Dendrite-Free Lithium Deposition with SelfAligned Columnar Structure in a Carbonate− Ether Mixed Electrolyte HaiLong Yu,†,‡,§ JunNian Zhao,†,‡,§ LiuBin Ben,†,‡ YuanJie Zhan,†,‡ YiDa Wu,†,‡ and XueJie Huang*,†,‡ †
P.R. China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Batteries with lithium metal anodes are promising because of lithium’s high energy density. However, the growth of Li dendrites on the surface of the Li electrode in a liquid electrolyte during cycling reduces the safety and cycle performance of batteries, hindering their commercial application. In this work, we observe for the first time a smooth and dendrite-free Li deposition with a vertically grown, self-aligned, and highly compact columnar structure formed during cycling in a mixed carbonate−ether electrolyte. The stable microsized (∼10 μm in diameter and ∼20 μm in length) Li deposits are aligned in arrays on the surface of the Li electrode. The columnar Li deposits still exhibit a dendrite-free morphology and a compact structure after 200 cycles at a current density of 1 mA/cm2 and a 1.5 mAh/cm2 cycling capacity in a mixed carbonate−ether electrolyte. This work shows an optimiztic outlook for Li batteries with liquid electrolytes.
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electrode, reducing the active Li content of the Li electrode. Moreover, if Li dendrites grow across the membrane of the battery, serious safety hazards may result, e.g., fire or explosion of the battery.2,3 Therefore, the growth of Li dendrites is one of the major problems restricting the practical application of Li metal in rechargeable batteries. The initial study of the formation of Li dendrites in liquid electrolytes can be traced back to the 1990s, and numerous experimental and computational works have been performed since that time.1,7−12 Many methods have been used to suppress the growth of Li dendrites in liquid electrolytes, such as adding a mechanically strong interface layer,13−16 packaging Li into a three-dimensional structure,3,7,14,17 producing an electrostatic shield, and increasing the Li+ concentration in the vicinity of the Li metal anode. 10,18−20 However, the accumulation of dead Li and the growth of Li dendrites after
nergy density is becoming the most critical indicator for energy storage systems because of the continuous increase in demand for consumer electronics, electric vehicles, and grid energy storage.1,2 The development of materials with high energy densities for batteries is a radical approach to overcoming the bottleneck created by the increasing requirement for high energy density. The rechargeable lithium (Li) metal anode as a “final anode” for energy storage systems has been considered for decades because of its extremely high theoretical specific capacity (3860 mAh/g) and its lowest negative electrochemical potential (−3.04 V vs the standard hydrogen electrode).1−4 However, Li dendrites unavoidably form during repeated charging−discharging cycles in liquid electrolytes.1,4 The growth of Li dendrites increases the specific surface area of the Li electrode, accelerating the reaction between the electrolyte and the highly chemically active Li surface to generate a solid electrolyte interphase (SEI) during cycles, thereby reducing the reversible capacity of the Li electrode.5,6 Furthermore, the growth of Li dendrites results in the formation of “dead Li,” i.e., Li dendrites fall from the Li © XXXX American Chemical Society
Received: March 29, 2017 Accepted: May 7, 2017
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Figure 1. SEM images of the surface morphology of the Li electrodes after the 50th cycle at 0.5 mA/cm2 in (a1−3) D/E-0, (b1−3) D/E-10, (c1−3) D/E-20, and (d1−3) D/E-30. The battery was disassembled at the fully charged state. The amounts of Li deposition were all 1 mAh/cm2.
of the mixed electrolyte were prepared by adding volume ratios of 0%, 10%, 20% and 30% DME, (D/E-0, D/E-10, D/E-20, and D/E-30, respectively). Both cyclic voltammetry (CV) measurements and galvanostatic charge−discharge cycling tests of the battery demonstrate that the carbonate−ether mixed electrolyte is stable under 3.8 V and that no significant side reaction between the anode and cathode occur, as shown in Figure S1. In addition, the battery can be cycled stably at potentials under 3.8 V in the electrolyte with 100% DME, as shown in Figure S2. Figure 1 shows typical SEM images of the surface morphology of the Li electrodes after the 50th cycle in electrolytes with various DME concentrations. In the absence of DME (D/E-0), a typical needlelike Li dendritic covering was observed on the surface of the Li electrode (Figure 1a1−3), which is similar to that reported previously.24,25 With the addition of DME (D/E-10 to D/E-30), the morphology of the deposited Li changed drastically. The needlelike Li dendrites gradually disappeared from the surface of the Li electrode with increasing DME concentration in the electrolyte (Figure 1b1−3−d1−3). The loose and irregularly deposited Li was transformed into a dense and uniform structure. The density and size of the Li deposits on the Li electrode increased substantially with increasing concentration of DME. In the case of the DME-30 electrolyte, the Li deposits exhibit a flat outer surface and an average diameter of ∼10 μm (Figure 1d1−3). In addition, the cross-sectional SEM image (Figure S3) shows that the deposited Li exhibits a columnlike structure. Note that the Li electrode cycled in D/E-30 shows Li deposits that are even denser than those of the electrode cycled in the electrolyte with 100% DME (Figure S4). The D/E-30 shows the best morphology of Li deposits. Zhang et al. have reported a similar morphology of Li deposits that were free of dendrites and exhibited a self-aligned nanorod structure when Cs+ or H2O
prolonged cycling or storage are still observed in almost all these methods.20−22 Therefore, a method that can suppress the formation of Li dendrites and improve the cycling performance of a Li electrode is strongly desired. In addition, the pores of current commercial separators for Li-ion batteries are submicrometer-sized, e.g., the average pore size of Celgard 2500, a typical separator, is 0.209 μm × 0.054 μm, which suggests that the deposited Li should have a much larger size to avoid a short circuit.23 Here, we report our discovery of an interesting phenomenon in which the formation of Li dendrites can be self-inhibited in a carbonate−ether mixed electrolyte. With increasing number of cycles, the deposited Li gradually forms a columnlike morphology, which remains stable up to 200 cycles under a current density of 1 mA/cm2. The columnlike Li deposits with a size of ∼10 μm substantially reduce the specific surface area compared to the needlelike Li dendrites, resulting in a reduced reaction rate between the Li electrode and the liquid electrolyte. To the best of our knowledge, this work represents the first report of these large columnlike Li deposits, which enhance the safety and cycling performance of a Li battery. The carbonate−ether mixed electrolyte was found to be stable between 2.8 and 3.8 V. Herein, the morphology of the Li deposits is investigated by both X-ray photoelectron spectroscopy (XPS) depth profiling and scanning electron microscopy (SEM). The battery in this work consisted of LiFePO4 (LFP) as the cathode electrode, a Li plate electrode as the anode, and a commercial membrane, resulting in an LFP|Li configuration. The carbonate−ether mixed electrolyte was prepared by adding 1,2-dimethoxyethane (DME) to ethylene carbonate (EC):dimethyl carbonate (DMC) = 1:1 (volume ratio) carbonate electrolyte containing 1 M LiPF6. Four different concentrations 1297
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Figure 2. Electrochemical performance of LFP|Li battery and SEM images of the morphology of the electrochemically deposited Li electrode in the D/E-30 electrolyte after the 200th cycling at 1 mA/cm2. (a) Capacity retention and Coulombic efficiency at 1 mA/cm2 (0.1 mA/cm2 in the first cycle). (b−d) Top-view SEM images of Li electrode surface. (e) Cross-sectional SEM image showing the columnar structure of the Li deposits. The amount of Li deposition is 1.5 mAh/cm2.
was added to the electrolyte.20,22 They explained that this characteristic structure could reduce the surface area of Li anodes, which could, in turn, minimize the loss of Li due to the chemical and electrochemical reactions between the Li metal and the electrolyte.20 However, because of the relatively small size of the Li deposits (10 cycles.20,22 In our work, however, the micrometer-sized (diameter ∼10 μm) columnar Li deposits can effectively reduce the specific area and the risk of piercing the separator. Furthermore, large, dense Li deposits can also reinforce the structure against deterioration. SEM images of the morphology of the Li electrode in the discharge state after cycling further suggest that
the columnar Li deposits provide reversible capacity and are not dead Li (Figure S5). To examine the stability of the battery during high-current and prolonged cycling, a battery with D/E-30 was further tested using a 1.5 mAh/cm2 cathode loading capacity. The electrochemical performance of the battery after 200 cycles at 1 mA/ cm2 shows excellent capacity retention and Coulombic efficiency (Figure 2a). The top-view surface morphology of the Li electrode after prolonged cycling (Figure 2b−d) demonstrates that the Li deposits retains their columnar structure without forming any needlelike Li dendrites on the surface of the Li electrode. The cross-sectional SEM image of the surface Li deposits further shows that they have a smooth outside surface (Figure 2e). The height of each columnar Li deposit is ∼24 μm. These results demonstrate that no obvious 1298
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Figure 3. XPS depth profile analysis of a Li electrode after the 50th cycle at 0.5 mA/cm2 current density in carbonate (D/E-0) and mixed carbonate−ether (D/E-30) electrolytes. Surface element contents of Li electrodes cycled in (a) D/E-0 and (b) D/E-30 electrolytes. (c) Fitted C 1s and O 1s XPS spectra of Li electrodes cycled in D/E-0 and D/E-30 electrolytes.
evaluated by plating−stripping and polarizing measurements with a Li|Li symmetric battery (Figure S7), which exhibited excellent cycling performance, with no obvious increase of voltage polarization.
morphology change or mechanical failure of Li deposits is evident after 200 cycles. In addition, the overpotential of the battery increased by only ∼10 mV after 200 cycles (Figure S6), consistent with the excellent structural stability. The cycling stability and the safety of the mixed electrolytes were also 1299
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Figure 4. SEM images of the transformation of Li deposits from isolated irregular structures to large-size columnar structures on the surface of Li electrodes cycled in D/E-30 electrolyte. (a1−3) Li electrode before cycling, (b1−3) Li electrode after the 1st cycle at 0.1 mA/cm2, (c1−3) Li electrode after the 10th cycle at 0.5 mA/cm2, (d1−3) Li electrode after the 15th cycle at 0.5 mA/cm2, and (e) a schematic of the evolution of the Li deposits with increasing cycle number. The batteries were disassembled at the fully charged state; the cathode capacity was approximately 1 mAh/cm2.
related to Li2O (528.4 eV), LiOH (531.2 eV), Li2CO3 (531.8 eV), ROCO2Li (532 eV), and −(CH2CH2O)n− (533.4 eV).26,27 The carbon peaks at 286.7 and 289.2 eV usually indicate the presence of Li carbide species such as ROCO2Li from the reduction of ethylene carbonate, which is still present even after 120 s of etching.26 When DME was present, the C 1s spectra of the SEI layer formed after cycling show much weaker peaks related to Li alkyl carbonate species at 286.7 and 289.2 eV. The peak located at 533.4 eV (O 1s) is much stronger for −(CH2CH2O)n−. Furthermore, the amounts of Li2O, Li2CO3, and LiOH are much lower than the corresponding amounts observed in batteries without DME. The aforementioned XPS results show that no difference exists in the composition of the SEI layer with or without DME added to the electrolyte. The amount of LiF and −(CH2CH2O)n− increased in the SEI layer formed on the Li
The excellent cycling performance of the battery with DME added to the electrolyte is attributable to the SEI formed on the surface of the Li deposits. Li electrodes after the 50th cycle in D/E-0 and D/E-30 were investigated by XPS depth profile analysis. The results of the elemental analysis and the fitted C and O spectra are shown in Figure 3. Compared to the D/E-0 electrolyte, for each etch time period, the F content substantially increased; by contrast, the O content substantially decreased for D/E-30 (Figure 3a,b). The fitted XPS spectra of elemental F, as shown in Figure S8, show that the majority of elemental F is associated with LiF in both electrodes. The fitted C XPS spectrum for D/E-0 (Figure 3c) shows peaks related to Li carbonate/Li alkyl carbonate [e.g., CO3 (289.8 eV)]; C−O in −(CH2CH2O)n− and ROCO2Li (286.7 eV); CO (289.2 eV); and −C−C− (284.8 eV) in the C 1s spectra. The fitted O 1s XPS spectrum (Figure 3c) shows peaks 1300
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electrode cycled in the D/E-30 electrolyte. Li2O and LiF are believed to enhance the mechanical strength of the SEI and to reinforce the ability to suppress Li dendrite formation because of their high thermodynamic and kinetic stablility.15,22 The flexible −(CH2CH2O)n− components also contribute to the flexibility of the SEI. This type of SEI layer contains many flexible Li organic species and stable inorganic Li salts, which can resist the attack of hydrogen fluoride in the electrolyte. Thus, the SEI layer could effectively protect the metallic Li from continuous reactions with the electrolyte, leading to better stability of the Li electrode and inhibiting the growth of Li dendrites.26,27 We investigated the process by which the columnar Li deposits form in detail by examining the morphology of the Li electrode (D/E-30) at the fully charged state after various numbers of cycles. Figure 4a1−a3 shows that the Li electrode exhibits a smooth surface before cycling. After the first charge at 0.1 mA/cm2, isolated Li deposits with a size of ∼15 μm appear on the surface of the Li electrode, as shown in Figure 4b1−3. Magnified SEM images (Figure S9) show that the passivation layer formed on the Li electrode starts to break, which may be caused by this nonuniform Li deposition. After 10 cycles at 0.5 mA/cm2, more and dense Li deposits with irregular sizes appear (average size ∼6 μm in diameter) on the Li electrode (Figure 4c1−3). The outside surface of the Li deposits is smoother than that after the first cycle. With additional cycling (15th cycle), the irregularly shaped Li deposits transform into columnar Li deposits (Figure 4d1−3). These results suggest that the columnar Li deposits are gradually formed by the increasing number of cycles, accompanied by the inhibition of growth of Li dendrites. The formation of Li deposits and the inhibition of Li dendrites are shown in the schematic in Figure 4e. In summary, we reported for the first time the smooth and dendrite-free deposition of Li as a vertically grown, self-aligned, and highly compact columnar structure. The growth of dendrite-free columnar Li deposits was achieved using a mixed carbonate−ether electrolyte. When DME was added to the electrolyte, the morphology of Li deposits changed from small, dendritelike loose Li deposits into large (10 μm in diameter and 20 μm in length) dense columnar Li deposits. The columnar Li deposits exhibited a stable structure and retained their dendrite-free morphology and dense structure even after 200 cycles under 1 mA/cm2 current density and 1.5 mAh/cm2 cycling capacity in D/E-30 electrolyte (30% DME was added to the carbonate electrolyte). This unique morphology greatly extends the cycle life and enhances the safety of Li metal batteries. The XPS depth profile analysis showed that the SEI formed in mixed carbonate−ether electrolytes contains a mass of LiF and −(CH2CH2O)−, which are thermodynamically and kinetically stable and have good flexibility, contributing to the suppression of the formation of Li dendrites. The evolution of the morphology of Li deposits with increasing number of cycles suggested that the columnar Li deposits were gradually formed, although the exact formation mechanism is not clear and needs further study. However, this work proves that electrolyte optimization is an effective approach for suppressing the formation of Li dendrites even at high currents and upon prolonged cycling. The fundamental findings revealed in this work open a new avenue for designing Li metal battery electrolytes.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00273. Details of experimental method, CV and galvanostatic results of batteries in various electrolytes, SEM images of Li electrode in various electrolytes, and the XPS fitting date (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
XueJie Huang: 0000-0001-5900-678X Author Contributions §
H.Y. and J.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is financially supported the National Program on Key Basic Research Project of China (973 Program, Grant 2013CB934002), “Tackling Key Technologies for Development and Industrialization of Power Lithium Ion Battery with High Specific Energy” of National Key Research and Development Program of China (Grant 2016YFB0100500), National Natural Science Foundation of China (Grant 51502334), and China Postdoctoral Science Foundation Funded Project (Grant 2016M591277).
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