0.006% capacity decrease at every cycle, while the capacity retention of bare-Li/LTO was 62.07% after 800 cycles. This may attribute to the highly improved interfacial stability of LiF layer and can essentially eliminate reactions with electrolyte. The voltage profiles at a rate of 2 C are further shown in Fig. 3(b and c), the 1hCFx-Li/LTO cells had a smaller polarization compared to bare-Li/ LTO. To further understand the electrochemical properties, impedance spectra and detailed fitting results of the Li/LTO cells are presented in Figs. 3(d, e) and S5 and Table S1. It can be observed that the 1hCFx-Li/LTO cells exhibit a smaller interfacial resistances and smaller change of resistances, which indicated the LiF layer has a favorable ionic conductivity. The interfacial resistances increased with cycling, which may be due to the that fact that SEI layer became thick and “dead Li” was formed with the discharge/charge
process. However, the interfacial resistances of 1hCFx-Li/LTO cells showed only small variation, indicating that the LiF layer was stable during cycling. In order to prove the capacity decay of Li/LTO cells were caused by the deterioration of Li anode rather than LTO, the bare-Li/LTO cell cycled 1000 cycles was disassembled and reassembled with a fresh Li anode. It was found that the capacity recovered to the initial state (Fig. S6). Symmetric cells were assembled to probe the performance in eater electrolyte systems, whereas the current density and the capacity were settled at 0.5 mA cm2 and 1 mAh cm2, respectively. As shown in Fig. 4, 1hCFx-Li exhibited a more stable deposition/ stripping behavior after 100 cycles comparing to the bare Li anode. Minor increases of the polarization can be observed on surfaceengineered Li anode, which may be benefited from the fragile nature of LiF films during cycling. And this provided a possible support for a more stable Li battery system such as Li-sulfur batteries. Hence, the optimization effect is further examined in the Li/S cells, which are widely considered as a promising candidate for the nextgeneration high energy storage devices. As is shown in Fig. S7 (a), the cycling of Li/S cells at a rate of 0.05 C for 10 cycles to 0.2 C for 60 cycles. The 1hCFx-Li anode exhibited much better cycling stability and higher specific capacity compared
Fig. 3. Cycling characteristics of Li/LTO cells at a high rate of 2 C. (a) The capacity retention of yhCFx-Li/LTO cells, (b) Voltage profiles of bare-Li/LTO cells, (c) Voltage profiles of 1hCFxLi/LTO cells, EIS spectra of (d) bare-Li/LTO cells and (e) 1hCFx-Li/LTO cells after different cycles.
C. Fang et al. / Materials Today Energy 17 (2020) 100415
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Fig. 4. (a) Symmetric-cell cycling curves of bare-Li (black curves) and 1hCFx-Li (red curves); (bed) the 1st, 100th and 300th cycle of symmetric-cells.
to the blank. The discharge/charge curves of cells with surfaceengineered Li and bare Li are shown in Fig. S7(b), respectively. Although at the first cycle the bare-Li/S cell has a high specific capacity, after 60 cycles the 1hCFx-Li/S cell has a higher specific capacity. The results may also imply the lighter corrosion of Li metal anode and minimal parasitic side reactions on cathode. 2.3. Morphology investigation after cycles The morphologies of Li metal anode after 1000 cycles (Li/LTO cells) were collected in Fig. 5. As shown in Fig. 5 (a-c), bare-Li showed a cracked interface consisting of pin-liked Li, which was considered as unfavorable for cyclic stability [34]. Correspondingly, the surface-engineered Li anode kept a stable and continuous coating layer, as shown in Fig. 5 (d-f), porous structure was retained even after 1000 cycles at a high rate of 2 C. This engineered surface layer can suppress the electrolyte erosion of lithium anode, thus provide a considerably electrochemical stable Li metal anode, which may be a key answer for the excellent performance listing in this work.
Overall, a facile and controllable method for engineering one superior LiF coating layer upon Li metal was achieved at room temperature for the first time via the reaction between CFx and Li. The resulting cycling stability of Li/LTO cell was significant improved, the capacity retention of 1hCFx-Li/LTO cell exhibited 94.79%, much higher than that of bare-Li/LTO (62.07%) ata high rate of 2 C. Additionally, the resistance of the 1hCFx-Li/LTO cell is smaller than the bare-Li/LTO after 800 cycles. Symmetric cells also proved that the engineered LiF layer can improve electrochemical stability, with small polarization. Moreover, Li/S cells also confirmed the enhanced cyclability. Nontoxic CFx ensures not only controllable reactivity, but also better permeability than solid/ liquid precursors, thus enabling more uniform LiF coating on Li metal. Highly improved interfacial stability and suppressed side reactions were achieved via the LiF coating engineering. Hence, we believe that this surface-engineering approach would promote strategies on Li metal surface modification and provide exciting possibilities for high-energy battery systems based on stable Li metal anode.
Fig. 5. SEM images for the surface of lithium electrode after 1000 cycles. (aec) bare-Li, (def) 1hCFx-Li.
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3. Experimental procedures
References
The bare Li foil was first covered with CFx electrode and reacted. Then, the CFx electrode was peeled off after y h. The modified Li foil was obtained and marked as yh-CFx-Li. Spinel Li4Ti5O12 (abbreviated as LTO) material, which is called ‘zero strain’ material, was selected for the evaluation of the stability difference of the yhCFx-Li and bare Li. The LTO/Li cells were assembled using one same batch Li4Ti5O12 electrode as the cathode and Celgard 2325 as separator. All electrochemical measurements were performed using 2016 coin-type cells. The electrolyte consisted of a solution of 1.20 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/ethylmethyl carbonate (3:7 by volume). The LTO material loading was around 3 mg cm2 for all samples. In this work, the specific current density of LTO at a rate of 1 C is 175 mA g1. Li/S cells were assembled using 60% S/C materials as the cathode and Celgard 2325 as separator. The electrolyte consisted of a solution of 1.0 M Bis(trifluoromethane) sulfonimide lithium (LiTFSI) in ethylene glycol monomethylether/methyl ether (1:1 by volume). The S/C material loading was around 2.5 mg cm2 for all samples. In this work, the specific current density of S/C at a rate of 1 C is 1 A g1. The symmetric-cells were assembled with lithium anode, celgard 2325 as separator in the 1.0 M Bis(trifluoromethane) sulfonimide lithium (LiTFSI) in ethylene glycol monomethylether/methyl ether (1:1 by volume) as the electrolyte. The particle morphology was examined by scanning electronic microscopy (SEM, Hitachi S- 4800) and Transmission electron microscope (TEM, FEI Tecnai G2 F20). X-ray photoelectron spectroscopy (XPS) was tested in a Perkin Elmer PHI 550 multi-technique spectrometer with the electron gun Perkin Elmer PHI 25-270AR precision electron analyzer. Data acquisition was achieved in the constant energy analyzer mode with a pass energy of △E ¼ 50 eV and Al Ka radiation from a monochromatic aluminum X-ray source. The binding energy calibration was calculated on the carbon C1s peak with the energy of 284.6 eV. Galvanostatic charge-discharge tests were performed using a LAND CT2001A battery test system.
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Maruta, et al., Arabidopsis NAC transcription factor, ANAC078, regulates flavonoid biosynthesis under high-light, Nano Lett. 14 (2014) 6016. [14] J.J. Woo, V.A. Maroni, L. Gao, et al., Symmetrical impedance study on inactivation induced degradation of lithium electrodes for batteries beyond lithium-ion, J. Electrochem. Soc. 161 (2014) A827eA830. [15] D. Lv, Y. Shao, T. Lozano, et al., Failure mechanism for fast-charged lithium metal batteries with liquid electrolytes, J. Adv. Mater. 5 (2015) 1400993. [16] H. Xiang, P. Shi, P. Bhattacharya, et al., Enhanced charging capability of lithium metal batteries based on lithium bis(trifluoromethanesulfonyl)imide-lithium bis(oxalato)borate dual-salt electrolytes, J. Power Sources 318 (2016) 170e177. [17] D. Lin, Y. Liu, W. Chen, et al., Conformal lithium fluoride protection layer on three-dimensional lithium by nonhazardous gaseous reagent freon, Nano Lett. 17 (2017) 3731e3737. [18] X.B. Cheng, T.Z. Hou, R. Zhang, et al., Dendrite-Free lithium deposition induced by uniformly distributed lithium-ions for efficient lithium metal batteries, J. Adv. Mater. 28 (2016) 2888. [19] W. Li, H. Yao, K. Yan, et al., The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth, Nat. Commun. 6 (2015) 7436. [20] Y. Han, X. Duan, Y. Li, et al., Improved cycling performances with high sulfur loading enabled by pre-treating lithium anode, J. Ionics 22 (2016) 151e157. [21] F. Ding, W. Xu, G.L. Graff, et al., Dendrite-Free lithium deposition via selfhealing electrostatic shield mechanism, JACS 135 (2013) 4450. [22] Y. Zhang, J. Qian, W. Xu, et al., Dendrite-free lithium deposition with selfaligned nanorod structure, Nano Lett. 14 (2014) 6889e6896. [23] G. Zheng, S.W. Lee, Z. Liang, et al., Interconnected hollow carbon nanospheres for stable lithium metal anodes, Nat. Nanotechnol. 9 (2014) 618. [24] M. Wu, Z. Wen, Y. Liu, et al., Electrochemical behaviors of a Li 3 N modified Lithium anode in secondary lithium batteries, J. Journal of Power Sources 196 (2011) 8091e8097. [25] A.C. Kozen, C.F. Lin, O. Zhao, et al., Stabilization of lithium metal anodes by hybrid artificial solid electrolyte interphase, Chem. Mater. 29 (2017) 6298e6307. [26] A.C. Kozen, C.F. Lin, A.J. Pearse, et al., Next-generation lithium metal anode engineering via atomic layer deposition, ACS Nano 9 (2015) 5884. [27] Y. Cao, X. Meng, J.W. Elam, Atomic layer deposition of LixAlyS solid-state electrolytes for stabilizing lithium-metal anodes, Chemelectrochem 3 (2016) 858e863. [28] M.D. Tikekar, S. Choudhury, Z. Tu, et al., Design principles for electrolytes and interfaces for stable lithium-metal batteries, Nat. Energy 1 (2016) 16114. [29] Y. Zhu, X. He, Y. Mo, Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first principles calculations, ACS Appl. Mater. 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Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgement This work was supported by the Shanghai Sailing Program (Grant No. 18YF1417000), Shanghai Science and Technology Commission Project (Grant No. 18DZ2284000) and Natural Science Foundation of Hebei Province (Grant No. 2015203406). Author contribution The design of the work was contributed by Yong Wang. The article drafting, data analysis and interpretation were completed by Congcong Fang, Yong Wang, Samuel S. Mao and Jingying Xie. Data collection was achieved by Guoju Dang, Yali Zhang, Haijuan Pei and Yong Li. Revision of the article was done by Rui Guo, Wen Liu and Shengxue Yu. All authors have given approval to the final version of the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2020.100415.
C. Fang et al. / Materials Today Energy 17 (2020) 100415 [31] S. Shiraishi, K. Kanamura, Z. Takehara, Surface condition changes in lithium metal deposited in nonaqueous electrolyte containing HF by dissolutiondeposition cycles, J. Electrochem. Soc. 146 (1999) 1633e1639. [32] R. Miao, J. Yang, Z. Xu, et al., A new ether-based electrolyte for dendrite-free lithium-metal based rechargeable batteries, Sci. Rep. 6 (2016) 21771.
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[33] Y. Zhang, J. Qian, W. Xu, et al., Dendrite-free lithium deposition with selfaligned nanorod structure, J. Nano Lett. 14 (2014) 6889e6896. [34] X. Li, J. Zheng, X. Ren, et al., Dendrite-Free and performance-enhanced lithium metal batteries through optimizing solvent compositions and adding combinational additives, J. Adv. Energy Mater. (2018) 1703022.
Contents lists available at ScienceDirect
Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/
Room temperature surface-engineering enabling stability of highenergy-density lithium batteries C. Fang a, b, f, Y. Wang b, e, f, W. Liu b, R. Guo b, G. Dang c, e, Y. Zhang a, H. Pei b, Y. Li b, S.S. Mao d, S. Yu a, J. Xie b, * a
College of Environment and Chemistry Engineering, Yanshan University, Qinhuangdao 066004, China State Key Laboratory of Space Power-Sources Technology, Shanghai Institute of Space Power-Sources, Shanghai 200245, China Shanghai Institute of Technology, Shanghai 201620, China d Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA e Department of Chemical Engineering, Shanghai Electrochemical Energy Device Research Center (SEED), Shanghai Jiao Tong University, Shanghai 200240, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2019 Received in revised form 4 April 2020 Accepted 8 April 2020 Available online xxx
Lithium (Li) ion batteries have witnessed a great success in areas ranging from portable electronics to electrical vehicles. Among various materials used for battery anodes, Li metal has been considered as the “Holy Grail” due to the highest theoretical capacity and lowest electrochemical potential. However, unmodified Li metal anode suffers from uncontrolled dendrite growth as well as inherent ultrahigh reactivity. Here we report a facile and universal approach to engineering Li anode by passivating anode surface through reactions with CFx, which yields a uniform porous LiF layer on the surface of Li metal. Retaining a high theoretical specific capacity of 3680 mAh/g, a marked extension of the cycle life for Li anode after the formation of a LiF surface layer was achieved, and characterization of Li/Li4Ti5O12 (LTO) cells with surface-engineered Li anode showed capacity retention of 94.79% after 800 cycles at 2 C, which is much higher than those in the cells with pure Li anode (62.07%). Further, enhanced stability and suppressed overpotential augment after 300 cycles were realized in symmetric cells of surfaceengineered Li metal. In addition, Li/S cells with surface-engineered Li anodes also exhibit significantly improved initial columbic efficiency, cyclability, and specific capacity simultaneously. These results suggest that an engineered LiF surface layer would enable an ideal Li metal anode for high-energydensity batteries of the future. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Lithium anode Surface engineering Facile method Excellent stability
1. Introduction A growing number of applications was required based on the lithium batteries for portable devices. Owning the highest theoretical capacity (3860 mAh g1, or 2061 mAh cm3) and the lowest electrochemical potential (3.04 V versus the standard hydrogen electrode), lithium metal attracted much attention among all possible candidates [1e4]. Further, Li anode was the indispensable part for Li-S and Li-air systems, both of which were considered as the most potential systems for next-generation high energy storage devices [5e9].
* Corresponding author. E-mail address: [email protected] (J. Xie). f This author contributes equally. https://doi.org/10.1016/j.mtener.2020.100415 2468-6069/© 2020 Elsevier Ltd. All rights reserved.
However, suffering from dendrite, which rooted in the intrinsic properties of Li metal, has limited the broad application for battery systems. On the other hand, the highly reactive property of Li metal triggered spontaneously reaction with the organic electrolyte and formed a solid-electrolyte interphase (SEI) layer. During the cycling, the SEI layer will break and reform with the formation of Li dendrite, which consumed tremendous amount of Li and the electrolyte, and finally caused the failure of the batteries [10e14]. To solve these problems, there has been a tremendous amount of efforts aimed to optimize Li anode, with the following three typical methods. The first is to change the morphology of Li, such as with a 3D host or using a Li powder anode [15e18]. The second method is to form better SEI by additives or a self-healing electrostatic shield [19e22], using, e.g., N2 [23]. Recently, a thin (~50 nm), uniform Li3PO4 artificial SEI on Li was demonstrated,
2
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which exhibited improved Li-ion conductivity and high Young's modulus, and a smooth interface without obvious dendrites after 200 cycles [24]. The third approach is to stabilize Li anode by interface structuring [23e27]. For example, 3D electrodes could decrease the effective current density by increasing specific area. A conformal layer consisting of LiF coating and Li-rGO was recently introduced for constructing a 3D electrode [18], in which protrusions would enable 3D Li-ion diffusion, with faster Li deposition on the tips rather than 1D diffusion for conventional flat surfaces. This method improved cycling stability, and with little overpotential augment for over 200 cycles in symmetric cells. LiF was always considered as an important role because of its highly electrochemical stability, negligible solubility in electrolyte and adjustable surface tension [28,29]. Trace amount of HF or fluorinated compounds was proved to form LiF in SEI in previous work, which would be able to stabilize the SEI during cycling [30]. However, dispersive LiF layers introduced by the methods mentioned above would easily fractured. What's more, HF and other fluorinated compounds owned hazardous properties, which are not easily to be realized in industrial scale [31,32]. Proper structure of uniform and porous coating would be necessary to enable Li metal anode for better cyclic stability. A uniform coating can also provide a better-proportioned current distribution at Li anode, which could eliminate dendrite inherently. On the other hand, porous structure can ensure smooth Li ion diffusion. It is therefore desirable to form a uniform, porous LiF layer on Li anode, without the need of high-cost processes or hazardous reagents. Here we demonstrate a facile approach to engineering a uniform, porous LiF layer on Li metal anodes based on the reaction, CFx þ xLi / xLiF þ C. The whole preparation process is illustrated in Scheme 1. The surface-engineered Li anodes obtained after Li reaction with CFx for 1 h were labeled as 1 h CFx-Li. Considering the excellent cycle stability of LTO electrode, the lifespan of surface-engineered Li anodes was evaluated in Li/LTO batteries. The effects of surface-engineered layer on the electrochemical performances of Li/LTO, Li/Li and Li/S batteries will be discussed (on the one hand, considering that LTO was marked as one of the most stable materials and always set as the evaluation method for lithium metal, on the other hand, the Li/S battery was examined for the applicability verification for high-energy-density systems).
2. Results and discussion 2.1. Morphology and physical properties Fig. 1 (a, b) and (c, d) show the top surface of a bare-Li foil and an 1hCFx-Li foil, respectively. For comparison, bare-Li foil was dipped in the electrolyte for 1 h simultaneously. Particle products causing from the reaction between the Li foil and electrolyte can be obviously observed in Fig. 1 (a, b). It is noted that the surface of 1hCFx-Li foil exhibits dense and porous morphology that like a sponge structure, and magnification indicates that the surface after reaction has a flake/plate shape. Differently, the surface of bare-Li foil is nonuniform with irregular agglomerated clusters. The results indicate that the reaction between CFx and Li changed the surface structure. And the surface of the reacted CFx electrode (Fig. S1) was consisted of a large number of little particles. Dense and porous morphology would be in favor of Li deposition/stripping and inhibited the growth of Li dendrites. Cross-section morphologies of the surface-engineered Li metal anode were captured and shown in Figs. 2(a, b) and S2. We prepared bare-Li immersing in electrolyte for 1 h, which were parallel to that of 1hCFx-Li. It was found that the 1hCFx-Li has a thicker modified layer (Fig. 2(a)), about 100 mm. As we can see in the EDX images, oxygen, carbon and fluorine elements were found on the 1hCFx-Li. And the fluorine and carbon elements only existed in the modified layer of 1hCFx-Li, which indicated that the modified layer may consist of the compounds of fluoride and carbide. Meanwhile, the cross section of the bare-Li (Fig. S2 (a, b)) only has oxygen element, it had no obvious SEI layer. Depth analysis based on X-ray photoelectron spectroscopy (XPS) further demonstrated the uniform nature of the modified layer (Fig. 2(c, d); Fig. S3). As shown in Fig. 2(c), the atomic ratio of F dropped with the Li increased gradually when etching, which indicated the gradient contribution of fluoride. Fig. 2(d) shows the peak at around 685 eV in the F 1s spectra consisting of two peaks, the peak at around 685.9 eV can be attributed to LiF, while the peak at 685.0 eV is most likely the reduction product of LiPF6, e.g., LixPF6-x [33]. The dropping intensity of F 1s indicated a thickness decrease of LiF layer and it was also confirmed by Li 1s spectra, as shown in Fig. S4. These results indicate that the reaction between Li and CFx introduced a LiF layer on the Li anode.
Scheme 1. Illustration of the preparation of 1hCFx-Li foil.
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Fig. 1. Scanning electron microscopy (SEM) images of the surface of Li electrode, (a) bare Li metal after dipping in the electrolyte for 1 h, (b) magnified image of (a), (c) 1hCFx-Li, (d) magnified image of (c).
Fig. 2. Characterizations of the modified layer on Li foils. (a) Cross-section of the 1hCFx-Li, (b) EDX images of 1hCFx-Li. Color mappings of red, green and blue denote oxygen, carbon and fluorine elements, (c, d) XPS depth profiles of (c) the change rate of elements content with etching time and (d) F 1s.
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2.2. Electrochemical performance The enhancement of the LiF layer of Li anode was examined in the Li-LTO cells. Fig. 3(a) showed the capacity retention of Li-LTO cells at a constant charge/discharge rate of 2 C, where the CFx-Li electrodes consistently performed a superior stability comparing to the blank ones. The 1hCFx-Li/LTO had the highest capacity retention 94.79%, with only 0.006% capacity decrease at every cycle, while the capacity retention of bare-Li/LTO was 62.07% after 800 cycles. This may attribute to the highly improved interfacial stability of LiF layer and can essentially eliminate reactions with electrolyte. The voltage profiles at a rate of 2 C are further shown in Fig. 3(b and c), the 1hCFx-Li/LTO cells had a smaller polarization compared to bare-Li/ LTO. To further understand the electrochemical properties, impedance spectra and detailed fitting results of the Li/LTO cells are presented in Figs. 3(d, e) and S5 and Table S1. It can be observed that the 1hCFx-Li/LTO cells exhibit a smaller interfacial resistances and smaller change of resistances, which indicated the LiF layer has a favorable ionic conductivity. The interfacial resistances increased with cycling, which may be due to the that fact that SEI layer became thick and “dead Li” was formed with the discharge/charge
process. However, the interfacial resistances of 1hCFx-Li/LTO cells showed only small variation, indicating that the LiF layer was stable during cycling. In order to prove the capacity decay of Li/LTO cells were caused by the deterioration of Li anode rather than LTO, the bare-Li/LTO cell cycled 1000 cycles was disassembled and reassembled with a fresh Li anode. It was found that the capacity recovered to the initial state (Fig. S6). Symmetric cells were assembled to probe the performance in eater electrolyte systems, whereas the current density and the capacity were settled at 0.5 mA cm2 and 1 mAh cm2, respectively. As shown in Fig. 4, 1hCFx-Li exhibited a more stable deposition/ stripping behavior after 100 cycles comparing to the bare Li anode. Minor increases of the polarization can be observed on surfaceengineered Li anode, which may be benefited from the fragile nature of LiF films during cycling. And this provided a possible support for a more stable Li battery system such as Li-sulfur batteries. Hence, the optimization effect is further examined in the Li/S cells, which are widely considered as a promising candidate for the nextgeneration high energy storage devices. As is shown in Fig. S7 (a), the cycling of Li/S cells at a rate of 0.05 C for 10 cycles to 0.2 C for 60 cycles. The 1hCFx-Li anode exhibited much better cycling stability and higher specific capacity compared
Fig. 3. Cycling characteristics of Li/LTO cells at a high rate of 2 C. (a) The capacity retention of yhCFx-Li/LTO cells, (b) Voltage profiles of bare-Li/LTO cells, (c) Voltage profiles of 1hCFxLi/LTO cells, EIS spectra of (d) bare-Li/LTO cells and (e) 1hCFx-Li/LTO cells after different cycles.
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Fig. 4. (a) Symmetric-cell cycling curves of bare-Li (black curves) and 1hCFx-Li (red curves); (bed) the 1st, 100th and 300th cycle of symmetric-cells.
to the blank. The discharge/charge curves of cells with surfaceengineered Li and bare Li are shown in Fig. S7(b), respectively. Although at the first cycle the bare-Li/S cell has a high specific capacity, after 60 cycles the 1hCFx-Li/S cell has a higher specific capacity. The results may also imply the lighter corrosion of Li metal anode and minimal parasitic side reactions on cathode. 2.3. Morphology investigation after cycles The morphologies of Li metal anode after 1000 cycles (Li/LTO cells) were collected in Fig. 5. As shown in Fig. 5 (a-c), bare-Li showed a cracked interface consisting of pin-liked Li, which was considered as unfavorable for cyclic stability [34]. Correspondingly, the surface-engineered Li anode kept a stable and continuous coating layer, as shown in Fig. 5 (d-f), porous structure was retained even after 1000 cycles at a high rate of 2 C. This engineered surface layer can suppress the electrolyte erosion of lithium anode, thus provide a considerably electrochemical stable Li metal anode, which may be a key answer for the excellent performance listing in this work.
Overall, a facile and controllable method for engineering one superior LiF coating layer upon Li metal was achieved at room temperature for the first time via the reaction between CFx and Li. The resulting cycling stability of Li/LTO cell was significant improved, the capacity retention of 1hCFx-Li/LTO cell exhibited 94.79%, much higher than that of bare-Li/LTO (62.07%) ata high rate of 2 C. Additionally, the resistance of the 1hCFx-Li/LTO cell is smaller than the bare-Li/LTO after 800 cycles. Symmetric cells also proved that the engineered LiF layer can improve electrochemical stability, with small polarization. Moreover, Li/S cells also confirmed the enhanced cyclability. Nontoxic CFx ensures not only controllable reactivity, but also better permeability than solid/ liquid precursors, thus enabling more uniform LiF coating on Li metal. Highly improved interfacial stability and suppressed side reactions were achieved via the LiF coating engineering. Hence, we believe that this surface-engineering approach would promote strategies on Li metal surface modification and provide exciting possibilities for high-energy battery systems based on stable Li metal anode.
Fig. 5. SEM images for the surface of lithium electrode after 1000 cycles. (aec) bare-Li, (def) 1hCFx-Li.
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3. Experimental procedures
References
The bare Li foil was first covered with CFx electrode and reacted. Then, the CFx electrode was peeled off after y h. The modified Li foil was obtained and marked as yh-CFx-Li. Spinel Li4Ti5O12 (abbreviated as LTO) material, which is called ‘zero strain’ material, was selected for the evaluation of the stability difference of the yhCFx-Li and bare Li. The LTO/Li cells were assembled using one same batch Li4Ti5O12 electrode as the cathode and Celgard 2325 as separator. All electrochemical measurements were performed using 2016 coin-type cells. The electrolyte consisted of a solution of 1.20 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/ethylmethyl carbonate (3:7 by volume). The LTO material loading was around 3 mg cm2 for all samples. In this work, the specific current density of LTO at a rate of 1 C is 175 mA g1. Li/S cells were assembled using 60% S/C materials as the cathode and Celgard 2325 as separator. The electrolyte consisted of a solution of 1.0 M Bis(trifluoromethane) sulfonimide lithium (LiTFSI) in ethylene glycol monomethylether/methyl ether (1:1 by volume). The S/C material loading was around 2.5 mg cm2 for all samples. In this work, the specific current density of S/C at a rate of 1 C is 1 A g1. The symmetric-cells were assembled with lithium anode, celgard 2325 as separator in the 1.0 M Bis(trifluoromethane) sulfonimide lithium (LiTFSI) in ethylene glycol monomethylether/methyl ether (1:1 by volume) as the electrolyte. The particle morphology was examined by scanning electronic microscopy (SEM, Hitachi S- 4800) and Transmission electron microscope (TEM, FEI Tecnai G2 F20). X-ray photoelectron spectroscopy (XPS) was tested in a Perkin Elmer PHI 550 multi-technique spectrometer with the electron gun Perkin Elmer PHI 25-270AR precision electron analyzer. Data acquisition was achieved in the constant energy analyzer mode with a pass energy of △E ¼ 50 eV and Al Ka radiation from a monochromatic aluminum X-ray source. The binding energy calibration was calculated on the carbon C1s peak with the energy of 284.6 eV. Galvanostatic charge-discharge tests were performed using a LAND CT2001A battery test system.
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Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgement This work was supported by the Shanghai Sailing Program (Grant No. 18YF1417000), Shanghai Science and Technology Commission Project (Grant No. 18DZ2284000) and Natural Science Foundation of Hebei Province (Grant No. 2015203406). Author contribution The design of the work was contributed by Yong Wang. The article drafting, data analysis and interpretation were completed by Congcong Fang, Yong Wang, Samuel S. Mao and Jingying Xie. Data collection was achieved by Guoju Dang, Yali Zhang, Haijuan Pei and Yong Li. Revision of the article was done by Rui Guo, Wen Liu and Shengxue Yu. All authors have given approval to the final version of the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2020.100415.
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