Very Stable Lithium Metal Stripping–Plating at a High Rate and High

May 8, 2017 - Very Stable Lithium Metal Stripping–Plating at a High Rate and High Areal Capacity in Fluoroethylene Carbonate-Based Organic Electroly...
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Very Stable Lithium Metal Stripping−Plating at a High Rate and High Areal Capacity in Fluoroethylene Carbonate-Based Organic Electrolyte Solution Elena Markevich,*,† Gregory Salitra,*,† Frederick Chesneau,‡ Michael Schmidt,‡ and Doron Aurbach*,† †

Department of Chemistry, Bar-Ilan University, Ramat Gan 5290002, Israel BASF SE, Ludwigshafen 67056, Germany

‡

S Supporting Information *

ABSTRACT: We report the highly stable galvanostatic cycling of lithium metal (Li) electrodes in a symmetrical Li|electrolyte solution| Li coin-cell configuration at a high rate and high areal capacity using fluoroethylene carbonate (FEC)-based electrolyte solutions [1 M LiPF6 in FEC/dimethyl carbonate (DMC)]. The FEC-based electrolyte solution shows cycling behavior that is markedly better than that observed for the cells cycled with an ethylene carbonate (EC)-based electrolyte solution (1 M LiPF6 in EC/DMC). With FEC-based electrolyte solution, Li|Li cells can be cycled at 2 mA cm−2 with an areal capacity of 3.3 mAh cm−2 for more than 1100 cycles, and full Li| LiNi0.6Co0.2Mn0.2O2 (NMC) cells with high areal loading cathode demonstrate stable cycling with the same capacity during 90 cycles. An increase in areal capacity up to 6 mA h cm−2 does not affect the shape of the voltage profile of the symmetric Li|Li cells. The reason for this high performance is the formation of a stable and efficient solid electrolyte interphase (SEI) on the surface of the Li metal electrodes cycled in the FEC-based solution. The composition of the SEI is analyzed by Fourier transform infrared spectroscopy. igh theoretical specific capacity (3860 mAh g−1) and low negative redox potential (−3.040 V vs a standard hydrogen electrode) make lithium metal (Li) an ideal anode for high-energy-density Li batteries. It was shown by Peled et al. that solid electrolyte interphase (SEI) is formed immediately on the Li surface once the Li is immersed in a nonaqueous electrolyte and during initial plating−stripping processes.1,2 Dendrite growth and side-electrolyte solution reactions limit the cycling life of the batteries and cause severe safety problems, hindering practical use of these anodes. It has been demonstrated that dendrite growth depends on the cycling conditions, namely, the current density and areal charge−discharge capacity,3−6 cell pressure,7 electrolyte solution composition or electrolyte mix formulation,7−12 and morphology of the Li metal surface.13 Many efforts have been made to stabilize Li metal anodes, including modification of the Li surface by various mechanical, physical, and chemical techniques;13−16 increase of the effective Li-electrode surface by use of an anode matrix with a very large surface area;17−19 the use of solid electrolytes;4−6 the addition of selected cations (such as cesium or rubidium) which protect the Li surface from

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dendrite formation according to the self-healing electrostatic shield mechanism;4,20 and the use of functional electrolyte additives for the in situ formation of a protective SEI.4 In terms of energy density, modification of the electrolyte solution is favorable compared to approaches based on additional integration of hosts and coatings on Li metal electrodes.21 Because Li metal anodes are important for elaborating rechargeable Li batteries with the highest energy density, the choice of the electrolyte solution is of great importance. The electrolyte solution for this type of battery must be compatible with a high-voltage cathode to extend the potential operating range for lithium batteries. Dahn et al. showed that the use of ethylene carbonate (EC)-free electrolytes is essential for the good performance of high-voltage (up to 4.4 V) lithium batteries.22 Earlier, we showed that replacing the commonly used EC by fluoroethylene carbonate (FEC) in electrolyte solutions for high-voltage Li-ion batteries results in the Received: April 6, 2017 Accepted: May 8, 2017 Published: May 8, 2017 1321

DOI: 10.1021/acsenergylett.7b00300 ACS Energy Lett. 2017, 2, 1321−1326

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Figure 1. Galvanostatic cycling results obtained for symmetric Li−Li cells cycled with current density of 2 mA cm−2 (a, b) and 0.5 mA cm−2 (c, d). Charge−discharge capacity was limited to 3.3 mAh cm−2 (a, c, d) or 2.5 mAh cm−2 (b). Electrolyte solution: (a, c) 1 M LiPF6/FEC/DMC, (b, d) 1 M LiPF6/EC/DMC (50 μL), Li foil 250 μm (blue curves) or 50 μm (red curves), 30 °C. Insets in panels a−c show voltage profiles measured in different periods of cycling life.

1100 cycles) at current density of 2 mA cm−2, with a voltage profile typical of a stable homogeneous lithium plating− stripping process.10,15 Cells cycled with the same current density in EC-based electrolyte solution demonstrate a much higher and more unstable voltage during both charge and discharge steps and fail after about 200 cycles (Figure 1b). Cycling at a lower current of 0.5 mA cm−2 also reveals the benefits of FEC-based electrolyte solution over the EC-based electrolyte (Figure 1c,d). The behavior of Li|Li cells with Li metal electrodes with a thickness of 50 μm is very similar to that of cells with Li metal electrodes with a thickness of 250 μm (Figure 1 c). Thus, the symmetric Li−Li cells cycled with FECbased electrolyte solution show cycling behavior that is markedly better than that observed for cells cycled with ECbased electrolyte solution. Importantly, cycling results obtained for symmetric Li−Li cells generally meet the requirements of commercial Li batteries imposed on anodes in terms of areal charge−discharge capacity, current density, and cycle life.31 It is remarkable that after prolonged cycling in FEC-based electrolyte solution, Li|Li symmetric cells demonstrate very stable voltage profile even with areal capacity of 6 mAh cm−2 (Figure S1). The Coulombic efficiency of Li plating/stripping process in the two electrolyte solutions was evaluated in Li/Cu asymmetric cells (Figure S2). The advantages of FEC-based solution over EC-based one are clearly seen. Coulombic efficiencies of 89% and 98.5% were obtained for EC- and FECbased solutions, respectively. Images of Li anodes and separators from the cells cycled in FEC- and EC-based electrolyte solutions (as presented in Figure 1 a,b) are shown in Figure S3. The cell cycled in FEC-

dramatically improved cycling behavior of LiCoPO4/Li cells, operating at up to 5.2 V because of the formation of effective protective surface films on these cathodes.23 We also observed excellent performance of a high-voltage Li-ion battery based on a 5 V LiNi0.5Mn1.5O4 spinel cathode and columnar silicon thinfilm anode in a 1 M LiPF6/FEC/dimethyl carbonate (DMC) electrolyte solution.24 For Li metal anodes, FEC was found to be one of the SEIforming additives that have a positive effect on their cycling efficiency and surface morphology.25−30 Zhang et al. demonstrated in ref 30 stable cycling of LiNi0.5Co0.2Mn0.3O2|Li cells with an areal capacity of 1.9 mAh cm−2 at a current density of 2.16 mA cm−2 for about 100 cycles in EC-based electrolyte solution containing 5% of FEC additive. However, prolonged cycling with a high current density and high areal capacity, comparable with that of commercial batteries,31 has not been reported for Li|Li cells in FEC-containing electrolyte solutions. Here, we demonstrate, for the first time, the extremely stable cycling of symmetric Li|Li cells in an EC-free FEC-based organic carbonate solution of 1 M LiPF6 /FEC/DMC with a current density of 2 mA cm−2 and specific areal capacity of 3.3 mAh cm−2 for more than 1110 cycles and 3500 h. We demonstrate also a stable cycling of Li|NMC cells with high loading of active cathode material with areal capacity of 3.3 mAh cm−2. These cycling results outperform those obtained with EC-containing electrolyte solutions with FEC additive. Typical galvanostatic cycling results obtained for Li|Li cells cycled in two electrolyte solutions with an areal charge− discharge capacity of 2.5−3.3 mAh/cm2 are shown in Figure 1. The cell cycled in FEC-based electrolyte solution (Figure 1a) demonstrates stable behavior for more than 3600 h (more than 1322

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Figure 2. SEM images of cycled Li electrodes obtained from the central part (a) and periphery (b) of the electrode cycled for 1100 cycles in 1 M LiPF6/FEC/DMC electrolyte solution, electrode cycled for 200 cycles in 1 M LiPF6/EC/DMC electrolyte solution (c), and pristine Li foil (d).

Figure 3. FTIR spectra of Li electrode surface after prolonged cycling in (a) 1 M LiPF6/FEC/DMC and (b) 1 M LiPF6/EC/DMC electrolyte solutions.

based electrolyte solution was stopped and disassembled after 1100 cycles of stable cycling. The separator was wet, indicating that no massive dendrite formation accompanied by the

depletion of the electrolyte solution occurred in this case. In contrast, all the separators from the cells cycled in EC-based solution were dry. It is remarkable that after more than 1100 1323

DOI: 10.1021/acsenergylett.7b00300 ACS Energy Lett. 2017, 2, 1321−1326

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Figure 4. Charge−discharge capacities vs cycling number (a) and voltage profiles (b) obtained for Li|NMC cells cycled with current density of 0.5 mA cm−2. Electrolyte solutions: 1 M LiPF6/FEC/DMC (red curves) and 1 M LiPF6/EC/DMC (black curves). Li foil 50 μm, 30 °C.

thicker surface films are formed on the surface of Li electrodes cycled in FEC-containing electrolyte solution. For EC-based solution, the main components of surface films are Li2CO3 (1511, 1454, and 873 cm−1),33−36 lithium ethylene dicarbonate (CH2OCO2Li)2, a reduction product of EC (1623, 1299, 1104, and 1072 cm − 1 ), 3 3 , 3 6 − 3 8 lithium-methyl carbonate (CH3OCO2Li), a reduction product of DMC (1645, 1196, 3003 cm−1),39 and the polycarbonate species R−O−CO−O−R with peaks in the 1750−1810 cm−1 region related to C=O stretching.40 FTIR spectra of Li electrodes cycled in FEC-based electrolyte solution contain IR bands of Li2CO3, Li-alkyl carbonates ROCO2Li,33,34,36,38 and polycarbonates, but the relative content of the polycarbonate species in the surface films formed in FEC-based solutions is higher than in the surface films formed in EC-based electrolyte solutions. The characteristic band of lithium-methyl carbonate at 1196 cm−1,39 as well as the characteristic shoulder at 3003 cm−1, are much more intense in the spectra of the Li anodes cycled in EC-based electrolyte solution, suggesting higher relative concentration of lithium-methyl carbonate in the SEI formed in this electrolyte solution, even though the content of the DMC in EC-based electrolyte solution is lower than in the FEC-based electrolyte. This means that the surface films formed on Li electrodes in FEC-based electrolyte solution contain a higher portion of the reduction products of the cyclic carbonate than the surface films that were formed in EC-based solution. This is, obviously, due to the high reactivity in the case of FEC. The SEI formed in FEC-based solutions contains a greater amount of LiPF6 salt decomposition products, namely, LiF (722 and 500 cm−1),41,42 LixPFy species with ν(P−F) stretching at 845 cm−1,27,41 and P(OR)3 components with ν(P−O−C) stretching at 970 cm−1.42 Thus, in FEC-containing solutions, the SEI on Li electrodes is enriched with FEC and salt-reduction products. This composition of surface films ensures very long-term stable cycling of Li metal electrodes in FEC-based electrolyte solution. FTIR spectra of pristine polypropylene (PP) separator (red curve) and a PP separator from a Li|Li symmetric cell cycled for 200 cycles in 1 M LiPF6/EC/DMC electrolyte solution rinsed with DMC are shown in Figure S5. As is seen, the main components of the jelly-like yellow film are PEO-like products of EC polymerization, polycarbonates, Li2CO3, and Li alkyl carbonates.39,43,44 Figure 4 exhibits the cycling performance of Li|NMC cells with FEC- and EC-based electrolyte solutions. The cells were cycled with the same high areal capacity as for Li|Li symmetric cells presented in Figure 1a, namely, about 3 mAh cm−2, which meets practical application demands. The cells demonstrate a voltage profile typical for NMC 622 cathodes.45 It is seen that

cycles in FEC-containing electrolyte solutions, the Li metal surface retains its bright metallic color. However, the morphology of the cycled anodes surface is not homogeneous and changes between its center and periphery. This observation obviously relates to the nonhomogeneous current distribution over the surface of the Li anodes caused by nonuniform pressure distribution in coin type cells. Separators from the cells cycled in EC-based electrolyte solution were covered with a soluble in water jelly-like yellow films (Figure S3). These films were formed because of massive reduction of solvents (mostly EC). The difference in the morphology between the center and periphery is more pronounced for Li anodes cycled in the FECbased electrolyte solution as these electrodes performed many more galvanostatic cycles than those cycled in EC-based solution. In addition, polymerization of EC probably affects the morphology. Scanning electron microscopy (SEM) images of cycled Li electrodes obtained from the central part and periphery of the electrode cycled for 1100 cycles in the FEC-based electrolyte solution (the locations are marked in Figure S3b) are shown in panels a and b of Figure 2, respectively. Figure 2c shows the electrode cycled for 200 cycles in the EC-based electrolyte solution, and a SEM image of pristine Li foil is presented in Figure 2d. Dendrite formation was not observed on the surface of Li electrodes cycled in both electrolyte solutions. However, a smoother surface was obtained for the case of FEC-based electrolyte solution, especially in the periphery shinny metallic part of the Li electrode. Typical Nyquist plots measured for a symmetric Li|Li cell before cycling and after 500 cycles in FEC-based electrolyte solution are presented in Figure S4a. A drastic decrease in the surface films and charge-transfer resistance of the cells due to cycling implies changes in the morphology of the Li metal electrodes with increase in the effective surface area of the electrodes.32 For the cells with EC-based solution, very high impedance before cycling is obviously caused by slow diffusion of Li ions in the pores of the PP separator due to its poor wettability in this electrolyte solution (Figure S4b). Failed cells (EC-based solutions) demonstrated impedance that was much higher than that of the cells cycled in FEC-based solution. Fourier transform infrared (FTIR) spectra of Li metal electrodes after prolonged cycling in FEC-based (a) and ECbased (b) electrolyte solutions are shown in Figure 3. The cycled electrodes were washed three times with pure DMC, dried, and hermetically closed in optical cells with a KBr window. The absorbance measured for the Li electrodes cycled in FEC-based electrolyte is higher than that of the Li electrodes after cycling in EC-based solution. This suggests that denser or 1324

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(3) Aurbach, D.; Zinigrad, E.; Teller, H.; Dan, P. Factors Which Limit the Cycle Life of Rechargeable Lithium (Metal) Batteries. J. Electrochem. Soc. 2000, 147, 1274−1279. (4) Xu, W.; Wang, J.; Ding, F.; Chen, X.; Nasybulin, E.; Zhang, Y.; Zhang, J.-G. Lithium Metal Anodes for Rechargeable Batteries. Energy Environ. Sci. 2014, 7, 513−537. (5) Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Wei, F.; Zhang, J.-G.; Zhang, Q. A Review of Solid Electrolyte Interphases on Lithium Metal Anode. Adv. Sci. 2016, 3, 1500213. (6) Vaughey, J. T.; Liu, G.; Zhang, J.-G. Stabilizing the Surface of Lithium Metal. MRS Bull. 2014, 39, 429−435. (7) Gireaud, L.; Grugeon, S.; Laruelle, S.; Yrieix, B.; Tarascon, J.-M. Lithium Metal Stripping/Plating Mechanisms Studies: A Metallurgical Approach. Electrochem. Commun. 2006, 8, 1639−1649. (8) Qian, J.; Henderson, W. A.; Xu, W.; Bhattacharya, P.; Engelhard, M.; Borodin, O.; Zhang, J.-G. High Rate and Stable Cycling of Lithium Metal Anode. Nat. Commun. 2015, 6, 6362. (9) Miao, R.; Yang, J.; Xu, Z.; Wang, J.; Nuli, Y.; Sun, L. A New Ether-Based Electrolyte for Dendrite-Free Lithium-Metal Based Rechargeable Batteries. Sci. Rep. 2016, 6, 21771. (10) Park, M. S.; Ma, S. B.; Lee, D. J.; Im, D.; Doo, S.-G.; Yamamoto, O. A Highly Reversible Lithium Metal Anode. Sci. Rep. 2015, 4, 3815. (11) Basile, A.; Bhatt, A. I.; O’Mullane, A. P. Stabilizing Lithium Metal Using Ionic Liquids for Long-Lived Batteries. Nat. Commun. 2016, 7, ncomms11794−11794. (12) Ma, Q.; Fang, Z.; Liu, P.; Ma, J.; Qi, X.; Feng, W.; Nie, J.; Hu, Y.-S.; Li, H.; Huang, X.; et al. Controlled Lithium Dendrite Growth by a Synergistic Effect of Multilayered Graphene Coating and an Electrolyte Additive. ChemElectroChem 2016, 3, 531−536. (13) Ryou, M.-H.; Lee, Y. M.; Lee, Y.; Winter, M.; Bieker, P. Mechanical Surface Modification of Lithium Metal: Towards Improved Li Metal Anode Performance by Directed Li Plating. Adv. Funct. Mater. 2015, 25, 834−841. (14) Kim, J.-S.; Kim, D. W.; Jung, H. T.; Choi, J. W. Controlled Lithium Dendrite Growth by a Synergistic Effect of Multilayered Graphene Coating and an Electrolyte Additive. Chem. Mater. 2015, 27, 2780−2787. (15) Kazyak, E.; Wood, K. N.; Dasgupta, N. P. Improved Cycle Life and Stability of Lithium Metal Anodes through Ultrathin Atomic Layer Deposition Surface Treatments. Chem. Mater. 2015, 27, 6457−6462. (16) Luo, J.; Lee, R.-C.; Jin, J.-T.; Weng, Y.-T.; Fang, C.-C.; Wu, N.L. A Dual-Functional Polymer Coating on a Lithium Anode for Suppressing Dendrite Growth and Polysulfide Shuttling in Li−S Batteries. Chem. Commun. 2017, 53, 963−966. (17) Liu, Y.; Lin, D.; Liang, Z.; Zhao, J.; Yan, K.; Cui, Y. LithiumCoated Polymeric Matrix as a Minimum Volume-Change and Dendrite-Free Lithium Metal Anode. Nat. Commun. 2016, 7, 10992. (18) Lee, H.; Song, J.; Kim, Y.-J.; Park, J.-K.; Kim, H.-T. Structural Modulation of Lithium Metal-Electrolyte Interface with ThreeDimensional Metallic Interlayer for High-Performance Lithium Metal Batteries. Sci. Rep. 2016, 6, 30830. (19) Lu, L.-L.; Ge, J.; Yang, J.-N.; Chen, S.-M.; Yao, H.-B.; Zhou, F.; Yu, S.-H. Free-Standing Copper Nanowire Network Current Collector for Improving Lithium Anode Performance. Nano Lett. 2016, 16, 4431−4437. (20) Ding, F.; Xu, W.; Graff, G. L.; Zhang, J.; Sushko, M.; Chen, X.; Shao, Y.; Engelhard, M. H.; Nie, Z.; Xiao, J.; et al. Dendrite-Free Lithium Deposition via Self-Healing Electrostatic Shield Mechanism. J. Am. Chem. Soc. 2013, 135, 4450−4456. (21) Choi, J. W.; Aurbach, D. Promise and reality of post-lithium-ion batteries with high energy densities. Nature Reviews Materials 2016, 1, 16013. (22) Ma, L.; Glazier, S. L.; Petibon, R.; Xia, J.; Peters, J. M.; Liu, Q.; Allen, J.; Doig, R. N. C.; Dahn, J. R. A Guide to Ethylene CarbonateFree Electrolyte Making for Li-Ion Cells. J. Electrochem. Soc. 2017, 164, A5008−A5018. (23) Markevich, E.; Salitra, G.; Fridman, K.; Sharabi, R.; Gershinsky, G.; Garsuch, A.; Semrau, G.; Schmidt, M. A.; Aurbach, D. Fluoroethylene Carbonate as an Important Component in Electrolyte

FEC-based solution significantly the outperforms EC-based solution. One possible reason for this difference is the better stability of electrodes in the FEC-based electrolyte solutions at high voltage, as is seen in Figure S7. In summary, excellent cycling performance of Li metal anodes was demonstrated, for the first time, in EC-free FECbased organic carbonate electrolyte solutions, which were shown to be the most promising electrolyte solutions for highenergy-density and high-voltage rechargeable Li batteries. Symmetric Li|Li cells demonstrated an extremely long cycle life and a stable voltage profile for more than 1100 cycles at a current density of 2 mA cm−2 and an areal capacity of 3.3 mAh cm−2 with a minimal amount of electrolyte solution, sufficient for wetting the separator in coin cells (50 μL/cell). The use of the FEC-based electrolyte made it possible to obtain stable cycling of Li|NMC cells with high loading of active cathode material with areal capacity of 3.3 mAh cm−2. These results are very promising, considering that the cycling parameters are comparable with those of commercial batteries. We attribute the high performance of the Li anodes presented in this work to the formation of a stable and efficient SEI on the surface of the Li metal electrodes cycled in FEC-based electrolyte solutions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00300. Experimental Section and Figures S1−S7 showing galvanostatic cycling results obtained for symmetric Li| Li cells with two areal capacities, Coulombic efficiencies measured for Li|Cu cells, images of Li electrodes and separators, Nyquist plots, FTIR spectra of PP separators, image of vacuum tight transferring cell from glovebox to SEM, and electrochemical windows of the two electrolyte solutions using Pt electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Doron Aurbach: 0000-0002-1151-546X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Israel Committee for High Education (CHE) in the framework of the INREP project.

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REFERENCES

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